How many times have you heard of people spending hundreds of dollars on the latest and greatest cell phone only to be disappointed by the bad signal? Dropping calls is another very annoying occurrence with cell phones. You need to look very carefully into the cell phone carrier that you wish to sign up with. You will be signing a contract usually for one year so make sure it’s money well spent.

Who are the main cell phone carrier?

* AT&T
* T-Mobile
* Verizon Wireless
* Cingular
* Nextel
* ALLTEL
* Sprint PCS

The above carriers are only a hand full in and every expanding mobile world. All will approach with special offers and incentives with camera cell phones etc to sign you up. The positives are obvious. You get a free cell phone and maybe some extra minutes talk time but they get a customer for a year. Most carriers have good coverage but it is worth your while looking at your options.

If you rely on your cell phone for work such as sales reps or drivers etc you need to look into the roaming charges. Some people think the charges may only vary slightly from one carrier to another so why bother. This is a lazy approach and untrue. You could save yourself hundreds of dollars per year simply by looking around. You can check the rates out online on most of the carrier’s websites. Roaming rates can be expensive so look long and hard before you decide.

I don’t need to travel so roaming charges are not a worry to me:

If you are happy enough using your phone mainly from the house or just plodding around you are not going to have any concern of high charges for roaming, but there are other ways to save money and lots of it. Many people never think too much about the SMS Text messages they send. Yes it saves money rather than calling and it is fast and generally reliable, however, different carriers have different text rates. You might not think that one-cent saving in not much and rightly so but if you are a regular Text user you need look at the overall yearly saving. Most cell phone carrier companies will offer special saving incentives on SMS Text so look into it.

Where else can I save money?

The latest and the greatest, the camera phone is as popular as a DVD. Everywhere you look people seem to have them. Great fun and very handy for that special moment for when you only wished you had a camera but very I repeat very expensive you decide to send many pictures to friends and family. Here by looking at your different options you can save plenty of you hard earned dollars. All it takes is a quick look around the web or a phone call; most of the carriers have free toll numbers. Monthly service rental will also vary from one company to another.

Another Tip

With so many cellular phone stores around you will be spoiled for choice. Remember stores make commission so if you are in a large shopping mall the chances are that there are a number of different cell stores. Check out the different rates and you will see a difference. Money is not everything, going back to the start of this article you need to make sure that you have an exceptional signal. If you are going to be a loyal customer for a year or so you should expect nothing but the best back in service.

What if I already have my own cell phone?

This is not a problem. If you are out of contract with one of the cell phone carriers you are free to look around just like from the beginning. You can either use your own cell or take them up on their offers, as most will offer you a free cell phone as a new user to the network.

What if I want to terminate my contract before it has officially ended?

Look long and hard at your contract before you sign, especially the smaller print. All carriers have different clauses in their contract but if you want to terminate early there usually is a penalty charge of some sort. One way out of this is to get a prepaid cell phone where you have no contract. You are free to swap from one carrier to another as you please. Be aware prepaid cell phones are more expensive pre minute talk time and Text than if you where on a monthly fee. Read more....!

GSM was the most successful second generation cellular telecommunications system, but the need for higher data rates spawned new developments to enable data to be transferred at much higher rates. The first system to make an impact on the market was GPRS. The letters GPRS stand for General Packet Radio System, and the system enables much higher data rates to be achieved.

GPRS became the first stepping-stone on the path between the second-generation GSM cell phone system and the W-CDMA / UMTS system. With GPRS offering data services with data rates up to 115 kbps, facilities such as web browsing and other services requiring data transfer became possible. Although some data could be transferred using GSM, the rate was too slow for real data applications.

Packet switching
The key element of GPRS is that it uses packet switched data rather than circuit switched data, and this technique makes much more efficient use of the available capacity. This is because most data transfer occurs in what is often termed a "bursty" fashion. The transfer occurs in short peaks, followed by breaks when there is little or no activity.

Using a traditional approach a circuit is switched permanently to a particular user. This is known as a circuit switched mode. In view of the bursty nature of data transfer it means that there are periods when it will not be carrying data.

To improve the situation the overall capacity can be shared between several users. To achieve this the data is split into packets and tags inserted into the packet to provide the destination address. Packets from several sources can then be transmitted over the link. As it is unlikely that the data burst for different users will occur all at the same time, by sharing the overall resource in this fashion, the channel, or combined channels can be used far more efficiently. This approach is known as packet switching, and it is at the core of many cellular data systems, and in this case GPRS.

Network
GPRS and GSM are able to operate alongside one another on the same network, and using the same base stations. However upgrades are needed. The network upgrades reflect many of those needed for 3G, and in this way the investment in converting a network for GPRS prepares the core infrastructure for later evolution to a 3G W-CDMA / UMTS.

The upgraded network, as described in later pages of this tutorial, has both the elements used for GSM as well as new entities that are used for the GPRS packet data service.

Mobiles
Not only does the network need to be upgraded for GPRS, but new GPRS mobiles are also required. It is not possible to upgrade an existing GSM mobile for use as a GPRS mobile, although GSM mobiles can be used for GSM speech on a network that also carries GPRS. To utilise GPRS new modes are required to enable it to transmit the data in the required format.

Network
Although designed to run alongside the GSM system, the core network structure updated for GPRS has several new elements added to enable it to carry the packet data. The network between the BSC and BTS is similar, but behind this there is a new infrastructure to support the packet data.

For GPRS, the data from the BSC is routed through what is termed a Serving GPRS Support Node (SGSN). This forms the gateway to the services within the network, and then a Gateway GPRS Support Node (GGSN) which forms the gateway to the outside world.

SGSN
The SGSN serves a number of functions for GPRS mobiles. It enables authentication to occur, and it then tracks the location of the mobile within the network, and ensures that the quality of service is to the required level.

For the network protocols there are two layers that are used and supported by GPRS, namely X25 and IP. In operation the protocols assign addresses (Packet Data Protocol or PDP addresses) to the devices in the network for the purpose of routing the data through the system. Thus the GGSN appears as a data gateway to the public packet network, and thus the fact that the users are mobiles cannot be seen.

In operation the mobile must attach itself to the SGSN and activate its PDP address. This address is supplied by the GGSN which is associated with the SGSN. As a result a mobile can only attach to one SGSN, although once assigned its address it can receive data from multiple GGSNs using multiple PDP addresses.

GPRS mobiles
Not all GPRS mobiles are designed to offer the same levels of service. As a result they are split into three basic categories according to their capabilities in terms of the ability to connect to GSM and GPRS facilities:

Class A: - This class describes mobile phones that can be connected to both GPRS and GSM services at the same time.
Class B: - These mobiles can be attached to both GPRS and GSM services but they can be used on only one service at a time. A Class B mobile can make or receive a voice call, or send and or receive a SMS message during a GPRS connection. During voice calls or texting the GPRS service is suspended but it is re-established when the voice call or SMS session is complete.
Class C: - This classification covers phones that can be attached to either GPRS or GSM services but user needs to switch manually between the two different types.

GPRS mobiles are also categorized by the data rates they can support. Within GSM there are eight time slots that can be used to provide TDMA, allowing multiple mobiles onto a single RF signal carrier. Within GPRS it is possible to use more than one slot to enable much higher data rates to be achieved when these are available. The different speed classes of the mobiles are dependent upon the number of slots that can be used in either direction. There are a total of 29 speed classes. Class one mobiles are able to send and receive in one slot in either direction, i.e. uplink and downlink, and class 29 mobiles are able to send and receive in all eight slots. The classes within these two limits are able to support sending and receiving in different combinations of uplink and downlink slots.

In order to accommodate the packet data within GPRS it has been necessary to develop the coding schemes. Additionally the layers based on the OSI system has become more important than it was for some of the previous systems and descriptions what are contained within these layers are found below.

GPRS coding
GPRS offers a number of coding schemes with different levels of error detection and correction. These are used dependent upon the radio frequency signal conditions and the requirements for the data being sent. These are given labels CS-1 to CS-4:

CS-1: This applies the highest level of error detection and correction. It is used in scenarios when interference levels are high or signal levels are low. By applying high levels of detection and correction, this prevents the data having to be re-sent too often. Although it is acceptable for many types of data to be delayed, for others there is a more critical time element. This level of detection and coding results in a half code rate, i.e. for every 12 bits that enter the coder, 24 bits result. It results in a throughput of 9.05 kbps actual throughput data rate.
CS-2: This error detection and coding scheme is for better channels. It effectively uses a 2/3 encoder and results in a real data throughput of 13.4 kbps which includes the RLC/MAC header etc.
CS-3: This effectively uses a 3/4 coder and results in a data throughput of 15.6 kbps.
CS-4: This scheme is used when the signal is high and interference levels are low. No correction is applied to the signal allowing for a maximum throughput of 21.4 kbps. If all eight slots were used then this would enable a data throughput of 171.2 kbps to be achieved.

