Friday, November 27, 2009

Unsolicited Grant Service (UGS)

Unsolicited grant service is a service flow in which the transmission system automatically and periodically provides a defined number of timeslots and fixed packet size that is used by a particular receiver. UGS is commonly used to provide services that require a constant bit rate (CBR) such as audio streaming or leased line (e.g. T1 or E1) circuit emulation.

UGS provides a constant bit rate for a single connection. A subscriber device may need additional bandwidth for an additional service that is added to a connection or to temporarily provide more bandwidth on the UGS connection. To request more bandwidth on a UGS connection, a poll me bit or slip indicator bit may be used.

A poll me bit is a signaling message in a data field within the header of a data packet that indicates that the device would like to be polled. The poll me bit indicates to the base station that the subscriber device needs to be polled for a service other than for the current UGS service.

For transmission to synchronous connections, timing inaccuracies may result in the need to transfer additional bits if the clock of one connection is slightly faster than the other connection. When the buffer of the faster connection indicates the number of bits to be transmitted may soon run out, a slip indicator bit may be used. The slip indicator is a signaling message within the header of a data packet that indicates that the data transmission queue of that device is changing (slipping) and that the device needs more bandwidth to keep up with the transmission queue. This allows the base station to temporarily assign additional bandwidth until the transmission buffer has caught up.

Figure 1 shows how WiMAX unsolicited grant service (UGS) operates. Subscriber stations are assigned to receive and transmit during assigned time intervals. The subscriber station may use the poll me bit in the header to indicate it wants to be polled so it can send data for another service. When the base station receives the poll me bit, it sends a polling message which allows the subscriber station to send a packet of data that is independent of the UGS packets.

Figure 1: Wireless Unsolicited Grant Service (UGS)
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