• Wireless Networks - WiMAX (802.16)

    WiMAX , which stands for Worldwide Interoperability for Microwave Access, was designed by the WiMAX Forum and standardized as IEEE 802.16. It was originally conceived as a last-mile technology. In WiMAX’s case that “ mile ” is typically 1 – 6 miles, with a maximum of about 30 miles, leading to WiMAX being classifi ed as a metropolitan area network (MAN). In keeping with a last-mile role, WiMAX does not incorporate mobility at the time of this writing, although efforts to add mobility are nearing completion as IEEE 802.16e. Also in keeping with the last-mile niche, WiMAX’s client systems, called subscriber stations , are assumed to be not end-user computing devices, but rather systems that multiplex all the communication of the computing devices being used in a particular building. WiMAX provides up to 70 Mbps to a single subscriber station.

    In order to adapt to different frequency bands and different conditions, WiMAX defi nes several physical layer protocols. The original WiMAX physical layer protocol is designed to use frequencies in the 10- to 66-GHz range. In this range waves travel in straight lines, so communication is limited to line-of-sight (LOS). A WiMAX base station uses multiple antennas pointed in different directions; the area covered by one antenna’s signal is a sector . To extend WiMAX to near-LOS and non-LOS situations, several physical layer protocols were added that use the frequencies below 11 GHz (in the 10- to 11-GHz range, WiMAX can use either the original physical layer or one of the newer ones). Since this range includes both licensed and license-exempt frequencies, each of these physical layer protocols defines a variant better adapted to the additional interference and the regulatory constraints of the license-exempt frequencies.

    The physical layer protocols provide two ways to divide the bandwidth between upstream
    (i.e., from subscribers to base station) and downstream traffic: time-division duplexing (TDD) and frequency-division duplexing (FDD). TDD is simply STDM of the two streams; they take turns using the same frequency, and the proportion of upstream to downstream time can be varied dynamically, adaptively , by the base station. FDD is simply FDM of the two streams: one frequency is used for upstream and another for downstream. In license-exempt bands, the protocols use only TDD.

    Both channels, upstream and downstream, must be shared not just among the many subscriber stations in a given sector, but also among the many WiMAX connections that each subscriber can have with the base station. WiMAX — unlike 802.11 and Ethernet — is connection oriented. One reason for this is to be able to offer a variety of QoS guarantees regarding properties such as latency and jitter, with the aim of supporting high-quality telephony and high-volume multimedia in addition to bursty data traffic. This is conceptually similar to some of the wired last mile technologies (such as DSL) with which WiMAX is intended to compete.

    Sharing of the upstream and downstream channels is based on dividing them into equal-sized time slots. A WiMAX frame generally takes up multiple slots, with different frames taking different numbers of slots. The downstream channel (from base to subscribers) is relatively easy to subdivide into connections since only the base station sends on that channel. The base station simply sends addressed frames, one after the other. Each subscriber station in the sector receives all the frames, but ignores those not addressed to one of its connections.

    In the upstream direction, how a connection gets handled depends on its QoS parameters.
    Some connections get slots at a fixed rate, some get polled to determine how many slots they need currently, and some must request slots whenever they need them. Connections in this last category must contend to place their requests in a limited number of upstream slots set aside for contention. They use an exponential back off algorithm to minimize the chance of a collision, even on the first attempt.

    A European alternative to WiMAX is HIPERMAN, which stands for high-performance radio metropolitan area network and uses the 2- to 11-GHz range. South Korea’s WiBro (for wireless broadband) technology operates at 2.3 GHz, and is being brought into line with the emerging IEEE 802.16e standard for mobile WiMAX.

    Source of Information : Elsevier Wireless Networking Complete 2010


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