WO2002015634A1

WO2002015634A1 - Method and system for managing traffic classes with different priorities in p-mp atm communication systems

Info

Publication number: WO2002015634A1
Application number: PCT/IT2000/000339
Authority: WO
Inventors: Alberto Profumo; Lucio D'ascoli
Original assignee: Italtel SpA
Current assignee: Italtel SpA
Priority date: 2000-08-11
Filing date: 2000-08-11
Publication date: 2002-02-21
Anticipated expiration: 2003-02-11

Abstract

A method of transferring ATM data packets carrying data belonging to traffic classes (Qos_prl, ..., Q-S_prN) with different priorities between a first transmitter device (2, TX_1, ..., TX_n) and a second device (3). The method provides for: the provision, in the first device, of a plurality of transmitter-device physical interfaces (TX_1, ..., TX_n) each comprising at least one respective queue (Q_pr1, ..., Q_prN) for storing data packets, the establishment of a one-to-one association (2) between each of the transmitter-device physical interfaces and a respective traffic class so that data packets belonging to the same traffic class are stored in the queue of the same device physical interface, the interfacing of the first device with the second device by means of a standard multi-physical interface (M_PHY), and the use of lines (S_TX_1, ..., S_TX_n) of the standard multi-physical interface for transferring information relating to the states of fullness of the queues of each transmitter device physical interface from the first device to the second device.

Description

"Method and system for managing traffic classes with different priorities in P-MP ATM communication systems"

DESCRIPTION

The invention relates in general to the communications field and in particular to ATM

("Asynchronous Transfer Mode") communication systems.

More specifically, the invention relates to point-to- multipoint (P-MP) ATM communication systems of the type which can manage the transfer of traffic classes with different priority levels.

As is known, in ATM communication systems, the information, whatever its type, is transferred in packets or cells. Each cell is a block of predetermined length constituted by a first portion known as the "header" and by a second, useful portion known as the "payload" . The header contains all of the information necessary to transfer the cell on the communication network, including the VPI and VCI identifiers; the payload, on the other hand, represents the actual information content of the cell.

ATM communication systems can manage the transfer of information belonging to different traffic classes (data, telephony, etc.) each of which may have, and usually has, a different priority level relative to the others. For example, voice transmission must clearly have a higher priority level than data transmission, to prevent the introduction of intolerable delays in voice communication between two individuals.

Amongst the possible access techniques in a telecommunications network, the point-to-multipoint technique is having considerable success by virtue of the advantages which it has in terms of flexibility and cost in comparison with the simpler point-to-point technique.

A point-to-multipoint communication system comprises a central unit or "master station" and a plurality of peripheral units or stations connected to the central unit by a communication means which is shared in accordance with a so-called "Time Division Multiple Access" (or "TDMA") method. The central unit regulates access to the shared communication means by the peripheral units by means of a control protocol; this control protocol defines the sequence in which the various peripheral units can access the shared communication means.

Each peripheral unit serves a predetermined number of connections. The data traffic on the connections generally belongs to several classes, each of which, as mentioned, is distinguished by a particular priority level .

Each peripheral unit comprises a respective plurality of queues, each queue being associated with a particular traffic class with its own associated priority level. The packets or cells relating to the corresponding traffic class are stored and held in each queue until the central unit arranges, by means of the control protocol, for the shared communication means to be assigned to the peripheral unit, thus permitting the transmission of one or more packets, according to the priority level of the traffic class to which the packet or packets belong.

To permit a correct sequence of access to the shared communication means by the peripheral units, the control protocol of the shared communication means has to know the current state of the queues of various priority levels of all of the peripheral units of the system. It is in fact necessary to prevent packets relating to a traffic class with a higher priority being kept waiting in the respective queue of a peripheral unit because the communication means is assigned at the time in question to another peripheral unit which is transmitting from the respective queue packets relating to a traffic class with lower priority. For this purpose, in addition to the data packets, each peripheral unit should also transmit to the central unit information relating to the state of its queues (for example, "queue empty", "queue not empty" for each of the queues of the peripheral unit) . In general, in each peripheral unit, it is possible to identify physically distinct devices for storing the packets in the queues, for the transmission of the packets, and for the storage and transmission of the information for the control protocol. This problem does not arise in point-to-point communication systems in which, instead of a plurality of peripheral units, there is only one. In this case, the peripheral unit is able to autonomously implement the correct management of the traffic classes with different priorities.

