Moving from B to A

As wireless networking evolves from 802.11b to 802.11a and beyond, access points that combine operation of the two standards offer the possibility of migration to higher speeds while continuing to use existing infrastructure.

We looked at two such combination access points, the Cisco Aironet 1200 with a 5GHz radio module, and the 3Com Access Point 8200. Either solution will suit the needs of companies looking to move from 802.11b to 802.11a, although from a management perspective, the Cisco unit edges out 3Com’s.

Eyeing the specs

Both access points have 100Mb Ethernet interfaces, 802.11a and 802.11b wireless interfaces, browser-based management software, and the option to power the unit through either direct transformer connection or Ethernet power injection with 48-volt injectors. One notable difference: Cisco shipped the Aironet with 11a and 11b cards, whereas 3Com included no cards with the Access Point 8200.

3Com provides a pair of internal diversity antennas for 802.11a. Cisco’s 1200 has four antennas: two function as diversity antennas for greatest range when the antenna panel is extended; the other two function as a patch antenna for widest coverage pattern when the antenna is folded. The antenna option is nice, although most users are likely to find that the antenna choice matters less than furnishings and walls when determining 802.11a performance.

Management differences

3Com’s browser-based management software is easier to navigate than Cisco’s. Categories of actions are listed in a navigation bar on the left side of the window, and options within the particular action are shown on the right. 3Com offers 64- and 128-bit WEP (Wired Equivalent Privacy) encryption for 802.11b, and 64-, 128-, and 152-bit WEP encryption for 802.11a. Filters are available to enforce policies on client-to-client communications and various packet types. The software also offers support for 802.1x security with either RADIUS (Remote Authentication Dial-In User Service) or local MAC address authentication. The Access Point 8200 provides an SNMP client and a variety of event logging levels, with logs generated in text format.

Cisco’s management software provides far more control over individual operating parameters and significantly greater built-in security options than does the 3Com management software. You can select data rates for each radio interface and decide which rates will be used for management packet signaling.

The software provides simple, useful radio diagnostics for both 11a and 11b. Cisco provides a wide range of security options, including 40- and 128-bit WEP, 802.1q VLAN support, 802.1x support with RADIUS, TACACS (Terminal Access Controller Access Control System), or local MAC address authentication, and a choice of three 802.1x protocol versions for EAP (Extensible Authentication Protocol). Cisco also provides Proxy Mobile IP, accounting support, and support for a variety of Cisco-only services (Cisco Discovery Protocol, distributing firmware and configuration to other devices) to round out it management capabilities.

The Aironet 1200 can generate SNMP traps, allowing for a variety of actions based on the type of event. There is an option for generating system log messages in the Cisco EMBLEM format for better integration with a Cisco enterprise network.

The drawback to Cisco’s management tool is its complexity. Early in the testing process, we set some parameters, such as the primary radio interface; finding them again required a long search through many individual screens. 3Com’s management application is much simpler to navigate. There are no deeply nested screens: Fourteen mouse clicks will let you see every management possibility.

Distance slowdowns

A common question is, how does the range for 802.11a compare to that of 802.11b? It’s understood that 802.11a will be faster, but at what cost in terms of distance? Because of the physics involved with radio-frequency signals at 11a frequencies (in the 5GHz range) versus 11b frequencies (2.4GHz), 11a signals will be far more affected to a much greater extent by walls and furnishings within the office environment.

We used the Airmagnet laptop console to monitor received signal levels on all 11a and 11b channels while moving around our office facility. As expected, the 11a signal was weaker than the 11b signal as we moved away from immediate proximity to the access point. We also found that, for purposes of a useable network link, it didn’t matter: 11a and 11b had approximately the same usable range in our tests. We attribute this to the fact that the spectrum used by 802.11b is used by many other devices and services — cordless telephones, microwave ovens, and a host of unregulated devices — which create a constant noise “floor” over which the networking signal must project. We disabled all other access points in our facility before testing began and still saw significant noise levels on every 802.11b channel during the time we were monitoring signals.

Our testing indicated that both 802.11a and 802.11b interfaces on both the 3Com and Cisco provided usable links within the confines of our testing facility. However, we found that performance stepped down rapidly from 54Mbps as distance increased. At the 70-foot range, performance was down to 18Mbps, and occasionally 12Mbps, for both 3Com and Cisco units. Although these results were better than those of 802.11b, they fell short of the 54Mbps speed promised by 11a supporters.

802.11b performance at all tested distances was generally at 11Mbps, with occasional drops to 6Mbps at 70 feet. The fact is that 54Mbps is difficult to sustain with any obstacles — walls, furniture or people — between access point and client. Both access points have sufficient processor capacity to maintain a performance level with multiple clients. But the performance level, especially for 802.11a, will be lower than maximum unless all clients are in the same room.

The Cisco Aironet yielded the most surprising test results. When we looked at signal strengths for both 11a and 11b radios, we found there was no signal from the 11b unit. The 11b internal radio didn’t begin broadcasting a signal until we designated it as the primary interface. The initial readings were taken at a range of about 5 feet from the access point. Although the signal strength was excellent, Airmagnet indicated that all traffic was being handled at just 6Mbps on the 802.11a link and at 1Mbps on the 802.11b link. We had to go into the management console and change parameters for the radio before the link speeds would shift to 54Mbps and 11Mbps.

Companies looking to move into 802.11a while maintaining the value of their 802.11b investments will find use for either the Cisco Aironet 1200 or the 3Com Access Point 8200. Both units provide solid radio and network performance. But the Aironet’s management software is superior to that of the 8200 because it provides far more options in crucial areas such security, and far greater control over operating parameters.