Bridge-A Connecting Device

Uses of Bridges

1) Many commercial department and institution of higher education have their own LANs, which are used to interconnect personal computers, workstations and servers. Different commercial departments have different goals so they choose different LANs without considering to other departments work. Bridges are used if there is need for communication.

2) The organization may be geographically spread by distances over several buildings. It may be costless to have separate LANs in each building and interconnect them with Bridges and infrared links that to run a single coaxial cable over the entire site.

3) It may be necessary to divide a single LAN into separate LANs to hold the load. Files are normally placed on file server’s machines and are downloaded to user machines upon request. The vast scale of this system prevents putting all the workstations on a single LAN so total bandwidth required is far too high. As an alternative multiple LANs connected by bridges are used. Each LAN includes a cluster of workstations with its own file server, so that most traffic is limited to a single LAN and does not add load to the backbone.

4) A single LAN would be sufficient in terms of the load but the physical distance between the most distant machines is too large. Even if placing the cable is easy to do, the network would not operate due to the large round-trip delay. The only solution is to divide the LAN and install bridges between segments. By using bridges the total physical distance required can be increased.

5) For reliability purpose, Bridges are used. A defective node on a single LAN that keeps outputting a continuous stream of garbage will cripple the LAN. Bridges can be placed at critical places like fire doors in a building to protect a single node, which has gone berserk from bringing down the entire system.

6) Bridges also provide organization’s security. Most LAN interfaces have immoral mode, in which all frames are sent to the computer not just addressed to it. By placing bridges at various points and being careful not to forward it to a sensitive traffic, it is possible to separate parts of the network so that its traffic cannot escape and fall into the wrong hands.

Different types of Bridges

1) Transparent Bridges
The first 802 bridge is known as transparent bridge or spanning Tree Bridge. A transparent bridge works in promiscuous mode, accepting every frame delivered all the LANs to which it is connected. When a frame receives, a bridge must determine whether to remove or forward it and on which LAN to put the frame. This is done by checking the destination address in a big table inside the bridge. The table lists each destination address and tells which output line it belongs on.

The entire hash table is empty when the bridges are first plugged in. Bridge does not know where destinations of frame are so they make use of flooding algorithm: every incoming frame for an unknown destination is output on all the LANs to which the bridge is interconnected except the one it arrived on. As time progresses the bridges learn where the destinations are. Once a destination is known in advance, frames destined for it are put on only the proper LAN and are not flooded.

Backward learning algorithm is used by transparent bridges. As bridges work in immoral mode so they see every frame sent on any of their LANs. By checking the source address, they can tell which machine is available on which LAN. Machines and bridges are powered up and down and moved around as the topology change takes place. Whenever a hash table entry is made to handle dynamic topologies the arrival time of the frame is noted in the entry. When a frame whose destination is already present in the table arrives, its entry is updated with the current time. Thus the time related with every entry tells the last time a frame from that machine was seen.

2)source routing bridge
Source routing bridges considers that the sender of each frame knows whether or not the destination is on its own LAN. While delivering a frame to a different LAN, the source machine sets the high-order bit of the source address to 1 to mark it. It is present in the frame header the exact path that the frame will follow. This path is created as follows. Each LAN consists of a unique 12-bit number, and each bridge consists of 4-bit number that uniquely specifies it in the contexts of its LANs. The two bridges far apart may both consists number 3 but two bridges between the same two LANs must consists different bridge numbers. A route is then order of sequence of bridge, LAN, bridge, LAN, bridge, LAN….

A source routing bridges are only considered those frames having higher-order bit of the destination set to 1. For each such frame it sees and checks the route looking for the number of the LAN on which the frame arrived. If this LAN number is followed by its own bridge number, the bridge forwards the frame onto the LAN whose number follows its bridge number in the route.

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