Admit None
You've outfitted the staff with sleek new Vo-Fi phones and gathered everyone in your conference room to demonstrate. But now you hear complaints that the devices aren't working. What gives? You tested each phone, and you ran through your presentation in the same room just an hour ago.
It may be difficult to believe, but even a newer WLAN may not be able to handle Vo-Fi. Although the G.711 codec with a 20-ms packetization rate may take only 75 Kbps to 85 Kbps on a wired network, overhead of the 802.11 protocol both in framing and timing, combined with the fact that most phones support only 802.11b, yields an upper limit of perhaps 14 to 15 handsets per AP, depending on how often users will be on the phone. Cisco recommends just seven to eight active calls per AP for its phones (see table, page 48).
No wonder early use cases favor verticals such as retail, health care and logistics that typically cover large areas with few people.
To properly engineer your WLAN for Vo-Fi, you'll need a dense AP deployment. If an AP also serves up data on the same radio, you'll need to accommodate fluctuating traffic flows and restrict phones from monopolizing bandwidth. That's where call-admission control can help, by enforcing a limit to the number of active calls an AP will accept.
Cisco, not one to shy away from creating its own standards, has made call-admission control a part of its proprietary CCX. Cisco licenses CCX code to client manufacturers, but reserves CCX support on APs for itself. According to the company, upwards of 95 percent of new wireless chipsets sold are CCX-compatible, meaning client devices from a variety of suppliers work with Cisco WLANs.
Through capabilities exposed in CCX, the Cisco handset will receive a fast busy when attempting to make an outgoing call on an AP that can't reserve enough capacity. In the reverse, callers to Cisco handsets that are associated to APs with insufficient capacity will receive the same feedback.
Vocera claims a higher call capacity than most--10 to 12 calls per AP--depending on the infrastructure vendor. By using an advanced audio codec, its packets are just one-third of the size of standard G.711 codecs, reducing overall wireless utilization.
SpectraLink implements both call-admission control and timing through its NetLink SVP (SpectraLink Voice Priority) Server. All voice traffic to and from its handsets must traverse the SVP Server, which can be configured to allow a set number of calls per AP. If a handset attempts to originate a call that would put the AP over capacity, the SVP Server informs the endpoint that the upper limit has been reached.
The most pertinent open standard related to call admission control is the Wi-Fi Alliance's WMM-SA (Scheduled Access), which is based on elements within the IEEE 802.11e specification. This standard will let clients reserve resources on the AP to ensure the necessary bandwidth, scheduling and low latency for a good telephony experience. The Wi-Fi Alliance had targeted a mid-2006 introduction, but the standard is still languishing, and our conversations with those involved in the process indicate WMM-SA is low on everyone's radar. It seems the Vo-Fi market is still too small to warrant bringing considerable resources to bear, and because Vo-Fi handset vendors have proprietary methods in place, they're not likely applying much pressure on chipset manufacturers.
It's unfortunate, because from where we're sitting, IEEE 802.11e has the process defined, it's just a matter of coding it up. SpectraLink has expressed support for WMM-SA, so hopefully we'll see it worked into its and other vendors' products once the Wi-Fi Alliance releases the standard.
