Enterprising AP

Proxim delivers a highly capable 802.11b/g AP, but 802.11g has limitations.

August 5, 2003

6 Min Read
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The AP-2000 sports a modular design that allows for the insertion of two radio cards to provide flexibility and a clear migration path. The product is plenum rated and supports the IEEE 802.3af PoE (Power over Ethernet) standard. It can handle the most demanding WLAN deployments and is hindered only by the limitations imposed by the 802.11g standard. (See "Pros & Cons of 802.11g" below.)

Straightforward Setup

Proxim's ScanTool discovery software found the AP-2000 on the network and indicated the IP address as assigned by the DHCP server. I logged into the unit through its Web configuration pages. After giving the AP-2000 a static address, I assigned it an SSID (Service Set ID). Although the AP-2000 automatically chooses a channel based on surrounding interference, in my tests it consistently selected channels that overlapped with the channel of another AP in the building, so I set the AP-2000 to an interference-free channel.

Orinoco AP-2000b/gclick to enlarge

Throughput and Range

To get a feel for how the AP-2000 handles traffic in a mixed-mode environment, and how it handles the traffic load of multiple nodes transmitting concurrently, I conducted throughput testing using 11b and 11g clients separately and then simultaneously. Using NetIQ's Chariot 4.3 network-performance tester,I inundated clients with 100 iterations of a 1-MB, TCP-based, long-file receive (filercvl) transfer. I used Proxim's 11a/b/g ComboCard for testing with 650-MHz Pentium III laptops running Windows 2000.

Results for 11b testing were impressive. Tests using both one and two clients yielded aggregate average throughputs just below 6.5 Mbps. Similarly, aggregate average throughputs for 11g clients were strong, returning aggregate results of just over 20 Mbps for both one and two clients--still about 30 percent slower than the fastest 11a products we've tested. Throughput didn't slow until I tested using both 11b and 11g clients at once. One client of each returned an aggregate average throughput of just over 8 Mbps, with each client sharing the bandwidth almost equally. In tests using two clients of each mode, aggregate throughput held steady at 8 Mbps, while each client came in at close to 2 Mbps.

The reason for the slowdown in an environment where both 11b and 11g clients are present is attributable not to the AP-2000 but to the way the 802.11g standard accommodates 11b traffic. The moment an 11b client associates to the 11g AP, a mechanism known as "protection" is implemented. To allow 11b clients to communicate with both 11g APs and clients, all communications are mediated with RTS/CTS (Request To Send/Clear To Send). This solves the backward compatibility problem by requiring that each node request to send data and then receive clearance to send that data--all mediated in a form of traffic that both 11g and 11b clients understand. Unfortunately, using RTS/CTS results in a throughput reduction because of the added control frames that must be transmitted before data may be sent.

To address this problem, Proxim suggests populating one slot of the AP-2000 with an Orinoco 11b radio and the second slot with an 11b/g radio configured to support only 11g traffic. This method will ensure the faster speeds of 11g while supporting existing 11b traffic. However, it requires doubling the use of channels in a band where channel allocation is already a nightmare. Also, with both slots full, supporting 802.11a becomes more difficult--requiring more than just a simple upgrade using Proxim's 11a Kit.

AP-2000b/g's Rangeclick to enlarge

When I set the AP-2000 to operate in 802.11g-only mode, throughput results were nearly identical to results of 11g client tests under the 802.11b/g compatibility mode. The AP-2000b/g can be set to operate in 802.11b-only mode as well.The Atheros-based chipset in the AP-2000b/g enables a proprietary turbo mode similar to that offered by Atheros' 802.11a products. By binding channels, the AP-2000b/g theoretically can reach speeds of 108 Mbps. When I enabled this mode, I noticed that channel selection was narrowed to only one channel. The practicality of this mode in an enterprise setting is unclear. Also, testing in this mode was impossible because the ComboCard doesn't support it.

To test the AP-2000's 11a and 11g ranges, I placed it in an elevated position and conducted a continuous ping to the unit's static IP address from various locations. Maximum range based on packet loss proved nearly identical for both 11b and 11g--an advantage for admins looking for ease of deployment. With coverage at about 85 percent of our 802.11b control--Cisco Systems'

350 AP--the AP-2000's range is only mildly disappointing. The product does allow for an external antenna, which would presumably increase range.Interoperability

To test 802.11b interoperability, I used Cisco's 350 client card. For 802.11g, I used Linksys' WPC54G and D-Link Systems' DWL-G650 802.11g cards. Operation was flawless, and throughputs were at the levels of the Proxim 11a/b/g ComboCard for all three client cards.

The AP-2000 is replete with security options. With support for WEP, WPA (Wi-Fi Protected Access), 802.1x and RADIUS, and promised support for the upcoming 802.11i security standard, the AP-2000 will likely meet any enterprise's security needs.

Management features include full SNMP support, telnet- and console-based CLI (command-line interface), and integration into Proxim's Wireless Network Manager software. The AP-2000 also supports load-balancing and up to two 802.1q VLANs.

Proxim's AP-2000 may not be a perfect fit for every install, but the modular design and availability of multiple radio cards make it an APto consider.Jesse Lindeman is a research associate at the Center for Emerging Network Technologies at Syracuse University. Write to him at [email protected].

Post a comment or question on this story.

802.11g is essentially an amalgamation of the two leading wireless LAN technologies: 802.11b and 802.11a. 802.11g operates in the same 2.4-GHz ISM (Industrial, Scientific and Medical) band used by 11b, but employs the OFDM (Orthogonal Frequency Division Multiplexing) coding technique of 11a to reach a 54 Mbps maximum theoretical data rate. Backward compatibility with 802.11b clients is achieved through 11g's ability to understand and transmit using CCK (Complimentary Code Keying), the coding scheme of 11b. It is from this backward compatibility with 11b that 802.11g's issues with performance and deployment arise.

The 802.11g standard is relegated to the interference-ridden 2.4-GHz band so that microwaves, cordless phones and Bluetooth devices can wreak havoc on the wireless segment. Operating in this frequency also means that 11g administrators are kept to the three nonoverlapping channels 11b uses--five short of what 11a administrators have to play with. The trade-offs for backward compatibility extend to performance as well; 11g's ability to accommodate 11b clients slows aggregate throughput because of the increased overhead associated with being able to communicate with clients of both 2.4-GHz standards.

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