WO2008133577A1

WO2008133577A1 - Method for selectively discarding data units in a radio communication system

Info

Publication number: WO2008133577A1
Application number: PCT/SE2008/050110
Authority: WO
Inventors: Stefan Wager; Magnus Lindström; Henning Wiemann; Mats SÅGFORS
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2007-04-27
Filing date: 2008-01-28
Publication date: 2008-11-06
Also published as: TW200910848A

Abstract

A sending node for transmitting data units over a radio interface to a receiving node is provided. Each data unit to be transmitted is associated with a sequence number. The method comprises the step of transmitting a stream of data units over the radio interface (115) to the receiving node (120). Each data unit is associated with a sequence number. The further step of selectively discarding a first data unit from the transmission window, which first data unit is associated with a first sequence number (SN4), and informing the receiving node (120) that first data unit associated with the first sequence number (SN4) is discarded, so that a gap in the in the associated sequence numbers of the stream of transmitted data units can be ignored by the receiving node (120).

Description

METHOD FOR SELECTIVELY DISCARDING DATA UNITS IN A RADIO COMMUNICATION SYSTEM

TECHNICAL FIELD The present invention relates to a method and an arrangement in a sending node and a method and an arrangement in a receiving node In particular, it relates to transmission of data units over a radio interface

BACKGROUND In current wireless communications systems, data is transmitted from sending nodes to receiving nodes over the radio interface Data is segmented into data packets in the sending node before being transmitted over the radio interface The Radio Link Control (RLC) protocol of Wideband Code Division Multiple Access (WCDMA) is a window based retransmission protocol, in which Service Data Units (SDU) to be transmitted to the receiving node, are segmented in Protocol Data Units (PDU) Each PDU is attached with an RLC header including a Sequence Number (SN) which is used for reassembly at the receiving node The SN is also used by the receiving node to request from the sending node retransmission of PDU s lost over the radio interface In the sending node the PDU s sent are also saved in a buffer for a while, e g in a transmitter window The buffered PDU s may then be retransmitted from the sending node upon request from the receiving node, until the maximum number of retransmissions (MaxDAT) is reached When this is reached, the RLC connection is reset and all buffers flushed

WCDMA comprises a SDU discard function which is used to discard SDUs from the transmission window before the maximum number of retransmissions is reached or a discard timer elapses The function is triggered by the transmitter sending a Move Receiving Window (MRW) command to the receiver telling it to advance the receiving window This results in that all SDUs not completely received, covered by the PDU SNs left outside the receiving window are discarded As the window is moved forward some PDUs that were not yet acknowledged by the receiver will become outside the window Effectively this will mean that transmission of these PDUs stops and they are discarded For the higher layers, it means that any SDU that was carried by the dropped PDUs will also be discarded One limitation of the current SDU discard function is that it only allows discarding of PDUs from the lower receiving window edge

One such limitation is related to Active Queue Management (AQM) Typically, an

AQM mechanism operates in the link layer transmitter and drops packets when they are released from the SDU (SDU (Service Data Unit) = Higher layer data unit ) queue and just before they enter the link layer, e g Automatic Repeat reQuests (ARQ) protocol loop E g in WCDMA no RLC sequence number has been assigned to such dropped SDUs so that the ARQ receiver will not even notice the drop and deliver consecutive data units without reporting errors and without causing additional delays

In order to reduce the protocol overhead, the link layer ARQ protocol could re-uses the sequence number of a higher layer protocol (e g Packet Data Convergence Protocol (PDCP) in Long Term Evolution (LTE)) The PDCP transmitter may assign sequence numbers before they enter the AQM queue If the AQM mechanism drops such a PDCP PDUs from the front of the queue this causes a gap in the stream of transmitted sequence numbers The ARQ receiver will notice such gaps and assume that they are due to transmission errors (that is the task of an ARQ receiver) The ARQ receiver will suspend delivery of consecutive data units to higher layers and send a negative status report to its transmitting peer The transmitter, however cannot transmit the requested data unit as it has been dropped by AQM

SUMMARY It is an object to provide a mechanism for improving the speed of transmitting data units over a radio interface

According to a first aspect of the present invention, the object is achieved by a method in a sending node for transmitting data units over a radio interface to a receiving node The method comprises the step of transmitting a stream of data units over the radio interface to the receiving node Each data unit is associated with a sequence number The method comprises the further step of selectively discarding a first data unit that will not be retransmitted or transmitted The first data unit is associated with a first sequence number The method comprises the yet further step of informing the receiving node that first data unit associated with the first sequence number is discarded, so that a gap in the in the associated sequence numbers of the stream of transmitted data units will be ignored by the receiving node The gap is related to the first sequence number associated to the discarded first data unit informed about Since the gap being ignored, no negative status report or request will be received from the receiving node for retransmission of the first data unit

