Scheduling Services

Overview

Scheduling services are the medium access control functions (data flow control) that define how and when devices will receive and transmit on a communication system. The types of services that WiMAX can provide range from guaranteed bandwidth with low delay unsolicited grant service (UGS) to random access best effort (BE) service. WiMAX systems use a grant management system to coordinate the request for new services and changes to existing services (such as requesting more bandwidth). The WiMAX system uses a combination of time division multiple access, polling and contention based flow control to provide specific types of services to users.

Time division multiple access (TDMA) is a process of sharing a single radio channel by dividing the channel into time slots that are shared between simultaneous users of the radio channel. When a subscriber communicates on a WiMAX system using TDMA, he/she is assigned a specific time position on the radio channel. By allowing several users to use different time positions (time slots) on a single radio channel, TDMA systems can guarantee a constant data rate with a minimal amount of flow control overhead.

Polling is the process of sending a request message (usually periodically) for the purpose of collecting events or information from a network device. The receipt of a polling message by a device starts an information transfer operation for a specific time period. Polling may be performed with specific units (unicast), to groups of units (multicast) or to all units (broadcast).

Unicast polls are requests for data transmission or responses to commands that are only sent between a sender (polling device) and receiver (polled device). When a subscriber station is responding to a unicast polling message, no other devices are allowed to transmit.

Multicast polls are requests for data transmission or responses to commands that are sent from a polling device to several receiving devices which are part of a multicast group. When a device receives a multicast polling message for its group, it will respond if it has data to send. When a subscriber station is responding to a multicast polling message, others may also have information to transmit. For multicast poll messages, subscriber stations must use contention based access (on the contention slot) to send their data.

Broadcast polls are requests for data transmission or responses to commands that are sent from a polling device to all devices that are able to receive its broadcasted polling message. When a device receives a broadcast polling message, it will respond if it has data to send. For broadcast poll messages, subscriber stations must use contention based access (on the contention slot) to send their data.

The amount of time between polling messages is called the polling cycle. The time between polling cycles is a balance between delay (more polling messages is less delay) and overhead (more polling messages increases the percentage of data that is used for control messages).

Figure 1 illustrates the different types of polling that are used in the WiMAX system. A device that is part of a multicast group, has received a multicast polling message, must compete for access to send its data. Finally, for a broadcast polling message, any device that has data will compete for access to send its data.

Figure 1: WiMax Polling Types

Contention based access control is the independent operation (distributed access control) of communication devices (stations). In a contention-based system, communication devices randomly request service from channels within a communication system. Because communication requests occur randomly, two or more communication devices may request service simultaneously. The access control portion of a contention based session usually involves requiring the communication device to sense for activity before transmitting and listening for message collisions after sending its service request. If the requesting device does not hear a response to its request, it will wait a random amount of time before repeating the access attempt. The amount of time waited between retransmission requests increases each time a collision occurs.

The WiMAX system defines time periods that subscriber stations can use for contention based access. When subscriber units desire to initiate requests to the system that are not scheduled from a polling message, they must access the process during the contention time slots period. The contention time slot period is periodically broadcast on the downlink channel along with other channel access control information.

Figure 2 shows how contention based access control can be performed on a WiMAX system. Channel descriptors are periodically broadcasted on the downlink radio channel that provides the time intervals for the contention slots. Subscriber devices that use contention based access must compete during these time periods. The WiMAX subscriber station will initially attempt to access the system at a relatively low power level. If the subscriber station does not hear a response to its request, it will wait a random amount of time, increase its transmitted power level and attempt access again. The subscriber station will continue to wait increasing amounts of time each time and increases its transmitted power level each time an access attempt fails until it receives a response from the system.

Figure 2: WiMax Contention Based Access Control

The WiMAX system uses a grant management process for the requesting and allocation (granting) of resources (such as transmission time or bandwidth). Subscriber stations can request changes to the type of services they require (e.g. increases or decreases in bandwidth) by transmitting a bandwidth request header and the system can decide to grant, adjust or not authorize the grant request.

The WiMAX system can grant resources based on a connection or based on a specific subscriber station. A grant per subscriber station is the allocation of transmission bandwidth that affects the transmission for all the connections associated with a subscriber station. A grant per connection is the assignment of bandwidth which only affects the transmission for a specific connection on a subscriber device.

Bandwidth requests can be in aggregate or incremental form. An aggregate request is a message that defines the amount of a resource (such as transmission bandwidth) that is requested to provide for a combined group of applications or services. An incremental request is a message that defines the additional amount of a resource (such as transmission bandwidth) that is requested to provide for an application or service. Bandwidth request messages may be sent as stand alone messages or they may be piggybacked in the payload of another packet of data.