WO2002100064A2 - Method and system for processing a data unit - Google Patents

Abstract

A method of processing a data unit of a first protocol layer (L3) for transmission in a data unit based communication system is described, comprising the steps of passing to a second protocol layer (L2) a given data unit of said first protocol layer (L3) that is to be transmitted, said second protocol layer (L2) lying below said first protocol layer (L3), determining a numeric value of a numerically quantifiable parameter associated with said given data unit of said first protocol layer (L3), embedding said given data unit of said first protocol layer (L3) into one or more data units of said second protocol layer (L2), performing transmission control for said one or more data units of said second protocol layer (L2) that embed said given data unit of said first protocol layer (L3), where said embedding and/or said transmission control is performed in accordance with said numeric value of said numerically quantifiable parameter.

Description

Method and system for processing a data unit

[Field of the invention]

The present invention relates to a method of processing a data unit in a data unit based communication system, and to a corresponding data unit based communication system.

[Background of the invention]

A well, known principle for data exchange in networks is that of data unit exchange. This means that data to be sent is broken down into individual units . Rules for sending and receiving such units, as well as rules for the structure of the units themselves, are determined by so-called protocols. Protocols are sets of rules that allow the communication between a sending end and a receiving end, as the rules specify how and in what form data to be sent has to be prepared such that the receiving end may interpret the data and react in accordance to protocol defined rules to which both partners in the communication adhere. The two ends of a communication adhering to a specific protocol are also referred to as peers. _

Such data units are sometimes referred to by different names, depending on the type of protocol involved, such as packets, frames, segments, datagrams, etc. For the purpose of clarity the present description uses the term "data unit" generically for any type of data unit associated with any type of protocol .

An important concept in communications using data unit exchange is that of protocol layering. This means that a number of protocols (sometimes also referred to as a suite) is organised in a hierarchy of layers, where each layer has specific functions and responsibilities. The concept of layering is well known in the art and described in many textbooks, for example "TCP-IP illustrated, volume 1, The Protocols" by W. Richard Stevens, Addison Wesley, 1994, such that a detailed description is not necessary here.

The TCP/IP protocol suite is an example of a layered protocol hierarchy. A basic structure of a protocol hierarchy is defined by the OSI (Open System Interconnection) layer model. At a lowest layer, which is also referred to as the physical layer or LI, the functions of directly transporting data over a physical connection are handled. Above the physical layer, a second layer L2, which is also referred as to the link layer is provided. The link layer L2 fulfils the function of handling the transport of data units over links between communication nodes . Above the link layer L2 a third layer L3 is provided, which is also referred to as the network layer. The network layer handles the routing of data units in a given network. An example of a network layer protocol is the internet protocol (IP) . Above the network layer, a fourth layer L4 is provided, which is also referred to as the transport layer. Examples of a transport layer protocol are the transmission control protocol (TCP) or the user datagram protocol (UDP)' .

In a data unit based communication system using a hierarchy of protocol layers, a communication comprises passing a given data unit downwards through the protocol hierarchy on the sending side, and passing a data unit upwards through the protocol hierarchy on the receiving side. When a data unit is passed downwards, each protocol will typically perform a certain function with respect thereto, e.g. add further information and change or adapt the structure to specific rules of that protocol layer. Typically each protocol layer will add its own header to a data unit received from a higher protocol layer and may also add delimiters . When a specific protocol layer receives a data unit from a higher protocol layer, it will embed the higher layer data unit into a data unit adhering to the rules of the given protocol layer. The term "embedding" shall refer to both encapsulation in which one data unit of a higher layer is placed into one data unit of a given layer, and to segmentation, where one data unit of a higher layer is segmented into a plurality of data units of the given protocol layer.

An important aspect of the layering scheme is that the different layers are "transparent" . This means that the peers in a layer are oblivious to what happens in another layer .

Typically, each protocol layer will perform some type of transmission control for its data units. Such transmission control can e.g. comprise the performing of a certain type of forward error correction, the setting of parameters associated with an automatic repeat request (ARQ) function, the scheduling of data units, or the performing of comparable operations .

