GB2401508A

GB2401508A - Radio link control layer operation

Info

Publication number: GB2401508A
Application number: GB0310635A
Authority: GB
Inventors: Jonathan Wallington
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2003-05-08
Filing date: 2003-05-08
Publication date: 2004-11-10
Anticipated expiration: 2023-05-08
Also published as: GB0310635D0; CN1551645A; GB2401508B; CN100399837C

Abstract

A method, and apparatus for, operating a radio link control layer in a cellular communications system; comprising: monitoring a load on a radio link control layer link (22) from a first peer entity (5) to a second peer entity (14); if the link is overloaded, reducing the rate at which status reports are sent from the first peer entity (5) to the second peer entity (14); and if the link is underloaded, increasing the rate at which status reports are sent from the first peer entity (5) to the second peer entity (14). A corresponding process may be performed in the reverse direction on the link. The status reporting rate may be changed by changing the rate at which polling requests are sent.

Description

240 1 508 - 1

RADIO LINK CONTROL LAYER OPERATION

Field of the Invention

The present invention relates to cellular communications systems, in particular to controlling the radio link of such systems. The present invention relates in particular, but not exclusively, to Universal Mobile Telecommunications System (UMTS) systems.

Background of the Invention

Cellular communications systems are well known. Examples of established harmonised cellular radio communications systems are Global System for Mobile Telecommunication (GSM) and General Packet Radio Service (GPRS) systems. A further harmonised standard being introduced is the Universal Mobile Telecommunications System (UMTS).

In a cellular communications system, communication service is provided to subscriber units, for example mobile telephones, via radio links from base stations. The radio link between the base station and the subscriber unit can be considered as being formed by two directional parts, i.e. a downlink from the base station to the subscriber, and an uplink from the subscriber unit to the base station. The area covered by a particular base station is known as a cell. A number of base stations are usually controlled by one base station controller. - 2

In UMTS, a base station is known as a Node-B, a base station controller is known as a radio network controller (RNC), and a subscriber unit is known as a user equipment (UE).

UMTS specifies various different protocol layers. One such protocol layer is called the radio link control (RLC) layer. This is used for various control processes related to the radio link. One such process is that the RLC layer is used for transmission and receipt of control messages, comprising protocol data units (PDU's), to and fro between the RNC and the UK. Acknowledged data transmission and error correction are performed. The RNC and the UE are considered as two peer entities.

The receiving entity (e.g. the UK) sends status reports to the transmitting entity (e.g. the RNC). The status reports provide feedback as to which PDU's have been received correctly and which have not been received, or are otherwise in error.

The status reports may be initiated or triggered autonomously by the receiving entity, based for instance on timers which may trigger or suppress status reports. Another possibility is the status reports may be initiated or triggered as responses to poll requests received from the transmitting entity, i.e. typically the receiving entity replies to each poll request with a status report.

The greater the frequency at which the status reports are sent, the better the feedback information. However, conventionally this is at a trade-off cost with - 3 higher load on the links. Conventionally the frequency at which status reports are sent is set, according to traffic type (e.g. voice, data), at a given value in a cellular communications system, or part thereof. Conventionally this is set to provide a compromise between outward link efficiency and return link efficiency.

The conventional compromise inherently does not optimise resource potentials.

Summary of the Invention

The present inventor has realised that it would be advantageous to improve the above described compromise. The present inventor has further realised that this would be particularly advantageous in systems with asymmetrically loaded links, i.e. where the RLC layer can be, or is, loaded differently in the uplink direction compared to the downlink direction.

In a first aspect, the present invention provides a method of operating a radio link control layer in a cellular communications system, as claimed in claim In a further aspect, the present invention provides a storage medium storing processor-implementable instructions, as claimed in claim 11.

In a further aspect, the present invention provides a method of operating a radio link control layer in a cellular communications system, as claimed in claim 12.

The present invention tends to alleviate or resolve the above described problems. Preferably the invention allows underloading or overloading in one or both directions on the RLC layer link to be alleviated by changing the rate at which status reports are transmitted. Preferably, the rate at which status reports are transmitted in one direction on the RLC layer link is l O changed by changing the rate at which polling requests are sent in the other direction. This avoids or reduces the need to send one or more reconfiguration messages, which could be subject to problems such as delay, loss etc.

