By Thor Olavsrud
January 31, 2003
A key amendment to a standard for wireless metropolitan area networks may help extend to customers the speed and capacity of fiber optic networks without the cost of laying fiber.
The Institute of Electrical and Electronics Engineers (IEEE) this week passed a key amendment to a standard which may open the floodgates for the creation of wireless metropolitan area networks (MANs).
On Wednesday, the IEEE approved 802.16a, an amendment to the IEEE Standard 802.16-2001, which the standards body approved in April 2002.
The original standard specifies the WirelessMAN-SC air interface, a single-carrier (SC) modulation scheme designed to operate in the 10-66 GHz spectrum. That spectrum supports continuously varying traffic levels at many licensed frequencies (including 10.5, 25, 26, 31, 38 and 39 GHz) for two-way communications. This opens the door for the creation of fixed Broadband Wireless Access (BWA), which could provide network access support to buildings with speeds that approach those offered by high-speed fiber optic networks.
While fiber is plentiful on the backbone, it is much more rarified at the network access level. The IEEE said less than 5 percent of commercial structures worldwide are served by fiber networks, largely due to the expense and time involved in extending fiber networks. With 802.16, the IEEE hopes to open the doors for thousands of users to share capacity for data, voice and video, while giving carriers a scalable solution that they can easily grow as subscribers demand more bandwidth.
Reaping the Full Benefits of a Hybrid Network. In the 802.16 vision, carriers would set up base stations connected to a public network. Each base station would support hundreds of fixed subscriber stations, probably mounted on rooftops. The base stations would then use the standard’s medium access control layer (MAC) — a common interface that makes the networks interoperable — to nearly instantaneously allocate uplink and downlink bandwidth to subscribers according to their needs.
Conceivably, 802.16 MANs could anchor 802.11 hot-spots, which serve as wireless local area networks (LANs), as well as servicing end-users directly.
However, the 10-66 GHz spectrum is strictly line-of-sight. That’s where the 802.16a amendment comes in. The amendment addresses the low-frequency 2-11 GHz spectrum, some of which is unlicensed, and which allows for non-line-of-sight operation.
The new 802.16a standard defines three physical layer modes. It retains the single-carrier access method for special-purpose networks, but adds a 256-carrier orthogonal frequency division multiplexed , or OFDM, layer which splits the radio signal into multiple smaller sub-signals that are then transmitted simultaneously at different frequencies to the receiver, allowing for the transmission of large amounts of digital data and reducing multipath (reflections of a signal — caused when the signal bounces off metal obstructions — which can create interference). The standard also defines a 2,048-carrier OFDMA, or orthogonal frequency division multiple access, layer, which offers advanced multiplexing in tiered MANs and supports selective multicast applications.