WO2017012668A1

WO2017012668A1 - Improved data unit reordering in dual connectivity scenarios

Info

Publication number: WO2017012668A1
Application number: PCT/EP2015/066812
Authority: WO
Inventors: Piotr LECHOWICZ; Henri Markus Koskinen
Original assignee: Nokia Solutions And Networks Oy
Priority date: 2015-07-23
Filing date: 2015-07-23
Publication date: 2017-01-26

Abstract

There are provided measures for improved data unit reordering in dual connectivity scenarios. Such measures, for ordering data units of a data transmission according to a sequence number associated with each of said data units, said data units being communicated utilizing a data communication via at least one of a first lower-layer protocol entity and a second lower-layer protocol entity, exemplarily comprise maintaining a value of a first variable for a delivery decision based on at least said sequence numbers associated with said data units received via said first lower-layer protocol entity, maintaining a value of a second variable for said delivery decision based on at least said sequence numbers associated with said data units received via said second lower-layer protocol entity, and detecting a predetermined indication of discontinuity in said data communication via at least one of said first lower-layer protocol entity and said second lower-layer protocol entity, and in response to said detecting, effecting a manipulation with respect to at least one of said value of said first variable and said value of said second variable.

Description

Title

Improved data unit reordering in dual connectivity scenarios Field

The present invention relates to improved data unit reordering in dual connectivity scenarios. More specifically, the present invention exemplarily relates to measures (including methods, apparatuses and computer program products) for realizing improved data unit reordering in dual connectivity scenarios.

Background

The present specification generally relates to dual connectivity scenarios in cellular network systems like 3^rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) and Long Term Evolution Advanced (LTE-A).

In 3GPP release 12 a dual connectivity operation mode is introduced, where data is delivered to a user equipment (UE) like a terminal over two evolved NodeBs (eNodeB, eNB) called Master eNB (MeNB) and Secondary eNB (SeNB). In Dual Connectivity, a configured set of serving cells for a UE consists of two subsets, namely a Master Cell Group (MCG) containing the serving cells of the MeNB, and a Secondary Cell Group (SCG) containing the serving cells of the SeNB.

An actual load sharing can take place either at a serving gateway (S-GW) or at the MeNB. The latter case, i.e., the load sharing taking place at the MeNB where the bearer's radio protocols are located in both eNBs, is known as a split bearer scenario. Figure 1 is a schematic diagram illustrating a split bearer scenario. In particular, Figure 1 illustrates a downlink user data flow for a split data radio bearer.

In relation to that and with respect to Packet Data Convergence Protocol (PDCP) the following procedures are defined.

In a procedure named re-establishment procedure, a ciphering key used in ciphering the payload of PDCP Protocol Data Units (PDU) is changed. The main use case is change of the UE's Primary serving cell (commonly referred to as handover), which always mandates such key change. It entails replacing all the PDCP PDUs ciphered with the previous key and being exchanged at lower layers, i.e., radio link control (RLC) layer, medium access control (MAC) layer, physical layer (PHY), with PDUs ciphered with the new key, which is the reason why it is carried out in conjunction with re-establishment/reset of the underlying protocols.

The RLC flushes all PDCP PDUs that it has received so far to PDCP, after which it erases everything in its memory. It then remains the responsibility of PDCP to retransmit the service data units (SDU) not yet delivered to the peer entity, within PDUs ciphered with the new key. In acknowledged-mode operation, a transmitting RLC continuously keeps the local PDCP informed of RLC SDUs, that is, PDCP PDUs (RLC SDUs = PDCP PDUs), that have been RLC acknowledged. Thanks to this the PDCP knows from which PDU to start the PDCP-level retransmissions when such a time comes.

In a procedure named PDCP Data Recovery procedure, when upper layers request a PDCP data recovery for a radio bearer, the UE shall - if the radio bearer is configured by upper layers to send a PDCP status report in the uplink, compile a status report and submit it to lower layers as the first PDCP PDU for the transmission, and shall

- perform retransmission of all the PDCP PDUs previously submitted to re- established acknowledged mode (AM) RLC entity in ascending order of the associated COUNT values from the first PDCP PDU for which the successful delivery has not been confirmed by lower layers.

Figure 2 is a schematic diagram illustrating a possible format of a COUNT value as cited above.

Such COUNT value may have a length of 32 bits and may be composed of a hyper frame number (HFN) and the PDCP sequence number (SN). The length of the PDCP SN may be configured by upper layers. As is derivable from the illustration on Figure 2, the size of the HFN part in bits may be equal to 32 minus the length of the PDCP SN.

When performing comparison of values related to COUNT, the UE shall take into account that COUNT may be a 32-bit value, which may wrap around (e.g., COUNT value of 2³² - 1 is less than COUNT value of 0).

The COUNT value may be maintained for ciphering and integrity.

The PDCP Data Recovery procedure represents a subset of the PDCP re- establishment procedure, in that the ciphering key does not change. The reason for its use is that the underlying RLC entity associated with the SCG ceases to exist in its current form, which is why the sequence of RLC re- establishment and PDCP-level retransmissions takes place. The main use cases are when a split bearer is in place for a UE, and the SeNB either changes to another SeNB or is de-configured entirely (i.e., the split bearer becomes a "regular" MCG bearer served by the MeNB only). With respect to re-establishment in relation to RLC it is further specified that when radio resource control (RRC) indicates that an RLC entity should be re-established, the RLC entity shall,

- if it is an AM RLC entity,

- if possible, reassemble RLC SDUs from any byte segments of acknowledged mode data (AMD) PDUs with SN < VR(MR) (which is a state variable) in the receiving side, remove RLC headers when doing so and deliver all reassembled RLC SDUs to upper layer in ascending order of the RLC SN, if not delivered before,

- discard the remaining AMD PDUs and byte segments of AMD PDUs in the receiving side,

- discard all RLC SDUs and AMD PDUs in the transmitting side,

- discard all RLC control PDUs;

- stop and reset all timers, and

- reset all state variables to their initial values.

The mentioned RLC-flushed PDUs are separately distinguished in the PDCP specification, i.e. an inter-layer indication enabling that is assumed between RLC and PDCP. For example, it can be identified that the PDCP PDU received by PDCP is not due to the re-establishment of lower layers. Conversely, it can be identified that the PDCP PDU received by PDCP is due to the re- establishment of lower layers.

Due to different queue lengths in the MeNB and the SeNB, packets of a split bearer may arrive to the UE out-of-sequence, and the UE PDCP may try to reorder them correctly. As reordering can not wait endlessly, a timeout may be used which is caused by expiry of the PDCP reordering timer (tReordering), which should suffice the maximum arrival-delay difference. However, if a packet is discarded from a transmission buffer (due to a queue management for example), the UE would wait unnecessarily until tReordering expires, which would increase jitter and latency and which would reduce the transmission control protocol (TCP) throughput. Figure 3 is a schematic diagram illustrating an example arrangement of communication entities in a network environment, namely a source of a data flow, an eNB, and a UE. Here, the transmission rate between the source of the data flow and the eNB is higher than the transmission rate between the eNB and the UE. Accordingly, queue management may be implemented in the eNB.