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 part of a UMTS/GPRS cellular communication system connected with the Internet; FIG. 2 schematically illustrates, in simplified form, an RLC relationship between an RNC and a UK; and - 5 FIG. 3 is a process flowchart showing certain steps carried out in an embodiment of the invention.

Description of a Preferred Embodiment

In this embodiment, the invention is applied to a cellular communications system compliant with, and containing network elements of, UMTS and GPRS.

The cellular communications system is connected to the Internet. However, it is to be appreciated the invention can be applied to other types of cellular system employing polling. It is also to be appreciated that the invention can be applied to communications emanating from sources other than the Internet.

FIG. 1 is a schematic illustration of part of a UMTS/GPRS cellular communication system 1 connected with the Internet 2.

The UMTS/GPRS cellular communication system 1 comprises a Gateway GPRS Support Node (GGSN) 3, which is arranged to provide a gateway connection with the Internet 4.

The system 1 further comprises a Serving GPRS Support Node (SGSN) 4 which is coupled to the GGSN 3. The SGSN 4 performs high level switching, including determining the location of a particular UE by means of accessing location registers (not shown). - 6

Together the GGSN 3 and SGSN 4 represent in effect a GPRS part of the UMTS/GPRS system 1, to which the UMTS parts will be connected as will be described below. However, in other UMTS system embodiments, UMTS parts may be connected to other elements, e.g. connected to a public switched telephone network (PSTN). In that case, the PSTN may be connected to UMTS parts such as those to be described below via a mobile services switching centre (MSC) The system 1 further comprises the following UMTS parts: a radio l O network controller (RNC) 5, and a Node-B 8. The RNC 5 is coupled to, and controls, the Node-B 8. The Node-B 8 has an antenna 10 from which it transmits and receives radio signals to and from UE's. In this example, Node-B 8 is transmitting and receiving radio signals to and from a UE 14 (for example a mobile telephone), which has an antenna 16, thus providing a radio link 20.

RNC 5 comprises a radio link control (RLC) module 7, which operates to implement the RLC protocol layer. UE 14 also comprises an RLC modulel8, which also operates to implement the radio link control layer; i.e. in this example UE 14 and RNC 5 are RLC layer peer entities of each other.

The geographical area covered, i.e. served, by Node-B 8 forms a respective cell of the cellular radio communication system 1. In this example, the Node-B 8 serves a cell 12, i.e. UE 14 is in, and is being served by, cell 12. - 7

In operation, control messages, comprising protocol data units (PDU's), are sent to and fro between the RNC 5 and the UE 14 using the RLC layer. These messages include status reports and poll requests.

S For example, status reports are included in the data sent from the UE 14 to the RNC 5. These status reports may be in response to poll requests included in the data sent from the RNC 5 to the UE 14 (these poll requests represent the RNC asking the UE 14 for a response). Additionally, or alternatively, these status reports may be initiated or triggered autonomously by the UE14.

Likewise, since the RNC 5 and UE 14 are peer entities in respect of the RLC layer, status reports are included in the data sent from the RNC 5 to the UE 14. These status reports may be in response to poll requests included in the data sent from the UE 14 to the RNC 5 (these poll requests represent the UE 14 asking the RNC 5 for a response). Additionally, or alternatively, these status reports may be initiated or triggered autonomously by the RNC5.

The frequency at which these status reports, and where appropriate poll requests, are sent is controlled by the RNC 5.

The system 1 as described above corresponds to a typical conventional arrangement and operates in conventional fashion, except as will now be described below in relation to embodiments of the present invention. - 8

In the embodiments described below, RNC 5 has been adapted, by provision of a rate control module 6, to offer, and provide for, an improved poll request and status report process involving adjustment of the frequency of status reports, as will be described in more detail below.

However, this adaptation may be implemented in any suitable manner to provide suitable apparatus. The module may consist of a single discrete entity added to a conventional RNC, or may alternatively be formed by adapting existing parts of a conventional RNC, for example by reprogramming of one or more processors therein. As such the required adaptation may be implemented in the form of processor-implementable instructions stored on a storage medium, such as a floppy disk, hard disk, PROM, RAM or any combination of these or other storage media. Furthermore, whether a separate entity or an adaptation of existing parts or a combination of these, the module may be implemented in the l 5 form of hardware, firmware, software, or any combination of these.

