Signaling Interfaces and Protocols
The interfaces and protocols for signaling between a MS and the PLMN are shown in Fig. 12.7-2 [10]. ETSI has defined two interfaces: the Um (radio) interface between a MS and the BSS, and the A (cable) interface between BSS and MSC. This is different from the U.S. cellular systems (AMPS, IS-54), where it is assumed that a MSC and its associated base stations (BSs) are supplied by the same manufacturer, and that the MSC-to-BSS interface is an internal manufacturer-specific interface.
The A-interface specifications of GSM enable the operator of a PLMN to purchase MSC and BS systems from different suppliers.
Um Interface. The signaling protocol on this interface has three layers.
Physical Layer (Layer 1). This consists of those parts of the RF channels that contain signaling channels (SACCH, FACCH, BCCH, SCH, FCCH, PAGCH, RACH, and SDCCH).
Data Link Layer (Layer 2). The protocol is known as LAPDm and is a modified version of the link access protocol for D-channels of DSS1 (Section 10.2). For details, the reader is referred to [15].
- Figure 12.7-2. Um and A interfaces. (From IEEE Commun. Mag. 31(4). Copyright # 1993 IEEE.)
Message Layer (Layer 3). In the MS, this layer consists of three parts. The radio resource management (RR) sublayer at a MS communicates with its peer in the BSS. For example, when RR at BSS allocates a TACH or a SDCCH channel to MS, it informs the MS with a RR message.
The mobility management (MM) and connection management (CM) sublayers at a MS communicate with their peers at the MSC. MM and CM messages traverse the Um and A interfaces and are transferred transparently across the BSS.
The MM sublayer messages support MS location updating (the GSM counterpart of "registration") and authentication.
The CM sublayer has three parts: call control (CC), supplementary services (SS), and short message service (SMS). CC contains the messages for the setup and release of connections to the MS. These messages are patterned after the Q.931 messages of DSS1 (Section 10.3). SMS is a service by which subscribers can send short (text) messages to a MS.
A Interface. The signaling protocol on this interface [16] consists of three layers that are similar to those in Signaling System No. 7.
Physical Layer (Layer 1). The BSS is connected to its MSC by digital El multiplexes (Section 1.5.2). The majority of the 64-kb/s multiplexed channels are digital trunks. During a call, a digital trunk, in tandem with a TCH channel, conveys the MS user's speech or data between the MS and the MSC [17]. Other E1 channels are signaling data links. Layers 2 and 3 of the protocol are outlined below.
Data Link Layer (Layer 2). This consists of the Message Transfer Part level 2 (MTP2) of Signaling System No. 7 (SS7)—see Chapter 8. MTP2 is responsible for the reliable transfer of signaling messages between the MSC and the BSS. Details can be found in [l8].
Message Layer (Layer 3). The Base Station System Application Part (BSSAP) is present at the MSC and BSS [19]. It is a user of the signaling connection control part (SCCP) of SS7 (Chapter 15). This is one of the few applications of connection-oriented SCCP. A signaling connection is established whenever a dedicated channel (SDCCH, FACCH, or FACCH) has been assigned to a MS.
BSSAP consists of two parts: the Direct Transfer Application Part (DTAP) and the BSS Management Application Part (BSSMAP).
We start with BSSMAP at BSS. The RR at a BSS is involved in the allocation, encryption, and release of dedicated radio channels and in the transmission and reception of RR messages on the common-control radio channels. The BSSMAP at BSS and MSC handles the transfer of RR-related BSSMAP messages. At a BSS, RR and BSSMAP communicate with each other. Figure 12.7-3 illustrates a few RR-BSSMAP interactions. In example (a), a MS sends a RR message—on the RACH—to request a dedicated radio channel. RR at BSS then allocates a channel, returns a RR message on PAGCH that includes the identity of the channel, and informs its BSSMAP, which then composes a message that informs the MSC and sends it to BSSMAP at the MSC. In example (b), the MSC sends a BSSMAP message requesting the transmission of a RR message on a common-control radio channel. The requested RR message may be a paging message,
- Figure 12.7-3. Transfer of RR messages. (From GSM 04.07 Version.10.0. Courtesy of ETSI.)
Figure 12.7-4. Transfer of MM and CM messages. (From GSM 04.07 Version 4.10.0. Courtesy of ETSI.)
