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by
Matthew Martin
Communications networks have greatly increased the usefulness of computer systems, providing near instant communications over great distances and allowing for the easy exchange of information, brining data to the desktop computer with an ease not previously known.
There are a number of ways of organising networks, here a few of the most prominent schemes are looked at in brief.
The topology of a network refers to the physical layout of the network.
Consisting of a central computer, usually acting as some form of server, and a number of smaller computers, forming a start-like pattern. A star network is useful where some form of centralised processing is performed by the central computer. If the central computer fails the whole network ceases to work.
In a bus network a main communication cable (or bus) is used that allows two-way communication. All computers and other network devices (such as printers) are independently connected to the bus, so if one device fails it in no way affects the use of the network by other devices. On a bus, data can travel in both directions, but this requires the use of protocols to cope with the collision of messages travelling along the bus.
In a ring network all computers and other network devices (such as printers) are linked by a ring of cable. It is very similar to the bus network but the communication only travels in one direction around the ring. As all the devices are connected to the cable independently, if one fails it does not affect the ability of the other devices to use the network.
In this section networks of different sizes (areas) are considered.
LANs cover small areas, typically a single office, school or building. The reach of the network is normally on the scale of tens of metres. Typical topologies for LANs are bus and ring.
WANs are used to connect two or more LANs. A WAN can connect offices in different cities, cover a business park or a university campus.
A VPN is used, typically by a company, in order to connect remote sites together where the cost of laying cable is prohibitive. Encryption systems ensure the privacy of communication over fast Internet connections. Essentially this is a virtual WAN.
The network of networks. There is only one of these.
A
networking protocol is a scheme that agrees the manner in which data is packaged,
sent and accounted for across a network. Vendors (companies) and industry
committees develop the agreements and individual companies write software
that conforms to the standards agreed. Usually they do not get it right first
time, and there follows several rounds of re-development before a satisfactory
implementation of the protocol is achieved.
There are a number of components that are important for the functioning of networks.
Repeaters are devices that are able to regenerate signals, effectively extending the range over which signals may be carried. This can be important where the LAN has to cover a large area. Repeaters work at the physical layer of the OSI model. It is worthwhile noting that networking hubs are a type of repeater, but typically when using the term repeater we refer to something that extends the reach of a LAN, rather than an actual hub.
A bridge connects different areas of the network and is discriminating in terms of what it will allow to pass through it. Only certain types of traffic will be allowed to pass through the bridge, from one area of the network to another. In this way security can be maintained. Bridges typically operate at the data-link layer of the OSI layer, using information provided by network device firmware.
A router connects LAN segments but has far greater capabilities for regulating what network traffic can flow through it. Routers use information about the network layer protocols for the regulation and directing of traffic.
A gateway connects networks that use different types of communications protocol. These are often incorporated into a router.
A PBX is a specialised computer used to handle telephone calls. This is today’s equivalent of the company switchboard. PBX systems can typically handle both voice and data. They provide services such as allowing multiple phones to connected into and out of the building via a limited number of lines supplied by the phone company and voice mail. PBX systems typically have a reach over a single office building, but it is possible to connect them to computer networks in order to extend their range.
by
Matthew Martin