It is also within the contemplation of the invention that the process to be described below may alternatively be controlled, implemented in full or implemented in part by a module added to or formed by adaptation of any other suitable part of the communication system 1. For example, operation of the RNC (and e.g. UE 14) in this way may be controlled remotely by an adapted form of the SGSN 4. - 9 -

Further, in the case of other network infrastructures or operating procedures, implementation may be at any appropriate switching node such as any other appropriate type of base station, base station controller etc. Another possibility is that the various steps involved in determining and carrying out such adaptation (as will be described in more detail below) can be carried out by various components distributed at different locations or entities within any suitable network or system.

FIG. 2 schematically illustrates, in simplified form, the RLC relationship between RNC 4 and UE 14. In terms of the RLC layer, the link between RNC 4 and UE 14 is effectively direct, i.e. Node-B 8 need not be considered. In FIG. 2, the effective RLC layer link between RNC 4 and UE 14 is represented in its two directionally distinct parts, namely a downlink 22 from RNC 4 to UE 14, and an l S uplink 24 from UE 14 to RNC 4.

The process steps carried out under the present embodiment will now be described with reference to the process flowchart of FIG. 3.

At step s2, the rate control module 6 monitors the load on both downlink 22 and the uplink 24. The respective loads are monitored in terms of any appropriate parameter or parameters representing or indicating a level of use of one or more relevant resources of the communications link employed. The choice is made according to the requirements of the particular system under consideration, and in view of any relevant technical or commercial influences. In - 10 this embodiment, the downlink load is monitored in terms of numbers of users, Orthogonal Variable Spreading Factor (OVSF) codes used (note OVSF codes are used to separate users in a cell in UNITS), and transmission power level; and the uplink is monitored in terms of numbers of users, OVSF codes used, and noise rise.

At step s4, the rate control module 6 determines whether each of the downlink 22 and uplink 24 are overloaded or underloaded. Appropriate predetermined overload and underload thresholds for the parameter or l 0 parameters monitored in step s2 are used. These thresholds are set according to the requirements of the particular system under consideration, and in view of any relevant technical or commercial influences. If the value of the parameter is over the overload threshold value, the relevant link is deemed to be overloaded.

If the value of the parameter is under the underload threshold value, the relevant link is deemed to be underloaded.

When more than one parameter is monitored for a respective link, the thresholds can be considered in combination in any suitable manner. For example, considering the overload threshold for the downlink in this embodiment, the following three parameters are monitored: numbers of users; OVSF codes used; and transmission power level. In this embodiment, each of these parameters is allocated a respective overload threshold. Then, for the downlink to be deemed to be overloaded, all three parameters must be over their respective overload thresholds. However, another possibility would be that for the downlink to be deemed to be overloaded, any two of the three parameters - 11 must be over their respective overload thresholds. Another possibility would be that the parameters may be divided into two or more groups, e.g. a first group and a second group, and then the downlink may be deemed overloaded if all the parameters in at least one of the groups are over their respective overload threshold. For example, in this example, the first group of parameters could be, say, simply a particular one of the parameters, e.g. numbers of users; and the second group could be the other two parameters, i.e. OVSF codes used and transmission power level. It will be appreciated by the skilled person that a wide variety of such arrangements may be used to set a criteria for whether the relevant link is to be determined as overloaded in view of the values of the respective parameters found during the monitoring step.

Another possibility is that more complicated functions or algorithms may be used when combining the effects of plural parameters. Such functions or algorithms may, for example, employ weightings that vary according to the time of day, or system variations, and so on.

The same considerations apply to the underload threshold.

A further detail is that, in this embodiment, the parameters are monitored for a given period of time, e.g. 10 seconds. A parameter is considered to have extended beyond the respective threshold if that parameter extends beyond its threshold at any time during that ten seconds. This is the case, even if, for the purposes of combining the outcome for different parameters as described in the preceding paragraph, the different parameters extend beyond their respective - 12 thresholds at different times during the 10 second period. (Indeed, the different parameters may be monitored at different non-overlapping or only partially overlapping periods of time.) However, in other embodiments, for the link to be deemed overloaded or underloaded, the assessment of one or more of the parameters may require that the parameter or parameters need to extend beyond the respective threshold for a certain amount of time, or a certain number or pattern of repeated confirmation type measurements, before the parameter or parameters is deemed to extend beyond the respective threshold for the purpose of combining the outcome with that of the other parameters to determine that the l O link is overloaded or underloaded.

