Describing GPRS (General Packet Radio Service)

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.
Posted by Admin, Saturday, September 30, 2006 8:46:00 AM

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