UMTS, the Universal Mobile Telecommunications System is the third generation (3G) successor to the second generation GSM based technologies including GPRS, and EDGE. Although UMTS uses a totally different air interface, the core network elements have been migrating towards the UMTS requirements with the introduction of GPRS and EDGE. In this way the transition from GSM to the 3G UMTS architecture does not require such a large instantaneous investment.

UMTS uses Wideband CDMA (WCDMA or W-CDMA) to carry the radio transmissions, and often the system is referred to by the name WCDMA. It is also gaining a third name. Some are calling it 3GSM because it is a 3G migration for GSM.

Specifications and Management
In order to create and manage a system as complicated as UMTS or WCDMA it is necessary to develop and maintain a large number of documents and specifications. For UMTS or WCDMA, these are now managed by a group known as 3GPP - the Third Generation Partnership Programme. This is a global co-operation between six organisational partners - ARIB, CCSA, ETSI, ATIS, TTA and TTC.

The scope of 3GPP was to produce globally applicable Technical Specifications and Technical Reports for a 3rd Generation Mobile Telecommunications System. This would be based upon the GSM core networks and the radio access technologies that they support (i.e., Universal Terrestrial Radio Access (UTRA) both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes).

Capabilities
UMTS uses Wideband CDMA - WCDMA - as the radio transmission standard. It employs a 5 MHz channel bandwidth. Using this bandwidth it has the capacity to carry over 100 simultaneous voice calls, or it is able to carry data at speeds up to 2 Mbps in its original format. However with the later enhancements of HSDPA and HSUPA (described in other articles accessible from the cellular telecommunications menu page ) included in later releases of the standard the data transmission speeds have been increased to 14.4 Mbps.

Many of the ideas that were incorporated into GSM have been carried over and enhanced for UMTS. Elements such as the SIM have been transformed into a far more powerful USIM (Universal SIM). In addition to this, the network has been designed so that the enhancements employed for GPRS and EDGE can be used for UMTS. In this way the investment required is kept to a minimum.

A new introduction for UMTS is that there are specifications that allow both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes. The first modes to be employed are FDD modes where the uplink and downlink are on different frequencies. The spacing between them is 190 MHz for Band 1 networks being currently used and rolled out.

However the TDD mode where the uplink and downlink are split in time with the base stations and then the mobiles transmitting alternately on the same frequency is particularly suited to a variety of applications. Obviously where spectrum is limited and paired bands suitably spaced are not available. It also performs well where small cells are to be used. As a guard time is required between transmit and receive, this will be smaller when transit times are smaller as a result of the shorter distances being covered. A further advantage arises from the fact that it is found that far more data is carried in the downlink as a result of internet surfing, video downloads and the like. This means that it is often better to allocate more capacity to the downlink. Where paired spectrum is used this is not possible. However when a TDD system is used it is possible to alter the balance between downlink and uplink transmissions to accommodate this imbalance and thereby improve the efficiency. In this way TDD systems can be highly efficient when used in picocells for carrying Internet data. The TDD systems have not been widely deployed, but this may occur more in the future. In view of its character, it is often referred to as TD-CDMA (Time Division CDMA).


UMTS or as it is often termed, Wideband CDMA, WCDMA is being widely deployed. It offers many advantages over GSM, GPRS, and EDGE in terms of much higher data rates and greater flexibility. These basic technical abilities reflect as a much richer number of applications and features that the 3G phones can be used to perform. This not only gives the user a much more useful 'phone', but this also translates into higher revenues for the operator. Read more....!

UMTS, the Universal Mobile Telecommunications System is the third generation (3G) successor to the second generation GSM based technologies including GPRS, and EDGE. Although UMTS uses a totally different air interface, the core network elements have been migrating towards the UMTS requirements with the introduction of GPRS and EDGE. In this way the transition from GSM to the 3G UMTS architecture does not require such a large instantaneous investment.

UMTS uses Wideband CDMA (WCDMA or W-CDMA) to carry the radio transmissions, and often the system is referred to by the name WCDMA. It is also gaining a third name. Some are calling it 3GSM because it is a 3G migration for GSM.

Specifications and Management
In order to create and manage a system as complicated as UMTS or WCDMA it is necessary to develop and maintain a large number of documents and specifications. For UMTS or WCDMA, these are now managed by a group known as 3GPP - the Third Generation Partnership Programme. This is a global co-operation between six organisational partners - ARIB, CCSA, ETSI, ATIS, TTA and TTC.

The scope of 3GPP was to produce globally applicable Technical Specifications and Technical Reports for a 3rd Generation Mobile Telecommunications System. This would be based upon the GSM core networks and the radio access technologies that they support (i.e., Universal Terrestrial Radio Access (UTRA) both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes).

Capabilities
UMTS uses Wideband CDMA - WCDMA - as the radio transmission standard. It employs a 5 MHz channel bandwidth. Using this bandwidth it has the capacity to carry over 100 simultaneous voice calls, or it is able to carry data at speeds up to 2 Mbps in its original format. However with the later enhancements of HSDPA and HSUPA (described in other articles accessible from the cellular telecommunications menu page ) included in later releases of the standard the data transmission speeds have been increased to 14.4 Mbps.

Many of the ideas that were incorporated into GSM have been carried over and enhanced for UMTS. Elements such as the SIM have been transformed into a far more powerful USIM (Universal SIM). In addition to this, the network has been designed so that the enhancements employed for GPRS and EDGE can be used for UMTS. In this way the investment required is kept to a minimum.

A new introduction for UMTS is that there are specifications that allow both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes. The first modes to be employed are FDD modes where the uplink and downlink are on different frequencies. The spacing between them is 190 MHz for Band 1 networks being currently used and rolled out.

