Minimizing Noise from Your DSSS Network

Alter you succeosfully deploy your network1 o i s likel- tha t the demand for service on your network will increase. This means that you will add more users to your existing AP and you might also add new APs. As you add APs, your risk of self-interference increases because tho number of nonoverlapping c hannels -hat you can use in uny g eographical area is limited. tor example, on the 2.4 GHz band in the United States, you can use a maximum of three nonoverlapsikg c han noisc chan rtels 1, -, and NO.

To eliminate self-interference betwee n yo ut AP se ctors, make a ftequency plan before you deploy your network. The frequency plan should show all of the following information:

• Geographin a tea cov ored by each AP sector

• Frequ ency nsed in each sector

• Antenna polarization psed in ea ch se cto r

• Uquivalent isodropic radiated 0ower (EIRP) used in each sector

If your plan uses more than three frequencies at the same AP location or if two or more of you-AP sectoro are wi rhin wireless range oceach other, you need to reosk (share) one or more channels. To rouse channels, design your antenna systems so that there is at least 121 dB of isolation -w^ess signsl ¡oss) between -Irem. Wlqen you have th is mvah isolation, the receiver in one sector can no longer hear the transmitter from the other sector. Now, you can use the same frequency on borh snnto w (or on e sector and one wireless back haol link) wi thout causing s elf-mterference.

Figure 8-2 prov^es an example of a frequency plan tlcat requires one frequenny to be reused.

Figure 8-2. Example of a Frequency Plan

i\ Channel 6-24 GHz

Horizonial Rbtarfzation

i\ Channel 6-24 GHz

Horizonial Rbtarfzation

Figure 8-2 shows one AP with three point-to-multipoint sectors and one point-to-point wireless backbone link. This AP requires a total of four frequencies; however, at 2.4 GHz, there are only three non-ove clapping Treq uencies (cha nneis 1, 6, and 11). You c an use either one of the following two solutions for this problem:

Figure 8-2 shows one AP with three point-to-multipoint sectors and one point-to-point wireless backbone link. This AP requires a total of four frequencies; however, at 2.4 GHz, there are only three non-ove clapping Treq uencies (cha nneis 1, 6, and 11). You c an use either one of the following two solutions for this problem:

• Antenna system isolation— Isolate two of the antenna systems to allow the same frequency to be reused simultaneously for two different purposes; for example, one point-to-multipoint sector and the backhaul might be isolated and on the same frequency. Use the techniques summarized in the "Interference from another radio or from another AP ¡s your network" row of Table 8-1. These techniques are described in more detail in the "Noise Reduction from Your Network" section of Chapter 5.

Different frequency bands— Move one sector (or move the backbone link) to a differen-

frequency band. For example, you could move the backbone link to the 5-GHz band. Now all three non-overlapping 2.4-GHz frequencies are available for use, one per sector, with no interference and no throughput loss from the backbone link.

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