WLANs VoWLAN Handoffs Another potential problem with VoWLAN has to do with handoffs. Voice users' requirements for mobility will be greater than those of data users, SO voice handoffs are more likely. A call handoff must occur quickly, and the authentication and encryption must remain intact. Handoffs can be several seconds long. The problem is that most traditional data-oriented authentication systems require that the device be reauthenticated when moving to a new WAP. The IEEE is looking for a 20-millisecond handoff capability for voice calls. Vendors today have introduced proprietary solutions to address the handoff concern, but eventually a standards-based solution will emerge. Meanwhile, a user must weigh the function- ality that can be delivered immediately against the potential risk of being locked in by a vendor's proprietary solution. The VoWLAN solution is typically implemented as an adjunct to an existing circuit-switched or IP-based PBX. In either case, there are three main elements in a VoWLAN solution: VoWLAN telephone sets, WLANs, and gateways. VoWLAN telephone sets include digital telephones and other voice-capable devices that sup- port an 802.11 WLAN interface. One issue regarding VoWLAN phones is the codec or voice-encoding rate. The main choices include G.711, the public network stan- dard that requires 64Kbps per voice channel, and G.729a, a more bandwidth- efficient low-bit-rate option, which calls for 8Kbps per voice channel. The main tradeoff between the two standards is a 15- to 30-millisecond increase in delay due to compression when using G. .729a. VoWLAN Capacity and QoS Capacity and QoS are discussed together here because the issues of providing adequate capacity while minimizing delay and jitter can be addressed by increasing the overall network capacity by offering higher bandwidth or by ensuring that voice packets are recognized and given priority over data packets-in other words, through managed bandwidth. Remember that WLANs were designed first and foremost for the requirements o