In addition to the error detection and coding schemes, GPRS also employs interleaving techniques to ensure the effects of interference and spurious noise are reduced to a minimum. It allows the error correction techniques to be more effective as interleaving helps reduce the total corruption if a section of data is lost.

As blocks of 20 ms data are carried over four bursts, with a total of 456 bits of information, a total of either 181, 268, 312, or 428 bits of payload data are carried dependent upon the error detection and coding scheme chosen, i.e. from CS-1 to CS-4, respectively.

Layers
Software plays a very large part in the current cellular communications systems. To enable it to be sectioned into areas that can be addressed separately, the concept of layers has been developed. It is used in GSM and other cellular systems but as they become more data-centric, the idea takes a greater prominence. Often these are referred to as layers, 1, 2, and 3.

Layer 1 concerns the physical link between the mobile and the base station. This is often subdivided into two sub-layers, namely the Physical RF layer that includes the modulation and demodulation, and the Physical link layer that manages the responses and controls required for the operation of the RF link. These include elements such as error correction, interleaving and the correct assembly of the data, power control, and the like.

Above this are the Radio Link Control (RLC) and the Medium Access Control (MAC) layers. These organise the logical links between the mobile and the base station. They control the radio link access and they organise the logical channels that route the data to and from the mobile.

There is also the Logical Link Layer (LLC) that formats the data frames and is used to link the elements of the core network to the mobile.

GPRS physical channel
GPRS builds on the basic GSM structure. GPRS uses the same modulation and frame structure that is employed by GSM, and in this way it is an evolution of the GSM standard. Slots can be assigned dynamically by the BSC to GPRS calls dependent upon the demand, the remaining ones being used for GSM traffic.

There is a new data channel that is used for GPRS and it is called the Packet Data Channel (PDCH). The overall slot structure for this channel is the same as that used within GSM, having the same power profile, and timing advance attributes to overcome the different signal travel times to the base station dependent upon the distance the mobile is from the base station. This enables the burst to fit in seamlessly with the existing GSM structure.

Each burst of information for GPRS is 0.577 mS in length and is the same as that used in GSM. It also carries two blocks of 57 bits of information, giving a total of 114 bits per burst. It therefore requires four bursts to carry each 20 mS block of data, i.e. 456 bits of encoded data.

The BSC assigns PDCHs to particular time slots, and there will be times when the PDCH is inactive, allowing the mobile to check for other base stations and monitor their signal strengths to enable the network to judge when handover is required. The GPRS slot may also be used by the base station to judge the time delay using a logical channel known as the Packet Timing Advance Control Channel (PTCCT).

Channel allocation
Although GPRS uses only one physical channel (PDCH) for the sending of data, it employs several logical channels that are mapped into this to enable the GPRS data and facilities to be managed. As the data in GPRS is handled as packet data, rather than circuit switched data the way in which this is organised is very different to that on a standard GSM link. Packets of data are assigned a space within the system according to the current needs, and routed accordingly.

The MAC layer is central to this and there are three MAC modes that are used to control the transmissions. These are named fixed allocation, dynamic allocation, and extended dynamic allocation.

The fixed allocation mode is required when a mobile requires a data to be sent at a consistent data rate. To achieve this, a set of PDCHs are allocated for a given amount of time. When this mode is used there is no requirement to monitor for availability, and the mobile can send and receive data freely. This mode is used for applications such as video conferencing.

When using the dynamic allocation mode, the network allocates time slots as they are required. A mobile is allowed to transmit in the uplink when it sees an identifier flag known as the Uplink Status Flag (USF) that matches its own. The mobile then transmits its data in the allocated slot. This is required because up to eight mobiles can have potential access to a slot, but obviously only one can transmit at any given time.

A further form of allocation known as extended dynamic allocation is also available. Use of this mode allows much higher data rates to be achieved because it enables mobiles to transmit in more than one slot. When the USF indicates that a mobile can use this mode, it can transmit in the number allowed, thereby increasing the rate at which it can send data.

Logical channels
There is a variety of channels used within GPRS, and they can be set into groups dependent upon whether they are for common or dedicated use. Naturally the system does use the GSM control and broadcast channels for initial set up, but all the GPRS actions are carried out within the GPRS logical channels carried within the PDCH.

Broadcast channels:
Packet Broadcast Central Channel (PBCCH): This is a downlink only channel that is used to broadcast information to mobiles and informs them of incoming calls etc. It is very similar in operation to the BCCH used for GSM. In fact the BCCH is still required in the initial to provide a time slot number for the PBCCH. In operation the PBCCH broadcasts general information such as power control parameters, access methods and operational modes, network parameters, etc, required to set up calls.

Common control channels:
Packet Paging Channel (PPCH): This is a downlink only channel and is used to alert the mobile to an incoming call and to alert it to be ready to receive data. It is used for control signalling prior to the call set up. Once the call is in progress a dedicated channel referred to as the PACCH takes over.
Packet Access Grant Channel (PAGCH): This is also a downlink channel and it sends information telling the mobile which traffic channel has been assigned to it. It occurs after the PPCH has informed the mobile that there is an incoming call.
Packet Notification Channel (PNCH): This is another downlink only channel that is used to alert mobiles that there is broadcast traffic intended for a large number of mobiles. It is typically used in what is termed point-to-point multicasting.
Packet Random Access Channel (PRACH): This is an uplink channel that enables the mobile to initiate a burst of data in the uplink. There are two types of PRACH burst, one is an 8 bit standard burst, and a second one using an 11 bit burst has added data to allow for priority setting. Both types of burst allow for timing advance setting.

Dedicated control channels:
Packet Associated Control Channel (PACCH): This channel is present in both uplink and downlink directions and it is sued for control signalling while a call is in progress. It takes over from the PPCH once the call is set up and it carries information such as channel assignments, power control messages and acknowledgements of received data.
Packet Timing Advance Common Control Channel (PTCCH): This channel, which is present in both the uplink and downlink directions is used to adjust the timing advance. This is required to ensure that messages arrive at the correct time at the base station regardless of the distance of the mobile from the base station. As timing is critical in a TDMA system and signals take a small but finite time to travel this aspect is very important if long guard bands are not to be left.

Dedicated traffic channel:
Packet Data Traffic Channel (PDTCH): This channel is used to send the traffic and it is present in both the uplink and downlink directions. Up to eight PDTCHs can be allocated to a mobile to provide high speed data.

When looking at the way in which GPRS operates, it can be seen that there are three basic modes in which it operates. These are: initialisation / idle, standby, and ready.

Initialisation / idle
When the mobile is turned on it must register with the network and update the location register. This is very similar to that performed with a GSM mobile, but it is referred to as a location update. It first locates a suitable cell and transmits a radio burst on the RACH using a shortened burst because it does not know what timing advance is required. The data contained within this burst temporarily identifies the mobile, and indicates that the reason for the update is to perform a location update.

When the mobile performs its location update the network also performs an authentication to ensure that it is allowed to access the network. As for GSM it accesses the HLR and VLR as necessary for the location update and the AuC for authentication. It is at registration that the network detects that the mobile has a GPRS capability. The SGSN also maintains a record of the location of the mobile so that data can be sent there is required.

Standby
The mobile then enters a standby mode, periodically updating its position as required. It monitors the MNC of the base station to ensure that it has not changed base stations and also looks for stronger base station control channels.

The mobile will also monitor the PPCH in case of an incoming alert indicating that data is ready to be sent. As for GSM, most base stations set up a schedule for paging alerts based on the last figures of the mobile number. In this way it does not have to monitor all the available alert slots and can instead only monitor a reduced number where it knows alerts can be sent for it. In this way the receiver can be turned off for longer and battery life can be extended.

Ready
In the ready mode the mobile is attached to the system and a virtual connection is made with the SGSN and GGSN. By making this connection the network knows where to route the packets when they are sent and received. In addition to this the mobile is likely to use the PTCCH to ensure that its timing is correctly set so that it is ready for a data transfer should one be needed.

With the mobile attached to the network, it is prepared for a call or data transfer. To transmit data the mobile attempts a Packet Channel Request using the PRACH uplink channel. As this may be busy the mobile monitors the PCCCH which contains a status bit indicating the status of the base station receiver, whether it is busy or idle and capable of receiving data. When the mobile sees this status bit indicates the receiver is idle, it sends its packet channel request message. If accepted the base station will respond by sending an assignment message on the PAGCH on the downlink. This will indicate which channel the mobile is to use for its packet data transfer as well as other details required for the data transfer.

This only sets up the packet data transfers for the uplink. If data needs to be transferred in the downlink direction then a separate assignment is performed for the downlink channel.

When data is transferred this is controlled by the action of the MAC layer. In most instances it will operate in an acknowledge mode whereby the base station acknowledges each block of data. The acknowledgement may be contained within the data packets being sent in the downlink, or the base station may send data packets down purely to acknowledge the data.

When disconnecting the mobile will send a packet temporary block flow message, and this is acknowledged. Once this has taken place the USF assigned to the mobile becomes redundant and can be assigned to another mobile wanting access. With this the mobile effectively becomes disconnected and although still attached to the network no more data transfer takes place unless it is re-initiated. Separate messages are needed to detach the mobile from the network. Read more....!

