Wireless Mesh Constraints
The following are a few system characteristics to consider when you design and build a wireless mesh network. Some of these characteristics apply to the backhaul network design and others to the CAPWAP controller design:
Wireless Backhaul Data Rate
Backhaul is used to create only the wireless connection between the access points. The backhaul interface is 802.11a/n/ac depending upon the access point. The rate selection is important for effective use of the available RF spectrum. The rate can also affect the throughput of client devices, and throughput is an important metric used by industry publications to evaluate vendor devices.
Dynamic Rate Adaptation (DRA) introduces a process to estimate optimal transmission rate for packet transmissions. It is important to select rates correctly. If the rate is too high, packet transmissions fail resulting in communication failure. If the rate is too low, the available channel bandwidth is not used, resulting in inferior products, and the potential for catastrophic network congestion and collapse.
Data rates also affect the RF coverage and network performance. Lower data rates, for example 6 Mbps, can extend farther from the access point than can higher data rates, for example 1300 Mbps. As a result, the data rate affects cell coverage and consequently the number of access points required. Different data rates are achieved by sending a more redundant signal on the wireless link, allowing data to be easily recovered from noise. The number of symbols sent out for a packet at the 1-Mbps data rate is higher than the number of symbols used for the same packet at 11 Mbps. Therefore, sending data at the lower bit rates takes more time than sending the equivalent data at a higher bit rate, resulting in reduced throughput.
A lower bit rate might allow a greater distance between MAPs, but there are likely to be gaps in the WLAN client coverage, and the capacity of the backhaul network is reduced. An increased bit rate for the backhaul network either requires more MAPs or results in a reduced SNR between MAPs, limiting mesh reliability and interconnection.
Note |
The data rate can be set on the backhaul on a per AP basis. It is not a global command. |
The required minimum LinkSNR for backhaul links per data rate is shown in Table 1.
802.11a Data Rate (Mbps) |
Minimum Required LinkSNR (dB) |
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The required minimum LinkSNR value is driven by the data rate and the following formula: Minimum SNR + fade margin.
Table 2 summarizes the calculation by data rate.
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Minimum SNR refers to an ideal state of noninterference, nonnoise, and a system packet error rate (PER) of no more than 10 percent.
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Typical fade margin is approximately 9 to 10 dB.
Minimum Required LinkSNR Calculations by Data Rate
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802.11n Date Rate (Mbps) |
Spatial Stream |
Minimum Required LinkSNR (dB) |
---|---|---|
15 |
1 |
9.3 |
30 |
1 |
11.3 |
45 |
1 |
13.3 |
60 |
1 |
17.3 |
90 |
1 |
21.3 |
120 |
1 |
24.3 |
135 |
1 |
26.3 |
157.5 |
1 |
27.3 |
30 |
2 |
12.3 |
60 |
2 |
14.3 |
90 |
2 |
16.3 |
120 |
2 |
20.3 |
180 |
2 |
24.3 |
240 |
2 |
27.3 |
270 |
2 |
29.3 |
300 |
2 |
30.3 |
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If we take into account the effect of MRC for calculating Minimum Required Link SNR. Table 3 shows the required LinkSNR for 802.11a/g (2.4 GHz and 5 GHz) for AP1552 and 1522 with 3 Rx antennas (MRC gain).
LinkSNR = Minimum SNR - MRC + Fade Margin (9 dB)
802.11a/g MCS (Mbps) |
Modulation |
Minimum SNR (dB) |
MRC Gain from 3 RXs (dB) |
Fade Margin (dB) |
Required Link SNR (dB) |
---|---|---|---|---|---|
6 |
BPSK 1/2 |
5 |
4.7 |
9 |
9.3 |
9 |
BPSK 3/4 |
6 |
4.7 |
9 |
10.3 |
12 |
QPSK 1/2 |
7 |
4.7 |
9 |
11.3 |
18 |
QPSK 3/4 |
9 |
4.7 |
9 |
13.3 |
24 |
16QAM 1/2 |
13 |
4.7 |
9 |
17.3 |
36 |
16QAM 3/4 |
17 |
4.7 |
9 |
21.3 |
48 |
64QAM 2/3 |
20 |
4.7 |
9 |
24.3 |
54 |
64QAM 3/4 |
22 |
4.7 |
9 |
26.3 |
If we consider only 802.11n rates, then Table 4 shows LinkSNR requirements with AP1552 for 2.4 and 5 GHz.
No. of Spatial Streams |
11n MCS |
Modulation |
Minimum SNR (dB) |
MRC Gain from 3 RXs (dB) |
Fade Margin (dB) |
Link SNR (dB) |
---|---|---|---|---|---|---|
1 |
MCS 0 |
BPSK 1/2 |
5 |
4.7 |
9 |
9.3 |
1 |
MCS 1 |
QPSK 1/2 |
7 |
4.7 |
9 |
11.3 |
1 |
MCS 2 |
QPSK 3/4 |
9 |
4.7 |
9 |
13.3 |
1 |
MCS 3 |
16QAM 1/2 |
13 |
4.7 |
9 |
17.3 |
1 |
MCS 4 |
16QAM 3/4 |
17 |
4.7 |
9 |
21.3 |
1 |
MCS 5 |
64QAM 2/3 |
20 |
4.7 |
9 |
24.3 |
1 |
MCS 6 |
64QAM 3/4 |
22 |
4.7 |
9 |
26.3 |
1 |
MCS 7 |
64QAM 5/6 |
23 |
4.7 |
9 |
27.3 |
2 |
MCS 8 |
BPSK 1/2 |
5 |
1.7 |
9 |
12.3 |
2 |
MCS 9 |
QPSK 1/2 |
7 |
1.7 |
9 |
14.3 |
2 |
MCS 10 |
QPSK 3/4 |
9 |
1.7 |
9 |
16.3 |
2 |
MCS 11 |
16QAM 1/2 |
13 |
1.7 |
9 |
20.3 |
2 |
MCS 12 |
16QAM 3/4 |
17 |
1.7 |
9 |
24.3 |
2 |
MCS 13 |
64QAM 2/3 |
20 |
1.7 |
9 |
27.3 |
2 |
MCS 14 |
64QAM 3/4 |
22 |
1.7 |
9 |
29.3 |
2 |
MCS 15 |
64QAM 5/6 |
23 |
1.7 |
9 |
30.3 |
Note |
With two spatial streams, the MRC gain is halved, that is the MRC gain is reduced by 3 dB. This is because the system has 10 log (3/2 SS) instead of 10 log (3/1 SS). If there were to have been 3 SS with 3 RX, then the MRC gain would have been zero.
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