Introduction to GPRS . . . .4.1 GPRS Logical Channels. . . .4.2 The GPRS Mobile. . . .4.3 GPRS Network Elements . . . .4.4 GPRS Network Element Basic Functions . . . .4.5 GPRS Network Architecture . . . .4.6 GPRS Resource Allocation. . . .4.7 EGPRS Resource Allocation . . . .4.8 GPRS Roaming . . . .4.9
GSM and UMTS Core Network
At the end of this section you will be able to:
OBJECTIVES
GPRS
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■ describe the basic functionality of the GPRS upgrade of GSM
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■ identify how the GSM air interface can be shared with GPRS services ■
■ outline the set of logical channels defined for use by GPRS
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■ identify the key component elements of the GPRS access and core networks ■
■ describe the functionality of the various GPRS Mobile Station classes
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■ outline the role played by the IP Backbone network in supporting GPRS functionality ■
■ discuss the role of the PDP Context
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■ describe how international roaming is supported for GPRS subscribers ■
GSM and UMTS Core Network
GPRS
Introduction to GPRS
Phase 1 GSM networks, when they began to deploy in the early 1990s, were designed to support circuit- switched voice services only. The ability to handle two-way SMS and CS dial-up data services were added later, but as the 1990s progressed and the growing importance of the Internet and other IP-based communications systems began to become apparent, it was decided that GSM needed a packet- switched data capability, which lead to the development of GPRS.
The GPRS can be considered to be a ‘bolt on’ or adjunct to the original GSM design; one of the main objectives of its development is to reuse as much of the existing GSM architecture as possible whilst causing the minimum disruption to existing services.
GPRS is a wireless bearer service providing mobile access to data applications, such as the Internet, to users on demand.
In GPRS, Internet and GSM technologies are brought together, offering improved applications and increased bit rates with an efficient use of network and radio resources. Sometimes referred to as an ‘always on’ service, GPRS allows subscribers to be online constantly while their phone is switched on, only being charged when they are transmitting or receiving data.
GPRS is optimized for ‘bursty’ data transfer: web browsing and information/application download, e-mails and multimedia messaging. It is not used for the transmission of speech.
GPRS is often referred to as a 2.5G (Generation 2.5) network, a stepping stone from 2G networks such as GSM towards 3G.
4.2
GSM and UMTS Core Network
GPRS Logical Channels
The GPRS logical channels are as follows:
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■ PBCCH (Packet Broadcast Control Channel) ■
■ PCCCH (Packet Common Control Channel) ■
■ PRACH (Packet Random Access Channel) ■
■ PPCH (Packet Paging Channel) ■
■ PAGCH (Packet Access Grant Channel) ■
■ PNCH (Packet Notification Channel) ■
■ PDTCH (Packet Data Traffic Channel) ■
■ PACCH (Packet Associated Control Channel) ■
■ PTCCH (Packet Timing-advance Control Channel)
This figure illustrates how these channels are mapped, and shows the subdivisions applicable to them.
GPRS
The GPRS Mobile
GPRS-enabled terminals evolved from standard GSM phones to much more sophisticated equipment. Some have colour screens and digital cameras; others incorporate touch-sensitive screens and voice recognition. Most GPRS terminals also support associated technologies such as WAP and Bluetooth™. Overall, functionality has improved, and most terminals have increased processing power.
A GPRS terminal can operate in one of three modes. The mode of operation will depend on the services to which the MS is attached, whether GPRS only or GPRS and GSM. The three mobile classes are class-A, class-B and class-C.
Class-A allows for simultaneous GSM and GPRS operation. Class-B terminals, which include most GPRS-enabled GSM phones, can be attached to both GSM and GPRS, but can only use the services alternately. Class-C allows for alternate GSM and GPRS operation, but can only be attached to one at a time; these are usually data-oriented terminals such as laptops or palmtops.
4.4
GSM and UMTS Core Network
GPRS Network Elements
The addition of GPRS has required significant modifications to the GSM network architecture to enable it to handle both packet- and circuit-switched connections. Three new entities have been added: the GGSN (Gateway GPRS Support Node), SGSN, and PCU (Packet Control Unit).
