June 2004

Speaking On Wireless

BY TONY Rybczynski

Wireless IP telephony, converged wireless LANs (WLANs), and Voice over WLANs (VoWLANs) are all different names for the same set of technologies. They all extend and leverage the ubiquity of Ethernet networks and the Internet to offer a new dimension in productivity for business users, by combining voice and data with an untethered or mobile connection. In so doing, they radically redefine the meaning of the workspace. Gartner Group says that, �The notion of the office as a fixed location will give way to a situation where �office� is just the act of paying attention to work through always-on access.� A Gartner study suggested that enterprises could expect a 22-percent productivity improvement by introducing WLANs. WLANs can provide mobile, high-speed connectivity when and where needed not only for data but also for telephony.

But wait a second! IP telephony users demand a high-quality telephony experience, which drives requirements for low delays, low jitter, effectively zero-percent packet loss and high reliability and seamless coverage. Isn�t a WLAN a shared media technology like Ethernet of old and hasn�t the industry rejected wired shared Ethernet for voice in favor of switched Ethernet? WLANs are not wired LANs. How will we make WLAN voice friendly? What about QoS standards for WLANs?

Priority Access For Voice
In WLANs, the radio channel is accessible by a number of mobile clients and the Access Point (AP). A mechanism is required to handle contention among these users, since only one user can transmit at a time. How are wireless LANs different from wired LANs, and more importantly what�s the impact on voice performance? In general, Ethernet uses a technique called Carrier Sense Multiple Access or CSMA, taking advantage of the fact that LAN devices can generally listen to or sense what is happening over the shared media. LAN devices can use this information to collectively provide stable throughput under overload conditions.

Wired LANs are based on IEEE802.3, which uses CSMA/ Collision Detection (CSMA/CD) to manage access to the medium; if a station detects a collision it randomly backs off its transmission and tries again. In WLANs, a station that is transmitting cannot detect if a collision is taking place, so collision avoidance is used (CSMA/CA). The current IEEE802.11 standard defines two WLAN modes of operation at the MAC layer: DCF (distributed coordination function) and PCF (point coordination function) modes with distributed and centralized controls respectively. In DCF mode, which is widely implemented, if a station detects a busy channel, it randomly backs-off and checks again. It treats all traffic as best-effort data. The PCF mode runs on DCF and uses polling to manage access to the medium with a view of better serving time bounded traffic such as voice; this mode has seen few or no commercial implementations to date but is an important component of the emerging IEEE802.11e QoS standard.
 

Best-effort VoWLANs can work reasonably well if the number of users is kept below some limit and data traffic is not too intensive; one brute force approach is to restrict voice telephones to 11b and data to 11a channels through MAC address checking. In an interference-free 802.11a network without data traffic, there is a break at 30 simultaneous voice calls, after which point the packet jitter increases significantly, particularly in the direction from the AP to the mobile user. This matches the typical number of mobile data clients supported by a single AP, in spite of the lower bandwidth required for voice (i.e., an effective bandwidth of 144Kbps and 200Kbps for G729 and G711 coding). Both these numbers would be roughly 70 percent lower with 802.11b. If more users need to be supported, then more APs need to be deployed.

The solution for voice over WLANs lies in QoS capabilities at both the packet and radio levels. The role of WLAN QoS is to consistently deliver voice quality over a broad range of conditions, including number of users and data traffic loads. Experienced enterprises have learned that without end-to-end packet QoS, the traffic may experience differing amounts of packet delay, loss, or jitter at any given time, which can in turn cause speech break-up, speech clipping, and audio pops and clicks. Even if bandwidth is over-engineered, growth of traffic, rapid changes of traffic patterns, and network connection failures may result in impairments that impact IP telephony (such as packet loss and excessive delays). While packet QoS mechanisms are sufficient in switched Ethernet and routed networks, WLANs are shared media technologies that additionally require wireless QoS mechanisms to support IP telephony.

There are two high-level thrusts of the IEEE802.11e standards work, the first will be subject to WiFi Alliance interoperability certification in mid-2004, under what is called Wireless Multimedia Enhancement (WME). The second will be subject to WiFi Alliance interoperability certification under what is called Wireless Multimedia Scheduling (WMS). The WiFi Alliance, previously called the Wireless Ethernet Compatibility Alliance, is a non-profit international association formed to certify interoperability of WLAN products based on IEEE 802.11 specifications.

