GB2375001A

GB2375001A - Re-transmission protocol

Info

Publication number: GB2375001A
Application number: GB0108644A
Authority: GB
Inventors: Walter Featherstone
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2001-04-06
Filing date: 2001-04-06
Publication date: 2002-10-30
Also published as: GB0108644D0

Abstract

Re-transmission of data packets (62) in a communication system (1) operating according to a protocol stack comprising a link layer and a physical layer is described. Copies of data packets are stored at an intermediate physical entity (4) of the physical layer. When negative acknowledgements are received at the transmit-end (8) of the link layer, instead of the rackets being re-transmitted all the way from the transmit-end to the receive-end (2) of the link layer, they are instead re-transmitted (62) using the copies stored at the intermediate physical entity. Application to cellular communication systems such as UMTS, GPRS, EDGE and EGPRS are described. In the case of UMTS, the intermediate physical entity (4) is a Node-B transceiver station (4).

Description

RE-TRANSMISSION PROTOCOL Field of the Invention This invention relates to the transmission and retransmission of data packets in communication systems operating according to a protocol stack. The invention is applicable to, but not limited to, cellular radio communication systems.

Background of the Invention In the field of this invention it is known that the transmission of data, in the form of data packets, across a digital communication network or system consisting of a number of individual links is typically performed using a mechanism known as a Protocol Stack. Protocols are used to organise the transmission of data by means of a hierarchy of protocol layers, the protocol layers being considered collectively as a protocol stack. The hierarchy of layers typically extends from a physical layer (which dictates the manner in which individual bits are transmitted), through to an application layer, which determines, for example, how two high-level computer programs interact with each other.

One example of an intermediate layer is a link layer, which controls the transmission of data across a single

One example of an intermediate layer is a link layer, which controls the transmission of data across a single organisational link in the network. The single organisational link at the link layer will often however correspond to multiple physical links in the physical layer. For example, in a cellular radio communication system according to the Universal Mobile Telephone Standard (UMTS), at the link layer a single organisational link exists between a Radio Network Controller (RNC) and a user station, usually a mobile station (MS) such as a mobile telephone, whereas in the physical layer the physical connection is implemented by a first physical link from the RNC to an intermediate physical entity, namely a Node-B (UMTS terminology for a base transceiver station) and a second physical link from the Node-B to the MS. In the above example, the RNC may be known as the transmit-end (Tx-end) of the link layer, and the MS as the receive-end (Rx-end).

Also, a split may be necessary in the protocol stack between the link layer and the physical layer, with an associated communication link between them (which in UMTS is comprised of the Iub interface plus the Iur interface in some specific circumstances), in order to co-ordinate the transmission of data across multiple physical links (for example transmission of data from a cellular phone which is received in multiple cells in a cellular telecommunications system).

Within the link layer, a mechanism called Automatic Repeat Request (ARQ) provides an error control mechanism for data transmission that allows the Rx-end to periodically advise the Tx-end as to whether data packets have been received without error or not. This allows the Tx-end to retransmit any packets that were transmitted in error in the previous period.

The message sent by the Rx-end is known as an acknowledgement/negative acknowledgement (Ack/Nack). The Ack/Nack message contains the Ack/Nack state of the previous transmitted packet data units (PDU), also termed data packets or blocks, sent to the Rx-end by the Tx-end.

On receiving the Ack/Nack message the Tx-end is able to retransmit those packets that were reported as being received in error (Nacked) by the Rx-end; typically the oldest Nacked PDU is retransmitted first.

However, a major disadvantage with the above described arrangement is that if the physical layer and the link layer are implemented by different physical entities connected by a communication link, the signalling delays across the communication link between the link layer protocol and the physical layer protocol can cause an unacceptably large delay in the re-transmission of negatively acknowledged RLC PDUs to the Rx-end. Such a delay increases the probability that the Quality of Service guarantees, that specify the maximum allowable delay in the transmission of the packet across the link,

the transmission of the packet across the link, may not be met.

Also, in the case of a split in the protocol stack between Link and Physical layers as described above, a major cause of unacceptable delays on the communication channel between the physical and link layers may be the limited bandwidth available on the communication link between the link layer and the physical layer.

