• 802.16d: Fixed WiMAX

• WiMAX Technical Panel

• QoS Implementation vs. WiFi

• Licensed and Unlicensed Backhauls

• Billing and Network Management

• Point-to-Point Backhauls

• WiMAX Business Case and Panel

• Peer-to-Peer in WiMAX

• WiMAX and QoS Training

The WiMAX 802.16d MAC was designed specifically for the Point-to-Multipoint wireless access environment. It supports higher layer or transport protocols such as ATM, Ethernet or Internet Protocol (IP), and is designed to easily accommodate future protocols that have not yet been developed. The MAC is designed for very high bit rates of the truly broadband physical layer, while delivering ATM compatible Quality of Service (QoS), UGS, rtPS, nrtPS and Best Effort.

The frame structure allows terminals to be dynamically assigned uplink and downlink burst profiles according to their link conditions. This allows a trade-off between capacity and robustness in real-time, and provides roughly a two times increase in capacity on average when compared to non-adaptive systems, while maintaining appropriate link availability.

The 802.16d MAC uses a variable length Protocol Data Unit (PDU) along with a number of other concepts that greatly increase the efficiency of the standard. Multiple MAC PDUs may be concatenated into a single burst to save PHY overhead. Additionally, multiple Service Data Units (SDU) for the same service may be concatenated into a single MAC PDU, saving on MAC header overhead. Fragmentation allows very large SDUs to be sent across frame boundaries to guarantee the QoS of competing services. In addition, payload header suppression can be used to reduce the overhead caused by the redundant portions of SDU headers.

The MAC uses a self-correcting bandwidth request/grant scheme that eliminates the overhead and delay of acknowledgements, while simultaneously allowing better QoS handling than traditional acknowledges schemes. Terminals have a variety of options available to them for requesting bandwidth depending upon the QoS and traffic parameters of their services. They can be polled individually or in groups. They can steal bandwidth already allocated or make requests for more. They can signal the need to be polled and can piggyback requests for additional bandwidth.

In the 802.16d specification, there are several scheduling services for the data handling mechanisms supported by the MAC. The notion of a “connection” here means a Layer-2 link established from each Subscriber Unit (SU) to an Access Point (AP). Each connection is associated with a single data service. Each data service is associated with a set of QoS parameters which quantify aspects of its behavior.

A SU can establish and hold-open several connections, each having different QoS parameters. In this way, a packet classifying function can be implemented in the SU and AP so that packets of differing types can be handled appropriately by the underlying network. What we can achieve with this arrangement is smooth integration of heavy demand for bulk data transfer (TCP transfers of files, video or other data), semi-realtime interactive flows (TCP for secure shell, telnet or other character based systems) and realtime UDP flows (Voice over IP, Video over IP, Video on Demand, multicast).

The WiMAX Specification supports four different data service types:                        

2.    Non Real-time Polling Services (nrtPS)