Neon-komputadór

Repeaters, Hubs and Switches

Whilst the network typology provides the means to connect a limited number of computers, anything beyond this requires additional means of means of filtering and directing network traffic, providing connectivity between different networks typologies (for example Ethernet and Token-Ring), different network protocols (for example TCP/IP and IPX/SPX), hierarchical addressing and signal regeneration. These large network connectivity devices include a range of devices including repeaters, hubs and switches, bridges, routers and brouters.

Repeaters are the simplest of these devices. They amplifies and regenerates data signals, as cables have a limited distance before signal loss becomes significant. Repeaters do not make any modification to the data and only four repeaters may be installed in succession without compromising data integrity.

The next step up in the range of connectivity devices are hubs and switches. They are the logical and physical centre of a local area network, consisting of multiple RJ-45 UTP connections and a number of diagnostic and activity lights. When a workstation sends a request for network information the Ethernet broadcasts the requests to all computers connected to the hub. When the destination computer receives the message, the hub broadcasts the results once again to all computers connected to it. Naturally enough only the computer that is interested in the information will act on it.

In general there are two types of hubs, active and passive. The main difference is that active hubs generally provide more ports than passive hubs. They regenerate received signals. They provide an extension of the physical medium connecting network components, similar to repeaters. Additional features in a hub include dual-speed, which is a necessity if adding Fast Ethernet to an existing Ethernet network and a stackable hubs. These include an uplink port which allows hubs to connected to other hubs, thus allowing the effective use of an existing hub even if it has run out of connectors. In any case it is usually considered worthwhile to purchase a hub with at 50% more connectors than is currently being used.

A switch is a special type of hub. They divide a local area network into virtual segments using a technique called micro-segmentation which reduces collisions and bandwidth use. Unlike a standard hub, where the data exists through all ports, the exit port in a switching hub is determined by the data packet's Media Address Control (MAC) layer destination address. Thus the data passes through virtual circuits from port-to-port.

It is usually recommended that the hub/switch is centrally located, although this is not a necessity. It is however strongly recommended that UTP cabling to a hub/switch is no more than 100 metres, as there is significant degrading of performance after that distance. This may result in what is termed a collision - when packets of information are unable to reach their intended destination. When this happens, the Ethernet protocol works by resending the packet. Whilst this virtually ensures that the information will eventually reach the desired destination on the local area network, it can cause delays. The higher the level of traffic, the greater the likelihood of collisions. To resolve this problem the use of switches, rather than hubs is recommended.