How Many Voice Callers Fit on the Head of an Access Point?
Pages: 1, 2, 3
Putting VoIP on a Perfect 802.11 Network
The simplest analysis to do is a calculation of the absolute maximum capacity of a network. Networks have a variety of effects that are hard to model. In 802.11, one of the hardest network effects to model is the loss of capacity due to contention for the network medium. A maximum-capacity analysis assumes that there is a medium coordinator with God-like powers to eliminate contention. As soon as one network transmission finishes, there is another one ready to start without delay. There is never contention for the medium, and no management traffic gets in the way of moving data. Although the resulting model is simplistic, it offers valuable insight into the maximum capacity of different types of access points. With time, the simple maximum capacity analysis is likely to better reflect real-world capabilities as improved wireless-LAN quality-of-service standards are developed. Cisco's calculation assumes that each frame must wait half the contention window before transmission, but many VoIP systems use various tricks to shorten that delay. As improved QoS reduces medium contention, the real-world performance will begin to more closely approach the theoretical maximum.
802.11 data transmission consists of a data frame plus an acknowledgement. Each IP packet must be put into an 802.11 data frame, and that data frame must be positively acknowledged. The simple exchange of a data frame consists of the following components:
- The distributed inter-frame space (DIFS). The data exchange is an atomic exchange that must begin by obtaining control of the medium.
- The 802.11 physical layer convergence procedure (PLCP) preamble, which depends on the PHY in use
- The 802.11 PLCP header
- The PLCP data field carries the 802.11 MAC frame, which consists of
- A 24-byte 802.11 MAC header
- A 6-byte SNAP header to indicate that the network protocol in use is IP
- The IP packet itself
- A 4-byte frame check sequence (FCS)
- If security protocols are used, there will be eight additional bytes with WEP, and 20 for either TKIP or CCMP.
Each frame must receive a positive acknowledgement. 802.11 is adding features for block acknowledgements, but the delay-sensitive nature of VoIP may limit their use. The acknowledgement consists of the following components:
- A short inter-frame space (SIFS). The ACK is a continuation of the same atomic exchange, and therefore uses the SIFS.
- The 802.11 PLCP preamble
- The 802.11 PLCP header
- The 802.11 acknowledgement itself, which is 14 bytes of data transmitted at the rate coded in the PLCP.
Take the case of a G.711 codec operating on an 802.11b network at 11Mbps. Breaking down each component of the encapsulation gives the results in Table 2.
| Component | Time (ms) |
Bytes |
|---|---|---|
| Data frame | ||
| DIFS | 50 | |
| PLCP preamble (short) | 72 | |
| PLCP header (short) | 24 | |
| MAC frame | ||
| Header | 24 | |
| SNAP | 6 | |
| IP packet | 200 | |
| FCS | 4 | |
| Security (WEP) | 8 | |
| Total size | 242 | |
| MAC time at 11 Mbps | 176 | |
| TOTAL: Data frame | 322 | |
| ACK | ||
| SIFS | 10 | |
| PLCP preamble (short) | 72 | |
| PLCP header (short) | 24 | |
| MAC frame | 14 | |
| MAC time at 11 Mbps | 11 | |
| TOTAL: ACK | 117 | |
| TOTAL for sequence | 439 | |
Each data frame transmitted by the codec requires 439 microseconds. Therefore, 2,277 codec packets can be transmitted per second. G.711 operates at a 20-millisecond period, and uses 50 frames per second for each voice stream. A phone call is bidirectional, and will therefore require 100 frames per second. Therefore, the maximum theoretical capacity of an 802.11b AP is 22 WEP-encrypted telephone calls.