However the TDD mode where the uplink and downlink are split in time with the base stations and then the mobiles transmitting alternately on the same frequency is particularly suited to a variety of applications. Obviously where spectrum is limited and paired bands suitably spaced are not available. It also performs well where small cells are to be used. As a guard time is required between transmit and receive, this will be smaller when transit times are smaller as a result of the shorter distances being covered. A further advantage arises from the fact that it is found that far more data is carried in the downlink as a result of internet surfing, video downloads and the like. This means that it is often better to allocate more capacity to the downlink.

Where paired spectrum is used this is not possible. However when a TDD system is used it is possible to alter the balance between downlink and uplink transmissions to accommodate this imbalance and thereby improve the efficiency. In this way TDD systems can be highly efficient when used in picocells for carrying Internet data. The TDD systems have not been widely deployed, but this may occur more in the future. In view of its character, it is often referred to as TD-CDMA (Time Division CDMA). Read more....!

CDMA2000 1x EV-DO cell phone system is a standard that has evolved from the CDMA2000 mobile phone system and it is now firmly established in many areas of the world. The letters EV-DO stand for Evolution Data Only or Data Optimised. From the title it can be seen that it is a data only mobile telecommunications standard that can be run on CDMA2000 networks.

The EV-DO cell phone system is capable of providing the full 3G data rates up to 3.1 Mbps now that release A of the standard has been issued. The first commercial CDMA2000 1xEV-DO network was deployed by SK Telecom (Korea) in January 2002. Now operators in Brazil Ecuador, Indonesia, Jamaica, Puerto Rico, Taiwan and the USA to mention but a few have all launched networks and more are to follow.




Basics
Th CDMA2000 1xEV-DO cell phone system is defined under IS-856 rather than IS-2000 that defines the other CDMA2000 standards, and as the name indicates it only carries data. In Release 0 of the standard the maximum data rate was 2400 Mbps in the forward (downlink) with 153 kbps in the reverse (uplink) direction, the same as CDMA2000 1X. However in the later release of the standard, Release A, the forward data rate rises to 3.1 Mbps, and 1.2 Mbps in the reverse direction.

The forward channel forms a dedicated variable-rate, packet data channel with signalling and control time multiplexed into it. The channel is itself time-divided and allocated to each user on a demand and opportunity driven basis. A data only format was adopted to enable the standard to be optimised for data applications. If voice is required then a dual mode phone using separate 1X channel for the voice call is needed. In fact the "phones" used for data only applications are referred to as Access Terminals or ATs.

Air interface
The EV-DO RF transmission is very similar to that of a CDMA2000 1X transmission. It has the same final spread rate of 1.228 Mcps and it has the same modulation bandwidth because the same digital filter is used. Although 1xEV-DO has many similarities with 1X transmissions, it cannot occupy the same channels simultaneously, and therefore requires dedicated paired channels for its operation. Accordingly new bands, often in the new 3G allocations are being dedicated for EV-DO in some areas.

Forward link
The forward link possesses many features that are specific to EV-DO, having been optimised for data transmission, particularly in the downlink direction. Average continuous rates of 600 kbps per sector are possible. This is a six fold increase over CDMA2000 1X and is provided largely by the ability of 1xEV-DO to negotiate increased data rates for individual ATs because only one user is served at a time.

The forward link is always transmitted at full power and uses a data rate control scheme rather than the power control scheme used with 1X, and the data is time division multiplexed so that only one AT is served at a time.

In order to be able to receive data, each EV-DO AT measures signal-to-noise ratio (S/N) on the forward link pilot every slot, i.e. 1.667 ms. Based on the information this provides the AT sends a data rate request to the base station. The AN receives requests from a variety of ATs, and decisions have to be made regarding which ATs are to be served next. The AN endeavours to achieve the best data transfer, and this is done by serving those ATs offering a good signal to noise ratio. This is achieved at the expense of users at some distance from the AN's antenna.

Accurate time synchronisation is required between the EV-DO Access Nodes. To achieve this time information is taken from the Global Positioning System as this is able to provide an exceedingly accurate time signal.

Forward link channels
A number of channels are transmitted in the forward direction to enable signalling, data and other capabilities to be handled. These channels include the Traffic channel, MAC channel, Control channel and Pilot. These are time division multiplexed.

Traffic Channel - This channels uses Quadrature Phase Shift Keying (QPSK) modulation for data rates up to 1.2288 Mbps. For higher data rates, higher order modulation techniques are used in the form of 8PSK with 3 bits per symbol or 16QAM with 4 bits per symbol. The levels of the I and Q symbols are chosen so that the average power becomes 1.

The Incoming data to be used as the modulation comes from the from the turbo coder and is scrambled by mixing it with a Pseudo Random Number (PN) sequence. The initial state of the PN is derived from known parameters, and is unique for each user. Every packet starts at the same initial value of the PN sequence.

At the beginning of the transmission to each user, there is a preamble that contains the user ID for the data. Its repeat rate is determined by the data rate because lower data rates require higher repeat values. However even at its largest, the preamble will fill no more than half the first slot.

Control Channel - This channel carries the signalling and overhead messages.

Pilot - The differentiator between the cell and the sector is still the PN offset of the pilot channel and the pilot signal is only gated on for 192 chips per slot.

Medium Access Control (MAC) Channel - This channel carries a number of controls including the Reverse Power Control (RPC), the Data Rate Control (DRC) Lock, and the reverse activity (RA) channels.

Reverse Link
The reverse link for 1xEV-DO has a structure similar to that for CDMA2000. In EV-DO all signalling is performed on the data channel and this means that there is no Dedicated Control Channel. The data channel can support 5 data rates which are separated in powers of 2 from 9.6 to 153.6 kbps. These rates are achieved by varying the repeat factor. The highest rate uses a Turbo coder with lower gain. The following channels are transmitted in addition to those used with 1X:

Reverse Rate Indicator (RRI) Channel - This indicates the data rate of the Reverse Data Channel.