It is known to implement protocol layers in such a way that they can be operated in-a specific mode with respect to thei transmission control. As an example, a so-called numbered mode (or I-mode) and a so-called unnumbered mode (Ul-mode) are known. In the numbered mode, if it is determined that a sent data unit was not correctly received by the receiving peer, then the sending peer performs retransmission of said data unit. In this way it can be assured that all packets are correctly transmitted, although this may increase the delay, depending on how many packets have to be transmitted. On the other hand, in the unnumbered mode, no retransmissions are provided. This has the advantage of less delay, but the transmission reliability depends on the quality of the physical connection. The possibility of setting a given protocol layer implementation into a specific transmission control mode has the advantage that the selection of the mode can e.g. be performed by a control procedure from a higher protocol layer, in order to optimise the sending of data units from said higher protocol layer. However, it does not provide very much flexibility, as a given protocol layer will typically handle a variety of different types of data units, that require different control settings with respect to the optimisation of the sending of the given type of data unit. As an example, if an application layer is sending a computer file, it desires to ensure a reliable transmission, and may therefore want to set a lower layer protocol implementation into the numbered mode, or the application layer may want to send data that requires realtime transmission, such as a video stream belonging to a video telephone, in which case transmission speed is more important than reliability, such that the application layer may want to set a lower layer protocol implementation into the unnumbered mode. However, if both computer file data and video data are being sent, then the setting of the lower layer protocol implementation into a given transmission control mode will not provide an optimum solution.

EP-0 973 302 Al addresses this problem and proposes a system-, in which a given protocol layer that receives higher data unit layers and embeds these higher data layers into data units of said given layer, is arranged to discriminate the higher layer data unit layers by reading the header information and determining the type of the higher layer data unit. Then, a classification is performed in accordance with the identified type. In this way, the given protocol layer can flexibly set the transmission reliability of its data units depending on the type of the higher layer data unit that is embedded in its own data units. As an example, if the system of EP- 0 973 02 Al is applied to a link layer in the TCP/IP suite, then the link layer can identify if the network layer IP data unit that it receives carries a TCP data unit, in which case the link layer data units are sent in the numbered mode, or if the network layer IP data unit carries UDP data unit, in which case the link layer data units are sent in the unnumbered mode.

However, the system of EP-0 973 302 Al is not always practical, as it requires the parsing of higher layer data units in order to identify type information, which e.g. does not work when the higher layer data unit has an encrypted header and/or payload.

[Object of the present invention]

It is desirable to provide an improved method and system of processing data units of a higher protocol layer at a given protocol layer, which is simple to implement, but flexibly provides improved transmission properties.

[Summary of the invention]

This object is achieved by the subject-matter of the independent claims. Advantageous embodiments are described in the dependent claims .

In accordance with an embodiment of the present invention, at a given protocol layer, e.g. a link layer L2, one or more numeric values of one or more numerically quantifiable parameters associated with a received given data unit of a higher protocol layer, e.g. the network layer L3 , are determined. In other words, one value of one numerically quantifiable parameter can be determined, or several values of one numerically quantifiable parameter, or one or more respective values for each of a plurality of numerically quantifiable parameters . The at least one numeric value is not derived from information contained in the given data unit of the higher protocol layer. In other words, instead of analysing the content of the higher layer data unit, one or more simple physical properties that are numerically evaluatable are measured, and the embedding and/or transmission control is performed in accordance with the determined value. Consequently, no parsing of higher layer data units or other similar complicated processing is necessary.

As an example, a numerically quantifiable parameter can be the size of the higher layer data unit. In other words, at a given protocol layer e.g. L2, a higher layer data unit, e.g. from the L3 layer, is received, and the size of said L3 data unit is measured. Then the embedding operation for embedding said L3 data unit into one or more L2 data units, or the transmission control operation for transmitting the one or more L2 data units into which said L3 data unit has been embedded, is performed in accordance with the result of said size measurement.

