EDGE GPRS TBF 2

General

The responsibility for the handling of a TBF is split up in the BSS between the so called connection control part and the transfer part.

The connection control part handles:

• set up of a TBF The goal of this UD is to describe 'the life of a TBF' meaning that the following chapters do not follow the functional order listed above. [1]. Any secondary TBF (requested while the first TBF is alive) is setup as described in this document.

GPRS/EGPRS Load Optimization is an optional feature intended to let TBFs sharing the same radio resources adapt the scheduling to the quality of the radio environment. A TBF with low throughput due to a bad radio environment is scheduled more seldom. A TBF in a better radio environment is then scheduled more often and this increases their throughput. The available radio resources are used more by a TBF that has a good radio environment and the number of users with a relatively high throughput in the network increases.

In order to put a TBF in Low Scheduling Mode the following conditions must be fulfilled:

A GPRS Attached Class B GPRS/EGPRS MS can indicate to the SGSN that it will temporarily suspend its GPRS/EGPRS service. Suspend is done when a circuit switched connection is about to be established. When the circuit switched connection is completed then the Resume procedure may be initiated by the BSC. A prerequisite for the Resume procedure is that the Suspend procedure was successful. If the resume procedure is not initiated by the BSC or it fails then the GPRS/EGPRS MS is expected to perform a Routing Area Update after the circuit switched connection is released to indicate to the SGSN that it has resumed the suspended GPRS service. to 48.018 maximum number of suspend retries. Then the suspend procedure is considered unsuccessful when the circuit switched connection is released.

The purpose of the TBF upgrade is to add PDCHs to a TBF without moving the TBF from its current PDCHs. The functionality described in the UD GPRS/EGPRS Channel Administration is responsible for the provision of eligible TBF upgrade candidates and for the PDCH Reservation. The following PDCH reservation enhancements are enabling the upgrade of a TBF:

• Upgrade of PDCH reservation.

• Dynamic downlink/uplink PDCH reservation.

See Reference [5] for more information about PDCH reservation in general and the enhancements mentioned above.

The purpose of the re-reservation is to get more resources and more bandwidth. The GPRS/EGPRS Channel Administration feature is responsible for the provision of eligible TBF re-reservation candidates and for the PDCH Reservation.

There are two different types of re-reservations: • Re-reservation due to adding resources which is moving an existing TBF from channels that in some way do not fulfil the requirement of the TBF, to channels that better correspond to the requirements, for example when the only way to upgrade a TBF is to move it first. A TBF occupying PDCHs of a PDCH type that it can not fully use is also a candidate for re-reservation.

• Re-reservation due to balancing which is also a way of re-reservation. This is not to improve a specific TBF but level out the total TBF load on the PDCHs. An existing TBF is moved from channels with high TBF load to channels with lower load. The imbalance may occur when TBFs sharing the same channels are released.

See Reference [5] for more information.

The re-reservation of a TBF differs depending on if both the uplink and the downlink TBF are to be moved or if only the downlink TBF is to be moved.

A TBF is released if:

• there is no more data to transfer in the downlink or uplink direction and the delayed downlink release timer or the extended uplink TBF timer expires.

• the GPRS/EGPRS MS makes a cell reselection to another cell.

• an uplink TBF is lost.

• the PDCH used for sending the continuous TA to the GPRS/EGPRS MS is preempted or becomes faulty.

• the GSL becomes faulty.

• the cell is halted.

• GPRS/EGPRS support is deactivated in the cell.

For a downlink streaming TBF also the timers 'Streaming Start Timer' and 'Streaming Pending Timer' must not be running.

Set Up of Uplink TBF Using PCCCH

The purpose is to establish an uplink TBF to support the transfer of data in the uplink direction. Since, in this scenario, there is a Master PDCH in the cell, a GPRS/EGPRS MS in Standby state is camping on the PBCCH.

The set up of an uplink TBF using PCCCH is always initiated at the reception of one of the following messages, on PRACH, from the GPRS/EGPRS MS: The set up of uplink TBF using PCCCH will fail and the 44.060 PACKET ACCESS REJECT message is sent to the GPRS/EGPRS MS on PAGCH if:

• No resources are available. E.g. no PDCH, TFI, USF and TAI.

• The access delay is greater than the maximum timing advance allowed in the cell.

The proceeding events described below depend on the access type received in the channel request from the GPRS/EGPRS MS.

The set up of uplink TBF using CCCH will fail and the message 44.018 IMMEDIATE ASSIGNMENT REJECT is sent to the GPRS/EGPRS MS on AGCH if:

• No resources are available. E.g. no PDCH, TFI, USF and TAI.

• The access delay is greater than the maximum timing advance allowed in the cell.

The proceeding events described below depend on the establishment cause or the access type received in the channel request from the GPRS/EGPRS MS. Access Type is regarded as establishment cause in the following text.

Establishment Cause is One Phase Packet Access

The GPRS/EGPRS MS may be capable of handling several channels, but only one PDCH can be reserved since the GPRS/EGPRS MS capability is unknown.

