Practical TCP/IP and Ethernet Networking- P40: The transmitter encodes the information into a suitable form to be transmitted over the communications channel. The communications channel moves this signal as electromagnetic energy from the source to one or more destination receivers. The channel may convert this energy from one form to another, such as electrical to optical signals, whilst maintaining the integrity of the information so the recipient can understand the message sent by the transmitter | LAN system components 177 Bridges Bridges operate at the data link layer of the OSI model layer 2 and are used to connect two separate networks to form a single large continuous LAN. The overall network however still remains one network with a single network ID NetID . The bridge only divides the network up into two segments each with its own collision domain and each retaining its full say 10 Mbps bandwidth. Broadcast transmissions are seen by all nodes on both sides of the bridge. The bridge exists as a node on each network and passes only valid messages across to destination addresses on the other network. The decision as to whether or not a frame should be passed across the bridge is based on the layer 2 address . the media MAC address. The bridge stores the frame from one network and examines its destination MAC address to determine whether it should be forwarded across the bridge. Bridges can be classified as either MAC or LLC bridges the MAC sublayer being the lower half of the data link layer and the LLC sublayer being the upper half. For MAC bridges the media access control mechanism on both sides must be identical thus it can bridge only Ethernet to Ethernet token ring to token ring and so on. For LLC bridges the data link protocol must be identical on both sides of the bridge . IEEE LLC however the physical layers or MAC sublayers do not necessarily have to be the same. Thus the bridge isolates the media access mechanisms of the networks. Data can therefore be transferred for example between Ethernet and token ring LANs. In this case collisions on the Ethernet system do not cross the bridge nor do the tokens. Bridges can be used to extend the length of a network as with repeaters but in addition they improve network performance. For example if a network is demonstrating fairly slow response times the nodes that mainly communicate with each other can be grouped together on one segment and the remaining nodes can be grouped together in .
