Introduction to Data Communications
Previous	41b. Purpose of a Bridge (cont'd)	Next

Bridges listen to the network traffic and build an image of the network on each side of the bridge. This image of the network indicates the location of each node and the bridge's port that accesses it. With this information, a bridge can make a decision whether to forward the packet across the bridge if the destination address is not on the same port or it can decide to not forward the packet if the destination is on the same port.

This process of deciding whether or not to forward a packet is termed filtering packets. Network traffic is managed by deciding which packets can pass through the bridge. The bridge filters packets.

Translate from one protocol to another

The MAC layer also contains the bus arbitration method used by the network. This can be CSMA/CD as used in Ethernet or Token Passing as used in Token Ring. Bridges are aware of the Bus Arbitration and special translation bridges can be used to translate between Ethernet and Token Ring.

41c. Bridge Segment to Segment Characteristics

Bridges are store and forward devices. They receive a packet on the local segment, store it and wait for the remote segment's to be clear before forwarding the packet.

There are 2 physical types of bridges: Local and Remote Bridges.

Local Bridges are used as in the previous examples where the network is being locally (talking physical location now) segmented. The 2 segments are physically close together: same building, same floor, etc... Only 1 bridge is required.

Remote Bridges are used in pairs and where the network is remotely segmented (again talking physical locations). The 2 segments are physically far apart: different buildings, different floors, etc... 2 x Half Bridges are required: one at each segment. The Remote bridges are 1/2 of a normal bridge and may use several different communications media inbetween.

Introduction to Data Communications
Previous	Table of Contents	Next