Establishment Cause is Single Block Packet Access

The Single Block Packet Access is actually a type of two-phase access (very similar to the PCCCH case). The Single Block Packet Access is requested by the GPRS/EGPRS MS in the channel request message. The purpose of the first phase is to assign a temporary PDCH to the GPRS/EGPRS MS that is used to send additional information to the BSS. The purpose of the second phase is to assign the PDCHs that are to be used for the actual data transmission.

Set Up of Uplink TBF Using PACCH

The purpose is to establish an uplink TBF to support the transfer of data in the uplink direction. A prerequisite, in this scenario, is that the GPRS/EGPRS MS is in Ready state and already assigned to a downlink TBF.

Set Up of Downlink TBF Using PCCCH

The purpose is to establish a downlink TBF to support the transfer of data in the downlink direction. Since, in this scenario, there is a Master PDCH in the cell, a GPRS/EGPRS MS is camping on the PBCCH.

A GPRS/EGPRS MS in standby state will first be paged according to procedures described in Reference [6]. The GPRS/EGPRS MS then responds to the page, with a set up of an uplink TBF using PCCCH described in Section 3.2.2 on page 6 (Access Type = Page Response), by sending any LLC PDU to the SGSN. The GPRS/EGPRS MS now enters ready state and the SGSN may start sending LLC-PDUs.

A GPRS/EGPRS MS in ready state can receive LLC-PDUs from the SGSN at any time.

The received LLC-PDUs may contain information about the capability of the GPRS/EGPRS MS. This information is used by the BSS to calculate the number of PDCHs that will be reserved for the TBF. If the LLC-PDU does not contain this information, an attempt is made to obtain this information from a recently released TBF that the same GPRS/EGPRS MS had been using. Downlink scheduling is started and the downlink TBF is considered established when an acknowledge of the first data has been received. If an acknowledge is not received then another six attempts to send 44.060 PACKET DOWNLINK ASSIGNMENT is made in accordance with the section above.

If Loss Free Preemption applies (see Section 3.11.4 on page 37) the buffer that was kept at release is used to continue the transfer.

Set Up of Downlink TBF Using CCCH

The purpose is to establish a downlink TBF to support the transfer of data in the downlink direction. Since, in this scenario, there is no Master PDCH in the cell, a GPRS/EGPRS MS is camping on the BCCH. Downlink scheduling is started and the downlink TBF is considered established when an acknowledge of the first data has been received. If an acknowledge is not received then another six attempts to send 44.060 PACKET DOWNLINK ASSIGNMENT is made in accordance with the section above.

If Loss Free Preemption applies (see Section 3.11.4 on page 37) the buffer that was kept at release is used to continue the transfer.

Normal Setup of Downlink TBF Using PACCH

The purpose is to establish a downlink TBF to support the transfer of data in the downlink direction. A prerequisite, in this scenario, is that the GPRS/EGPRS MS is already assigned to an uplink TBF.

Normal setup of Downlink TBF using PACCH is initiated in the BSS when a LLC-PDU has been received from the SGSN. If this message is received, the downlink scheduling is started and the downlink TBF is established.

PDCHs and TBF is finally released if/when BSS timer T3193 expires after which normal set up of downlink using PCCCH, CCCH or PACCH will have to be used to set up a new downlink TBF.

If Loss Free Preemption applies (see Section 3.11.4 on page 37) the buffer that was kept at release is used to continue the transfer.

Ongoing Uplink TBF

Introduction

After having set up the uplink TBF according to one of the set up of uplink TBF procedures described in Section 3.2 on page 6, the GPRS/EGPRS MS starts monitoring the downlink transmission on each of the PDCH(s) that has been assigned to it. It is explicitly informed when the GPRS/EGPRS MS has the permission to send an uplink radio block.

Scheduling of Uplink Data Blocks and Uplink Control Blocks

An GPRS/EGPRS MS can be assigned several PDCHs, and several GPRS/EGPRS MSs can be assigned to each PDCH. See Figure 1

Reserved USF

The Reserved USF is a USF value which is not assigned to any one particular GPRS/EGPRS MS. It is used to prevent more than one GPRS/EGPRS MS from attempting to transmit on the same PDCH in the same block period.

The PCU may inform an GPRS/EGPRS MS that one or more future uplink blocks have been reserved for it on a particular PDCH. The GPRS/EGPRS MS may have an uplink or downlink TBF, or no TBF at all. When the reserved radio block period is reached, the network sends the Reserved USF. Only the GPRS/EGPRS MS to which this block has been assigned is allowed to transmit.

The Reserved USF is used in the following situations:

• During the second phase of a two-phase access. • If USF Granularity is in use (see below). The GPRS/EGPRS MS uses its three reserved blocks to send RLC data blocks and/or control messages.

Free USF

The free USF is only available on a Master PDCH. It is used to denote that the next uplink block period is for PRACH only. This channel is utilized during the first phase of a set up procedure. All GPRS/EGPRS MSs are allowed to transmit a 44.060 PACKET CHANNEL REQUEST or 44.060 EGPRS PACKET CHANNEL REQUEST on PRACH. The free USF is used in all downlink blocks if there are no GPRS/EGPRS MSs being scheduled on uplink and according to PRACH allocation. For PRACH allocation see Reference [6].