The "ATM Forum Technical Committee - UTOPIA Specification - Level 1, Version 2.01 - af-phy-0017.000 " of 21st March 1994 describes an interface, known as "UTOPIA Level 1", between the physical level (PHY level) and the ATM level, for connecting a single physical-level device to a single ATM-level device. The UTOPIA Level 1 interface can be used for the transfer of packets belonging to traffic classes with different priority levels in a manner consistent with their priority level. In general, however, it is impossible to determine, by means of a single standard UTOPIA Level 1 interface, whether a particular queue is empty or not .

To prevent this problem, it is possible to use the bytes which are reserved for the user in the standard which defines the UTOPIA Level 1 interface. In this case, however, it is necessary for both the transmitter and the receiver to agree the meaning attributed to the bytes reserved for the user and to be able to interpret them correctly. Naturally, this is limiting, particularly with regard to compatibility with existing systems or with components produced by different producers .

Alternatively, it would be possible to consider using a plurality of UTOPIA 1 interfaces, each associated in a one-to-one relationship with a priority level. In this case, it would be possible to determine whether a particular queue were empty or not, but this arrangement would be too complex since it would involve the use of too many signal lines between the data-transfer devices. Another possible technique would provide for the states of the queues in the devices for storing the packets of a given peripheral unit to be communicated, by means of a suitable software module, to a register of the transmitter device resident in that given peripheral unit. In that case, the transmitter device would send the information relating to the states of the queues of the respective peripheral unit to the central unit by means of the shared communication means in order for the control protocol of the communication means to be able to define the correct sequence of access to the shared communication means by the peripheral units. This technique is disadvantageous, since it would require a microcontroller in each peripheral unit for implementing the software module in order to communicate the states of the queues to the transmission device. As well as being complex because of the use of a software module, the technique mentioned would not be able to ensure the necessary speed of transfer of the information relating to the states of the queues of packets. In principle, a technique in which the states of the queues are read not by software but by hardware, with the provision of suitable hardware modules for performing the reading, is also possible. In this case, however, it would be necessary to create an interface physically distinct from and additional to the interface used for the transfer of the packets from the queues. This would involve increased complexity and hence cost of the technique .

In view of the prior art outlined above, an object of the present invention is to provide a ^"method of transferring information relating to the states of fullness of the queues of packets from a device for storing the packets of data in the queues with different priorities, to a device for transmitting the packets of data as well as information for the control protocol of the shared communication means, which method overcomes the limitations and the disadvantages of the techniques mentioned above.

According to the present invention, this object is achieved by means of a method of transferring ATM data packets carrying data belonging to traffic classes with different priorities between a first device or transmitter device comprising queues for storing the packets and a second device or receiver device for receiving the packets from the transmitter device and for transmitting the packets on a communication means, in which the transfer is performed in accordance with the priority of the packets, characterized by: the provision, in the first device, of a plurality of transmitter-device physical interfaces each comprising at least one respective queue for storing data packets, the establishment of a one-to-one association between each of the transmitter-device physical interfaces and a respective traffic class so that the data packets belonging to the same traffic class are stored in the queue of the same device physical interface, the interfacing of the first device with the second device by means of a standard multi-physical interface, and the use of lines of the standard multi-physical interface for transferring information relating to the states of fullness of the queues of each transmitter- device physical interface from the first device to the second device.

