Interfaces and Protocols

The previous section introduced GSM network architecture, and this section introduces the SS7/C7 protocols that are used. It also discusses interfaces, because different protocols are used on different interfaces. The SS7/C7 protocols MTP, SCCP, TUP, ISUP are protocols that were used before digital wireless networks were available. The final part of this section introduces SS7/C7 protocols that were specifically developed for GSM.

Table 12-2 summarizes the interfaces and protocols that are used in GSM.

Table 12-2. GSM Interfaces and Protocols

Interface	Between	Description
Um	MS-BSS	The air interface is used for exchanges between a MS and a BSS. LAPDm, a modified version of the ISDN LAPD, is used for signaling.
Abis	BSC-BTS	This is a BSS internal interface that links the BSC and a BTS; it has not been standardized. The Abis interface allows control of radio equipment and radio frequency allocation in the BTS.
A	BSS-MSC	The A interface is between the BSS and the MSC. It manages the allocation of suitable radio resources to the MSs and mobility management. It uses the BSSAP protocols (BSSMAP and DTAP).
B	MSC-VLR	The B interface handles signaling between the MSC and the VLR. It uses the MAP/B protocol. Most MSCs are associated with a VLR, making the B interface "internal." Whenever the MSC needs to access data regarding an MS that is located in its area, it interrogates the VLR using the MAP/B protocol over the B interface.
C	GMSC-HLR or SMSG-HLR	The C interface is between the HLR and a GMSC or a SMSC. Each call that originates outside of GSM (such as an MS terminating call from the PSTN) must go through a gateway to obtain the routing information that is required to complete the call, and the MAP/C protocol over the C interface is used for this purpose. Also, the MSC can optionally forward billing information to the HLR after call clearing.
D	HLR-VLR	The D interface is between the HLR and VLR, and uses the MAP/D protocol to exchange data related to the location of the MS and subsets of subscriber data.
E	MSC-MSC	The E interface connects MSCs. The E interface exchanges data that is related to handover between the anchor and relay MSCs using the MAP/E protocol. The E interface can also be used to connect the GMSC to an SMSC.
F	MSC-EIR	The F interface connects the MSC to the EIR and uses the MAP/F protocol to verify the status of the IMEI that the MSC has retrieved from the MS.
G	VLR-VLR	The G interface interconnects two VLRs of different MSCs and uses the MAP/G protocol to transfer subscriber information—for example, during a location update procedure.
H	MSC-SMSG	The H interface is located between the MSC and the SMSG and uses the MAP/H protocol to support the transfer of short messages. Again, GSM as well as ANSI-41 is unknown, but H in ANSI-41 is used for HLR–AC interface.
I	MSC-MS	The I interface is the interface between the MSC and the MS. Messages exchanged over the I interface are transparently relayed through the BSS.

In terms of the physical layer, the air interface (MS-BTS) uses RF radio transmission. The A-bis interface (BTS-BSC) uses 64 kbps over whatever medium is most convenient for installation: wire, optical, or microwave. All other interfaces in the GSM system use SS7/C7s MTP1 at the physical layer.

The data link layer that is used at the air interface (MS-BTS) is LAP-Dm; LAP-D is the data link layer that is used at the A-bis interface (BTS-BSC). All other interfaces in the GSM system use SS7/C7s MTP2 at the data link layer.

The air interface (MS-BTS) and the Abis interface (BTS-BSC) do not have a network layer. All other interfaces in the GSM system use SS7/C7s MTP3 and SCCP at the network layer.

The transport, session, and presentation layers are not used in SS7/C7—these functions are grouped together at the application layer, which is known as Level 4 in SS7/C7. GSM interfaces to fixed-line networks using ISUP or TUP (TUP is never used in North America).

Figure 12-6 shows the SS7 protocols that operate at each interface.

All of the interfaces around the MSC use SS7/C7-based protocols. The B, C, D, F, and G interfaces are referred to as MAP interfaces. These either connect the MSC to registers or connect registers to other registers. The E interface supports the MAP protocol and calls setup protocols (ISUP/ TUP). This interface connects one MSC to another MSC within the same network or to another network's MSC.