At step s6, the rate control module 6 sends instructions to the RLC module 7 of the RNC and the RLC module 18 of the UE 14, dependent upon whether either or both of the downlink 22 and uplink 24 have been determined as l 5 overloaded or underloaded in the preceding step.

More particularly, if, say, the downlink 22 has been determined to be overloaded, the rate control module 6 sends an instruction to the UE 14. The instruction is for the UE 14 to reduce the frequency or rate at which it sends polling requests to the RNC 5.

Likewise, if the uplink 24 has been determined to be overloaded, the rate control module 6 sends an instruction to the RNC 5. The instruction is for the RNC 5 to reduce the frequency or rate at which it sends polling requests to the UE 14. - 13

In this embodiment, each instructions is implemented as a "CRLCCONFIG-Req primitive" (as defined in UMTS).

At step s8, the RLC module 7 of the RNC 5 and the RLC module 18 of the UE 14 adjust the frequencies at which they send polling requests, in accordance with any such instructions that have been sent to them in step s6. Thus, in this example, both the RLC module 7 of the RNC 5 and the RLC module 18 of the UE 14 decrease the frequencies at which they send polling requests. The frequency of l 0 polling requests is decreased by increasing the "TIMER_Poll_Periodic value" and/or the "Poll_PDU value" and/or the "Poll_SDU value" and/or the "Timer_Poll value" and/or the "Timer_Poll_Prohibit value" (all as defined in UMTS). In the present example the change in the number or rate of poll requests in themselves (i.e. before any response thereto by the respective receiving entity) l 5 makes no difference to the loads on the downlink 22 and the uplink 24, since the polling requests are sent in the form of changing given poll bits, i.e. they are merely sent with a "not set" binary value 0 instead of sent with a "set" binary value 1.

In response, at step s10, the RLC module 7 of the RNC and the RLC module 18 of the UE 14 send their status reports back at accordingly decreased frequencies. Thus, the change of the polling request rates in step s8 acts as a triggering or initiation step for changing the rate at which the consequential status reports are sent back. This use of the polling request represents a preferred way of initiating or triggering the change in status report rate, as it avoids or - 14 reduces the need to send one or more reconfiguration messages, which could be subject to problems such as delay, loss etc. The description so far describes the situation when, at step s4, the downlink 22 and the uplink 24 are determined to be overloaded. Now, the situation for underloaded will be described.

If, in the above example, at step s4 the downlink 22 is determined to be underloaded, the rate control module 6 sends an instruction to the UE 14. The instruction is for the UE 14 to increase the frequency or rate at which it sends polling requests to the RNC 5.

Likewise, if the uplink 24 has been determined to be underloaded, the rate control module 6 sends an instruction to the RNC 5. The instruction is for the l 5 RNC 5 to increase the frequency or rate at which it sends polling requests to the UE 14.

In these circumstances, at step s8, both the RLC module 7 of the RNC 5 and the RLC module 18 of the UE 14 increase the frequencies at which they send polling requests. The frequency of polling requests is increased by decreasing the "TIMER_Poll_Periodic value" and/or the "Poll_PDU value" and/or the "Poll_SDU value" and/or the "Timer_Poll value" and/or the "Timer_Poll_Prohibit value" (all as defined in UMTS). -

Again, at step s10, the RLC module 7 of the RNC and the RLC module 18 of the UE 14 send their status reports back at accordingly increased frequencies.

If, at step s4, it is determined that one of the downlink 22 and the uplink 24 is underloaded and the other is overloaded, then the appropriate actions are taken for each link at steps s6-s10. Thus, it will be appreciated that the two examples described in the preceding paragraphs are particular cases where there is symmetery between the downlink and uplink in respect of load. Nevertheless, the method is also applied when there is asymmetry. Indeed, this can be a lO particularly advantageous situation for implementation of the method.