Figure 12.7-4. Transfer of MM and CM messages. (From GSM 04.07 Version 4.10.0. Courtesy of ETSI.)
which is then transmitted by BSS on the PAGCH, or a broadcast message, which is transmitted on the BCCH.
The DTAP at a BSS transparently (without processing by BSS) transfers MM and CM messages, received on dedicated radio channels, to a SS7 data link, which transports them to the MSC—see Fig. 12.7-4. Also, messages for a MS received from DTAP at the MSC are transferred to the radio channel that is currently dedicated to the MS.
The transfer of BSSMAP and DTAP messages involves the SCCP and MTP in the MS and BSS [12]. The SCCP at the sending end adds a discrimination parameter to the messages, which indicates whether the message belongs to BSSMAP or DTAP, and which is used by SCCP at the receiving end to deliver the message to the proper entity.
12.7.4 Identification of GSM Entities
Since GSM systems are deployed in many countries, ITU-T and ETSI have standardized the identification of GSM entities according to a numbering plan specified in ITU-T Rec. E.212 [20]. This plan is different from the ITU-T Rec. 163/164 numbering plan for fixed networks.
PLMN Identity. A PLMN is uniquely identified by its mobile country code (MCC) and mobile network code (MNC). MCC consists of three digits, of which the first one indicates a world zone:
2 Europe
3 North America
4 The Mideast and Western Asia
5 Eastern Asia and Australia
6 Africa
7 Latin America
The second and third digits represent individual nations in these zones. For example, the MCCs of the United Kingdom and Malaysia are 234 and 502.
The MNC identifies a PLMN in a country. It consists of two digits and is allocated by individual national organizations for mobile telecommunication standards.
Location Area Identity (LAI). This uniquely identifies a GSM location area. The service area of a PLMN is divided into a number of MSC service areas, and each of these is subdivided into location areas that consist of a number of adjacent cells. A MSC keeps track of the location areas of mobiles currently registered in its service area. When a MS has to be paged, paging messages are sent out in all cells of the mobile's present location area. The LAI format is
LAI = MCC-MNC-LAC
where the location area code (LAC) identifies a location area within a PLMN. The code consists of up to four hexadecimal digits. LACs are allocated by the operators of individual PLMNs.
Mobile Station Identity. We begin by pointing out a significant difference between U.S. mobiles and GSM mobiles. A GSM mobile consists of two parts: the mobile equipment (ME) and the subscriber identity module (SIM). The ME is an "almost complete" mobile station, containing the RF equipment, keypad, mouthpiece, earphone, and so on. The SIM is a small package (smart card) with semiconductor chips that store permanent and temporary information about the ME user. A user can insert her SIM into—and extract it from—any ME. The ME is operable only when a SIM has been inserted.
Splitting a MS into two parts allows a ME to be used—at different times—by different people. For example, a business may have a number of employees, each of whom has a SIM. The business also owns a smaller number of MEs which are used by the employees on a "when needed" basis.
Mobile station identities can be in two forms.
International Mobile Station Identity (IMSI). This uniquely identifies a mobile station in any GSM network. Its format is
IMSI = MCC-MNC-MSIN
where MCC-MNC identifies the PLMN selected by the MS owner for mobile services, and MSIN (mobile station identity number) identifies a MS in that PLMN. The maximum length of MSIN is nine BCD digits.
An IMSI is allocated by the operator of the selected PLMN and entered into permanent memory of the SIM.
The second MS identity format is a combination of LAI and TMSI.
Temporary Mobile Station Identity (TMSI). This is a 32-bit binary number that uniquely identifies the MS within one location area, or a group of adjacent location areas, of a PLMN. TMSI is a temporary identification and is usually changed by the network when the MS enters a new location area. LAI and TMSI are stored in temporary SIM memory.
Most messages on the Um (radio) interface identify a MS by TMSI and LAI. IMSIs are used only in exceptional cases. TMSI gives protection against cloning (obtaining MS identifications for fraudulent use), because an intercepted TMSI no longer identifies the mobile after it has left the location area.
International Mobile Equipment Identity (IMEI). This is the counterpart of the mobile serial number (MSN) in AMPS. It uniquely identifies a ME. IMEI is a 15-digit number, entered into permanent ME memory by the manufacturer.
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