Acknowledgement (Ack) Channel - This channel is transmitted after the AT detects a frame with the preamble detailing it to be the recipient of the data.

Data Rate Control (DRC) Channel - This channel contains a four bit word in each slot to allow the choice of 12 different transmission rates. Read more....!

IS-95 was the first CDMA mobile phone system to gain widespread use and it is found widely in North America. Its brand name is cdmaOne and the initial specification for the system was IS95A, but its performance was later upgraded under IS-95B. It is this later specification that is synonymous with cdmaOne. Apart from voice the mobile phone system is also able to carry data at rates up to 14.4 kbps for IS-95A and 115 kbps for IS-95B.

The IS-95 system was introduced by Qualcomm. They had been investigating the use of direct sequence spread spectrum techniques for military use when it was realised that it could be used as a multiple access technology for mobile communications. Previous systems had used frequency division multiple access (FDMA) to time division multiple access (TDMA). The principle of FDMA is that different users use different frequencies. This techniques was used for the analogue systems such as AMPS, TACS and NMT. The TDMA principle is used in GSM. Here in different users are allocated different time slots on a given channel.

CDMA
The CDMA or code division multiple access system used for IS-95 is very different. Although a complete summary of CDMA will not be included here, the basic principle of CDMA is that different codes are used to distinguish between the different users. CDMA uses a form of modulation known as direct sequence spread spectrum. Here a signal is generated that spreads out over a wide bandwidth. A code known as a spreading code is used to perform this action. By using a group of codes known as orthogonal codes, it is possible to pick out a signal with a given code in the presence of many other signals with different orthogonal codes. In fact many different baseband "signals" with different spreading codes can be modulated onto the same carrier to enable many different users to be supported. By using different orthogonal codes interference between the signals is minimal. Conversely when signals are received from several mobile stations, the base station is able to isolate each one as they have different orthogonal spreading codes. In fact the system has been likened to hearing many people in a room speaking different languages. Despite a very high noise level it is possible to pick out the person speaking your own language - English for example.

The advantage of using CDMA over FDMA and TDMA is that it enables a greater number of users to be supported. The improvement in efficiency is hard to define as it depends on many factors including the size of the cells and the level of interference between cells and several other factors.

Unlike the more traditional cellular systems where neighbouring cells use different sets of channels, a CDMA system re-uses the same channels. Signals from other cells will be appear as interference, but the system is able to extract the required signal by using the correct code in the demodulation and signal extraction process. Often more than one channel is used in each cell, and this provides additional capacity because there is a limit to the amount of traffic that can be supported on each channel.

Downlink signal
The downlink transmission (i.e. base station to the mobile) within IS-95 consists of a number of elements. There is the pilot channel and other further channels each with their own functions. The pilot channel corresponds to the control channel in GSM and enables the mobile to estimate the path loss and as a result of this to set its power level accordingly. In addition to this there are other channels for paging, speech, data etc. The speech is encoded using a voice encoder. Error correction is then applied to this data to enable it to be carried even under poor conditions. This brings the data rate up to 19.2 kbps. The next stage in the generation of the signal is to multiple the data by a Walsh code - the form of orthogonal code used to spread the signal when generating the CDMA signal itself. As this is a 64 bit code, this multiplies the data rate by 64 to bring the overall data rate to 1.228 Mbps. This signal is then transmitted.

Uplink signal
The uplink signal for IS-95 is generated in a different way. Although the same voice encoder is used, the resulting data has a greater degree of error correction or protection applied. Accordingly the resulting data rate is brought up to 28.8 kbps. A more complicated method of spreading using a Walsh code is used. However this only results in 307 kbps data stream. Further spreading is required. This is provided by using a different form of orthogonal spreading code known as a PN code. This is multiplied with the signal to increase its data rate by four to bring it up to the final data rate of 1.228 Mbps, the same as the downlink signal.

Soft handover
The reason that the uplink and downlink transmissions for IS-95 are generated in a different way results from the fact that it is difficult to synchronise the mobile handsets. Each one is a different distance away from the base station and the time delays will be different. As a result synchronisation is not possible. For the Walsh codes to maintain their orthogonality and to operate correctly they must be properly synchronised. PN codes do not require synchronisation and can be used more successfully under these circumstances.

One of the advantages of CDMA is the fact that handover can be made easier and more reliable. Normally when handing over from one from a base station in one cell to the base station in the next, it is necessary for the system to arrange for a new channel to be used. The mobile then changes channel and hopes to be able to receive the signal on the new one satisfactorily. Obviously there is a degree of risk, and occasionally a hand over does not proceed smoothly. With CDMA it is possible to use what is termed a soft hand over. As transmissions from the base stations in adjacent cells may be made on the same frequency, it is possible for a mobile to receive signals from two base stations at once. Normally the mobile would reject the signal from the second base station, but it is possible to arrange for it to receive signals from the two stations and this proves to be very useful during handover. During the period of the handover the two base stations transmit the same signal enabling the mobile to receive the signal via two routes at the same time. This means that during this handover phase the mobile should not loose the signal. Then as the mobile moves further into the second cell and the signal is firm, it can rely on one station only and the handover is complete.

This approach considerably reduces the risk of loosing the connection during handover, and it also minimises the risk of a short break in the speech during this period. However it is not free and there is an associated cost. The mobile needs two decoders to monitor and decode the two signals and this increases the complexity of the mobile. On the network side it means that two channels are used instead of one and this reduces the overall capacity. Some estimate this could be as high as 40%. This is dependent upon the speed of handover and the degree of overlap in the cells. The figure given is obviously a worst case scenario, but despite this the advantages are deemed to outweigh the reduction in capacity and increased mobile complexity.

IS-95 has been successfully installed in many areas of the world, chiefly in North America. IS 95 also has the advantage that it has an evolutionary migration path to 3G with CDMA2000 to give the higher data rates that are needed for video streaming and data transfer whilst retaining compatibility with the existing networks. Read more....!