Multiplexing EGPRS/GPRS mode TBFs using USF Granularity

When allocating uplink resources to a GPRS only capable MS, downlink radio blocks must use a GMSK-based coding scheme to allow a GPRS MS to read its USF value.

When there is no MPDCH in the cell, the following applies:

• If a GPRS only capable MS has an UL TBF on EGPRS channels, USF Granularity is applied.

• In all other cases USF Granularity is not applied.

When there is a MPDCH in the cell, the following applies:

• USF Granularity is not applied.

The reason for not applying USF Granularity for the MPDCH case is that USF Granularity would (if used) degrade performance of an EGPRS DL transfer.

When USF Granularity is used, a GPRS MS that reads its own USF value, in the header of a downlink RLC/MAC block, may send uplink data in the next four radio block periods on the PDCH. The network then transmits the Reserved USF with the next three downlink RLC/MAC blocks. Hence, with USF Granularity a GMSK radio block need only be sent every fourth downlink block, limiting the negative impacts on the EGPRS downlink throughput (for any EGPRS MSs using the same channels as were USF Granularity is applied).

If a GPRS MS has an ongoing uplink or downlink GPRS TBF, it must receive at least one GMSK radio block every 360 ms in order to maintain synchronization. The GMSK radio block actually does not need to be destined for that specific GPRS MS.

No Reply to a USF

A GPRS/EGPRS MS is given a certain limited number of times to transmit an RLC/MAC block in response to its USF. If no RLC/MAC block is received from the GPRS/EGPRS MS, the connection is presumed lost and TBF is released.

Handling of LLC-PDUs and RLC/MAC Blocks

Every 20 ms the Physical Link layer in BSS can receive an RLC/MAC block on each PDCH.

RLC data blocks contain a TFI to indicate which TBF they belong to. Within each TBF, blocks are numbered sequentially to allow them to be reassembled into LLC-PDUs.

LLC-PDUs are assembled using data from one or more RLC data blocks, and are transmitted together with control information to the SGSN over the Gb interface in the order in which they were received.

RLC Mode

Two RLC modes exist, acknowledged and unacknowledged mode. The GPRS/EGPRS MS indicates which RLC mode it will use in the uplink TBF set up procedure. This is indicated by the GPRS If a Stall Condition arises, the GPRS/EGPRS MS sets the Stall Indicator (SI) in uplink RLC/MAC blocks. The GPRS/EGPRS MS will then retransmit not yet acknowledged blocks continuously. The network transmits a 44.060 PACKET UPLINK ACK/NACK when it receives the SI for the first time. As long as the SI bit is set in uplink blocks, the interval between successive 44.060 PACKET UPLINK ACK/NACK messages is shorter than normal.

There are two RLC modes, acknowledged and unacknowledged. Each 48.018 DL-UNITDATA message indicates which mode should be used for its transfer.

If the RLC mode is changed by the SGSN during a data transfer, all LLC-PDUs with the original RLC mode are transferred first. A new TBF is then established with the new mode.

After a certain number of RLC data blocks have been transmitted to a GPRS/EGPRS MS, the S/P Bit is set in the header of the next RLC/MAC block sent. The RRBP header field indicates how many TDMA frames the GPRS/EGPRS MS should wait after receiving the block before returning a reply. The header of the downlink RLC/MAC block in the frame reserved for this reply will contain the Reserved USF (see Section 3.3.2 on page 14).

For EGPRS RLC/MAC blocks, the ES/P field is used to indicate the validity of the RRBP field. It also indicates what fields the specified uplink control block shall contain.

Note that using the S/P or ES/P field to schedule an uplink control block is referred to as 'sending a polling request', or simply 'polling the GPRS/EGPRS MS'.

Unacknowledged mode

In unacknowledged mode no retransmissions are made. LLC-PDUs are padded with filler octets in place of missing RLC data blocks. 44.060 PACKET UPLINK ACK/NACK messages are transmitted to the GPRS/EGPRS MS at fixed intervals. The size of this interval is optimized for the radio resources used. In unacknowledged mode the purpose of these messages is to inform the GPRS/EGPRS MS that the network is still receiving data.

Flow Control

In an overload situation, the scheduling of USFs for uplink data transfer is gradually restricted for the entire PCU.

Reallocate an Ongoing Uplink TBF

Handling of LLC PDUs and RLC/MAC Blocks

48.018 DL-UNITDATA messages are sent from SGSN to BSS over the Gb Interface. Each message contains an LLC-PDU and control information necessary for the transmission of the LLC-PDU across the radio interface. Within a TBF, LLC-PDU containing user signalling data are always queued at a higher priority than LLC-PDUs containing user data. When the QoS feature is activated, data PDUs may be queued at different priorities -see Reference [7].

The LLC-PDUs are disassembled into RLC data blocks. For GPRS an RLC/MAC block is created for each RLC data block which are then being scheduled.

For EGPRS a RLC/MAC block may contain one or two RLC data blocks depending on the MCS used. For MCS-1 to MCS-6 there is one RLC data block, whereas for MCS-7 to MCS-9 there are two RLC data blocks. See Reference [2] for more information.