Most united communication could be carried out through various networks. With softswitch, the network more andal and economical without sacrificing the other network available.

Convergence between the network of the circuit (circuit networks) and the network of the package Cellular (packet network) was the evolution of multifunctional network technology in the future.
Eventually, the user of the telephone Cellular will communicate with many lines starting from when telephoning the house, the internet telephone in PC, telephoned the office, or conversely, with not only involved the voice, but also the data.

His process need not replace all the network of the available circuit, the migration cost of the cheap network, and in stages did upgraded headed the network of the package.
One of his methods was with technology softswitch.
This implement could connect between the network of the circuit and the network of the package, including inside was the network of the telephone continue to (PSTN), the based internet IP, the TV cable but also the network Cellular available uptil now. Read more....!

The GSM system is the most widely used mobile telecommunications system in use in the world today. The letters GSM originally stood for the words Groupe Speciale Mobile, but as it became clear this standard was to be used world wide the meaning of GSM was changed to Global System for Mobile Communications. Since it was first deployed in 1991, the use of GSM has grown steadily, and it is now the most widely cell phone system in the world. GSM reached the 1 billion subscriber point in February 2004, and continued to grown in popularity.

System idea
The GSM system was designed as a second generation (2G) cellular communication system. One of the basic aims was to provide a system that would enable greater capacity to be achieved than the previous first generation analogue systems. GSM achieved this by using a digital TDMA (time division multiple access approach). By adopting this technique more users could be accommodated within the available bandwidth. In addition to this, ciphering of the digitally encoded speech was adopted to retain privacy. Using the earlier analogue systems it was possible for anyone with a scanner receiver to listen to calls and a number of famous personalities had been "eavesdropped" with embarrassing consequences.

Services provided
Speech or voice calls are obviously the primary function for the GSM system. To achieve this the speech is digitally encoded and later decoded using a vocoder. A variety of vocoders are available for use, being aimed at different scenarios.

In addition to the voice services, GSM supports a variety of other data services. Although their performance is nowhere near the level of those provided by 3G, they are nevertheless still important and useful. A variety of data services are supported with user data rates up to 9.6 kbps. Services including Group 3 facsimile, videotext and teletex can be supported.

One service that has grown enormously is the short message service. Developed as part of the GSM specification, it has also been incorporated into other cellular systems. It can be thought of as being similar to the paging service but is far more comprehensive allowing bi-directional messaging, store and forward delivery, and it also allows alphanumeric messages of a reasonable length. This service has become particularly popular, initially with the young as it provided a simple, low fixed cost.

Basic concept
The GSM system had a number of design aims when the development started. It should offer good subjective speech quality, have a low phone or terminal cost, terminals should be able to be handheld, the system should support international roaming, it should offer good spectral efficiency, and the system should offer ISDN compatibility.

The system that developed provided for all of these. The overall system definition for GSM describes not only the air interface but also the network. By adopting this approach it is possible to define the operation of the whole network to enable international roaming as well as enabling network elements from different manufacturers to operate alongside each other, although this last feature is not completely true, especially with older items.

GSM uses 200 kHz RF channels. These are time division multiplexed to enable up to eight users to access each carrier. In this way it is a TDMA / FDMA system.

The base transceiver stations (BTS) are organised into small groups, controlled by a base station controller (BSC) which is typically co-located with one of the BTSs. The BSC with its associated BTSs is termed the base station subsystem (BSS).

Further into the core network is the main switching area. This is known as the mobile switching centre (MSC). Associated with it is the location registers, namely the home location register (HLR) and the visitor location register (VLR) which track the location of mobiles and enable calls to be routed to them. Additionally there is the Authentication Centre (AuC), and the Equipment Identify Register (EIR) that are used in authenticating the mobile before it is allowed onto the network and for billing. The operation of these are explained in the following pages.

Last but not least is the mobile itself. Often termed the ME or mobile equipment, this is the item that the end user sees. One important feature that was first implemented on GSM was the use of a Subscriber Identity Module. This card carried with it the users identity and other information to allow the user to upgrade a phone very easily, while retaining the same identity on the network. It was also used to store other information such as "phone book" and other items. This item alone has allowed people to change phones very easily, and this has fuelled the phone manufacturing industry and enabled new phones with additional features to be launched. This has allowed mobile operators to increase their average revenue per user (ARPU) by ensuring that users are able to access any new features that may be launched on the network requiring more sophisticated phones.

Specification Summary of GSM Cell Phone System

Multiple Access Technology FDMA / TDMA

Duplex Technique FDD

Uplink frequency band 933 - 960 MHz
(basic 900 MHz band only)

Downlink frequency band 890 - 915 MHz
(basic 900 MHz band only)

Channel spacing 200 kHz

Modulation GMSK

Speech coding Various - Original was RPE-LTP/13

Speech channels per RF channel 8

Channel data rate 270.833 kbps

Frame duration 4.615 mS

The architecture of the GSM system with its hardware can broadly be grouped into three main areas: the mobile station, the base station subsystem, and the network subsystem. Each area performs its own functions and when used together they enable the full operational capability of the system to be realised.

Mobile station
Mobile stations (MS), mobile equipment (ME) or as they are most widely known, cell or mobile phones are the section of a GSM cellular network that the user sees and operates. In recent years their size has fallen dramatically while the level of functionality has greatly increased. A further advantage is that the time between charges has significantly increased.

There are a number of elements to the cell phone, although the two main elements are the main hardware and the SIM.

The hardware itself contains the main elements of the mobile phone including the display, case, battery, and the electronics used to generate the signal, and process the data receiver and to be transmitted. It also contains a number known as the International Mobile Equipment Identity (IMEI). This is installed in the phone at manufacture and "cannot" be changed. It is accessed by the network during registration to check whether the equipment has been reported as stolen.

The SIM or Subscriber Identity Module contains the information that provides the identity of the user to the network. It contains are variety of information including a number known as the International Mobile Subscriber Identity (IMSI).

Base station subsystem
The Base Station Subsystem (BSS) section of the GSM network is fundamentally associated with communicating with the mobiles on the network. It consists of two elements, namely the Base Transceiver Station (BTS) and the Base Station Controller (BSC).

The BTS used in a GSM network comprises the radio transmitter receivers, and their associated antennas that transmit and receive to directly communicate with the mobiles. The BTS is the defining element for each cell. The BTS communicates with the mobiles and the interface between the two is known as the Um interface with its associated protocols.

The BSC forms the next stage back into the GSM network. It controls a group of BTSs, and is often co-located with one of the BTSs in its group. It manages the radio resources and controls items such as handover within the group of BTSs, allocates channels and the like. It communicates with the BTSs over what is termed the Abis interface.

Network subsystem
The network subsystem contains a variety of different elements, and is often termed the core network. It provides the main control and interfacing for the whole mobile network. It includes elements including the MSC, HLR, VLR, Auc and more as described below:

The main element within the core network is the Mobile switching Services Centre (MSC). The MSC acts like a normal switching node within a PSTN or ISDN, but also provides additional functionality to enable the requirements of a mobile user to be supported. These include registration, authentication, call location, inter-MSC handovers and call routing to a mobile subscriber. It also provides an interface to the PSTN so that calls can be routed from the mobile network to a phone connected to a landline. Interfaces to other MSCs are provided to enable calls to be made to mobiles on different networks.

To enable the MSC to perform its functions it requires data from a number of databases. One is known as the Home Location Register (HLR). It contains all the administrative information about each subscriber along with their last known location.

When a user switches on their phone, the phone registers with the network and from this it is possible to determine which BTS it communicates with so that incoming calls can be routed appropriately. Even when the phone is not active (but switched on) it re-registers periodically to ensure that the network (HLR) is aware of its latest position.

There is one HLR per network, although it may be distributed across various sub-centres to for operational reasons.

Another of the databases is known as the Visitor Location Register (VLR). This contains selected information from the HLR that enables the selected services for the individual subscriber to be provided.

The VLR can be implemented as a separate entity, but it is commonly realised as an integral part of the MSC, rather than a separate entity. In this way access is made faster and more convenient.

The third register is the Equipment Identity Register (EIR). The EIR is the entity that decides whether a given mobile equipment may be allowed onto the network. Each mobile equipment has a number known as the International Mobile Equipment Identity. This number, as mentioned above, is installed in the equipment and is checked by the network during registration. Dependent upon the information held in the EIR, the mobile may be allocated one of three states - allowed onto the network, barred access, or monitored in case its problems.

The final register is the Authentication Centre (AuC). The AuC is a protected database that contains the secret key also contained in the user's SIM card. It is used for authentication and for ciphering on the radio channel.

Another element in the network is the Gateway Mobile Switching Centre (GMSC). The GMSC is the point to which a ME terminating call is initially routed, without any knowledge of the MS's location. The GMSC is thus in charge of obtaining the MSRN (Mobile Station Roaming Number) from the HLR based on the MSISDN (Mobile Station ISDN number, the "directory number" of a MS) and routing the call to the correct visited MSC. The "MSC" part of the term GMSC is misleading, since the gateway operation does not require any linking to an MSC.