The following table shows a result of a simulation of a queue management which discards 1% of the packets. It is derivable that that reordering delay, possibly caused by timer tReordering, can have a destructive influence on the TCP traffic.

Reordering Throughput

[ms] [kbps]

10 5911

20 4174

30 3211

40 2514

50 2121

60 1739

70 1576

80 1396

90 1286

100 1171

110 1072

120 1004

130 948

140 896

150 835

160 790

170 746

180 691

190 654

200 626

Figure 4 is a schematic diagram illustrating an impact of a reordering latency on the throughput corresponding to the table recited above. Further, a split bearer implementation is affected by the following concepts from established standards.

On the one hand, a lower-numbered PDCP PDU which arrives later than higher-numbered PDUs already delivered by PDCP to higher layers is discarded by the UE. On the other hand, it is assumed that the PDCP transmitter performs prioritized retransmission of packets that it has found to have gone missing, which implies a possibility of out-of-sequence reception of PDCP PDUs over a given eNB's RLC.

Hence, the problem arises that the throughput can be unnecessarily affected to the negative in case of certain situations in a split bearer scenario. In particular, the UE may no longer apply the single connectivity logic of not waiting for a packet when a higher-numbered one arrives and instead rely only on the tReordering timeout, which will produce hiccup traffic if any SN gaps occur. Further, due to the fact that late packets will be discarded, the tReordering needs to be relatively long.

Any PDCP SN gap, for example due to an active queue management (AQM), may cause delivery of data to higher layers by the UE to stall until the tReordering expires.

Hence, there is a need to provide for improved data unit reordering in dual connectivity scenarios. Summary

Various example embodiments of the present invention aim at addressing at least part of the above issues and/or problems and drawbacks. Various aspects of example embodiments of the present invention are set out in the appended claims. According to an example aspect of the present invention, there is provided a method for ordering data units of a data transmission according to a sequence number associated with each of said data units, said data units being communicated utilizing a data communication via at least one of a first lower-layer protocol entity and a second lower-layer protocol entity, the method comprising maintaining a value of a first variable for a delivery decision based on at least said sequence numbers associated with said data units received via said first lower-layer protocol entity, maintaining a value of a second variable for said delivery decision based on at least said sequence numbers associated with said data units received via said second lower-layer protocol entity, and detecting a predetermined indication of discontinuity in said data communication via at least one of said first lower- layer protocol entity and said second lower-layer protocol entity, and in response to said detecting, effecting a manipulation with respect to at least one of said value of said first variable and said value of said second variable.

According to an example aspect of the present invention, there is provided an apparatus for ordering data units of a data transmission according to a sequence number associated with each of said data units, said data units being communicated utilizing a data communication via at least one of a first lower-layer protocol entity and a second lower-layer protocol entity, the apparatus comprising a maintaining circuitry configured to maintain a value of a first variable for a delivery decision based on at least said sequence numbers associated with said data units received via said first lower-layer protocol entity, and to maintain a value of a second variable for said delivery decision based on at least said sequence numbers associated with said data units received via said second lower-layer protocol entity, a detecting circuitry configured to detect a predetermined indication of discontinuity in said data communication via at least one of said first lower- layer protocol entity and said second lower-layer protocol entity, and a manipulation circuitry configured, in response to said detecting circuitry detecting said predetermined indication, to effect a manipulation with respect to at least one of said value of said first variable and said value of said second variable.

According to an example aspect of the present invention, there is provided a method for controlling a data communication via at least one of a first lower-layer protocol entity and a second lower-layer protocol entity, the method comprising determining that data communication via any of said first lower-layer protocol entity and said second lower-layer protocol entity is inactive, and in response to said determining, transmitting a signaling indicating that any of said first lower-layer protocol entity and said second lower-layer protocol entity is inactive.

According to an example aspect of the present invention, there is provided an apparatus for controlling a data communication via at least one of a first lower-layer protocol entity and a second lower-layer protocol entity, the apparatus comprising a determining circuitry configured to determine that data communication via any of said first lower-layer protocol entity and said second lower-layer protocol entity is inactive, and a transmitting circuitry configured, in response to said determining circuitry determining an inactivity, to transmit a signaling indicating that any of said first lower-layer protocol entity and said second lower-layer protocol entity is inactive.

According to an example aspect of the present invention, there is provided a computer program product comprising computer-executable computer program code which, when the program is run on a computer (e.g. a computer of an apparatus according to the aforementioned apparatus- related exemplary aspect of the present invention), is configured to cause the computer to carry out the method according to the aforementioned method-related exemplary aspect of the present invention.

Such computer program product may comprise (or be embodied) a (tangible) computer-readable (storage) medium or the like on which the computer-executable computer program code is stored, and/or the program may be directly loadable into an internal memory of the computer or a processor thereof.

Any one of the above aspects enables an efficient avoidance or reduction of negative impacts of certain situations related to the above mentioned procedures on the quality of the dual connectivity traffic to thereby solve at least part of the problems and drawbacks identified in relation to the prior art. By way of example embodiments of the present invention, there is provided improved data unit reordering in dual connectivity scenarios. More specifically, by way of example embodiments of the present invention, there are provided measures and mechanisms for realizing improved data unit reordering in dual connectivity scenarios.

Thus, improvement is achieved by methods, apparatuses and computer program products enabling/realizing improved data unit reordering in dual connectivity scenarios. Brief description of the drawings

In the following, the present invention will be described in greater detail by way of non-limiting examples with reference to the accompanying drawings, in which

Figure 1 is a schematic diagram illustrating a split bearer scenario,

Figure 2 is a schematic diagram illustrating a possible format of a COUNT value,

Figure 3 is a schematic diagram illustrating an example arrangement of communication entities in a network environment, Figure 4 is a schematic diagram illustrating an impact of a reordering latency on throughput,

Figure 5 is a block diagram illustrating an apparatus according to example embodiments of the present invention,

Figure 6 is a schematic diagram of a procedure according to example embodiments of the present invention, Figure 7 is a block diagram illustrating an apparatus according to example embodiments of the present invention,

Figure 8 is a schematic diagram of a procedure according to example embodiments of the present invention,

Figure 9 is a schematic diagram illustrating an example of data unit transmissions in a dual connectivity scenario, and

Figure 10 is a block diagram alternatively illustrating the apparatus according to example embodiments of the present invention.

