In GSM network architecture, signaling plane (control plane) is defined for transmitting, and controlling of signals between network elements. Few essential protocol entities can be identified in this signaling plane architecture.
Connection Elements
Signaling plane consists of three distinguished connection elements.
1. Radio-interface connection element – between MS & BTS
2. BSS-interface connection element – between BTS & BSC
3. A-interface connection element – between BSC & MSC
Let’s try to focus on the protocols by relating to the much familiar ISO network model.
Layer 1: Physical Layer
In simple terms this is where the signal meets the air interface e. Thus the physical layer protocols implement the logical signaling channels. Signaling messages will be transport over Abis and A interaces on a digital line.
Layer 2: Data-link Layer – LAPDm
LAPDm (Link Access Procedure in Dm channels) is the protocol defined at this layer. LAPDm is dedicated to transparent transport of messages between protocol entities of layer 3 through services such as information framing, peer-to-peer transmission of data in defined frames, recognition of frame formats and establish, maintain and terminate of one or more data links on signaling channels.
Layer 3: Networking Layer
Protocols in layer 3 are used for communication of network resources, code format, call management etc. The services offered by layer 3 protocols can be categorized under three sub layers.
1. Radio Resource Management (RR) – Performs the administration tasks related to frequencies and channels
2. Mobility Management (MM) – Dedicated to handle tasks resulting from mobility such as TMSI assignment, location related activities, identification, authentication and detaching IMSI.
3. Connection Management (CM) – Consists of three sub-services
- Call Control (CC) – Set-up, maintenance and take down calls
- Supplementary Services – To utilize the services of USSD
- SMS – Point-to-point short message transmission
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