LAN Interconnection

The existence of different LANs inside an organization can be necessitated through a number of factors (like as too various users for one network to support, too huge a geographic distance to cover, various network require of several work groups, or a require for special network security). The Organizations holding separate LANs frequent required passing data back and forth among them. The generic term "relay" refers to the equipment which connects LANs. Relays are commonly implemented in the lower three layers of the OSI reference model.

At the Physical Layer (Layer 1), relays are repeaters in which copy signals from one LAN segment to another. The Repeaters extend LANs, thus inexpensively solving distance problems at a single location.

At the Data Link Layer (Layer 2), relays are bridges which store and forward data packets. LANs use 48-bit source  and  destination addresses  which  are  administered through  the  Institute of  Electrical  and  Electronic Engineers (IEEE) so in which every system connected to a LAN has a worldwide unique source address. A bridge examines the source and destination addresses in every data packet transmitted over the LANs to that it is connected. It constructs (and frequently updates) an address table which relates each source address with the LAN on that it is seen. Every packet's destination address is compared to the source-address table. The packet is filtered (discarded) if the address is found to be related with the LAN on that the packet was found. Or else, the packet is forwarded. A few bridges can connect various LAN categories and are thus known as translation bridges.

Bridges have a number of benefits as a means of connecting LANs; the biggest benefits are in which they do not need end-user systems to execute any special routing protocols or services.   Subsequently, bridges are said to be transparent. Through filtering out a few data packets, bridges can considerably decrease traffic on related LANs, therefore increasing LAN throughput. Several bridges permit network managers to further optimize, or raise the security of their networks through giving additional filters which discard specified categories of packets or packets which have particular destination addresses.

The greatest drawback of connecting LANs with a bridge is which, whenever filtering is inadequate, the bridge  can  saturate  the  network  with  propagated  packets,  therefore  degrading  network  performance. Thus, bridges are a low-cost, high-performance means of connecting LANs if the networks hold a limited number of LAN nodes which are not spread over too wide an area.

At the Network Layer (Layer 3), relays are either routers or gateways. A router stores and forwards data packets which are straight addressed to it through the user systems. To a router and the source and destination systems have a adhere to a general routing protocol.  Unlike bridges, routers need the active participation of all end users.