Detailed description of drawings and embodiments of the present invention

The present invention is described herein with reference to particular non- limiting examples and to what are presently considered to be conceivable embodiments of the present invention. A person skilled in the art will appreciate that the invention is by no means limited to these examples, and may be more broadly applied. It is to be noted that the following description of the present invention and its embodiments mainly refers to specifications being used as non-limiting examples for certain example network configurations and deployments. Namely, the present invention and its embodiments are mainly described in relation to 3GPP specifications being used as non-limiting examples for certain example network configurations and deployments. As such, the description of example embodiments given herein specifically refers to terminology which is directly related thereto. Such terminology is only used in the context of the presented non-limiting examples, and does naturally not limit the invention in any way. Rather, any other communication or communication related system deployment, etc. may also be utilized as long as compliant with the features described herein. Further, while the description of the present invention and its embodiments mainly refers to an implementation of a split bearer and the data splitting on the split bearer in downlink, it is noticed that Release 13 standardization is introducing the data splitting on a split bearer also in uplink. Therefore all the principles recited for the description of the present invention and its embodiments should be understood as equally applicable in both directions, e.g., to a PDCP receiver regardless of whether that is at the UE or the network.

Hereinafter, various embodiments and implementations of the present invention and its aspects or embodiments are described using several variants and/or alternatives. It is generally noted that, according to certain needs and constraints, all of the described variants and/or alternatives may be provided alone or in any conceivable combination (also including combinations of individual features of the various variants and/or alternatives).

According to example embodiments of the present invention, in general terms, there are provided measures and mechanisms for (enabling/realizing) improved data unit reordering in dual connectivity scenarios.

In order to overcome the above identified drawbacks of the prior art, it is suggested to enhance the established (Release 12) PDCP reordering behavior by considering that AM RLC entity mostly delivers RLC SDUs in- sequence and PDCP would not retransmit PDCP PDUs without a separate PDCP procedure. Consequently, if a PDU with SN=X is received from an AM RLC entity, the UE can know that in absence of a separate PDCP procedure, missing PDUs with SN < X will not be received from the AM RLC entity. For split bearers, if a PDU with SN=X1 is received from AM RLC 1 and a PDU with SN=X2 is received from AM RLC 2, then the UE can know that in absence of a separate PDCP procedure, missing PDUs with SN < XI and X2 will not be received from any of the AM RLC entities.

This behavior is similar to pre-Release-12 in-order delivery function where when a PDU with SN=X is received from lower layers but not due to re- establishment of lower layers, all the missing PDUs with SN < X are considered as lost.

The most basic implementation of the above suggestion would be that the PDCP receiver maintains, separately for both the reception branches MCG and SCG of the split bearer, in a new state variable the highest sequence number of a PDCP PDU received over that branch. In the following, the respective state variables are denoted with Highest_COUNT_MCG for the MCG and Highest_COUNT_SCG for the SCG. As a further part of the most basic implementation, following every reception of a PDU, all stored SDUs with associated COUNT value not exceeding min(Highest_COUNT_MCG, Highest_COUNT_SCG) are delivered to higher layer.

However, the following issues are identified as being entailed with this most basic implementation.

When one of the split-bearer branches is not used for data delivery for a period of time, during that period, the above-mentioned state variable for the unused branch will not progress, and will therefore not allow delivery to higher layer to progress either.

Further, the behaviour upon various possible transitions and discontinuities, such as PDCP re-establishment and PDCP data-recovery procedures, where the underlying RLC operation interrupts and flushes any stored SDUs to PDCP, and PDCP-level retransmissions take over, are identified as disadvantageous. Namely, the RLC-flushed PDUs would update the state variables Highest_COUNT_MCG and Highest_COUNT_SCG as defined above, and based on this, delivery of SDUs to higher layers by PDCP can take place, without accounting for the fact that PDCP-level retransmissions of the PDUs that the RLC was missing before the flush are still to be expected. Once these PDCP-level retransmissions are received, they are received outside the PDCP reception window and hence considered as duplicate PDUs and discarded. This means loss of higher-layer data.

Further potential measures in order to overcome the above identified drawbacks of the prior art and in particular to improve the UE reordering latency are identified.

Namely, gaps among PDCP SNs received by PDCP should be avoided, which means, that

- SeNB should not discard PDCP PDUs, and PDCP PDUs lost in the X2 interface should be sent again by the MeNB directly,

- the AQM and the PDCP-discard procedure should not operate on PDCP PDUs, but should manage packets before receiving their SNs and before ciphering (PDCP SDUs), which delays the feedback given to TCP source, or forwards too old streaming packets, and that

- for the case of discarding by the transmitting PDCP entity, e.g., for the purpose of AQM, only the SDU within a numbered PDU should be discarded, while the PDU should still be sent with its SN to the peer entity, thereby avoiding a gap in received SNs at reception. As a further measure, a move receiver window (MRW) control PDU may be implemented by which a transmitting protocol entity indicates to the receiver which PDUs it should no longer expect to receive. As a further measure, the real packet discard may be replaced with a so- called virtual discard, which inserts duplicate acknowledgments (ACK) which are identical copies of already transmitted ACKs in the reverse ACK flow, thereby creating a fake signalling of packet loss towards the TCP sender. However, it was identified that these measures may restrict flexibility of load balancing algorithms and do not guarantee that UE PDCP will never stall.

Hence, according to example embodiments of the present invention, mechanisms for application at PDCP reception are provided under the assumption that the PDCP transmitter does not perform out-of-order transmissions in continuous operation.

Figure 5 is a block diagram illustrating an apparatus according to example embodiments of the present invention. The apparatus may be a communication node 50 such as a base station or access node of a cellular system or a terminal, user equipment, mobile station or modem in a cellular system, and in particular an apparatus for ordering data units of a data transmission according to a sequence number associated with each of said data units, said data units being communicated utilizing a data communication via at least one of a first lower-layer protocol entity and a second lower-layer protocol entity. The apparatus 50, in its most basic form, comprises a maintaining circuitry 51, a detecting circuitry 52 and a manipulation circuitry 53. The maintaining circuitry 51 maintains a value of a first variable for a delivery decision based on at least said sequence numbers associated with said data units received via said first lower-layer protocol entity. Further, the maintaining circuitry 51 maintains a value of a second variable for said delivery decision based on at least said sequence numbers associated with said data units received via said second lower- layer protocol entity. The detecting circuitry 52 detects a predetermined indication of discontinuity in said data communication via at least one of said first lower-layer protocol entity and said second lower-layer protocol entity. The manipulation circuitry 53 effects a manipulation with respect to at least one of said value of said first variable and said value of said second variable in response to said detecting circuitry detecting said predetermined indication of discontinuity in said data communication via at least one of said first lower-layer protocol entity and said second lower-layer protocol entity.

