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GPRS introduces new interface definitions in the network as well as over the air. The interfaces are open standards as described by 3GPP and are shown in Figure 4.3. This enables a multi-vendor network to be constructed with minimum amount of modification. Since GPRS uses much of the GSM network, standardized interfaces are also required between the GSM equipment and the GPRS equipment.

• Ga: this is used for transferring the charging records known as call detail records

(CDRs) from the SGSN and the GGSN to a CG. It uses an enhanced version of GTP known as GTP.

• Gb: the Gb interface resides between the SGSN and the BSS. Its function is to transport both signalling and data traffic. This interface is based on frame relay and is described in more detail in Section 4.7.

• Gc: this interface is between the GGSN and the HLR, and provides the GGSN with access to subscriber information. The protocol used here is MAP and the interface is used for signalling purposes only. This interface can be used to activate the mobile device for mobile terminated packet calls. It requires that the mobile device is given a unique IP address, which is often not the case and thus it may not be implemented. The GGSN is essentially an IP device and may not have MAP capabilities, so the specification allows the GGSN to pass requests to the SGSN so that they can be forwarded to the HLR on its behalf. The Gc is an optional interface.

• Gd: the Gd interface connects the SGSN to an SMS gateway, thus enabling the SGSN to support SMS services.

• Gf: this interface connects the SGSN to the EIR and allows the SGSN to check the status of a particular mobile device, such as whether it has been stolen or is not type approved for connection to the network.

BSS BSC GGSN BG IP Network GRX HLR/AuC EIR MSC/VLR Gb Gn Gp Gi Gs SMS-GW CGW Gf Gd Ga Gr Gc Ga Billing Centre SGSN Figure 4.3 GPRS interfaces

• Gi: this is a reference point rather than an interface and refers to the connection between the GGSN and some external network. Currently IPv4, IPv6 and PPP1_{are supported by}

GPRS and the Gi interface simply has to be able to support the required protocol for this particular access point. For example, the access point may be required to transport IPv4 packets; the underlying network is not specified and may be Ethernet, asynchronous transfer node (ATM), frame relay or any other transport protocol.

• Gn: the Gn interface resides between the GSNs. It consists of a protocol stack which includes IP and GTP. GTP is explained in detail in Section 4.8. The GTP tunnel is also used between two SGSNs and also between an SGSN via a BG to another operator’s GGSN. It is not used between two GGSNs unless they have BG functionality. This tunnel ensures that the operator’s IP network is completely separated from the IP used for the mobile device to connect to the external network. The GTP tunnel actually consists of two parts, the GTP-U which is used to carry user data and the GTP-C which is used to carry control data.

• Gp: this has similar functionality to the Gn interface and also consists of a GTP pro- tocol. It is required when the SGSN and GGSN are in different PLMNs. It introduces further routing and security functions to the Gn interface. Connection is via BGs and possibly an intermediate inter-PLMN network which may be owned by a third party, hence the increased security functions.

• Gr: this interface is between the SGSN and the HLR, providing the SGSN with access to subscriber information. The SGSN and HLR will be in different networks in the case of roaming users. The protocol used here is MAP and the interface is used for signalling purposes only.

• Gs: this is another optional interface. It is used for signalling between the SGSN and the visitor location register (VLR), which is usually co-located with the mobile switching centre (MSC) and an SGSN, it uses the BSS application part plus (BSSAP+) protocol. This is a subset of the BSSAP protocol to support signalling between the SGSN and MSC/VLR. To some extent, the SGSN appears to be a BSC when communicating with the MSC/VLR. This interface enables a number of efficiency saving features by coordinating signalling to the mobile device such as combined updates of the location area (LA) and routing area (RA) and IMSI attach/detach which reduces the amount of signalling over the air interface.

In addition to the ‘G’ interfaces, two relevant interfaces are those across the air, for both GPRS and UMTS:

• Um: this is the modified GSM air interface between the mobile device and the fixed network which provides GPRS services.

• Uu: this is the UMTS air interface between the mobile device and the fixed network which provides GPRS services.

4.4.1

Network operation mode

A network can be in three different modes of operation. These modes depend on whether the Gs interface is present and how paging of the mobile device is executed.

• Network operation mode 1. A network which has the Gs interface implemented is referred to as being innetwork operation mode 1. CS and PS paging is coordinated in this mode of operation on either the GPRS or the GSM paging channel. If the mobile device has been assigned a data traffic channel then CS paging will take place over this data channel rather than the paging channel (CS or PS).

• Network operation mode 2. The Gs interface is not present and there is no GPRS paging channel present. In this case, paging for CS and PS devices will be transferred over the standard GSM common control channel (CCCH) paging channel. Even if the mobile device has been assigned a packet data channel, CS paging will continue to take place over the CCCH paging channel and thus monitoring of this channel is still required.

• Network operation mode 3. The Gs interface is not present. CS paging will be trans- ferred over the CCCH paging channel. PS paging will be transferred over the packet CCCH (PCCCH) paging channel, if it exists in the cell. In this case the mobile device needs to monitor both the paging channels.

Thenetwork operation mode being used is broadcast assystem information to the mobile devices.