Bridges

Bridges act as data link layer relays between LANs.

A bridge participates as a device on the networks to which it is attached, exchanges information with devices on those networks, and forwards information between the networks selectively through the MAC address.
There are four types of bridges and they are classified by their hardware and
software capabilities.

Simple Bridges

Simple Bridges consist of two or more linked network interfaces connecting local area networks. Bridges interconnect separate local area networks (LANs) by relaying data frames between the separate MAC (media access control) entities of the bridged LANs.











Figure: Simple bridge connecting two homogeneous LANs.







The main functions of a simple bridge may be summarized as follows:

• The bridge reads all data frames transmitted on LAN A and receives those addressed to LAN B. Simple bridges make no changes to the content or format of the data frames that they receive. They also do not encapsulate frames with any additional headers. Most simple bridges contain routing addressing and routing intelligence. At a minimum, the bridge must know which addresses are on each connected network so that it can know which frames to pass on.
• The bridge retransmits the data frames addressed to LAN B using the MAC protocol for that LAN. Bridges should have enough buffer space to meet peak data traffic demands since data frames may arrive faster than the bridge can transmit them.
• The bridge does the same for LAN B-to-LAN A data frame traffic.

Complex Bridges

Complex Bridges carry out more sophisticated functions than simple bridges. These functions may include the bridge maintaining status information on the other bridges. This information includes the communication path cost as well as the number of hops required to reach each connected network. Periodic exchanges of information between bridges update all bridge information. These types of exchanges allow for dynamic routing between bridges.
Complex bridges can also modify frames and recognize and transmit packets from different LAN technologies (for example, token-ring and Ethernet). In this case the bridge is sometimes referred to as a translational bridge.
The Adaptive Source-Routing Transparent (ASRT) Bridge is the IBM 2210¢s
implementation of bridge technology. The ASRT Bridge is a collection of software components capable of several of the bridging options just described and more.

Local Bridges

Local Bridges provide connections between several LAN segments in the same geographical area. An example of this would be a bridge used to connect the various LANs located in your company's main headquarters.


Remote Bridges

Remote Bridges connect multiple LAN segments in different geographical areas. An example of this would be bridges used to connect LANs located in your company¢s main headquarters to LANs in other branch offices around the country. Because of the geographical differences, this configuration moves from a local area network configuration to a wide area network (WAN) configuration.
Remote bridges can differ from local bridges in several ways. One major
difference is found in the speed in which data is transmitted. WAN connections are generally slower than LAN connections. This difference in speed can make quite a difference when running time-sensitive applications. Another difference is found in the physical way that remote and local bridges are connected to LANs. In local bridges, the connections are made through local cabling media (for example, Ethernet). Remote bridge connections are made over the serial lines.