According to a second aspect of the present invention, the object is achieved by a method in a receiving node for receiving a transmission of data units over a radio interface from a sending node Each data unit to be transmitted by the transmitting node is associated with a sequence number The method comprises the step of receiving a stream of transmitted data units from the sending node The method comprises the further step of obtaining information from the sending node that a first data unit is discarded, which first data unit thus can not be retransmitted or transmitted to the receiving node, and that the first data unit is associated with a first sequence number The method comprises the yet further step of ignoring a gap in the associated sequence numbers of the stream of transmitted data units which gap is related to the first sequence number associated to the discarded first data unit and since ignored no negative status report or request for retransmission of the first data unit, will be sent to the sending node

According to a third aspect of the present invention the object is achieved by a use of the methods according to the first or second aspect of the present invention in an inter base station handover procedure, wherein the first data unit is detected to be lost by the sending node instead of being selectively discarded while being forwarded from a source base station to a target base station The target base station is represented by the sending node

According to a fourth aspect of the present invention the object is achieved by an arrangement in a sending node arranged to transmit data units over a radio interface to a receiving node The sending node arrangement comprises a transmitting unit configured to transmit a stream of data units over the radio interface to the receiving node Each data unit is associated with a sequence number The sending node arrangement further comprises a discarding unit The discarding unit is configured to selectively discard a first data unit that will not be retransmitted or transmitted The first data unit is associated with a first sequence number The sending node arrangement further comprises an informing unit configured to inform the receiving node that first data unit associated with the first sequence number is discarded so that a gap in the in the associated sequence numbers of the stream of transmitted data units can be ignored by the receiving node The gap is related to the first sequence number associated to the discarded first data unit informed about Since the gap is ignored, no negative status report or request will be received from the receiving node for retransmission of the first data unit

According to a fifth aspect of the present invention, the object is achieved by an arrangement in a receiving node for receiving a transmission of data units over a radio interface from a sending node Each data unit to be transmitted by the transmitting node is adapted to be associated with a sequence number The receiving node arrangement comprises a receiving unit configured to receive a stream of transmitted data units from the sending node The receiving node arrangement further comprises an information unit configured to obtain information from the sending node that a first data unit that can not be retransmitted or transmitted is discarded and that the first data unit is associated with a first sequence number The first data unit thus will not be transmitted to the receiving node The receiving unit is further configured to ignore a gap in the associated sequence numbers of the stream of transmitted data units The gap is related to the missing first sequence number associated to the discarded first data unit When ignored no negative status report or request for retransmission of the first data unit will be sent to the sending node

Since the receiving node is informed that first data unit associated with the first sequence number is discarded and therefore the gap in the associated sequence numbers of the stream of transmitted data units can be ignored by the receiving node no negative status report or request will be sent by the receiving node for retransmission of the first data unit Instead the received data stream can be acknowledged despite the missing data unit, allowing the sending node to start sending the next stream of data units This implies that the speed of transmitting data units over a radio interface is improved since with conventional data unit discard based on moving receiver window all data units below the data unit to be discarded must first be correctly received and acknowledged to the transmitter before the move receiving window command can be transmitted An advantage of the present solution is that a selective discard allows an earlier discard of the data unit, and a window advance and delivery of out of sequence data units at the receiving node is faster.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in more detail with reference to attached drawings illustrating exemplary embodiments of the invention and in which:

Figure 1 is a sche matic block diagram illustrating embodiments of a wireless communication system.

Figure 2 is a sche matic block diagram illustrating embodiments of data units.

Figure 3 is a sch ematic block diagram illustrating embodiments of a method.

Figure 4 is a sche matic block diagram illustrating embodiments of a method.

Figure 5 is a flow chart illustrating embodiments of a method in a sending node.

Figure 6 is a schematic block diagram illustrating embodiments of a sending node arrangement.

Figure 7 is a flow chart illustrating embodiments of a method in a receiving node.

Figure 8 is a schematic block diagram illustrating embodiments of a receiving node arrangement.

DETAILED DESCRIPTION

The invention is defined as a method and an arrangement which may be put into practice in the embodiments described below. Figure 1 depicts a sending node 100 in a wireless communication system 110. The wireless communication system 1 10 may be a cellular system and/or a system such as e.g. Long Term Evolution (LTE), Evolved - Universal Terrestrial Radio Access (E-UTRA), Worldwide Interoperability for Microwave Access (WiMAX), Universal Terrestrial Radio Access (UTRA), Wideband Code Division Multiple Access (WCDMA). GSM, Ultra Mobile Wideband (UMB) or any other wireless communication system using technologies that perform transmission of data units over a radio interface. The sending node 100 is capable of transmitting data units over a radio interface 115 to a receiving node 120.

The sending node 100 may be any type of a base station such as e.g. a eNB as in LTE. The receiving node 120 may be a user equipment (UE) such as e.g. a mobile phone or a Personal Digital Assistant (PDA). It may also be the other way around, that the sending node 100 may be a UE such as e.g. a mobile phone or a Personal Digital Assistant (PDA) and the receiving node 120 may be any type of a base station such as e.g. a NodeB. In the example of Figure 1. the sending node 100 is a base station and the receiving node 120 is a user equipment. In addition, the sending node 100 and the receiving node 120 may constitute arbitrary wireless devices in communication with each other and with no particular hierarchical ordering.

