Friday, December 8, 2006

How Bridges Work

Bridges are hardware and software combinations that connect different parts of a single network, such as different sections of an intranet. They connect local area networks LANs to each other. They are generally not used, however, for connecting entire networks to each other, for example, for connecting an intranet to the Internet, or an intranet to an intranet, or to connect an entire subnetwork to an entire subnetwork. To do that, more sophisticated pieces of technology called routers are used.

1. When there is a great amount of traffic on an Ethernet local area network, packets can collide with one another, reducing the efficiency of the network, and slowing down network traffic. Packets can collide because so much of the traffic is routed among all the workstations on the network.

2. In order to cut down on the collision rate, a single LAN can be subdivided into two or more LANs. For example, a single LAN can be subdivided into several departmental LANs. Most of the traffic in each departmental LAN stays within the department LAN, and so it neednt travel through all the workstations on all the LANs on the network. In this way, collisions are reduced. Bridges are used to link the LANs. The only traffic that needs to travel across bridges is traffic bound for another LAN. Any traffic within the LAN need not travel across a bridge.

3. Each packet of data on an intranet has more information in it than just the IP information. It also includes addressing information required for other underlying network architecture, such as for an Ethernet network. Bridges look at this outer network addressing information and deliver the packet to the proper address on a LAN

4. Bridges consult a learning table that has the addresses of all the network nodes in it. If a bridge finds that a packet belongs on its own LAN, it keeps the packet inside the LAN. If it finds that the workstation is on another LAN, it forwards the packet. The bridge constantly updates the learning table as it monitors and routes traffic.

5. Bridges can connect LANs in a variety of different ways. They can connect LANs using serial connections over traditional phone lines and modems, over ISDN lines, and over direct cable connections. CSU/DSU units are used to connect bridges to telephone lines for remote connectivity.

6. Bridges and routers are sometimes combined into a single product called a brouter. A brouter handles both bridging and routing tasks. If the data needs to be sent only to another LAN on the network or subnetwork, it will act only as a bridge delivering the data based on the Ethernet address. If the destination is another network entirely, it will act as a router, examining the IP packets and routing the data based on the IP address.