The method according to the present invention provides for interfacing, by means of a standard multi- physical interface, between a first device which comprises a set of queues for storing data packets belonging to traffic classes with different priorities, and a second device, for the transmission, on a shared communication means, of the said packets as well as information for the control protocol of the communication means . The first device acts as a transmitter device and the second device acts as a receiver device; the first device with the queues of packets with different priorities in fact transmits the packets and the information relating to the states of fullness of its own queues to the second device. For example, the interface may advantageously be the multiple physical "UTOPIA Level 2" interface defined by the ATM Forum Technical Committee (document "UTOPIA LEVEL 2, Version 1.0, af-phy-0039-00, June 1995"). A particular characteristic of the method according to the present invention is that it enables the information relating to the states of fullness of the queues of packets of traffic classes with different priorities to be transferred from the transmitter device to the receiver device via hardware, by means of the standard interface, making use of signals already provided for this purpose in an interface of this type.

Further characteristics and advantages of the present invention will become clear from the following detailed description of possible practical embodiments thereof, given purely by way of non-limiting examples and illustrated in the appended drawings, in which:

Figure 1 is a schematic representation of a point- to-multipoint (P-MP) communication system, Figure 2 shows schematically a peripheral unit of the point-to-multipoint system according to the present invention,

Figure 3 shows the peripheral unit of Figure 2 in a first embodiment of the invention, and Figure 4 shows a second embodiment of the peripheral unit of Figure 2.

With reference to Figure 1, this shows schematically a point-to-multipoint (P-MP) ATM communication system in which a central unit UC or "master station" communicates, by means of a communication means MC shared by time division (by the TDMA technique) , with a plurality of peripheral units or stations UP.

The shared communication means MC may, for example, be of the wireless type, in which case, the communication system is via radio, or may be constituted by a passive optical network (or "PON"). According to the ATM technique, both the central unit UC and the peripheral units are organized so as to include so-called "ATM- level" devices (shown schematically in the drawing by functional blocks marked by the reference ATM) and so- called "physical-level" devices (shown schematically by functional blocks marked by the reference PHY) .

Each peripheral unit UP receives input data coming from a plurality of connections QoS_prl, ..., QoS_prN which generate different traffic classes each distinguished by a respective priority level. For example, the connection QoS_prl generates the traffic class with the highest priority level, and the connection QoS_prN generates the traffic class with the lowest priority level. The central unit UC performs a periodic interrogation ("polling") of the various peripheral units and assigns the communication means MC to one of the peripheral units UP at a time; the peripheral unit to which the central unit UC has assigned the communication means MC can thus transfer the data to the central unit UC.

There are various techniques for the assignation of the shared communication means to the various peripheral units, and these provide for the allocation of either static or dynamic bands. One of these techniques is described, for example, in the Applicant's European patent application No. EP-A1-1017244.

To ensure that the data packets relating to the traffic classes with the highest priority level are always transmitted first, the peripheral units UP have to inform the central unit UC not only of the presence of data for transmission in the peripheral units UP but also of the priority level of the data. With reference now to Figure 2, this shows a peripheral unit UP according to the present invention, in greater detail.

In the peripheral unit UP, the ATM-level devices generate a flow of ATM data packets or cells CP1, ... , CPN, on the basis of the data input to the peripheral unit UP. Each cell comprises, in accordance with the ATM protocol, a first portion or "header" and a second portion or "payload". Each cell CP1, ..., CPN carries data associated with a particular traffic class QoS_prl , ... , QoS_prN with which traffic class a particular priority level is associated. In general, upon the assumption that there are N traffic classes, there will be N different priority levels to manage. In the example shown, the cell CP1 is assumed to be associated with the traffic class QoS_prl with the highest priority level

("1"), the cells of which ^'should be transmitted first, if possible, and the cells CPN are assumed to be associated with the traffic class QoS_prN with the lowest priority level ("N"), the cells of which can therefore be transferred last, after the cells associated with the traffic classes with higher priorities have been transmitted.

