Switches

A switch is sometimes described as a multiport bridge. While a typical bridge may have just two ports linking two network segments, the switch can have multiple ports depending on how many network segments are to be linked. Like bridges, switches learn certain information about the data packets that are received from various computers on the network. Switches use this information to build forwarding tables to determine the destination of data being sent by one computer to another computer on the network. Essentially, a LAN switch creates a series of instant networks that contain only the two devices communicating with each other at that particular moment.
Although there are some similarities between the two, a switch is a more sophisticated device than a bridge. A bridge determines whether the frame should be forwarded to the other network segment based on the destination MAC address. A switch has many ports with many network segments connected to them. A switch chooses the port to which the destination device or workstation is connected. Ethernet switches are becoming popular connectivity solutions because, like bridges, switches improve network performance by improving speed and bandwidth.

Switching is a technology that reduces congestion in Ethernet LANs by reducing the traffic and increasing the bandwidth. Network switches look nearly identical to hubs, but a switch generally contains more "intelligence" (and a slightly higher price tag) than a hub. Unlike hubs, network switches are capable of inspecting the data packets as they are received, determining the source and destination device of that packet, and forwarding that packet appropriately. By delivering messages only to the connected device that it was intended for, network switches conserve network bandwidth and offer generally better performance than hubs
In data communications today, all switching equipment performs two basic operations:
• The first operation is called switching data frames. Switching data frames is the process by which a frame is received on an input medium and then transmitted to an output medium. This consists of the learning process. The switch gets the first frame (packet of data) from a Node. It reads the MAC address and saves it to the lookup table. The switch now knows where to find that Node anytime a frame is addressed to it. This process is called learning. This is followed by the bridging process.
• The second is the maintenance of switching operations where switches build and maintain switching tables and search for loops. Learning and flooding continue as the switch adds nodes to the lookup tables. Most switches have plenty of memory in a switch for maintaining the lookup tables; but to optimize the use of this memory, they still remove older information so that the switch doesn't waste time searching through stale addresses. To do this, switches use a technique called aging. Basically, when an entry is added to the lookup table for a node, it is given a timestamp. Each time a packet is received from a node, the timestamp is updated. The switch has a user-configurable timer that erases the entry after a certain amount of time with no activity from that node. This frees up valuable memory resources for other entries.
Switches operate at much higher speeds than bridges and can support new functionality, such as virtual LANs.
An Ethernet switch has many benefits. One benefit is that an Ethernet switch allows many users to communicate in parallel through the use of virtual circuits and dedicated network segments in a virtually collision-free environment. This maximizes the bandwidth available on the shared medium. Another benefit is that moving to a switched LAN environment is very cost effective because existing hardware and cabling can be reused.