Cisco Goes Bigger In 802.11nCisco Goes Bigger In 802.11n

Anyone planning WLAN deployments today should be focusing on 802.11n, and most importantly they should be planning how it can best be deployed.

Cisco today announced a new 802.11n capable wireless LAN access point dubbed the Aironet 1140 that can operate on standard Power over Ethernet (PoE). The new unit also includes a beam-forming technology called ClientLink that is aimed at improving performance in mixed-mode networks. Industry estimates put Cisco's share of the enterprise WLAN market at roughly 65%, so with the migration to 802.11n, you can be certain that the new access point will sell in large numbers.The basic specs for the 1140 are competent but not extraordinary. It comes in a 2.4 GHz single radio ($1,099) and a 2.4 GHz/5GHz dual radio ($1,299) configuration, both of which feature packaging with no external antennas. They also use the same mounting brackets as the Aironet 1200 series units, making changeouts faster and cheaper. The dual radio model will operate in either the 2.4 GHz or 5 GHz unlicensed bands, and can actually support two separate networks simultaneously out of the same unit. Like most other n-capable devices, it can support either standard 20 MHz or 40 MHz double-wide channels for higher transmission rates.

The major performance gain with 802.11n comes from the ability to transmit multiple independent signals, called transmit chains, in the same frequency band; the more transmit chains, the higher the transmission rate. The bad news with the new Cisco AP is that it can only support two transmit chains; it does include three receive antennas (i.e. a 2x3 configuration). The 802.11n standard defines up to 4 transmit chains--though products today (not just Cisco's) have a maximum of 2 or 3.

Still, even with 2-3 transmit chains, the new AP supports a significant increase in bandwidth. The maximum bit rate on a 20-MHz channel with 2 chains is 144 Mbps, and 217 Mbps with 3. The transmission rates on a 40 MHz channel are roughly twice that.

Most notably, both models can operate using standard 802.3af Power over Ethernet (PoE). PoE support is critical for WLAN access points as they are typically installed on the ceiling where there are no available electrical receptacles. With PoE, only a LAN connection is needed to install an access point.

The ability to use standard PoE is crucial: Given the power requirements of 802.11n, most early-generation n-capable access points cannot be supported on standard PoE, necessitating either special powering options, dual LAN connections, or reducing some of the capabilities of the unit (e.g. reducing the number of transmit chains). Siemens delivered the first access point that could be supported on standard PoE almost a year ago, and that is a 3x3 configuration.

The other feature of the 1140 is the incorporation of a beam-forming technique called ClientLink that is designed to optimize performance when n-devices share channels with legacy 802.11a/b/g devices. The essential idea of beam-forming is to build a steerable directional antenna. The basic technology was developed during World War II and has been used in phased array antennas for years. It involves sending radio signals from two or more antennas (two in Cisco's case), and by shifting their relative phases, the radiation pattern can be reinforced in a particular direction. In effect, the access point will be able to point a stronger transmission signal in the direction of the client device, thereby improving link performance. The access point notes the antenna on which the incoming transmission appears stronger to decide how to shape the outbound transmission.

For inbound transmissions, the access point uses Maximum Ratio Combining, a standard feature in 802.11n to improve the receive signal strength. Cisco's announcement cites tests from Miercom that showed an increase in throughput up to 65% for legacy devices using ClientLink, along with a reduction in the incidence of retransmissions. How well this works in the real world remains to be seen.

The 802.11n radio link has been a major step forward in wireless LANs, and will eventually outstrip the earlier 802.11a, b, and g interfaces. However, power consumption is still a challenge for 802.11n, so PoE support is an important step. With the installed base predominantly made up of pre-n devices, the ClientLink technology should help us make the best use of "the old stuff." However, as I noted in a post last year, anyone planning WLAN deployments today should be focusing on 802.11n, and most importantly they should be planning how it can best be deployed.Anyone planning WLAN deployments today should be focusing on 802.11n, and most importantly they should be planning how it can best be deployed.