Networks and Topologies

1 State two differences between a LAN and a WAN. 2

One mark for each of the following, to a maximum of 2.

• A LAN covers a small geographical area, a WAN covers a large one
• LAN hardware is owned by the organisation, WAN infrastructure is often leased from third parties

Model answer. A LAN covers a small geographical area such as a single building or site, whereas a WAN covers a large geographical area and may span cities or countries. In addition, the organisation using a LAN owns all of the hardware and cabling itself, whereas a WAN relies on infrastructure leased from telecommunications companies that the organisation does not own.

2 Explain the difference between the role of a switch and the role of a router. 4

One mark for each of the following, to a maximum of 4.

• A switch connects devices within a single network or LAN
• A switch uses MAC addresses to send data only to the intended device
• A router connects different networks together
• A router uses IP addresses to direct data between networks

Model answer. A switch operates inside a single network such as a LAN, connecting the devices on that network to one another. It reads the MAC address in each frame and forwards it only to the port where the intended recipient is connected, which avoids sending unnecessary traffic to every device. A router works at a higher level, connecting entirely different networks together, such as joining a home LAN to the internet. It uses IP addresses to decide which network a packet must be sent towards, and selects the appropriate route to get it there.

3 A small business with six computers is deciding between a peer to peer network and a client server network. Recommend which they should use and justify your answer. 6

One mark for each of the following, to a maximum of 6.

• Recommends peer to peer
• Cheaper because no dedicated server hardware needs to be bought
• Simple to set up without specialist network staff
• With only six computers the management overhead of client server is not justified
• Notes the drawback that there is no central backup
• Notes that files are unavailable if a computer is switched off, and suggests a mitigation

Model answer. The business should use a peer to peer network. With only six computers, the main advantage is cost: a peer to peer network needs no dedicated server hardware and no server software licences, so the setup cost is far lower than a client server arrangement. It is also simple enough to configure and maintain without employing a network technician, which matters for a business of this size where there is unlikely to be dedicated IT staff. A client server network would bring central backup, central security and central account management, but with six machines the administrative burden those features remove is small, so the extra expense is difficult to justify. The business should be aware of the drawbacks. There is no central backup, so each computer's files must be backed up separately and it would be easy for this to be neglected until data is lost. Files are also only available while the computer holding them is switched on, which can be inconvenient. Both problems can be reduced cheaply by adding a shared network attached storage drive or a cloud storage subscription for important files, which keeps the simplicity of peer to peer while covering its weakest point.

4 Compare a star topology and a mesh topology in terms of reliability and cost. 4

One mark for each of the following, to a maximum of 4.

• Star has a single point of failure at the central switch
• Mesh has multiple paths so data can be rerouted if a connection fails
• Star uses less cable and is cheaper to install than a full mesh
• A wired mesh requires many connections making it expensive and complex

Model answer. In terms of reliability, a mesh topology is stronger. Every device in a mesh is connected to several others, so if one link fails the data is simply rerouted along an alternative path and the network continues to function, meaning there is no single point of failure. A star topology is reliable against individual cable faults, because a broken cable only disconnects the one device attached to it, but it depends entirely on the central switch, and if that switch fails then every device loses connectivity. In terms of cost, the star topology wins clearly. It needs only one cable per device running back to the switch, whereas a full wired mesh requires a connection between every pair of devices, so the amount of cabling and the complexity of installation rise very steeply as more devices are added.

5 Explain three factors that can affect the performance of a network. 6

One mark for each of the following, to a maximum of 6.

• Bandwidth is the amount of data that can be carried per second
• Higher bandwidth allows more data to be transferred, but it is shared between users
• The number of users affects performance because bandwidth is divided between them
• Transmission media matters, with fibre optic faster and less prone to interference than copper
• Interference and errors cause data to be resent, reducing effective speed
• Latency is the delay before data arrives and affects interactive applications

Model answer. The first factor is bandwidth, which is the amount of data the connection can carry each second. A higher bandwidth connection can move more data in the same time, but bandwidth is a shared resource, so the capacity available to any one device depends on what everyone else is doing. That leads directly to the second factor, the number of users. As more devices connect and transmit at the same time, the same total bandwidth is divided between more of them, so each device receives a smaller share and everything slows down, which is why school networks feel slowest at the start of a lesson when every student logs on at once. The third factor is the transmission media. Fibre optic cable carries far more data over much greater distances than copper and is immune to electromagnetic interference because it transmits light rather than electricity, whereas copper signals degrade over distance and can be disrupted by nearby electrical equipment. Wireless is more convenient but weakens with distance and is blocked by walls, and when signal quality is poor data has to be retransmitted, which reduces the effective throughput even though the nominal bandwidth has not changed.