The SMS-G or SMS gateway is the term that is used to collectively describe the two Short Message Services Gateways defined in the GSM standards. The two gateways handle messages directed in different directions. The SMS-GMSC (Short Message Service Gateway Mobile Switching Centre) is for short messages being sent to an ME. The SMS-IWMSC (Short Message Service Inter-Working Mobile Switching Centre) is used for short messages originated with a mobile on that network. The SMS-GMSC role is similar to that of the GMSC, whereas the SMS-IWMSC provides a fixed access point to the Short Message Service Centre.

There are a number of elements to the GSM radio or air interface. There are the aspects of the physical power levels, channels and the like. Additionally there are the different data channels that are employed to carry the data and exchange the protocol messages that enable the radio subsystem to operate correctly.

Basic signal characteristics
The GSM system uses digital TDMA technology combined with a channel bandwidth of 200 kHz. Accordingly the system is able to offer a higher level of spectrum efficiency that that which was achieved with the previous generation of analogue systems. As there are many carrier frequencies that are available, one or more can be allocated to each base station. The system also operates using Frequency Division Duplex and as a result, paired bands are needed for the up and downlink transmissions. The frequency separation is dependent upon the band in use.

The carrier is modulated using Gaussian Minimum Shift Keying (GMSK). GMSK was used for the GSM system because it is able to provide features required for GSM. It is resilient to noise when compared to some other forms of modulation, it occupies a relatively narrow bandwidth, and it has a constant power level.

The data transported by the carrier serves up to eight different users under the basic system. Even though the full data rate on the carrier is approximately 270 kbps, some of this supports the management overhead, and therefore the data rate allotted to each time slot is only 24.8 kbps. In addition to this error correction is required to overcome the problems of interference, fading and the like. This means that the available data rate for transporting the digitally encoded speech is 13 kbps for the basic vocoders.

Power levels
A variety of power levels are allowed by the GSM standard, the lowest being only 800 mW (29 dBm). As mobiles may only transmit for one eighth of the time, i.e. for their allocated slot which is one of eight, the average power is an eighth of the maximum.

Additionally, to reduce the levels of transmitted power and hence the levels of interference, mobiles are able to step the power down in increments of 2 dB from the maximum to a minimum 13 dBm (20 milliwatts). The mobile station measures the signal strength or signal quality (based on the Bit Error Rate), and passes the information to the BTS and hence to the BSC, which ultimately decides if and when the power level should be changed.

A further power saving and interference reducing facility is the discontinuous transmission (DTx) capability that is incorporated within the specification. It is particularly useful because there are long pauses in speech, for example when the person using the mobile is listening, and during these periods there is no need to transmit a signal. In fact it is found that a person speaks for less than 40% of the time during normal telephone conversations. The most important element of DTx is the Voice Activity Detector. It must correctly distinguish between voice and noise inputs, a task that is not trivial. If a voice signal is misinterpreted as noise, the transmitter is turned off an effect known as clipping results and this is particularly annoying to the person listening to the speech. However if noise is misinterpreted as a voice signal too often, the efficiency of DTX is dramatically decreased.

It is also necessary for the system to add background or comfort noise when the transmitter is turned off because complete silence can be very disconcerting for the listener. Accordingly this is added as appropriate. The noise is controlled by the SID (silence indication descriptor).

Multiple access and channel structure
GSM uses a combination of both TDMA and FDMA techniques. The FDMA element involves the division by frequency of the (maximum) 25 MHz bandwidth into 124 carrier frequencies spaced 200 kHz apart as already described.

The carriers are then divided in time, using a TDMA scheme. The fundamental unit of time is called a burst period and it lasts for approximately 0.577 mS (15/26 mS). Eight of these burst periods are grouped into what is known as a TDMA frame. This lasts for approximately 4.615 ms (i.e.120/26 ms) and it forms the basic unit for the definition of logical channels. One physical channel is one burst period allocated in each TDMA frame.

There are different types of frame that are transmitted to carry different data, and also the frames are organised into what are termed multiframes and superframes to provide overall synchronisation.

GSM uses a variety of channels in which the data is carried. In GSM, these channels are separated into physical channels and logical channels. The Physical channels are determined by the timeslot, whereas the logical channels are determined by the information carried within the physical channel. It can be further summarised by saying that several recurring timeslots on a carrier constitute a physical channel. These are then used by different logical channels to transfer information. These channels may either be used for user data (payload) or signalling to enable the system to operate correctly.

Common and dedicated channels
The channels may also be divided into common and dedicated channels. The forward common channels are used for paging to inform a mobile of an incoming call, responding to channel requests, and broadcasting bulletin board information. The return common channel is a random access channel used by the mobile to request channel resources before timing information is conveyed by the BSS.

The dedicated channels are of two main types: those used for signalling, and those used for traffic. The signalling channels are used for maintenance of the call and for enabling call set up, providing facilities such as handover when the call is in progress, and finally terminating the call. The traffic channels handle the actual payload.

The following logical channels are defined in GSM:

TCHf - Full rate traffic channel.

TCH h - Half rate traffic channel.

BCCH - Broadcast Network information, e.g. for describing the current control channel structure. The BCCH is a point-to-multipoint channel (BSS-to-MS).

SCH - Synchronisation of the MSs.

FCHMS - frequency correction.

AGCH - Acknowledge channel requests from MS and allocate a SDCCH.

PCHMS - terminating call announcement.

RACHMS - access requests, response to call announcement, location update, etc.

FACCHt - For time critical signalling over the TCH (e.g. for handover signalling). Traffic burst is stolen for a full signalling burst.

SACCHt - TCH in-band signalling, e.g. for link monitoring.

SDCCH - For signalling exchanges, e.g. during call setup, registration / location updates.

FACCHs - FACCH for the SDCCH. The SDCCH burst is stolen for a full signalling burst. Function not clear in the present version of GSM (could be used for e.g. handover of an eight-rate channel, i.e. using a "SDCCH-like" channel for other purposes than signalling).

SACCHs - SDCCH in-band signalling, e.g. for link monitoring.

If digitised in a linear fashion, the speech would occupy a far greater bandwidth than any cellular system and in this case the GSM system would be able to accommodate. To overcome this, a variety of voice coding systems or vocoders are used. These systems involve analysing the incoming data that represents the speech and then performing a variety of actions upon it to reduce the data rate. At the receiving end the reverse process is undertaken to re-constitute the speech data so that it can be understood. In GSM a variety of vocoders are used, including LPC-RPE, EFR, etc as described in the following paragraphs.

The vocoder that was originally used in the GSM system was the LPC-RPE (Linear Prediction Coding with Regular Pulse Excitation) vocoder. This vocoder took each 20 mS block of speech and then represented it using just 260 bits. This actually equates to a data rate of 13 kbps.

In GSM it is recognised that some bits are more important than others. If some bits are missed or corrupted, it is more important to the voice quality than others. Accordingly the different bits are classified:

Class Ia 50 bits - most important and sensitive to bit errors
Class Ib 132 bits - moderately sensitive to bit errors
Class II 78 bits - least sensitive to bit errors

The 50 Class 1a bits are given a 3 bit Cyclic Redundancy Code (CRC) so that errors can be detected. This makes a total length of 53 bits. If there are any errors, the frame is not used, and it is discarded. In its place a version of the previously correctly received frame is used. These 53 bits, together with the 132 Class Ib bits with a 4 bit tail sequence, are entered into a 1/2 rate convolutional encoder. The total length is 189 bits. The encoder encodes each of the bits that enter as two bits, the output also being dependent upon a combination of the previous 4 input bits. As a result the output from the convolutional encoder consists of 378 bits. The remaining 78 Class II bits are considered the least sensitive to errors and they are not protected and simply added to the data. In this way every 20 ms speech sample generates a total of 456 bits. Accordingly the overall bit rate is 22.8 kbps. Once in this format the data is interleaved to add further protection against interference and noise.

The 456 bits output by the convolutional encoder are divided into 8 blocks of 57 bits, and these blocks are transmitted in eight consecutive time-slots, i.e. a total of four bursts as each burst takes two sets of data.

Later another vocoder called the Enhanced Full Rate (EFR) vocoder was added in response to the poor quality perceived by the users. This new vocoder gave much better sound quality and was adopted by GSM. Using the ACELP (Algebraic Code Excitation Linear Prediction) compression technology it gave a significant improvement in quality over the original LPC-RPE encoder. It became possible as the processing power that was available increased in mobile phones as a result of higher levels of processing power combined with their lower current consumption.

There is also a half rate vocoder. Although this gives much inferior voice quality, it does allow for an increase in network capacity. It is used in some instances when network loading is very high to accommodate all the calls. Read more....!