In an embodiment at least some of the functionalities of the apparatus shown in Figure 5 may be shared between two physically separate devices forming one operational entity. Therefore, the apparatus may be seen to depict the operational entity comprising one or more physically separate devices for executing at least some of the described processes.

Figure 6 is a schematic diagram of a procedure according to example embodiments of the present invention. The apparatus according to Figure 5 may perform the method of Figure 6 but is not limited to this method. The method of Figure 6 may be performed by the apparatus of Figure 5 but is not limited to being performed by this apparatus.

As shown in Figure 6, a procedure for ordering data units of a data transmission according to a sequence number associated with each of said data units, said data units being communicated utilizing a data communication via at least one of a first lower-layer protocol entity and a second lower-layer protocol entity according to example embodiments of the present invention comprises an operation of maintaining (S61) a value of a first variable for a delivery decision based on at least said sequence numbers associated with said data units received via said first lower-layer protocol entity, an operation of maintaining (S62) a value of a second variable for said delivery decision based on at least said sequence numbers associated with said data units received via said second lower-layer protocol entity, an operation of detecting (S63) a predetermined indication of discontinuity in said data communication via at least one of said first lower- layer protocol entity and said second lower-layer protocol entity, and an operation of effecting (S64) a manipulation with respect to at least one of said value of said first variable and said value of said second variable in response to said detecting.

According to a variation of the procedure shown in Figure 6, example additional operations and example details of the detecting operation (S63) and the effecting operation (S64) are given, which are inherently independent from each other as such.

According to such variation, an example method according to example embodiments of the present invention may comprise an operation of determining a delivery threshold value based on a minimum among the value of said first variable and the value of said second variable.

Further, such example detecting operation (S63) according to example embodiments of the present invention may comprise an operation of identifying that communication via any of said first lower-layer protocol entity and said second lower-layer protocol entity is inactive.

Further, such example effecting operation (S64) according to example embodiments of the present invention may comprise an operation of ignoring said value of said first variable when determining said delivery threshold value, if said communication via said first lower-layer protocol entity is inactive, and an operation of ignoring said value of said second variable when determining said delivery threshold value, if said communication via said second lower-layer protocol entity is inactive.

According to further example embodiments of the present invention, said communication via any of said first lower-layer protocol entity and said second lower-layer protocol entity is identified inactive, if an inactivity timer is expired, or if a de-configuration of a lower-layer protocol entity from said data transmission is detected, or if a signaling indicating that any of said first lower-layer protocol entity and said second lower-layer protocol entity is inactive is received.

In more concrete terms, as a first concrete measure according to example embodiments of the present invention, a deactivation-activation mechanism for a CG at least in use for the split bearer in question (e.g., MCG or SCG) is provided.

In particular, the above identified most basic implementation that following every reception of a PDU, all stored SDUs with associated COUNT value not exceeding min(Highest_COUNT_MCG, Highest_COUNT_SCG) are delivered to higher layer, is modified to consider the minimum of only such state variables representing the CGs currently active in reception of the split bearer in question, i.e. the minimum taken over the active set of CGs only.

The trigger for the receiver to consider a CG as inactive in reception may be implicit, such as based on an inactivity timer or at split-MCG bearer reconfiguration (after which the SCG branch should be considered inactive). Alternatively, the trigger for the receiver to consider a CG as inactive in reception may be an explicit indication from the transmitter such as a new PDCP control PDU defined for the purpose.

Similarly, the trigger for considering a CG active again can be implicit based on another PDU received over the CG, or an explicit indication.

According to a variation of the procedure shown in Figure 6, example details of the detecting operation (S63) and the effecting operation (S64) are given, which are inherently independent from each other as such. Such example detecting operation (S63) according to example embodiments of the present invention may comprise an operation of receiving a data unit due to a re-establishment of any of said first lower- layer protocol entity and said second lower-layer protocol entity, and an operation of deducing, based on said receiving, which of said first lower- layer protocol entity and said second lower-layer protocol entity is subject to said re-establishment.

Further, such example effecting operation (S64) according to example embodiments of the present invention may comprise an operation of prohibiting adjusting said value of said first variable based on at least said sequence number associated with said received data unit, if said first lower- layer protocol entity is subject to said re-establishment, and an operation of prohibiting adjusting said value of said second variable based on at least said sequence number associated with said received data unit, if said second lower-layer protocol entity is subject to said re-establishment.

Here, it is noted that a normal behavior in relation to said maintaining said value of said first variable may be - for example - an updating, if a data unit is received via said first lower-layer protocol entity, said value of said first variable based on at least said sequence number associated with said received data unit, if no data unit lower in said sequence number is still expected to be received via said first lower-layer protocol entity, and a normal behavior in relation to said maintaining said value of said second variable may be - for example - an updating, if a data unit is received via said second lower-layer protocol entity, said value of said second variable based on at least said sequence number associated with said received data unit, if no data unit lower in said sequence number is still expected to be received via said second lower-layer protocol entity. Hence, according to these examples, prohibiting an adjusting according to the mentioned example embodiments of the present invention may be effected by prohibiting the updating of said value of said first variable or second variable based on at least said sequence number associated with said received data unit.

In more concrete terms, as a second concrete measure according to example embodiments of the present invention, a specified handling of PDCP PDUs received when flushed by an underlying RLC entity may be performed.

Namely, because reception of PDCP-level retransmissions following PDCP PDUs flushed by RLC at RLC re-establishment violates the baseline assumption that the lower layer of each CG provides in-sequence delivery, according to this provided measure, when a PDCP PDU is received due to re-establishment of RLC of a specific CG, the state variable Highest_COUNT_xCG for that CG is not incremented.

According to a variation of the procedure shown in Figure 6, example details of the detecting operation (S63) and the effecting operation (S64) are given, which are inherently independent from each other as such. Such example detecting operation (S63) according to example embodiments of the present invention may comprise an operation of identifying a request for a re-establishment procedure or a data recovery procedure with respect to said data communication. Further, such example effecting operation (S64) according to example embodiments of the present invention may comprise an operation of setting said value of said first variable and said value of said second variable to a minimum among said value of said first variable and said value of said second variable.

In more concrete terms, as a third concrete measure according to example embodiments of the present invention, a specified handling of PDCP re- establishment and PDCP data recovery may be performed. Namely, as discussed above, one of these procedures is invoked whenever at least one of the underlying RLCs is re-established and/or released, and entails possible PDCP-level retransmissions to compensate for the possible RLC-data loss. Hence, according to this provided measure, whenever either of these procedures is invoked (regardless of whether any RLC-flushed PDUs are received), both the variables Highest_COUNT_MCG and Highest_COUNT_SCG are assigned the value min(Highest_COUNT_MCG, Highest_COUNT_SCG).

