How Many Voice Callers Fit on the Head of an Access Point?
Pages: 1, 2, 3

Table 3 shows the calculated number of telephone calls with no security overhead. Including WEP encapsulation overhead drops most of the numbers by one. Using CCMP or TKIP instead drops many by two. Table 3 assumes the use of the short PLCP headers in 802.11b, which are now commonplace. Figure 1 shows the same results graphically.

Figure 1. Maximum theoretical number of voice calls per 802.11b AP (longer bar is better)

Most 802.11 telephones on the market today are based on 802.11b, but that will not be the case forever. 802.11b has low power consumption, but limited capacity. Many of the newer dual-mode (802.11/cellular) telephones use 802.11a. If the same maximum capacity analysis is repeated for 802.11a or 802.11g (without protection), it leads to the much higher capacities shown in Table 4.

However, 802.11g protection imposes a significant limit on throughput. Protection is triggered by any 802.11b frame in the area, so it is a practical reality of 802.11g that it almost always operates in protected mode. Using the most common and lowest-overhead form of protection, CTS-to-self, the results are quite striking. Depending on the codec and data transmission rate, about a quarter of the voice capacity is lost. The loss is worse at higher data rates, and the percentage loss is slightly higher with the more efficient codecs. Table 5 shows the number of telephone calls lost to protection overhead.

Figure 2 shows the loss due to protection as a fraction of the capacity for representative data rates. High-efficiency codecs suffer greater losses because they already devote a large part of the network time to traffic service. G.711 does not have as big a proportional loss because it is inefficient. As a rule of thumb, expect a loss of between one quarter and one third of the network capacity when protection is lost. Alternatively, that is likely to be the difference between using 802.11a and 802.11g.

Figure 2. Percentage loss of capacity due to 802.11g protection (shorter is less loss)

As a final comparison, the three 802.11 physical layers can be compared. Figure 3 compares the capacity of 802.11b, 802.11g with protection, and 802.11a for roughly comparable data rates. Even though the data rate on 802.11a and 802.11g is only slightly higher, the improved physical layer efficiency gives it much greater call-handling capacity.

Figure 3. Comparison capacity of 802.11a, b, and g (longer is higher)

Effects of Security

Enabling encryption protocols has only minor effects on the call capacity. In 802.11b, enabling WEP reduces the capacity by a call for most data rates and codecs, with TKIP or CCMP reducing the capacity by one or two calls. In 802.11g, the capacity loss is numerically higher (one or two calls for WEP, three or four for TKIP/CCMP), but roughly equivalent on a percentage basis. As a rule of thumb, expect WEP to cost 1 to 3 percent of the call capacity, and TKIP or CCMP to cost 3 to 5 percent of the call capacity.

Matthew Gast is the director of product management at Aerohive Networks responsible for the software that powers Aerohive's networking devices.

Return to O'Reilly Emerging Telephony

Comments on this article

Showing messages 1 through 3 of 3.

• wrong question -> irrelevant numbers
  2005-12-16 01:14:01  fireless [View]
  
  
  Quite a disappointing article adding to common misconceptions about 802.11. Calculations are technically correct, but are giving answer to a wrong questions.
• wrong question -> irrelevant numbers
  2006-01-20 12:21:17  Ted Wallingford | [View]
  
  
  WHAT? Try again, Fireless. This is probably one of the soundest analysis I've seen. The only thing it doesn't really take into account is poor radio performance, which is entirely peripheral anyway.
• wrong question -> irrelevant numbers
  2006-11-13 15:55:07  MaxDowney [View]
  
  
  Nah, the analysis doesn't take into account the delay inherent in 802.11b's Distributed Coordination Function.
  
  Back of the envelope calculations would show that using G.711 A-Law encoded audio transmitted in 20 ms blocks (say 160bytes of audio per frame) at 11Mbps would give a maximum of 12 VoIP calls possible and this still doesn't take into account a noisy channel which increases the amount of time wasted in the DCF.
  
  Max