LANs, WANs and Topology
The Local Area Network (LAN)
A LAN is a network that is restricted to one geographic site or location. Typically, nodes in such a LAN are all connected by a common cabling system (although are increasingly using cable-less media, such as radiofrequency - RF - infrastructure). The major plus points for LAN infrastructure is that it is cheap to implement and bandwidth (i.e. amount of data that can flow through the network in a given time period) is high, allowing fast communication between nodes.
A group of computers in an office will be sitting on one local area network. Similarly, home networks are becoming more and more common, whether the purpose of the network is for work, for sharing a single Internet connection amongst several computers in a household, or perhaps for gaming. These home networks are also examples of LANs.
The Wide Area Network (WAN)
Two or more LANs that are linked together form a WAN. Typically, inter-communication between LANs is slower than intra-communication within the individual LANs. This is because WAN communication is usually achieved using a slower medium, such as a dial-up modem, an Integrated Services Digital Network (ISDN) telephone line, or dedicated cable. Furthermore, implementing WAN infrastructure is usually very expensive. A simple example of LAN and WAN infrastructure might be a home network. You may have several computers in your home that are all connected to each other by an Ethernet LAN. One of these machines maybe connected to the Internet by a modem or cable. In this way, your home LAN is connected to the WAN that is the Internet.
LAN Architecture and Topology
Within any LAN, all nodes need to be connected to the same network. Once upon a time, this was done using a strategy known as point-to-point, where every node would be connected to every other node. For small networks, this is simple... With only two nodes, there would be only one connection. With three nodes, three connections are required, and with four nodes, six connections are required:
Cleary this just isn't feasible for larger networks. With just 10 nodes, 45 connections would be required! And if you were to add another node, you'd have to cable this node to the other 10 machines with 10 individual stretches of cable. Clearly this approach is way too expensive!
Thus was born the modern LAN. Such a LAN provides a common medium through which all nodes connect, minimising duplication and providing a mechanism to coordinate the transmission of data.
LANs are usually based around one of three popular network architectures:
Ethernet
Of these three, Ethernet is by far the prominent type of LAN architecture used today, both in the home and in the office. A thorough description of Ethernet will be covered in the next section. For now, I will simply state that the early versions of Ethernet used a bus topology, where all nodes connected to a single shared cable, while modern Ethernet typically uses the star topology, where nodes connect to a central point such as a switch or hub. These terms will all be explained in the sections to come.
(Note that the IEEE 802.3 standard is a newer implementation of the original Ethernet standard and is not much used. Be aware that some people use the terms interchangeably, but in fact the packet structures of Ethernet and IEEE 802.3 are slightly different. If you don't understand any of this, don't worry as it will all be made clear later.)
Token Ring
Token Ring networks (which come under the IEEE - Institute of Electrical and Electronics Engineers - 802.5 standard) utilise (not surprisingly) a ring topology and transmissions are unidirectional. (Note that this topology is logical, not physical, i.e. the computers don't literally need to be physically arranged in a circle!)
Imagine a crowd of people all sitting around a table. Everyone wants to have their say, but if they all talk at the same time, the noise level goes up and nobody can understand anything that is said. One way to improve communication around our table is to give a single individual a token which he or she must hold. While holding this token, this person can make one statement, but then must pass the token on to the next person along. The person who receives the token can now have his say... and so on.
In simple terms, this is exactly how a token ring network operates. A token (which is in fact just a small piece of data) is passed from node to node along the ring network. Only the node which currently possesses the token is permitted to transmit data on the ring. Having sent a packet of data, the token is then passed on to the next node in the ring. In this way, token-passing is a mechanism of ensuring that collisions of data do not occur, and that every machine gets an equal opportunity to transmit. (A collision is a bad thing and results in loss of the information that is being passed.)
A ring topology network
FDDI
Fibre distributed data interface (FDDI) networks have big advantages over other cabling mechanisms in that they have an effective range of 200km and are immune to electromagnetic interference. This is because data is passed as light along optic fibres. Conversely, Ethernet and token ring mechanisms rely on electrical signals being passed along (usually) copper cables. As discussed previously in transmission media, such signals degrade with distance and so the effective range is much shorter. More on this later. Furthermore, FDDI allows for transmission at 100 megabits per second, some eight times quicker than Token Ring.
FDDI still uses token passing, but circumvents one of the major limitations of the Token Ring architecture previously described. In Token Ring networks, if a node fails or there is a break in the cable, the entire network fails. FDDI, on the other hand, builds in redundancy in the form of a second ring. If a break occurs in one ring (or even in both), then the two rings effectively 'join' so that a complete ring is still available. This is known as 'self-healing' and makes the FDDI much more reliable than traditional Token Ring.
The drawback of FDDI is the large expense of implementation.
What's next?
Move on to look at Ethernet and cabling.
| Just Too Good Last updated: June, 2006 (DJL)
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