Avoiding noise

The unlicensed ISM and U-NII bands represent a very tiny piece of the known electgromagnetic spectrum . Since this region can be utilized without paying license fees , many consumer devices use it for a wide range of applications . Cordless phones , analog video senders , Bluetooth , baby monitors , and even microwave ovens compete with wireless data networks for use of the very limnited 2.4 GHz band . These signals , as well as othr local wireless networks , can cause significant problems for long range wireless links . here are some steps you can use to reduce reception of unwanted signals .

- Increase antenna gain on both sides of a point-to-point link . Antennas not only add gain to a link , but their increased directionalityu tends to reject noise from areas around the link . Two high gain dishes that are pointed at each other will reject noise from directions that are outside the path of the link . Using omnidirectional antennas will receive nolise from all directions

- Use sectorials instead of using an omnidirectional . By making use of several sectorial antennas , you can reduce the overall noise received at a distribution point .
By staggering the channels used on each sectorial , you can also increasethe available bandwidth to your clients .

- Dont use an amplifier . As we will see in Chaptr 4 , amplifiers can make interference issues worse by indiscriminately amplifying all received signals , including sources of interference . Amplifiers also cause interference problems for other nearby users of the band .

- Use the best available channel . Remember that 802.11b/g channels are 22MHz wide , but are only separate by 5 MHz .Perform a site survey , and select a channel that is as far as possible from exisiting sources of interference . Remmeber that the wireless alndscape can change at any time  as people add new devices ( cordless phones , other netwrks , etc . ) if your link suddenly has trouible sending packets , you may need to perfrom an other site survey and pick a different channel .

- Use smaller hops and repeaters , rather than a single long distance shot . Keep your point-to-point links a short as possible . While it may be posible to create a 12km  links that cuts across the middle of a city , you will likely have all kinds of interference problems . if ypou can break that link into two or three shorter hops  , the link will likely be more stable . Obviously this isn't possible on long distance rural links where power and mounting structuress are unabialable , but noise problems are also unlikely in those settings .

- If  possible , use 5.8 Ghz , 900Mhz , or another unlicensed band . While this is only a short term solution , there is currently far more consumer equiment installed in the field that uses 2.4Ghz . Using 802.11a  or a 2.4GHz to 5.8 GH step-up device will let you avoid this congestion altogether . if you can find it  , some old 802.11 equipment uses unlicensed spectrum at 900MHz ( unjfortunately at much lower bit rates 0 / Other technologies , such as Ronja use optical technology for short distance , noise - free links .

- If all else fails , use licensed spectrum . These are places where all available unlicensed spectrm is effectively used . In these cases , itmay make sense t spend the additional mone for proprietary equipment that uses a less congested band . for long distance poin-to-point  links that require very high throughput and maximum uptime , this is certainly an potion . of course , thjese features come at a much higher price tag compared to unlicensed equipment .

To indentify source of noise , you need tools that will show you what is happening in the air at 2.4 GHz . We will see some exaples of these tools in Chapter 6.