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Explain the importance of bandwidth when transmitting data and how different types of communication media (cables, wireless, optical) govern the bandwidth available (knowledge of examples of different communication media is expected)

Figure 3.9 A bandwidth analogy

Bandwidth Bandwidth measures how much data can be transferred along a communications channel. The more frequencies available to the communications channel, the more data that can be transferred at once. While bandwidth is officially measured as a frequency (Hz), it is more generally reported in bits per second (bps). For example, a bandwidth of 8 Mbps means that 8 megabits of data can be transferred at once every second. Megabits are often confused with megabytes. A byte consists of eight bits. Therefore, a bandwidth of 8 Mbps is equivalent to one megabyte per second.

A higher bandwidth means that more data can flow per second which has a positive effect on the speed of data transfer as data is likely to arrive more quickly.

A way of understanding bandwidth is to consider a busy motorway. In Figure 3.9, you can see how four lanes of traffic are stuck in a traffic jam. The motorway has a 'bandwidth' of four cars. In other words, four cars can travel at once along the motorway. If the number of lanes was increased to six, then six cars could travel at once along the motorway. The same applies with bandwidth. The more frequencies (lanes) available on a channel (motorway), then the more data (cars) that can travel at once.

Figure 3.10 The bottleneck analogy

Figure 3.11 The original bottleneck

Figure 3.12 An internet 'bottleneck'

Router

< Activity > Find out what bandwidth you have available by visiting http://www.speedtest.net/ Try this at school and at home. Also try it at different times of the day. Now try downloading a large file and measure how long it takes per megabyte. Then divide that figure by eight and this will tell you how many bits per second you were transferring. You will probably notice that this number is lower than your bandwidth. Why do you think this is the case?

Bottlenecks You may have found using the activity above that your bandwidth is higher than your download speed. This is caused by 'bottlenecks' somewhere within the internet. A bottleneck is the smallest bandwidth that exists between the user and the place that data is being downloaded from.

Imagine some road works on a motorway. There are normally three lanes of traffic but for half a mile, there are only two lanes due to a lane closure. Before the closure, three cars were able to travel at once, now only two cars can. This causes a 'bottleneck'.

The term 'bottleneck' comes from the idea that water can only pour out through the neck (the narrowest part) of the bottle. This effectively slows down the flow rate.

Similarly, with communication channels, bottlenecks are caused by the narrowest bandwidth.

In Figure 3.12, the user is connected to their own ISP by an 8 Mbps bandwidth. The user is downloading a file from a website that is connected using a 2 Mbps bandwidth. Therefore, the bottleneck is 2 Mbps, meaning that the user can only download the data from the website at a maximum rate of 2 Mbps. If lots of people are downloading at once from the same website, then the 2 Mbps is shared among those users and so the effective download rate will be much lower.

Importance of bandwidth The importance of bandwidth depends on the application that the user is running and how much data needs to be downloaded in a short space of time. If a user only sends an occasional email with no attachments, then a small bandwidth (e.g. 56 kbps through a modem) is perfectly acceptable.

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2 Mbps

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However, if a user is participating in a video conference, then lots of data needs to be transferred every second, meaning that a high bandwidth is required, The video and sound associated with video conferences consist of large packets of data. If these are not delivered on time, then the video conference will have time lags resulting in broken or missing pictures and sound.

If the user is listening to live radio (streamed), then a high bandwidth is required in order that the sound is delivered on time and so that breaks are not experienced within the broadcast.

However, if the user is downloading a pre-recorded radio programme, then it does not matter as much how long it takes to download as it can be listened to once the whole programme has been downloaded. This may be inconvenient to the user in terms of having to wait, but it does not affect the final user experience of listening to the programme.

The importance of bandwidth is therefore important when accessing content that is being delivered in real time (i.e. it is live). Without a high bandwidth, images and sound are broken up, become out of sync and freeze, making the conversation, viewing or listing very difficult. However, if the content is not live, then it can be downloaded for later listening or viewing and, even though it may take a long time to download, once it is downloaded it will be able to be listened to or watched without distortion. If a user wants to download something quickly, so they can turn their computer off, then bandwidth is important.

_Q_U_E_S_T_IO_N_S ______________________________________ _

1 How many megabytes can be downloaded each second at a bandwidth of 16 Mbps?

2 Describe the term bandwidth. 3 If a user has an 8 Mpbs connection to the internet, why might it take

30 seconds to download 1 M8 of data? 4 Give two examples of when it is important to have a high

bandwidth 5 Why is bandwidth unimportant if downloading a television

programme from the internet to watch at a later time?

Cable Copper cable is one option for transmitting data. Older networks used co-axial (coax) cables which consisted of two wires. Coax cables are still used within the television industry due to its capacity for high bandwidths. The outer cable acts as a shield to electromagnetic interference which reduces signal loss.

Figure 3.14 Twisted pair cable

Figure 3.15 An Ethernet cable and connector (with twisted pair wires visible)

Figure 3.16 Bundled optical cable

Copper cables in modern networks are usually in the form of twisted pairs of wire. These are narrow strands of wire, insulated with plastic, then twisted together. Most of these cables are unshielded which means there is no earth wire - this is known as unshielded twisted pair (UTP) or more commonly as Ethernet cables. This can result in data packets being lost, particularly data being transmitted at higher frequencies.

There are also Shielded Twisted Pair cables (STP) where each pair of wires has a metal shield which reduces electromagnetic interference. These cables are sometimes used in high-speed networks where higher bandwidths are required.

Copper cable is used within local area networks and can support data transmission of up to 1 Gbps using standard CAT Se cables (four twisted pairs). CAT 6 and CAT 7 cables can transmit up to 10 Gbps.

While copper cables are cheaper than optical cables, they are limited to a maximum of 100 metres. Beyond this the data signal becomes too weak. The bandwidth is limited by the frequencies that the copper cable is able to cope with. Copper cables are also prone to electrical interference which results in lost data packets that will reduce the overall transmission rate.

< Activity > Fill in the table to compare the different types of cables available.

Co-axial UTP STP

Main use

Shielding

Number of cable cores

Optical Fibre-optic cables are minute glass tubes that reflect light along the length of the tube. They are used where more than 100 metres of cable is required and in modern cable TV networks. Fibre optics are not susceptible to electrical interference and suffer very little loss of data, meaning that the bandwidths available are much higher than copper. The bandwidth available is increasing all the time as technology develops. In 2000, Siemens carried out a successful test of transmitting 7.04 Tbps (terra bits per second) along a single fibre-optic cable. In 2008, Virgin Media were offering 20 Gbps broadband connections to home users using fibre-optic cable.

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Wireless Wireless communication can consist of many methods, such as radio, satellite and infrared . These are explained in more depth later in this chapter. There is a smaller range of frequencies available for wireless transmission than for copper cable or fibre optics, which means that the bandwidth available is less with wireless communication methods. Further to this, obstacles such as walls and steel can interrupt wireless signals causing lost packets and wireless is susceptible to other interference which reduces the transmission rate.

_Q_U_E_S_TI_O_N_S __________________________________ ___

1 Why can't copper cable be used over long distances? 2 What is the difference between UTP and STP? 3 In what circumstances might STP be used instead of UTP? 4 Why are fibre.optic bandwidths higher than copper bandwidths? 5 Why are wireless bandwidths lower than cable bandwidths? 6 Why might wireless signals be weakened?