Wireless LAN – Do you Know What you’re Buying?

David Confalonieri
Vice President, Corporate Marketing
Extricom

Wireless LAN is one of the most misunderstood technologies in IT. There, I’ve said it. But before I’m accused of raining on the wireless and mobility parade, let me say that I wholeheartedly believe in the transformational powers of wireless. In fact, few technologies can change an enterprise’s operational fabric more than wireless LAN.

The benefits of WLAN have been so extensively written about, that the value proposition and promise of WLAN is clear. Consequently, the momentum toward broader and deeper use of WLAN is increasing. The goal: the multi-application WLAN, providing voice and data services of all kinds.

So, what’s the issue? Well, consider what would happen if I plugged into the wired network and got a 100 MBit connection, while you did the same but received, say, a 49 MBit connection? Someone would probably say “Hey, there must be something wrong with your cable”. In other words, the physical layer of the network would be seen as the culprit.

Interestingly, this is exactly what happens today in a WLAN system. It’s typical for two people to receive a different grade of wireless service, based solely on where they happen to sit relative to the access points. The culprit: the physical wireless medium. Another related example: you’re handed a brand new, really cool Wi-Fi phone. Then you’re told to stand still (don’t move!) while initiating a phone call. In this case, the fault lies in the way the wireless channels are being deployed in the system.

Where’s the disconnect (no pun intended)? Simply this: the business case is based on the value of the wireless application, and rightly so. But, when it comes to implementation, the attention has been on the ‘N’ in WLAN (i.e., “Network”). Discussion of WLAN is dominated by a networking mindset and the better known names in enterprise WLAN are, you guessed it, “networking” companies. What an irony, then, to realize that the IEEE 802.11 specification (a.k.a. WLAN, “Wi-Fi”) pertains to the physical and link layers – the “W” in WLAN (i.e., “Wireless”). So the misunderstanding comes from not being sufficiently aware of, and dealing with those wireless traits that will be the ultimate determinants of project success.

As enterprises move forward with broader-scale deployments, the architecture of the physical wireless transport can present showstoppers. The early adopters have already seen many of these: radio interference that kills system throughput and capacity, unstable wireless connectivity leading to dropped communications, the great irony of wireless without mobility, and the tremendous burden on IT personnel who need to become radio experts in order to have a fighting chance. This all adds up to project risk – those factors that will make the difference between reaping the maximum rewards of a powerful technology and an investment that fails to live up to its billing.

The Wi-Fi Standard – Friend and Foe

So you defined what you want the wireless network to do, for whom, with what hoped-for level of performance. Next, you’ll dramatically improve your chances of success if you take a step back, and separate the hype and perception from the reality of what the IEEE 802.11 standard really does and does not do. The following are some basic facts that will be of significance in the context of an enterprise deployment.

Channels Are Scarce. WLANs operate in a very limited set of channels in two frequency bands. The 2.4 GHz band (by far the most popular choice) has only 3 non-overlapping channels available. The 5 GHz band offers up to 13 non-overlapping channels (depending on regulatory constraints). By any measure, this represents a frequency scarce environment. Since the channel is the pipe that transports the communications traffic, the more efficient the solution is in its utilization of this precious resource, the higher the overall capacity and bandwidth of the resulting network. What you should ask for: proof for how the channels will be used to simultaneously deliver no co-channel interference and maximum throughput.

Mileage Will Vary. The farther you are from an access point (AP), the slower the communication rate that your device will experience. What does this mean for your WLAN users? First, a lower rate means lower total bandwidth. Second, a user located farther away from an AP will receive a different grade of service than one that is closer in – hardly a desirable trait, since it prevents you from specifying a Service Level Agreement (SLA) for users. To address this, the goal must be to find a solution that provides all users, everywhere, the same connection rate performance. Bottom line, connecting to wireless should be as consistent as plugging into the wall. What you should ask for: a guaranteed SLA, defined in terms of a minimum communications rate for each client.

It May be Wireless, but not Mobile. 802.11 is designed for portability (i.e., users sitting in one place), not for mobility (users on the move). That’s because, in a Wi-Fi network, and unlike cellular and other wireless systems, it is the client device that makes the decision to “roam”, rather than that being governed by the infrastructure. How could this impact your success? Applications that are real-time (e.g. voice, streaming data, VPN security sessions) will suffer degraded operation, including dropped calls, buffering, and transmission retries, when the user moves. What you should ask for: seamless mobility through the elimination of the overhead and delays of hand-off transactions.

Mixed Mode 802.11b/g Degrades System Capacity. The vast majority of WLANs operate in the 2.4GHz range, where there are two operating standards - 802.11b and the newer 802.11g. To enable backward compatibility, networks can operate in “mixed mode”. But this backward compatibility comes with a price: significant capacity degradation. In fact, adding just 10% 802.11b users to a system will degrade the overall throughput by 50%, compared to a system that had only 802.11g users on it. In a broad deployment, this represents a significant trade-off between capacity, and the ability to support a full range of device types. What you should ask for: how the mixed mode contention will be reduced or outright eliminated.

Voice and Data Don’t Mix Well. Voice over WLAN (VoWLAN) is one of the most attractive propositions for WLAN in the enterprise. Especially if the same network is also intended to serve all of the data and video needs – the vaunted triple play. Unlike data applications, however, voice requires real-time communications that cannot withstand buffering and retries, a condition that can arise when competing with data users on a congested system. Therefore, if voice and data users are to share the same channel, some mechanism will be needed to give voice traffic a higher priority than data. Recently ratified standards (802.11e) use statistical methods to prioritize voice traffic, but these statistical methods don’t always work when many users are competing for the same bandwidth. The result can be poor Quality of Service for everyone. What you should ask for: how the voice and data contention is managed or outright eliminated

Selecting a Solution

What do the above tell us about Wi-Fi in general? This is a standard whose DNA is based in the consumer electronics arena. When we’re talking about home-based or small office environments (e.g. a handful of access points and users), everything outlined above really has no material impact on system performance. But the above realities of the 802.11 specification can severely impair project success, when the project is on an enterprise (or even just department) level.

So, the first step in evaluating possible solutions is to look at each architecture and ask: “What is its underlying premise?” Seek to understand how the architecture addresses the above wireless traits of Wi-Fi, in the context of an enterprise-scale project. Just as important, insist that your team adopt a systems way of thinking, rather than focusing on speeds and feeds of individual components. That is how they may effectively deal with all of the interrelated design factors that affect WLAN performance.

Once you start your research, you will find that there are generally two types of architecture approaches for you to evaluate, commonly known as Cell based and Channel Blanket based. The Cell based architecture is the more traditional approach taken for data-centric solution, and is geared to optimize either coverage or capacity. The Channel Blanket approach is a more recent topology, targeted at the real-time application environment of voice, delay sensitive applications, multi-tenant/multi-device requirements, mobility demands, and simplicity of design and implementation.

Summary

The wireless LAN has been part of IT’s vocabulary for some time. But a focus on its networking nature, rather than its wireless side, has given rise to a great deal of misunderstanding about what it really can and cannot do. In short, all the applications design and network integration will be for naught, if, at the end of the day, the wireless transport itself is not able to support the desired user scenarios. Armed with sound knowledge of Wi-Fi’s physical layer traits, capabilities, and constraints, the road to success is there for the taking.