Is Your WLAN Ready for Voice?

Voice-over-WLAN can cut costs, increase productivity and improve customer service. Here's how to prepare your WLAN infrastructure for voice.

September 12, 2005

8 Min Read
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Mobility brings organizations considerable improvements in employee productivity, customer satisfaction and cost savings. Wireless voice is emerging from a specialized, proprietary application used in vertical industries to become a productivity enabler in a wide variety of businesses and educational institutions. As voice becomes an application across your wireless LAN infrastructure, making the deployment successful means meeting the key challenges of wireless coverage, graceful scalability, quality of service, seamless roaming, and of course security.


Once IT unleashes wireless LANs to enable mobile employees to stay constantly connected through their laptops and PDAs, those same workers are quick to ask when they will be able to make and receive phone calls over the WLAN infrastructure. With wireless voice, employees can place and receive phone calls and hear pages no matter where they are—at their desks, in the hallways, in conference rooms or in the warehouse. Workers can use either a wireless IP phone or a soft phone on a PDA or laptop for these calls. In addition, push-to-talk applications and paging can be run over the WLAN.

This article highlights some of the best practices in designing an IP telephony solution for mobility as well as unique considerations of running voice over a wireless LAN.

Business BenefitsMore and more, voice over IEEE 802.11 WLANs is becoming a mainstream application at enterprises and educational institutions. Proprietary wireless voice systems have long been used in warehouses, distribution facilities, retailers and healthcare organizations. But today, wireless voice is increasingly being built using industry standards, namely the Session Initiation Protocol (SIP) for call control over an 802.11 WLAN infrastructure. Wireless voice can bring many benefits to an organization, including:

  • Improved Employee Productivity. Wireless voice gives workers more flexibility. They don’t miss calls when they are not at their desks because their phones are always with them. For instance, the adoption of a wireless voice system in a hospital can reduce the communications delay among clinicians, patients and support specialists. If a nurse needs to consult a physician, she typically has to go back to the nursing station to page the doctor, wait for a response and return to the patient with the answer.

  • Improved Customer Satisfaction. Customers get answers faster when employees have wireless voice. For instance, in a customer service center, customers spend less time on hold because your employees stay connected even when they have to get up from their desks to find the answer to a customer’s question.

  • Cost Savings. Being able to reach someone even when they’re not at their desks also delivers cost savings, because your company isn’t footing the bill of returning missed calls. Wireless voice can also reduce operational costs associated with cell phones and private radio walkie-talkie systems in the enterprise. And of course, wireless voice eliminates or reduces the need to wire buildings, which can be a significant cost savings in new deployments.

  • Improved Safety. Wireless voice can be integral in improving the ability to stay connected and in communication in remote locations. For example, with wireless voice, a school’s faculty and staff are always in touch even when traveling across campus. And when location-based capabilities are available, their physical locations will always be known in emergencies.

  • A WLAN infrastructure that is ready for the demands of voice must meet strict performance requirements. Voice was not usually the prime consideration when the WLAN was designed and deployed. In particular, network designers will want to pay close attention to wireless coverage, network scalability, quality of service (QoS), and seamless roaming. Security is, of course, a pre-requisite for any enterprise WLAN deployment. Requirements include:

    Sufficient RF Coverage. Deploying voice demands ubiquitous wireless coverage, so that roaming users don’t hit “dead” spots as they turn corners or walk up and down the stairs while talking on their 802.11 phones. In contrast, many WLANs were designed to provide coverage where users sit and work – at their desks, in conference rooms and in common areas such as lobbies and lunchrooms.Having a good understanding of the number of simultaneous users and the bandwidth requirements of their applications, including voice, is essential in the WLAN design phase. WLANs are a shared medium with bandwidth speeds ranging from 54Mbps with IEEE 802.11a or 802.11g to 11Mbps with IEEE 802.11b. Because of the shared medium, actual throughput is significantly slower than switched Ethernet at 10Mbps, 100Mbps or even 1Gbps.

    Each WLAN must be designed to accommodate the unique characteristics of the environment. A WLAN is a radio frequency (RF) network, so a building’s physical characteristics impact how the RF signals flow through the physical space. Building materials like sheetrock, steel and glass absorb RF signals differently. Whether the facility has cubicles or offices impacts the WLAN design.

    In a physical site survey, a network designer gathers the building’s physical characteristics and assesses the RF coverage needs. Automated WLAN design tools are helpful to assess how many access points (APs) and wireless switches are needed to provide sufficient coverage and capacity.

