After reading this article you will learn about the use of bridges in LAN.
Organisations that use multiple LANs and of different designs generally need to connect them to each other. They do this by using bridges. Bridges operate in the data link layer.
This means that bridges do not examine the network layer header and can, therefore, copy IP, IPX and OSI packets equally well. An IP, IPX or OSI router can handle only its own native packet. Bridge design has to concentrate on connecting 802.3, 802.4 and 802.5—the three important LAN standards.
First, it is necessary to look at the reasons that an organisation may need to have multiple LANs. Many universities or corporate offices have their own LANs, primarily to connect their own personal computers, servers and work stations. Since the goals of different departments differ each chooses the LAN most suitable for the department, irrespective of the LANs of other departments.
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There will also be a need for interaction with other departments; hence bridges are going to be required. The organisation may be spread out geographically over some distance and it may be cheaper to have separate LANs and to connect them through bridges, than to have a single coaxial cable over the entire site.
Then there is the question of load. It may be worthwhile to split a single LAN into separate LANs to balance the load. In some cases while the load is not a consideration, but the physical distances involved may be too great. A classic example is the proposed network of Patna University. Colleges at distances of 5 to 6 km may need to be connected to each other. These would require separate LANs.
There is also the question of reliability. On a LAN, a single defective node that keeps outputting a stream of garbage can cripple the entire LAN. Bridges inserted at critical places act like fire doors in buildings. They will help in isolating the defective node while keeping the entire LAN network operational.
Finally, bridges can contribute to an organisation’s security. Most LAN interfaces send all frames to the computer, and by having bridges at various places and being careful to isolate sensitive traffic, the network can make sure that sensitive data does not fall into the wrong hands. Clearly, these reasons make having bridges almost a compelling necessity.
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Bridges from One 802 LAN to another LAN:
This might appear to be a trivial problem but sadly, it is not. Each of the nine combinations of 802.x to 802.y has its own problems. There is no valid technical reason for different frame formats. It is just that none of the three giants supporting the three standards was willing to change their format.
As a result copying between different LANs requires reformatting. This may introduce errors. A second problem may be the fact that interconnected LANs do not necessarily run at the same data rate. A third and probably most serious problem may be the fact that all 802 LANs have different maximum frame lengths. Bridges must overcome these problems to become useful devices.