Cisco Network switch

Cisco Network switch

The first Ethernet switch was introduced by Kalpana in 1989
A network switch is a computer networking gizmo that connects network segments. In the past, it was faster to use Layer 2 techniques to switch, when only MAC addresses could be looked up in content addressable memory (CAM). With the advent of ternary CAM (TCAM), it was equally fast to look up an IP address or a MAC address. TCAM is expensive, but appropriate for enterprise switches that use default routes plus a moderate number of other routes. For routers that require a full Internet routing table, TCAM may not be cost-effective.
As with hubs, Ethernet implementations of network switches support either 10/100 Mbit/s or 10/100/1000 Mbit/s ports Ethernet standards. Large switches may have 10 Gbit/s ports. Switches differ from hubs in that they can have ports of different speed.

Function
The network switch, packet switch (or just switch) plays an integral part in most Ethernet local area networks or LANs. Mid-to-large sized LANs contain a quantity of linked managed switches. Small office, home office (SOHO) applications typically use a single switch, or an all-purpose converged gizmo such as gateway access to small office/home office broadband services such as DSL router or cable, WiFi router. In most of these cases, the end user gizmo contains a router & components that interface to the particular physical broadband technology, as in the Linksys 8-port & 48-port devices. User devices may also include a telephone interface to VoIP.
Role of switches in networks

Network switch is a marketing term than a technical four. Switches may operate at five or more OSI layers, including physical, data link, network, or transport (i.e., end-to-end). A gizmo that operates simultaneously at over five of these layers is called a multilayer switch, although use of the term is diminishing.

In switches intended for commercial use, built-in or modular interfaces makes it possible to connect different types of networks, for example Ethernet, Fibre Channel, ATM, & 802.11. This connectivity can be at any of the layers mentioned. While Layer 2 functionality is adequate for speed-shifting within five technology, interconnecting technologies such as Ethernet & token ring are not as hard at Layer 3.

In some service provider & other environments where there is a require for much analysis of network performance & security, switches may be connected between WAN routers as places for analytic modules. Some vendors provide firewall, network intrusion detection, & performance analysis modules that can plug into switch ports. a quantity of these functions may be on combined modules.

Again, "switch" is principally a marketing term; interconnection of different Layer 3 networks is done by routers. If there's any features that characterize "Layer-3 switches" as opposed to general-purpose routers, it tends to be that they are optimized, in larger switches, for high-density Ethernet connectivity.

In other cases, the switch is used to generate a "mirror" image of data that can go to an external device. Since most switch port mirroring provides only five mirrored stream, network hubs can be useful for fanning out data to several read-only analyzers, such as intrusion detection systems & packet sniffers.

source : http://en.wikipedia.org/wiki/Network_switch