Tci Ip Vs Osi

TCP/IP versus OSI

Comparison between TCP/IP and OSI

This chapter gives a brief comparison between OSI and TCP/IP protocols with a special focus on the similarities and on how the protocols fromboth worlds map to each other. The adoption of TCP/IP does not conflict with the OSI standards because the two protocol stacks were developed concurrently. In some ways, TCP/IP contributed to OSI,and vice-versa. Several important differences do exist, though, which arise from the basic requirements of TCP/IP which are:

A common set of applications
Dynamic routing
Connectionless protocols atthe networking level
Universal connectivity
Packet-switching
The main differences between the OSI architecture and that of TCP/IP relate to the layers above the transport layer (layer 4) and thoseat the network layer (layer 3). OSI has both, the session layer and the presentation layer, whereas TCP/IP combines both into an application layer. The requirement for a connectionless protocol alsorequired TCP/IP to combine OSI’s physical layer and data link layer into a network level.

Physical Layer

The physical layer may be either ethernet, SDH-DCC, or some timeslot of a PDH signal.Either OSI protocols and TCP/IP protocols build on the same physical layer standards, thus there is no difference between OSI and TCP/IP in this aspect.

Data Link Layer

The purpose of the data linklayer is to provide error free data transmission even on noisy links. This is achieved by framing of data and retransmission of every frame until it is acknowledged from the far end, using flowcontrol mechanisms. Error detection is done by means of error detection codes.

The data link layer in the OSI world makes use of the Q.921 LapD protocol which must support an information field length ofat least 512 octets according to G.784. LapD is based on HDLC framing.

In the internet world there is no real data link layer protocol, but the subnet protocol which has quite many similarities....