主題： IEEE 802.16 standard標準技術淺釋
The IEEE 802.16 standard, the specifications for the global deployment of broadband wireless metropolitan area networks, was approved in December 2001 and was followed by three amendments: 802.16a, 802.16b and 802.16c to address issues of radio spectrum, quality of service and interoperability respectively. It implements the “first-mile/last-mile” connection in wireless metropolitan area networks. An IEEE 802.16 wireless service provides a communication path between a subscriber site and a core network. Examples of a core network are the public telephone network and the Internet. The IEEE 802.16 is concerned of the air interface between a subscriber’s transceiver station and a base transceiver station.
The IEEE 802.16 standard is organized into a three-layer architecture:
1. The physical layer (lowest layer) specifies the frequency band, modulation scheme, error-correction techniques, synchronization between transmitter and receiver, data rate and the time-division multiplexing (TDM) structure.
For transmission from subscribers to a base station, the standard uses the Demand Assignment Multiple Access-Time Division Multiple Access (DAMA-TDMA) technique. It 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 TDM schemes.
2. The media access control (MAC) layer above the physical layer is associated with providing services to subscribers. This is the functions including transmitting data in frames and controlling access to the shared wireless medium. The MAC protocol defines how and when a base station or subscriber station may initiate transmission on the channel.
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 compete for access, resulting in a more complex MAC protocol. In both directions, a TDMA technique is used in which the datastream is into a series of time slots. The sequence of time slots across multiple TDMA frames that are dedicated to one subscriber and forms a logical channel, and MAC frames are transmitted over this logical channel. The IEEE 802.16.1 is intended to support individual channel data rates from 2M to 155M bit/sec.
3. Above the MAC layer is a convergence layer that provides functions specific to the service being provided. The services include digital audio/video multicast, digital telephony, ATM, Internet access, wireless trunks in telephone networks and frame relay. It supports point-to multipoint architecture with transimitting at data rates up to 120Mbps.
The following diagram shows how wireless traffic works between subscribers and core networks under the IEEE 802.16 standards.
The IEEE 802.16 standard focuses on the efficient use of bandwidth between 10 and 66 GHz and defines a medium access control (MAC) layer that supports multiple physical layer specifications customized for the frequency band of use. It supports both frequency and time division duplexing (FDD and TDD).
The intention of IEEE 802.16 application : Owing to the insufficient IEEE802.11 standards, more and more companies desires to deploy wireless connectivity over larger areas, such as cities and rural areas with less expensive but much effective standards-based solutions. The IEEE 802.16 Broadband Wireless Access (BWA) systems offer true differentiated broadband services at lowest cost and higher speed. It is also a scalable solution to extend optical fibre backbone. More and more users can share capacity for data, voice and video easily. For the time being, there is no physical products yet with IEEE 802.16 standard.
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802.16a能提供語音與影片傳輸等這類要求低延遲 (Low Latency)的服務，語音服務包括傳統的分時多工(TDM)及網路電話(VoIP)。