One of the major cell / mobile phone or cellular telecommunications technologies today is the cdmaOne / CDMA2000 system. One of its strengths is that it has focussed on being an evolutionary technology moving from standards such as IS-95 (IS-95A and IS-95B) for cdmaOne through to standards including IS-2000 and IS-856 for CDMA2000 1X, 1xEv, 1xEV-DO and 1xEV-DV. Currently the standard uses one standard channel under a system known as 1X RTT, although for the future three channels (3X RTT) may be used).

In view of the fact that the CDMA2000 system has been designed to be an evolutionary standard, it is relatively easy to introduce upgrades to the system. This has made it particularly popular with operators because the cost of upgrading to the new standards is much less, and they can have users with a variety of types of phone on the same network. Thus users may operate cdmaOne phones on the same network as CDMA2000 1X or CDMA2000 1xEV-DV phones.

The story of how the system was developed is particularly interesting, and it reveals much about the nature of the system as well as telling its significant successes.


The idea for using the form of modulation known as direct sequence spread spectrum (DSSS) for a multiple access system for mobile telecommunications came from a California based company called Qualcomm in the 1980s. Previously DSSS had been mainly used for military or covert communications systems as the transmissions were hard to detect, jam and eavesdrop.,span class="fullpost">

The system involved multiplying the required data with another data stream with a much higher data rate. Known as a spreading code, this widened the bandwidth required for the transmission, spreading it over a wide frequency band. Only when the original spreading code was used in the reconstruction of the data, would the original information be reconstituted. It was reasoned that by having different spreading codes, a multiple access system could be created for use in a mobile phone system.

In order to prove that the new system was viable a consortium was set up and Qualcomm was joined by US network operators Nynex and Ameritech to develop the first experimental code division multiple access (CDMA) system. Later the team was expanded as Motorola and AT&T (now Lucent) joined to bring their resources to speed development. As a result the new standard was published as IS-95A in 1995 under the auspices of the Cellular Telecommunications Industry Association (CTIA) and the Telecommunications Industry Association (TIA). As part of the development of CDMA an organisation called the CDMA Development Group (CDG) was formed from the founding network and manufacturers. Its purpose is to promote CDMA and evolve the technology and standards, although today most of the standards work is carried out by 3GPP2.

It then took a further three years before Hutchison Telecom became the first organisation to launch a system. It is now widely deployed in North America, and the Asia Pacific region, but there are also networks in South America, Africa, and the Middle East as well as some in Eastern Europe.

System Basics
The CDMA system was totally unlike any system used before. In the UK the original TACS system had used a channel spacing of 25 kHz and AMPS in the US had used 30 kHz. The new GSM system used 200 kHz channels whilst the US -TDMA standard kept compatibility with AMPS and was based around 30 kHz channels. CDMA, IS-95A, used a 1.25 MHz bandwidth and this was much wider than anything that had been used before. CDMA operates well with a wide bandwidth, but it was limited to 1.25MHz to remain compatible with the spectrum allocations that were available.

There were other differences as well. CDMA mobiles did not have SIM cards, although recently this has changed. Instead the subscriber data has simply been stored in memory of the mobile with a method of over-the-air programming of this data being available.

cdmaOne
The first offerings of CDMA under the guise of IS-95 catered for voice as well as data up to a speed of 14.4 kbps. However with the market moving towards data applications, the IS-95 specification was upgraded to IS-95B to cater for the needs of operators. This new specification allowed packet switched data transmission up to a speed of 64 kbps. IS-95B was first deployed in September 1999 in Korea and has since been adopted by operators in Japan and Peru.

Often IS-95 A and B versions are marketed under the brand name cdmaOne. This is a registered trademark of the CDMA Development Group.

CDMA2000 1X
cdmaOne had been very successful and was introduced into many countries, but with operators seeing revenue from voice services levelling off, the pressure to migrate to 3G technologies where data speeds were higher and revenue growth could be maintained. As a result of this the IS-2000 standard was developed to enable the higher 3G data rates to be provided.

Within IS-2000 a number of further developments were included. It was envisaged that with many more areas moving towards 3G standards and the old AMPS systems being made obsolete it would be possible to have systems operating on a wider bandwidth. As a result of this the new standards allowed for systems that would use the single channel bandwidth (1X or 1X RTT) and also ones that would use three times the bandwidth (3X). Currently all work is focussed on the 1X systems, with the idea for the 3X (or 3X RTT) systems to be used some time in the future.

CDMA2000 1X can double the voice capacity of cdmaOne networks and delivers peak packet data speeds of 307 kbps in mobile environments although today's commercial CDMA2000 1X networks (phase 1) support a peak data rate of 153.6 kbps. CDMA2000 1X has been designated a 3G standard and it is now widely deployed.

Evolution
CDMA2000 1X is the basic 3G standard, in fact some people only consider it a 2.75G system, and it is being developed beyond this. In what is termed CDMA2000 1xEv, there are further developments to bring it in line with the UMTS or Wideband CDMA system that is being deployed in Western Europe and many other areas.

The first of these known as CDMA2000 1xEV-DO (EVolution Data Only) is something of a sideline from the main evolutionary development of the standard. It is defined under IS-856 rather than IS-2000, and is as the name indicates is only carries data, but at speeds up to 2.4Mbps in the forward direction and the same as 1X in the reverse direction.

The forward channel forms a dedicated variable-rate, packet data channel with signalling and control time multiplexed into it. The channel is itself time-divided and allocated to each user on a demand and opportunity driven basis. A data only format was adopted so that the system could be optimised for data applications, and if voice is required then a dual mode phone using separate 1X channel for the voice call is required. In fact the "phones" used for data only applications are referred to as Access Terminals or ATs.

The first commercial CDMA2000 1xEV-DO network was deployed by SK Telecom (Korea) in January 2002. Now operators in Brazil Ecuador, Indonesia, Jamaica, Puerto Rico, Taiwan and the USA to mention but a few have all launched networks and more are to follow.

