In effect, layered architecture/OSI model logically separate the functions required (for telecommunications) in layers, with each layer handling its own set of functions.
Physical layer (also called L1) consist of mostly hardware and firmware interfaces. Layers above are mainly softwares.
Let us see rules of game !
Every layer provide services to layer above, which in turn mean, each layer use services of layer below.
Like Network layer say to Transport layer, "Hey, I will take care of finding the peer entity which you wanna connect to in network". Transport layer uses this service to do its own function of creating transport.
In software terminology, each layer provide a interface to a layer above. This interface could be message based (asynchronous) or function/API based (synchronous). Typically this interface is message based. Technical name for these inter-layer messages is "primitives". These primitives are actually queued up to layer and layer processes it one by one. This queue is known as SAP (Service Access Point). There could be multiple number of SAPs between two layers.
As shown in OSI model diagram below, each layer is likely to have a "peer layer" to which it "talks".
Let us see how it works.
Each layer while using services of lower layer hands over its own information - that is meant to be delivered to its peer layer - while requesting service from lower layer. Lower layer in turn appends its own information to this parcel (typically without modifying it but not necessarily) and passes it to its lower layer while requesting service from it. Refer a diagram below.
At peer entity (and in-between entities) this happen in reverse way and bottom to top.
Now what does standards/specifications do ? Standards specify or recommend functions of each layer, interface details, various scenarios (may be in terms of states- events or procedures), messages (that goes in & out of entity), message formats, message parameters, values etc.
Examples would be 3GPP GSM specifications 24.007, 24.008 (which talk about Layer 3 and above of GSM) available at www.3gpp.org or SS7 standard Q.931.
Many times inter layer messages (even though specified by standard) are proprietary in nature as two layers next to each other are present on one entity, but the messages between two peer layers need to follow specifications properly so that a entity implemented by one vendor can inter-work with peer entity implemented by another vendor. This is where conformance tests come into picture. Typically specifications mention these tests like 3GPP specs for GSM or it may be done by industry wide body like WiMAX forum (with collaboration with ETSI) for 802.16 standard.
Last point which need mentioning about layers is: today's protocols/standards are based on their area of application and need not follow OSI model layers religiously. So may be functionalities of few layers are put together in one layer and/or layer may be split etc. But yes, OSI model forms an excellent reference for protocol design, implementation, and study.
© Copyright Samir Amberkar 2010
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