Preferably, the size measurement is used as a basis for adjusting the transmission control to optimise _ predetermined target properties. More specifically; the L2 data units are transmitted with parameters set for optimising throughput if the L3 data unit falls into a predetermined size range, and otherwise the L2 data units are transmitted with optimised delay. Namely, if the L3 data unit is found to have a size indicative of a maximum size e.g. falls into a range around or is equal to the TCP maximum segment size if the L3 data units carry TCP data units, then the transmission of the L2 data units, into which said L3 data unit has been embedded, is optimised for throughput, as it may be assumed that the maximum size L3 data unit belongs to a larger amount of data being sent from above the L3 layer. On the other hand, if the L3 data unit is smaller in size, then the transmission control is ω > to t μ> H in o σi o U1 o LΠ

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Fig. 1 shows a flow chart for explaining an embodiment of the present invention;

Fig. 2 shows a flow chart for explaining another embodiment of the present invention, in which the numeric value is determined based on data unit size;

Fig. 3 shows a flow chart for explaining another embodiment of the present invention, in which higher layer data units are additionally discriminated;

Fig. 4 shows a flow chart for explaining another embodiment of the present invention, in which the numeric value is determined based on the discrimination result;

Fig. 5 shows an overview of a protocol structure to which the present invention can be applied;

Fig. 6 schematically illustrates the concept of flow- splitting in the context of the example shown in Fig. 5;

Figs. 7a-7c show routines of an embodiment of the invention in which data units are buffered and a data unit dropping procedure is provided;

Fig. 8 shows an example of a data unit dropping procedure;

Fig. 9 shows a further example of a data unit dropping procedure; and Fig. 10 shows a schematic block diagram of a system that is arranged to perform the routines and procedures of Figs . 7 to 9.

[Detailed description of embodiments]

In the following, detailed embodiments of the present invention shall be described in order to give the skilled person a full and complete understanding. However, these embodiments are illustrative and not intended to be limiting, as the scope of the invention is defined by the appended claims .

The following examples shall be described in the context of TCP/IP. However, it may be noted that the present invention is applicable to any protocol hierarchy in which higher layer data units are embedded into lower layer data units, and where a transmission control is performed for the lower layer data units.

The following embodiments shall be described in the context of applying the invention to a link layer L2 , said link layer embedding L3 data units from the network layer . However, this is only an example, and the present invention' can be applied at any level of a protocol hierarchy i.e. also below or above the link layer.

Fig. 1 shows a flow chart describing a first embodiment of the present invention. As can be seen, in a first step SI a L3 data unit is passed to the link layer L2 below the layer L3. Then, in step S2, a numeric value of a numerically quantifiable parameter associated with the L3 data unit is determined. It may be noted that the term "associated with the L3 data unit" is to be understood broadly as relating to any numerically quantifiable parameter derivable from the L3 data unit, where said parameter can relate to the L3 data unit as such, or to a data unit from a higher layer than L3 embedded in the L3 data unit. Then, in step S3, the received L3 data unit is embedded into one or more L2 data units. Finally, in step S4, transmission control for the one or more L2 data units and/or LI data units that embed the received L3 data unit is performed according to the numeric value determined in step S2.

Although the example of Fig. 1 shows steps SI to S4 in a specific sequence, this is only an example and other arrangements of steps SI to S4 are possible.

The adjustments performed in step S4 will depend on the specific circumstances and the system under consideration. For example, the transmission control may comprise adjusting a forward error correction for the data units of the L2 protocol layer or for the LI data units below the L2 protocol layer. The type of forward error correction can be chosen as is desirable or appropriate. For example, the transmission control may comprise adjusting a transmission power and/or a data rate (e.g. by selecting a spreading factor in a CDMA system) over a given link and/or a degree of interleaving. The mentioned link can e.g. be a wireless link.

If the L2 protocol layer comprises a function of providing automatic retransmission of L2 data units under predetermined conditions, then the transmission control may comprise adjusting said retransmission function.

If the L2 protocol layer comprises a function for the scheduling of the L2 data units, then the transmission control may comprise adjusting said scheduling.

In the event that the embedding in step S3 is a segmentation operation, then the embodiment of Fig. 1 can be arranged such that the segmentation operation is performed according to said numeric value. In other words, LO LO to to μ> i >

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Namely, L3 data units for which the throughput should be optimised in general belong to a larger application data amount, which is segmented into transport layer data units, e.g. TCP segments. Therefore, these transport layer data units, which are then embedded in network layer (L3) data units generally have a maximum transfer unit size, e.g. the TCP maximum segments size (MSS) .

Consequently, as already mentioned, step S41 of Fig. 2 can be implemented in such a way that the comparison is conducted with respect to a reference value or reference range .indicative of a predetermined maximum transfer unit size. Typically, the value of the maximum transfer unit size is 256 byte, 512 byte, 536 byte or 1460 byte, such that taking into account the IP header of 40 byte, suitable reference size values used in step S41 could be 296 byte, 552 byte, 576 byte or 1500 byte.