A demultiplexer 2 (DEMUX) receives a input flow 1 of cells CP1, ..., CPN, identifying their priority levels, and shunting the cells CP1, ..., CPN to a plurality (n=N) of transmitter-device physical interfaces TX_1, ..., TX_n, according to the priority level associated with each cell. The physical interfaces TX__1, ..., TX_n form a transmitter side (TX side) of a multi-physical interface M-PHY with a receiver side RX. In particular and advantageously, the multi-physical interface M-PHY is of the standard type defined, for example, in the ATM Forum Technical Committee's document UTOPIA LEVEL 2, VERSION 1.0, AF-PHY-0039.000, June 1995. Each transmitter physical interface TX_1,..., TX_n generally comprises a plurality of queues Q_prl, ..., Q_prN for the data cells and a states register S_TX_1 , ... , S_TX_n for holding information relating to the states ("queue full "/"queue not full") of the queues of the transmitter-device physical interface.

According to the present invention, a one-to-one association is established between a given transmitter- device physical interface TX_1, ..., TX_n and a corresponding data-cell priority level, that is, a given traffic class.

Specifically, only one of the N queues normally provided for in each interface TX_1, ..., TX_n is used for storing the cells which are received from the flow 1 and are to be transmitted to the central unit UC. For example, in the interface TX_1, the queue Q_prl is used, in the interface TX_2 (not shown) the queue Q_pr2 is used, and so on up to the interface TX_n, in which the queue Q_prN is used. It is worth pointing out that, if the N queues in each interface TX_1, ..., TX_n are managed in accordance with a shared buffer scheme, there is no wastage of memory.

The demultiplexer 2 can identify the priority levels of the cells CP1, ..., CPN of the flow 1 and can sort the cells in a manner such that all of the cells distinguished by the same priority level are stored in the preselected queue of the same interface TX_1, ..., TX_n. For example, all of the cells which are distinguished, as the cell CP1 is, by priority level 1, are stored in the queue Q_prl of the interface TX_1, all of the cells with priority level 2 are stored in the queue Q_pr2 of the interface TX_2 , and so on up to the cells with priority level N, which are stored in the queue Q_prN of the interface TX_n.

In each interface TX_1, ..., TX_n, the respective state register S_TX_1, ..., S_TX_n will contain the data relating to the state ("queue full"/"queue not full") of the respective queue Q_prl, ..., Q_prN of the interface.

The transmitter-device physical interfaces TX_1, ... , TX_n which are on the transmitter side (TX side) of the multi-physical interface M-PHY communicate the contents of the respective state registers S__TX_1, ..., S_TX_n to a block 3 which indicates schematically the devices constituting the receiver side (RX side) of the multi-physical interface M-PHY. The devices of the block 3 can receive the contents of the respective queues Q_prl, ..., Q_prN from the transmitter-device physical interfaces TX_1, ... , TX_n and transfer the data cells on the shared communication means MC when the communication means is assigned to the peripheral unit UP to which they belong. The devices of the block 3 also communicate to the central unit UC of the communication system P-MP the information relating to the states of fullness of the queues of cells.

In particular, the standard UTOPIA Level 2 multi- physical interface enables transmission to be controlled by a plurality of transmitter-device physical interfaces by means of standard signal lines known as CLAVs (CelL Available) . The generic i-th transmitter-device physical interface TX_i can indicate, by means of the respective line CLAV__i, the presence in the respective queue Q_pri of at least one data cell to be transmitted.

Either the transmitter side or the receiver side of the multi-physical interface M-PHY may be configured as the master side of the interface. Figure 3 shows schematically the case in which the master side of the multi-physical interface M-PHY is the receiver side (RX side) . A buffer 4 on the receiver side

(RX side) of the multi-physical interface M-PHY is intended to receive the data cells from the queues Q_prl, ..., Q_prN of the physical interfaces TX_1 , ..., TX_N of the transmitter side TX of the multi-physical interface M-PHY for the subsequent transmission of these cells to the central unit UC by means of the shared communication means MC. A control unit 5 of the multi-physical interface M-PHY receives from the physical interfaces TX_1, ..., TX_n, by means of the standard lines CLAV_1 , ... , CLAV_n of the interface M-PHY, the information relating to the states of fullness of the respective queues Q_prl, .-., Q_prN, that is, the information relating to the presence or absence of cells in each of the queues. The standard lines are the TX CelL Available lines provided for by the multi-physical interface standard.

