Early 802.11a/b/g WLANs were used for only the basics: Giving employees and guests convenient access to the public Web and email services in public areas. Even newer WLANs evolved only to support business applications in narrowly targeted areas. But now those legacy WLANs must handle more complex mobility services and mobile device connectivity. In order to establish WLANs for mobile data, voice and video services, aging
Why 802.11n WLANs for mobile devices and applications?
No matter how well designed at inception, those legacy 802.11a/b/g WLANs are inherently limited in several critical dimensions that 802.11n technology can address.
Speed: 802.11a/g data rates were capped at 54Mbps, delivering no more than half that application throughput. 802.11nWLANs immediately benefit high-throughput applications by boosting maximum data rates to 300, 450 or 600Mbps, depending on product. Better yet, 802.11n users can sustain speeds that are four to five times faster at greater distances. These characteristics make 802.11n extremely competitive with 100Mbps Fast Ethernet, letting enterprise networks transition to all wireless LAN access. Once workers are no longer tied to their desks, many more business processes can be enhanced and optimized through mobilization.
Comparing 802.11 technologies
|Technology||Band||Max. Data Rate||Throughput|
|802.11b||2.4 GHz||11 Mbps||6 Mbps|
|802.11n (2x2MIMO)||2.4 and 5 GHz||300Mbps||150Mbps|
|802.11n (4x4MIMO)||2.4 and 5 GHz||600Mbps||300Mbps|
Capacity: Of course, the traffic conveyed by new business applications cannot grow without spare capacity. 802.11b/g networks were constrained by the overcrowded and noisy 2.4 GHz band in which they operated. 802.11a operates in the less congested 5 GHz band but still limits data rates. Fortunately, 802.11n can increase both WLAN speed and capacity by an order of magnitude.
Contemporary 802.11n access points (APs) can operate in both bands, creating a bigger and cleaner "wireless sandbox" with dozens of channels— including double-wide high-throughput channels. As a result, 802.11n can better exploit a valuable but scarce shared medium—the airwaves inside your offices—to support a multitude of mobile applications and users.
Coverage: Using a converged WLAN for mobile unified communications may be compelling, but doing so requires comprehensive coverage. In particular, voice over IP (VoIP) benefits from greater capacity but absolutely requires 802.11n's broader reach. Most legacy WLANs—including those that support voice—are limited in size and plagued by corners and stairwells where users experience drop-outs and disconnects.
New 802.11n APs can use multiple input, multiple output (MIMO) antennas and advanced signal processing to bypass those dead-spots, defeat interference and better focus transmissions on each individual user. As a result, networks that deliver far more ubiquitous wireless coverage can now be deployed at a lower cost.
Reliability:Those MIMO and signal processing advances let 802.11n deliver the higher reliability required by mission-critical applications. In the past, when 802.11a/g best-effort data connections were slow or failed, laptop users could always fall back on Ethernet as a tried-and-true access method. By comparing and combining radio waves that bounce off walls and doors, 802.11n leverages spatial redundancy, overcomes errors and improves signal strength. Some newer 802.11n APs even offer beamforming: tailoring transmissions to maximize throughput for each individual user. In a WLAN based on 802.11n APs, all users—even older 802.11b/g VoIP handsets—experience far more robust connections.
Designing WLANs for mobile services and applications: Focusing on architecture
When APs have been upgraded to 802.11n, your bigger, faster, more robust wireless network will be inherently capable of supporting many different applications, including those involved in mobile unified communications. But using 802.11n technology does not by itself ensure success—next-generation WLANs must be designed, deployed and managed with mobility in mind.
Planning: Legacy WLANs often delivered limited VoIP support because they could not satisfy voice coverage and quality requirements. APs must be explicitly positioned to deliver the gap-free coverage required to sustain toll-quality calls.
APs must be spaced at intervals estimated to satisfy vendor specifications for the lowest common denominator in your fleet—typically, at least -65 dBm. To build a cost effective converged WLAN capable of supporting a diverse collection of devices and applications, use a predictive planning tool to convert complex coverage area and performance targets into an optimized AP layout.
Prioritization: By combining the power of 802.11n with 802.11e quality-of-service (QoS) controls, contemporary APs can deliver simultaneous high-quality video and real-time interactive messaging while leaving ample room for latency-sensitive voice. To accomplish this, a converged WLAN must be designed to use Wi-Fi Multimedia (WMM) to prioritize voice transmissions before video, and video above best-effort data. These over-the-air priorities must then be mapped onto wired network QoS, using conventional techniques like 802.1p and DSCP to tag mobile traffic that needs preferential treatment.
