3.4 Connectivity Methods

Local Area Network (LAN)

A Local Area Network (LAN) is a network in which the computer systems are all located relatively close to each other, for example, in the same building or on the same site.

 

The network topologies described in the previous section (3.3 Network Characteristics) are all examples of local area networks, such as the token ring network.

 

Another type of LAN is ethernet. Technically, ethernet is actually a network protocol that controls how data is transmitted over a LAN. Unlike token ring networks ethernet LANs can be half-duplex - allowing data packets to be sent in both directions along a cable (but not simultaneously).

When a computer sends data to another system on the network it checks to see if the cable is free. If it is clear, it sends its data packets and each device checks the destination address to see if it is the intended recipient (accepting the packet if it is or passing it on if it isn't). If the cable is in use the computer delays for thousandths of a second before retrying.


If more than one data packet is placed on the network at the same time and collide the network will be fail (this is known as data collision). To recover, the LAN will use the control method CSMA/CD (Carrier Sense Multiple Access with Collision Detection) which calculates back off - a randomly generated amount of time before trying to resend the packets. Each system generates its own back off time so they don't just immediately collide again.

 

Metropolitan Area Network (MAN)

A metropolitan area network (MAN) is formed by connecting many LANs together, to cover the size of a town, city or

university campus for example. A MAN is smaller than a WAN but bigger than a LAN.

MANs are usually very efficient at providing fast communication for cities, with high connection speeds through the use of fibre optic cables.

 

A Wide Area Network (WAN) is a network spanning a large geographical area, such as multiple cities or countries.

The internet is the ultimate example of a WAN as it stretches across the entire world. There are different types of WAN rules and standards that are used:

ADSL (asymmetric digital subscriber) is a method of transferring high amounts of data across copper wire telephone lines. It is much faster than previously used traditional voice modems but, because it uses the telephone system, it requires a modem at both the sending and receiving end of the connection. While bandwidth is high with this method, is not as secure as other methods such as leased lines so a firewall and/or VPN (Virtual Private Network) would be needed to improve security. ADSL is contended which means that connection speeds are affected by other systems using the network.

Leased Line is a method of providing an uncontended, fixed-bandwidth data connection. The user maintains a dedicated connection that is more secure and, because it is uncontended, will have the same speed all of the time, regardless of how busy the network is. Bandwidth is high, security is high, connection speed is constant and the network is full duplex (allowing for data transmission both ways simultaneously but these advantages come at a high price tag - hundreds of pounds per month.

ISDN (integrated services digital network) are standards for transmitting data, such as video and voice, over traditional copper telephone wires. This method uses a circuit-switched network, similar to leased line, where all data packets take the same route between computer systems.

Wide Area Network (WAN)

Voice

 

PSTN (public switched telephone network) is the global collection of wired public telephone networks that are used to transmit data over a long distance. Using a wired connection like a PSTN is more reliable and communication will be clearer than alternatives such as cellular or satellite methods.

Cellular networks require a cell or transmitting tower to be in close proximity to the communication device. This point-to-point communication (where line of sight is necessary) can be disrupted by buildings and poor weather; cell towers in cities have an approximate range of half a mile, whilst towers in rural areas may transmit up to 10 times further. Each cell tower is also connected to the PSTN.

Satellite networks use point-to-multipoint communication by using satellites above the Earth's atmosphere that receive a

transmission and rebroadcast them back to Earth. Because of the distance between the communication device and the satellite

(roughly 45,000 miles) there is a delay between data transmission and it being received. Transmitting large files using satellites may

take longer.

 

Diagrammatical Representation

In an exam you may be asked to draw a diagram that represents how networks are connected to each other. This is different from drawing a network topology (e.g. ring or mesh) and refers to:

  • Connection type (e.g. ADSL / Leased Line).

  • Devices (e.g. router / modem),

  • Security methods (e.g. firewall / VPN)

Two LANs connected using an ADSL line

Two LANs connected using a Leased line

Rules for drawing a network diagram:

  1. Label each device and label your LANs.

  2. Use appropriate symbols and be neat. There are no set symbols, just be consistent (e.g. both modems are the same shape).

  3. ADSL must have a modem and VPN router + Firewall.
    (ADSL is not very secure so it needs to be protected with the VPN router and firewall. It also uses the telephone line across the internet so it requires a modem at both ends).

  4. Leased Line must have a router, a direct connection and CSU/DSU*.  
    (Leased line is a secure and direct connection - so it doesn’t need a firewall or VPN router and should be direct (not across the internet)).

*CSU/DSU is used to connect a router to a leased line network.

Question Corner

Local Area Network: 

a. Explain how a ethernet LAN differs to a token ring network.

b. Explain what happens when data collision occurs and how the problem is resolved.

Wide Area Network: 

a. What is the difference between a MAN and a WAN?

b. Compare and contrast ADSL and Leased Line as WAN connection methods.

c. What is ISDN?

Voice Network:

a. For each of the three types of voice networks, suggest a scenario in which each would be best to use.

b. Describe one drawback of cellular and one drawback of satellite networks.

Diagrammatical Representation:

Volcanic Games and Orient Entertainment have just been bought by SuperTech Limited. SuperTech will be linked to both company’s LANs by ADSL and the two smaller companies will have a leased line between them.

Draw a diagram to show how these networks would be connected.

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