In the control unit 5 of the multi-physical interface M-PHY, the scheduling unit 6 reads the states of the lines CLAV_1, ..., CLAV_n and, by means of enabling signals EN_1, ..., EN_n, enables the data cells stored in the queues Q_jprl, ..-, Q_prN of the transmitter-device physical interfaces TX_1 , ..., TX_n to be transferred to the buffer 4, in the correct order, according to their respective priority levels.

The peripheral unit UP also communicates information ST_Q relating to the states of fullness of all of its own queues Q_prl, ..., Q_prN to the central unit UC of the communication system P-MP, by means of the respective control unit 5, so that the central unit UC can establish to which of the various peripheral units UP of ^• the system to assign the shared communication means MC. This information is supplied by the peripheral unit UP by means of the states of the lines CLAV_1, ..., CLAV_n, which are indicative of the states of the various physical interfaces TX_1, ..., TX_n, that is, of the presence or absence of data cells in the queues Q_jprl,

... , Q_pr . The central unit UC can thus determine which of the various peripheral units UP has a cell awaiting transmission and corresponding to a traffic class with higher priority than the other peripheral units so as to assign the communication means MC to that peripheral unit. It is thus ensured that the cells which carry data relating to the traffic class with the highest priority level are transferred first .

Figure 4 shows schematically the case in which the master side of the interface is the transmitter side of the multi-physical interface M-PHY. In this case, N buffers RX_1, ..., RX_N which are present on the receiver side (RX side) of the interface M-PHY are used. Each of the N buffers RX_1, ..., RX_2 is associated with (that is, receives) the cells stored in a respective queue Q_prl, ..., Q_prN of the queues used by the transmitter side of the interface, according to its priority level. In other words, a one-to-one association is established between the generic queue Q_pri held in the transmitter- device physical interface TX_i and associated with the i- th priority level and a buffer RX_i of the N buffers present on the receiver side. Still on the receiver side of the interface M-PHY, a respective line CLAV_1, ... , CLAV_N (RX CelL Available lines of the standard multi - physical interface) is associated with each buffer RX_1 , ... , RX_N, and is intended to indicate the availability of the respective buffer to receive cells from the associated queue Q_prl, ..., Q_prN. A respective states register Stat_l, ..., Stat_N is also associated with each buffer RX_1/ •••/ RX_N and is intended to hold information relating to the state of fullness of the buffer.

The buffers RX_1, ..., RX_N and the respective state registers Stat_l, ... , Stat_N communicate with a device MAC ("medium access control") for managing the access of the peripheral unit UP to the shared communication means MC.

A control unit 5 of the multi-physical interface M- PHY, including a scheduler 6, receives from the lines CLAV_1, ... , CLAV_N on the receiver side of the interface M-PHY the information relating to the states of the buffers RX_1 , ..., RX_N, that is, the availability of the buffers to receive cells; the control unit 5 enables the interfaces TX_1, ..., TX_n to transmit cells disposed in the respective queues Q__pτl , ..., Q_prN of the interfaces TX_1, ..., TX__n on the basis of this information, by means of enabling signals EN_1, ..., EN_n.

The peripheral unit UP can communicate the states of the respective buffers RX_1, ..., RX_N to the central unit CU of the P-MP system by means of the device MAC. The central unit UC can thus establish which of the peripheral units UP has cells which are awaiting transmission and are associated with the traffic class having the highest priority level and can consequently assign the communication means MC to that peripheral unit .

The present invention thus provides a method of transferring the information relating to the states of fullness of the queues of ATM data packets or cells from a device for storing the data packets in queues with different priorities to a device for transmitting the packets of data and information for the control protocol of the shared communication interface means, by means of a standard multi-physical interface between a transmitter device comprising a set of queues associated with traffic classes with different priority levels, and a receiver device .