The sending node 100 and the receiving node 120 may use a radio link transmission protocol such as e.g. the RLC protocol of WCDMA or LTE. In this radio link transmission protocol data units are transmitted between the sending node 100 and the receiving node 120 over the radio interface 115. The radio link transmission protocol may be a window based retransmission protocol, in which incoming data units from higher protocol layers such as e.g. RLC SDU:s from a higher level of Layer 2 RLC are segmented into smaller "lower protocol data units " such as RLC PDU:s of a lower level of Layer 2 RLC. An example of how this is performed is schematically depicted in Figure 2. Each of the data units is attached, i.e. associated with a respective sequence number such as SN1 , SN2, SN3, SN4, SN5, SN6 and SN7 depicted in Figure 2. It may also be the other way around, in which incoming data units from higher protocol layers are concatenated into larger "lower protocol data units". Also in this case each of the data units such as e.g. RLC PDUs is associated with a sequence number. The sequence number is used for reassembly at in the receiving node 120 and to keep a check on whether all data units are received or if any data unit is missing. If the receiving node 120 detects that a data unit is missing it may send to the sending node 100, a negative status report or request for retransmission of the first data unit In some embodiments related to RLC the data units may comprise a respective RLC header comprising the associated sequence number The sequence numbering may be continuous or e g some offset mechanisms could be used This offset could for example be signaled derived, or randomly selected

Furthermore, according to a specific embodiment of the invention, the sequence number is continuous also between base stations in the case of handover, so that the user equipment can perform reordering between packets received from source and target base station Automatic Repeat reQuests (ARQ) may be triggered by the receiver to correct errors in the radio transmission

To improve throughput, the sending node 100 and the receiving node 120 may use a window mechanism In this case the sending node 100 comprises a transmission window and the receiving node 120 comprises a receiver window The window mechanism allows the sending node 100 to send the multiple data units within the transmission window without waiting for acknowledgements of each data unit before sending the next one within the sender window The window can be seen as a buffer

The sending node 100 may be informed about the size of the receiver window of the receiving node 120 to enable the sending node 100 to avoid sending more data units than the receiving node 120 can concurrently handle The sender can send a whole sender window of data units before receiving an acknowledgement of the first data unit in the window while still ensuring lossless operation Upon reception of acknowledgements the sender window is moved forward such that the first data unit in the window is again the oldest transmitted but not yet acknowledged data unit

Hence the transmission window defines the range of packets which are permissible to send, but for which acknowledgment of the oldest packet has not been received The receiver window defines the range of packets which can be/are accepted by the receiver, but for which reception of the oldest packet has not been successful and acknowledged For lossless operation, the receiver window may start at the oldest not successfully received and acknowledged packet Moving the sender or receiver window past unacknowledged packets may result in packet losses

A scenario 300 illustrating a transmission protocol which may be used in some embodiments will be described with reference to Figure 3 Incoming data units A B₁ C₁ D come in to a transmitter 310 in the sending node 100 for transmission to the receiving node. The sending node 100 may use a retransmission protocol, such as e.g. the RLC, deploying selective retransmissions to provide reliability over an unreliable link. In the transmitter 310, each data unit A. B, C, D is associated to a sequence number SN1 , SN2. SN3, SN4, referred to as A1 , B2, C3 and C4 in Figure 3, when transmitted according to the retransmission protocol. The transmitted data units A1 , B2, C3 and C4 are buffered e.g. in a transmission window 315. The transmission window 315 defines how many data units can be transmitted without receiving a status report from the receiving node 120. The transmission of the data units A1 , B2, C3 and C4 is illustrated by the arrow 320 and the transmitted data units A1 , B2, C3 and C4 on their way to the receiving node 120 are illustrated in the dashed circle 330. In the exemplary scenario 300 of Figure 3, the data unit C3 is lost on the radio link on the way to the receiving node 120.

A receiver 340 in the receiving node 120 receives the data units A1 , B2, and C4 e.g. in a receiver window 350. The receiving node 120 sends status reports to the sending node 100, e.g. comprising the sequence numbers of data units correctly received. In the scenario 300 of Figure 3. the receiving node 120 sends a status report to the sending node 100 e.g. by acknowledging the sequence numbers SN1 , SN2 and SN4 of the data units A1 , B2, and C4 correctly received. This is illustrated by the arrows 360 and 365 and the box 370. This may also be made in other ways as well, e.g. the receiver 340 may send an acknowledgement for SN4 and a non-acknowledgement for SN3, indicating that everything was received OK until SN4, except SN3.

Based on this information, the transmitter 310 in the sending node 100 performs retransmissions of data units lost on the radio link. In this exemplary scenario 300, the transmitter will retransmit the data unit C3 (not shown). This continues until the data units are correctly received by the receiver 340 in the receiving node 120 or until the data unit has been transmitted a predetermined maximum amount of times.

In some embodiments the transmitter 310 is able to advance its transmission window 315 to the sequence number up to which the receiver 340 has received every data unit in sequence. In the scenario 300 of Figure 3, the transmission window is advanced to data unit C3 which advanced transmission window 315 is illustrated by the dashed transmission window 315 and the dashed arrow 375.