The reasoning behind is that the minimum of these two values represents the point passed so far by the reordering algorithm. In the PDCP-level retransmissions that are part of this procedure, the retransmitting PDCP should have no constraints as to which PDUs to retransmit over which branch (i.e. CG), which would depend on the exact pre-existing values of these parameters. Namely, the value of these parameters (related to given direction of data) is known only by the receiving PDCP, not by the transmitting one. According to a variation of the procedure shown in Figure 6, example additional operations are given, which are inherently independent from each other as such. According to such variation, an example method according to example embodiments of the present invention may comprise an operation of buffering said received data units, an operation of deciding whether to deliver, to a higher protocol layer, buffered data units based on said sequence numbers associated with said buffered data units and on said delivery threshold value, and an operation of delivering said buffered data units according to a respective result of said deciding. According to a variation of the procedure shown in Figure 6, example details of the deciding operation (deciding whether to deliver buffered data units based on said sequence numbers associated with said buffered data units and on said delivery threshold value) are given, which are inherently independent from each other as such.

Such example deciding operation according to example embodiments of the present invention may comprise an operation of prescribing to deliver buffered data units, if said sequence numbers associated with said buffered data units are less than or equal to said delivery threshold value.

According to a variation of the procedure shown in Figure 6, example details of the maintaining operation (S61, maintaining said value of said first variable based on at least said sequence numbers associated with said data units received via said first lower-layer protocol entity) and the maintaining operation (S62, maintaining said value of said second variable based on at least said sequence numbers associated with said data units received via said second lower-layer protocol entity) are given, which are inherently independent from each other as such.

Such example maintaining operation (S61) according to example embodiments of the present invention may comprise an operation of applying a respective sequence number associated with a respective data unit received via said first lower-layer protocol entity as said value of said first variable, if no data unit lower in said sequence number is still expected to be received via said first lower-layer protocol entity. Further, such example maintaining operation (S62) according to example embodiments of the present invention may comprise an operation of applying a respective sequence number associated with a respective data unit received via said second lower-layer protocol entity as said value of said second variable, if no data unit lower in said sequence number is still expected to be received via said second lower-layer protocol entity. According to further example embodiments of the present invention, said first lower-layer protocol entity and said second lower-layer protocol entity are mapped to serving-cell groups in a split-bearer scenario. According to still further example embodiments of the present invention, said data unit is a Packet Data Convergence Protocol Protocol Data Unit.

Figure 7 is a block diagram illustrating an apparatus according to example embodiments of the present invention. The apparatus may be a communication node 70 such as a base station or access node of a cellular system or a terminal, user equipment, mobile station or modem in a cellular system, and in particular an apparatus for controlling a data communication via at least one of a first lower-layer protocol entity and a second lower- layer protocol entity. The apparatus 70, in its most basic form, comprises a determining circuitry 71 and a transmitting circuitry 72. The determining circuitry 71 determines that data communication via any of said first lower- layer protocol entity and said second lower-layer protocol entity is inactive. The transmitting circuitry 72 transmits a signaling indicating that any of said first lower-layer protocol entity and said second lower-layer protocol entity is inactive in response to said determining circuitry 71 determining that data communication via any of said first lower-layer protocol entity and said second lower-layer protocol entity is inactive.

In an embodiment at least some of the functionalities of the apparatus shown in Figure 7 may be shared between two physically separate devices forming one operational entity. Therefore, the apparatus may be seen to depict the operational entity comprising one or more physically separate devices for executing at least some of the described processes. Figure 8 is a schematic diagram of a procedure according to example embodiments of the present invention. The apparatus according to Figure 7 may perform the method of Figure 8 but is not limited to this method. The method of Figure 8 may be performed by the apparatus of Figure 7 but is not limited to being performed by this apparatus.

As shown in Figure 8, a procedure for controlling a data communication via at least one of a first lower-layer protocol entity and a second lower-layer protocol entity according to example embodiments of the present invention comprises an operation of determining (S81) that data communication via any of said first lower-layer protocol entity and said second lower-layer protocol entity is inactive, and an operation of transmitting (S82) a signaling indicating that any of said first lower-layer protocol entity and said second lower-layer protocol entity is inactive in response to said determining (S81).

On the basis of such signaling, another communication node according to the present invention may identify that communication via any of said first lower-layer protocol entity and said second lower-layer protocol entity is inactive.

According to example embodiments of the present invention, said signaling is a Packet Data Convergence Protocol control Protocol Data Unit.

Figure 9 is a schematic diagram illustrating an example of data unit transmissions in a dual connectivity scenario. Based on the example of data unit transmissions in a dual connectivity scenario illustrated in Figure 9, the effects of the above presented concrete measures according to example embodiments of the present invention are explained below. For the explanatory example it is assumed that the SeNB changes into another SeNB, which would mean that the SCG RLC is re-established and the PDCP data-recovery procedure is invoked. Figure 9 shows the split of PDCP PDUs between MCG and SCG for their initial PDCP transmissions, i.e., before these procedures (re-establishment and PDCP data-recovery procedure) are invoked. In this explanatory example, it is assumed that at the time these two procedures are invoked, the situation at the SCG RLC is as follows.

The PDCP PDUs #1 and #9 have been received (dotted) by the SCG RLC, while the PDUs #5 and #8 have not. Accordingly, PDU #1 has already been passed to PDCP before these procedures are invoked, implying that at this time the state variable Highest_COUNT_SCG= l.

It is further assumed that by this time each of PDUs #3, #4, #6, and #7 have been received (dotted) by the MCG RLC, i.e. Highest_COUNT_MCG=7.

From this assumed situation it follows from the invocation of the mentioned procedures that the SCG-RLC re-establishment results in PDU #9 being flushed to PDCP. Here, due to the above presented second concrete measure according to example embodiments of the present invention, this does not increment Highest_COUNT_SCG to value 9. If it would, PDUs #6, #7 currently stored at the receiving PDCP would be delivered to higher layers, i.e. the PDCP reordering would not wait for the PDCP-level retransmission of PDU #5.

From the assumed situation it further follows from the invocation of the mentioned procedures that the PDUs #5 and #8 will undergo PDCP-level retransmissions. For the present example, it is assumed that PDU #5 is retransmitted over MCG, and PDU #8 is retransmitted over SCG and reaches the receiving PDCP before PDU #5.

Here, due to the above presented third concrete measure according to example embodiments of the present invention, at the time these procedures are invoked, the Highest_COUNT_MCG is set back from value 7 to value 1. If this would not be done, at the time PDU #8 reaches the receiving PDCP over SCG (before PDU #5) and increments Highest_COUNT_SCG to value 8, again PDUs #6, #7 currently stored at the receiving PDCP would be delivered to higher layers, i.e. the PDCP reordering would not wait for the PDCP-level retransmission of PDU #5.

