2.7 WIRELESS TRANSPORT

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2.7WIRELESS TRANSPORT

This is a very broad area, which could fill an entire book of its own. However, in the limited space available, I’ll try to cover the basic issues, which any operator of a wireless network will need to address.

The term wireless covers a multitude of transmission techniques and access formats. Wireless transport mechanisms can support point-to-point, point-to-multipoint and broadcast systems. The challenges associated with building wireless networks combine many of the same challenges associated with using any other media along with a whole list of unique problems linked with the air interface. Regulatory constraints, interference, obstructions and even atmospheric conditions all place extra burdens on the network designer. Each of these is discussed below.

2.7.1REGULATORY CONSTRAINTS

There are a number of different regulatory environments throughout the world, each of which has slightly different rules governing the use of particular frequencies, the maximum transmission power, even the type of service, which may be offered using particular spectrum. Some bands require the payment of licence fees and are more heavily regulated while others are either unlicensed or licence exempt. Even the licence-exempt bands are subject to certain regulations.

2.7.2INTERFERENCE

Interference refers to any signal that causes the desired signal to be indistinguishable from the surrounding noise. This can be an unwanted signal being received at a very high power level, which masks the required signal being received at a much lower power signal. Alternatively, it can be another source transmitting simultaneously on the same frequency.

2.7.3OBSTRUCTIONS

The term wireless covers both radio and optical-based systems. Optical systems clearly require line of sight between transmitter and receiver. The line of sight has the diameter equal to the diameter of the aperture of the transmitter. Any obstruction in the path will effectively stop all transmission.

The issue of what constitutes an obstruction is less clear with respect to radio systems. Certain frequencies work better than others without clear line of sight between transmitter and receiver.

18	TRANSPORT MEDIA

2.7.3.1 The Fresnel Zone

It is also important to note the exact implications of the phrase line of sight. Intuitively, you might think that line of sight for radio would be the same as line of sight for an optical system, represented as a tube with a diameter equal to that of the aperture of the transmitting antenna. However, that is certainly not the case.

In fact, there is a rugby ball-shaped (football-shaped for the American readers) zone around the centre line connecting the two antennae. This is called the Fresnel zone. In order to effectively maintain line of sight, it is necessary to ensure that there are no obstructions within 60% of the distance from the centre line to the outside surface of the Fresnel zone. So long as that constraint is met, any calculations can assume clear line of sight.

Some frequencies are particularly susceptible to obstructions; for example, transmissions at 2.4 GHz are exceptionally susceptible to trees in the path. This can be explained by taking a look at the function of microwave ovens. Microwave ovens use this frequency because 2.4 GHz microwaves are particularly well absorbed by water, causing the water molecules to vibrate and get hot. Trees are great big reservoirs of water so it is hardly surprising that they soak up transmissions at 2.4 GHz causing an attenuation of between 0.25 and 0.5 dB per metre.

This is particularly unfortunate because 2.4 GHz is almost universally licence-exempt, making it extremely popular for WLAN systems, as used in 802.11b and 802.11g. Technically, these systems can be used to transmit over distances in excess of 60 km. However, this is severely limited in the presence of trees or other obstructions.

2.7.4 ATMOSPHERIC CONDITIONS

As mentioned above, some frequencies are particularly susceptible to water in the path. Previously, we have discussed this in terms of trees, but the same applies to other sources of water in the path. Rain, fog and snow all cause significant problems for wireless transmission, both radio and optical. For example, rain imposes losses of around 0.05 dB per kilometre on a 2.4 GHz transmission. Even moderately heavy snow or fog will effectively prevent transmission of an optical system beyond a few hundred metres.

These constraints mean that wireless technologies, particularly those in the frequencies vulnerable to absorption by water, are not able to supply high priority links carrying high value traffic without back-up from a more traditional transport media, e.g. fibre or copper-based transmission media.

2.7.5 IF IT IS SO BAD . . .

Given all these issues, it might appear that building a wireless network is not worth the trouble. However, these issues have to be balanced with the ability of wireless networks to reach places where wired services just cannot go, to roll out a network in timescales and at costs within which you could barely plan a wired network.

2.7 WIRELESS TRANSPORT

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Wireless is widely seen as a possible solution to the non-availability of broadband services in rural locations. The ability to deliver point-to-point and point-to-multipoint services quickly and cost effectively to areas where there is no cable, where the copper is either of too poor quality or the exchange is too far away to deliver broadband services, is highly attractive to service providers keen to reach those customers and to customers keen to access the broadband services.

3

Router and Network

Management

Once your network is operating as expected, it is obviously essential to be able to keep it running smoothly and to know when it stops operating smoothly. Having identified that there is a failure, it is also important to be able to access the network elements affected by and surrounding the fault in order to analyse and repair the failure.

It is also important to be able to correlate events on several devices. Network Time Protocol (NTP) provides a mechanism to synchronize the internal clocks of your network devices. Since the internal clocks are used to timestamp logs, their synchronization is critical to be able to accurately correlate events.

In addition, it is essential to be able to manage the software on your network elements. This is necessary both to add functionality and to overcome faults in previous software.

Similarly, the ability to manage the configuration of your network elements will be critical to the continued smooth operation of your network. Having offline copies of your configurations and a planned regular process for backing up and restoring configurations will allow you to recover more quickly and accurately from a complete failure or loss of a network device.

3.1THE IMPORTANCE OF AN OUT-OF-BAND (OOB) NETWORK

An out-of-band network is fundamental to the continued operation of a scalable IP network. This is an independent network, which in the event of a failure of part of your network, gives you access to all network devices via an unaffected path. If part of your network

Designing and Developing Scalable IP Networks G. Davies

2004 Guy Davies ISBN: 0-470-86739-6