Acknowledged Mode

In If an expected control message is not received by the network, the N3105 counter is incremented and the GPRS/EGPRS MS is re-polled. If after a number of consecutive polling requests this counter reaches a value N3105max, then a timer is started. The TBF is released after expiry of the timer.

Unacknowledged Mode

In unacknowledged mode no retransmissions are made. However, there is one exception: the final RLC/MAC block in a TBF can be retransmitted a certain limited number of times. This is done to ensure that the GPRS/EGPRS MS is aware that the TBF is finished.

In an unacknowledged mode GPRS TBF, the reply to a polling request will be a 44.060 PACKET DOWNLINK ACK/NACK unless the RLC/MAC block is the last in a TBF, for which a 44.060 PACKET CONTROL ACKNOWLEDGEMENT is returned.

In an unacknowledged EGPRS TBF, the reply to a polling request that is not the last in a TBF is a 44.060 EGPRS PACKET DOWNLINK ACK/NACK control message. Otherwise the handling is that same as for GPRS TBFs.

In the case where an expected control message is not received by the network, the N3105 counter is incremented and the mobile is re-polled. When a reply to a polling request is not received by the network, the regularity of the polling requests is increased. If after a number of consecutive polling requests this counter reaches a value N3105max, then a timer is started. The TBF is released after expiry of the timer.

Scheduling of Downlink Data Blocks

Each downlink TBF is assigned to one or more PDCHs. coding scheme can be chosen to suit the prevailing radio conditions. See Reference [9] and Reference [2] for more information.

Multiplexing EGPRS/GPRS mode TBFs

If a GPRS MS has an ongoing uplink or downlink GPRS TBF, it must receive at least one GMSK radio block every 360 ms to stay synchronized. This is done by maintaining a minimum level of the scheduling of GMSK blocks. The GMSK radio block actually does not need to be destined for that specific GPRS MS.

Downlink Flow Control

When GPRS support is activated in a cell, a 48.018 FLOW-CONTROL-BVC message is transmitted to the SGSN. This message indicates that the cell is ready to receive downlink LLC data. It includes a buffer size and data flow rate. These are set high enough to accommodate the maximum GPRS traffic volume in the cell.

The 48.018 FLOW-CONTROL-BVC message also contains default flow control parameters for any GPRS/EGPRS MS that enters the cell. The SGSN uses these values for a GPRS/EGPRS MS until it receives a 48.018 MS-FLOW-CONTROL message with individual parameters for the GPRS/EGPRS MS. This message is sent at the beginning of a TBF. Buffer size and leak rate depend on the MS multislot capability, and whether the TBF is using GPRS or EGPRS. As buffer occupancy and transmission speed vary during the lifetime of the TBF, additional messages may be sent to alter the flow control parameters.

The Current Bucket Level Procedure is supported which involves synchronizing buffer levels in the PCU of the BSC and the SGSN. This procedure makes flow control in the SGSN more responsive to changing conditions in the PCU.

Time Supervision of LLC-PDUs

Each 48.018 DL-UNITDATA message includes a PDU lifetime timer specifying how long the enclosed LLC-PDU may be queued in the PCU of the BSC. The PCU checks how long each LLC-PDU has been queued before it is disassembled into RLC data blocks. If PDU lifetime has expired, it is discarded and a 48.018 LLC-DISCARDED message is sent to the SGSN.

The SGSN may also specify that an LLC-PDU should be queued indefinitely. In this case, PDU lifetime is never checked. All LLC-PDUs are discarded at RA change.

Flush LL Procedure

See Reference [4] for more information about cell reselection.

Scheduling of Downlink Control Blocks

The

Active Queue Management

The Active Queue Management (AQM) feature is a queue management feature for the radio link in downlink. With the AQM feature the data buffers are kept in the BSS instead of SGSN. AQM can be used for QoS class Interactive and Background. AQM can not be used for QoS class Streaming and signalling. For applications using TCP as transmission it is important with a rapid feedback of the radio link data rate to the TCP protocol in the server. Feedback of the radio link data rate is reported to TCP sender in the server by discarding IP packets. In this way the TCP sender will faster adjust its send rate according to the radio link capacity and TCP slow-starts are avoided.

Whether the AQM feature is activated or not is shown by the BSC Exchange Property AQMSUPPORT. When the AQM feature is activated the Flow Control function is not used for the MSs.

If the feature "QoS and Scheduling" in BSS is active, the queue management feature is applied to Interactive class and Background class data. The Active Queue Management feature is applied per Traffic Class for each MS. The queue management feature is not applied to GMM/SM signalling messages, SMS, LLC acknowledged mode data and Streaming class data when QoS is active. For SMS, LLC acknowledged mode data and Streaming class data a maximum buffer is set and when the buffer is overflowed packets are discarded starting from front of the buffer.

If the feature "QoS and Scheduling" in BSS is passive, the queue management feature is applied to all payload data, but not to GMM/SM signalling messages and LLC acknowledged mode data. For LLC acknowledged mode data a 24 206/1553-HSC 103 12/7 Uen B 2005-08-18 maximum buffer is set and when the buffer is overflowed packets are discarded starting from front of the buffer.