Accordingly, the proposed measures according to example embodiments of the present invention clearly encourage overcoming the original problems of the prior art impacting heavily the quality of dual connectivity traffic.

The above-described procedures and functions may be implemented by respective functional elements, processors, or the like, as described below. In the foregoing example description of the network entities, only the units that are relevant for understanding the principles of the invention have been described using functional blocks. The respective network entity may comprise further units that are necessary for its respective operation. However, a description of these units is omitted in this specification. The arrangement of the functional blocks of the devices is not construed to limit the invention, and the functions may be performed by one block or further split into sub-blocks.

When in the foregoing description it is stated that the apparatus, i.e. network entity/communication node is configured to perform some function, this is to be construed to be equivalent to a description stating that a (i.e. at least one) processor or corresponding circuitry, potentially in cooperation with computer program code stored in the memory of the respective apparatus, is configured to cause the apparatus to perform at least the thus mentioned function. Also, such function is to be construed to be equivalently implementable by specifically configured circuitry or means for performing the respective function (i.e. the expression "unit configured to" is construed to be equivalent to an expression such as "means for"). In Figure 10, an alternative illustration of apparatuses according to example embodiments of the present invention is depicted. As indicated in Figure 10, according to example embodiments of the present invention, the apparatus (communication node) 50' (corresponding to the communication node 50) comprises a processor 101, a memory 102 and an interface 103, which are connected by a bus 104 or the like, the apparatus (communication node) 70' (corresponding to the communication node 70) comprises a processor 105, a memory 106 and an interface 107, which are connected by a bus 108 or the like, and the apparatuses may be connected with each other via link 109.

The processor 101/105 and/or the interface 103/107 may also include a modem or the like to facilitate communication over a (hardwire or wireless) link, respectively. The interface 103/107may include a suitable transceiver coupled to one or more antennas or communication means for (hardwire or wireless) communications with the linked or connected device(s), respectively. The interface 103/107is generally configured to communicate with at least one other apparatus, i.e. the interface thereof.

The memory 102/106 may store respective programs assumed to include program instructions or computer program code that, when executed by the respective processor, enables the respective electronic device or apparatus to operate in accordance with the example embodiments of the present invention.

In general terms, the respective devices/apparatuses (and/or parts thereof) may represent means for performing respective operations and/or exhibiting respective functionalities, and/or the respective devices (and/or parts thereof) may have functions for performing respective operations and/or exhibiting respective functionalities. When in the subsequent description it is stated that the processor, the circuitry (or similar) is configured to perform some function, this is to be construed to be equivalent to a description stating that at least one processor, potentially in cooperation with computer program code stored in the memory of the respective apparatus, is configured to cause the apparatus to perform at least the thus mentioned function. Also, such function is to be construed to be equivalently implementable by specifically configured means for performing the respective function (i.e. the expression "processor configured to [cause the apparatus to] perform xxx-ing" is construed to be equivalent to an expression such as "means for xxx-ing").

According to example embodiments of the present invention, an apparatus representing the communication node 50 (for ordering data units of a data transmission according to a sequence number associated with each of said data units, said data units being communicated utilizing a data communication via at least one of a first lower-layer protocol entity and a second lower-layer protocol entity) comprises at least one processor 101, at least one memory 102 including computer program code, and at least one interface 103 configured for communication with at least another apparatus. The processor (i.e. the at least one processor 101, with the at least one memory 102 and the computer program code) is configured to perform maintaining a value of a first variable for a delivery decision based on at least said sequence numbers associated with said data units received via said first lower-layer protocol entity (thus the apparatus comprising corresponding means for maintaining), to perform maintaining a value of a second variable for said delivery decision based on at least said sequence numbers associated with said data units received via said second lower- layer protocol entity, to perform detecting a predetermined indication of discontinuity in said data communication via at least one of said first lower- layer protocol entity and said second lower-layer protocol entity (thus the apparatus comprising corresponding means for detecting), and in response to said detecting, to perform effecting a manipulation with respect to at least one of said value of said first variable and said value of said second variable (thus the apparatus comprising corresponding means for effecting).

Further, according to example embodiments of the present invention, an apparatus representing the communication node 70 (for controlling a data communication via at least one of a first lower-layer protocol entity and a second lower-layer protocol entity) comprises at least one processor 105, at least one memory 106 including computer program code, and at least one interface 107 configured for communication with at least another apparatus. The processor (i.e. the at least one processor 105, with the at least one memory 106 and the computer program code) is configured to perform determining that data communication via any of said first lower-layer protocol entity and said second lower-layer protocol entity is inactive (thus the apparatus comprising corresponding means for determining), and in response to said determining, to perform transmitting a signaling indicating that any of said first lower-layer protocol entity and said second lower-layer protocol entity is inactive (thus the apparatus comprising corresponding means for transmitting). For further details regarding the operability/functionality of the individual apparatuses, reference is made to the above description in connection with any one of Figures 1 to 9, respectively.

For the purpose of the present invention as described herein above, it should be noted that

- method steps likely to be implemented as software code portions and being run using a processor at a network server or network entity (as examples of devices, apparatuses and/or modules thereof, or as examples of entities including apparatuses and/or modules therefore), are software code independent and can be specified using any known or future developed programming language as long as the functionality defined by the method steps is preserved; - generally, any method step is suitable to be implemented as software or by hardware without changing the idea of the embodiments and its modification in terms of the functionality implemented;

- method steps and/or devices, units or means likely to be implemented as hardware components at the above-defined apparatuses, or any module(s) thereof, (e.g., devices carrying out the functions of the apparatuses according to the embodiments as described above) are hardware independent and can be implemented using any known or future developed hardware technology or any hybrids of these, such as MOS (Metal Oxide Semiconductor), CMOS (Complementary MOS), BiMOS (Bipolar MOS), BiCMOS (Bipolar CMOS), ECL (Emitter Coupled Logic), TTL (Transistor- Transistor Logic), etc., using for example ASIC (Application Specific IC (Integrated Circuit)) components, FPGA (Field-programmable Gate Arrays) components, CPLD (Complex Programmable Logic Device) components or DSP (Digital Signal Processor) components;

- devices, units or means (e.g. the above-defined network entity or network register, or any one of their respective units/means) can be implemented as individual devices, units or means, but this does not exclude that they are implemented in a distributed fashion throughout the system, as long as the functionality of the device, unit or means is preserved;

- an apparatus like the user equipment and the network entity /network register may be represented by a semiconductor chip, a chipset, or a (hardware) module comprising such chip or chipset; this, however, does not exclude the possibility that a functionality of an apparatus or module, instead of being hardware implemented, be implemented as software in a (software) module such as a computer program or a computer program product comprising executable software code portions for execution/being run on a processor;

- a device may be regarded as an apparatus or as an assembly of more than one apparatus, whether functionally in cooperation with each other or functionally independently of each other but in a same device housing, for example. In general, it is to be noted that respective functional blocks or elements according to above-described aspects can be implemented by any known means, either in hardware and/or software, respectively, if it is only adapted to perform the described functions of the respective parts. The mentioned method steps can be realized in individual functional blocks or by individual devices, or one or more of the method steps can be realized in a single functional block or by a single device.