Thus there exists a need in the field of the present

I invention to provide a re-transmission protocol wherein the abovementioned disadvantages with prior art arrangements may be alleviated.

Statement of Invention In a first aspect, the present invention provides a method of re-transmitting data packets, as claimed in claim 1.

In a second aspect, the present invention provides a communication system, as claimed in claim 10.

I In a third aspect, the present invention provides an apparatus for use in an intermediate physical entity of physical layer in a communication system, as claimed in claim 19.

In a fourth aspect, the present invention provides a storage medium, as claimed in claim 26.

Further aspects are as claimed in the dependent claims.

According to the invention, copies of data packets are stored or cached at an intermediate physical entity, such that when the original data packets are not properly acknowledged, they may be re-transmitted from the intermediate physical entity of the physical layer, rather than all the way from the transmit-end of the link layer. This can save time and/or communication resource.

Brief Description of the Drawings Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: FIG. 1 is a schematic illustration of transmission and re-transmission of data packets performed in accordance with the prior art in a cellular communication system; FIG. 2 is a schematic illustration of protocol layers of a protocol stack in an embodiment of the present invention;

FIG. 3 is a flowchart showing process steps employed in an embodiment of the present invention; and FIG. 4 is a schematic illustration of transmission and re-transmission of data packets performed according to an embodiment of the present invention in the cellular communication system shown earlier in FIG. 1.

Description of Preferred Embodiments In general, this invention may be applied to any communication system where the protocol stack is split between two or more entities separated by a communications link, such that the link layer is separated from the physical layer via a communications link.

In the first embodiment, the communication system is a cellular communication system according to the Universal Mobile Telephone Standard (UMTS). The parts of such a communication system 1 which are helpful for understanding this embodiment are illustrated schematically in FIG. 1. A mobile station (MS) 2, e. g. a mobile telephone, for use by an end-user, is coupled with a base transceiver station, known in UMTS as a Node-B, 4, via a radio link 6 operating according to the UMTSspecified Uu interface. In terms of this invention, the radio link 6 represents a physical link. The Node-B 4 is

coupled to a Radio Network Controller (RNC) 8 via a physical link (e. g. a landline) 10 operating according the UMTS-specified Iub interface. The RNC 8 is coupled to a core network 12, e. g. the Internet, via a physical link (e. g. a landline) 14 operating according to the UMTSspecified Iu interface.

This embodiment relates to data packets being sent from the RNC 8 to the MS 2, and as such the RNC 8 represents the transmit-end of the link layer under consideration, and the MS 2 represents the receive-end of the link layer. The corresponding physical layer from the RNC 8 to the MS 2 are implemented in the MS2 and the node B4, where the Node B forms an intermediate physical entity of the physical layer between the Ms 2 and the RNC 8. The RNC 8 implementing the transit end link layer is connected to the Node B4 implementing the transmit end of the physical layer by a landline 10. FIG. 1 thus schematically shows original i. e. new data packets 16 being sent from the RNC 8 to the MS 2.

For each data packet, the MS 2 sends back an Ack/Nack message 18 to the RNC, i. e. an acknowledgement (in the case of proper receipt) or a negative acknowledgement (in the case of improper receipt). In prior art arrangements, the ARQ mechanism is operated entirely from the RNC 8, by organisationally using only the link layer, in the radio link control (RLC) layer protocol. Thus, in prior art arrangements, negatively acknowledged data packets 20 are

re-transmitted from the RNC 8 to the MS 2, as shown in FIG. 1.

However, in this embodiment of the invention, a Tx-end Link Layer Cache is additionally collocated with the Physical layer. The Cache stores all link layer PDUs passed to the Physical Layer, for a given limited time period after their reception by the Physical Layer. FIG.

2 shows a protocol stack arrangement 28 (for the system of FIG. 1) adapted to perform this. The RX-end 30 (corresponding to the MS 2) of the protocol stack contains a link layer 32 and a physical layer 34. The Txend 36 (corresponding to the RNC 8) of the protocol stack contains a link layer 38, associated with the link A from the MS 2 to the RNC 8. The transmit end physical layer is implemented in an intermediate physical entity, namely the Node-B 4, and this adds to the protocol stack the additional physical layer part 46, as shown in FIG. 2.