Data and voice
The next logical evolution of the system is to incorporate both data and voice into the standard. This is exactly what CDMA2000 1xEV-DV achieves. This is catered for under Release C of the IS-2000 standard. And is effectively 1X with additional high speed data channels. In this way it is able to provide complete backward compatibility with both CDMA2000 1X and cdmaOne. In addition to this the migration requires comparatively few upgrades to a 1X system and as such it is a very attractive option for network operators. Further developments are available under Release D of the IS-2000 standard. This provides for 3.1 Mbps data in both directions as well as many other upgrades.

The first CDMA networks in the form of IS-95 / cdmaOne were the first deployments of CDMA technology, the technology that is being used for all 3G cell phone systems. This formed the basis for a unique evolutionary system as CDMA2000. CDMA2000 is a well established 3G technology, and it is particularly successful in the USA, and Asia Pacific regions as well as having a significant presence in many other parts of the world. It was able to offer 3G services well before W-CDMA became established, and it is now continuing to build on this success. Read more....!

ISDN or Integrated Services Digital Network is an international standard for end to end digital transmission of voice, data and signaling. It can operate over copper based systems and allows the transmission of digital data over the telecommunications networks, typically ordinary copper based systems and providing higher data speeds and better quality than analogue transmission. The ISDN specifications provide a set of protocols that enable the set up, maintenance and completion of calls.

ISDN, Integrated Services Digital Network, provides a number of significant advantages over analogue systems.
In is basic form it enables two simultaneous telephone calls to be made over the same line simultaneously Faster call connection. It typically takes a second to make connections rather than the much longer delays experienced using purely analogue based systems. Data can be sent more reliably and faster than with the analogue systems Noise, distortion, echoes and crosstalk are virtually eliminated The digital stream can carry any form of data from voice to faxes and internet web pages to data files - this gives the name 'integrated services'

Usage
ISDN is in use around the world, but with the introduction of ADSL it is facing strong competition. The technology never gained much market share in the USA, although it used in other countries. In Japan it became reasonably popular in the late 1990s although it is now in decline with the advent of ADSL. The system was also introduced in Europe where providers such as BT, France Telecom and Deutsche Telekom introduced services.

Configurations
There are two types of channel that are found within ISDN. These are the 'B' and 'D' channels. The B or 'bearer' channels are used to carry the payload data which may be voice and / or data, and the d or 'Delta' channel is intended for signalling and control, although it may also be used for data under some circumstances.

Additionally there are two levels of ISDN access that may be provided. These are known as BRI and PRI.

BRI (Basic Rate Interface) - This consists of two B channels, eac of which provides a bandwidth of 64 kbps under most circumstances. One D channel with a bandwidth of 16 kbps is also provided. Together this configuration is often referred to as 2B+D.

The basic rate lines connect to the network using a standard twisted pair of copper wires. The data can then be transmitted simultaneously in both directions to provide full duplex operation. The data stream is carried as two B channesl as mentioned above, each of which carry 64 kbps (8 k bytes per second). This data is interleaved with the D channel data and this is used for call management: setting up, clearing down of calls, and some additional data to maintain synchronisation and monitoring of the line.

The network end of the line is referred to as the 'Line Termination' (LT) while the user end acts as a termination for the network and is referred to as the 'Network Termination' (NT). Within Europe and Australia, the NT physically exists as a small connection box usually attached to a wall etc, and it converts the two wire line (U interface) coming in from the network to four wires (S/T interface or S bus). The S/T interface allows up to eight items or 'terminal equipments' to be connected, although only two may be used at any time. The terminal equipments may be telephones, computers, etc, and they are connected in what is termed a point to point configuration. In Europe the ISDN line provides up to about 1 watt of power that enables the NT to be run, and also enables a basic ISDN phone to be used for emergency calls. In North America a slightly different approach may be adopted in that the terminal equipment may be directly connected to the network in a point to point configuration as this saves the cost of a network termination unit, but it restricts the flexibility. Additionally power is not normally provided.

PRI (Primary Rate Interface) - This configuration carries a greater number of channels than the Basic Rate Interface and has a D channel with a bandwidth of 64 kbps. The number of B channels varies according to the location. Within Europe and Australia a configuration of 30B+D has been adopted providing an aggregate data rate of 2.048 Mbps (E1). For North America and Japan, a configuration of 23B+1D has been adopted. This provides an aggregate data rate of 1.544 Mbps (T1).

The primary rate connections utilise four wires - a pair for each direction. They are normally 120 ohm balanced lines using twisted pair cable. Primary rate connections always use a point to point configuration.

Primary rate lines are widely used to conenct to Private Branch eXchanges (PBX) in an office etc. Typically this may be used to provide a number of POTS (Plain Old Telephone System) or basic rate ISDN lines to the users. Read more....!

The Mobile WiMAX race is on. There's still a question as to how big the potential market for the technology will be, but a lot of big vendors seem to be betting that it will be huge. Ever since Sprint announced its commitment to the technology, it looks like those predictions may pan out.

Over the next two weeks, leading into the WiMAX World conference in Boston the next month, Telephony will be examining the vendors that hope to make an impact on this new promising technology sector. With the help of Current Analysis analyst Peter Jarich, we've selected six vendors to profile -- each vendor for different reasons. There are some obvious choices like Motorola and Samsung, which have unquestionable momentum after being named primary vendors for Sprint's multimillion-dollar deployment, but there may be one or two surprises in the mix. Telephony isn't being so bold as to say that every vendor on the list will be a dominant force in WiMAX, and we're certainly not predicting which vendors of the six will wind up on top. But each of the vendors either stands a good chance of making its mark on the WiMAX industry--through momentum, technology or scale--or represents a sector of the WiMAX community that presents a challenge to the established order of telecom infrastructure players.