These elements are connected with each other and with the GSM network elements. The SGSN and GGSN are connected via the operator’s backbone IP network. The SGSN is connected to the PCU using Frame Relay over an E1 physical interface. The GSM RAN (Radio Access Network) provides ‘connections’ from the MS to the SGSN for both signalling and traffic.
GPRS shares some network resources with GSM, including the HLR, EIR and AuC The HLR has been modified to cater for GPRS subscription data and in cooperation with the SGSN keeps track of a mobile’s location for GPRS services.
The PCU allocates air interface resources for GPRS operation like the BSC does for CS GSM resources. Since the air interface is a shared resource, the PCU is generally co-located with the BCS to coordinate air interface resource allocation more efficiently .
GPRS provides a packet-switched bearer service between the MS and an external packet network accessed via a GGSN. For example, GPRS may be used to provide an IP bearer service to the public Internet or to a private IP network, intranet, or third-party content provider. To achieve this the MS initially attaches itself to an SGSN, essentially a similar process to location updates. The MS then requests an IP service via the SGSN indicating which network it wishes to reach and what QoS (Quality of Service) it requires. The SGSN forwards a request to the appropriate GGSN in order to establish the bearer. The GGSN allocates the appropriate QoS and an IP address for the MS, following which IP datagrams may be transferred between the mobile and external IP-based devices.
The IP bearer together with its QoS definition is referred to as a PDP (Packet Data Protocol) context. The PDP context is managed by the SGSN and GGSN; the PCU measures the QoS delivered to the end user against the negotiated QoS for the PDP context.
GPRS
GPRS Network Element Basic Functions
The PCU provides radio access control. It allocates radio channels for data transfer, ensures packets are the correct size for transmission over the radio interface, and makes QoS measurements in respect of the radio link with the user’s mobile.
Like an MSC, an SGSN is responsible for a service area containing a number of mobiles. Within this service area the main functions of the SGSN include the authorization and authentication of mobiles; ciphering of packets across the air interface; the routing of data packets to and from mobiles; and location management, noting the location of mobiles new to the service area and tracking their subsequent position within the service area.
The SGSN also has charging functionality. It gathers data relating to a subscriber’s use of the radio network.
The GGSN performs the functions necessary to allow mobiles to communicate with external networks. For incoming calls it contains routing tables so that incoming packets of data can be routed to the SGSN that is supporting the destination mobile. In addition, the GGSN can allocate IP addresses to the served mobile and also act as a firewall to prevent unwanted access.
In respect of charging, the GGSN is responsible for gathering data relating to packets transmitted to and received from external networks.
4.6
GSM and UMTS Core Network
GPRS Network Architecture
The introduction of GPRS has required significant modifications to the GSM network architecture to enable it to handle both packet- and circuit-switched connections.
The three new entities that have been added are the GGSN, the SGSN and the PCU.
These are connected to each other, and to the existing GSM network elements, via a series of interfaces that all carry the prefix ‘G’. The interfaces deal with both traffic connections and signalling connections. Another new element has been added to assist in the management of charging and billing procedures. This is the CGF (Charging Gateway Function).
Some GPRS network resources shared the existing GSM network. These include the VLR, the HLR, the AuC, the EIR and the SMSC. The HLR and the VLR require software upgrades to cater for GPRS, so that both GSM and GPRS networks are able to keep track of the mobile’s location.
By allowing access to these common units GSM/GPRS information is maintained from centralized resources, making it easier for the effective management, hence interworking, of the two systems.
GPRS
GPRS Resource Allocation
In GSM, a timeslot – effectively a channel – is allocated to a user for their sole use for the duration of their call. In GPRS, users may share resources within a timeslot or across multiple timeslots. This concept is illustrated in the diagram. Note that in ‘standard GPRS’ the modulation scheme used on the air interface is GMSK (Gaussian Minimum Shift Keying).