As already mentioned, step S41 can be implemented in such a way that the measured size of the L3 data unit is compared with a plurality of reference sizes, e.g. the previously mentioned series of discrete values 296, 552, 576 and 1500 bytes, or with a suitable continuous range, or with a combination of discrete values and continuous values. Then, if the measured size of the L3 unit is equal to any one of the plurality of reference sizes, or falls into the predetermined range or ranges, then step S42 is enabled, to thereby optimise the transmission control for throughput.

In this connection, it can be mentioned that the reference size values may not only take into account the IP header, but also the possibility of header compression. In other words, the plurality of size reference values used in step S41 also contains a set of values that take into account header compression. LO LO to to μ> »

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where it can be buffered until further processing of the SDU takes place.

It may be noted that step S2 does not necessarily have to be performed prior to the buffering, and could also be performed subsequently, as will be explained in connection with the embodiment of Fig. 9.

The buffering of received SDUs is especially advantageous in situations where the immediate processing at the receiving layer is not guaranteed or feasible, e.g. if the receiving layer is a link layer for sending data over a wireless link.

Fig. 7b shows a routine for processing buffered SDUs, namely for embedding the SDUs into L2 PDUs. In step S31 an SDU is taken out of the SDU buffer and embedded into one or more L2 PDUs . In step S7 the one or more resulting L2 PDUs are buffered in a PDU buffer. The buffered L2 PDUs can then be transmitted according to anyone of the transmission control procedures S4, S40, S41 to S43 explained in connection with Figures 1 to 4. It may be noted that the processing routine of Fig. 7b could also be embodied by directly passing the L2¹ PDUs from step S31 to a transmission control procedure S4, S40, S41-S43.

Fig. 7c shows a basic routine for managing the contents of one or both of the SDU buffer and PDU buffer. More specifically, in a step S8 it is determined whether a triggering condition for performing dropping of buffered data units is met or not, and if it is, a data unit dropping procedure S9 is conducted. The triggering conditions can be chosen as is suitable or desirable, e.g. a data unit dropping procedure can be called for if the buffer is an overflow state (e.g. the amount of data in the buffer exceeds an overflow limit) , and/or if tne link over which the L2 PDUs are to be transmitted is in an overload LO LO to t μ¹ μ>

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dropping at the highest sub-layer, i.e. at the SDU buffer, in order to avoid unnecessary embedding operations for data that is possibly dropped.

Figure 8 shows an example of a data unit dropping procedure S9. In a first step S91, a first data unit is considered. The selection of the data unit to be considered can be done as is suitable or desirable, and can depend on the way the data units are buffered. For example, if the data units are queued, then a data unit at a predetermined position in the queue can be selected (such as the first or last data unit) ,, or any other type of selection routine can be chosen, such as the random selection of a data unit.

Then, in step S92, the above mentioned decision step is performed, i.e. it is determined whether the numeric value (e.g. data unit size or inter-arrival time) of the L3 data unit or SDU associated with the data unit under consideration (either the SDU itself or an L2 PDU embedding the SDU) fulfils a predetermined condition or not. If the predetermined condition is fulfilled, the procedure branches to step S96 and drops the data unit under consideration. If not, the procedure branches to step S93, where' it is determined whether there 'are further data units present in the buffer, beyond those already considered. If yes, then the procedure branches to step S94, in which a next data unit is selected, and then the procedure loops back to step S92. The selection of a next data unit to be considered can be performed in any desirable or suitable way and will generally be linked to the method used in step

S91. For example, S94 can consist in simply selecting the following or preceding data unit in the queue, or in performing another random selection that only excludes previously selected data units.

If step S93 shows that no more data units are left that have not yet been considered for step S92 (which indicates LO LO to to H μ>

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different source, where the congestion indication will still have an effect.

Dropping ACK messages does not improve the traffic situation as TCP ACKs are cumulative.

It may be added that it is also preferable to drop larger data units in place of smaller ones because the dropping of small data units will generally not improve the buffering delay at the link. In other words, more is done to alleviate the condition that triggered the data unit dropping (e.g. link overload or buffer overflow) if large data units are dropped than if small ones are dropped.