Thus, more generally, steps s6 to s10 are implemented, dependent on the outcome of step s4, as any combination of the following: l 5 a) When the downlink 22 is underloaded, this results in an instruction for the UE 14 to increase the frequency or rate at which it sends polling requests to the RNC 5. The consequence is that status reports are sent with increased frequency from the RNC 5 to the UE 14, thus increasing the load on the downlink 22.

b) When the downlink 22 is overloaded, this results in an instruction for the UE 14 to decrease the frequency or rate at which it sends polling requests to the RNC 5. The consequence is that status reports are sent with decreased frequency from the RNC 5 to the UE 14, thus decreasing the load on the downlink 22. - 16

c) When the downlink 22 is found to be neither underloaded nor overloaded, no instruction occurs with respect to the UE 14 changing the frequency or rate at which it sends polling requests to the RNC 5.

d) When the uplink 24 is underloaded, this results in an instruction for the RNC 5 to increase the frequency or rate at which it sends polling requests to the UE 14. The consequence is that status reports are sent with increased frequency from the UE 14 to the RNC 5, thus increasing the load on the uplink 24.

e) When the uplink 24 is overloaded, this results in an instruction for the RNC 5 to decrease the frequency or rate at which it sends polling requests to the UE 14. The consequence is that status reports are sent with decreased frequency from the UE 14 to the RNC 5, thus decreasing the load on the uplink 24.

f) When the uplink 24 is found to be neither underloaded nor overloaded, no instruction occurs with respect to the RNC 5 changing the frequency or rate at which it sends polling requests to the UE 14.

In this embodiment the process ends after step s10. In other embodiments, the process may be repeated by returning to step s2. This may take place at regular time intervals, or according to any other operational requirement. For example, the monitoring step s2 may be performed continuously, with the determination step s4 taking place intermittently, or the monitoring step s2 may also be performed only intermittently. Cycles of the process may alternatively be - 17 initiated by operator input, or by some external event triggering automatic start or repeat of the process. The way in which the process may be repeated may be made dependent upon the level of changes that have taken place in preceding cycles of the process.

In this embodiment, the process is applied to only one entity at each end of the link under consideration. However, in other embodiments, the link being monitored may be such that more than one entity is present at one or both ends thereof. For example, a number of UE's may be considered, and we may l O consider the part of the link being assessed for overload or underload as, say, the coupling from the RNC 5 to the Node-B 8. In this case, if that link is found to be overloaded or underloaded in the uplink direction, then the RNC 5 is instructed to respectively decrease or increase the frequency or rate at which it sends polling requests to one or more of the UE's. Similarly, if in this case that link is l 5 found to be overloaded or underloaded in the downlink direction, then one or more of the UE's are instructed to respectively decrease or increase the frequency or rate at which they send polling requests to the RNC 5.

In this embodiment, the process is applied to both directions of the RLC link, i.e. both the downlink direction and the uplink direction. However, in other embodiments, the process may be applied to just one of these directions.

In this embodiment, the process is such that both the possibility of overload and underload are investigated, with appropriate responses. However, in other embodiments, the process may be implemented such as to only - 18 investigate overload, with appropriate responses, or only to investigate underload, with appropriate responses.

In this embodiment, when it is determined (in response to overload or underload) that the rate at which status reports are to be transmitted by a peer entity is to be changed, the change is initiated or triggered by changing the rate at which polling requests are sent to that entity from the other peer entity.

However, in other embodiments, when it is determined (in response to overload or underload) that the rate at which status reports are to be transmitted is to be lO changed, the change may be initiated or triggered in any other suitable way (including in combination with the above described polling request way). For example, status reporting may be triggered autonomously by the responding peer entity, based for instance on timers which may trigger or suppress status reports, e.g. by changing values such as the Timer_Status_Periodic value and/or l 5 the Timer_Status_Prohibit value. - 19

Claims

1. A method of operating a radio link control layer in a cellular communications system (1); the method comprising: monitoring a load on a radio link control layer link (22) from a first peer entity (5) to a second peer entity (14); and responsive to the monitored load, changing a rate at which status reports are sent from the first peer entity (5) to the second peer entity (14) over the radio link control layer link (22).

2. A method according to claim 1, wherein the step of changing a rate at which status reports are sent from the first peer entity (5) to the second peer entity (14) over the radio link control layer link (22) is performed in response to an initiating step comprising changing a rate at which polling requests are sent l S from the second peer entity (14) to the first peer entity (5) over the radio link control layer link.

3. A method according to claim 1 or 2, wherein when the monitored load is determined to be overloaded, the step of changing the rate at which status reports are sent from the first peer entity (5) to the second peer entity (14) comprises reducing the rate.

4. A method according to claim 1 or 2, wherein when the monitored load is determined to be underloaded, the step of changing the rate at which status - 20 reports are sent from the first peer entity (5) to the second peer entity (14) comprises increasing the rate.