A scenario 400 according to some embodiments, illustrating a method wherein a selectively discard data unit mechanism added to a transmission protocol is used, will be described with reference to Figure 4. In these embodiments the protocol may have the ability to selectively discard any data unit in the transmission window 315 Data units in the transmission window 315 have typically already been transmitted once, but whether the transmission was successful or not is not yet known to the transmitter 310 In case data units incoming to the transmitter 310 are received out of sequence, some of the data units in the transmission window 315 have not yet been transmitted In the exemplary scenario 400 of Figure 4 four data units E1 , F2, G3 and H4 are comprised in the transmission window 315 of the transmitter 310 of the sending node 100 In the transmitter 310, each data unit E, F, G and H has been associated to a sequence number SN1 , SN2, SN3, SN4, referred to as E1 F2 G3 and H4 in Figure 4 when they were transmitted according to the retransmission protocol The transmission may be performed in accordance of the scenario 300 described with reference to in Figure 3 I e the transmitted data units E1 , F2, G3 and H4 are buffered in the transmission window 315

The transmitter 310 selectively discards a data unit G3 from the transmission window 315 An example of a trigger for the discard function may be AQM, where an incoming buffer of the transmitter 310 grows large and it wants to drop a data unit to inform higher layers of congestion It may be decided to discard a data unit due to e g out of sequence reception of incoming data units Another example of this may the handover of LTE where incoming data units may be out of order and the transmitter 310 has already assigned sequence number and started transmission of incoming data units, i e E1 F2 G3 and H4 in Figure 4, when it detects that incoming data units the data unit G3 that should have been transmitted before the already transmitted data units were lost on the incoming link

To prevent the receiving node 120 from sending a negative status report i e instead triggering the receiving node 120 to acknowledge the sequence of data units lacking the discarded data unit G3, the receiving node 120 is informed that the data unit G3 is discarded, e g by informing that the sequence number SN3 associated to the discarded data unit G will never come and shall be ignored This information sent is illustrated by the box 410 and the arrows 420 and 430 Of course, more than one data unit may be selectively discarded and informed about The receiving node 120 having received the data units E1 F2, and H4 has detected a gap in the sequence of the received data units associated to the discarded data unit G3 Therefore no acknowledgement for data unit G3 associated with sequence number SN3 has been sent to the transmitter 310 in the sending node 100 When the receiving node 120 has received the information about the discarded data unit G3 which information comprises the sequence number SN3 of the discarded data unit G3, the receiving node 120 ignores the gap and sends an acknowledgement for the discarded data unit G3 and the received data units E1 , F2, and H4 e.g. by acknowledge the sequence numbers SN1- 4. This is illustrated the arrows 440 and 450 and the box 460.

When the transmitter 310 in the sending node 100 has received the acknowledgement for the discarded data unit G3 and the received data units E1. F2, and H4. the transmitter 310 stops retransmitting these data units. In some embodiments the transmitter 310 is able to advance its transmission window 315 to the sequence number up to which the receiver 340 has received every data unit in sequence. In the scenario 400 of Figure 4, the transmission window is advanced to subsequent of data unit H4 which advanced transmission window 315 is illustrated by the dashed transmission window 315 and the dashed arrow 470. The receiver window 350 may also be correspondingly advanced as illustrated by the dashed transmission window 350 and the dashed arrow 480.

Described below, the data unit that is discarded is called 'the first data unit'^' and its associated sequence number is called "the first sequence number". However, this should not be interpreted as the first data unit in order to be transmitted among a stream of data units. The expressions "the first data unit" and 'the first sequence number" are just used to separate the discarded data unit and its associated sequence number from the other data units and their respective sequence numbers in the stream of data units. "The first data unit'^' associated to the first sequence number" may appear anywhere in the order of the stream of data units to be transmitted.

That "a" data unit is discarded as described in this document shall be interpreted as "one or more" data units are discarded, each discarded data unit thus being associated with a respective sequence number.

The method steps in the sending node 100 for transmitting data units over the radio interface 115 according to some embodiments will now be described with reference to a flowchart depicted in Figure 5. The method comprising the steps of:

501. The sending node 100 transmits a stream of data units over the radio interface

115 to the receiving node 120. Each data unit is associated with a sequence number when transmitted. The stream of data units may e.g. comprise the data units associated with the respective sequence number SN1 , SN2, SN3, SN4, SN5 SN6, SN7 as depicted in Figure 2 The stream of data units may be comprised in a transmission window 315 for possible retransmission in the case the receiving node 120 do not receive one or more data units that were transmitted

By some reason the sending node 100 will drop i e discard a data unit to be transmitted before transmitting it This is performed by selectively discard a first data unit that will not be retransmitted, or transmitted (if not yet being transmitted) The first data unit is associated with a first sequence number The discarded first data unit may e g be the data unit with the associated sequence number SN4 as depicted in Figure 2

The sequence numbers SN1 SN2, SN3 SN4, SN5 SN6, SN7 were associated to the data units before the first data unit was discarded thus the discarded first data unit will cause a gap in the associated sequence numbers of the stream of data units to be received by the receiving node 120 e g in the receiver window

