The OSI Reference Model:
1. A layer should be created where a different level of abstraction is needed.
2. Each layer should perform a well defined function.
3. The function of each layer should be chosen with an eye toward defining internationally standardized protocols.
4. The layer boundaries should be chosen to minimize the information flow across the interfaces.
5. The number of layers should be large enough that distinct functions need not be thrown together in the same layer out of necessity, and small enough that the architecture does not become unwieldy.
Fig. . The OSI reference model.
The OSI model is not a network architecture – it does not specify the exact services and protocols. It just tells what each layer should do. However, ISO has also produced standards for all the layers as a separate international standards.
1.The Physical Layer
The main task of the physical layer is to transmit raw bits over a communication channel.
Typical questions here are:
- how many volts should be used to represent 1 and 0,
- how many microseconds a bit lasts,
- whether the transmission may proceed simultaneously in both directions,
- how the initial connection is established and how it is turn down,
- how many pins the network connector has and what each pin is used for.
- The design issues deal with mechanical, electrical, and procedural interfaces, and the physical transmission medium, which lies below the physical layer.
- The user of the physical layer may be sure that the given stream of bits was encoded and transmitted.He cannot be sure that the data came to the destination without error. This issue is solved in higher layers.
2. The Data Link Layer
The issues that the layer has to solve:
- to create and to recognize frame boundaries – typically by attaching special bit patterns to the beginning and end of the frame,
- to solve the problem caused by damaged, lost or duplicate frames (the data link layer may offer several different service classes to the network layer, each with different quality and price),
- to keep a fast transmitter from drowning a slow receiver in data,
- if the line is bi-directional, the acknowledgment frames compete for the use of the line with data frames.
- Broadcast networks have an additional issue in the data link layer: how to control access to the shared channel. A special sublayer of the data link layer (medium access sublayer) deals with the problem.
- The user of the data link layer may be sure that his data were delivered without errors to the neighbor node. However, the layer is able to deliver the data just to the neighbor node.
3. The Network Layer
- to implement the routing mechanism,
- to control congestions,
- to do accounting,
- to allow interconnection of heterogeneous networks.
- In broadcast networks, the routing problem is simple, so the network layer is often thin or even nonexistent.
- The user of the network layer may be sure that his packet was delivered to the given destination. However, the delivery of the packets needs not to be in the order in which they were transmitted.
4. The Transport Layer
The issues that the transport layer has to solve:
- to realize a transport connection by several network connections if the session layer requires a high throughput or multiplex several transport connections onto the same network connection if network connections are expensive,
- to provide different type of services for the session layer,
- to implement a kind of flow control.
- The transport layer is a true end-to-end layer, from source to destination. In other words, a program on the source machine carries on a conversation with a similar program on the destination machine. In lower layers, the protocols are between each machine and its immediate neighbors.
- The user of the transport layer may be sure that his message will be delivered to the destination regardless of the state of the network. He need not worry about the technical features of the network.
5. The Session Layer
Some of these services are:
- Dialog control – session can allow traffic to go in both directions at the same time, or in only one direction at a time.
- If traffic can go only in one way at a time, the session layer can help to keep track of whose turn it is.
- Token management – for some protocols it is essential that both sides do not attempt the same operation at the same time.
- The session layer provides tokens that can be exchanged. Only the side holding the token may perform the critical action.
- Synchronization – by inserting checkpoints into the data stream the layer eliminates problems with potential crashes at long operations. After a crash, only the data transferred after the last checkpoint have to be repeated.
- The user of the session layer is in similar position as the user of the transport layer but having larger possibilities.
6. The Presentation Layer
7. The Application Layer