The series will kick off Wednesday with Motorola, which has an unquestionable lead in terms of mind share if not market share (remember, not a single vendor has yet recorded a dime of Mobile WiMAX revenues).
info by http://telephonyonline.com Read more....!

EDGE is an enhancement to the GSM mobile cellular phone system. The name EDGE stands for Enhanced Data for GSM Evolution and it enables data to be sent over a GSM TDMA system at speeds up to 384 kbps. In some instances EDGE systems may also be known as EGPRS, or Enhanced General Packet Radio Service systems. Although strictly speaking a "2.5G" system, it is anticipated that it will be used to provide data services by operators who have not been able to secure the full 3G licences.

EDGE is intended to build on the enhancements provided by the addition of GPRS (General Packet Radio Service) where packet switching is applied to a network. It then enables a three fold increase in the speed at which data can be transferred by adopting a new form of modulation. GSM uses a form of modulation known as Gaussian Minimum Shift Keying (GMSK), but EDGE changes the modulation to 8PSK and thereby enabling a significant increase in data rate to be achieved.

Technical Overview
It is generally expected that EDGE will be applied to networks where the enhancements provided by GPRS have already been added. Under the original GSM system, a circuit would be allocated to a given user whether data was being transmitted or not. This was fine for voice communications because there would normally be some data present for most of the time. However this is not the case for data transmissions where high levels of data are transmitted in short bursts. TO make more efficient use of the available capability, packet switching is used. Here individual packets of data are routed to the user, enabling the channel or channels to be shared by several users.

To achieve this requires the addition of two additional nodes to the network, namely the Gateway GPRS Service Node (GGSN) and the Serving GPRS Service Node (SGSN). Here the GGSN connects to packet-switched networks such as the Internet and other GPRS networks. The SGSN provides the packet-switched link to mobile stations.

In terms of implementation EDGE systems require an EDGE transceiver unit to be added to each cell along with software upgrades to allow its use. This software upgrades may be implemented remotely. This change means that the inclusion of EDGE onto a network requires a significant investment in the infrastructure and as a result it is these upgrades will normally be implemented over a period of time. However GSM, GPRS and EDGE can all co-exist on the same network.

As both GPRS and EDGE represent significant upgrades to handsets and they are not just software upgrades, new mobile handsets are required.

Modulation
One of the key elements of EDGE is the form of modulation that is used. GPRS, being essentially a packet switched version of GSM uses GMSK, along with GSM itself. This form of modulation limits the data rate that can be transmitted over the air interface. EDGE uses a form of modulation known as 8 PSK. This is a form of phase shift keying where 8 phase states are used. The advantage is that it can transmit high data rates, although it is not as immune to interference and noise. The network therefore switches to 8PSK to allow the high data transfer rates when signal strengths are sufficient to permit the data transfer with a sufficiently low Bit Error Rate. By using 8PSK it is possible to transfer data at 48 kbps per channel rather than 9.6 kbps that is possible using GMSK. By allowing the use of multiple channels the technology allows the transfer of data at rates up to 384 kbps. However it should be remembered that these data transfer rates are only possible when the network is not highly loaded as access to all the channels would not be allowed. Read more....!

Code Division Multiplexing (Synchronous CDMA)

Synchronous CDMA, also known as Code Division Multiplexing (CDM), exploits at its core mathematical properties of orthogonality. Suppose we represent data signals as vectors. For example, the binary string "1011" would be represented by the vector (1, 0, 1, 1). We may wish to give a vector a name, we may do so by using boldface letters, e.g. a. We also use an operation on vectors, known as the dot product, to "multiply" vectors, by summing the product of the components. The operation is denoted with a dot between the vectors. For example, the dot product of \mathbf a=(1, 0, 1, 1) and \mathbf b=(1, -1, -1, 0), written as \mathbf a\cdot \mathbf b, would be (1)\times(1)+(0)\times(-1)+(1)\times(-1)+(1)\times(0)=1+(-1)=0. For the special case when the dot product of two vectors is identically 0, the two vectors are said to be orthogonal to each other.

Example
An example of 4 orthogonal digital signals.
Enlarge
An example of 4 orthogonal digital signals.

Suppose now we have a set of vectors that are mutually orthogonal to each other. Usually these vectors are specially constructed for ease of decoding—they are columns or rows from Walsh matrices that are constructed from Walsh functions—but strictly mathematically the only restriction on these vectors is that they are orthogonal. An example of orthogonal functions is shown in the picture on the right. Now, associate with one sender a vector from this set, say v, which is called the chip code. Associate a zero digit with the vector -v, and a one digit with the vector v. For example, if v=(1,-1), then the binary vector (1, 0, 1, 1) would correspond to (1,-1,-1,1,1,-1,1,-1). For the purposes of this article, we call this constructed vector the transmitted vector.

Each sender has a different, unique vector chosen from that set, but the construction of the transmitted vector is identical.

Now, the physical properties of interference say that if two signals at a point are in phase, they will "add up" to give twice the amplitude of each signal, but if they are out of phase, they will "subtract" and give a signal that is the difference of the amplitudes. Digitally, this behaviour can be modelled simply by the addition of the transmission vectors, component by component. So, if we have two senders, both sending simultaneously, one with the chip code (1, -1) and data vector (1, 0, 1, 1), and another with the chip code (1, 1), and data vector (0,0,1,1), the raw signal received would be the sum of the transmission vectors (1,-1,-1,1,1,-1,1,-1)+(-1,-1,-1,-1,1,1,1,1)=(0,-2,-2,0,2,0,2,0).