This would be present in a prior art arrangement, however, in this embodiment, as already mentioned, there is additionally a Tx-end link layer cache collocated with the physical layer, and there is correspondingly the additional component of link layer 38.

Also, the link layer and the physical layer are effectively separated by a further communications link 52 (called Link B in FIG. 2), as shown, and being physically

implemented by the Iub interface in this embodiment (and sometimes, in other UMTS arrangements, by the Iub interface and the Iur interface).

The communication over this Link B52 uses a similarly organised protocol stack and has an associated transmit and receive link layer 42,48 and physical layer 40,50 as is known in the art.

Apparatus for implementing the above arrangement, and performing the method steps to be described later below, is provided by adapting conventional apparatus and/or providing additional modules. In particular, additional apparatus may be provided at the intermediate physical entity, i. e. the Node-B 4. The apparatus may be in the form of hardware, firmware, or software, or a combination of these. The apparatus may comprise one or more processors, for implementing instructions and using data stored in a storage medium such as a computer disk or PROM.

The process steps performed in this embodiment are shown in the flowchart of FIG. 3.

Data packets are transmitted from the RNC 8 (the TX-end) to the MS 2 (the Rx-end). Routinely, the MS 2 sends back acknowledgment messages, containing acknowledgements (Acks) for those packets received satisfactorily and negative acknowledgements (Nacks) for those packets not received satisfactorily. At step s2, the RNC 8 receives the acknowledgements and negative acknowledgements.

Additionally, as part of the process of transmitting the packets to the MS 2, copies of the packets are stored (step s4) in the above described cache, which is located at the Node-B 4, i. e. the intermediate physical entity.

As described above, when a packet is transmitted erroneously, a Nack will be passed back to the Link Layer. The Link Layer may now prepare for the erroneous packet to be re-transmitted. When this retransmission is due, rather than re-transmit the entire packet across Link B, the Link Layer sends the Link Layer Cache a trigger message containing a reference or pointer to the packet (for example a PDU ID no), which enables the Link Layer Cache to identify the packet, and pass it to the Physical Layer for re-transmission across Link A. Thus the stored copy of the packet is used, and retransmission of the negatively acknowledged packet is implemented by transmitting a copy from the Node-B to the MS 2 (step s6). Alternatively, a block of packets may be processed in this manner instead of a single data packet.

To summarise, in this embodiment the downlink (DL) Link Layer RLC/MAC PDUs, passed to the Physical Layer for transmission across the air, are cached at the Node-B 4.

The result is that the RLC/MAC PDUs do not need to be retransmitted from the Link Layer to the Physical Layer if the Rx-end negatively acknowledges them. This reduces the round trip delay (RTD) between the Rx-end negatively acknowledging a PDU and receiving the next retransmission of the PDU.

This is shown in FIG. 4, which schematically illustrates the communication system 1 of FIG. 1 adapted to operate according to the present embodiment. Like parts are numbered the same as in FIG. 1. As before, original i. e. new data packets 16 are sent from the RNC 8 to the MS 2. Also as before, for each data packet, the MS 2 sends back an Ack/Nack message 18 to the RNC, i. e. an acknowledgement (in the case of proper receipt) or a negative acknowledgement (in the case of improper receipt). However, in contrast to the FIG. 1 procedure, in this embodiment re-transmission of negatively acknowledged packets is implemented by transmission of a trigger signal 60 from the RNC 8 to the Node-B 4, followed by re-transmission of copies of the negatively acknowledged packets 62 from the Node-B 4 to the MS 2.

In this embodiment the copies of the data packets are stored at the Node-B 4 for a given time, and then discarded. In other embodiments, the RNC 8 informs the Node-B 4 of all acknowledgements and negative acknowledgements it receives, and the Node-B 4 then discards those packets for which acknowledgements have been received (and re-transmits copies of those packets for which negative acknowledgements have been received).

