PLMN Configurations and Interfaces

The fixed connections for transport of signaling and user data in a GSM PLMN (Figure 3.8) are standard transmission lines. Within the SMSS, lines with a transmission rate of 2 Mbit/s (or 1.544 Mbit/s in North America) are typically used (fixed lines, mostly microwave links or leased lines). The BSS uses mostly 64 kbit/s lines. Signaling has two fundamentally different parts: GSM-specific signaling within the BSS, including the air interface, and signaling within the SMSS and with other PLMN in conformity with Signalling System Number 7 (SS#7). User data connections are processed with an SS#7 protocol for signaling between network nodes, the ISDN User Part (ISUP). For the mobile

Plmn Interface Gsm

signating data

Figure 3.8: Signaling and user data transport in a GSM PLMN

signating data

Figure 3.8: Signaling and user data transport in a GSM PLMN

network specific signaling, MSC, HLR, and VLR hold extensions of SS#7, the so-called Mobile Application Part (MAP). Signaling between MSC and BSS uses the Base Station System Application Part (BSSAP). Within the BSS and at the air interface, signaling is mobile-specific, i.e. no SS#7 protocol is used here for signaling transport.

3.5.1 Interfaces

This results in a large number of communication relationships for user data transport and signaling; for simpler structuring and standardization, these relationships have been separated by introducing a number of interfaces (Figure 3.9).

Plmn Figure

Figure 3.9: Interfaces in a GSM PLMN

Figure 3.9: Interfaces in a GSM PLMN

The A interface between BSS and MSC is used for the transfer of data for BSS management, for connection control, and for mobility management. Within the BSS, the Abis interface between BTS and BSC and the air interface Um have been defined.

An MSC which needs to obtain data about a mobile station staying in its administrative area, requests the data from the VLR responsible for this area over the B interface. Conversely, the MSC forwards to this VLR any data generated at location updates by mobile stations. If the subscriber reconfigures special service features or activates supplementary services, the VLR is also informed first, which then updates the HLR.

This updating of the HLR occurs through the D interface. The D interface is used for the exchange of location-dependent subscriber data and for subscriber management. The VLR informs the HLR about the current location of the mobile subscriber and reports the current MSRN. The HLR transfers all the subscriber data to the VLR that is needed to give the subscriber his or her usual customized service access. The HLR is also responsible for giving a cancellation request for the subscriber data to the old VLR once the acknowledgement for the location update arrives from the new VLR. If, during location updating, the new VLR needs data from the old VLR, it is directly requested over the G interface. Furthermore, the identity of subscriber or equipment can be verified during a location update; for requesting and checking the equipment identity, the MSC has an interface F to the EIR.

An MSC has two more interfaces besides the A and B interfaces, namely the C and E interfaces. Charging information can be sent over the C interface to the HLR. Besides this, the MSC must be able to request routing information from the HLR during call setup, for calls from the mobile network as well as for calls from the fixed network. In the case of a call from the fixed network, if the fixed network's switch cannot interrogate the HLR directly, initially it routes the call to a gateway MSC (GMSC), which then interrogates the HLR. If the mobile subscriber changes during a conversation from one MSC area to another, a handover needs to be performed between these two MSCs, which occurs across the E interface.

3.5.2 Configurations

As already mentioned, the configuration of a PLMN is largely left to the network operator. Figure 3.10 shows a basic configuration of a GSM mobile communication network. This basis configuration contains a central HLR and a central VLR. All database transactions (updates, inquiries, etc.) and handover transactions between the MSC are performed with the help of the MAP over the SS#7 network. For this purpose, each MSC and register is known as a Signalling Point (SP) and is known by its Signalling Point Code (SPC) within the SS#7 network. The VLR is mainly a database which stores the location information of the mobile stations. At each change of the location area, this information must be updated. Furthermore, this database has to be interrogated: the MSC needs subscriber parameters besides location data for successful connection setup, such as service restrictions and supplementary services to be activated. Thus, there is a significant message traffic between MSC and VLR, which constitutes an ensuing load on the signaling network.

It is logical, therefore, that these two functional units are combined in one physical unit, i.e., the entire VLR is implemented in distributed form and a VLR is associated with each

PSTN ISDN

PSTN ISDN

Basic Plmn

Figure 3.10: Basic configuration of a GSM PLMN

Figure 3.10: Basic configuration of a GSM PLMN

MSC (see Figure 3.11). The traffic between MSC and VLR then does not need to be transported through the SS#7 network.

PSTN ISDN

Gmsc Pstn Plmn

Figure 3.11: Configuration of a GSM PLMN with a VLR for each MSC

PSTN ISDN

Figure 3.11: Configuration of a GSM PLMN with a VLR for each MSC

One could go one step further and also distribute the database of the HLR and thus introduce several HLRs in a mobile network. This is especially interesting for a growing pool of subscribers, since a centralized database leads to a high traffic load for this database. If there are several HLR in a PLMN, the network operator has to define an association rule between MSISDN and HLR, such that for incoming calls the routing information to an MSISDN can be derived from the associated HLR. One possible association is geographic partitioning of the whole subscriber identification space (SN field in the MSISDN, see Section 3.2.3), where, for example, the first two digits of the SN indicate the region and the associated HLR.

In extreme cases, the HLR can be realized with the VLR in a single physical unit. In this case, an HLR would also be associated with each MSC.

GSM Switching, Services and Protocols: Second Edition. Jorg Eberspacher, Hans-Jorg Vogel and Christian Bettstetter Copyright © 2001 John Wiley & Sons Ltd Print ISBN 0-471-49903-X Online ISBN 0-470-84174-5

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Responses

  • craig
    How to configure your own PLMN?
    2 years ago
  • chester
    How to configrure plmn?
    12 months ago
  • vappu
    What is plmn in gsm network?
    7 months ago
  • Miranda
    How to setup new PLMN?
    3 months ago

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