    Scalability. The WLAN infrastructure must scale to handle the real-time demands of voice. 802.11 WLANs were originally designed to support data, not voice or video. 802.11 is modeled on 802.3 Ethernet, which allows stations to transmit on the network only when no other stations are transmitting. If two stations try to send data at the same time, the packets “collide” and one must wait a random amount of time before retransmitting. While this approach works well for data, it does not work well for latency-sensitive traffic like voice and video.

    Increasing the WLAN capacity—essentially deploying more APs and switches—is one approach to the problem. However, the more APs covering a particular area means the greater the possibility of co-channel interference, which can result in poor performance for both data and voice applications. In fact, early adopters of wireless voice often deployed separate infrastructures for voice and data traffic; however, doubling the acquisition cost and management overhead is simply impractical for enterprise deployment.Scalability also means that the WLAN infrastructure should load balance traffic among APs to ensure that the user associates with the “best possible” AP. In 802.11, the client decides which AP to associate with based on the strongest signal; however, the client doesn’t take into account the APs’ existing traffic load. In reality, the AP with the strongest signal could be overloaded, and poor voice quality could result. Plus, clients are “sticky,” meaning that they tend to associate with the same AP they last associated with, so a bad connection once may mean a bad connection again.

    The IEEE is working on a standard that allows a wireless IP phone to discover its neighboring APs and select the optimal AP for service. Pre-standard solutions are available that provide this enterprise-grade scalability.

    Deliver Quality of Service. Quality of service (QoS) is required whether voice is running over a wired or wireless infrastructure. Guaranteeing voice the right of way over other applications can help minimize the packet loss, delay and jitter that cause poor voice quality, clicks and silent periods.

    Because the 802.11 standard was developed without a way to distinguish voice and other delay-sensitive traffic, the IEEE is developing the 802.11e standard to address QoS at Layer 2 for wireless networks. In the interim, the Wi-Fi Alliance released Wireless MultiMedia (WMM) as a subset of the capabilities in 802.11e. Vendors are bringing WMM implementations to market now.

    WMM defines four priority levels to support different kinds of traffic, including voice, video, “best effort” for data, and background traffic. Enhancements to the 802.11 media-access control (MAC) arbitrate access to the WLAN medium. These additions are backward compatible with existing 802.11 devices. Voice traffic destined for the network backbone can also be marked using DiffServ/Type-of-Service (ToS) or 802.11p bits so that voice priority is preserved across the network.However, 802.11e or WMM may be just the first step for meeting enterprise requirements for voice. 802.11e or WMM doesn’t provide QoS per application. If a user on a laptop is running a soft phone and checking e-mail, the device receives the high priority assigned to it, not the voice application. As wireless usage grows, systems will likely be enhanced for improved QoS solutions.

    Allow Seamless Roaming. In the enterprise, AP densities are high, which means that roaming can occur every few seconds at normal walking speeds. As users roam from AP to AP and across subnets, the underlying WLAN infrastructure must “hand off” these users quickly enough that their voice connections don’t drop. The necessary re-association and re-authentication to the APs, plus the necessary wireless encryption for airtight security, also contribute delay.

    Associating and de-associating with an AP may take from 150 ms to 500 ms. VoIP performs optimally with a delay of less than 50 ms, otherwise callers will hear noise and degraded voice quality. And VoIP calls drop when delays approach 150 ms.

    Standards are under development to address the client-side issues. The IEEE 802.11i security standard will allow for users to be pre-authenticated to neighboring APs before roaming, which will reduce overall roaming time. Some vendors include a pre-standard version of this fast roaming capability in their WLAN solutions today.

    Ensure Airtight Security. Security continues to be a top concern of enterprises deploying WLANs, whether for data or voice.Effective security is predicated on knowing who the users are. IEEE 802.1X authentication should be used whenever possible to verify a user’s identity to the network. While 802.1X was originally developed for network access control for wired networks, many organizations are beginning to adopt it for their wired networks as well. Laptops and handhelds can support 802.1X authentication; however, less powerful devices such as wireless IP phones may need less processor-intense authentication methods, such as by MAC address or username and password.

    Preparing for the Future

    As mobility becomes pervasive in the enterprise, employees will want to do far more than use their laptops in conference rooms. In the same way employees on the road were quick to take Wi-Fi hotspots in their hotel rooms and airports for granted, users will expect their key business tools to be available over the wireless network. And there’s no doubt that voice will become their most critical tool.

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