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1 Describe two disadvantages of fax.

2 Give three reasons for using email instead of fax. 3 Describe each of the terms:

• Discussion forum

• Thread • Post.

4 Identify two facilities of video conferencing that are not available in

teleconferencing .

5 If you wanted to send a private message to multiple recipients,

which method of communication would be best to use?

6 Describe how IRe could be used during a lesson when a teacher is absent.

Compare different types of broadband connection and

give suitable situations where the use of each would

be

appropriate: asymmetric digital subscriber

line (ADSLJ,

cable, wireless, leased line, satellite

Asymmetric digital subscriber line

Asymmetric digital subscriber line (ADSL) is when a range of frequencies are used on an existing copper telephone line to provide a broadband connection to the internet. Asymmetric

means that the downstream bandwidth is different to the

upstream bandwidth. As the vast majority of ADSL connections are used for downloading data from the internet, more

frequencies are allocated to downstream bandwidth than to upstream bandwidth. A typical ADSL line with 8 Mbps

downstream bandwidth, may only have 512 kbps or 1 Mbps

upstream bandwidth. This is why it takes longer to upload a file to a web page than to download a file and why it takes longer to

send an email than receive an email. A much smaller range of frequencies is left available for the public switched telephone

network (PSTN).

ADSL provides 'always on' internet access and does not prevent

Figure 3.28 ADSL frequency plan

o

4 kHz 25.875 kHz 138 kHz 1104 kHz

PSTN Upstream Downstream

• Distance from the telephone exchange.

• Local AM radio stations causing interference (this may reduce

the number of usable frequencies and thus reduces the

bandwidth).

• Other devices, such as fax machines, connected to the

telephone line.

• Electrical interference on the copper cable.

ADSL is mainly used in homes as no extra installation of cable is

required. While it is used in businesses, it is not favoured in large

businesses where more upstream bandwidth is required. A

common problem with ADSL is the contention ratio, which refers

to the number of subscribers that are sharing a bandwidth. A

larger contention ratio means that there is more chance of not

being able to download data using the full bandwidth - see the

section on bottlenecks on page 79.

<

Actillity

>

Look at http://www.uswitch.com/broadband/ and compare the

different ADSL broadband packages.

Cable

Broadband internet access can be provided by cable television

companies. The main infrastructure uses fibre-optic links meaning

that higher bandwidths are available. In 2009, in the UK,

downstream bandwidths of 50 Mbps were available for business

users and 20 Mbps were available for home users. Cable, like

ADSL, is asymmetric because it has different upstream and

downstream bandwidths. While some cable television providers

claim that there are no contention ratios with cable broadband,

this is not actually true because there may still be a bottleneck

when a number of households share the same fibre-optic link from

the street to the cable company.

Figure 3.29

Cable

broadband

<

Activity>

Compare

the deals

available for

mobile

broadband at

http

://

www

.b

roadbandwatchdog

.

co.uk

/

mobile-broadband-providers.php

ISP

Wireless

Since

the introduction

of

3G

on

mobile

telephone

networks,

wireless

broadband access

has become

far

more popular.

3G

was

introduc

e

d to enable

v

ideo calls

and high

bandwidth

internet

connections.

In 2009, mobile

telephone

networks

were offering

bandwidths

of

up to

7.2 Mbps, although

3G

is

capable of

providing up

to

14.4 Mbps

downstream and

5.8

Mbps

upstream,

meaning

that it

is

asymmetric.

Wireless broadband enables users to use the internet while not at

home

and

also while travelling. However, it does require

a good

3G

signa

l

to

be

availab

l

e

.

A computer

can

connect to a wire

l

ess

broadband

connection

by

either connecting through a

mobile

phone u

s

ing Bluetooth@

, a

USB

cable or

by

using a USB modem

stick.