GPRS provides the ability to vary the bit rate within a timeslot by using four coding schemes, CS1 to CS4. With CS1 the timeslot contains 9.1 kbit/s of data with a high level of forward error correction (suitable for poor radio channel performance) while CS4 has 21.4 kbit/s data with no error correction (suitable for excellent radio channel condition). CS4 is rarely used. This is because there is no error protection provided with this coding scheme and the system has no way of a dynamically and quickly adapting between coding schemes in order to combat changes in radio channel conditions. Operators therefore err on the side of caution and use the lower-level coding schemes.
The maximum theoretical bit rate, 171.2 kbit/s, is achieved by allocating all eight timeslots. However, in typical operation four timeslots may be allocated with a user rate of between 40 and 50 kbit/s.
It should be noted that as in GSM, GPRS resources are finite. If many subscribers are using the network, fewer timeslots can be allocated to each subscriber, resulting in slower data rates.
4.8
GSM and UMTS Core Network
EGPRS Resource Allocation
EDGE is an enhancement to GSM’s air interface that may be applied to both CS and PS operation. However, in practice it has only been applied to GPRS and is commonly referred to as EGPRS.
EGPRS (Enhanced General Packet Radio Service) introduces a new coding scheme, 8PSK (eight Phase Shift Keying ), which provides a three-times increase in the air interface bit rate but is more susceptible to errors in noisy radio channels. Note that EDGE-enhanced devices may switch between GMSK and 8PSK modulation schemes.
EDGE also introduces nine new coding schemes, MCS-1 (8.8 kbit/s, high error protection) to MCS-9 (59.2 kbit/s, no error protection). MCS-1 to MCS-4 are used with GMSK while MCS-5 to MCS-9 are used with 8PSK.
EDGE is able to rapidly adapt between both modulation schemes and coding schemes in order to combat a noisy radio channel albeit at the expense of higher bit rates. For example, it may be that all eight timeslots are allocated to EGPRS using 8PSK and MCS-9 for the highest bit rate when the radio conditions are favourable. However, as the channel worsens, the coding scheme may be ‘throttled’ down to say MCS-5. If inference increases further then this may cause a change of modulation scheme to GMSK and, say, MCS-2.
The maximum theoretical bit rate, 473.6 kbit/s, is achieved by allocating all eight timeslots. However, in typical operation four timeslots may be allocated with a user rate of between 100 and 120 kbit/s.
GPRS
GPRS Roaming
As with GSM, International Roaming is supported in GPRS using very similar mechanisms.
The discovery and use of foreign networks by roaming Mobile Stations follows the procedures laid down for GSM; roaming is only possible on networks that support GPRS, it is only possible on networks that have a roaming agreement with the subscriber’s home network and it is made possible by the interaction between visited SGSNs and home HLRs.
Once a roaming subscriber has been authorised to use the resources of a foreign network, their MS is able to request that a PDP Context be established.
Visited SGSNs have two options when processing connection requests from roaming users; they are able to establish a PDP Context to a local GGSN and provide ‘local breakout’ to the requested network or service, or they can ‘tunnel’ a PDP Context back to the subscriber’s home network to allow ‘home breakout’ to take place. The choice can be based on subscription information contained in user profiles or on bilateral agreements made between the operators. In most cases, networks elect to have roaming connections tunnelled home as this allows them greater visibility of roaming user’s requested services and the ability to provide roaming users with access to the same set of services available to home users. The international links between partner networks can be carried via dedicated point-to-point connections between border gateways, or they can go via specialized interconnection backbones known as GRX (GPRS Roaming Exchange) services, or could just be tunnelled across the general Internet.
GPRS roaming has traditionally been a somewhat underused service, partly due to the high costs often associated with it, but the widespread use of smartphones (such as Blackberrys and iPhones) with applications that automatically access the Internet to retrieve email and other services has lead to rapid growth of roaming traffic in recent years. Regulators in some regions have made efforts to impose ‘roaming charge caps’ on operators, which has helped to stimulate interest in these services still further.
4.10