The GSM cell or mobile phone system is the most popular in the world. GSM handsets are widely available at good prices and the networks are robust and reliable. The GSM system is also feature-rich with applications such as SMS text messaging, international roaming, SIM cards and the like. It is also being enhanced with technologies including GPRS and EDGE. To achieve this level of success has taken many years and is the result of both technical development and international cooperation.

The first cell phone systems that were developed were analogue systems. Typically they used frequency-modulated carriers for the voice channels and data was carried on a separate shared control channel. When compared to the systems employed today these systems were comparatively straightforward and as a result a vast number of systems appeared. Two of the major systems that were in existence were the AMPS (Advanced Mobile Phone System) that was used in the USA and many other countries and TACS (Total Access Communications System) that was used in the UK as well as many other countries around the world.

Another system that was employed, and was in fact the first system to be commercially deployed was the Nordic Mobile Telephone system (NMT). This was developed by a consortium of companies in Scandinavia and proved that international cooperation was possible.
The success of these systems proved to be their downfall. The use of all the systems installed around the globe increased dramatically and the effects of the limited frequency allocations were soon noticed. To overcome these a number of actions were taken. A system known as E-TACS or Extended-TACS was introduced giving the TACS system further channels. In the USA another system known as Narrowband AMPS (NAMPS) was developed.

New approaches
Neither of these approaches proved to be the long-term solution as more efficient systems were required. With the experience gained from the NMT system, showing that it was possible to develop a system across national boundaries, and with the political situation in Europe lending itself to international cooperation it was decided to develop a new Pan-European System. Furthermore it was realized that economies of scale would bring significant benefits. This was the beginnings of the GSM system.

To achieve the basic definition of a new system a meeting was held in 1982 under the auspices of the Conference of European Posts and Telegraphs (CEPT). They formed a study group called the Groupe Special Mobile ( GSM ) to study and develop a pan-European public land mobile system. Several basic criteria that the new system would have to meet were set down for the new GSM system to meet. These included: good subjective speech quality, low terminal and service cost, support for international roaming, ability to support handheld terminals, support for range of new services and facilities, spectral efficiency, and finally ISDN compatibility.

With the levels of under-capacity being projected for the analogue systems, this gave a real sense of urgency to the GSM development. Although decisions about the system were not taken at an early stage, all had been working toward a digital system. This decision was finally made in February 1987. This gave a variety of advantages. Greater levels of spectral efficiency could be gained, and in addition to this the use of digital circuitry would allow for higher levels of integration in the circuitry. This in turn would result in cheaper handsets with more features. Nevertheless significant hurdles still needed to be overcome. For example, many of the methods for encoding the speech within a sufficiently narrow bandwidth needed to be developed, and this posed a significant risk to the project. Nevertheless the GSM system had been started.

Launch dates
Work continued and a launch date for the new GSM system of 1991 was set for an initial launch of a service with limited coverage and capability to be followed by a complete roll out of the service in major European cities by 1993 and linking of the areas by 1995.

Meanwhile technical development was taking place. Initial trials had shown that time division multiple access techniques offered the best performance with the technology that would be available. This approach had the support of the major manufacturing companies which would ensure that with them on board sufficient equipment both in terms of handsets, base stations and the network infrastructure for GSM would be available.

Further impetus was given to the GSM project when in 1989 the responsibility was passed to the newly formed European Telecommunications Standards Institute (ETSI). Under the auspices of ETSI the specification took place. It provided functional and interface descriptions for each of the functional entities defined in the system. The aim was to provide sufficient guidance for manufacturers that equipment from different manufacturers would be interoperable, while not stopping innovation. The result of the specification work was a set of documents extending to more than 6000 pages. Nevertheless the resultant phone system provided a robust, feature-rich system. The first roaming agreement was signed between Telecom Finland and Vodafone in the UK. Thus the vision of a pan-European network was fast becoming a reality. However this took place before any networks went live.

The aim to launch GSM by 1991 proved to be a target that was too tough to meet. Terminals started to become available in mid 1992 and the real launch took place in the latter part of that year. With such a new service many were sceptical as the analogue systems were still in widespread use. Nevertheless by the end of 1993 GSM had attracted over a million subscribers and there were 25 roaming agreements in place. The growth continued and the next million subscribers were soon attracted.

Global usage
Originally GSM had been planned as a European system. However the first indication that the success of GSM was spreading further a field occurred when the Australian network provider, Telstra signed the GSM Memorandum of Understanding.

Frequencies
Originally it had been intended that GSM would operate on frequencies in the 900 MHz cellular band. In September 1993, the British operator Mercury One-to-One launched a network. Termed DCS 1800 it operated at frequencies in a new 1800 MHz band. By adopting new frequencies new operators and further competition was introduced into the market apart from allowing additional spectrum to be used and further increasing the overall capacity. This trend was followed in many countries, and soon the term DCS 1800 was dropped in favour of calling it GSM as it was purely the same system but operating on a different frequency band. In view of the higher frequency used the distances the signals travelled was slightly shorter but this was compensated for by additional base stations.

In the USA as well a portion of spectrum at 1900 MHz was allocated for cellular usage in 1994. The licensing body, the FCC, did not legislate which technology should be used, and accordingly this enabled GSM to gain a foothold in the US market. This system was known as PCS 1900 (Personal Communication System).


A great success
With GSM being used in many countries outside Europe this reflected the true nature of the name which had been changed from Groupe Special Mobile to Global System for Mobile communications. The number of subscribers grew rapidly and by the beginning of 2004 the total number of GSM subscribers reached 1 billion. Attaining this figure was celebrated at the Cannes 3GSM conference held that year. Read more....!

Orthogonal Frequency Division Multiplex, the modulation concept being used for many radio and wireless applications from DAB, DVB, Wi-Fi and Mobile Video.
Orthogonal Frequency Division Multiplex or OFDM is a modulation format that is finding increasing levels of use in today's communications scene. OFDM has been adopted in the Wi-Fi arena where the 802.11a standard uses it to provide data rates up to 54 Mbps in the 5 GHz ISM (Industrial, Scientific and Medical) band. In addition to this the recently ratified 802.11g standard has it in the 2.4 GHz ISM band. If this was not enough it is also being used for digital terrestrial television transmissions as well as DAB digital radio. A new form of broadcasting called Digital Radio Mondiale for the long medium and short wave bands is being launched and this has also adopted COFDM. Then for the future it is being proposed as the modulation technique for fourth generation cell phone systems that are in their early stages of development and OFDM is also being used for many of the proposed mobile phone video systems.

OFDM concept
An OFDM signal consists of a number of closely spaced modulated carriers. When modulation of any form - voice, data, etc. is applied to a carrier, then sidebands spread out either side. It is necessary for a receiver to be able to receive the whole signal to be able to successfully demodulate the data. As a result when signals are transmitted close to one another they must be spaced so that the receiver can separate them using a filter and there must be a guard band between them. This is not the case with OFDM. Although the sidebands from each carrier overlap, they can still be received without the interference that might be expected because they are orthogonal to each another. This is achieved by having the carrier spacing equal to the reciprocal of the symbol period.

Traditional view of receiving signals carrying modulation

To see how OFDM works, it is necessary to look at the receiver. This acts as a bank of demodulators, translating each carrier down to DC. The resulting signal is integrated over the symbol period to regenerate the data from that carrier. The same demodulator also demodulates the other carriers. As the carrier spacing equal to the reciprocal of the symbol period means that they will have a whole number of cycles in the symbol period and their contribution will sum to zero - in other words there is no interference contribution.

OFDM Spectrum

One requirement of the OFDM transmitting and receiving systems is that they must be linear. Any non-linearity will cause interference between the carriers as a result of inter-modulation distortion. This will introduce unwanted signals that would cause interference and impair the orthogonality of the transmission.

In terms of the equipment to be used the high peak to average ratio of multi-carrier systems such as OFDM requires the RF final amplifier on the output of the transmitter to be able to handle the peaks whilst the average power is much lower and this leads to inefficiency. In some systems the peaks are limited. Although this introduces distortion that results in a higher level of data errors, the system can rely on the error correction to remove them.

Data
The data to be transmitted on an OFDM signal is spread across the carriers of the signal, each carrier taking part of the payload. This reduces the data rate taken by each carrier. The lower data rate has the advantage that interference from reflections is much less critical. This is achieved by adding a guard band time or guard interval into the system. This ensures that the data is only sampled when the signal is stable and no new delayed signals arrive that would alter the timing and phase of the signal.

Guard Interval

The distribution of the data across a large number of carriers in the OFDM signal has some further advantages. Nulls caused by multi-path effects or interference on a given frequency only affect a small number of the carriers, the remaining ones being received correctly. By using error-coding techniques, which does mean adding further data to the transmitted signal, it enables many or all of the corrupted data to be reconstructed within the receiver. This can be done because the error correction code is transmitted in a different part of the signal. It is this error coding which is referred to in the "Coded" word in the title of COFDM which is often seen.

Other variants
Flash OFDM - This is a variant that was developed by Flarion and it is a fast hopped form of OFDM. It uses multiple tones and fast hopping to spread signals over a given spectrum band.

