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802.11n Is Coming -- Here's What To Expect

Besides going through the busy work of arranging for briefings at next week's Interop in Las Vegas, I've had the pleasure over the last few weeks of receiving some roadmap and pre-announcement briefings regarding IEEE 802.11n. For those keeping score, Xirrus, Meru and Colubris have all announced their plans, while Aruba, Cisco, Extricom, Motorola, Siemens and Trapeze have still to do so. I predict that by the end of third quarter 2007 most, if not all, of the vendors will have a story to tell.
Without breaking any embargoes, I've gathered some interesting points regarding 802.11n that deserve further scrutiny, and I hope to get a better handle on these in weeks and months ahead. If you have any insight or thoughts, please drop me a line (at [email protected]).

Central Vs. Distributed: If you thought the debate was over, I can tell you that it has only just begun. Those unwieldy 'fat' APs (access points) of yore were trashed for 'thin' APs, with all the traffic flowing to the core. With 802.11n holding out the promise of raw data throughputs of 600 Mbps (actual payload throughputs about one-half to two-thirds, with rates dependent on the number of radio chains, environmental conditions and channel bonding), some vendors are changing their pitch, pointing out that wireless controllers/switches will not be able to handle the tsunami of 802.11n traffic. Their answer has been to separate the management and control plane from the data plane. What this means is that the AP performs the switching of client traffic onto the wired network rather than the controller. APs are now required to
perform all the security key management, session state management, network and security policy, and any firewall/access control list functions that used to be performed at the core. This is the route that Aerohive, Colubris and Trapeze are following. Meru is taking a hybrid approach by pushing its controllers to the edge switching closet, essentially scaling out its operations, but now gaining an interface into wireline traffic at the edge vs. the core.

There are several purported benefits: Splitting the planes eliminates the likely need to upgrade or add controllers at the core. Although this is true, it adds a significant amount of complexity to the AP's data switching. All the tasks previously performed in a centralized manner at the core now need to be performed on the limited processing capabilities and resources of an access point. Any policy-based user or group rule sets need to be implemented at the edge, even if there are 40 users attached to the AP.

Shifting wireless client traffic from the AP directly onto the edge switching infrastructure also requires the switch port to expose the necessary VLANs (virtual LAN). Previously, the APs were VLAN-agonistic, as all the traffic was tunneled to the core; now, network administrators need to make sure their VLAN design can support the wireless network. Because network administrators are likely to be reticent to deploy every wireless client VLAN to the switch port of every AP, roaming now introduces a new complexity. Whereas before the controller anchored every client connection and created tunnels between controllers as necessary, an AP now may need to act as that anchor if the Wi-Fi client roams to an AP that does not have direct access to the client's VLAN. It's not clear at this point how Colubris and Trapeze implement this L3 roaming (Aerohive has this task on its roadmap for the third quarter '07) and what kind of scalability limits there are, but it will be an important question.

Another purported benefit of AP-based client traffic switching is reduced traffic levels, lower latency and decreased jitter. In a larger installation, where there may be a department server and other resources, a local switching solution avoids sending the traffic to the core and back again. It's not clear how much east-west vs. north-south traffic there is in a typical organization, but this makes eminent sense in a headquarters/branch-office configuration. On the other hand, in a campus deployment most wireless usage is to access e-mail, Web pages and other Internet content. File and print may be the only local resources. Concern about saturating the distribution layer with unnecessary and high volumes of traffic seems unlikely. Most networks have 1-Gbps links at the distribution layer. While 802.11n will support higher access speeds, Wi-Fi still makes up a small percentage of overall LAN traffic and higher link rates do not necessarily translate into greater usage, except perhaps for higher bursts. There's no reason to believe that wireless is the straw that broke the gigabit link.

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