* Meru manages contention in mixed 802.11b/g networks
There seems to be virtually unlimited numbers of challenges to tackle in the wireless LAN arena. Radio-frequency interference and bandwidth contention are generally high on user-lists, and Meru Networks, a WLAN switch start-up, last week said it can now keep 802.11g clients from suffering performance penalties when operating in mixed 802.11b/g networks. Mixed b/g networks notoriously suffer in that when g clients transmit, the entire network generally reverts back to b speeds.
At the same time, Meru announced a $795 dual-radio AP200 access point, in which each radio can be programmed to run 802.11b, 802.11b/g, or 802.11a. The company now says it has figured out how to achieve a logical separation between b and g transmissions in the 2.4-GHz band.
802.11b runs at a maximum throughput of 11M bit/sec and uses direct sequence spread spectrum (DSSS) modulation with Complementary Code Keying (CCK) to achieve the 11M-bit/sec speeds. 802.11g, though it runs in the same frequency band as 802.11b, runs at a maximum throughput of 54M bit/sec (albeit at shorter distances) and uses orthogonal frequency-division multiplexing (OFDM) modulation.
Meru says that its scheme keeps 802.11g from operating in backward-compatibility mode by having 802.11g clients send control frames telling the others to back off after a random period using the old CCK encoding scheme, then falling into OFDM mode. This prevents significant OFDM overhead from being incurred by each and every client as the overhead is spread across all clients, speeding things up, says Kamal Anand, Meru vice president of marketing. He says there are “g” transmission periods and “b” transmission periods, but no prioritization of one type of packet over another.
Meru designed its own MAC chip from the ground up, rather than buying off-the-shelf products, changing the interface between the upper and lower portions of the MAC to be synchronous. In this way, its own APs coordinate transmission schedules in order to manage contention.
Between this capability and the new 802.11b and g management, says Joel Vincent, Meru director of product marketing, enterprises should be able to design their WLANs once – say, for b (because of the ubiquity of b clients) and a (for its speed and channel availability). As more g clients become available, g network can be built without having more APs around to adjust for the distance variability between b and a.
CORRECTION: In my recent newsletter, “AES: The cornerstone of 802.11i,” I indicated that AES is a 256-bit encryption mechanism that replaces shorter encryption key lengths in earlier 802.11-specified ciphers. While the AES-Counter Mode with Cipher Block Chaining Message Authentication Code Protocol (CCMP) mode is specified by 802.11i, and AES-CCMP does allow 128-, 192- and 256-bit encryption, the 802.11i standard calls specifically for the 128-bit flavor of AES-CCMP. I regret the error.
