Physical layer
Here we get down to physical characteristics of medium. Modulation and Access techniques (like TDMA, FDMA, OFDM(A), CDMA) are of concern here. Aim is to use physical medium efficiently taking into account its specific characteristics. Examples are POTS, MTP-1(SS7), SONET, X.25, GSM L1, UMTS L1, 802.11 PHY, 802.16 PHY.
Data link layer
Aim of data link layer is to provide "reliable" ways to "establish", "maintain", and "release" links between network entities. Examples are MTP-2 (SS7), ATM, X.25-LAPB, LAPDm(GSM), LLC/RLC/MAC (GPRS), 802.11 MAC(WiFi), MAC(802.16/WiMAX).
Network layer
Though Date link layer takes care of links, entity decision with which the link is to be made is decided by network layer. This layer takes care of "addressing" to make sure that "end-to-end" connection is established between concerned entities. Examples are MTP (SS7), SCCP (SS7), IP (IP suite).
Transport layer
Sitting on network layer, transport layer provide "transports" between network entities. These transports could be connection oriented or connectionless. Reliable and efficient transmission is also of concern here. Famous example is TCP/UDP (IP suite).
Session layer
The transport provided by transport layer may have multiple sessions. For examples, two project engineers working one project may talk about different areas in the project. So though it is about one project, there could be different areas to work on. Session layers take care of multiple sessions created by presentation layer. In many cases, session layer is closely bound to transport layer. Good examples are SIP(VoIP), RTP(VoIP). NAS (Non-Access stratum) of GSM may be considered as combination of Session/Transport/Network layer.
Presentation layer
Till now the actual information that all the above entities worked on did not have particular meaning. Presentation layer brings the meaning to this information. This is mainly concerned with format of the bits received. MIME, ASCII are popular examples.
Application layer
Application layer is where we are only a step away from user. Application layer provide a way for actual UI (User Interface) to use a stacked layers below for the purpose. MAP/TCAP/ISUP (SS7), HTTP (IP suite), AT interface (GSM/GPRS/3G) are examples of application layer.
Let us take a simple analogy to remember major functions of these layers.
Say you are buyer B in a fish market, full of other buyers and sellers (buyers more than sellers). You would like to know the price of Salmon from S, but not able to reach S. Luckily you spot your friend F who can help you with.
You call your friend, say hi/hello, and then request him to ask S the price of Salmon.
F start talking to S and tells him that B would like to know the price of Salmon.
S tell F the price of Salmon, which F tell back to B.
Let us see if you can separate out various parts of this communication in OSI layers.
B "talk" to F in their "common language". Sound waves (modulated as per the language) become a Physical layer interface with "Air" being transmission media.
When F listen to B (and vice versa), they try "to hear" each other peoperly among lot of people's voices; they may even have to repeat or confirm to make sure they heard it correctly. This process is Data link layer function.
B and F directly talk to each other, but when B tell F to "ask S" "the price of Salmon", this is an example of Network layer function. S is destination (entity address) and F only passes B's message to S.
During this conversation, note that F only passes the B's message to S and S's message back to B without modifying it. There is (virtual) transport created between B and S, with the help of F. This is Transport layer function.
Now B and S can talk about "lot of things" (like world economy, global warming in addition to price of Salmon) with the help of F. These different things/topics become "Sessions" over the same transport.
Language that B, S, and F speak is nothing but the presentation of modulated sound they hear. Presentation layer does this function of presenting the information in agreed format to application.
Next step is to give meaning to words that was provided. After hearing "What is the price of Salmon ?", S understands that price of Salmon is needed. This is Appication layer working. S's logical mind does the later work of checking the price. This is nothing but the user of application layer.
Though we tried our best to separate out various OSI layers in above analogy, we may not be able to see distinct and all OSI layers in all protocols. This is because protocols are specific to type of traffic (like IP suite for Internet traffic), physical media (like GSM/GPRS/3G/WiMAX protocols for wireless traffic), area of application of protocol (like VoIP protocols) etc. Depending on these factors, specifications/standards may have to combine, separate out, or eliminate the layers and/or their functions.
© Copyright Samir Amberkar 2010
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