VOFDM - Vector OFDM. This form of OFDM uses the concept of MIMO technology. It is being developed by CISCO Systems. MIMO stands for Multiple Input Multiple output and it uses multiple antennas to transmit and receive the signals so that multi-path effects can be utilised to enhance the signal reception and improve the transmission speeds that can be supported.

WOFDM - Wideband OFDM. The concept of this form of OFDM is that it uses a degree of spacing between the channels that is large enough that any frequency errors between transmitter and receiver do not affect the performance. It is particularly applicable to Wi-Fi systems.


OFDM and COFDM have gained a significant presence in the wireless market place. The combination of high data capacity, high spectral efficiency, and its resilience to interference as a result of multi-path effects means that it is ideal for the high data applications that are becoming a common factor in today's communications scene. Read more....!

CDMA or Code Division Multiple Access is now in widespread use for mobile or cell phone (cellular telecommunications) systems around the world. It was first used for the IS-95 mobile phone system also known by the trade name cdmaOne, and in its later 3G developments as CDMA2000. CDMA is also being used in the other major 3G cell phone system, Wideband-CDMA system originally called UMTS.

CDMA technology is based on a form of transmission known as Direct Sequence Spread Spectrum (DSSS). This form of transmission originally used for military and police communications because the transmissions were difficult to detect in many instances, and even if they were received they were very difficult to decipher without the correct codes. However the possibilities of using this technology to provide a multiple access scheme for mobile telecommunications and have now been exploited in a major way.

Previous cellular telecommunications technologies used either frequency division multiple access (FDMA) where different users were allocated different frequencies, or time division multiple access (TDMA) where they were allotted different time slots on a channel. CDMA is different. Using the CDMA system, different users are allocated different codes to provide access to the system. It can be likened to many different people standing in a room talking to others in many different languages. Although the ambient noise level is very high, it is nevertheless still possible to pick out someone speaking in the same language as yourself.

DSSS basics
The key element of code division multiple access CDMA is its use of DSSS. In essence the required data signal is multiplied with what is known as a spreading or chip code data stream. This has a higher data rate than the data itself and it enables the overall signal to be spread over a much wider bandwidth. Signals with high data rates occupy wider signal bandwidths than those with low data rates.

To decode the signal and receive the original data, the CDMA signal is multiplied with the spreading code to regenerate the original data. When this is done, then only the data with that was generated with the same spreading code is regenerated, all the other data that is generated from different spreading code streams is ignored

This is a powerful principle and using code division multiple access technique, it is possible to transmit several sets of data independently on the same carrier and then reconstitute them at the receiver without mutual interference. In this way a base station can communicate with several mobiles on a single channel. Similarly several mobiles can communicate with a single base station, provided that in each case an independent spreading code is used.

The CDMA spreading codes can either be a random number (or pseudo random), or more usually orthogonal codes are used. Two codes are said to be orthogonal if when they are multiplied together and then the result is added over a period of time they sum to zero. For example a codes 1 -1 -1 1 and 1 -1 1 -1 when multiplied together give 1 1 -1 -1 which gives the sum zero. Although pseudo random number codes can be used there is possibility of data errors being introduced into the system.

Advantages
There are several advantages to using code division multiple access CDMA. The main reason for its acceptance is that it enables more users to use a given amount of spectrum. Its use also enables adjacent base stations to operate on the same channel, allowing more efficient use of the spectrum and it provides for an easier handover.
In view of these advantages CDMA has been adopted for all the 3G technologies and will be around for many years to come. Read more....!

The concept of a cellular phone system is that it has a large number base stations covering a small area (cells), and as a result frequencies are able to be re-used. Cell phone systems also provide mobility. As a result it is a very basic requirement of the system that as the mobile handset moves out of one cell to the next, it must be possible to hand the call over from the base station of the first cell, to that of the next with no discernable disruption to the call. There are two terms for this process: handover is used within Europe, whereas handoff is the term used in North America.

The handover or handoff process is of major importance within any cellular telecommunications network. It is necessary to ensure it can be performed reliably and without disruption to any calls. Failure for it to perform reliably can result in dropped calls, and this is one of the key factors that can lead to customer dissatisfaction, which in turn may lead to them changing to another cellular network provider. Accordingly handover or handoff is one of the key performance indicators monitored so that a robust handover / handoff regime is maintained on the cellular network.

Handover basics
Although the concept of handover or handoff is relatively straightforward, it is not an easy process to implement in reality. The cellular network needs to decide when handover or handoff is necessary, and to which cell. Also when the handover occurs it is necessary to re-route the call to the relevant base station along with changing the communication between the mobile and the base station to a new channel. All of this needs to be undertaken without any noticeable interruption to the call. The process is quite complicated, and in early systems calls were often lost if the process did not work correctly.

Different cellular standards handle hand over / handoff in slightly different ways. Therefore for the sake of an explanation the example of the way that GSM handles handover is given.

There are a number of parameters that need to be known to determine whether a handover is required. The signal strength of the base station with which communication is being made, along with the signal strengths of the surrounding stations. Additionally the availability of channels also needs to be known. The mobile is obviously best suited to monitor the strength of the base stations, but only the cellular network knows the status of channel availability and the network makes the decision about when the handover is to take place and to which channel of which cell.

Accordingly the mobile continually monitors the signal strengths of the base stations it can hear, including the one it is currently using, and it feeds this information back. When the strength of the signal from the base station that the mobile is using starts to fall to a level where action needs to be taken the cellular network looks at the reported strength of the signals from other cells reported by the mobile. It then checks for channel availability, and if one is available it informs this new cell to reserve a channel for the incoming mobile. When ready, the current base station passes the information for the new channel to the mobile, which then makes the change. Once there the mobile sends a message on the new channel to inform the network it has arrived. If this message is successfully sent and received then the network shuts down communication with the mobile on the old channel, freeing it up for other users, and all communication takes place on the new channel.

Under some circumstances such as when one base transceiver station is nearing its capacity, the network may decide to hand some mobiles over to another base transceiver station they are receiving that has more capacity, and in this way reduce the load on the base transceiver station that is nearly running to capacity. In this way access can be opened to the maximum number of users. In fact channel usage and capacity are very important factors in the design of a cellular network.

Types of handover / handoff
With the advent of CDMA systems where the same channels can be used by several mobiles, and where it is possible to adjacent cells or cell sectors to use the same frequency channel there are a number of different types of handover that can be performed:

* Hard handover


* Soft handover


* Softer handover


Although all of these forms of handover or handoff enable the cellular phone to be connected to a different cell or different cell sector, they are performed in slightly different ways and are available under different conditions.

Hard handover
The definition of a hard handover or handoff is one where an existing connection must be broken before the new one is established. One example of hard handover is when frequencies are changed. As the mobile will normally only be able to transmit on one frequency at a time, the connection must be broken before it can move to the new channel where the connection is re-established. This is often termed and inter-frequency hard handover. While this is the most common form of hard handoff, it is not the only one. It is also possible to have intra-frequency hard handovers where the frequency channel remains the same.

Although there is generally a short break in transmission, this is normally short enough not to be noticed by the user.

Soft hand over
The new 3G technologies use CDMA where it is possible to have neighbouring cells on the same frequency and this opens the possibility of having a form of handover or handoff where it is not necessary to break the connection. This is called soft handover or soft handoff, and it is defined as a handover where a new connection is established before the old one is released. In UMTS most of the handovers that are performed are intra-frequency soft handovers.

Softer handover
The third type of hand over is termed a softer handover, or handoff. In this instance a new signal is either added to or deleted from the active set of signals. It may also occur when a signal is replaced by a stronger signal from a different sector under the same base station. This type of handover or handoff is available within UMTS as well as CDMA2000.


Handover and handoff are performed by all cellular telecommunications networks, and they are a core element of the whole concept of cellular telecommunications. There are a number of requirements for the process. The first is that it occurs reliably and if it does not, users soon become dissatisfied and choose another network provider in a process known as "churn". However it needs to be accomplished in the most efficient manner. Although softer handoff is the most reliable, it also uses more network capacity. The reason for this is that it is communicating with more than one sector or base station at any given instance. Soft handover is also less efficient than hard handover, but again more reliable as the connection is never lost.
It is therefore necessary for the cellular telecommunications network provider to arrange the network to operate in the most efficient manner, while still providing the most reliable service. Read more....!

(1)Wireless LAN 802.11b
he IEEE 802.11b specification builds on the original IEEE 802.11 standard by providing up to 11Mbps data rates in a direct sequence spread spectrum air interface using Barker codes for spreading 1 and 2 Mbps data rates and CCK modulation for 5.5Mbps and 11Mbps data rates. Nuntius has expertise in WLAN technologies and can provide solutions on multiple platforms. The diagram below shows an 802.11b solution that includes RF, analog baseband, digital baseband (with the PHY), and the MAC. For more information, contact Nuntius Systems, Inc.