350 in the receiving node 120 The gap is associated to the missing first sequence number SN4 of the discarded first data unit

If the sending node 100 uses a window mechanism the first data unit is discarded from the transmitter window 315 and thus causing the gap in the associated sequence numbers in the transmitter window 315

This selective discard step may be performed by the link layer between sending node 100 and the receiving node 120

In some embodiments an Active Queue Management (AQM) mechanism is used in the sending node 100 In that case the AQM mechanism may perform the step of selectively discarding 502 the first data unit

According to a specific embodiment regarding a handover process, the sending node is a target base station The discarded first data unit and its associated sequence number SN4 may be a data unit lost when forwarded from the source base station to the target base station

To e g prevent the receiving node 120 from wondering why the first data unit not is received, the sending node 100 informs the receiving node 120 that first data unit associated with the first sequence number SN4 is discarded, so that a gap in the in the associated sequence numbers SN1 SN2, SN3, SN5, SN6, SN7 of the stream of transmitted data units can be ignored by the receiving node 120 The gap is related to the first sequence number SN4 associated to the discarded first data unit informed about. Since the gap is ignored, no negative status report or request will be received from the receiving node 120 for retransmission of the first data unit. In some embodiments wherein the RLC protocol is used in the sending node

100, this step is performed by sending a RLC control data unit message to the receiving node 120. The message comprises the first sequence number SN4 associated with the discarded first data unit. This may be sent in a Super Field (SUFI) of the RLC status report. Similar mechanism as the Move Receiving Window mechanism, except that it point out an individual PDU or set of PDUs to be dropped, not a moving of the window.

According to some other embodiments this step of informing may be performed by sending a dummy data unit to the receiving node 120. The dummy data unit comprises the first sequence number associated with the discarded first data unit, and an indication that the first data unit is discarded. The dummy data unit may be a PDU and may comprise only the RLC header plus a length indicator indicating that the PDU is empty. The indication that the first data unit is discarded may be given by a special value of a length indicator in the dummy data unit. The indication that the first data unit is discarded may also be given by a special discard bit in a header of the dummy data unit.

Selectively discarding a data unit means that the link layer gives up on retransmitting that particular data unit. This means that the receiving node 120 will not request retransmissions for this data unit and also that the reordering function will not wait for the data unit anymore. Selective means that any data unit in a transmission window may be discarded.

Since the gap in the associated sequence numbers of the stream of transmitted data units will be ignored by the receiving node 120 and no negative status report or request will be sent by the receiving node 120 for retransmission of the first data unit, the received data stream can be acknowledged despite the missing data unit, allowing the sending node 100 to start sending the next stream of data units. The method may be used in an inter base station handover procedure wherein the first data unit is detected to be lost by the sending node 100, instead of being selectively discarded while being forwarded from a source base station to a target base station The target base station is in this case represented by the sending node 100 A handover from a source base station to a target base station may include forwarding of data units from the source base station to the target base station to minimize data loss To ease reordering in the user equipment between data units received from source and target base station the data units may contain a continuous sequence number To reduce link layer overhead, this sequence number could also be used for ARQ purposes In such scenario, the target base station instead of selectively discarding data units, detects data units lost while being forwarded from the source base station from the ARQ transmission window, using the selective discard mechanism presented above

Data loss may occur on the physical layer if the receiving node 120 is unable to decode a data unit Lost data units are normally retransmitted by the link layer In the handover example data units may also be lost on the link between the base stations when moving the link layer network side termination point from one base station to another In that case, a loss will mean that the target base station will not have the corresponding data unit for retransmission when the receiver requests it Therefore the selective discard function is useful in these cases

To perform the method for transmitting of data units over a radio interface 115 to a receiving node 120 described above the sending node 100 comprises an arrangement 600 depicted in Figure 6 As mentioned above, each data unit to be transmitted is adapted to be associated with a sequence number SN1 , SN2, SN3, SN4, SN5, SN6, SN7

The arrangement 600 comprises a transmitting unit 610 configured to transmit a stream of data units over the radio interface 115 to the receiving node 120, each data unit being associated with a sequence number SN1 , SN2 SN3, SN4, SN5, SN6, SN7

The arrangement 600 further comprises a discarding unit 620 configured to selectively discard a first data unit that will not be retransmitted or transmitted which first data unit is associated with a first sequence number SN4

In some embodiments an Active Queue Management (AQM) mechanism is adapted to be used in the sending node 100 In these embodiments the discarding unit 620 is represented by the AQM mechanism being configured to selectively discarding the first data unit

The arrangement 600 further comprises an informing unit 630 configured to inform the receiving node 120 that first data unit associated with the first sequence number SN4 is discarded so that a gap in the in the associated sequence numbers SN1 SN2, SN3, SN5, SN6 SN7 of the stream of transmitted data units can be ignored by the receiving node 120 The gap is related to the first sequence number SN4 associated to the discarded first data unit informed about Since the gap being ignored no negative status report or request will be received from the receiving node 120 for retransmission of the first data unit

