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IEEE 802.16 for Broadband Wireless Local Loop

Bill Stallings describes a new standard for fixed wireless access for home and office.
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IEEE 802.16 for Broadband Wireless Local Loop

In recent years, there has been increasing interest in wireless technologies for subscriber access in place of the traditional twisted-pair local loop. These approaches are generally referred to as wireless local loop (WLL) or fixed wireless access. To provide a standardized approach to WLL, the IEEE 802 committee set up the 802.16 working group in 1999 to develop broadband wireless standards. In essence, IEEE 802.16 standardizes the air interface and related functions associated with WLL. Three working groups have been chartered to produce standards:

  • IEEE 802.16.1: Air Interface for 10 to 66 GHz

  • IEEE 802.16.2: Coexistence of Broadband Wireless Access Systems

  • IEEE 802.16.3: Air Interface for Licensed Frequencies, 2 to 11 GHz

The work of 802.16.1 is the farthest along and is the most likely to generate the most industry interest, as it's targeted at available frequency bands.

An 802.16 wireless service provides a communications path between a subscriber site and a core network (the network to which 802.16 is providing access). Examples of a core network include the public telephone network and the Internet. IEEE 802.16 standards are concerned with the air interface between the subscriber's transceiver station and the base transceiver station (see Figure 1).

Figure 1 Wireless local loop (WLL).

Protocols defined specifically for wireless transmission address issues relating to the transmission of blocks of data over the network. The standards are organized into a three-layer architecture. The lowest layer, called the physical layer, specifies the frequency band, the modulation scheme, error-correction techniques, synchronization between transmitter and receiver, data rate, and the time-division multiplexing structure. For transmission from subscribers to the base station, the standard uses a DAMA-TDMA (demand assignment multiple access-time division multiple access) technique. DAMA is a capacity assignment technique that adapts as needed to optimally respond to demand changes among the multiple stations. TDMA is simply the technique of dividing time on a channel into a sequence of frames, each consisting of a number of slots, and allocating one or more slots per frame to form a logical channel. With DAMA-TDMA, the assignment of slots to channels varies dynamically. For transmission from the base station to subscribers, the standard specifies two modes of operation—one targeted to support a continuous transmission stream (mode A), such as audio or video, and one targeted to support a burst transmission stream (mode B), such as IP-based traffic. Both are time-division multiplexing schemes.

Above the physical layer are the functions associated with providing service to subscribers. These include transmitting data in frames and controlling access to the shared wireless medium. These functions are grouped into a medium access control (MAC) layer. The MAC protocol defines how and when a base station or subscriber station can initiate transmission on the channel. Because some of the layers above the MAC layer, such as ATM, require specified service levels (QoS), the MAC protocol must be able to allocate radio channel capacity so as to satisfy service demands. In the downstream direction (base station to subscriber stations), there is only one transmitter and the MAC protocol is relatively simple. In the upstream direction, multiple subscriber stations are competing for access, resulting in a more complex MAC protocol. In both directions, a time-division multiple access technique is used, in which the data stream is divided into a number of time slots. The sequence of time slots across multiple TDMA frames dedicated to one subscriber forms a logical channel, and MAC frames are transmitted over that logical channel. IEEE 801.16.1 is intended to support individual channel data rates from 2 Mbps to 155 Mbps.

Above the MAC layer is a convergence layer that provides functions specific to the service being provided. For IEEE 802.16.1, the bearer services include digital audio/video multicast, digital telephony, ATM, Internet access, wireless trunks in telephone networks, and frame relay.

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