GPRS Protocol Layers [Under GPRS] §
In article 1, we saw overall requirements for making packet traffic available in GSM network. Let us now see: how GPRS has split those functions in layers and nodes.
Below diagram shows signaling plane (called as control plane in
specifications) and transmission plane (called as "user plane" in
specifications) layers from MS till GGSN.
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GPRS description - 23.060 «
MS protocol layers [Under GPRS >> Protocol Layers]
GMM/SM - GPRS Mobility Management/Session Management
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MS-SGSN; SM/GMM - 24.007/8 «
When (class A or B) MS powers on, it can do normal GSM registration/location update followed by GPRS registration/routing area update or both at together (known as combined attach). GMM layer takes for these and other mobility related procedures. Note that SGSN talks to MSC/VLR over Gs interface for combined attach.
In GSM, CC (part of CM) layer takes care of connection setup and teardown. In addition, it takes decision about characteristics of connection, known as "bearer capability", like type of bearer (speech or data), speech rate (full rate - 16/13 kpbs, half rate - 8/6.5 kpbs) etc. In GPRS, for packet traffic, "Session" is created with certain characteristics like maximum/mean bit rate, reliability of transmission etc. SM layer manages these and related procedures.
SNDCP - Sub-Network Dependent Control Protocol
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MS-SGSN; SNDCP - 04.65 «
SNDCP understands Packet protocol - data of which need to transmitted - (like TCP/IP, X.25) and so it helps in bridging the gap between this protocol characteristics and those provided by GPRS access. For example, it can compress IP header (by removing redundant information) for efficient transmission.
LLC - Logical Link Layer
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MS-SGSN; LLC - 04.64 «
LLC maintain a reliable logical link between MS and SGSN. Note that LLC is common in transmission and signaling plane (this is because packet traffic channel is used for both signaling and transmission data). RLC (Radio Link Control) also maintain a reliable point-to-point link between MS and BSS, so strictly speaking, we do not need LLC. The main advantage of having LLC is LLC acts like a wrapper hiding radio access related intricacies from network. So in future if radio access is modified/enhanced, network modification would be nil or minimal. Another important function of LLC is Ciphering.
RLC/MAC - Radio Link Control/Medium Access Control
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MS-BSS Interface; RLC/MAC - 04.60 «
RLC maintain reliable radio link between MS and BSS. As mentioned in article 2, we need procedures for knowing when and how to send and receive the packet traffic. MAC takes care of these access resolution procedures.
GSM RF
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GPRS radio channels - 5.01/2 «
As mentioned in article 2, Physical layer remain same as GSM with addition of Packet channels like PRACH (Packet RACH), PAGCH (Packet Access Grant Channel), PDTCH (Packet Data Traffic channels or just Packet channels) etc.
References: GPRS in Practice by McGuiggan, GPRS whitepaper from Christian Bettstetter et al.
© Copyright Samir Amberkar 2010
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