In some embodiments a Radio Link Control (RLC) protocol is adapted to be used in the sending node 100 In these embodiments the informing unit 630 further is configured to inform the receiving node 120 that first data unit associated with the first sequence number SN4 is discarded by sending a RLC control data unit message to the receiving node 120 The message comprises the first sequence number SN4 associated with the discarded first data unit

In some other embodiments, the informing unit 630 further is configured to inform the receiving node 120 that first data unit associated with the first sequence number SN4 is discarded by sending a dummy data unit The dummy data unit comprises the first sequence number SN4 associated with the discarded first data unit and an indication that the first data unit is discarded The indication that the first data unit is discarded may e g be given by a special value of a length indicator in the dummy data unit or by a special discard bit in a header of the dummy data unit

The method steps in the receiving node 120 for receiving a transmission of data units over a radio interface from a sending node 100 according to some embodiments will now be described with reference to a flowchart depicted in Figure 7 Each data unit to be transmitted by the transmitting node 100 is associated with a sequence number SN1 SN2, SN3 SN4 SN5 SN6 SN7 The method comprises the steps of

701 In this step the receiving node 120 receives a stream of transmitted data units from the sending node 100 702. The receiving node 120 obtains information from the sending node 100. The information says that a first data unit is discarded, which first data unit thus can not be retransmitted or transmitted to the receiving node 120, and that the first data unit is associated with a first sequence number SN4. In some embodiments wherein the RLC protocol is used, this step may be performed by obtaining a RLC control data unit message from the sending node 100. The message comprises the first sequence number SN4 associated with the discarded first data unit.

In some other embodiments, this step is performed by receiving a dummy data unit. The dummy data unit may comprise the first sequence number SN4 associated with the discarded first data unit, and an indication that the first data unit is discarded.

In one embodiment, the indication that the first data unit is discarded is given by a special value of a length indicator in the dummy data unit. In another embodiment, the indication that the first data unit is discarded is given by a special discard bit in a header of the dummy data unit.

If the receiving node 120 uses a window mechanism, the first data unit is discarded from the receiver window and thus causing the gap in the associated sequence numbers SN1 , SN2, SN3, SN5, SN6, SN7 in the receiver window.

703. Since the receiving node 120 received the information that the first data unit is discarded and that the first data unit is associated with the first sequence number SN4 associated to the discarded first data unit, the receiving node 120 ignores the gap in the associated sequence numbers SN1 , SN2, SN3, SN5, SN6, SN7 of the stream of transmitted data units. Since ignored, no negative status report or request will be sent to the sending node 100 for retransmission of the first data unit, which would have caused additional delay.

If the receiving node 120 uses a window mechanism, the gap in the associated sequence numbers SN1 , SN2, SN3, SN5, SN6, SN7 in the receiver window is ignored by the receiving node 120.

If the information obtained in step 702 is performed by receiving a dummy data unit comprising the first sequence number SN4 associated with the discarded first data unit, the gap is filled out with associated sequence number SN4 of the dummy data unit, and the receiving node 120 would not even notice the ignorance of the gap or the missing first data unit. The method may be used in an inter base station handover procedure wherein the first data unit is detected to be lost by the sending node 100, instead of being selectively discarded while being forwarded from a source base station to a target base station. The target base station is in this case represented by the sending node 100

To perform the method for receiving a transmission of data units over a radio interface from a sending node 100 above, the receiving node 120 comprises an arrangement 800 depicted in Figure 8. As mentioned above, each data unit to be transmitted by the sending node 100 is adapted to be associated with a sequence number SN1 , SN2, SN3, SN4, SN5, SN6, SN7.

The receiving node arrangement 800 further comprises a receiving unit 810 configured to receive a stream of transmitted data units from the sending node 100.

The receiving node arrangement 800 comprises an information unit 820 configured to obtain information from the sending node 100 that a first data unit that can not be retransmitted or transmitted is discarded. The first data unit thus will not be transmitted to the receiving node 120. The first data unit is associated with a first sequence number SN4.

In some embodiments, a RLC protocol is adapted to be used. In these embodiments the information unit 820 further is adapted to obtain information from the sending node 100 that a first data unit that would have been transmitted is discarded, by obtain a RLC control data unit message from the sending node 100. The message comprises the first sequence number SN4 associated with the discarded first data unit.

In some embodiments, the information unit 820 further is adapted to obtain information from the sending node 100 that a first data unit that would have been transmitted is discarded, by receiving a dummy data unit: The dummy data unit comprises the first sequence number SN4 associated with the discarded first data unit, and an indication that the first data unit is discarded. The indication that the first data unit is discarded may e.g. be given by a special value of a length indicator in the dummy data unit or a special discard bit in a header of the dummy data unit.

