" c " E. " u. Asymmetric digital subscriber line

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1 Ul ';:; " co u ' " E E " u " c co I iii! M 1 Describe two disadvantages of fax. 2 Give three reasons for using 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. ompare 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 lin e 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 than receive an . 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 the telephone from being used at the sa me time. In 2009, ADSL was available at up to 24 Mbps. In order to be able to use ADSL, a subscriber needs to be connected to a compatible telephone exchange within a workable distance. A number of factors can affect the available bandwidth: 98

2 o 4 khz khz 138 khz 1104 khz Figure 3.28 ADSL frequency plan 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. Look at and compare the different ADSL broadband packages. able < Actillity > 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. able, 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., 3: c o 111 '" 'iii :E 111 S. "ti '" 111 'H c 8 "ti.! "ti '- 111 a. '- ' 0,a.. a. o 111, g;:e g '" c " ", '- u.!! 111 II) "ti. 0 '- 111 a.", E U o, 99

3 ISP Figure 3.29 able broadband < Activity> ompare the deals available for mobile broadband at Wireless Since the introduction of 3G on mobile telephone networks, wireless broadband access has become far more popular. 3G was introduced to enable video 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 signal to be available. A computer can connect to a wireless broadband connection by either connecting through a mobile phone using Bluetooth@, a USB cable or by using a USB modem stick. 100 _-"--Ll gu re33.(lwi reless_broadba",dl

4 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. 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. dish (one-way) ite,,,, '" " III Q '0; '".t! III :t; '". III '". "0 '" Q 'H '" c Q u "0", c.!!! "Or a. Q Q r r 0 a.. a. Q ",0 III '" i::e '" Q ;:: "'", r u -flo :t: Q) "0. '" Q r III '" a.", E Q", '" U Figure 3.31 One-way satellite broadband Telephone socket up/down link 101

5 c 0 '':; II U 'c " E E 0 U 't:i II. 0 2 ') Two-way satell ite 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). dish (two-way) modem, Internet Figure 3.32 Two-way satellite broadband omputer up/down link 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 s, 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. Bandwidth ost ADSL 24 Mbps heap Asymmetric/ symmetric Asymmetric able Wireless Leased line 50 Mbps 14.4 Mbps 100 Mbps 20 kbps heap Medium Highest High Asymmetric Asymmetric Symmetric Asymmetric onnection type opper cable Fibre-optic Mobile phone Fibre optic Microwaves backbone radio waves ontention Yes Yes Yes No Yes (very high ratio) 102

6 _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 ab le 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 ellular phone network How it works ellular 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). ells that have base station transceivers (BSn - often known as a mobile phone mast that gives signal coverage to a cell. Mobile switching centre (MS) - this controls all the calls taking place on the mobile telephone network. Base station controller (BS) - this manages the communication between a set of BSTs and the MS. 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. When a phone call is placed from a mobile phone to a landline phone, a number of actions take place, which include the following: " :s c I ra 'iii.c "''' E :l.c> " Q.c a. ni:: IQ Q 8"E I a= 3:Gi c " Q.c I a. ""S :g= E. Q 3: :fl Q B Q '".c c Q ' ii u. " 103