Generally, any method step is suitable to be implemented as software or by hardware without changing the idea of the present invention. Devices and means can be implemented as individual devices, but this does not exclude that they are implemented in a distributed fashion throughout the system, as long as the functionality of the device is preserved. Such and similar principles are to be considered as known to a skilled person.

Software in the sense of the present description comprises software code as such comprising code means or portions or a computer program or a computer program product for performing the respective functions, as well as software (or a computer program or a computer program product) embodied on a tangible medium such as a computer-readable (storage) medium having stored thereon a respective data structure or code means/portions or embodied in a signal or in a chip, potentially during processing thereof. The present invention also covers any conceivable combination of method steps and operations described above, and any conceivable combination of nodes, apparatuses, modules or elements described above, as long as the above-described concepts of methodology and structural arrangement are applicable.

In view of the above, there are provided measures for improved data unit reordering in dual connectivity scenarios. Such measures, for ordering data units of a data transmission according to a sequence number associated with each of said data units, said data units being communicated utilizing a data communication via at least one of a first lower-layer protocol entity and a second lower-layer protocol entity, exemplarily comprise maintaining a value of a first variable for a delivery decision based on at least said sequence numbers associated with said data units received via said first lower-layer protocol entity, maintaining a value of a second variable for said delivery decision based on at least said sequence numbers associated with said data units received via said second lower-layer protocol entity, and detecting a predetermined indication of discontinuity in said data communication via at least one of said first lower-layer protocol entity and said second lower-layer protocol entity, and in response to said detecting, effecting a manipulation with respect to at least one of said value of said first variable and said value of said second variable.

Even though the invention is described above with reference to the examples according to the accompanying drawings, it is to be understood that the invention is not restricted thereto. Rather, it is apparent to those skilled in the art that the present invention can be modified in many ways without departing from the scope of the inventive idea as disclosed herein.

List of acronyms and abbreviations

3GPP 3^rd Generation Partnership Project

ACK acknowledgment

AM acknowledged mode

AMD acknowledged mode data

AQM active queue management

CG cell group

DL downlink

DRB data radio bearer

eNB evolved NodeB, eNodeB

HFN hyper frame number

LTE Long Term Evolution LTE-A Long Term Evolution Advanced

MAC medium access control

MCG Master Cell Group

MeNB Master eNB

MRW move receiver window

PDCP Packet Data Convergence Protocol

PDU Protocol Data Unit

PHY physical layer

RLC radio link control

RRC radio resource control

SCG Secondary Cell Group

SDU service data unit

SeNB Secondary eNB

SN sequence number

S-GW serving gateway

TCP transmission control protocol

UE user equipment

UL uplink

Claims

1. A method for ordering data units of a data transmission according to a sequence number associated with each of said data units, said data units being communicated utilizing a data communication via at least one of a first lower-layer protocol entity and a second lower-layer protocol entity, the method comprising

maintaining a value of a first variable for a delivery decision based on at least said sequence numbers associated with said data units received via said first lower-layer protocol entity,

maintaining a value of a second variable for said delivery decision based on at least said sequence numbers associated with said data units received via said second lower-layer protocol entity, and

detecting a predetermined indication of discontinuity in said data communication via at least one of said first lower-layer protocol entity and said second lower-layer protocol entity, and

in response to said detecting, effecting a manipulation with respect to at least one of said value of said first variable and said value of said second variable.

2. The method according to claim 1, further comprising

determining a delivery threshold value based on a minimum among the value of said first variable and the value of said second variable, and in relation to said detecting, said method further comprises

identifying that communication via any of said first lower-layer protocol entity and said second lower-layer protocol entity is inactive, and in relation to said effecting said manipulation, said method further comprises

ignoring said value of said first variable when determining said delivery threshold value, if said communication via said first lower-layer protocol entity is inactive, and ignoring said value of said second variable when determining said delivery threshold value, if said communication via said second lower-layer protocol entity is inactive.

3. The method according to claim 2, wherein

said communication via any of said first lower-layer protocol entity and said second lower-layer protocol entity is identified inactive, if

an inactivity timer is expired, or

a de-configuration of a lower-layer protocol entity from said data transmission is detected, or

a signaling indicating that any of said first lower-layer protocol entity and said second lower-layer protocol entity is inactive is received.

4. The method according to claim 1 to 3, wherein

in relation to said detecting, said method further comprises

receiving a data unit due to a re-establishment of any of said first lower-layer protocol entity and said second lower-layer protocol entity, and deducing, based on said receiving, which of said first lower-layer protocol entity and said second lower-layer protocol entity is subject to said re-establishment, and

in relation to said effecting said manipulation, said method further comprises

prohibiting adjusting said value of said first variable based on at least said sequence number associated with said received data unit, if said first lower-layer protocol entity is subject to said re-establishment, and

prohibiting adjusting said value of said second variable based on at least said sequence number associated with said received data unit, if said second lower-layer protocol entity is subject to said re-establishment.

5. The method according to claim 4, further comprising

determining a delivery threshold value based on a minimum among the value of said first variable and the value of said second variable.

6. The method according to any of claims 1 to 4, wherein in relation to said detecting, said method further comprises

identifying a request for a re-establishment procedure or a data recovery procedure with respect to said data communication, and

in relation to said effecting said manipulation, said method further comprises

setting said value of said first variable and said value of said second variable to a minimum among said value of said first variable and said value of said second variable.

7. The method according to claim 6, further comprising

determining a delivery threshold value based on said minimum among the value of said first variable and the value of said second variable.

8. The method according to any of claims 2, 3, 5 and 7, further comprising buffering said received data units,

deciding whether to deliver, to a higher protocol layer, buffered data units based on said sequence numbers associated with said buffered data units and on said delivery threshold value, and

delivering said buffered data units according to a respective result of said deciding.

9. The method according to claim 8, wherein

in relation to said deciding whether to deliver buffered data units based on said sequence numbers associated with said buffered data units and on said delivery threshold value, said method further comprises

prescribing to deliver buffered data units, if said sequence numbers associated with said buffered data units are less than or equal to said delivery threshold value.