In the above embodiments, the acknowledgement messages are received (i. e. step s2) at the RNC 8, i. e. the transmit-end of the link layer. In an alternative embodiment, the Node-B 4 intercepts the acknowledgement

messages (i. e. the Node-B performs step s2) and decides for itself which packets to re-transmit. In this alternative, the physical layer (at the Node-B) performs the ARQ functionality conventionally performed by the link layer.

It is within the contemplation of the invention that, in addition to UMTS systems, any other wireless communication system using re-transmission. protocols, such as other wideband code division multiplex access (WCDMA) systems, General Packet Radio Service (GPRS), and Enhanced Data Rate for Global Evolution (EDGE) which encompasses Enhanced GPRS (EGPRS), would benefit from employing the inventive concepts described herein. For example, in a GPRS or EGPRS system, the above embodiments may be implemented in the same way as described above, except that, referring for example to FIG. 4, a GPRS base transceiver station (BTS) is used instead of the Node-B 4, a GPRS base station controller (BSC), comprising a packet control unit (PCU), is used instead of the RNC 8, and a Um interface and an Abis interface are used instead of the Uu interface and the Iub interface respectively.

It will be understood that the re-transmission protocol described above may provide the following advantages: (i) reduction of the necessary bandwidth required on Link B by reducing the bandwidth required to handle ARQ, thereby reducing transmission delays across Link B;

(ii) reduction of delays across Link A for the retransmission of erroneous packets; and (iii) improvement of the Quality of Service achievable across Link A.

Claims

Claims 1. A method of re-transmitting data packets in a communication system operating according to a protocol stack comprising a link layer and a physical layer, the method comprising: receiving, for data packets transmitted from a transmit-end of the link layer to a receive-end of the link layer, acknowledgements and negative acknowledgements from the receive-end of the link layer ; storing copies of the data packets at an intermediate physical entity of the physical layer which is intermediate between first and second physical entities of the physical layer corresponding respectively to the transmit-end and the receive-end of the link layer; and using the copies of the data packets stored at the intermediate physical entity to re-transmit, from the intermediate physical entity to the receive-end of the link layer, those data packets for which negative acknowledgements have been received.
2. A method according to claim 1, further comprising, prior to the step of receiving the acknowledgements and negative acknowledgements, transmitting the data packets from the transmit-end of the link layer to the receiveend of the link layer.

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3. A method according to claim 1 or 2, wherein the step of receiving the acknowledgements and negative acknowledgements is performed at the transmit-end of the link layer.
4. A method according to claim 3, wherein in response to receiving a negative acknowledgement, the transmit-end of the link layer transmits a trigger message to the intermediate physical entity for indicating to the intermediate physical entity that the corresponding data packet should be re-transmitted to the receive-end.
5. A method according to claim 1 or 2, wherein the intermediate physical entity receives at least the negative acknowledgements during the step of receiving the acknowledgements and negative acknowledgements by intercepting at least the negative acknowledgements being transmitted from the receive-end to the transmit-end of the link layer.
6. A method according to any preceding claim, wherein the copy of a data packet is stored at the intermediate physical entity for a given time.
7. A method according to any of claims 1 to 5, wherein the copy of a data packet is stored at the intermediate physical entity until the intermediate physical entity is aware that the corresponding acknowledgement or negative acknowledgement has been received.

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8. A method according to any preceding claim, wherein the communication system is a UMTS system, the first physical entity of the physical layer corresponding to the transmit-end of the link layer is an RNC, the second physical entity of the physical layer corresponding to the receive-end of the link layer is a mobile station, and the intermediate physical entity is a Node-B.
9. A method according to any of claims 1 to 7, wherein the communication system is a GPRS/EGPRS system, the first physical entity of the physical layer corresponding to the transmit-end of the link layer is a BSC, the second physical entity of the physical layer corresponding to the receive-end of the link layer is a mobile station, and the intermediate physical entity is a BTS.
10. A communication system operable according to a protocol stack comprising a link layer and a physical layer, the system comprising: means for receiving, for data packets transmitted from a transmit-end of the link layer to a receive- end of the link layer, acknowledgements and negative acknowledgements from the receive-end of the link layer; means for storing copies of the data packets at an intermediate physical entity of the physical layer which is intermediate between first and second

