GSM Network Connection to SS7 Networks

The MSC is the Central Switching function of the GSM network. The MSC is connected to a SS7 network for the purpose of signaling and performing database queries. The SS7 network uses a network node called the STP, which is a packet switching node (can be SS7, IP, or X.25). Using a 64 Kbps channel connection between STPs, the network can process its signaling information.

Next in a SS7 network is the use of the SCP, which houses the databases congruent to the network. In many cases these databases interact with the HLR, VLR, EIR, AuC, and PSTN nodes. The SCP is used whenever a Global Title Translation is required, which converts numbers (800-322-2202 equates to 480-706-0912) and whenever the Mobile Application Part (MAP) is used . These services link across an SS7 interface. The GSM architecture using the SS7 protocol is shown in Figure 8-14.

Gsm Topology

Figure 8-14: SS7 protocol stack and GSM

The Signaling Protocol Stack for GSM

Figure 8-14: SS7 protocol stack and GSM

The Signaling Protocol Stack for GSM

The normal SS7 network uses the bottom three layers in what is called the Message Transfer Part (MTP) 1-3. These parts use a different layer of the OSI model to provide the routing and data link layers across the physical link. Between the layers three and applications, is the Signaling Connection Control Part (SCCP), which is used when database queries are required and when providing both connection and connectionless access to the SS7 networks. The combination of the MTP1-3 and SCCP creates what is called the actual MTP.

When looking at the upper layers, the SS7 protocols support the use of the following protocols shown in Figures 8-15 and 8-16.

D

MAP

T

T

BSSAP

A P

U P

SCCP

MTP 3

MTP 2

MTP 1

Figure 8-15: The protocols for GSM and SS7 networks

Figure 8-15: The protocols for GSM and SS7 networks

Um Abis A

Figure 8-16: The protocols for the wireless GSM architecture

Um Abis A

Figure 8-16: The protocols for the wireless GSM architecture

• Telephony User Part (TUP) For a voice circuit-switched call across the PSTN (refer to Figure 8-15).

• ISDN User Part A newer implementation and replaces the TUP (refer to Figure 8-15).

• Transaction Capabilities Application Part (TCAP) An application layer that supports the features and functions of a network (refer to Figure 8-15).

• MAP Sits on top of the TCAP as a means of supporting the difference application service entities for mobile users (refer to Figure 8-15).

• Base Station Systems Application Part (BSSAP) A combination of the BSSMAP and DTAP (refer to Figure 8-16).

• BSSMAP Transmits messages that the BSC must process. This applies generally to all messages to and from the MSC where the MSC participates in RR management (refer to Figure 8-16).

• Direct Transfer Application Part (DTAP) Transports messages between the mobile and the MSC, where the BSC is just a relay function, transparent for the messages. These messages deal with MM and Cm (refer to Figure 8-16).

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