The reason for not using the queue management feature for data using the Streaming class is that streaming applications use UDP as transport protocol. UDP has other rate controlling mechanisms than TCP/IP for which discarding of data would be harmful. Discarding of data for Streaming will only be done in case of buffer overflow in the BSS.

When the queue management feature is turned off the GPRS Flow Control function will be active. The data flow from the SGSN is then regulated per BVC and per MS.

GPRS/EGPRS Load Optimization

Downlink TBF

A DL TBF is considered to be in a bad radio environment when it has a low throughput.

A sliding window is used to store radio link bitrate values for the last 50 RLC/MAC blocks. The radio link bitrate is calculated for every 44.060 (EGPRS) PACKET DOWNLINK ACK/NACK that is received from the MS. When the calculated radio link bitrate is lower than or equal to the threshold LOPTGTHR or LOPTETHR the TBF is put in Low Scheduling Mode. At this point the sliding window is cleared. This is to avoid that old values put the TBF back in Low Scheduling Mode after 5 seconds.

The data buffer level for the TBF must be medium or large, see Reference [5], to fulfill the criteria for low scheduling. If the data buffer level and the recommended coding scheme for the DL TBF is lower than or equal to 2 the sampling of the radio link bit rate starts.

The radio link bitrate calculation continues at every reception of 44.060 (EGPRS) PACKET DOWNLINK ACK/NACK to evaluate if the DL TBF should be put back in Low Scheduling Mode after 5 seconds.

In case of an update of the QoS priority for the TBF, during Low Scheduling Mode, a check is made to see if the TBF still fulfill the QoS priority criterion specified by the parameter LOADOPT. If not, the Low Scheduling Mode is aborted. The Low Scheduling Mode is also aborted if the coding scheme is changed above CS-2 or MCS-5.

In case an MS, with a DL TBF in Low Scheduling Mode, is performing a NACC the basic System Information messages needed for NACC is sent to that MS with the same scheduling priority as if the DL TBF was in Normal Scheduling Mode.

Uplink TBF

An UL TBF is considered to be in a bad radio environment when it has low throughput. An UL TBF is only put in Low Scheduling Mode if it is in Dynamic Allocation mode. An UL TBF that is in Extended Dynamic Allocation mode is first switched to Dynamic Allocation mode before the evaluation is done.

A sliding window is used to store radio link bitrate values for at least 100 RLC/MAC blocks. The radio link bitrate is calculated for every 44

Suspend and Resume

Suspend GPRS Service in Packet Transfer Mode

Suspend GPRS Service in Packet Idle Mode

Resume

The resume procedure can be initiated when the circuit switched connection is completed. A 48.018 RESUME PDU is sent to the SGSN if the Suspend procedure was successfully accomplished and the call was released in the same RA as it was set up in.

If the SGSN answers with a 48.018 RESUME-ACK PDU message then the GPRS service is resumed. If the SGSN answers with a 48.018 RESUME-NACK PDU message then the network has failed to resume the GPRS service.

If nothing is received from the SGSN within 48.018 T4 timer, no retries will be made meaning that the network has not been able to resume the GPRS service.

If the CS call is released in another RA than it was setup in then the network is not able to perform the resume procedure for the MS. The MS will have to send a routing area update.

Upgrade of a TBF

Upgrade of an Uplink and Downlink TBF

Re-reservation of a TBF

Re-reservation of an Uplink and Downlink TBF

The following events will occur when an eligible uplink and downlink TBF re-reservation candidate is found:

Re-reservation of a Downlink TBF

The following events will occur when an eligible downlink TBF re-reservation candidate is found: A downlink TBF is downgraded if:

• The DL TBF is in Delayed Release Mode and restricts the number of PDCHs that the corresponding UL TBF is reserved for and it is believed that the main direction of the transmission is in the uplink. See Section 3.9.2 on page 31 for more information.

Downgrade of a Downlink TBF in Delayed Release Mode

Change of TBF Mode

Change of TBF mode is a feature intended to make it possible to change the TBF mode for an ongoing TBF.

This feature is for packet data connections only. MSs having only an UL TBF or TBFs belonging to DTM connections are not handled by this feature.

The availability of the feature is defined by TBFMODEACT. Change of TBF mode is triggered by the following cases:

• Change of QoS to traffic class Streaming is requested but only possible by changing TBF mode. The procedure to change TBF mode is initiated immediately.

• The requested GBR or higher than the negotiated GBR for a streaming TBF is possible to reach by changing TBF mode 3.11 Release of a TBF

Uplink TBF Release

Uplink TBFs can be set up in two different modes depending on the GPRS/EGPRS MS capability and if the MS RAC is known or not in the BSS:

• 'non extended UL TBF mode' if the GPRS/EGPRS MS is Release 99 (or earlier) compatible or if the MS RAC is unknown in the BSS.

• 'extended UL TBF mode' if the GPRS/EGPRS MS is Release 4 (or later) compatible and the MS RAC is known in the BSS.

Note that a TBF may change mode from 'non extended' to 'extended' if BSS receives information that the GPRS/EGPRS MS is Release 4 compatible.

The proceeding events described below depend on the Uplink TBF Mode.

Non extended UL TBF mode

The GPRS/EGPRS MS initiates the release of an uplink TBF by setting the countdown value (CV) to zero in the header of the last RLC/MAC block. No new data will now be sent in this TBF.

