GSM Protocol Stack - GSM

What is GSM Protocol Stack?

GSM architecture is a layered model designed for allowing communications between two different systems. Lower layers will assure services of the upper-layer protocols. Each layer will pass appropriate notifications to make sure that the transmitted data is formatted, transmitted and received accurately.

GMS protocol stacks diagram is shown below:

MS Protocols

Based on the interface, GSM signaling protocol will be assembled into three general layers:

  • Layer 1 : Physical layer. It makes use of the channel structures over the air interface.
  • Layer 2 : Data-link layer. Across the Um interface, data-link layer is a tailored version of Link access protocol for D channel (LAP-D) protocol used in ISDN, called Link access protocol on the Dm channel (LAP-Dm). Across the A interface, the Message Transfer Part (MTP), Layer 2 of SS7 is used.
  • Layer 3 : GSM signaling protocol’s third layer can be divided into three sublayers:
  • Radio Resource Management (RR),
  • Mobility Management (MM), and
  • Connection Management (CM).

MS to BTS Protocols

RR layer is the lower layer which manages a link, both radio and fixed, between MS and MSC. Main components involved for this formation are MS, BSS, and MSC. Job of RR layer is to manage the RR-session, the time when a mobile is in a committed mode and the radio channels which include the allocation of dedicated channels.

MM layer is stacked above the RR layer. It will handle the functions that arise from the mobility of the subscriber and also the authentication and security aspects. Location management is concerned with the procedures which enable the system to identify the current location of a powered-on MS to compete the incoming call routing.

CM layer is the topmost layer of the GSM protocol stack and this layer is responsible for Call Control, Supplementary Service Management, and Short Message Service Management. Each of these services will be treated as individual layer within the CM layer. Other functions of the CC sublayer include call establishment, selection of the type of service (including alternating between services during a call), and call release.

BSC Protocols

BSC makes use of a different set of protocols after receiving the data from BTS. Abis interface is used between BTS and BSC. At this level, radio resources at the lower portion of Layer 3 will be changed from RR to Base Transceiver Station Management (BTSM). BTS management layer is a relay function at BTS to BSC.

RR protocols will be responsible for the allocation and reallocation of traffic channels between MS and BTS. These services contain controlling the initial access to the system, paging for MT calls, handover of calls between cell sites, power control, and call termination. BSC still has some radio resource management in place for the frequency coordination, frequency allocation and management of the overall network layer for the Layer 2 interfaces.

To transit from BSC to MSC, BSS mobile application part or the direct application part will be used and SS7 protocols will be applied by the relay. Therefore the MTP 1-3 can be used as the main architecture.

MSC Protocols

At MSC, starting from BSC, information will be mapped across A interface to the MTP Layers 1 through 3. Here, Base Station System Management Application Part (BSS MAP) will be the equal set of radio resources. Relay process is finished by the layers that are stacked on top of Layer 3 protocols, they are BSS MAP/DTAP, MM, and CM. This will complete the relay process. To find and connect to the users across the network, MSCs will interact using the control-signalling network. Location registers will be included in the MSC databases to help in the role of determining how and whether connections should be made to roaming users.

Every GSM MS user will be given a HLR which in turn consists of the user’s location and subscribed services. VLR is a separate register used for tracking the location of a user. When the user moves out of the HLR covered area, VLR will be notified by the MS to discover the location of the user. VLR in turn, with the help of the control network, signals HLR of the MS’s new location. With the help of location information contained in the user’s HLR, MT calls will be routed to the user.

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