From the above discussion, it can be seen that the setting of the threshold Th is preferably done in dependence on the possible signalling messages sent at higher layers. As an example, if the above embodiment is applied to a layer 2 on top of which TCP is run, then the threshold Th should be set such SYN, ACK and FIN messages are not dropped, i.e. the threshold Th should be set larger or equal to the expected size of the SYN, ACK and FIN messages .

It may be added that the step of discriminating a group_! into an L3 data unit belongs (e.g. whether it belongs to a specific flow) , described above in connection with step S5 in Figures 3 and 4, can also be used in connection with data unit dropping procedure. Namely, in addition to making the data unit dropping procedure dependent on the numeric value, the result of such a discrimination step can also be taken into account .

For example, if the discrimination step S5 consists in determining which flow an L3 data unit belongs to, then the decision step S92 or S920 can be amended to also take the discriminated flow into account, e.g. such that no data units from a specific flow or group of flows are dropped, LO LO to to H H

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Claims

Claims

1. A method of processing a data unit of a first protocol layer (L3) for transmission in a data unit based communication system, comprising the steps of:

passing to a second protocol layer (L2) a given data unit of said first protocol layer (L3) that is to be transmitted, said second protocol layer (L2) lying below said first protocol layer (L3) ;

determining one or more numeric values, said one or more numeric values belonging to at least one numerically quantifiable parameter associated with said given data unit of said first protocol layer (L3);

embedding said given data unit of said first protocol layer (L3) into one or more data units of said second protocol layer (L2) ,

performing transmission control for said one or more data units of said second protocol layer (L2) that embed said given data unit of said first protocol layer (L3) ,

where said embedding and/or said transmission control is performed in accordance with said one or more numeric values of said at least one numerically quantifiable parameter.

2. The method of claim 1, wherein said at least one numerically quantifiable parameter is the size of said given data unit of said first protocol layer (L3) .

The method of claim 1 or 2, wherein said at least one numerically quantifiable parameter is associated with a buffer fill level of a buffer holding data units of said first protocol layer (L3) or said second protocol layer (L2) .

4. The method of one of claims 1 to 3 , wherein said at least one numerically quantifiable parameter is an inter-arrival time of data units of said first protocol layer (L3) .

5. The method of one of claims 1 to 4, furthermore comprising a step of discriminating a group of data units of said first protocol layer (L3) to which said given data unit of said first protocol layer (L3) belongs .

6. The method of claim 5, wherein said group is discriminated on the basis of source information and/or destination information and/or a protocol identifier contained in said data units of said first protocol layer (L3) .

7. The method of claim 5 or 6, wherein said transmission control for said one or more data units of said second protocol layer (L2) that embedi said given data unit of said first protocol layer (L3) is also performed in accordance with a result of said discriminating step.

8. The method of claim 5 or 6, wherein said numeric value is determined on the basis of a result of said discriminating step.

9. The method of claim 8 , wherein said numerically i quantifiable parameter is associated with a buffer fill level of a buffer holding data units of said second protocol layer (L2) , said numerically quantifiable parameter being the number of data units of said second protocol layer (L2) in said buffer that embed data units of said first protocol layer (L3) belonging to said group.

10. The method of claim 8, wherein said numerically quantifiable parameter is the inter-arrival time of data units of said first protocol layer (L3) belonging to said group.

11. The method of one of claims 1 to 10, wherein said transmission control comprises adjusting a forward error correction for said data units of said second protocol layer (L2) or for data units of a third protocol layer (LI) below said second protocol layer (L2) .

12. The method of claim 11, wherein a function (RLC) controls the sending of said data units of said second protocol layer (L2) over a link, and said transmission control comprises adjusting a transmission power and/or a data rate over said wireless link and/or a degree of interleaving.

13. The method of one of claims 1 to 12, wherein said second protocol layer (L2) comprises a function of providing automatic retransmission of data units of said second protocol layer (L2) under predetermined conditions, and where said transmission control comprises adjusting said retransmission function.

14. The method of one of claims 1 to 13, wherein said second protocol layer (L2) comprises a function (MAC) for scheduling of said data units of said second protocol layer (L2) , and where said transmission control comprises adjusting said scheduling.

