Information and Communication Technology

4.1.7.1 Internetworking devices

4.1.7.1 Interworking devices 

The devices which interconnect the sub networks are often called bridges, routers or gateways. The network of the sub networ ks so formed is called internetwork.

The problem of interconnection can be visualized as the problem of

Matching the protocols of the peer layers

Matching services offered by different layers

Relaying the data units from one sub network to another and

R outing the data units from one sub network to another

These functions are carried out by the bridges, routers and gateways.

Bridge :The bridge is a device that is attached to two or more local area networks to create an extended LAN. It takes MAC frames from one LAN and sends them across to the other LAN. A frame is transferred across the bridge only if its destination is in other LAN. The interconnected LANs need not be of the same type.

Bridge operations

A bridge performs three basic functions

Frame f iltering

Learning the addresses of the stations

Routing

Routers

Router is an interconnecting device between dissimilar subnetworks.

A bridge can interconnect LANs having different media access control sub layers but with a common LLC protocol. If there ar e differences in the protocols at the Data Link layer and Network Layer of two sub networks , another internetworking device called a router is required to interconnect them. It operates at the Network layer and accommodates all the differences of the sub networks upto this layer to provide a uniform network service to the transport layer entities in the end systems.

Since the router operates at the Network layer, its configuration is determined by

The type of network service, connectionless mode networ k service (CLNS) or connection-mode Network service (CONS) to be provided to the Transport entities and

The type of underlying sub network service.

The sub network service could be a Logical link control (LLC) service, Data Link service provided by H DLC protocol or connection oriented virtual circuit service of the X.25 sub networks. Internetworking the sub networks, with different underlying services and providing a Uniform Network service , alls for matching sets of protocols to be implemented in the Network layer of the end systems and in the routers. The Network entities in the end systems provide the required kind of network service and interface with the router. The Network layer of the router contains the protocols of the sub networks being interconnected and also a routing and relaying function which transfers the N-PDU s from one sub network to the other. Network layer contains the three sub layers to provide its diverse functionality. We need to select a suitable stack of the Networ k layer protocols to meet the above requirements. Nevertheless , it is a fact that most of the LAN installations provide connectionless-mode Network service. ISO had originally formulated CONS but later came out with the standards for CLNS as well. It ap pears that CLNS will continue to gain ground and push CONS into the corner in the future. ISO internet Protocol provides Connectionless-mode Network service (CLNS).

Gateways

A gateway is a general term describing a device which is used to bridge differe nt network architectures. For example , a gateway will be required to interconnect two systems, one based on OSI model and the other on SNA . It is at the Application layer , that a gateway bridges two systems.