Network Protocol Layers [Under GPRS >> Protocol Layers]
Relay
Relay function of BSS relays LLC PDUs from Um to Gb interface and vice versa. In SGSN, relay happen for Packet Data Protocol (like IP) PDUs (this would be original user data) between Gb and Gn interfaces.
BSSGP - BSS GPRS Protocol
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BSS-SGSN; BSSGP - 8.18 «
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BSS-SGSN; Network Service - 8.16 «
BSSs and SGSN are connected over frame relay network. Network Service is network/transport layer of underlying frame relay network. As can be seen, BSSGP is an interface between RLC/MAC (sitting in BSS) and LLC (sitting in SGSN); in BSS is invokes appropriate RLC/MAC functions as per QoS and in SGSN, it gives (MS radio related) information derived from RLC/MAC to LLC. In addition, BSSGP also need to create/flush frame relay links between BSS and SGSN for packet flows. QoS is associated with these packet flows. In SGSN, it also does downlink flow control for per MS and per Cell (by maintaining contexts). Like BSSMAP, it plays part in paging and RA update procedures.
GTP - GPRS Tunnelling Protocol
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GTP - 29.060 «
GSNs are connected over IP network backbone. GTP tunnels signaling messages (GTP-C) and PDP PDUs (GTP-U) between GSNs using UDP services.
Gs (SGSN - MSC/VLR) interface
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SGSN-MSC/VLR, Gs - 29.016 «
Gs interface is based on SS7/SCCP as shown below:
BSSAP+ procedures are subset of BSSAP procedures, meant for (combined) mobility management like combined attach.
GSN - HLR, EIR interfaces
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MAP spec - 29.002 «
As with GSM, HLR/EIR interfaces come under MAP as shown below.
This complete brief end-to-end view of GPRS protocol layers.
As seen, GPRS not only reuse GSM protocols but also add lot of concepts and protocols. Major of them is RLC/MAC concepts, separation of radio access from core network protocols with the help of LLC, and QoS concepts.
References: GPRS in Practice by McGuiggan, GPRS whitepaper from Christian Bettstetter et al.
© Copyright Samir Amberkar 2010
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