Switching

Switching uses temporary connections for routing information. This communications method is widely implemented in local area networks and provides better performance on the LAN than routers in two ways:

• First, switches provide a means of directing a frame to the appropriate output port, typically using the data-link layer addressing. The hardware itself forwards the frame to the output port. Routers use software programs on the network to route frames. Because the data-link layer does not use software to forward frames to the proper route, performance is improved.

• Second, switches improve performance by enabling communication between logical groupings of users. Only one communication at a time can take place in a routing environment. The computers must wait until the line is free to communicate. Switches act as bridges, allowing the messages to be routed from one port on the switch to another. There is no requirement to wait until the communication finishes before communicating.

Switching technology uses switches, which are multiport devices that create temporary paths to send frames directly to a device based on its MAC address. There are three basic switching technologies: Configuration, Frame, and Cell. Another technology, Frame-to-Cell translation, enables the migration to cell-based backbones without changes to the host network's interface.

Configuration switching, or port switching, allows individual ports to be assigned to individual segments within a multi-segmented network hub. This provides the equivalent of an intelligent patch panel for network centers and wiring closets. Port assignments are performed when devices are initially attached to networks. This type of switching offers the ability to segment networks quickly for better performance. Configuration switching operates at the physical level and is transparent to end systems and upper-layer protocols.

Frame switching is a connectionless technology that provides cost-effective bandwidth to the workgroup and to multiplexed, low-speed traffic from the wiring closet onto high-speed down links. Operating at the data-link layer, frame switches "learn" the destination MAC addresses of each attached computer. Through the MAC address of each received frame, the switch forwards the packet to the output port attached to the computer with the same MAC address. The output port can be directly attached to the computer, to a shared access segment with multiple users, or to another frame switch, which, in turn, is connected to the destination. Frame switching supports both Ethernet Frame switching and Token Ring switching. Ethernet Frame switching supports shared or dedicated 10-Mbps or 100-Mbps connections. A Token Ring switch, also referred to as a segment switch, is a high-capacity, high port-density frame-relaying engine that forwards data simultaneously, among all ports, at wire speed (4/16-Mbps).

Cell switching, also called asynchronous transfer mode (ATM), is a high-speed technology that switches fixed-length, 53-byte cells at speeds of 155-Mbps and higher. Designed to carry voice, video, and data traffic, cell switching supports asynchronous traffic. Cell switching also supports different classes of traffic: loss-sensitive, delay-sensitive, delay-variance sensitive, and various combinations of these.