5. A method according to claim 1 or 2, wherein: when the monitored load is determined to be overloaded, the step of changing the rate at which status reports are sent from the first peer entity (5) to the second peer entity (14) comprises reducing the rate; and when the monitored load is determined to be underloaded, the step of changing the rate at which status reports are sent from the first peer entity (5) to the second peer entity (14) comprises increasing the rate.

6. A method according to any of claims 1 to 5, further comprising: monitoring a second load on the radio link control layer link from the second peer entity (14) to the first peer entity (5); and I S responsive to the monitored second load, changing a rate at which status reports are sent from the second peer entity (14) to the first peer entity (5).

7. A method according to claim 6, wherein the step of changing a rate at which status reports are sent from the second peer entity (14) to the first peer entity (5) over the radio link control layer link (22) is performed in response to an initiating step comprising changing a rate at which polling requests are sent from the first peer entity (5) to the second peer entity (14) over the radio link control layer link. - 21

8. A method according to any of claims 1 to 7, wherein the load comprises any one or any combination of the following group: (i) number of users; (ii) OVSF codes; (iii) transmission power level; and (iv) noise rise.

9. A method according to any of claims 1 to 8, wherein the cellular communications system (1) is a UMTS system.

10. A method according to claim 9, wherein the first peer entity (5) is one of a radio network controller and a user equipment, and the second peer entity (14) is the other of the radio network controller and the user equipment.

11. A storage medium storing processor-implementable instructions for controlling a processor to carry out the method of any of claims 1 to 10.

12. Apparatus for operating a radio link control layer in a cellular communications system (1); comprising: means for monitoring a load on a radio link control layer link (22) from a I first peer entity (5) to a second peer entity (14); and means for changing, responsive to the monitored load, a rate at which status reports are sent from the first peer entity (5) to the second peer entity (14) over the radio link control layer link (22). - 22

13. Apparatus according to claim 12, wherein the means for changing a rate at which status reports are sent from the first peer entity (5) to the second peer entity (14) over the radio link control layer link (22) comprises means for performing an initiating step comprising changing a rate at which polling requests are sent from the second peer entity (14) to the first peer entity (5) over the radio link control layer link.

14. Apparatus according to claim 12 or 13, arranged such that when the monitored load is determined to be overloaded, the rate at which status reports are sent from the first peer entity (5) to the second peer entity (14) is reduced.

15. Apparatus according to claim 12 or 13, arranged such that when the monitored load is determined to be underloaded, the rate at which status reports are sent from the first peer entity (5) to the second peer entity (14) is increased.

16. Apparatus according to claim 12 or 13, arranged such that: when the monitored load is determined to be overloaded, the rate at which status reports are sent from the first peer entity (5) to the second peer entity (14) is reduced; and when the monitored load is determined to be underloaded, the rate at which status reports are sent from the first peer entity (5) to the second peer entity (14) is increased.

17. Apparatus according to any of claims 12 to 16, further comprising: 23 means for monitoring a second load on the radio link control layer link from the second peer entity (14) to the first peer entity (5); and means for changing, responsive to the monitored second load, a rate at which status reports are sent from the second peer entity (14) to the first peer entity (5).

18. Apparatus according to claim 17, wherein the means for changing a rate at which status reports are sent from the second peer entity (14) to the first peer entity (5) over the radio link control layer link (22) comprises means for performing an initiating step comprising changing a rate at which polling requests are sent from the first peer entity (5) to the second peer entity (14) over the radio link control layer link.

19. Apparatus according to any of claims 12 to 18, wherein the load comprises any one or any combination of the following group: (i) number of users; (ii) OVSF codes; (iii) transmission power level; and (iv) noise rise.

20. Apparatus according to any of claims 12 to 19, wherein the cellular communications system is a UMTS system. - 24

21. Apparatus according to claim 20, wherein the first peer entity (5) is one of a radio network controller and a user equipment, and the second peer entity (14) is the other of the radio network controller and the user equipment.

22. A cellular communications system base station or base station controller, comprising apparatus according to any of claims 12 to 21.

23. A method of operating a radio link control layer in a cellular communications system substantially as hereinbefore described with reference to the accompanying drawings.

24. Apparatus for operating a radio link control layer in a cellular communications system substantially as hereinbefore described with reference to the accompanying drawings.