Best Practices for Voice over Wi-Fi – Part 3

Matt V.
New Contributor

3. Signal Strength

 

To provide reliable service, wireless networks should be engineered to deliver adequate signal strength in all areas where the wireless telephones will be used. The required minimum signal strength for all handsets depends on the 802.11 frequency band it is operating in, modulation used, data rates enabled on the AP, and data rate used by the handset at any particular time. 

 

Recommended signal strength characteristics are summarized in Table 1 and Table 2.  Use these values to determine RF signal strength at the ‘limit of AP A’ or ‘limit of AP B’, illustrated in Figure 1.  The handset should be in the assessment area for 2–3 seconds to allow for smooth roaming handoffs.

 

 

2.4GHz 802.11b/802.11g (CCK)

2.4GHz 802.11g (OFDM)

Rate (Mb/s)

1

2

5.5

11

6

9

12

18

24

36

48

54

Best Practices (dBm)

-75

-70

-69

-65

-67

-66

-64

-62

-60

-56

-52

-47

 

Table 1 – 2.4GHz

 

 

 

5GHz 802.11a (OFDM)

5GHz 802.11n (OFDM)

Rate (Mb/s)

6

9

12

18

24

36

48

54

6.5

13

19.5

26

39

52

58.5

65

Best Practices (dBm)

-67

-65

-63

-61

-58

-54

-52

-50

-67

-65

-63

-61

-58

-54

-52

-50

 

  

 

 

 

5GHz 802.11a (OFDM)

Rate (Mb/s)

6

9

12

18

24

36

48

54

Best Practices (dBm)

-60

-59

-58

-56

-53

-49

-47

-45

 

Table 2 – 5GHz

 

The critical factor is the lowest data rate set to “Required” or “Mandatory”[1]. Other data rates can be set to “Supported”. The highest AP data rate set Mandatory determines the RF power required by the wireless telephone for proper operation. Broadcast frames (beacons) utilize the lowest “Basic” [2]data rate and multicast frames (used for the push-to-talk feature) also use the lowest data rate set Mandatory.

 

Unicast frames (data) utilize the ‘best or highest’ data rate which supports low frame errors and low retry rates but will rate scale up or down to use the ‘best’ rate of all available rates.

 

Referencing Table 1 and Table 2, the lowest rate set Mandatory (Required) determines the signaling requirements for the wireless telephone in all areas (limit of AP) where they are used.

 

  • For example, if an 802.11b/g access point has 1Mbps, 2Mbps, 5.5Mbps and 11Mbps all set Mandatory, the handset requires -75dBm in all areas.
  • For example, if an 802.11b/g access point has 1Mbps Mandatory and other rates set Supported (or “Enabled”) the handset requires -75dBm in all areas.
  • For example, if an 802.11a access point has 6Mbps, 12Mbps & 24Mbps set Mandatory and all other data rates set to Supported the handset requires -58dBm60dBm in all areas. 
  • Some AP vendors use a parameter to set the data rate used by broadcast (beacons) and multicast packets. The data rate used by broadcast & multicast packets determine the signal strength required by the wireless handset from table 1 & 2 above.
  • Other AP  vendors pick a data rate set Basic for transmission of broadcast & multicast packets. The data rate used by broadcast & multicast packets determine the signal strength required by the wireless handset from table 1 & 2 above.

SpectraLink handsets have a Site Survey mode that can be used to validate the signal strength it is receiving from the AP. The handset also has a Diagnostics mode which can show AP signal strength, as well as other details, as received during a call.  See the SpectraLink 8400 Wireless Telephone DeploymentAdministration Guide for details on using the Run Site Survey and Wi-Fi Diagnostics mode features, Section 4 Troubleshooting SpectraLink 8400 Series Handsets.

 

Although it is possible that for example, the SpectraLink handsets may operate at signal strengths which are weaker than those provided in Table 1 and Table 2; real world deployments involve many RF propagation challenges such as physical obstructions, interference, and multipath effects that impact both signal strength and quality.  Designing RF coverage to the required levels will provide an adequate buffer for these propagation challenges, enabling a more reliable and consistent level of performance with low retry rates.   

 

Full, bi-directional, access point diversity, using both antennas, is critical for improved communications between the AP and wireless handset to keep retry rates low, to improve voice quality and to provide a different & unique path between the AP and handset on any packet retries.

 

We will continue our discussion with Deployment Considerations in part 4 of this series. 



[1] Access Point (AP) vendors refer to this configuration setting differently but the value indicates a data rate that clients must be capable of utilizing in order to associate with the access point. These data rates are also used for different data traffic types by clients and APs that should be considered when designing for coverage requirements.

[2] The 802.11-2007 Standard defines any data rate set as required to be basic rates. See 802.11-2007 for additional details. (http://www.ieee.org)

 

 

 

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‎12-05-2011 12:17 PM
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