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A computer network exists where two or more computers are linked together to share data and/or hardware or software resources. In turn, this may facilitate electronic human-to-human communications, e-business, and alternative working practices such as teleworking.
Computer networking is a large and technically very complex topic upon which a great deal of online information is widely available. The scope of this section is therefore constrained to a largely non-technical overview of the practicalities involved in connecting computers together to form a local area network (LAN) or a personal area network (PAN) using the most commonly available wired and wireless technologies. Connecting a computer to the Internet is covered separately in the Internet pages, whilst networking-related security issues receive attention in the security pages.
When they were first introduced in the late 1970s and 1980s, most personal computers were entirely stand-alone devices. These days of course this is no longer the case, with it being unusual to find a computer that is not at least occasionally connected to a LAN or the wider Internet. Indeed, due to the growth of their interconnection as communications devices, most computers could now best be described as "interpersonal" rather than merely "personal".
Most computer networks are based around a "client-server" model in which the majority of the computers on the network -- the "clients" -- have their useful capabilities extended via connection to one or more "server" computers that provide them with greater functionality. Typically a server on a local area network (LAN) may provide its clients with additional services such as private and/or sharable storage space, access to software applications that run across the network, access to shared peripherals (most commonly printers), and access to wider networks (most notably the Internet).
It is important to realise that the term "client-server" refers purely to the relationship between two computers on a network, and not necessarily to their hardware specification. Most network servers are fairly big, powerful computers with large storage capacities. However, this need not be the case, and indeed when first establishing many home or even small business networks it is not uncommon for older and less powerful personal computers to be pressed into service as servers.
Network clients are often categorised as being either "thick" or "thin". A thick client refers to a computer -- such as a typical modern PC -- that has significant functionality (such as the ability to run complex software applications) even when not connected to the network. In contrast a thin client refers to a networked computing device than can perform little or no useful independent action without network connectivity. A thin client may, for example, be a computer which only ever runs software as a service applications accessed over the Internet.
Whilst client-server networks are the most common, it is perfectly possible to have a network with no server computer. Such usually small networks are described as "peer-to-peer" indicating the equal status of all computers on the network. Peer-to-peer networks most commonly exist for very simple purposes such as to facilitate the sharing of files, a printer or an Internet connection between two or more computers (see also the example networks pictured at the end of this section).
NETWORK BUILDING BLOCKS
The basic building blocks of any network are at least two computing devices with wired or wireless points of connection -- known as "network ports" -- and usually at least one "network hub" or "network switch" that permits their interconnection. Both hubs and switches are hardware devices featuring multiple network ports for connecting computers or the different "segments" that make up larger networks.
The difference between a network hub and a network switch is that a network switch has more internal "intelligence". This allows the network switch to inspect the "packets" of data that it receives and to send them on only to the intended recipient. In contact, network hubs simply broadcast all received data to all connected computers. On all but the smallest networks, the use of network switches rather than hubs hence increases network performance as the network traffic is managed intelligently. Network hubs are becoming obsolete -- even for small networks -- as the cost of network switches continues to fall.
On a small wireless network, the network switch (often both wired and wireless) is often integrated into a device commonly termed a "wireless access point" or "wireless router" (see below). In this context it should be noted that the terms "hub", "switch" and "router" are often liberally bandied around in a technically dubious manner in the popular marketing and labelling of network hardware. This is largely irrelevant in most practical contexts, but can rightly annoy anybody with more specialist technical networking knowledge.
Wired computer networks are most commonly based on a standard known as Ethernet. This uses UTP (unshielded twisted pair) cables, and today almost all most new personal computers come with a UTP Ethernet network port as standard. Many printers now also feature an Ethernet port, allowing them to be easily shared amongst the users of a network.
