cisco!!! >>OSI

Det her burde give svaret:

Physical layer
Concerned with the description of the physical circuits and the transmission of bits. It ensure that when one side sends a 1, the other side receives a 1. Voltages used to represent a 1 and a 0 must be determined. The length of time a bit may occupy (usually in microseconds) must also be agreed.


Data link layer
Turns the raw (possibly errorant ) transmission facility into an error free digital link. The data link layer breaks data into frames, provides error detection and correction mechanisms upon these frames.
The Data link needs to: recognize the boundaries of each frame, determine the correct sequence of frames and regulate how many frames arrive over a specific period.


Network layer
network layer is very complex, it provides the transfer of data between two hosts without the transport layers on the machines knowing about the mechanism. Eg. the simplest connection may be a direct link between two stations. Alternatively, at the other extreme, the connection may be over many different networks which are linked together via gateways
The network layer must therefore be responsible for the establishment, maintenance and termination of the connection between two hosts across any intervening communications facility. It must deal with problems of addressing, routing and prevention of bottlenecks and some accounting - to detect bottlenecks before they arise!


The Transport Layer
Primary task is to hide all the network dependent characteristics from the layers above it. Provides transparent data transfer so all the protocols defined for the transport layer will only need to be implemented on the host computers and not on any intermediate computers in the network
The transport layer establishes and maintains a logical connection with the corresponding transport layer on a remote host uses this connection to transfer data

se evt. her:

http://www.geocities.com/SiliconValley/Monitor/3131/ne/osimodel.html

Jeg vil tor det er 4! ik?

What is the function of the session layer?

Det er 4...transport layer

session layer sørger for at 2 applikationer kan synkronisere deres kommunikation og udveksle data....

Session layer:
The session layer allows users on different machines to establish sessions between them. A session allows ordinary data transport, as does the transport layer, but it also provides some enhanced services useful in a some applications. A session might be used to allow a user to log into a remote time-sharing system or to transfer a file between two machines.

One of the services of the session layer is to manage dialogue control. Sessions can allow traffic to go in both directions at the same time, or in only one direction at a time. If traffic can only go one way at a time, the session layer can help keep track of whose turn it is.

A related session service is token management. For some protocols, it is essential that both sides do not attempt the same operation at the same time. To manage these activities, the session layer provides tokens that can be exchanged. Only the side holding the token may perform the critical operation.

Another session service is synchronization. Consider the problems that might occur when trying to do a two-hour file transfer between two machines on a network with a 1 hour mean time between crashes. After each transfer was aborted, the whole transfer would have to start over again, and would probably fail again with the next network crash. To eliminate this problem, the session layer provides a way to insert checkpoints into the data stream, so that after a crash, only the data after the last checkpoint has to be repeated.

se evt. her for mere info.:
http://www.rad.com/networks/1994/osi/session.htm

(NB kunne du ikke bruge svaret på dit oprindelige spørgsmål?)

og ja, dios har ret jvf. beskrivelsen må dit oprindelige spørgsmål gå på transportlaget