Hubs, Switches, Bridges

A hub is a device that lets a single network cable to split into multiple cables leading to nodes (network devices such as computers, printers, other hubs).

Hubs and switches do essentially the same job. A PC's network card is connected by cable to a hub or switch. The hub/switch is in turn connected to the file server. Switches are rather more intelligent and efficient than hubs and reduce the amount of network traffic.

When the file server transmits a message to a PC via a hub, the hub sends the message to every PC connected to it. A switch, however, knows which PC the message is meant for and directs the message just to that PC. The other PCs on the network aren't flooded with irrelevant traffic.

(Think of it like this: when asked to pass on a message a hub picks up a megaphone and shouts to everyone in the neighbourhood. A switch picks up the phone and dials the PC it needs to talk to.)

Switches used to be far more expensive than hubs, but the price differences have pretty well disappeared and the use of switches is becoming standard.

Above: an 24 port switch. Switches usually have up to 24 ports - though there are BIG switches with 48 or more ports - if you need more, most manufactureres make switches can be linked together to act as a HUGE single switch. You need one port for every PC on the network. This type of switch is designed to fit in a rack. McKinnon SC has about 10 big switches to connect about 200 student PCs to the file server, proxy server, CD-ROM towers and CD-ROM server.

Older hubs and switches could communicate at 10 megabits per second. More recent switches and network cards can cope with 100 Mbps (10 times faster). Of course, to get that speed all components between the user and the fileserver (PC and fileserver network cards, cables, switches) have to be rated at the same - or higher - speed.

One cable is split into many cables by a hub.

Hubs and switches let a network be split into smaller segments, for example a single backbone cable can might have a cable branching from it to a hub from which 24 cables split off to computers in a school computer room. The hub and its connected computers would form a segment of the network.

A typical simple network - each room is a segment

The problem with hubs is network congestion. On a basic network, every network message (packet) is sent to every node on the network, but most messages are only intended for one of the nodes. Each node has to inspect every packet to see if it's addressed to him.

To make matters worse, only one message can pass along a network cable at a time, so while one node is transmitting, every other node must wait. If a network had hundreds of nodes, you can imagine the delays that would happen as each computer fought for a chance to speak.

It's like every person in a shopping centre using the public address system to talk to each other. Not only does every other shopper have to listen to the message in case it's meant for them, but no one else could speak while the PA was in use.

Hubs are dumb creatures: they pass on every network packet they receive to every node in their segment, regardless of whether the packet is addressed to one of their nodes or not. On a large network, the number of irrelevant packets received by each node is enormous, and delays caused by congestion can rapidly cripple a large network.

Switches (switching hubs) are the modern equivalent of hubs, and have now largely replaced hubs. Switches have the intelligence to examine each packet they receive, and only pass on packets that are addressed to one of their nodes. In fact, if a node is connected directly to a switch, it is treated like it is on a segment by itself: no other node  - not even other nodes in the same segment - will hear a packet except the node it is intended for.

The red line shows the path of a packet being routed directly from a server to a single node through switches.  The blue lines do not see the traffic.

Hubs and switches come in a variety of sizes (from 4 to 24 ports), speeds (10, 100 or 1000 bits per second), management modes (managed or unmanaged - managed modes can be controlled remotely), and can be stackable (so two or more hubs can be linked to split a single incoming cable into 48 or more outgoing cables.

Bridges were devices used to connect network segments. They only passed on packets relevant to their own segment. Nowadays, switches are used as bridges.