1.1 OSI NETWORK MODEL

 

• Advantages

- Computers not necessarily from the same manufacturer

- Allow computers to communicate information.

- Sharing of equipment such as printers, disks, etc.

- Programs can run on multiple machines improving performance

- Access to machines with better/different resources.

 

• Several types: Ring, Star, Linear, Point-to-Point

 

• Local Area Network (LAN), Wide Area Network (WAN)

 

1.1.1 Why Use A Network?

 

• Medium for Communication:

- Satellite link

- Phone line

- High Speed (Multiplexed) phone lines

- Coax Cable

- Fibre Optical Cable

- Twisted pair cable

- (Theoretically anything can be used)

 

• General OSI Diagram contains seven layers

 

 

 

1.1.1.1 - Physical Layer

 

• Physical layer is concerned with transmission of raw bits over a physical circuit.

 

• Deals with voltages, timing, connections, etc.

 

• Responsible for bit synchronization and the identification of a signal element as either a 0 or a 1.

 

• Protocols: RS-232, RS-449, CCITT X.25 and X.21, IEEE 802

 

 

1.1.1.2 - Data Link Layer

 

• Data link layer breaks input data into “data frames” and processes acknowledgments.

 

• Object is to provide a error-free transmission line to the network layer.

 

• Responsible for the reliable delivery of information over a point-to-point or multipoint link.

 

• Supervises interchange of both link control data and user information

 

• Protocols: ANSI X3.28, HDLC, X.25, ISDN, IEEE 802

 

 

1.1.1.3 - Network Layer

 

• Network layer determines the interface between the computer and the intermediate system, how packets are routed.

 

• Chooses a route from the available data links that form the network.

 

• Object is to take messages, convert them to packets and send them towards the destination.

 

• Protocols: CCITT X.25, X.21, IP, CCITT Q.931, ISO 8473

 

 

1.1.1.4 - Transport Layer

 

• Transport layer takes data from the session layer, splits it up if necessary, and passes this to the network layer.

 

• Ensures that pieces all arrive correctly at the other end.

 

• Isolates the user from any concern for the actual movement of the information.

 

• Protocols: TCP, ISO 8073

 

 

1.1.1.5 - Session Layer

 

• Session layer defines a connection between users (presentation layer processes).

 

• Includes specification of the remote machine, authorization, options for the communication, and recovering from errors in broken transport connections.

 

• The set-up of communications is called binding.

 

• Protocols: ISO 8327, CCITT X.225, T.62, ECMA 75, NFS, RPC.

1.1.1.6 - Presentation Layer

 

• Ensures compatible syntax among the communicating processes by adjusting data structures, formats, and codes.

 

• Presentation layer is generally represented by library routines which the user accesses to perform network operations.

 

• This layer can also perform transformations such as compression and encryption.

 

• Protocols: DIS 8823, 8824, 8825, CCITT X.409, T.61

 

 

1.1.1.7 - Application Layer

 

• Application layer is written by the user, or is a program that performs some function for the user.

 

• Provides a window by which the user gains access to the communication services provided by the architecture.

 

• Protocols: DIS 8571, 8832, 9040