15. The method of one of claims 1 to 14, wherein said embedding comprises a segmentation operation for data units of said first protocol layer (L3) , and where said transmission control comprises adjusting said segmentation operation.

16. The method of claim 15, wherein the adjusting of said segmentation operation comprises adjusting the size of the data units of said second protocol layer (L2) into which said given data unit of said first protocol layer (L3) is segmented.

17. The method of one of claims 1 to 16, wherein said transmission control comprises discriminating said one or more data units of said second protocol layer (L2) in which said given data unit of said first protocol layer (L3) is embedded on the basis of said numeric value, and placing each of said one or more data units of said second protocol layer (L2) into one of a plurality of predetermined transmission categories on the basis of said discrimination result.

18. A computer program arranged to execute the method of one of claims 1 to 17.

I¹9. A computer program storage medium storing' a¹ computer program according to claim 18.

20. A data unit based communication system comprising implementations of a first protocol layer (L3) and a second protocol layer (L2) , said first protocol layer (L3) lying above said second protocol layer (L2) , said system being arranged to pass a given data unit of said first protocol layer (L3) that is to be transmitted to said second protocol layer (L2) , and said implementation of said second protocol layer (L2) being arranged to determine one or more numeric values belonging to at least one numerically quantifiable parameter associated with said given data unit of said first protocol layer (L3) , embed said given data unit of said first protocol layer (L3) into one or more data units of said second protocol layer (L2) , and perform transmission control for said one or more data units of said second protocol layer (L2) that embed said given data unit of said first protocol layer (L3) in accordance with said one or more numeric values of said at least one numerically quantifiable parameter.

21. The data unit based communication system of claim 20, wherein said at least one numerically quantifiable parameter is the size of said given data unit of said first protocol layer (L3) .

22. The data unit based communication system of claim 20 or 21, wherein said at least one numerically quantifiable parameter is associated with a buffer fill level of a buffer holding data units of said first protocol layer (L3) or said second protocol layer (L2) .

23. The data unit based communication system of one of claims 20 to 22, wherein said at least one numerically quantifiable parameter is an inter-arrival time of data units of said first protocol layer (L3) .

24. The data unit based communication system of one of claims 20 to 23, wherein said implementation of said second protocol layer (L2) is arranged to discriminate a group of data units of said first protocol layer

(L3) to which said given data unit of said first protocol layer (L3) belongs.

25. The data unit based communication system -of claim 24, wherein said implementation of said second protocol layer (L2) is arranged to da scriminate said group on the basis of source information and/or destination information and/or a protocol identifier contained in said data units of said first protocol layer (L3) .

26. The data unit based communication system of claim 24 or 25, wherein said implementation of said second protocol layer (L2) is arranged to also perform said transmission control for said one or more data units of said second protocol layer (L2) that embed said given data unit of said first protocol layer (L3) in accordance with a result of said discriminating.

27. The data unit based communication system of claim 24 or 25, wherein said implementation of said second protocol layer (L2) is arranged to determine said numeric value on the basis of a result of said discriminating step.

28. The data unit based communication system of claim 27, comprising a buffer for holding data units of said second protocol layer (L2) , said numerically quantifiable parameter being the number of data units of said second protocol layer (L2) in said buffer that embed data units of said first protocol layer (L3) belonging to said 'group . ^~~

29. The data unit based communication system of claim 27, comprising a timer for measuring an inter-arrival time of data units of said first protocol layer (L3) belonging to said group, where said numerically quantifiable parameter is the inter-arrival time of data units of said first protocol layer (L3) belonging to said group .

30. The data unit based communication system of one of claims 20 to 29, wherein said transmission control comprises adjusting a forward error correction for said data units of said second protocol layer (L2) or for data units of a third protocol layer (LI) below • said second protocol layer (L2) .

31. The data unit based communication system of claim 30, further comprising a function (RLC) for controlling the sending of said data units of said second protocol layer (L2) over a link, and said transmission control comprises adjusting a transmission power and/or a data rate over said link and/or a degree of interleaving.

32. The data unit based communication system of one of claims 20 to 31, wherein said implementation of said second protocol layer (L2) comprises a function of providing automatic retransmission of data units of said second protocol layer (L2) under predetermined conditions, and where said transmission control comprises adjusting said retransmission function.

