Mobile Equipment Protocol layers [Under GSM]
Let us start with Mobile Equipment protocol stack. To do, let us first list out "ME need to"s on a broader level based on our understanding till now.
1) ME need to interface with SIM
2) ME need to understand physical nature of air or radio access to
talk to BTS
3) As seen earlier, ME need to listen to broadcast done by BTS. In
essence, ME need to manage these radio resources (listening them,
using them etc.)
3) ME need to update network about its mobility. Though not
apparent here, ME need to take decision about when this location
update is to be made.
4) ME need to make a call and should be able accept the call
5) ME need to provide interface to user
6) As radio access is "open" in nature, GSM specifies
"authentication" and "ciphering" procedures. Ciphering prevents
eavesdropping by making data in question encrypted. ME need to
support this.
Below diagram shows ME protocol stack layers.
SIM layer takes care of interface with SIM layer. Note that
SIM is separate physical entity, so ME will have physical
interface to access SIM.
» SIM
ME interface - 11.11 «
Physical layer (Layer 1 or L1) implements "physical"
functions related to radio access.
»
Radio interface - 5.01 «
For (OSI) data link layer functions, modified Link Access
Protocol for D channel (LAPDm) is used. It provide a reliable
point-to-point signaling link between ME and BTS (on top of
physical access provide by L1) It map to OSI Layer 2. LAPD is
actually ISDN protocol used for D-channel (signaling link) -
specified Q.720/721. GSM modified it for its purposes.
»
Data link layer - 4.05, 4.06 «
GSM air access is in terms of channels, some of which are shared channels like used for "paging", "broadcast" and some are of dedicated type like signaling channel (created during call setup) for signaling with ME. Radio Resource (RR) layer takes care of these channel (radio resource) management (which include handover scenarios). RR layer related parameters are of value to BTS, BSS, and to MSC as well. That's the reason RR is considered as part of Layer 3.
Mobility Management (MM) layer implements functions related to mobility like location update. L1, L2, and RR takes care of accessing radio resources, but which Network to access decision is done by MM layer. MM is also given a job of authentication and ciphering signaling. MM information is passed to MSC/VLR through BSS. In fact, part of MM data is used by BSS as will be explained later.
Connection Management (CM) layer has the main function of setting up connections with MSC. These connections might be meant for voice call or short message service (SMS). Accordingly, CM is further divided in Call Control (CC) and SMS entities.
Specification also divide these layers in Access Stratum (AS) and Non-Access Stratum (NAS). L1, L2, and RR are part of AS where as MM and CM are part of NAS.
Though CM, MM, and RR are shown in layered way, layer 3 messages are marked to which entity it is to be passed. Protocol Discriminator parameter in layer 3 message distinguishes between CC, SS, SMS, MM, and RR. MM and RR will typically have logic to forward the message to layer above (incoming messages) or below (outgoing messages).
Also note that for many of the functions, RR need to talk to L1 directly wherein reliable L2 connection is not needed or not possible e.g. sending initial access request sent over Random Access Channel (RACH).
The layering shows the dependency required during
signaling procedures.
» Layer 3 - 4.18 (RR), 24.007 (Overall), 24.008 (MM/CC), 24.011 (SMS), 24.010 (SS), 24.08x (SS), 24.09x (SS) «
ME also has TAF (Terminal Adaptation Functions) in transmission
plane to take care of rate adaptation between GSM data rate and
external device date rate.
»
TAF - 27.001 «
ADAPTation layer (not directly mentioned in specification) provide
AT command interface to Protocol Stack. AT commands are simple
text based commands for example ATD to make a call, AT+CMGS to
send SMS, AT+COPS for Network registration, AT+CPBR for Phonebook
reading etc.
» AT commands -
27.007 «
We will continue with GSM interfaces and protocol layers in next article.
References: GSM book by Mouly and Pautet, GSM Networks book by Heine
© Copyright Samir Amberkar 2010
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