| Local area networks (LANs) are limited both in their reach and in the number of stations that can be connected. For example, only a maximum of 30 stations per segment can be connected to Ethernet based on the 10Base2 standard; and even if you connect fewer than the maximum number of stations, but use an extremely traffic-intensive application, it can happen that the traffic in a LAN is so high that the throughput of the entire network drops rapidly. This degradation is due mainly to the fact that local area networks are broadcast networks when station A sends a data frame to station B, then the data packet is concurrently transported to all other stations. The bandwidth in a local area network is used only by the sending station at that time (asynchronous time division multiplexing TDM). The more stations there are in a local area network, the smaller is the share of each single station. Depending on the network technology, a lot of additional time might be used to decide which station may send next (Medium Access Control). For the above reasons, it is meaningful to divide a heavily loaded or very large local area network into several subnetworks. Similarly, several local area networks can be linked by single coupling elements to form one large internetwork. In this regard, the parts of the original local area network should not be split into different subnetworks (as is possible in IP), but should always represent themselves as one single (sub)network to the network layer. The two networks are connected transparently, for the network layer. One coupling element that can link different local area networks to form one single logical LAN is called a bridge. A bridge connects several local area networks on the data-link layer (layer 2 in the OSI reference model) and distributes the traffic over the subnetworks. Stations that communicate often are generally grouped into one subnetwork. Grouping frequently communicating stations within the same subnetwork means that the entire network has less load to carry, because these stations can exchange traffic within their subnetwork regardless of the traffic in other subnetworks. | 