Switch
Look at a switch as a device that allows PCs to connect to it. For example, a switch could have say 24 connections which will allow 24 PCs to connect to it. Each PC would connect to what is called a switch-port on the switch using a (Ethernet) cable. Now look visualize an office environment on one floor. Say there are 16 people on the office floor and each person’s PC connects to a switch-port. The switch setup in this way allow the sixteen people to communicate with each other. You can also connect a printer to one of the switch-ports thereby enabling everyone to access to print documents.
You can also connect switches to each other using a (crossover Ethernet) cable. So for example, a office block had 6 floors with 20 people each. Each switch on the floor could connect to each other (trunking) thereby allowing everybody in the building to communicate with each other. Switches traditionally operates at Layer2 of the OSI Layer (Data-link layer) but is common these days to see hybrid ones with Layer 3 functionality. Where routers prevent broadcasts, switches allow broadcasting of packets so that every switch-ports can learn if data is destined for them. You can contain broadcasts by turning on Layer3 features using VLANs. A switch stores the MAC Address of every device which is connected to it.
The switch will then evaluate every frame that passes through it. The switch will examine the destination MAC Address in each frame. Based upon the destination MAC Address, the switch will then decide which port to copy the frame to. If the switch does not recognize the MAC Address, it will not know which port to send the frame to and broadcasts all ports. If the switch does not have layer3 options then it can connect to a router to prevent broadcasts and route packets to different networks.
Routers and switches vary in size and performance to meet differing needs. High performance switches allow modules to be connected or inserted to enable routing, security services and voice over IP (VOIP).
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 number of linked managed switches. Small office/home office (SOHO) applications typically use a single switch, or an all-purpose converged device such as gateway access to small office/home broadband services such as DSL router or cable Wi-Fi router. In most of these cases, the end user device contains a router and components that interface to the particular physical broadband technology, as in the Linksys 8-port and 48-port devices. User devices may also include a telephone interface to VoIP.
In the context of a standard 10/100 Ethernet switch, a switch operates at the data-link layer of the OSI model to create a different collision domain per switch port. If you have 4 computers A/B/C/D on 4 switch ports, then A and B can transfer data between them as well as C and D at the same time, and they will never interfere with each others’ conversations. In the case of a “hub” then they would all have to share the bandwidth and run in Half duplex. The result is that there would be collisions and retransmissions. Using a switch is called micro-segmentation. It allows you to have dedicated bandwidth on point to point connections with every computer and to therefore run in Full with no collisions.
Role of Switches in networks
Switches may operate at one or more OSI layers, including physical, data link, network, or transport (i.e., end-to-end). A device that operates simultaneously at more than one of these layers is known as a multilayer switch.
In switches intended for commercial use, built-in or modular interfaces make it possible to connect different types of networks, including Ethernet, Fibre Channel, ATM, ITU-T G.hn and 802.11. This connectivity can be at any of the layers mentioned. While Layer 2 functionality is adequate for speed-shifting within one technology, interconnecting technologies such as Ethernet and token ring are easier at Layer 3.
Interconnection of different Layer 3 networks is done by routers. If there are 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 some service provider and other environments where there is a need for a great deal of analysis of network performance and security, switches may be connected between WAN routers as places for analytic modules. Some vendors provide firewall, network intrusion detection, and performance analysis modules that can plug into switch ports. Some of these functions may be on combined modules.
In other cases, the switch is used to create a mirror image of data that can go to an external device. Since most switch port mirroring provides only one mirrored stream, network hubs can be useful for fanning out data to several read-only analyzers, such as intrusion detection systems and packet sniffers.
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