an ethernet switch

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introduction

An ethernet switch is a specialised computer used to form a star network. It receives and sends packets of data through several ports. It works at the 2nd layer of the OSI 7 layer model. The job it does is defined by the ethernet standard.

An ethernet switch is also called a LAN (Local Area Network) switch or network switch, because it is used to build a LAN.

Cisco, Nortel and Lucent are companies that make switches.

what it is

an ethernet switch is a box with a lot of sockets and LEDs marked Rx and Tx. Rx and Tx are weird network words for Receive and Transmit. This page only deals with ethernet switches. There are other sorts of switches such as KVM (Keyboard Video Mouse) switches, but we don't care about them so we don't bother with the LAN part of the name in the rest of this page. It is a specialised computer that is connected to a bunch of other computers to form a star network. The plugs on a network cable are plugged into the sockets on the switch and another computer, and little packets of data are sent back and forth. Every time the switch receives a packet through this socket, its Rx LED winks. Every time it sends a packet through this socket, its Tx light winks.

A small switch for a home network has 4 sockets. Bigger switches have 8 sockets, 16, 24, and even 48 sockets. Where are the switches with an odd number of sockets? This is a market opportunity, surely. Switches that connect dozens of computers together are are bulky boxes made for rackmounting. They are 19" across and have mounting screw holes.

A switch forwards packets by matching up intended destinations with the addresses of computers it is attached to. Each packet has its destination address written on it. A switch keeps a list of addresses and which sockets lead to those addresses. When a packet arrives at the switch it knows which socket to send it to.

A switch works at the 2nd layer of the OSI 7 layer model. The addresses it keeps track of are MAC addresses. A switch builds up its list of MAC addresses automatically using cunning little techniques.

Being a layer 2 thingy means all of the higher level protocols, such as IP, IPX and Appletalk, can be sent across the switch and it won't be able to tell the difference. It also means that one switch can only handle one of the lower level protocols. You can tell what sort of protocol the switch can handle by the way it looks. An ethernet switch has lots of RJ-45 sockets which are sort of rectangular and an FDDI switch has lots of SC sockets which are sort of rectangular.

A switch can be used to split an ethernet LAN into smaller pieces, called segments. An ethernet LAN using switches to connect LAN segments is called- wait for it- a switched ethernet LAN.

The speed a switch runs at is not dependant on how powerful a computer it is. It doesn't take much power to shift packets around. It depends on the lower level protocol it understands. Ethernet runs at a maximum of 10 Mbps, fast ethernet at 100 Mbps, Gigabit ethernet at 1000 Mbps, FDDI at, er, a million mph.

computers connected to a switch
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A star network connected by a hub and CAT-5 cables only goes at half the speed of a star network connected by a switch. A hub allows half duplex and a switch allows full duplex. A hub is a multiport repeater: any packet it receives through one of its sockets it immediately forwards copies to all the other sockets. This means that every computer can either receive or transmit data (half duplex). The computer must keep half an ear out for screwups called packet collisions. A switch receives a packet then forwards it through only one socket. The computers don't have to watch for collisions and can receive and transmit data at the same time (full duplex).

Some switches have optional extras like VLANs, leather upholstery and spanning trees. A VLAN (Virtual LAN) is a LAN made up of a few of the computers connected to the switch. One switch and several VLANs acts like several smaller switches connecting several LANS. The switch does not allow communication between the VLANs.

what it isn't

A switch is not a router. A router works on layer 3, higher protocols.

where it is

The LIC has a few switches. The easiest setup is to have one switch in each DMZ, with each LAN actually a VLAN. A more tricky setup with lots more cables and switches is to have a hub or switch on each LAN connected to a main switch.

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history

The first switches appeared in the 1990s. They were meant to replace hubs where more bandwidth was needed. Apparently.