The GPRS network is designed as an overlay network on an existing GSM network. Additional components are added to handle packet data and interface to external PDNs.
Figure 4-1 shows the additional components required to implement GPRS. It also shows the interfaces between new components, as well as the existing GSM components.
Anew mobile station is required to support GPRS. The GPRS terminals are available in many forms and are backward compatible to support GSM voice and circuit-switched data.
The existing GSM BTSs need software upgrades to support a new air interface, new coding schemes, and logical channels and their mapping. No hardware upgrades are required. The BTS connects to the BSC, using the Abis interface as in GSM.
The BSCs require both hardware and software upgrades. The software upgrade is needed to support mobility and paging of GPRS terminals. The hardware upgrade is needed to add new functionality to control and handle the packet data. As shown in Figure 4-1, a packet control unit (PCU) is added to the BSC. The PCU connects with the SGSN node by using the Gb interface, which is based on frame relay.
GPRS introduces two new nodes to handle the packet-switched data. The gateway GPRS support node (GGSN) has capabilities similar to those of the GMSC. It provides an interface (Gi) to external packet data networks (PDNs). The GGSN has mobility management and access server functionality built in.
The serving GPRS support node (SGSN) is an MSC/VLR equivalent. It controls the connection between the MS and the network. The SGSN provides mobility and session management functionality. It connects with the GGSN via the Gn interface. It also has connectivity to HLR, EIR, MSC, and SMS-IWMSC via the Gr, Gf , Gs, and Gd interfaces, respectively.
Figure 4-1 GPRS Network Architecture.
To support inbound roamers, it connects to other PLMNs via the Gp interface.
The GSM HLR needs a software upgrade to support GPRS subscription data and routing information. The HLR communicates with the SGSN via the Gr interface, using the CCS7 MAP protocol. For roaming MSs, the HLR is in a different PLMN than the serving SGSN.
The existing MSC/VLR needs a software upgrade to support terminals that are attached to the both GSM and the GPRS. The MSC/VLR communicates with the SGSN via the Gs interface, using BSSAP+ protocol. The EIR/AUC does not require any upgrades.
GPRS is also used as an efficient bearer to carry SMS messages. The Gd interface is defined to exchange SMS with the SMS-IWMSC.
The next section describes the new GPRS components and their functionality in more detail.
GPRS terminals
Currently, several different options are available in the market. Some of these have the look and feel of normal mobile phones while others are designed specifically to make better use of enhanced data capabilities. PC cards, Smartphones, and PDAs are very popular GPRS terminals. The ETSI specification defines three different classes of mobiles for the hybrid GPRS/GSM networks:
Class A mobiles can attach to both GSM and GPRS networks simultaneously. These mobiles can make and receive voice and data calls at the same time. In order to achieve this, mobiles monitor both the GSM and the GPRS for incoming calls and have an additional receiver.
Class B terminals can attach to both the GSM and the GPRS networks simultaneously, but can handle only one service at a time. It is possible to switch between the calls. For example, a Class B mobile can suspend an outgoing packet transfer, when it gets an incoming voice call, and resume the packet transfer once the voice call is over.
Class C terminals can attach to only one network, i.e., GSM or GPRS. For example, if a Class C mobile is attached to a GPRS network, it will not be able to make or receive a voice call from a GSM network.
GPRS BSS—Packet control unit
As described earlier, the GSM BSS requires new software for both the BTS and the BSC and additional hardware for the BSC to support GPRS. The new piece of hardware is generally termed a packet control unit (PCU). Each BSC will require at least one PCU. One PCU cannot serve multiple BSCs. The PCU connects to the SGSN via a physical and logical data interface, i.e., Gb. In most of the implementations, the PCU is collocated with the BSC, as discussed earlier. However, it is possible that the PCU resides within the BTS or outside the BSC near the SGSN. Figure 4-2 illustrates the three possible locations. The channel control unit (CCU), as shown in Figure 4-2, resides in the BTS and is responsible for channel coding, radio channel measurement, and management functions. It is a software-only implementation.
The Gb interface connects the BSC to the SGSN. This is based on frame relay on the E1/T1 interface. To achieve efficient use of transmission bandwidth, a switched frame relay network is used between the BSC and the SGSN. The newer implementation deploys Gb over IP.
GPRS support nodes
SGSN. The serving GPRS support node (SGSN) provides packet routing to and from the mobile stations currently in its coverage area. For establishing data calls, the GPRS users need to attach to the SGSN via the base station. The SGSN performs functions to support mobility, session, and security management. The SGSN is also responsible for charging functions. To perform its tasks, it communicates with other subsystems using G-interfaces as shown in Table 4-1.
Currently, several vendors supply SGSN with varying performance and capacities. Some of the network providers prefer several SGSNs of smaller capacity, while others like to consolidate and implement only a few higher-capacity SGSNs covering the whole network.
Figure 4-2 PCU location in the BSS.
