Voice over wireless LAN: Time to pick up

One of the most compelling aspects of the wireless LAN (WLAN) is seamless mobility: the capability of maintaining applications on the networks while the client is in motion. The ability to support real-time applications such as IP voice and other multimedia services has moved from "nice-to-have" to a critical component of next-generation WLAN networks. One key driver is reducing the load on cell phones; the other is that, when an employee uses a cell phone, he usually also has a desk phone, meaning there are two voicemail boxes and two phone numbers for each employee. This is inefficient, and it's that much more work for the employee to manage. 

A recent business users study conducted by Instat/MDR finds that voice over WLAN (VoWLAN) has reached nearly 10% of respondents and 48% of respondents are considering implementing it. While the business case for VoIP in the enterprise has taken several years to solidify, the added benefits of mobility are catalyzing adoption of this new technology in a rapid way. Companies in the healthcare, education, retail/warehousing/distribution and manufacturing industries, for example, are early adopters of VoWLAN technology as the need for real-time data and voice services prove mission-critical.

Proper support of voice means that enterprises must rethink their approach to WLANs. While several standards are emerging to help in the delivery of real-time applications, there are fundamental capabilities that are required in a WLAN to support a mixed voice and data environment, such as quality of service (QoS), fast roaming, RF management and security. Whether deploying voice applications today or planning for it in the near future, any enterprise planning a WLAN rollout should understand the specific attributes required to support voice and how best to implement these features in a cost-effective manner.

Applications such as voice are sensitive to digital transmission errors. When transmitted over data networks, lost packets and retransmissions due to heavy traffic loads increase "jitter" in the signal which can result in poor voice quality. Implementing QoS in wireless networks can help reduce jitter and improve the overall quality of VoWLAN.

Today, there is no unified standard for QoS. As vendors are proposing their own proprietary ways of handling QoS, enterprises need to understand what effect this will have on their ability to support VoWLAN services in the future. In addition, different WLAN vendors handle mobility in different ways. Those that require mobile IP, for example, add client-dependencies and require a reconfiguration of backbone routers to support voice. These changes are not trivial, having a major impact on the cost, management and even reliability of a VoWLAN deployment.

Fortunately, some assistance is coming from the standards bodies. The existing 802.11 standard, for example, has an optional provision called the point coordination function (PCF) that attempts to provide some standardized QoS capabilities by controlling when wireless devices can transmit traffic over the RF domain. PCF is a contention-free protocol that enables stations to transmit data frames synchronously, with regular time delays between data frame transmissions. This makes it possible to more effectively support information flows with sensitive performance characteristics. However, as it is not mandatory to support this provision, few wireless clients support PCF, limiting its widespread availability.

The 802.11 Working Group is also in the process of defining a new amendment to the WLAN medium access control, called 802.11e, which defines QoS extensions for the first hop between the mobile device and the access point. This standard introduces two options for handling real-time traffic:

• Enhanced Distribution Coordination Function (EDCF)

  , which uses the same contention-based access mechanism as current devices but adds "offset contention windows" that separate high priority packets from low priority packets by assigning a larger random backoff window to lower priorities than to higher priorities. The result is "statistical priority," where high-priority packets usually are transmitted before low-priority packets. 

• Hybrid Coordination Function (HCF)

  , which adds AP-controlled client access on top of EDCF. HCF extends the definition of PCF with traffic specifications that are agreements between the AP and client for a unidirectional traffic stream, providing policed bandwidth, polling, delay and jitter definitions.  

It is important to note that the Wi-Fi Alliance is attempting to accelerate the adoption of the EDCF version of 802.11e via a new standard called Wireless Multimedia Extensions (WME). HCF is attempting to be delivered in a standard called Wireless Scheduled Multimedia (WSM). WME is expected out this summer, WSM towards the end of 2004.

The 802.11e standard will go a long way towards defining how QoS is handled in wireless networks. However, even upon completion, there are numerous additional aspects that a WLAN infrastructure must address to support voice services. These include:

• Seamless Roaming.

   If a voice client hits a coverage hole, the call will be dropped. Furthermore, if significant time is required to re-authenticate when a client moves from one AP to another, call quality will suffer. As a result, fast roaming is required when supporting voice applications, with seamless handoff across APs an absolute must. The emerging 802.11r working group was developed to improve VoWLAN roaming. Its focus is to reduce the time required to authenticate when roaming, which will help support real-time applications, such as voice. Newer WLAN systems take things one step further, centralizing security, QoS and other policies within a WLAN controller (i.e., a WLAN switch or appliance) for network-wide visibility. This eliminates the need for individual APs to handle these functions and creates an environment whereby proactive handoffs can take place across APs. By acting as a complete wireless system as opposed to autonomous RF domains, these platforms improve mobility and provide higher WLAN performance. 

• Voice Privacy.

   Security is a concern for any IT administrator, regardless of the types of services deployed over a WLAN. Voice applications take WLAN security concerns to a new level--who wants someone listening in on the CEO's phone conversation? As each VoWLAN client has unique security capabilities, a WLAN must be flexible enough to simultaneously support multiple options. For example, most VoWLAN handsets today support WEP or WPA; softphones might be able to use web-authentication; a laptop running a voice application might be able to leverage 802.1x.  A WLAN needs to be able to support different security policies to accommodate these different VoWLAN devices, including the newest additions to the security line up 802.11i.  In addition, these security policies need to be implemented on each AP to avoid scalability and management issues. Centralized authentication and admission control can also prove invaluable in a VoWLAN implementation.  By removing this functionality from individual APs and placing it within a single controller, a WLAN infrastructure can reduce the number of times this process occurs.  The result will be improved handover speeds as a VoWLAN client roams from one AP to another, without compromising security. 

• RF Management.

  RF management is essential to any WLAN environment. However, it becomes especially critical if you are supporting real-time applications, such as voice. RF management can be used to provide consistent performance under heavy load, which is vital as many clients (voice and data) are using a wireless network simultaneously. RF management can also be used to dynamically balance load across different APs to avoid congestion and to change channels on APs to avoid interference or noise. In addition, with real time RF management, AP signal strengths can be adjusted in real-time to reduce packet loss and to adjust coverage areas for optimal throughput and latency, and to eliminate any coverage holes that might cause a voice call to be dropped. 

• Emergency 911 (e911).

  Many states have specific requirements for e911, indicating that emergency dispatch personnel must be able to locate a distressed caller to within a few meters. To help achieve this, a WLAN system should incorporate advanced location tracking software to help locate 802.11 handsets. This location tracking software should use RF fingerprinting technology to account for specific RF characteristics within a building. Otherwise, the accuracy of the location tracking software will suffer, preventing the WLAN from adhering to these standards.

Wireless LANs unlock the true potential of packetized voice deployments in enterprise environments. By combining industry standards with best-of-breed WLAN capabilities such as centralized intelligence and real-time RF management capabilities, WLAN vendors are increasingly delivering reliable platforms that support the convergence of voice and data applications. With an intelligent WLAN system, Wi-Fi is ready for real-time. 

Alan Cohen is the vice president of marketing and product management for Airespace and may be contacted at acohen@airespace.com.

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