Chapter 2: Communication and networking technologies

Question 1

Today, a typical WAN is characterised by the following.

Answer 1

• It will be used by an organisation or a company to connect sites or branches.
• It will not be owned by the organisation or company.
• It will be leased from a public switched telephone network company (PSTN).
• A dedicated communication link will be provided by the PSTN.
• The transmission medium will be fibre-optic cable.
• Transmission within the WAN will be from switch to switch.
• A switch will connect the WAN to each site.
• There will not be any end-systems connected directly to the WAN.

Question 2

Wide area network (WAN)

Answer 2

a network connecting computers on different sites, possibly thousands of kilometres apart

Question 3

Local area network (LAN)

Answer 3

a network connecting computers in a single room, in a single building or on a single site

Question 4

Today, a typical LAN is characterised by the following.

Answer 4

It will be used by an organisation or a company within a site or branch.
* It will be owned by the organisation or company.
* It will be one of many individual LANS at one site.
* The transmission medium will be twisted pair cable or WiFi.
* The LAN will contain a device that allows connection to other networks.
* There will be end-systems connected which will be user systems or servers.

Question 5

The client-server model

Answer 5

The client-server model (or architecture) was first used in large organisations when they
had installed internal networks. Typically, the organisation would have individual LANs connected via an organisation-wide WAN. An individual LAN might have had an application server attached. The organisation was likely to need a powerful central computer. The central computer could be connected to the WAN as a server. It would probably not have individual users connected to it directly. A PC, attached to a LAN, could access the server as a client.
The client-server mode of operation nowadays is different. The client is a web browser connected to the Internet. The server is a web server hosted on the Internet.
The server provides an application and the client uses the application. There are two options for how the client functions.

Question 6

Client-server

Answer 6

an architecture where a client runs an application provided by a server on a network

Question 7

Thin-client

Answer 7

a client that only provides input and receives output from the application

• chooses an application to run on the server
• sends input data to the server when requested by the application
• receives output from the application.

Question 8

thick-client

Answer 8

a client that carries out at least some of the processing itself

• chooses an application provided by the server
• possibly carries out some processing before running the application on the server and also after receiving output from the application
• alternatively, possibly downloads the application from the server and runs the application itself.

In thick-client mode the processing on the client can be controlled by the use of a scripting language.

Question 9

The client-server approach is the choice in the following circumstances.

Answer 9

The server stores a database which is accessed from the client system.
* The server stores a web application which allows the client system to find or, sometimes, supply information.
* The server stores a web application which allows the client system to carry out an e-commerce or financial transaction.

Question 10

File sharing

Answer 10

If a user uploads files to a file server then the client-server operation can be used by another user to download these from the server.
An alternative mode of operation for sharing files is peer-to-peer networking. Instead of having one server that many clients access, a peer-to-peer network operates with each peer (networked computer) storing some of the files. Each peer can therefore act as a client and request a file from another peer or it can act as a server when another peer requests the download of a file.

Question 11

peer-to-peer advantages

Answer 11

it avoids the possibility of congestion on the network when many clients are simultaneously attempting to download files
* parts of a file can be downloaded separately
* the parts are available from more than one host.

Question 12

client-server model advantages

Answer 12

It allows an organisation to control the downloading and use of files.
* The files can be better protected from malware attacks because the files are stored on
one server which will be regularly scanned using appropriate anti-virus software.

Question 13

There are five requirements for a data communications system:

Answer 13

a sender, a receiver, a transmission medium, a message and a protocol

Question 14

A transmission medium can be

Answer 14

air (e.g. for WiFi) or cables (e.g. for Ethernet)

Question 15

Data can be sent through the medium in different modes:

Answer 15

simplex mode where data flow is one-way only
* half duplex where data can flow either way but not simultaneously
* full duplex where simultaneous both-ways data flow is possible.

Question 16

A ‘message’ is any type of data, which can be sent as either:

Answer 16

• a broadcast, which is a one-to-all communication (as used traditionally for radio and television)
• a multicast, which is from one source to many destinations
• a unicast, which is a one-to-one communication.

Question 17

The simplest topology

Answer 17

point-to-point connection, which is a dedicated link. Transmission might be simplex or duplex and a message can only be unicast.
where two systems are connected by a network link

Question 18

Bus topology

Answer 18

contains one shared link to which all devices are attached

Early LAN topologies used either a ring or a bus topology. We don’t need to cover the ring topology as it is not used very often now. A bus topology has only one link but it is shared
by a number of end-systems and is therefore described as a multi-point connection. The configuration is shown in Figure 2.02. There is no direct connection between any pair of end- systems. A message must therefore be broadcast even though it might only be intended for one end-system. The topology is resilient because a fault in an end-system or in the link to it does not affect the use of the network by the other end-systems.