The receiving unit 810 is further configured to ignore a gap in the associated sequence numbers SN1 , SN2, SN3, SN5, SN6, SN7 of the stream of transmitted data units The gap is related to the missing first sequence number SN4 associated to the discarded first data unit When ignored, no negative status report or request for retransmission of the first data unit will be sent to the sending node 100

According to some embodiments the invention may be described as a method in a transmitter for transmission of DUs over a radio interface between a transmitter and a receiver Each DU is attached with a sequence number (SN) The method comprises the step of selectively discarding, from a receiving window of the receiver DUs that will not be transmitted by informing the receiver of the DUs to be discarded, whereby the receiver is able to ignore the gap in the stream of transmitted SNs instead of sending a negative status report to the transmitter

The transmitter may inform the receiver of the DUs to be discarded by transmitting a control message including the SNs of the DU to be discarded The transmitter may inform the receiver of the DUs to be discarded by transmitting dummy DUs, including only the SN and an indication that this DU is discarded

The indication of discarded DU may be given by a special value of the length indicator or by a special discard bit in the DU header

According to some embodiments, the invention may be described as a method in a receiver for reception of DUs over a radio interface between a transmitter and a receiver whereby each DU is attached with a sequence number (SN) The method comprises the steps of

- receiving information from the transmitter of DUs to be discarded from a receiving window of the receiver

- ignoring the gaps in the stream of transmitted SNs that corresponds to the DUs to be discarded in accordance with the received information,

- delivering consecutive DUs without sending negative status report to the transmitter The methods may be used in an inter base station handover procedure where DUs are lost while being forwarded from one base station to another or where DUs are lost due to dropping of DUs in accordance with Automatic Queue Management in the transmitter The present mechanism for transmitting of data units from a sending node 100 to a receiving node 120 over a radio interface may be implemented through one or more processors, such as a processor 640 in the sending node arrangement 600 depicted in Figure 6 or the processor 830 in the receiving node arrangement 800 depicted in Figure 8, together with computer program code for performing the functions of the present solution. The program code mentioned above may also be provided as a computer program product, for instance in the form of a data carrier carrying computer program code for performing the present solution when being loaded into the sending node 100 or the receiving node 120. One such carrier may be in the form of a CD ROM disc. It is however feasible with other data carriers such as a memory stick. The computer program code can furthermore be provided as pure program code on a server and downloaded to the sending node 100 or the receiving node 120 remotely.

When using the word "comprise" or "comprising " it shall be interpreted as non- limiting, i.e. meaning "consist at least of.

The present invention is not limited to the above described preferred embodiments. Various alternatives, modifications and equivalents may be used. Therefore, the above embodiments should not be taken as limiting the scope of the invention, which is defined by the appending claims.

Claims

1. A method in a sending node (100) for transmitting data units over a radio interface (115) to a receiving node (120), the method comprising the steps of: transmitting (501 ) a stream of data units over the radio interface (115) to the receiving node (120), each data unit being associated with a sequence number (SN1 , SN2, SN3, SN4, SN5, SN6, SN7), selectively discarding (502) a first data unit that will not be retransmitted or transmitted, which first data unit is associated with a first sequence number (SN4), informing (503) the receiving node (120) that first data unit associated with the first sequence number (SN4) is discarded, so that a gap in the in the associated sequence numbers (SN1 , SN2, SN3, SN5. SN6, SN7) of the stream of transmitted data units can be ignored by the receiving node (120), which gap is related to the first sequence number (SN4) associated to the discarded first data unit informed about, and since the gap being ignored, no negative status report or request will be received from the receiving node (120) for retransmission of the first data unit.

2. The method according to claim 1 , wherein an Active Queue Management (AQM) mechanism is used in the sending node (100). and wherein the AQM mechanism performs the step of selectively discarding (502) the first data unit.

3. The method according to any of the claims 1-2, wherein a Radio Link Control (RLC) protocol is used in the sending node (100), and wherein the step of informing (503) is performed by sending a RLC control data unit message to the receiving node (120), the message comprising the first sequence number (SN4) associated with the discarded first data unit.

4. The method according to any of the claims 1-2, wherein the step of informing (503) is performed by sending a dummy data unit, which dummy data unit comprises the first sequence number (SN4) associated with the discarded first data unit, and an indication that the first data unit is discarded.

5. The method according to claim 4, wherein the indication that the first data unit is discarded is given by a special value of a length indicator in the dummy data unit.

6. The method according to claim 5, wherein the indication that the first data unit is discarded is given by a special discard bit in a header of the dummy data unit.

7. A method in a receiving node (120) for receiving a transmission of data units over a radio interface (130) from a sending node (100), wherein each data unit to be transmitted by the sending node (100) is associated with a sequence number (SN1 , SN2, SN3, SN4, SN5, SN6, SN7), the method comprising the steps of: receiving (701) a stream of transmitted data units from the sending node (100), obtaining (702) information from the sending node (100) that a first data unit is discarded, which first data unit thus can not be retransmitted or transmitted to the receiving node (120), and that the first data unit is associated with a first sequence number (SN4), ignoring (703) a gap in the associated sequence numbers (SN1 , SN2, SN3, SN5, SN6, SN7) of the stream of transmitted data units, which gap is related to the first sequence number (SN4) associated with the discarded first data unit, and since ignored no negative status report or request for retransmission of the first data unit, will be sent to the sending node (100).

