A Cellular And WiFi Rendezvous

With cellular phones in virtually everyones pocket and wireless LAN deployments progressing rapidly across the residential and commercial landscapes, there are obvious benefits to be gained by enabling roaming between the two types of networks with the same device. Cellular networks can deliver mobile connectivity when users are away from the home or office, while 802.11-based wireless LANs can carry those connections indoors when they reach their residence or desk, effectively and inexpensively extending the service coverage area. With such convergence just over the horizon, users can realize the dream of single-number access being able to reach each individual through the same phone number, regardless of his or her location and corporations can leverage IP voice within their facilities to reduce cellular phone charges and improve productivity.

There is tremendous momentum behind the convergence of cellular and WiFi networks. Recent studies indicate that over 30 percent of subscribers are already using their mobile minutes within the home and office. Unfortunately, users often experience difficulty maintaining call quality and consistent connection in these areas. Installing additional base stations for all indoor coverage quickly becomes cost prohibitive for mobile carriers. Convergence of the cellular network would provide advantages for both these mobile carriers and end users. With convergence, end users could consistently use a single phone number for all their communications business, personal, desk-based, voice mail and more. In addition, end users would gain a very-high-speed data connection right to their mobile device. At the same time, cellular carriers benefit from inexpensive coverage and capacity expansion to their GSM networks, and increased customer satisfaction through better service coverage.

Because of these advantages, the momentum for a converged solution is steadily building. Already, handset vendors have announced telephones that can support GSM mobile networks and 802.11 wireless LANs. In February of 2005, a specification for Unlicensed Mobile Access (UMA) technology which details how a dual-mode phone roams from a licensed spectrum mobile network to the unlicensed spectrum (or WiFi) network, was formally approved by the 3GPP (3rd Generation Partnership Program) and will be included in 3GPP Release 6, the next major update of the standards for GERAN (GSM/EDGE Radio Access Network) TSG (Technical Specifications Group) the international standards body.

With all of this market momentum and the drive to have single-number GSM/VoIP access, one important industry requirement has yet to be met: the need for wireless LANs that can support IP voice that not only meets, but improves, on the quality of service and coverage that users have come to expect from their current cell phones.

Traditional Access Points Cant Deliver Quality VoIP
The current generation of WLAN access points (APs) are a key obstacle to delivering quality voice communications over wireless LANs. The problems encountered a few years ago when designing WLAN equipment are not the same problems that must be solved to use a WLAN as an extension to the mobile voice network. Previously, the design focus for WLANs in the enterprise was to ensure that they were manageable and secure. That design focus drove innovation in the deployment architecture of the WLAN the trend moved from distributing independent APs that needed to be individually configured and maintained, to distributing the wireless access but centralizing the management and security of those APs into a control unit or switch. This type of centralized architecture was a tremendous leap forward for controlling and managing a network of many access points and moved the WLAN industry in an important direction: out of the coffee shop and into the enterprise to support improved productivity. However, this still did little to solve the problems created by asking the same network to support VoIP. The technology transporting the VoIP packets from the network to the client and back is still the same technology being used in the coffee shops. In some instances, vendors have placed fancy management software wrappers on the access points and some have even gone so far as to distribute their code to these coffee shop hotspot providers so that their hotspots can be managed by the vendors centralized switches. Unfortunately, software is not enough.

There are two significant limitations of these software-wrapping approaches that make them unsuitable for carrying voice: the inability to prioritize and reliably deliver data packets both to and from the client, and the inability to maintain predictable performance between the client and an AP as the client roams from one APs coverage area to anothers. The problems experienced by hotspot technology can be traced to their Media Access Control (MAC) technologies. APs based on these technologies have access to some general wireless information such as signal strength, station identification, link status, and other asynchronous information, allowing the WLAN switch or controller to set the aggregate transmit powers for the cells, for example, and to crudely shuttle clients onto different APs on the basis of some optimization criteria. But the WLAN has no control over the details of the RF transmission (the latency, jitter, and error rate) because this information isnt being presented to the APs software and therefore isnt being used to make any real-time, synchronous decisions. It is precisely these details the latency, jitter, and error rate that must be understood and coordinated to ensure predictable quality of service for voice.

