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ery one making use of a fraction of the total number of sub-channels at the same time. The standard IEEE 802.16-2004 makes better last-mile delivery in numerous key features:

Multi-path interference
Delay spread
Robustness
The multi-path interference and delay spread enhance performance in situations When there is no straight LOS path among the BS and the SS. The MAC management messages contained in 802.16 specification permit the BS to inquire the SS, but a definite amount of time delay is involved there. TDD is used by WiMAX products functioning in un-licensed bands, products functioning in licensed bands will use either TDD or FDD [25].

3.2 RANGE and SCALABILITY
The standard 802.16-2004 is dependent on a grant-request access protocol that doesn't permit data collisions, as is in contention-based access used in 802.11 and, consequently, the available bandwidth is utilized more effectively. The BS synchronizes entire communication [25]. Further characteristics of the standard 802.16-2004 comprise:

Improved user connectivity
The 802.16-2004 standard maintains more users connected due to its flexible channel widths and adaptive modulation. Since it uses channels narrower instead of fixed 20-MHz channels used in 802.11, lower data-rate subscribers can be served by the 802.16-2004 standard without misusing bandwidth. When subscribers come across noisy conditions or low signal strength, they might be dropped but the adaptive modulation scheme keeps them connected.

Higher quality of service
This standard also facilitates WISPs to guarantee QoS for customers who need it and to tailor service levels to meet different customer requirements. For example, the 802.16-2004 standard can promise high bandwidth to business customers or low latency for voice and video applications, whereas residential Internet surfers can only be provided with best-effort and lower-cost service.

Full support for WMAN service
In contrast to last-mile implementations based on the 802.11g standard, 802.16-2004 is capable of supporting more users and deliver faster data rates at longer distances.

Robust carrier-class operation
The standard was intended for carrier-class operation. As more users join, they have to share the cumulative bandwidth and their individual throughput reduces linearly. The reduction, however, is not as dramatic as experienced under 802.11. This ability is termed “efficient multiple access.”

3.3 FLEXIBLE CHANNEL BANDWIDTH
When subscriber moves by walking or driving in a car, the distance among a subscriber and the base station (or AP) increases, then it becomes more difficult for that subscriber to transmit successfully at a given power level back to the base station. For power-sensitive devices such as laptop computers or handheld devices, if the channel bandwidth is broad, then it's not possible for them to transmit to the base station over long distances. To make possible transmission over longer ranges and to different types of subscriber devices, 802.16 has elastic channel bandwidths between 1.5 and 20 MHz. In addition, this flexibility of channel bandwidth is also important for cell planning, particularly in the licensed spectrum. For scalability, an operator with 14 MHz of accessible spectrum, may have multiple sectors (transmit/receive pairs) on the same base station by dividin