Suppose a receiver gets such a signal, and wants to detect what the transmitter with chip code (1, -1) is sending. The receiver will make use of the property described in the above foundation section, and take the dot product to the received vector in parts. Take the first two components of the received vector, that is, (0, -2). Now, (0, -2).(1, -1) = (0)(1)+(-2)(-1) = 2. Since this is positive, we can deduce that a one digit was sent. Taking the next two components, (-2, 0), (-2, 0).(1,-1)=(-2)(1)+(0)(-1)=-2. Since this is negative, we can deduce that a zero digit was sent. Continuing in this fashion, we can successfully decode what the transmitter with chip code (1, -1) was sending: (1, 0, 1, 1).

Likewise, applying the same process with chip code (1, 1): (1, 1).(0,-2) = -2 gives digit 0, (1, 1).(-2,0)=(1)(-2)+(1)(0)=-2 gives digit 0, and so on, to give us the data vector sent by the transmitter with chip code (1, 1): (0, 0, 1, 1).

Asynchronous CDMA

The previous example of orthogonal Walsh sequences describes how 2 users can be multiplexed together in a synchronous system, a technique that is commonly referred to as Code Division Multiplexing (CDM). The set of 4 Walsh sequences shown in the figure will afford up to 4 users, and in general, an NxN Walsh matrix can be used to multiplex N users. Multiplexing requires all of the users to be coordinated so that each transmits their assigned sequence v (or the complement, -v) starting at exactly the same time. Thus, this technique finds use in base-to-mobile links, where all of the transmissions originate from the same transmitter and can be perfectly coordinated.

On the other hand, the mobile-to-base links cannot be precisely coordinated, particularly due to the mobility of the handsets, and require a somewhat different approach. Since it is not mathematically possible to create signature sequences that are orthogonal for arbitrarily random starting points, unique "pseudo-random" or "pseudo-noise" (PN) sequences are used in Asynchronous CDMA systems. These PN sequences are statistically uncorrelated, and the sum of a large number of PN sequences results in Multiple Access Interference (MAI) that is approximated by a Gaussian noise process (via the theorem of the "law of large numbers" in statistics). If all of the users are received with the same power level, then the variance (e.g., the noise power) of the MAI increases in direct proportion to the number of users.

All forms of CDMA use spread spectrum process gain to allow receivers to partially discriminate against unwanted signals. Signals with the desired chip code and timing are received, while signals with different chip codes (or the same spreading code but a different timing offset) appear as wideband noise reduced by the process gain.

Since each user generates MAI, controlling the signal strength is an important issue with CDMA transmitters. A CDM (Synchronous CDMA), TDMA or FDMA receiver can in theory completely reject arbitrarily strong signals using different codes, time slots or frequency channels due to the orthogonality of these systems. This is not true for Asynchronous CDMA; rejection of unwanted signals is only partial. If any or all of the unwanted signals are much stronger than the desired signal, they will overwhelm it. This leads to a general requirement in any Asynchronous CDMA system to approximately match the various signal power levels as seen at the receiver. In CDMA cellular, the base station uses a fast closed-loop power control scheme to tightly control each mobile's transmit power.

Advantages of Asynchronous CDMA over other techniques

Asynchronous CDMA's main advantage over CDM (Synchronous CDMA), TDMA and FDMA is that it can use the spectrum more efficiently in mobile telephony applications. TDMA systems must carefully synchronize the transmission times of all the users to ensure that they are received in the correct timeslot and do not cause interference. Since this cannot be perfectly controlled in a mobile environment, each timeslot must have a guard-time, which reduces the probability that users will interfere, but decreases the spectral efficiency. Similarly, FDMA systems must use a guard-band between adjacent channels, due to the random doppler shift of the signal spectrum which occurs due to the user's mobility. The guard-bands will reduce the probability that adjacent channels will interfere, but decrease the utilization of the spectrum.

Most importantly, Asynchronous CDMA offers a key advantage in the flexible allocation of resources. There are a fixed number of orthogonal codes, timeslots or frequency bands that can be allocated for CDM, TDMA and FDMA systems, which remain underutilized due to the bursty nature of telephony and packetized data transmissions. There is no strict limit to the number of users that can be supported in an Asynchronous CDMA system, only a practical limit governed by the desired bit error probability, since the SIR (Signal to Interference Ratio) varies inversely with the number of users. In a bursty traffic environment like mobile telephony, the advantage afforded by Asynchronous CDMA is that the performance (bit error rate) is allowed to fluctuate randomly, with an average value determined by the number of users times the percentage of utilization. Suppose there are 2N users that only talk half of the time, then 2N users can be accommodated with the same average bit error probability as N users that talk all of the time. The key difference here is that the bit error probability for N users talking all of the time is constant, whereas it is a random quantity (with the same mean) for 2N users talking half of the time.

In other words, Asynchronous CDMA is ideally suited to a mobile network where large numbers of transmitters each generate a relatively small amount of traffic at irregular intervals. CDM (Synchronous CDMA), TDMA and FDMA systems cannot recover the underutilized resources inherent to bursty traffic due to the fixed number of orthogonal codes, time slots or frequency channels that can be assigned to individual transmitters. For instance, if there are N time slots in a TDMA system and 2N users that talk half of the time, then half of the time there will be more than N users needing to use more than N timeslots. Furthermore, it would require significant overhead to continually allocate and deallocate the orthogonal code, time-slot or frequency channel resources. By comparison, Asynchronous CDMA transmitters simply send when they have something to say, and go off the air when they don't, keeping the same PN signature sequence as long as they are connected to the system.

Soft handoff

Soft handoff (or soft handover) is an innovation in mobility. It refers to the technique of adding additional base stations (in IS-95 as many as 5) to a connection to be certain that the next base is ready as you move through the terrain. However, it can also be used to move a call from one base station that is approaching congestion to another with better capacity. As a result, signal quality and handoff robustness is improved compared to TDMA systems.

In TDMA and analog systems, each cell transmits on its own frequency, different from those of its neighbouring cells. If a mobile device reaches the edge of the cell currently serving its call, it is told to break its radio link and quickly tune to the frequency of one of the neighbouring cells where the call has been moved by the network due to the mobile's movement. If the mobile is unable to tune to the new frequency in time the call is dropped.