For an acknowledged mode TBF, all RLC data blocks must be successfully received by the network before the release procedure can begin. For an unacknowledged mode TBF, reception of the block with CV set to zero is sufficient to initiate the uplink TBF release procedure. Regardless of the RLC mode used, a higher protocol layer in the network will probably want to send a reply to the GPRS/EGPRS MS. A new downlink TBF would be needed to send this reply.

Therefore, a timer is started to delay the uplink TBF release, to allow time for a downlink TBF to be setup on PACCH instead of (P)CCCH. If a downlink TBF is already ongoing, this delay is unnecessary and the release procedure proceeds immediately.

To release the TBF, the network sends a 44.060 PACKET UPLINK ACK/NACK with the FAI bit set to '1' and resets a counter, N3103. The GPRS/EGPRS MS replies with a 44.060 PACKET CONTROL ACKNOWLEDGEMENT message. The TBF is released on successful reception of this message.

If the 44.060 PACKET CONTROL ACKNOWLEDGEMENT message is not received in the radio block specified by the RRBP field, counter N3103 is incremented and the 44.060 PACKET UPLINK ACK/NACK is resent. If the counter exceeds a maximum value, N3103max, then the TBF is released.

Extended UL TBF mode

In this mode when the GPRS/EGPRS MS has no more RLC data to send and has indicated that CV=0, the UL TBF is kept alive for a temporary inactivity period which is the duration of a timer ULDELAY, possible to set by the operator. Then if more data are to be sent from GPRS/EGPRS MS before the expiry of the timer, the RLC data transmission continues on the same UL TBF instead of setting up a new TBF. The GPRS/EGPRS MS is also allowed to interrupt the countdown of a UL TBF and send more data on the very same TBF. If no more data are to be sent from GPRS/EGPRS MS after the expiry of the timer, the UL TBF is released using the non extended Uplink TBF release procedure.

Delayed Release of Downlink TBF

It has been observed that a typical user session consists of several uplink and downlink TBFs. Overall transmission speeds can be increased by keeping TBFs alive for a longer time. This reduces the number of TBFs in a user session, and thereby significantly cuts the time spent on TBF setups.

The purpose of the delayed release of downlink TBF is to keep a downlink TBF alive during the temporary inactive periods when there are no data to send. When new data arrives from the SGSN, setup of a new TBF is not needed.

For an acknowledged mode TBF, the delayed release procedure is initiated when all data have been acknowledged by the GPRS/EGPRS MS. For an unacknowledged mode TBF, it begins when all data has been sent from the PCU.

To avoid ending the TBF, the PCU adds dummy LLC-PDUs to the back of its data queue. Dummy LLC data are added at the end of the RLC/MAC block containing the last 'real' data. This allows all 'real' data to be transmitted without the FBI bit set to '1' which would end the TBF.

Typically, the GPRS/EGPRS MS will want to respond to the downlink transfer with an uplink TBF. To allow this TBF to be established on the PACCH, new RLC/MAC blocks containing dummy LLC-PDUs are sent periodically with a polling request. The GPRS/EGPRS MS transmits a 44.060 PACKET DOWNLINK ACK/NACK in response to each poll that it receives. When it is ready to send a response to the downlink TBF, it can include a request for an uplink TBF in one of these messages. Persistent Uplink Scheduling is used the interval of sending dummy LLC-PDUs with polling requests are done according to the setting of the PULSCHEDINT parameter. See Section 5.1 on page 43 for more information about the parameter. If Persistent Uplink Scheduling is not used the frequency of sending polling requests are reduced (compared to during data transfer) so that the scarce radio resources can be freed up for real data traffic but at the same time ensuring a relatively short delay if the MS shall transmit real data.

When more data arrive from the network side in response to the uplink TBF, it can be sent immediately as part of the existing TBF. If no new data arrive within a certain time, a dummy LLC-PDU is sent in a new RLC/MAC block. The FBI bit is set to '1' in this block to end the TBF.

Parameter DLDELAY specifies how long the Delayed Release Procedure should last before the FBI bit set to '1' is sent. Its value should be set to the length of time during which new data are likely to arrive from the SGSN. Note that if the downlink TBF was established using the Early Setup procedure, the duration of the TBF is determined by parameter ESDELAY, and DLDELAY is not used.

The delayed release procedure is identical for an EGPRS TBF, except that the GPRS/EGPRS MS returns a 44.060 EGPRS PACKET DOWNLINK ACK/NACK in response to a polling request. Release of an EGPRS TBF is identical to the procedure described above, except that the GPRS/EGPRS MS returns a 44.060 EGPRS PACKET DOWNLINK ACK/NACK in response to a polling request.

Release of Acknowledged Mode Downlink TBF

Release of Unacknowledged Mode Downlink TBF

To release an unacknowledged mode TBF, the network sets the FBI bit to The TBF is released when the GPRS/EGPRS MS's reply is successfully received in the BSS.