Without having detailed information of the transmission over the air, APs designed with these simplistic technologies are unable to differentiate voice clients from other clients, leaving the client to work in a best-effort method to get their chance to transmit. Since data traffic is much faster and more aggressive, it will crowd out the slow and steady voice calls because the collision-avoidance algorithm in WLANs (CMSA/CA) responds to the most aggressive incoming traffic. Its this inherent randomness in the last 100 feet of the communications that make these hotspot APs and even their enterprise big brothers unsuitable for use in a VoIP network.

Just as these APs make predictable packet delivery impossible, they also prevent reliable handoffs when the user roams from one APs coverage area to anothers. The hotspot APs act as an independent entity in terms of managing their connections, so in this architecture the client collects information about the network on its own and makes the decision of when and where to hand off. A client that switches from one AP to another is forced to perform a handshake each time it re-associates. This handshake has many components, and if any part is lost due to congestion or interference, the entire handoff is delayed. According to David Newman, writing in Network World, recent industry tests have shown that these commodity-based systems experience delays in their handoff processes, consuming half a second to multiple seconds during which no data is being transmitted and call quality is severely impacted.

A Better Solution Is Required
Fortunately, the 802.11 standard is specifically written to enable innovation and encourages methods by which vendors can design enterprise-grade access points that actually do provide precision control over the quality of client connections, and which thereby solve the problems of voice quality.

As it turns out, the real solution to these problems isnt coming up with ever-more-clever software to force legacy and commodity-based independent APs to work together. It lies in re-thinking the design of WLANs and APs to create a global system that meets the requirements of large-scale deployments that are intended to be voice networks. This is where the concept of a Cellular WLAN becomes the next logical evolution for WLAN technology. In a cellular network, each AP coordinates with the others to optimize the connection with each client as those clients roam. These APs use MAC technologies that have fully controlled, over-the-air interface with each client, and can control the ordering of both transmitted and received packets to adjust precisely for the quality of the connection required by each caller. This precise control over the timing and transmission and packet-by-packet scheduling, delivers traffic management over-the-air not only for each AP, but also for coordination across APs. Such a WLAN sy stem creates awareness across APs so they can coordinate transmissions to provide clear voice communications for every client without fear of interference from APs that are acting independently on the network.

In addition, by asserting control over the MAC, access points can cooperate without knowledge or involvement of the client to seamlessly transfer ownership of a connection in exactly the same way as the cellular telephone network, i.e., replicating the connection at the next AP and transferring the transmission to the new AP while simultaneously tearing down the old connection. When a dynamically controlled MAC is used, maintaining voice quality during handoff becomes a simple matter of having the central controller choose which AP is best. In other words, the wireless infrastructure decides which AP is best for the client at any given time, rather than the client deciding which AP is best.

Because the coordinated APs and wireless controller are maintaining awareness and control of all the transmissions and the quality of service to each client, the Cellular WLAN approach brings order and predictable service quality to an otherwise random communication medium. As such, this approach resolves the voice quality and WiFi handoff problems in a similar fashion to the proven method used in cellular telephone networks.

As the momentum continues in the convergence between Cellular and WiFi networks and WLANs move from convenient data networks to critical, revenue generating extensions of the voice network, it will be essential for WLAN vendors to use hardware architectures that provide precise control over the quality of each AP-to-client connection. While hotspot AP technology may have been sufficient for coffee shop coverage and even for data coverage in an enterprise with centralized management and security, it is ineffective for IP voice applications or for deploying a reliable extension to the mobile voice network. New problems require new solutions and the Cellular WLAN approach represents the ideal solution for the emerging, converged wireless networks. IT

Joel Vincent is director of product marketing at Meru Networks, Inc., a VoIP mobility infrastructure solutions specialist. For more information, please visit the company online at www.merunetworks.com.

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