GPRS Protocol Stack

The GPRS protocol stack enables the software within a GPRS handset or mobile station to be organised in a logical fashion enabling the different functions to be layered in a logical fashion - layers include SNDCP, LLC, RLC and MAC areas.

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The GPRS protocol stack is a key element within the overall GPRS handset or base station. It enables the different functions to be split into different layers so that the correct priorities.

The protocol stack enables the exchanges to take place in the required fashion and to be managed in an orderly fashion.

OSI layer model

Protocol stacks are layered and in broad terms follow the OSI layer model. The OSI, Open Systems Interconnection model is a conceptual model that characterizes and standardises the communication functions of a system like a telecommunications or computer system, but in this case the GPRS system. The OSI model does this without regard of their underlying internal structure and technology.

The aim of the OSI model is the interoperability of diverse communication systems with standard protocols. The model partitions a communication system into abstraction layers.The original version of the model defined seven layers, although this is often tailored to suit an individual application or system.

OSI Layer Model used for GPRS Protocol Stack  
Layer Layer Name Data Unit Function
7 Application Data High level applications including resource sharing, remote file access, directory services, virtual terminals
6 Presentation Data Translation of data between networking service and an applications. This includes character encoding, encryption / decryption, data compression
5 Session Data Managing communication sessions, etc.
4 Transport Segments Reliable transmission of data segments between points on a network, including segmentation, acknowledgement and multiplexing.
3 Network Packet / datagram Structuring and managing a multi-node network, including addressing, routing and traffic control.
2 Data Link Bit / Frame Reliable transmission of data frames between two nodes connected by a physical layer
1 Physical Bit Transmission and reception of the raw data over the physical medium.

GPRS handset protocol stack

The MS or mobile station GPRS protocol stack has seven layers:

GPRS Protocol Stack
Layer Layer Name
7 Application
6 IP / X.25

SubNetwork Dependent Convergence Protocol, SNDCP

The SNDCP is equivalent to layer 5 in the GPRS protocol stack. The SNDCP is transparent to the base station subsystem, BSS< and is the highest later in the actual GPRS protocol stack itself. It provides interfaces to the higher layers that are device or application specific including IP and PPP.

There is a tight link and dependency between the SNDCP and the LLC which are adjacent layers in the GPRS protocol. The SNDCP relies totally on the LLC for error recovery and transmission capabilities.

Logical Link Control, LLC

The higher later data is encapsulated into LLC frames.

The LLC also hosts error correction and it also provides different service levels through the Link Access Protocol on the G channel, LAPG. The LLC also provides asynchronous balanced aand unbalanced modes as well as UI, SABM and I frames - all of which are present in the GSM protocol stack.

Radio Link Control, RLC, & Medium Access Control, MAC

Within the GPRS protocol stack the RLC and MAC layers virtually merge into one and are often termed the RLC/MAC layer.

GSM RF layer

The GSM RF function forms the layer 1 of the GPRS protocol stack.

This layer of the stack controls all the operation of the physical data being transmitted and received for payload data and also for the control of the link.

The GPRS protocol stack is an entity within the GPRS handset, even if it is software and not a defined hardware element.

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