Network Switches

Like a hub, a switch is a device that connects individual devices on an Ethernet network so that
they can communicate with one another. But a switch also has an additional capability; it
momentarily connects the sending and receiving devices so that they can use the entire bandwidth
of the network without interference. If you use switches properly, they can improve the
performance of your network by reducing network interference.

Switches have two benefits: (1) they provide each pair of communicating devices with a fast
connection; and (2) they segregate the communication so that it does not enter other portions of
the network. (Hubs, in contrast, broadcast all data on the network to every other device on the
network.)

Different models of network switches support differing numbers of connected devices. Most consumer-grade network switches provide either four or eight connections for Ethernet devices. Switches can be connected to each other, a so-called daisy chaining method to add progressively larger number of devices to a LAN.

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, run in Half Duplex and 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 duplex with no collisions.

Role of switches in networks:

Switches may operate at one or more OSI layers, including physical, data lin, network or transport. 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 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.

Purchase Considerations

When you purchase and install a switch, you should review and
apply the following criteria:

• Your switches must be compatible with your physical and data link
level protocols. If you are running a 10BaseT Ethernet network, then
you must purchase a 10BaseT switch.
• Some switches can accommodate more than one physical or data link
level protocol. For example, modern switches accommodate both
10BaseT and 100BaseTX protocols. It is wise to purchase a switch
with at least one 100BaseTX port, since you can interconnect your
switches via their high speed ports to improve network performance
(even if the remainder of your network uses 10BaseT).
• If you purchase a switch that accommodates more than one protocol,
then make sure that it automatically senses which protocol is being
used on each port. Autosensing switches ensure that you can connect
any part of the network to any switch port. (Older switches required
that you attach each segment of the network to a port compatible
with its physical and data link level protocol. Keeping the segments
and ports straight presents a management headache.)
• Purchase switches from a known manufacturer whose support you
trust. Make sure the manufacturer provides a competitive warranty.
• Install your switches in a room that is cool and free of dust, if
possible. Additionally, plug your switches into an uninterruptible
power supply (UPS) to ensure that they receive clean power.