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physical entities of the physical layer corresponding respectively to the transmit-end and the receive-end of the link layer; and means for using the copies of the data packets stored at the intermediate physical entity to re- transmit, from the intermediate physical entity to the receive-end of the link layer, those data packets for which negative acknowledgements have been received.
11. A communication system according to claim 10, further comprising means for transmitting the data packets from the transmit-end of the link layer to the receive-end of the link layer prior to receiving the acknowledgements and negative acknowledgements.
12. A communication system according to claim 10 or 11, wherein the means for receiving the acknowledgements and negative acknowledgements is located at the transmit-end of the link layer.
13. A communication system according to claim 12, arranged such that, in response to receiving a negative acknowledgement, the transmit-end of the link layer transmits a trigger message to the intermediate physical entity for indicating to the intermediate physical entity that the corresponding data packet should be retransmitted to the receive-end.

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14. A communication system according to claim 10 or 11, wherein the intermediate physical entity is adapted to receive at least the negative acknowledgements by intercepting at least the negative acknowledgements being transmitted from the receive-end to the transmit-end of the link layer.
15. A communication system according to any of claims 10 to 14, wherein the means for storing the copies of the data packets is adapted to store the copy of a data packet at the intermediate physical entity for a given time.
16. A communication system according to any of claims 10 to 14, wherein the means for storing the copies of the data packets is adapted to store the copy of a data packet at the intermediate physical entity until the intermediate physical entity is aware that the corresponding acknowledgement or negative acknowledgement has been received.
17. A communication system according to any of claims 10 to 16, wherein the communication system is a UMTS system, the first physical entity of the physical layer corresponding to the transmit-end of the link layer is an RNC, the second physical entity of the physical layer corresponding to the receive-end of the link layer is a mobile station, and the intermediate physical entity is a Node-B.

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18. A communication system according to any of claims 10 to 16, wherein the communication system is a GPRS/EGPRS system, the first physical entity of the physical layer corresponding to the transmit-end of the link layer is a BSC, the second physical entity of the physical layer corresponding to the receive-end of the link layer is a mobile station, and the intermediate physical entity is a BTS.
19. Apparatus for use in an intermediate physical entity of a physical layer of a communication system operable according to a protocol stack comprising a link layer and the physical layer, the apparatus comprising: means for forwarding, to a receive-end of the link layer, data packets being transmitted from a transmit-end of the link layer to the receive-end of the link layer; means for storing copies of the data packets; and means for re-transmitting data packets, using the stored copies of the data packets; to the receive- end of the link layer, in response to a trigger means or message.
20. Apparatus according to claim 19, wherein the trigger means or message is an instruction received from the transmit-end of the link layer.

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21. Apparatus according to claim 19, wherein the trigger means or message is trigger means, comprised by the apparatus, and operable to: intercept acknowledgements and negative acknowledgements being transmitted from the receive- end of the link layer to the transmit-end of the link layer; detect therefrom the negative acknowledgements ; and identify the data packets corresponding to the respective negative acknowledgements.
22. Apparatus according to any of claims 19 to 21, wherein the means for storing copies of the data packets is adapted to store the copy of a data packet for a given time.
23. Apparatus according to any of claims 19 to 21, wherein the means for storing copies of the data packets is adapted to store the copy of a data packet until informed to delete the copy by receipt of a delete message from the transmit-end of the link layer.
24. Apparatus according to any of claims 19 to 23, adapted for use in a Node-B forming the intermediate physical entity in a UMTS communication system.
25. Apparatus according to any of claims 19 to 23, adapted for use in a BTS forming the intermediate physical entity in a GPRS/EGPRS communication system.

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26. A storage medium storing processor-implementable instructions for controlling a processor to carry out the method of any of claims 1 to 9.
27. A method of re-transmitting data packets substantially as hereinbefore described with reference to the accompanying drawings.
28. A communication system substantially as hereinbefore described with reference to the accompanying drawings.
29. Apparatus for use in an intermediate physical entity of a physical layer of a communication system as hereinbefore described with reference to the accompanying drawings.