100

---Figure 3.31 One-way satellite

broadband

Leased

line

A leased line exists between two locations only. There are no

contention ratios as the line is a direct connection between one

router and another router. This means the bandwidth is

guaranteed. The line must be installed especially for an

organisation and so this makes leased lines very expensive. Leased

lines are symmetric, meaning that there is the same amount of upstream bandwidth available as downstream bandwidth.

Therefore leased lines are preferred by large businesses and

organisations providing access into their own networks using an

extranet.

Many local education authorities have a 'Grid for Learning', which

provides each school with a leased fibre-optic line that connects to

the local authority. In 2009, the Birmingham Grid for Learning

was providing 10 Mbps leased lines to all of its schools. Secondary

schools have hundreds of computers that could be accessing the

internet at the same time, including multimedia applications that

use up a lot of bandwidth. Some local authorities are now looking

at the possibility of upgrading the 10 Mbps connections to

100 Mbps. A bottleneck can still exist between a local authority

and the internet though, particularly if lots of schools are

downloading lots of data at once.

Satellite

Satellite broadband is used in remote areas where physical and

wireless connections are not available. These areas could include

some locations in the countryside or ships at sea. There are two

types of satellite broadband:

• One-way satellite broadband uses a satellite connection for

downloading data from the internet, but uses a modem and

telephone line for uploading data.

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Figure 3.32 Two-way satellite

broadband

ADSL

Bandwidth 24 Mbps

Cost Cheap

Asymmetric/symmetric Asymmetric

•

Two-way satellite broadband allows both downloading and uploading of data through the satellite connection. It is

asymmetric meaning downstream has a higher bandwidth than

upstream (like ADSL).

Satellite dish (two-way) Satellite

---

~

Satellite modem

,

Internet Computer Satellite up/down link

Satellite broadband uses geosynchronous satellites which are

approximately 37,000 km above the Earth. This means that there is a latency (delay) between data being sent and data being received of about 0.7 seconds. This is not too much of a problem

for downloading data and sending/receiving emails, but it means interactive applications, such as internet gaming, are not

practically possible. Using this method for video conferencing can

also be very frustrating due to the delay.

Line of sight is required, so low Earth orbit satellites cannot be used as they do not remain in a fixed point above the Earth. This table compares the different types of broadband connection using 2009 data.

Cable Wireless Leased line Satellite

50 Mbps 14.4 Mbps 100 Mbps 20 kbps

Cheap Medium Highest High

Asymmetric Asymmetric Symmetric Asymmetric

Connection type Copper cable Fibre-optic Mobile phone Fibre optic Microwaves

backbone radio waves

Contention Yes Yes Yes No Yes (very high

_Q_U_E_S_T_IO_N_S __________________________________ ___

1 What does ADSL stand for?

2 What does asymmetric mean?

3 Describe two factors that could affect the bandwidth available on ADSL lines.

4 Why are cable broadband operators able to offer higher bandwidths?

S How can internet access be obtained using a laptop without a cable and no wireless access point near by?

6 Why is bandwidth guaranteed on a leased line?

7 How might a bottleneck occur when using leased lines?

8 Why a user want to use satellite broadband?

9 Describe the term 'latency'.

Describe how a mobile phone network operates (cellular

and satellite) and the advantages and disadvantages of

cellular and satellite mobile phone systems and their use

Cellular phone network

How it works

Cellular telephone networks are named as such because the area covered by the network is divided into' cells'. A cellular telephone network consists of a number of components that enable mobile

communication to take place:

• Mobile stations (MS) (e.g. a mobile phone).

• Cells that have base station transceivers (BSn - often known as

a mobile phone mast that gives signal coverage to a cell.

• Mobile switching centre (MSC) - this controls all the calls

taking place on the mobile telephone network.

• Base station controller (BSC) - this manages the

communication between a set of BSTs and the MSC.

• Public switched telephone network (PSTN) - when a call is being made from one mobile phone company to another mobile phone company or to a landline, then the call needs to go through the PSTN.