Features
2.4GHz ISM Frequency Band
AGC
Antenna Diversity
Rake Receiver Functionality

Data Rates
1 Mbps
2 Mbps
5.5 Mbps
11 Mbps

Modulation
DBPSK
DQPSK
CCK
PBCC (Optional)

(2)Wireless LAN 802.11a
The IEEE 802.11a specification provides higher data rates (up to 54Mbps) in the 5GHz frequency band. It also employs a spreading technique known as Orthogonal Frequency Division Multiplexing (OFDM). Nuntius has expertise in these WLAN technologies and can provide solutions on multiple platforms. The diagram below shows an 802.11a solution that includes RF, analog baseband, digital baseband (with the PHY), and the MAC. For more information, contact Nuntius Systems, Inc.


Features
5 GHz Frequency Band
OFDM
AGC
Channel Coding Supported (K=7)
Carrier Recovery
Data Puncturing

Data Rates
6 Mbps
9 Mbps
12 Mbps
24 Mbps
36 Mbps
48 Mbps
54 Mbps

Modulation
BPSK
QPSK
16-QAM
64-QAM Read more....!

The earliest wireless networking products came to market about a decade ago, operated in the 900 MHz band. Because these were proprietary designs, an effort soon ensued to pursue a vendor-independent standard, to promote interoperability. This resulted in the formation of the IEEE 802.11 committee, which quickly began to focus on the 2.4-GHz band WLAN. The approval of the 2.4-GHz 802.11 standard was finally achieved in June 1997. The WLAN market finally gained acceptance as a legitimate enterprise technology in 2000 and is now gaining momentum. With the release of Wi-Fi, IEEE 802.11b standard products from several prominent network equipment suppliers have driven WLAN gear into wider acceptance. The market continues to thrive as suppliers unveil new products with higher speeds, increased interoperability and lower prices.

The original 802.11 specification identified 1Mbps and 2Mbps data speeds in a variety of physical medium access methods for the 2.4GHz ISM frequency band. These physical layer modulation methods included frequency hopping spread spectrum (FHSS) and direct sequence spread spectrum (DSSS). This was followed by the 802.11b specification that added 5.5Mbps and 11Mbps data rates using CCK modulation. IEEE also released a physical layer implementation that uses the 5 GHz band supporting data rates up to 54Mbps called 802.11a. The spreading scheme used in this specification is Orthogonal Frequency Division Multiplexing (OFDM). Other standards are being proposed that will augment these standards well into the future.

Building on their established expertise in spread spectrum communications systems, Nuntius has the technology to build Wireless LAN solutions for today's markets as well as the future markets.
Read more....!

Internet telecom began as a revolution in technology. The concept of sending voice across a data network, in packet format, has been nurtured by a hope of eventually creating new markets, but for a long time that hope remained latent.

Today customers are demanding a different kind of communications infrastructure to line up more closely with the different kind of business environment (shaped by the Internet) in which they find themselves operating. Likewise, service providers are beginning to come forth with new ideas about how to deliver features and applications to their customers.

The local exchange carrier market has become ground-zero for much of the work being done in Internet telecom today. Particularly at the local level, competitive and incumbent carriers alike are realizing that just providing dial tone (for Internet or telephony) won't be enough to succeed long term. Carriers are starting to have to do the one thing they've always avoided: respond to the needs of individual business customers. As a result more intelligence will be pushed from the customer premises into the local loop and network EDGE.

The technology to make it all happen, of course, is being put in place today. Just look at the development of the gateways, softswitch, and the broadband voice-over-DSL.and DOCSIS cable deployments to see how the new generation of voice/data infrastructure is as much about facilitating service creation and delivery as it is about creating a more economical transport. Today, service providers are finding alternative and innovative methods for the delivery of voice services on top of DSL lines and coaxial cable lines.

At the same time as the carrier world is being shaken up by convergence, the enterprise communications market is undergoing changes of equal if not greater proportions. As e-business practices become a more integral aspect of enterprises across the board, reliance on communications will only increase. The latest trends in IP PBXs have already begun to reflect this concept. In general, hardware will be de-emphasized at the enterprise level, and displaced by distributed, network-based software platforms, managed remotely.

On the public network side, mobile wireless has become the most visible intersection of telephony and the Internet, a crossing that will be galvanized by third-generation technology. Within the enterprise, the convergence promise of "one-wire" infrastructure is quickly giving way to a "no-wire" model.

Nuntius technology enables the transmission of voice, fax and modem traffic over an IP or ATM backbone network. The software products accomplish this in three functional areas. These functions are designed to execute in a distributed fashion on programmable Digital Signal Processors (DSPs) and RISC and CISC Microprocessors. The three functional modules are Signal Processing Software, Telephony Processing Software and Protocol Processing Software.

Signal Processing Software
The Signal Processing software provides the interface between the analog world we live in and the digital binary world of embedded processors. This software prepares analog or PCM voice samples for transmission over the packet network. Its components perform tone detection and generation, echo cancellation, voice compression, voice activity detection, jitter removal, re-sampling and voice packetization. It also performs equalization, modulation and demodulation in support of fax relay applications.

Telephony Processing Software
The Telephony Processing software addresses the complexities of translating between traditional telephony signals and modern data networks. It interacts with telephony equipment, translating signaling into state changes used by the Protocol Processing software to set up connections.

Protocol Processing Software
The Protocol Processing software receives signaling information that has been interpreted by the Telephony Processing software and converts it from the telephony signaling protocols to the specific packet signaling protocol that is used to set up connections over the data network. It also adds appropriate protocol headers to both voice and signaling packets before transmission. Standard formats for IP, Frame Relay and ATM networks are common. Read more....!

On any cellular telecommunications system the way in which registration and call set-up occur needs to be carefully managed. Not only does the cellular telecommunications network need to provide quick and efficient service for its rightful customers, but it also needs to be able to offer high levels of security for the user and the network.

There are many different cellular telecommunications systems in use around the globe. Older ones are being phased out, and newer cellular systems are being introduced. Accordingly there is no single way in which registration and call set up are managed. However there are some general principles that are used, and these are illustrated here.

Basic requirements
When the mobile phone is turned on it needs to be able to communicate with the cellular telecommunications network. However the phone does not have an allocated channel, time slot or chip code (dependent upon the type of access method used). It is therefore necessary for there to be some methods or allocated means within the cellular telecommunications network, whereby a newly switched on mobile can communicate with the network and set up the standard communication.

Even if a call is not to be made instantly, the network needs to be able to communicate with the mobile to know where it is. In this way the network can route any calls through the relevant base station as the network would be soon overloaded if the notification of an incoming call had to be sent via several base stations.

Registration
There are a variety of tasks that need to be undertaken when a phone is turned on. This can eb seen by the fact that it takes a few seconds from switching the phone on before it is ready for use. Part of this process is the software start-up for the phone, but the majority comes from the registration process with the cellular network. There are several aspects to the regristration. The first is to make contact with the base station, and next the mobile has to register to allow it to have access to and use the network.

In order to make contact with the base station the mobile uses a paging or control channel. The name of this channel, and the exact way in which it works will vary from one cellular standard to the next, but it is a channel that is used that the mobile can access to indicate its presence. The message sent is often called the "attach" message. Once this has been achieved it is necessary for the mobile to register with the cellular network, and to be accepted onto it.

Network elements
It is necessary to have a register or database of users allowed to register with a given network. With mobiles often being able to access the all the channels available in a country, methods of ensuring the mobile registers with the correct network, and to ensure the account is valid are required. Additionally it is required for billing purposes. To achieve this, an entity on the network often known as the Authentication Centre (AuC) is used. The network and the mobile communicate and numbers giving the identity of the subscriber. Here the user information is checked to provide authentication and encryption parameters that verify the user's identity and ensure the confidentiality of each call protecting users and network operators from fraud.

Once accepted onto the network two further registers are normally required. These are the Home Location Register (HLR) and the Visitors Location Register (VLR). These two registers are required to keep track of the mobile so that the network knows where it is at any time so that calls can be routed to the correct base station or general area of the network. These registers are used to store the last known location of the mobile. Thus at registration the register is updated and then periodically the mobile updates its position. Even when the mobile is in what is termed its idle mode it will periodically communicate with the network to update its position and status.

When the mobile is switched off it sends a detach message. This informs the network that it is switching off, and enables the network to update the last known position for the mobile.

Home and abroad
The two registers are required, one for mobiles for which the network is the home network, i.e. the one with whom the contract exists, and the other for visitors. If there was only one register then every time the mobile sent any message to the foreign network, this would need to be relayed back to the home network and this would require international signalling. The approach which is adopted is to send a message back to the HLR when the mobile first enters the new country saying that the mobile is in a different network and that any calls for that mobile should be forwarded to the foreign visited network. Read more....!

