Design Considerations

This chapter describes important design considerations and provides an example of a wireless mesh design.

Each outdoor wireless mesh deployment is unique, and each environment has its own challenges with available locations, obstructions, and available network infrastructure. Design requirements driven by expected users, traffic, and availability needs are also major design criteria. This chapter contains the following sections:

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/g 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.

Table 1. Backhaul Data Rates and Minimum LinkSNR Requirements

802.11a Data Rate (Mbps)

Minimum Required LinkSNR (dB) 


54


31


48


29


36


26


24


22


18


18


12


16


9


15


6


14

  • 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.

    • Minimum SNR refers to an ideal state of noninterference, nonnoise, and a system packet error rate (PER) of no more than 10 percent.

    • Typical fade margin is approximately 9 to 10 dB.

      Minimum Required LinkSNR Calculations by Data Rate

Table 2. Backhaul Data Rates and Minimum LinkSNR Requirements for 802.11n

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

  • 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)

Table 3. Required LinkSNR Calculations for 802.11a/g

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.

Table 4. Requirements for LinkSNR 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.

  • Number of backhaul hops is limited to eight but we recommend three to four hops.

    The number of hops is recommended to be limited to three or four primarily to maintain sufficient backhaul throughput, because each mesh access point uses the same radio for transmission and reception of backhaul traffic, which means that throughput is approximately halved over every hop. For example, the maximum throughput for 24 Mbps is approximately 14 Mbps for the first hop, 9 Mbps for the second hop, and 4 Mbps for the third hop.

  • Number of MAPs per RAP.

    There is no current software limitation on how many MAPs per RAP you can configure. However, it is suggested that you limit the number to 20 MAPs per RAP.

  • Number of controllers

    • The number of controllers per mobility group is limited to 72.

  • Number of mesh access points supported per controller.

Controller Planning

The following items affect the number of controllers required in a mesh network:

  • Mesh access points (RAPs and MAPs) in the network.

    The wired network that connects the RAP and controllers can affect the total number of access points supported in the network. If this network allows the controllers to be equally available to all access points without any impact on WLAN performance, the access points can be evenly distributed across all controllers for maximum efficiency. If this is not the case, and controllers are grouped into various clusters or PoPs, the overall number of access points and coverage are reduced.

  • Number of mesh access points (RAPs and MAPs) supported per controller. See Table 1.

    For clarity, nonmesh access points are referred to as local access points in this document.

Table 5. Mesh Access Point Support by Controller Model 

Controller Model

Local AP Support (nonmesh)1

Maximum Possible

Mesh AP Support

55082

500

500

25043

75

75

3504 150 150

WiSM2

500

500

5520 1500 1500
8540 6000 6000
1 Local AP support is the total number of nonmesh APs supported on the controller model.
2 For 5508, controllers, the number of MAPs is equal to (local AP support - number of RAPs).
3 For 2504, controllers, the number of MAPs is equal to (local AP support - number of RAPs).

Note

Mesh is fully supported on Cisco 2500, 3504, 5508, 5520, 8540 and WiSM–2 Controllers. The Base License (LIC-CT508-Base) is sufficient for indoor and outdoor APs. The WPlus License (LIC-WPLUS-SW) is merged with the base license. The WPlus License is not required for indoor mesh APs.