Also look for features that go beyond WMM to better allocate limited resources, such as steering 802.11a/n laptops onto 5 GHz to reduce congestion on the scarce 2.4 GHz channels used by 802.11b/g voice handsets.
Call Admission Control: Even in an 802.11nWLAN, there is only so much airtime available to be shared. Voice capable WLANs should use Call Admission Control to constrain the number of active mobile voice sessions and gracefully reject or redirect excess requests that would otherwise degrade QoS. For example, you might reserve a percentage of channel capacity for all voice calls, with a subset specifically reserved to sustain roaming voice sessions. Some wireless clients can also submit reservation requests, using 802.11e Transmission Specification (TSPEC) parameters like data rate, burst size, and service interval to signal their anticipated airtime needs in advance.
Power Save: Battery-powered 802.11 devices that run real-time applications face a tough challenge: They must communicate at frequent, regular intervals without consuming too much power. Laptops used to check email or surf the Web can doze for scheduled periods to conserve battery; during these power-save intervals, APs automatically queue wireless traffic for later delivery. But a voice handset can't afford to doze for hundreds of milliseconds at a time, nor can a laptop running real-time applications over wireless.
To master this problem, deploy newer 802.11 clients that support WMM Power Save. This standard mechanism lets sleeping clients request queued traffic without waiting for their scheduled wake-up call, extending battery life up to 40%without introducing unacceptable latency for real-time applications.
Seamless roaming: In fact, when it comes to voice quality, latency (delay between packets), jitter (variation in delay), and packet loss are critical metrics. A converged WLAN should be designed with VoIP QoS needs in mind, using WMM prioritization and Call Admission Control to meet those goals during each call. But mobile users complicate matters by changing location mid-session—depending upon topology, this may involve roaming between APs or roaming between subnets. When upgrading your WLAN, choose controllers that support 802.11i and/or 802.11r fast handoff and can maintain client IP addresses while completing a subnet roam without exceeding VoIP latency tolerance.
Typical VoIP requirements
|Bit rate per cell||G.711||64 Kbps|
Redundancy: No matter what your intended application mix, mobilizing more business processes will make your business increasingly dependent upon your WLAN. Design and build your wireless network for high availability using contemporary WLAN hardware features like redundant AP, radios, backhaul links, dynamic frequency selection, and dynamic power control. VoIP requires stability, however, so special care is required to survive channel changes or AP failure. For example, a controller that quickly increases transmit power on surviving APs to fill a down AP's coverage gap might actually disrupt voice calls still in progress. Look for solutions that execute automated radio power and frequency changes slowly or with active calls in mind.
Service assurance: Finally, every mission-critical network requires continuous monitoring and proactive problem resolution. Surround your upgraded WLAN with tools and processes to monitor, detect and respond to over-the-air operational, security and performance events.
Deploy an 802.11n-aware wireless IPS to continuously monitor RF utilization and interference, accompanied by automated end-to-end tests to periodically measure throughput, latency and voice quality. Expand wired network operations and help-desk processes to incorporate WLAN troubleshooting, investing in tools to rapidly pinpoint or rule out wireless problems. Doing so may increase initial deployment cost but will greatly reduce operating expenses. In a well-designed WLAN, most help-desk calls will not end up requiring costly RF expertise to resolve; determining this quickly will speed problem resolution and minimize business disruption.
Upgrading mobile clients for mobility services
Upgrading your WLAN infrastructure to support a rich mix of data, voice and video services is a good start, but to fully realize that potential, you must also upgrade wireless clients. Take stock of legacy wireless devices to plan for their migration to 802.11n, and budget for purchase of new wireless devices to support enhanced mobility applications.
For example, most corporate laptop fleets are already at least a year into equipment refresh cycles that will upgrade embedded WLAN adapters to 802.11n. This portion of your wireless workforce is likely to complete migration quickly and can start leveraging 802.11n straight away for mobility applications like video conferencing and telepresence.
On the other hand, today's wireless VoIP devices—from single-mode 802.11-only handsets to dual-mode Wi-Fi/3G cellular smartphones— do not yet use 802.11n radio. MIMO power demands, small handheld devices are slower to incorporate 802.11n. In the interim, reduce drag on your entire WLAN by retiring elderly 802.11b VoIP devices, and use dual-band 802.11n APs to support older 802.11g VoIP clients in the 2.4 GHz band while rapidly transitioning new 802.11n clients onto the 5 GHz band.
Finally, consider whether and how to leverage new kinds of wireless devices to enhance workforce mobility, from stationary 802.11n media servers and 802.11n network storage devices to 802.11n handheld media players and dual-mode Wi-Fi/3G netbooks. A reliable high-capacity WLAN will not only expand business applications but may well utilize innovative client devices to deliver those services more effectively to mobile users.
This was first published in July 2010