GPRS Protocol Stack
- like any other system using various levels of communication, GPRS has its own protocol stack and uses various protocols including SNDCP, LLC, RLC and MAC areas.
The GPRS protocol stack forms a key element in the overall structure of the GPRS system.
Like other forms of communication system, GPRS has a protocol stack that separates out the different functions into layers.
This enables communication to be established and maintained in an orderly fashion, with exchanges taking place between the base-station and the handset in a way that efficiently supports the different levels of functionality within the system.
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 protocol stack in handset
The MS or mobile station GPRS protocol stack has seven layers:
|GPRS Protocol Stack|
|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.
By Ian Poole
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