10. The method according to any of claims 1 to 9, wherein in relation to said maintaining said value of said first variable based on at least said sequence numbers associated with said data units received via said first lower-layer protocol entity, said method further comprises, applying a respective sequence number associated with a respective data unit received via said first lower-layer protocol entity as said value of said first variable, if no data unit lower in said sequence number is still expected to be received via said first lower-layer protocol entity, and

in relation to said maintaining said value of said second variable based on at least said sequence numbers associated with said data units received via said second lower-layer protocol entity, said method further comprises,

applying a respective sequence number associated with a respective data unit received via said second lower-layer protocol entity as said value of said second variable, if no data unit lower in said sequence number is still expected to be received via said second lower-layer protocol entity.

11. The method according to any of claims 1 to 10, wherein

the method is operable at or by a base station or access node of a cellular system and/or at or by a terminal, user equipment, mobile station or modem, and/or

the method is operable in at least one of a LTE and a LTE-A cellular system, and/or

said first lower-layer protocol entity and said second lower-layer protocol entity are mapped to serving-cell groups in a split-bearer scenario, and/or

said data unit is a Packet Data Convergence Protocol Protocol Data

Unit.

12. An apparatus for ordering data units of a data transmission according to a sequence number associated with each of said data units, said data units being communicated utilizing a data communication via at least one of a first lower-layer protocol entity and a second lower-layer protocol entity, the apparatus comprising a maintaining circuitry configured to

maintain a value of a first variable for a delivery decision based on at least said sequence numbers associated with said data units received via said first lower-layer protocol entity, and to

maintain a value of a second variable for said delivery decision based on at least said sequence numbers associated with said data units received via said second lower-layer protocol entity,

a detecting circuitry configured to detect a predetermined indication of discontinuity in said data communication via at least one of said first lower-layer protocol entity and said second lower-layer protocol entity, and a manipulation circuitry configured, in response to said detecting circuitry detecting said predetermined indication, to effect a manipulation with respect to at least one of said value of said first variable and said value of said second variable.

13. The apparatus according to claim 12, further comprising

a determining circuitry configured to determine a delivery threshold value based on a minimum among the value of said first variable and the value of said second variable, and

an identifying circuitry configured to identify that communication via any of said first lower-layer protocol entity and said second lower-layer protocol entity is inactive, and

an ignoring circuitry configured to

ignore said value of said first variable when determining said delivery threshold value, if said communication via said first lower-layer protocol entity is inactive, and to

ignore said value of said second variable when determining said delivery threshold value, if said communication via said second lower- layer protocol entity is inactive.

14. The apparatus according to claim 13, wherein

said communication via any of said first lower-layer protocol entity and said second lower-layer protocol entity is identified inactive, if an inactivity timer is expired, or

15. The apparatus according to claim 12 to 14, further comprising

a receiving circuitry configured to receive a data unit due to a re- establishment of any of said first lower-layer protocol entity and said second lower-layer protocol entity,

a deducing circuitry configured to deduce, based on said receiving circuitry receiving said data unit due to a re-establishment, which of said first lower-layer protocol entity and said second lower-layer protocol entity is subject to said re-establishment, and

a prohibiting circuitry configured to

prohibit adjusting said value of said first variable based on at least said sequence number associated with said received data unit, if said first lower-layer protocol entity is subject to said re-establishment, and to prohibit adjusting said value of said second variable based on at least said sequence number associated with said received data unit, if said second lower-layer protocol entity is subject to said re-establishment.

16. The apparatus according to claim 15, further comprising

a determining circuitry configured to determine a delivery threshold value based on a minimum among the value of said first variable and the value of said second variable.

17. The apparatus according to any of claims 12 to 15, further comprising an identifying circuitry configured to identify a request for a re- establishment procedure or a data recovery procedure with respect to said data communication, and a setting circuitry configured to set said value of said first variable and said value of said second variable to a minimum among said value of said first variable and said value of said second variable.

18. The apparatus according to claim 17, further comprising

a determining circuitry configured to determine a delivery threshold value based on said minimum among the value of said first variable and the value of said second variable.

19. The apparatus according to any of claims 13, 14, 16 and 18, further comprising

a buffering circuitry configured to buffer said received data units, a deciding circuitry configured to decide whether to deliver, to a higher protocol layer, buffered data units based on said sequence numbers associated with said buffered data units and on said delivery threshold value, and

a delivering circuitry configured to deliver said buffered data units according to a respective result of said deciding.

20. The apparatus according to claim 19, wherein

said deciding circuitry is configured to prescribe to deliver buffered data units, if said sequence numbers associated with said buffered data units are less than or equal to said delivery threshold value.

21. The apparatus according to any of claims 12 to 20, further comprising said maintaining circuitry is further configured to

apply a respective sequence number associated with a respective data unit received via said first lower-layer protocol entity as said value of said first variable, if no data unit lower in said sequence number is still expected to be received via said first lower-layer protocol entity, and to apply a respective sequence number associated with a respective data unit received via said second lower-layer protocol entity as said value of said second variable, if no data unit lower in said sequence number is still expected to be received via said second lower-layer protocol entity.

22. The apparatus according to any of claims 12 to 21, wherein

the apparatus is operable as or at a base station or access node of a cellular system and/or as or at a terminal, user equipment, mobile station or modem, and/or

the apparatus is operable in at least one of a LTE and a LTE-A cellular system, and/or

said data unit is a Packet Data Convergence Protocol Protocol Data

Unit.

23. A method for controlling a data communication via at least one of a first lower-layer protocol entity and a second lower-layer protocol entity, the method comprising

determining that data communication via any of said first lower-layer protocol entity and said second lower-layer protocol entity is inactive, and in response to said determining, transmitting a signaling indicating that any of said first lower-layer protocol entity and said second lower-layer protocol entity is inactive.

24. The method according to claim 23, wherein

said first lower-layer protocol entity and said second lower-layer protocol entity are mapped to serving-cell groups in a split-bearer scenario, and/or said signaling is a Packet Data Convergence Protocol control Protocol Data Unit.

25. An apparatus for controlling a data communication via at least one of a first lower-layer protocol entity and a second lower-layer protocol entity, the apparatus comprising

a determining circuitry configured to determine that data communication via any of said first lower-layer protocol entity and said second lower-layer protocol entity is inactive, and

a transmitting circuitry configured, in response to said determining circuitry determining an inactivity, to transmit a signaling indicating that any of said first lower-layer protocol entity and said second lower-layer protocol entity is inactive.

26. The apparatus according to claim 25, wherein

said signaling is a Packet Data Convergence Protocol control Protocol Data Unit.

27. A computer program product comprising computer-executable computer program code which, when the program is run on a computer, is configured to cause the computer to carry out the method according to any one of claims 1 to 11, 23 and 24.

28. The computer program product according to claim 27, wherein the computer program product comprises a computer-readable medium on which the computer-executable computer program code is stored, and/or wherein the program is directly loadable into an internal memory of the processor.