OSI Layer

[vishalpatil86]

Hi,

What exactly OSI layer is?

I know what are the layers, what are the protocols uder these layers, the flow everything.

But still not cleared with the concept. Pls explain how it used with the help of example.

[Joseph W. Doherty · Accepted solution]

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Posting

The concept is just a nice way of defining logical demarcations between different functions.  Conceptually, very similar to a programming language that supports functional subroutines.

For example, when the L3 layers passes a packet down to the L2 layer, L3 doesn't have to concern itself with how the packet is going to encapsulated into a frame.  What's the structure of the frame? How big is the MAC?  Is there CRC checks?  Should L3 care?  For purposes of the OSI model, the last question's answer is no.

Conversely, L2 doesn't care about the structure of the L3 packet.  To layer 2, the packet it just payload.  L2 doesn't need to care about IP protocols, or even if what's being passed to it is IP.

The logical demarcation between layers often is implemented with programming demarcation so that the code for the various layers can be interchanged.  Today my "stack" has IPX/SPX for layers 3 and 4, tomorrow I'll use a stack with those layers replaced so that it supports IP/TCP instead, or perhaps the revised stack will support IPv4 and IPv6.

Since the OSI is a conceptual model, actual implements don't always slaveishly follow it.  There are other models, or sometimes you'll see implementations that "cross" layers which might be given names like layer 2.5.

[burleyman]

Hope this helps as well.

Layer 1 the Physical Layer

This layer defines the connection, electrical and wiring specifications. This layer operates in units of Bits.

Layer 2 the Logical Link Layer

This layer provides the hardware addressing and error detection/correction. This layer operates in units of Frames

Layer 3 the Network Layer

This layer addresses and routes datagrams. Also performs fragmentation and reassembly. This layer operates in units of Packets.

Layer 4 the Transport Layer

This layer manages the connections and provides reliable packet delivery. This layer operates int units of messages.

Layer 5 the Session Layer

This layer establishes sessions between services. Synchronizes and performs translations for naming services such as DNS, NetBIOS, RPC, DHCP, BOOTP, and SIP.

Layer 6 the Presentation Layer

This layer performs data format conversion and also provides compression, encoding, and encryption of data.

Layer 7 the Application Layer

This layer provides standard services to applications and end-user interfaces such as HTTP, FTP, Telnet and SMTP.

Mike

[wvaldivia]

I've some few "keywords" for the first four layers which might help you in trying to remember the concepts.

Ex.

Layer 1's (physical) PDU are called "bits" --- short for 'binary digits'

Layer 2 (data link) deals with "frames" (as PDU) which contain "MAC addresses". Switches are layer 2 devices.

Layer 3 (network) deals with "packets" (as PDU) which contain "IP addresses". Routers are layer 3 devices.

Layer 4 (transport) has "segments" as it's PDU. It uses "ports" (think trans-'port') as its addressing mechanism.

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