TABLE 4-1 SGSN Interfaces
|
Connection |
Mandatory/ optional |
Interface |
Common implementation |
|
SGSN-PCU |
Mandatory |
Gb |
Frame relay-based, E1/T1 interface, channelized, nonchannelized, and fractional. In most of the implementations, each Gb link consists of n x 64 timeslots, depending on traffic. |
|
SGSN-HLR |
Mandatory |
Gr |
CCS7-based, E1/T1 interface. Multiple timeslots can be used for signaling, if required |
|
SGSN-EIR |
Optional |
Gf |
CCS7-based, E1/T1 interface. Multiple timeslots can be used for signaling, if required |
|
SGSN-MSC/ VLR |
Optional |
Gs |
CCS7-based, E1/T1 interface. Multiple timeslots can be used for signaling, if required |
|
SGSN-SMS GMSC SGSN-SMS IWMSC |
Optional |
Gd |
CCS7-based, E1/T1 interface. Multiple timeslots can be used for signaling, if required |
|
SGSN-GGSN |
Mandatory |
Gn |
IP-based IP over Ethernet/Fast Ethernet IP over ATM IP over PPP |
|
SGSN-external GSNs |
Optional |
Gp |
IP-based IP over Ethernet/Fast Ethernet IP over ATM IP over PPP |
The SGSN performance and capacity are defined by the following parameters:
■ Maximum number of simultaneously attached users
■ Maximum number of PDP contexts
■ Maximum throughput
PDP stands for packet data protocol. In the GPRS context, it is X.25 or IP.
In most of the implementations, the SGSN and the GGSN functions reside in separate physical nodes. However, it is possible to combine the SGSN and the GGSN functionalities in a single node. In such cases, the Gn interface is not visible.
Mobility management. Like an MSC in GSM, SGSN is responsible for supporting MS mobility. It keeps track of all the subscribers in its coverage area. The MM functions include the following procedures:
■ GPRS attach
■ GPRS detach
■ Paging
■ Routing area update
These procedures are discussed later in this topic.
Session management. The SGSN is responsible for relaying the data PDUs between the MS and the GGSN. For this purpose, SGSN establishes a session, which is defined as the period between opening and closing the connection. The SM functions include the following procedures:
■ Activate PDP context
■ Modify PDP context
■ Delete PDP context
These procedures are discussed later in this topic.
Security management. The SGSN authenticates the subscriber at the very first request to attach to the network. This is necessary to prevent unauthorized users from gaining access to the network services.
As the air interface is most vulnerable to fraudulent access, the SGSN also initiates procedures with the MS to cipher the data. The ciphering, however, is a network feature, which can be put off if the network operator so desires. The security functions include following procedures.
■ Identity request
■ Authentication and ciphering
PDU handling. The SGSN and the GGSN use this function to transport packet data units (PDUs) between the MS and the external packet data network. The SGSN and the GGSN use a tunneling concept to transport PDUs over the Gn interface. The PDUs are encapsulated into an IP datagram to facilitate transfer of PDUs of any format across the Gn link.
Charging. The charging functions include the call detailed record (CDR) generation and charging gateway function (CGF). The CDR includes information necessary for the service providers to invoice the customers. The CDRs are generated for each PDP context and contain parameters such as user identity, PDP address, volume of data transfer, time, and QoS requested and assigned. The CGF functions include collection, temporary storage, and transportation of CDRs to downstream billing system.
Operation and maintenance. O&M functions are vendor specific. They allow the service provider to manage the nodes. Generally, all the FCAPS (fault, configuration, accounting, performance, and security) functions are supported.
GGSN. As the name suggests, the gateway GPRS serving node acts as a gateway between the GPRS network and the external PDN. Several SGSNs can use one GGSN to access an external PDN. On the other hand, a SGSN can send its packets by using different GGSNs to reach different packet data networks. The GGSN functions include session management, PDU handling, PDP address management, QoS negotiation, and authentication through RADIUS. To perform its tasks, it communicates with other subsystems using G interfaces as shown in Table 4-2.
Session management. The GGSN works in conjunction with the SGSN to establish a session. The GGSN is also responsible for assigning an IP address to the MS. It supports both dynamic and static IP address allocation. For dynamic address allocation, the GGSN either provides an IP address from its own allocated IP address range or uses a RADIUS server located at the ISP or a company’s intranet.
Mobility management. The GGSN makes sure that the PDUs related to an MS get tunneled to its current serving SGSN.
Interface to external PDN and PDU handling. GGSNs act as an interface point to external networks such as the Internet, enterprise intranets, ISPs, and to other GPRS PLMNs. To external PDNs, the GPRS network is another data network. It hides GPRS network complexity such as mobility from the external networks and acts as a router for the IP addresses of all the MSs served by the GPRS network.
Security. The GGSNs include a firewall to protect the GPRS network from intrusion. The GGSNs also include a RADIUS client, which queries an external RADIUS server on behalf of MS for authentication purposes.
TABLE 4-2 GGSN Interfaces
|
Connection |
Mandatory/ optional |
Interface |
Common implementation |
|
GGSN-SGSN |
Mandatory |
Gn |
IP-based IP over Ethernet/Fast Ethernet IP over ATM IP over PPP |
|
GGSN-HLR |
Optional |
Gc |
CCS7-based, E1/T1 interface. Multiple timeslots are used for signaling, if required. |
|
GGSN-external PDN |
Mandatory |
Gi |
IP-based IP over Ethernet/Fast Ethernet IP over ATM IP over PPP |
Charging. Like SGSNs, the GGSNs also have charging and CGF functions. The CDRs from both the SGSN and the GGSN are needed to support billing and invoicing functions. For example, the GGSN is unaware of MS location; hence the CDRs available with GGSNs are not sufficient for roaming charging.