Most wireless computer networks are currently based around one of two technology standards known as WiFi and Bluetooth. Of these, Bluetooth is a low power, short-range wireless technology largely used to interconnect computing devices into a personal area network (PAN). As the name suggests, a personal area network exists around an individual, and typically includes devices such as a laptop, mobile phone, headset, digital camera, personal digital assistant (PDA) or other form of mobile computer.
Wireless local area networks (WLANs) almost always use WiFi. This incorporates a set of standards for wireless local area networking which are certified by an organization known as the Wi-Fi Alliance.
Wi-Fi is based on a set of wireless networking technologies known as 802.11. These include 802.11b, 802.11a, 802.11g and 802.11n (all of which are in common usage). The 802.11b standard supports data transfers at 11Mbps (megabits per second), whilst 802.11a -- which for various reasons came later! -- and 802.11g support 54Mbps, and 802.11n a theoretical 100Mbps.
At present the range of Wi-Fi network transmission is about 30-40 metres indoors and up to about 100 metres or so outdoors. This said, some building construction types can significantly impede or even totally block Wi-Fi signals. A forthcoming standard known as 802.11y is intended to boost outdoor range to up to 5000 metres.
Any two devices with a wireless network connection can establish a so-called "ad-hoc" wireless network. However, most usually a wireless network is established using a piece of hardware known as a "wireless access point" (also termed a wireless router) that facilitates the wireless communications between the different devices on the network. To communicate with the access point, all computers on the network must either have an internal wireless network card (as built-in to most modern laptops and mobile computers), or an external wireless adapter. The later most usually plugs in to a USB socket and is about the size of a USB memory stick.
Wireless access points commonly feature an ADSL modem to facilitate a broadband connection to the Internet for all computers on the network (and as discussed in the Internet section). Such devices are typically termed "wireless ADSL routers", and usually also incorporate one or more Ethernet ports to allow the creation of a network of both wired and wireless devices. Where wireless access points are made available to facilitate public Internet access -- either free or for a fee -- they are usually termed "wireless hotspots".
A third wireless technology is WiMax. Created by the WiMax Forum, and standing for "Worldwide Interoperability for Microwave Access", WiMax is based on the IEEE standard known as 802.16 and facilitates highspeed wireless network links to both fixed and mobile devices. To cite the WiMax Forum, WiMax was created "to deliver non-line-of-sight (LoS) connectivity between a subscriber station and base station", and as an alternative to broadband/DSL link (as discussed in the Internet section). The range of a WiMax wireless connection is around three to ten kilometers. To use WiMAX, a mobile or desktop computer needs to be connected to a WiMax base station or network card.
Increasingly on small LANs, hardware devices known as network attached storage (NAS) drives are connected to provide network storage capacity. NAS units are effectively just large external hard disk drives (as discussed in the storage section) with one or more Ethernet ports to facilitate their connection to a network.
NAS drives are usually configured by via web browser access from a computer on the network. Some NAS units can also be connected as "standard" external hard disk drives via a USB, firewire or E-SATA connector. NAS drives are particularly handy for providing network storage on small peer-to-peer networks. And on client-server networks where enough server-based storage is available, they can be usefully be employed as a back-up storage facility.
Networking is a complex topic that dominates the computer industry. As an illustrative summary, the following provides two examples of how some basic network hardware could be connected to create two simple networks in a SoHo (small office home office) environment.
In the first example, four personal computers are connected with Ethernet cables to a network switch that is also connected to a network server. An ADSL modem is also connected to the resultant wired client-server network to facilitate access to the Internet (and as explained in the Internet section), as is a network printer,
In the second example, a wireless ADSL router (wireless access point) is connected wirelessly to two laptops, and also by an Ethernet cable to one desktop computer, hence permitting all three network users broadband Internet access. A NAS drive is additionally connected by an Ethernet cable to this peer-to-peer network to provide network storage. This sort of network is increasingly typical in many homes, with the NAS drive often used to store audio and video files that can then be "streamed" to any connected device.
A computer network exists where two or more computers are linked together to share data and/or hardware or software resources.