As for an acknowledged mode TBF, abnormal release can occur if the GPRS/EGPRS MS fails to respond to N3105max successive polling requests. If some LLC-PDUs are still queued for the GPRS/EGPRS MS, a timer is started to allow time for a 48.018 FLUSH LL message to be received. Otherwise, the TBF is released immediately.

Release of an unacknowledged mode EGPRS TBF is the same as for a GPRS TBF. When the buffer is kept for a certain time period and no successful DL TBF has been setup the remaining parts of the buffer is discarded and the SGSN is informed by sending 48.018 LLC-DISCARDED.

Loss Free Preemption

Related Statistics

Impact on Legacy Counters

The counters DLTHP1GTHR, DLTHP2GTHR, DLTHP3GTHR, DLBGGTHR are expected to have an increased value when the AQM feature is activated.

The counters DLTHP1EGTHR, DLTHP2EGTHR, DLTHP3EGTHR, DLBGEGTHR are expected to have an increased value when the AQM feature is activated.

Statistics for Performance Management

Two counters related to the AQM feature:

AQMDELIVDATA

Total amount of data delivered to the MS by AQM. This is generated per BSC.

AQMRECDATA

Total amount of data received from SGSN by AQM. This is generated per BSC. • too long is a waste of radio resources and TFIs

Main Changes in Ericsson GSM

• too short makes the response from the core network to be set up on PCCCH or CCCH, which takes significantly much more time than setting up the response on PACCH.

DLDELAY defines the time a downlink TBF is kept alive after there is no more real data to send in downlink. The intention of the parameter value is to keep the downlink TBF alive between arrival of consecutive LLC-PDUs from the core network. The parameter is set per BSC. Scenarios to consider when setting this parameter are:

• • more frequently may significantly reduce roundtrip times for traffic such as TCP handshakes and Ping benchmarking, but reduces battery lifetime in the mobiles • less frequently increases roundtrip times for traffic such as TCP handshakes and Ping benchmarking, but does not reduce the battery lifetime in the mobiles.

Since Persistent Uplink Scheduling is used when it does not affect any ongoing active data transfer(s) there are no negative affects from schedule the mobile more frequently except for possibly shorter battery lifetime in the mobiles. An important note is that setting the parameter to any of the values 6-10 will increase roundtrip times compared to having the feature deactivated.

PULSCHEDINT=99 and =1 uses the same scheduling frequency, but the parameter set to 99 schedules the mobile on all assigned PDCHs UL. For example a mobile with 2 slot UL will be scheduled 2 times every 20 ms with the parameter set to 99 but only 1 time with the parameter set to 1. This means that the parameter set to 99 will increase battery consumption compared to the parameter set to 1, but the reduced roundtrip times with the parameter set to 99 compared to the parameter set to 1 is neglectible. Also, QoS scheduling can have a negative impact on the mobile with lower QoS weight when the parameter is set to 99, since the mobile with higher QoS weight (absolute priority) will be scheduled all the time. Therefore if the choice is between parameter setting 1 and 99, then 1 is the recommended setting.

Note that for EIT it is important to consider the parameter settings for ESDELAY, DLDELAY and ULDELAY. To keep a constant flow of data during an EIT transfer, high values of the mentioned parameters give benefits for the end-users running EIT. This is important to avoid noise for the end-user caused by temporary interrupts in the data flow, for example from temporary periods of silence. With low values for the mentioned parameter this may lead to unnecessary releases of TBFs and thus extra delays from having to setup TBFs again.

Note that for the feature Dual Transfer Mode (DTM) it is important to consider the parameter settings for ESDELAY, DLDELAY and ULDELAY. If the TBF(s) for a mobile in DTM mode is released a re-allocation of the complete DTM connection might be necessary. Therefore a too short setting of the three parameters may have negative impact on the DTM service.

A higher value than the default value of the parameter AQMRTTCONST results in that LLC-PDUs will be discarded earlier.

AQMMINBUFF

This BSC Exchange Property gives the value of T min which ensures that at least a few IP packets are buffered at very low peak rate.

AQMMINSIZE

The BSC Exchange Property indicates the lower limit of the IP packet size.

AQMRTTCONST

The BSC Exchange Property indicates the constant part of the roundtrip time. This includes delays in server, core network, BSC, MS and laptop as well as transmission delays on the air interface such as Gn, Gb and Abis. The parameter is changed in steps of 10 ms.

AQMSUPPORT

Support of AQM

Contention Resolution

A procedure used to uniquely identify a GPRS/EGPRS MS during an MS originated access procedure. The procedure is completed in the network side at reception of the TLLI of the GPRS/EGPRS MS. The procedure is completed in the GPRS/EGPRS MS side when the same TLLI is returned in a PACKET UPLINK ACK/NACK or in a PACKET UPLINK ASSIGNMENT message.

Dedicated PDCH

A dedicated PDCH is allocated from the CSD to the PSD, and will be dedicated for GPRS/EGPRS traffic only. The allocation of dedicated PDCHs will be made by the system according to operator request (ordered number of PDCHs). The operator sets the number of dedicated PDCHs in a cell.

Dual Transfer Mode

An MS in dual transfer mode has simultaneously a PS and a CS connection. The allocated radio resources are co-ordinated by BSS.