Question 19

End-system

Answer 19

a computer or server connected to a network

here is no direct connection between any pair of end- systems. A message must therefore be broadcast even though it might only be intended for one end-system. The topology is resilient because a fault in an end-system or in the link to it does not affect the use of the network by the other end-systems.

Question 20

Mesh topology

Answer 20

contains direct links between devices

An example of a fully-connected mesh topology is shown in Figure 2.03. In this configuration, each end-system has a point-to-point connection to each of the other end- systems. Transmission is duplex; messages might be unicast, multicast or broadcast.

Question 21

Topology

Answer 21

the configuration of a network that defines how the various devices on the network are connected

A data communications system may consist of a single isolated network. There are several possibilities for the topology of an isolated network.

Question 22

Star topology

Answer 22

each end-system is linked to a central device

Figure 2.04 could have been drawn so that it looked like a star but has been drawn to show the physical configuration that is used in a real life installation. In a star topology, each end- system has a point-to-point connection to the central device. Transmission is duplex and messages from the central device might be unicast, multicast or broadcast. As with the bus topology, the failure of an end-system, or its link, leaves the other end-systems unaffected. However, the central device must not fail.
In the bus topology most of the end-systems might be user workstations and the others are servers. However, in the star topology, the end-systems might be user workstations or servers but the central device is different. It is a specialised device with the purpose of connecting other devices in the network. Currently, the star topology is the usual way to configure a network. There are several reasons for this. The most important is that the central device can be used to connect the network to other networks and, in particular, to the Internet.

Question 23

Hybrid network

Answer 23

a collection of connected LANs where some of them have different topologies or supporting technologies

In a situation where several LANs are connected, they can have different topologies or supporting technologies. This collection of LANs then becomes a hybrid network. A special connecting device is needed to ensure that the hybrid network is fully functional. It is often an advantage to be able to connect a new topology LAN to existing LANs where it is not sensible or not possible to use the existing topology for the new LAN. An example is when a wired LAN is already installed but a new wireless LAN is to be connected to it.

Question 24

Cable

Answer 24

a transmission using copper wire or fibre-optic

A network cable can be twisted pair, coaxial or fibre-optic. The twisted pair and coaxial cables both use copper for the transmission medium. In discussing suitability for a given application there are a number of factors to consider. One of these is the cost of the cable and connecting devices. Another is the best bandwidth that can be achieved. The bandwidth governs the possible data transmission rate. There are then two factors that can cause poor performance: the likelihood of interference affecting transmitted signals and the extent of attenuation (deterioration of the signal) when high frequencies are transmitted. These factors will dictate whether repeaters or amplifiers are needed in transmission lines and how many will be needed. Table 2.01 shows some comparisons of the different cable types.
You need to understand that for each of the three types of cabling there are defined standards for different grades of cable which must be considered when you decide which type of cable to use. Fibre-optic cable performs best but costs more than the other kinds. For a new installation the improved performance of fibre-optic cable is likely to be the factor that governs your choice. However, where copper cable is already installed the cost of replacement by fibre-optic cable may not be justified.
Currently, twisted pair cable is normally used to connect telephone handsets to telephone lines. This type of cable is illustrated in Figure 2.05. It is also the technology of choice for high- speed local area networks.

Coaxial cable is used extensively by cable television companies and in metropolitan area networks. It is not usually used for long-distance telephone cabling. Fibre-optic cable is the technology of choice for long-distance cabling. As shown in Figure 2.06, coaxial cable is not bundled but a fibre-optic cable contains many individual fibres.

Question 25

Bandwidth

Answer 25

a measure of the amount of data that can be transmitted per second

Question 26

Twisted pair

cable comparison

Answer 26

Cost LOWEST
Bandwidth or data rate LOWEST
Attenuation at high frequency AFFECTED
Interference WORST AFFECTED
Need for repeaters MORE OFTEN

Question 27

Coaxial

cable comparison

Answer 27

Cost HIGHER
Bandwidth or data rate HIGHER
Attenuation at high frequency MOST AFFECTED
Interference LESS AFFECTED
Need for repeaters MORE OFTEN

Question 28

Fibre optic

cable comparison

Answer 28

Cost HIGHEST
Bandwidth or data rate MUCH HIGHER
Attenuation at high frequency LEAST AFFECTED
Interference LEAST AFFECTED
Need for repeaters LESS OFTEN

Question 29

Wireless

Answer 29

a transmission using radio, microwave or infrared

The alternative to cable is wireless transmission. The three options here are radio, microwave or infrared. These are all examples of electromagnetic radiation; the only intrinsic difference between the three types is the frequency of the waves.