Local area network bridges
Local area network bridgesLocal Area Network Bridges
Both the physical distance that Local Area Network (LAN) can cover and the number of hosts that can be attached to it are limited. To overcome this limitation, bridges are introduced as devices which connect LANs at the MAC layer. The purpose of bridges is to allow hosts attached to different LANs to communicate as if they were located on the same LAN. In contrast to repeaters, that act at the physical layer and allow all traffic to cross LAN segments, bridges are more intelligent and limit the traffic to the section of the network on which it is relevant. To accomplish this, bridges must make a forwarding decision with each received frame regarding where to send the frame so that it reaches its destination. There are two different bridge standards: Source-Routing (SR), which is common in Token Ring environment, and Transparent Spanning Tree (TST), which common in Ethernet environment. The IEEE standard for MAC bridges is ANSI/IEEE 802.1D: MAC Sub-layer Interconnection.
Bridge Routing Requirements
In general, LANs are low-cost, low-delay, high-bandwidth (e.g., 1-10 Mbps) broadcast channels. A bridged LAN environment preserves the low-delay and high-bandwidth feature but its topology may be more dynamic than in a single LAN due to possible bridge or LAN failures and hosts being moved around.
Bridge routing algorithms should meet the following requirements:
1) A bridges LAN environment should resemble a single LAN environment as closely as possible. In other words, the extension should be transparent to hosts.
2) The transparency requirement extends to performance requirements, such as low transmission delays, low undetected data corruption and FIFO maintenance.
3) Bridges routing algorithms should be able to adapt quickly to environment changes.
Source Routing Bridges
Source Routing is mostly used to interconnect token ring LANs. In Source-Routing, the source station must determine, in advance, the route to the LAN of the destination station, and include this route in the header of each frame. To determine the routing information, the source stations first issues a search frame, which is general an LCC TEST commands on its ring. If a response is received from the desired destination station, it indicates that both source and destination stations are on the same ring and that no routing information is required. However, if no respond is received, the source station issues a route discovery frame, which fans-out on every ring in the LAN segment. As the frame is forwarded from one ring to another, each bridge updates the routing information in the search frame. When the search frame reaches the destination, it contains the route between the source and destination station. The destination station then sends a response frame to the source station, with the routing information. Both station then use the routing information in each subsequent sent to each other.
Source Routing uses two key parameters to identify a route between a source station and a destination station. These parameters are ring numbers and...
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