EDGE

Enhanced Data for Global Evolution. An add on to GPRS which uses different modulation (8-PSK or GMSK) and/or coding schemes to allow greater bandwidth and thus provide greater spectral efficiency.

EGPRS

Enhanced GPRS (EGPRS) supports the GMSK and 8-PSK modulation methods and defines nine modulation and coding schemes. EGPRS increases the bitrate over the radio interface up to 59.2 kbit/s per timeslot and offers increased system capacity.

GPRS

GPRS is a feature that makes it possible to send packet data over the GSM network with GMSK coding schemes (CS-1 to CS-4). GPRS supports net bit rates up to 20.0 kbps per timeslot.

GPRS Attach

A GPRS/EGPRS MS shall perform a GPRS Attach to the network in order to obtain access to the GPRS/EGPRS services.

GPRS/EGPRS MS Class Mode of Operation

• Class A mode of operation allows a GPRS/EGPRS MS to have a CS connection at the same time as it is involved in a packet transfer.

• Class B mode of operation allows a GPRS/EGPRS MS to be attached to both CS and PS connections, but it cannot use both services at the same time. However, a GPRS/EGPRS MS that is involved in a packet transfer can receive a page for a CS connection which requires the Gs Interface between the MSC/VLR and SGSN to be present. The GPRS/EGPRS can then suspend the packet transfer for the duration of the CS connection and afterwards resume the packet transfer.

• Class C mode of operation allows a GPRS/EGPRS MS only to be attached to one service at a time.

GPRS/EGPRS MS States and Modes

There are three GPRS/EGPRS MM states:

• Idle state, when the GPRS/EGPRS MS is turned on but not GPRS Attached. The GPRS/EGPRS MS is "invisible" to GPRS/EGPRS, e.g. when the GPRS/EGPRS MS is outside of the coverage area for GPRS/EGPRS.

Low Scheduling Mode

A TBF that is in Low Scheduling Mode is scheduled with lower frequency than a similar TBF that is not in Low Scheduling Mode.

Master PDCH

A PDCH carrying the Packet Broadcast Control Channel and the Packet Common Control Channel. PS traffic can also be carried on the Master PDCH. The Master PDCH is a dedicated PDCH.

MCS

A Modulation and Coding Scheme (MCS) is a specific coding scheme in combination with a modulation method.

MS multislot class

MS capability to handle multiple time slots. The multislot classes 1-45 are defined. They are all supported but multislot classes 30-45 are mapped into lower multislot classes, 33-45 if the feature Five Downlink Timeslots is available.

MS RAC

The purpose of the MS RAC is to provide the BSS with additional information concerning the radio capabilities of the GPRS/EGPRS MS. The contents of the MS RAC affects the way in which the BSS handles the GPRS/EGPRS MS.

On-demand PDCH

A PDCH which is set up and released dynamically depending on the need for GPRS/EGPRS traffic. An on-demand PDCH may be PDCH pre-empted by CS traffic.

PCU

The Packet Control Unit (PCU) is a logical unit in the BSS which is responsible for the handling of the RLC/MAC and parts of the physical link layers of the radio interface. The B-PDCH requires a 16 kbit/s Abis connection per timeslot

PDCH

The G-PDCH and the E-PDCH require a 64 kbit/s Abis connection per timeslot.

PDCH Reservation

When a TBF is to be set up in order to support a certain service for a mobile user, transmission resources will be 51 206/1553-HSC 103 12/7 Uen B 2005-08-18 reserved on one or more PDCHs. Among other things, the PDCH reservation is taking the GPRS/EGPRS MS capabilities (Multislot Class, and support for EGPRS) and the requested QoS Class for the service into account.

PSET

A set of PDCHs possible to use together in a TBF. A PSET can contain up to 16 on-demand and/or dedicated and/or semi-dedicated PDCHs. Maximum one PSET can be allocated on the same TCHGRP.

R97 QoS

Quality of Service as specified in the 3GPP TS R97. Includes the attributes Precedence class, Peak Throughput etc.

R99 QoS

Quality of Service as specified in the 3GPP TS R99. Includes the PFM procedure and the classes Conversational, Streaming, Interactive and Background.

RA

A Routing Area consists of one or a number of cells. The RA is identical to the LA in the Ericsson BSS.

RLC/MAC

Radio Link Control (RLC) and Medium Access Control (MAC) are protocols responsible for segmentation of data and multiplexing of users.

RTT

The time for a peer to receive an answer for a transmitted message.

Semi-dedicated PDCH

A semi-dedicated PDCH is permanently allocated in a cell by operator command but not always activated. That is, a hardware device is not tied to the channel until the PS traffic requires it. A semi-dedicated PDCH can not be preempted by CS traffic, but it can be deallocated by operator command. Pre-emption is possible though at Abis congestion if the feature Flexible Abis is available.

TAI individual

The PDCH, among the PDCHs used by a TBF, that is chosen to carry the TAI. The TAI is actually sent to GPRS/EGPRS MSs sharing the PTCCH on the same PDCH.

TBF

A Temporary Block Flow (TBF) is a logical connection between the BSS and the MS. A TBF is set up when there is data to send at the BSS or MS side.

TBF MODE

Indicates whether an MS is using GPRS or EGPRS.