10-06-2014, 04:27 PM
Differences between OSI model & TCP/IP model
Differences between OSI.doc (Size: 37 KB / Downloads: 117)
INTRODUCTION
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 at the 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 those at 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 also required 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 link layer 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 flow control mechanisms. Error detection is done by means of error detection codes. For ethernet based networks e.g. LANs (Local Area Network), the data link protocol LLC (Logical Link Control) is equally used in OSI and TCP/IP networks.
Network Layer
The network layer provides routing capabilities between source and destination system.
OSI uses the CLNS (Connection Less Network Service) protocols ES-IS for communication of an end system to an intermediate system and IS-IS for communication between intermediate systems.
TCP divides messages in datagram’s of up to 64k length. Each datagram consists of a header and a text part. Besides some other information, the header contains the source and the destination address of the datagram. IP routes these datagram’s through the network using e.g. the protocol OSPF (Open Shortest Path First) or RIP (Route Information Protocol) for path calculation purposes.
Transport Layer
The transport layer provides a reliable end-to-end connection between source and destination system on top of the network layer. It builds an integral part of the whole OSI layering principle and of the internet protocol.
The OSI transport layer protocol (TP4) and the internet transport protocol (TCP) have many similarities but also some remarkable differences. Both protocols are built to provide a reliable connection oriented end-to-end transport service on top of an unreliable network service.