f 802.11 wireless LANs promise to untether corporate users from their wired network connections, then Voice over Wireless IP (VoWIP) promises to free them from their telephone handsets. VoWIP is an emerging technology that enables IP voice to be sent over an 802.11 wireless LAN. People may use their PDAs for walkie-talkie style communications within an office building, for instance, or VoWIP may bring mobile office phones to everyone on a corporate campus. Today VoWIP is mostly used in specialized vertical applications, but as both VoIP and WLANs proliferate, VoWIP is likely to become more widespread.
Over the past several years many companies have adopted VoIP. IP PBXs enable a company to run its voice traffic over its IP data network while gaining telephony features such as voice mail and "follow me" services. Follow me or find me services enable users to have their phone calls find them regardless of their locationat home, work, or in the car. With VoIP, companies can leverage their data networks so the voice traffic rides for free, rather than build a separate voice network. Companies can also save significantly on long distances charges by routing VoIP calls through their internal networks nationwide, instead of placing calls through the public phone network. This is referred to as toll bypass. The same benefits apply when the IP voice runs over a WLANplus a few additional benefits such as increased employee productivity and additional cost savings from not having to wire an office for phones.
Because VoWIP delivers mobility to office phones, it promises to boost worker productivity. VoWIP may be used by corridor-cruising information-workers (Gartner calls these "micro-mobile workers") who want to be able to place or receive phone calls whether they are at their desks, in a conference room, or roaming the corporate campus. VoWIP can provide mobile office phones to vertical industries such as retail, healthcare, and manufacturing where workers don't typically sit at desks. While many people use their cell phones inside their offices, using your cell phone as your office phone is very expensive, not to mention that cell coverage inside a building is often spotty.
What's Inside VoIP on WLANs
Quite simply, a VoWIP system works by translating a PBX telephone call to IP and then sending those IP packets over an 802.11 WLAN. The packets are then reassembled by the 802.11 wireless telephone or softphone running on your PC. A softphone is a software-based IP phone that is installed on a user's PC, giving the PC a full range of phone capabilities, including call forwarding, conference calling and integration with Outlook for automatic phone dialing. Softphones may work alone or in conjunction with an IP PBX.
On the device side, the wireless handsets, softphones, or PDAs typically support either H.323 or SIP for call control. Originally developed by the International Telecommunications Union (ITU) for desktop videoconferencing, H.323 is commonly used for IP telephony phones and IP PBXs. Because H.323 was originally a videoconferencing standard, it defines more functionality than is necessary for most VoIP applications.
SIP is a relative newcomer, developed by the Internet Engineering Task Force (IETF). SIP is an application-layer signaling protocol that lets customers use IP networks to establish sessions, instead of just phone calls, which can include voice, video, or instant messaging. SIP can be used in "presence" applications where users can list themselves as available, similar to a buddy list. With a SIP client on your PDA, you can turn your PDA into an 802.11 phone. Unlike H.323, which is based on binary-encoding, SIP is a text-based protocol so applications are easier to implement.
Because quality is so important with voice, it's necessary to prioritize VoIP traffic ahead of data traffic on the WLAN. A data application won't care about a momentary blip in the session, but latency will cause a voice call to drop. The VoWIP market is nascent, so the two standards for QoS on VoWIP have been developed by their vendors, SpectraLink and Symbol Technologies. SpectraLink has the SpectraLink Voice Priority protocol, which is an open standard for 802.11b networks. Symbol supports both its own VoWIP prioritization as well as SpectraLink's with its NetVision phones. These standards are supported by a number of the WLAN system vendors.
Over the longer term, the IEEE is developing a Quality of Service (QoS) standard called 802.11e, which is expected to be finalized in late 2003. In the meantime, however, most WLANs will implement the standard QoS functions, such as IEEE 802.1p, 802.1q and DiffServe for QoS for data traffic or the VoIP specific ones for voice.
Ready, Set, Grow
Today VoIP over WLANs has all the bumps and bruises that come with the territory of emerging technology. Reliability of the underlying WLAN infrastructure will be essential for the success of the VoIP over WLAN market. If it's annoying for your 802.11 data connection to drop when you move, it's simply unacceptable for a voice call to drop.
Being tightly integrated with the underlying WLAN infrastructure will be critical. A robust solution will require a WLAN system infrastructure that will support speedy secure mobility with session persistence. Session persistence means that the users application session is maintained as the user roams across subnetseven if that application is time-sensitive.
WLAN vendors take many different approaches to secure mobility, which directly impacts their ability to support real-time applications like VoIP. Secure mobility solutions based on creating IPsec VPN tunnels often require a user to re-login when roaming across subnets. Cisco uses Mobile IP, which is an overlay routing protocol, to enable secure roaming. Unfortunately, Mobile IP isn't known for being very speedy.
Support for video on WLANs is also challenging, as the bandwidth is so limited. With an 802.11b WLAN, the 11Mbps data rate translates into 4Mbps to 5Mbps of real throughput. On an 802.11a WLAN, the 54Mbps data rate translates into 20Mbps to 25Mbps of real throughput. And that bandwidth is shared by all users on the access point. Because video is such a bandwidth hog, most IT managers will restrict streaming video on the WLAN. Many WLAN vendors are developing solutions to support IP Multicast on the WLAN though QoS, so we may see more streaming video over the WLAN as those solutions come to market and 802.11a becomes more widespread.