Cisco Access Points - FAQs

Q. How many clients can associate to the AP?

A. The AP has the physical capacity to handle 2048 MAC addresses, but, because the AP is a shared medium and acts as a wireless hub, the performance of each user decreases as the number of users increases on an individual AP. Ideally, not more than 24 clients can associate with the AP because the throughput of the AP is reduced with each client that associates to the AP.

Q. How many service set identifiers (SSIDs) can you have per VLAN?

A. You can have only one SSID per VLAN. The use of multiple SSIDs over a single VLAN is not supported with Aironet APs.

Q. How many wireless IP telephony handsets are recommended per AP?

A. IP telephony network sizing is essential to ensure that adequate bandwidth and resources are available to carry mission-critical voice traffic. In addition to the usual IP telephony design guidelines for sizing components, such as PSTN gateway ports, transcoders, WAN bandwidth, and so forth, also consider these 802.11b issues when sizing your wireless IP telephony network:

Number of 802.11b devices per AP: Cisco recommends that you have no more than 15 to 25.

Number of 802.11b phones per AP

Before any discussion about network plans can take place, it helps to understand the basics of the overall network capacity. These network capacity guidelines apply to sizing the Wireless IP Telephony network:

No more than seven concurrent G.711 calls per AP

No more than eight concurrent G.729 calls per AP

Note:  These design recommendations assume that Voice Activity Detection (VAD) has been disabled on the Cisco 7920 Wireless IP Phones.

Use of VAD on the Cisco 7920 phones can conserve bandwidth, but Cisco recommends that you disable VAD on all Cisco CallManager servers to provide better overall voice quality. In addition to the determination of how much bandwidth is needed for an 802.11b VoIP call, you must also consider overall radio contention for a particular RF channel. The general rule is that you should not deploy any more than 20 to 25 802.11b endpoints per AP. The more endpoints you add to an AP, the more you reduce the amount of overall bandwidth and potentially increase transmission delays. The maximum number of phones per AP depends on the calling patterns of individual users (based on Erlang ratios). Cisco recommends that no more than seven concurrent calls use G.711 or eight concurrent calls use G.729. Beyond that number of calls, when excessive background data is present, the voice quality of all calls becomes unacceptable. Packetization rates for these recommendations are based on 20-ms sample rates with VAD disabled. This rate generates 50 packets per second (pps) in each direction. A larger sample size (such as 40 ms) can result in a larger number of simultaneous calls, but it also increases the end-to-end delay of the VoIP calls.

The number of 802.11b phones you can deploy per Layer-2 subnet or VLAN depends on these factors:

Use no more than seven G.711 or eight G.729 active calls per AP.

The calling ratio is used to determine the number of active and non-active calls. This ratio is often determined with Erlang calculators. Based on these factors and normal business-class Erlang ratios (between 3:1 and 5:1), Cisco recommends that you deploy no more than 450 to 600 Cisco 7920 phones per Layer-2 subnet or VLAN.

Refer to the Network Sizing section of Wireless Network Infrastructure, as well as Is Your WLAN Ready for Voice? for more detailed information.

For more information on how many clients can connect to an AP, refer to the How many clients can associate to the unit? section of  Cisco Aironet Access Point FAQ.