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A network switch, commonly referred to as just a switch, is a network device that is used to connect segments of a LAN (local area network) or multiple LANs and to filter and forward packets among them. Switches have an appearance similar to hubs (because both are box-like devices that contain a number of RJ-45 jacks), but they are actually multi-port bridges. A bridge is a device that functions at the data link layer (the second layer) of the seven-layer OSI model to connect and control the flow of data between two LANs or two segments of the same LAN. Bridges have three main functions: (1) creating a bridging table to keep track of devices on each segment, (2) filtering packets based on their MAC addresses and (3) dividing a single network into multiple collision domains, thereby reducing the number of collisions on each segment and effectively increasing its bandwidth. Hubs, as is the case with repeaters and in contrast to switches and bridges, operate at the physical layer (i.e., the bottom layer) of the OSI model. This layer provides no filtering of packets. Switches, however, because of their operation at the data link layer, provide a greater degree of control, including the filtering of packets by their MAC addresses so that they are forwarded only to their destination network segments rather than just broadcasting them to all segments. This limits the collision domain, and thereby conserving system bandwidth. Switches also improve network performance over hubs by providing full duplex (i.e., data flow in both directions simultaneously) operation instead of half-duplex (data flow in only one direction at a time) operation. This results in a doubling of the maximum bandwidth. Switches can operate in full duplex mode because they prevent collisions, and thus no collision detection system is needed. Switches thus eliminate the conventional CSMA/CD (carrier-sense multiple-access with collision detection) method and utilize a far more efficient communication method. LANs that use switches to join segments are called switched LANs. In the case of Ethernet LANs, which account for most LANs, they are also called switched Ethernets. Switches are self learning, and thus they are as easy to install as hubs and bridges. All that is necessary is to plug them in and they are ready to use. Switches have become extremely sophisticated in recent years. For example, models have been developed that include support for optical fiber networks, the addition of services at higher layers of the OSI model and the ability to create virtual LANs (VLANs). A VLAN is a LAN in which devices are logically configured to communicate as if they were attached to the same network, without regard to their physical locations. Switches can also include the functionality of routers, which are devices and/or software that connect at least two networks and forward packets among them according to the information in the packet headers and routing tables. However, in general, switches are simpler and faster than routers, which require knowledge about the network and how to determine the route. Routing occurs at the network layer (i.e., the third layer) of the OSI model, in contrast to the data link layer operation of switches. The superior performance of switches together with their ease of use and a narrowing of the price gap has resulted in their increased use in place of hubs in recent years. Created December 1, 2005. |