In CDMA, a set of neighbouring cells all use the same frequency for transmission and distinguish cells (or base stations) by means of a number called the "PN offset", a time offset from the beginning of the well-known pseudo-random noise sequence that is used to spread the signal from the base station. Because all of the cells are on the same frequency, listening to different base stations is now an exercise in digital signal processing based on offsets from the PN sequence, not RF transmission and reception based on separate frequencies.

As the CDMA phone roams through the network, it detects the PN offsets of the neighbouring cells and reports the strength of each signal back to the reference cell of the call (usually the strongest cell). If the signal from a neighbouring cell is strong enough, the mobile will be directed to "add a leg" to its call and start transmitting and receiving to and from the new cell in addition to the cell (or cells) already hosting the call. Likewise, if a cell's signal becomes too weak the mobile is directed to drop that leg. In this way, the mobile can move from cell to cell and add and drop legs as necessary in order to keep the call up without ever dropping the link.

It should be noted that this "soft handoff" does not happen via CDMA from cell tower to cell tower. A group of cell sites are linked up with wire and the call is synced over wire, over TDM, ATM, or even IP.

When there are frequency boundaries between different carriers or sub-networks, a CDMA phone behaves in the same way as TDMA or analog and performs a hard handoff in which it breaks the existing connection and tries to pick up on the new frequency where it left off. Read more....!

Code division multiple access (CDMA) is a form of multiplexing (not a modulation scheme) and a method of multiple access that does not divide up the channel by time (as in TDMA), or frequency (as in FDMA), but instead encodes data with a special code associated with each channel and uses the constructive interference properties of the special codes to perform the multiplexing. CDMA also refers to digital cellular telephony systems that make use of this multiple access scheme, such as those pioneered by Qualcomm, and W-CDMA by the International Telecommunication Union or ITU.

CDMA has since been used in many communications systems, including the Global Positioning System (GPS) and in the OmniTRACS satellite system for transportation logistics.

Usage in mobile telephony

A number of different terms are used to refer to CDMA implementations. The original U.S. standard defined by QUALCOMM was known as IS-95, the IS referring to an Interim Standard of the Telecommunications Industry Association (TIA). IS-95 is often referred to as 2G or second generation cellular. The QUALCOMM brand name cdmaOne may also be used to refer to the 2G CDMA standard. The CDMA has been submitted for approval as a mobile air interface standard to the ITU International Telecommunication Union.

Whereas the Global System for Mobile Communications (GSM) standard is a specification of an entire network infrastructure, the CDMA interface relates only to the air interface—the radio part of the technology. For example GSM specifies an infrastructure based on internationally approved standard while CDMA allows each operator to provide the network features as it finds suited. On the air interface, the signalling suite (GSM: ISDN SS7) work has been progressing to harmonise these.

After a couple of revisions, IS-95 was superseded by the IS-2000 standard. This standard was introduced to meet some of the criteria laid out in the IMT-2000 specification for 3G, or third generation, cellular. It is also referred to as 1xRTT which simply means "1 times Radio Transmission Technology" and indicates that IS-2000 uses the same 1.25 MHz shared channel as the original IS-95 standard. A related scheme called 3xRTT uses three 1.25 MHz carriers for a 3.75 MHz bandwidth that would allow higher data burst rates for an individual user, but the 3xRTT scheme has not been commercially deployed. More recently, QUALCOMM has led the creation of a new CDMA-based technology called 1xEV-DO, or IS-856, which provides the higher packet data transmission rates required by IMT-2000 and desired by wireless network operators.

The QUALCOMM CDMA system includes highly accurate time signals (usually referenced to a GPS receiver in the cell base station), so cell phone CDMA-based clocks are an increasingly popular type of radio clock for use in computer networks. The main advantage of using CDMA cell phone signals for reference clock purposes is that they work better inside buildings, thus often eliminating the need to mount a GPS antenna on the outside of a building.

The US CDMA system is frequently confused with a similar but incompatible technology called Wideband Code Division Multiple Access (W-CDMA) which forms the basis of the W-CDMA air interface. The W-CDMA air interface is used in the global 3G standard UMTS and the Japanese 3G standard FOMA, by NTT DoCoMo and Vodafone; however, the CDMA family of US national standards (including cdmaOne and CDMA2000) are not compatible with the W-CDMA family of International Telecommunication Union (ITU) standards.

Another important application of CDMA — predating and entirely distinct from CDMA cellular — is the Global Positioning System, GPS.


Coverage and Applications

The size of a given cell depends on the power of the signal transmitted by the handset, the terrain, and the radio frequency being used. Various algorithms can reduce the noise introduced by variations in terrain, but require extra information be sent to validate the transfer. Hence, the radio frequency and power of the handset effectively determine the cell size. Long wavelengths need less energy to travel a given distance vs. short wavelengths, so lower frequencies generally result in greater coverage while higher frequencies result in shorter coverage. These characteristics are used by mobile network planners in determining the size and placement of the cells in the network. In cities, many small cells are needed; the use of high frequencies allows sites to be placed more-closely together, with more subscribers provided service. In rural areas with a lower density of subscribers, use of lower frequencies allows each site to provide broader coverage. (See also the Market situation section of GSM.)

Various companies use different variants of CDMA to provide fixed-line networks using Wireless local loop (WLL) technology. Since they can plan with a specific number of subscribers per cell in mind, and these are all stationary, this application of CDMA can be found in most parts of the world.

CDMA is suited for data transfer with bursty behaviour and where delays can be accepted. It is therefore used in Wireless LAN applications; the cell size here is 500 feet because of the high frequency (2.4 GHz) and low power. The suitability for data transfer is the reason for why W-CDMA seems to be "winning technology" for the data portion of third-generation (3G) mobile cellular networks. Read more....!