33. The data unit based communication system of one of claims 20 to 32, wherein said implementation of said second protocol layer (L2) comprises a function (MAC) for scheduling of said data units of said second protocol layer (L2) , and where said transmission control comprises adjusting said scheduling.

34. The data unit based communication system of one of claims 20 to 33, wherein said implementation of said second protocol layer (L2) is arranged to perform a segmentation operation for data units of said first protocol layer (L3) , and where said transmission control comprises adjusting said segmentation operation.

35. The data unit based communication system of claim 34, wherein the adjusting of said segmentation operation comprises adjusting the size of, the data units of said second protocol layer (L2) into which said given data unit of said first protocol layer (L3) is segmented.

36. The data unit based communication system of one of claims 20 to 35, wherein said transmission control comprises discriminating said one or more data units of said second protocol layer (L2) in which said given data unit of said first protocol layer (L3) is embedded on the basis of said numeric value, and placing each of said one or more data units of said second protocol layer (L2) into one of a plurality of predetermined transmission categories on the basis of said discrimination result.

37. The data unit based communication system of one of claims 20 to 35, wherein said system is contained in a mobile communication device.

38. The data unit based communication system of claim 37, wherein said mobile communication device operates in accordance the standard of the Universal Mobile Telecommunication System.

39i A method¹ of processing a¹ data unit of a first protocol layer (L3) for transmission in a data unit based communication system, comprising the steps of :

passing to a second protocol layer (L2) a given data unit of said first protocol layer (L3) that is to be transmitted, said second protocol layer (L2) lying below said first protocol layer (L3) ;

performing a buffering operation (S6, S7) for one or both of - said data unit of said first protocol layer

(L3) and LO LO to to μ> Ή

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42. The method of claim 41, wherein said decision step (S92) comprises comparing (S920) the size of said given data unit of said first protocol layer (L3) with a predetermined threshold (Th) , and performing the congestion alleviation measure with respect to the data unit under consideration if the threshold (Th) is exceeded.

43. The method of one of claims 39 to 42, wherein said at least one numerically quantifiable parameter is an inter-arrival time of data units of said first protocol layer (L3) .

44. The method of one of claims 39 to 43, furthermore comprising a step of discriminating a group of data units of said first protocol layer (L3) to which said given data unit of said first protocol layer (L3) belongs .

45. The method of claim 44, wherein said group is discriminated on the basis of source information and/or destination information and/or a protocol identifier contained in said data units of said first protocol layer (L3)¹. ^~

46. The method of claim 44 or 45, wherein said decision step of said congestion alleviation procedure also depends on a result of said discriminating step.

47. The method of claim 46, wherein said group is discriminated on the basis of a protocol identifier in said given data unit of said first protocol layer (L3) , and said decision step comprises determining whether the congestion alleviation measure should consist in dropping the data unit under consideration or marking the given data unit of the first protocol layer (L3) , where the data unit under consideration is dropped if the protocol identifier belongs to a predetermined category.

48. A data unit based communication system comprising implementations of a first protocol layer (L3) and a second protocol layer (L2) , said first protocol layer (L3) lying above said second protocol layer (L2) , said system being arranged to pass a given data unit of said first protocol layer (L3) that is to be transmitted to said second protocol layer (L2) , and said implementation of said second protocol layer (L2) cpmprising a buffer (101, 104) for buffering one or both of

- said data unit of said first protocol layer (L3) and

- one or more data units of said second protocol layer into which said data unit of said second protocol layer (L2) is embedded,

a controller (102) connected to said buffer (101, 104) for performing a congestion alleviation procedure (S9) for buffered data units if one or more predetermined triggering conditions are fulfilled, said congestion alleviation procedure (S9) comprising a decision step (S92) for deciding whether a congestion alleviation measure is to be performed with respect to a buffered data unit under consideration or not, where said decision step depends on one or more numeric values that belong to at least one numerically quantifiable parameter associated with a given data unit of said first protocol layer (L3) , said given data unit of said first protocol layer (L3) being the data unit under consideration if the data unit under consideration is a data unit of the first protocol layer (L3) , and said given data unit of said first protocol layer (L3) being a data unit of said first protocol layer (L3) embedded in said data unit under^" consideration if the data unit under consideration is a data unit of said second protocol layer (L2) .