By this point, you can gather that different functional entities (e.g. HLR, MSC, and so on) run the required and therefore differing stack of SS7/C7 protocols. In relation to the following diagram, remember that the MSC runs MAP-MSC, and that MAP-VLR and the HLR run MAP-HLR.

Figure 12-7. Protocols Required for Functional Entities

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BSSAP (DTAP/BSSMAP)

On the A interface, an application part known as the BSSAP is used. BSSAP can be further separated into the base station subsystem management application part (BSSMAP) and the direct transfer application part (DTAP).

Neither the BTS nor the BSC interpret CM and MM messages. They are simply exchanged with the MSC or the MS using the DTAP protocol on the A interface. RR messages are sent between the BSC and MSC using the BSSAP.

BSSAP includes all messages exchanged between the BSC and the MSC that the BSC actually processes—examples include PAGING, HND_CMD, and the RESET message. More generally, BSSAP comprises all messages that are exchanged as RR messages between MSC and BSC, and messages that are used for call-control tasks between the BSC and the MSC.

The DTAP comprises all messages that the subsystem of the NSS and the MS exchange. DTAP transports messages between the MS and the MSC, in which the BSC has just the relaying function.

Mobile Application Part (MAP)

The MAP is an extension of the SS7/C7 protocols that are added to support cellular networks. It defines the operations between the MSC, the HLR, the VLR, the EIR, and the fixed-line network. It comes in two incompatible variants: GSM-MAP and ANSI-41 MAP. While GSM-MAP only supports GSM, ANSI-41 supports AMPS, NAMPS, D-AMPS/TDMA, CDMA (cdma One and cdma 2000), and GSM. GSM-MAP is the international version, while ANSI-41 is the North American version.

The MAP is used to define the operations between the network components (such as MSC, BTS, BSC, HLR, VLR, EIR, MS, and SGSN/GGSN in GPRS). This involves the transfer of information between the components using noncircuit-related signaling. MAP signaling enables location updating, handover, roaming functionality, authentication, incoming call routing, and SMS. MAP specifies a set of services and the information flows between GSM components to implement these services. MAP can be considered an extension of the SS7/C7 protocol suite created specifically for GSM and ANSI-41 networks.

MAP uses TCAP over SCCP and MTP. TCAP correlates between individual operations. The TCAP transaction sublayer manages transactions on an end-to-end basis. The TCAP component sublayer correlates commands and responses within a dialog. Chapter 10, "Transaction Capabilities Application Part (TCAP)," describes TCAP in more detail.

MAP protocols are designated MAP/B–MAP/H, according to the interface on which the protocol functions. For example, the MAP signaling between the GMSC and the HLR is MAP/F.

Figure 12-8 shows the specific MAP-n protocols. The PCS 1900 specifications use the same MAP interfaces, but PCS 1900 also defines MAP-H.

Figure 12-8. MAP-n Protocols

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MAP allows implementation of functions such as location updating/roaming, SMS delivery, handover, authentication, and incoming call routing information. The MAP protocol uses the TCAP protocol to transfer real-time information (between NSS components).

• MAP provides the functionality to route calls to and from the mobile subscribers—it has the mechanisms necessary for transferring information relating to subscribers roaming between network entities in the PLMN.

• The U.S. version is known as ANSI-41-MAP (standardized by EIA/TIA).

• The international version is known as GSM-MAP (standardized by ITU/ETSI).

MAP only makes use of the connectionless classes (0 or 1) of the SCCP.

Table 12-4 shows the SCCP Subsystem Numbers (SSNs) that are specified for MAP.

Table 12-3. SSNs Used by MAP

SCCP Subsystem Numbers	Use
0 0 0 0 0 1 0 1	For the entire MAP (reserved for possible future use)
0 0 0 0 0 1 1 0	HLR
0 0 0 0 0 1 1 1	VLR
0 0 0 0 1 0 0 0	MSC
0 0 0 0 1 0 0 1	EIR
0 0 0 0 1 0 1 0	Allocated for evolution (possible Authentication centre)