Topic: What is TBF?

TBF - Temporary Block Flow

A Temporary Block Flow (TBF) is a logical connection used by two General Packet Radio Service (GPRS) entities to support the unidirectional transfer of lower layer compatibility (LLC) protocol data units (PDUs) on packet data physical channels(PDCHs)(or timeslots). In a typical GPRS system, the network needs to establish a downlink (DL) TBF to transfer data in the DL direction and an uplink (UL) TBF to transfer data in the UL direction. TBFs are typically short-lived and are generally only active during data transfers.

A mobile station (MS)with an active downlink TBF, requests uplink resources by sending a channel request tothe network as a part of a Packet Downlink Ack/Nack (PDAK) message. The PDAK message is also used to acknowledge the radio link control (RLC) data blocks received in the downlink direction. The network solicits a PDAK message by setting the "poll" bit in a DL RLC data block. The network, or packet control unit (PCU), sends the mobile a Packet Uplink Assignment (PUA) message to assign resources for an uplink TBF. ThePUA contains a timeslot allocation for the MS's uplink TBF. The MS can then start sending uplink RLC data blocks on the timeslot(s) allocated in PUA.

The MS initiates the release of the uplink TBF by beginning the countdown process (as described in 3GPP specification 04.60, sub clause 9.3.1). During an uplink TBF, when the mobile station has only BS_CV_MAX number of blocks remaining to be transmitted in this TBF, the mobile station decrements the Countdown Value (CV) in theRLC/MAC data block header of each subsequent RLC data block sent to the network until the mobile sends the last RLC data block with CV=0.
If the network has received all the RLC data blocks when it detects the end of theTBF (i.e., when CV=0), it will send the Packet Uplink Ack/Nack (PUAK) message with the Final Ack Indicator bit set to `1`. When the received PUAK message has the FinalAck Indicator bit set to `1`, the mobile station shall transmit the Packet Control Acknowledgment (PCA) message and release the TBF.

In existing GPRS systems, if the mobile has a limited amount of data to send to the network, it establishes a UL TBF and starts counting down soon after it reaches BS_CV_MAX number of blocks remaining to be transmitted. The count down mechanism allows the network to realize that the uplink data transfer is ending and allows the network to better utilize uplink resources. However, once the mobile starts its countdown procedure, it cannot append more RLC data blocks (than it originally intended for this TBF) with new data into the same uplink TBF. In order to send any new data, the mobile needs to terminate the present UL TBF and re-establish a new UL TBFafter the current TBF is terminated.

In existing GPRS systems, UL TBF establishment delays (during a DL TBF) are costly, ranging from 180 ms to 360 ms depending on the round-trip delay between the base transmitter station (BTS) and the base station controller (BSC)/PCU. In multiple-wireless application protocol (WAP)-type GPRS sessions, several UL TBFs are established, typically during ongoing DL TBFs. Each such UL TBF suffers the round-tripdelay. However, when poor wireless conditions are prevalent, the delays experienced can exceed 5 seconds. This can occur when UL requests, during an ongoing DL TBF, are lost because of the poor wireless conditions. Subsequent requests may not be sent for over 5 seconds, causing a serious delay in UL traffic.

Therefore, a need exists for an apparatus and method for providing uplink communication resources that reduce some of the existing delays in acquiring these resources.



Re: What is TBF?

Thanks a lot for the doc.