8. The method according to claim 7, wherein a Radio Link Control (RLC) protocol is used, and wherein the step of obtaining information (702) is performed by obtaining a RLC control data unit message from the sending node (100), the message comprising the first sequence number (SN4) associated with the discarded first data unit.

9. The method according to claim 8. wherein the step of obtaining information (702) is performed by receiving a dummy data unit, which dummy data unit comprises the first sequence number (SN4) associated with the discarded first data unit, and an indication that the first data unit is discarded.

10. The method according to claim 9, wherein the indication that the first data unit is discarded is given by a special value of a length indicator in the dummy data unit.

11. The method according to claim 9, wherein the indication that the first data unit is discarded is given by a special discard bit in a header of the dummy data unit. 2 Use of the methods according to any of claims 1-1 1 in an inter base station handover procedure wherein the first data unit is detected to be lost by the sending node (100), instead of being selectively discarded while being forwarded from a source base station to a target base station which target base station is represented by the sending node (100) 3 An arrangement (600) in a sending node (100) arranged to transmit data units over a radio interface (115) to a receiving node (120), the sending node arrangement (600) comprising a transmitting unit (610) configured to transmit a stream of data units over the radio interface (115) to the receiving node (120) each data unit being associated with a sequence number (SN1 , SN2 SN3 SN4, SN5 SN6, SN7), a discarding unit (620) configured to selectively discard a first data unit that will not be retransmitted or transmitted, which first data unit is associated with a first sequence number (SN4) an informing unit (630) configured to inform the receiving node (120) that first data unit associated with the first sequence number (SN4) is discarded so that a gap in the in the associated sequence numbers (SN 1 SN2 SN3 SN5, SN6 SN7) of the stream of transmitted data units can be ignored by the receiving node (120) which gap is related to the first sequence number (SN4) associated to the discarded first data unit informed about, and since the gap being ignored, no negative status report or request will be received from the receiving node (120) for retransmission of the first data unit

14 The sending node arrangement (600) according to claim 13 wherein an Active Queue Management (AQM) mechanism is adapted to be used in the sending node (100), and wherein the discarding unit (620) is represented by the AQM mechanism being configured to selectively discarding the first data unit

15 The sending node arrangement (600) according to any of the claims 13-14 wherein a Radio Link Control (RLC) protocol is adapted to be used in the sending node (100) and wherein the informing unit (630) further is configured to inform the receiving node (120) that first data unit associated with the first sequence number (SN4) is discarded by sending a RLC control data unit message to the receiving node (120), the message comprising the first sequence number (SN4) associated with the discarded first data unit

16 The sending node arrangement (600) according to any of the claims 13-14, wherein 5 the informing unit (630) further is configured to inform the receiving node (120) that first data unit associated with the first sequence number (SN4) is discarded by sending a dummy data unit, which dummy data unit comprises the first sequence number (SN4) associated with the discarded first data unit and an indication that the first data unit is discarded 10

17 The sending node arrangement (600) according to claim 16, wherein the indication that the first data unit is discarded is given by a special value of a length indicator in the dummy data unit

15 18 The sending node arrangement (600) according to claim 16 wherein the indication that the first data unit is discarded is given by a special discard bit in a header of the dummy data unit

19 An arrangement (800) in a receiving node (120) for receiving a transmission of data

20 units over a radio interface from a sending node (100), wherein each data unit to be transmitted by the sending node (100) is adapted to be associated with a sequence number (SN1 SN2 SN3, SN4, SN5, SN6 SN7), the receiving node arrangement (800) comprising a receiving unit (810) configured to receive a stream of transmitted data units

25 from the sending node (100), an information unit (820) configured to obtain information from the sending node (100) that a first data unit is discarded, which first data unit thus can not be retransmitted or transmitted to the receiving node (120), and that the first data unit is associated with a first sequence number (SN4),

30 the receiving unit (810) further being configured to ignore a gap in the associated sequence numbers (SN1 , SN2 SN3 SN5, SN6, SN7) of the stream of transmitted data units, which gap is related to the first sequence number (SN4) associated to the discarded first data unit and when ignored no negative status report or request for retransmission of the first data unit, will be sent to the sending node (100)

35

20. The receiving node arrangement (800) according to claim 19, wherein a Radio Link Control (RLC) protocol is adapted to be used, and wherein the information unit (820) further is adapted to obtain information from the sending node (100) that a first data unit that would have been transmitted is discarded, by obtain a RLC control data unit message from the sending node (100), the message comprising the first sequence number (SN4) associated with the discarded first data unit.

21. The receiving node arrangement (800) according to claim 20, wherein the information unit (820) further is adapted to obtain information from the sending node (100) that a 10 first data unit that would have been transmitted is discarded, by receiving a dummy data unit, which dummy data unit comprises the first sequence number (SN4) associated with the discarded first data unit, and an indication that the first data unit is discarded.

15 22. The receiving node arrangement (800) according to claim 21, wherein the indication that the first data unit is discarded is given by a special value of a length indicator in the dummy data unit.

23. The receiving node arrangement (800) according to claim 21 , wherein the indication 20 that the first data unit is discarded is given by a special discard bit in a header of the dummy data unit.