Prioritized Channel Access For Telephony (WME): The standard includes an enhanced DCF contention mode (formally called Enhanced Distributed Channel Access or EDCA) with different treatment for each of eight Traffic Classes. Technically, it defines a new interframe spacing parameter with a value that is larger for best effort data and smaller for voice and video. This provides voice prioritized access to the radio channel. In addition, the maximum back-off time is also dependent on the Traffic Class, to bound the maximum delay in case of retries, again an important attribute for voice. WLAN products that support the WiFi Alliance WME certification ensure wireless telephony users get priority on the shared radio channel.

Deterministic Voice Through Polling (WMS): The IEEE802.11e working group is also defining a Hybrid Coordination Function (HCF) to support more intensive multimedia applications. HCF allows a QoS-enabled mobile unit to alternate between EDCA and what it calls HCF Controlled Channel Access or HCCA, a PCF-like polling mode. Technically, HCF defines a superframe, which is split between a contention period, during which time EDCA is used, and a contentionless period, when polling is used. It leverages the already defined but unimplemented PCF mode to provide policing and deterministic channel access. Since the value of the interframe spacing parameter is shorter than that used with WME, this traffic always has faster access to the radio channel. In this mode, a signaling protocol can be used to facilitate admission control and specify service rate requirements. There is still research underway to validate the effectiveness of HCCA and to address issues of �selfish� mobile units that could handle all traffic as high priority.

IEEE802.11e is pivotal to converged WLANs, providing a standards toolkit to meet a broad range of enterprise converged network needs. Fortunately, enterprise IT will not have to concern themselves with the above details, which will be implemented in WiFi certified products.

Planning WLAN Coverage
Each WLAN device (mobile unit and AP) has an antenna and RF capabilities, which allow it typically to operate on one of three 11Mbps or 54 Mbps channels under IEEE802.11b and 11g respectively; and/or on one of thirteen 54Mbps channels (in the US) under 802.11a. The IEEE802.11a standard actually allows more than this, but the number allowed depends on which country you operate in (e.g., eight in France and UK; 19 in Sweden; more coming in the United States).
One of the attributes of 802.11 systems is that they use relatively low power spectral density (compared to a cell phone). They therefore operate over limited distances measured in hundreds of feet, not miles (though high-gain directional antennas could be used in special cases). The 802.11 standard includes adaptive coding, which lowers the bandwidth of the channel as the signals weakens (to minimize loss), whether because of distance or because of obstructions along the path.

So 11 Mbps capacity near AP can become 1Mbps at the edge of the cell. In fact, 802.11 allows different bandwidths to be dynamically assigned, allowing for example, a device close to the AP to transmit at full 11 Mbps one moment, and a moment later for the AP to transmit to a user half-way to the edge of the cell at 5Mbps. Other impairments can result in weakening of the signal (e.g., trying to get through walls or partitions) or undesirable reflections (off walls and partitions). This is highly dependent on the nature of the material encountered. Clearly, these are not an issue in line of sight transmissions (e.g., out-of-doors). The implications for voice are that it is critical that voice be given priority access to the radio channel, and that the capacity of the WLAN be properly engineered to handle voice and data traffic.

To provide the broader coverage for wireless telephony, cell planning is required to ensure that APs are properly distributed throughout the building, and that adjacent APs aren�t using the same frequencies, thus preventing interference. This is complicated by the fact that radio signals operate in three dimensions. Today, many APs in enterprises are deployed to serve a particular environment, such as conference room or executive office, rather than to provide enterprise-wide roaming. Most deployed WLAN systems are based on 802.11b, the first widely deployed standard; most wireless IP telephones also use 11b to maximize battery life. To overcome the limitations of having only three channels available with 11b (and newer 11g) systems, some form of directional antennas and dynamic RF management are highly desirable to minimize interference in multi-story buildings.

Conclusions
Multi-vendor QoS-enabled wireless LAN interoperability based on WME will set the stage for converged WLANs. Once implemented in both WLAN Access Points, deployed ubiquitously across the enterprise, and in mobile devices, wireless IP telephony will enter the mainstream. Converged WLANs will be used to extend the reach of enterprise IP telephony and multimedia communication systems. They will spur further innovation in mobile devices, particularly in multimode operation enabling seamless voice and data mobility between enterprise and third generation public wireless systems.

Tony Rybczynski is Director of Strategic Enterprise Technologies at Nortel Networks. He has over 30 years experience in the application of packet network technology. For more information, please visit www.nortelnetworks.com.

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