The network forms the heart of any cellular telephone system. The cellular network fulfils many requirements. Not only does the network enable calls to be routed to and from the mobile phones as well as enabling calls to be maintained as the cell phone moves from one cell to another, but it also enables other essential operations such as access to the network, billing, security and much more. To fulfil all these requirements the cellular network comprises many elements, each having its own function to complete.
The most obvious part of the cellular network is the base station. The antennas and the associated equipment often located in a container below are seen dotted around the country, and especially at the side of highways and motorways. However there is more to the network behind this, as the system needs to have elements of central control and it also needs to link in with the PSTN landline system to enable calls to be made to and from the wire based phones, or between networks.

Different cellular standards often take slightly different approaches for the cellular network required. Despite the differences between the different cellular systems, the basic concepts are very similar. Additionally cellular systems such as GSM have a well defined structure, and this means that manufacturers products can be standardised.

Basic cellular network structure
An overall cellular network contains a number of different elements from the base transceiver station (BTS) itself with its antenna back through a base station controller (BSC), and a mobile switching centre (MSC) to the location registers (HLR and VLR) and the link to the public switched telephone network (PSTN).

Of the units within the cellular network, the BTS provides the direct communication with the mobile phones. There may be a small number of base stations then linked to a base station controller. This unit acts as a small centre to route calls to the required base station, and it also makes some decisions about which of the base station is best suited to a particular call. The links between the BTS and the BSC may use either land lines of even microwave links. Often the BTS antenna towers also support a small microwave dish antenna used for the link to the BSC. The BSC is often co-located with a BTS.

The BSC interfaces with the mobile switching centre. This makes more widespread choices about the routing of calls and interfaces to the land line based PSTN as well as the HLR and VLR.

Base transceiver station, BTS
The base transceiver station or system, BTS consists of a number of different elements. The first is the electronics section normally located in a container at the base of the antenna tower. This contains the electronics for communicating with the mobile handsets and includes radio frequency amplifiers, radio transceivers, radio frequency combiners, control, communication links to the BSC, and power supplies with back up.

The second part of the BTS is the antenna and the feeder to connect the antenna to the base transceiver station itself. These antennas are visible on top of masts and tall buildings enabling them to cover the required area. Finally there is the interface between the base station and its controller further up the network. This consists of control logic and software as well as the cable link to the controller.

BTSs are set up in a variety of places. In towns and cities the characteristic antennas are often seen on the top of buildings, whereas in the country separate masts are used. It is important that the location, height, and orientation are all correct to ensure the required coverage is achieved. If the antenna is too low or in a poor location, there will be insufficient coverage and there will be a coverage "hole". Conversely if the antenna is too high and directed incorrectly, then the signal will be heard well beyond the boundaries of the cell. This may result in interference with another cell using the same frequencies.

The antennas systems used with base stations often have two sets of receive antennas. These provide what is often termed diversity reception, enabling the best signal to be chosen to minimise the effects of multipath propagation. The receiver antennas are connected to low loss cable that routes the signals down to a multicoupler in the base station container. Here a multicoupler splits the signals out to feed the various receivers required for all the RF channels. Similarly the transmitted signal from the combiner is routed up to the transmitting antenna using low loss cable to ensure the optimum transmitted signal.

Mobile switching centre (MSC)
The MSC is the control centre for the cellular system, coordinating the actions of the BSCs, providing overall control, and acting as the switch and connection into the public telephone network. As such it has a variety of communication links into it which will include fibre optic links as well as some microwave links and some copper wire cables. These enable it to communicate with the BSCs, routing calls to them and controlling them as required. It also contains the Home and Visitor Location Registers, the databases detailing the last known locations of the mobiles. It also contains the facilities for the Authentication Centre, allowing mobiles onto the network. In addition to this it will also contain the facilities to generate the billing information for the individual accounts.

In view of the importance of the MSC, it contains many backup and duplicate circuits to ensure that it does not fail. Obviously backup power systems are an essential element of this to guard against the possibility of a major power failure, because if the MSC became inoperative then the whole network would collapse. Read more....!

The mobile phone or cell phone as it is often called is equally important to the network in the operation of the complete cellular telecommunications network. Despite the huge numbers that are made, they still cost a significant amount to manufacture, discounts being offered to users as incentives to use a particular network. Their cost is a reflection of the complexity of the mobile phone electronics. They comprise several different areas of electronics, from radio frequency (RF) to signal processing, and general processing.

The design of a cell phone is particularly challenging. They need to offer high levels of performance, while being able to fit into a very small space, and in addition tot his the electronics circuitry needs to consume very little power so that the life between charges can be maintained.

Mobile phone contents
Mobile phones contain a large amount of circuitry, each of which is carefully designed to optimise its performance. The cell phone comprises analogue electronics as well as digital circuits ranging from processors to display and keypad electronics. A mobile phone typically consists of a single board, but within this there are a number of distinct functional areas, but designed to integrate to become a complete mobile phone:

* Radio frequency - receiver and transmitter


* Digital signal processing


* Analogue / digital conversion


* Control processor


* SIM or USIM card


* Power control and battery


Radio frequency elements
The radio frequency section of the mobile phone is one of the crucial areas of the cell phone design. This area of the mobile phone contains all the transmitter and receiver circuits. Normally direct conversion techniques are generally used in the design for the mobile phone receiver.

The signal output from the receiver is applied to what is termed an IQ demodulator. Here the data in the form of "In-phase" and "Quadrature" components is applied to the IQ demodulator and the raw data extracted for further processing by the phone.

On the transmit side one of the key elements of the circuit design is to keep the battery consumption to a minimum. For GSM this is not too much of a problem. The modulation used is Gaussian Minimum Shift Keying. This form of signal does not incorporate amplitude variations and accordingly it does not need linear amplifiers. This is a distinct advantage because non linear RF amplifiers are more efficient than linear RF amplifiers.

Unfortunately EDGE uses eight point phase shift keying (8PSK) and this requires a linear RF amplifier. As linear amplifiers consume considerably more current this is a distinct disadvantage. To overcome this problem the design for the mobile phone is organised so that phase information is added to the signal at an early stage of the transmitter chain, and the amplitude information is added at the final amplifier.

Analogue to Digital Conversion
Another crucial area of any mobile phone design is the circuitry that converts the signals between analogue and digital formats that are used in different areas. The radio frequency sections of the design use analogue techniques, whereas the processing is all digital.

The digital / analogue conversion circuitry enables the voice to be converted either from analogue or to digital a digital format for the send path, but also between digital and analogue for the receive path. It also provides functions such as providing analogue voltages to steer the VCO in the synthesizer as well as monitoring of the battery voltage, especially during charging. It also provides the conversion for the audio signals to and from the microphone and earpiece so that they can interface with the digital signal processing functions.

Another function that may sometimes be included in this area of the mobile phone design or within the DSP is that of the voice codecs. As the voice data needs to be compressed to enable it to be contained within the maximum allowable data rate, the signal needs to be digitally compressed. This is undertaken using what is termed a codec.

There are a number of codec schemes that can be used, all of which are generally supported by the base stations. The first one to be used in GSM was known as LPC-RPE (Linear Prediction Coding - Regular Pulse Excitation). However another scheme known as AMR (Adaptive Multi-Rate) is now widely used as it enables the data rate to be further reduced when conditions permit without impairing the speech quality too much. By reducing the speech data rate, further capacity is freed up on the network.

Digital Signal Processing
The DSP components of the mobile phone design undertake all the signal processing. Processes such as the radio frequency filtering and signal conditioning at the lower frequencies are undertaken by this circuitry. In addition to this, equalisation and correction for multipath effects is undertaken in this area of the design.

Although these processors are traditionally current hungry, the current processors enable the signal processing to be undertaken in a far more power effective manner than if analogue circuits are used.

Control processor
The control processor is at the heart of the design of the phone. It controls all the processes occurring in the phone from the MMI (Man machine interface) which monitors the keypad presses and arranging for the information to be displayed on the screen. It also looks after all the other elements of the MMI including all the menus that can be found on the phone.

Another function of the control processor is to manage the interface with the mobile network base station. The software required for this is known as the protocol stack and it enables the phone to register, make and receive calls, terminate them and also handle the handovers that are needed when the phone moves from one cell to the next. Additionally the software formats the data to be transmitted into the correct format with error correction codes included. Accordingly the load on this processor can be quite high, especially when there are interactions with the network.

The protocols used to interact with the network are becoming increasingly complicated with the progression from 2G to 3G. Along with the increasing number of handset applications the load on the processor is increasing. To combat this, the design for this area of the phone circuitry often uses ARM processors. This enables high levels of processing to be achieved for relatively low levels of current drain.

A further application handled by this area of the design of the mobile phone is the monitoring the state pf the battery and control of the charging. In view of the sophisticated monitoring and control required to ensure that the battery is properly charged and the user can be informed about the level of charge left, this is an important area of the design.

Battery
Battery design and technology has moved on considerably in the last few years. This has enabled mobile phones to operate for much longer. Initially nickel cadmium cells were used, but these migrated to nickel-metal-hydride cells and then to lithium ion cells. With phones becoming smaller and requiring to operate for longer from a single charge, the capacity of the battery is very important, and all the time the performance of these cells is being improved. Read more....!