The information relating to the states of the various queues is transferred from the transmitter device to the receiver device via hardware means, by means of lines already provided in the same standard multi- physical interface.

The method according to the present invention is based on the association between a transmitter-device physical interface comprising at least one queue for the packets of data, and a traffic class with a given priority level. This association is achieved by a demultiplexing of the flow of cells or packets which directs cells or packets which carry data belonging to a particular traffic class with a given priority level, to the physical device interface which is associated with that traffic class, that is, with that priority level.

In general, a generic transmitter-device physical interface comprises a plurality of queues each of which is conventionally associated with a traffic class, that is, with a particular priority. In the method according to the invention, however, for each transmitter-device physical interface, only one of its queues is used and the remaining queues are not used. All of the cells which carry data belonging to a particular traffic class and which therefore have a given priority level are arranged in a FIFO arrangement in a single queue of the device physical interface.

A one-to-one association is thus established between a particular traffic class, that is, between a given priority level, and a respective transmitter-device physical interface.

The- states of the various queues, each . of which is associated with a traffic class having a particular priority, thus correspond to the states of the transmitters. The states of the transmitters can easily be transferred by the standard multi-physical interface with the use of lines (such as the TX CelL Available lines or the RX CelL Available lines) already provided by the standard interface.

A suitable standard interface is the UTOPIA Level 2 interface defined in the ATM Forum.

An advantage of the method according to the present invention is that it uses a standard UTOPIA Level 2 interface and does not require the provision of separate, additional interfaces. By virtue of the method according to the invention, the standard UTOPIA Level 2 interface is used to transfer the information relating to the states of the queues associated with traffic classes with different priority levels. In the method according, to the invention, the transfer of the information relating to the states of the queues associated with the traffic classes with different priority levels is achieved by hardware and not software and is thus very quick in comparison with techniques which provide for the transfer of the information via software, by means of dedicated software modules.

Moreover, the transfer of the information relating to the states of the queues associated with the traffic classes with different priority levels takes place by means of the standard UTOPIA Level 2 interface and does not require operations to be performed to read states registers by means of a channel separate from those provided by the standard interface . Naturally, variations and/or additions may be provided for the embodiments described and illustrated without thereby departing from the scope of protection defined in the appended claims.

Claims

1. A method of transferring ATM data packets carrying data belonging to traffic classes (QoS_prl, ..., Q-S_prN) with different priorities between a first device (2, TX_1 , ..., TX_n) or transmitter device comprising queues (Q_prl, ..., Q_prN) for storing the packets (CP1, ... , . CPN) and a second device (3) or receiver device for receiving the packets from the transmitter device and for transmitting the packets on a communication means (MC) , in which the transfer is performed in accordance with the priority of the packets, characterized by: the provision, in the first device, of a plurality of transmitter-device physical interfaces (TX_1, ..., TX_n) each comprising at least one respective queue (Q_prl, ..., Q_prN) for storing data packets, the establishment of a one-to-one association (2) between each of the transmitter-device physical interfaces and a respective traffic class so that data packets belonging to the same traffic class are stored in the queue of the same device physical interface, the interfacing of the first device with the second device by means of a standard multi-physical interface (M-PHY) , and the use of lines (S_TX_1, ..., S_TX_n; CLAV1, ..., CLAV_N) of the standard multi-physical interface for transferring information relating to the states of fullness of the queues of each transmitter-device physical interface from the first device to the second device .

2. A method according to Claim 1 in which the standard multi-physical interface (M-PHY) is a UTOPIA Level 2 interface.

3. A method according to Claim 2 in which the establishment of a one-to-one association provides for a demultiplexing (2) of data packets (CP1, ... , CPN) of a flow (1) .

4. A method according to Claim 3 in which each of the transmitter-device physical interfaces (TX_1, ...,

TX_n) is of the type comprising a respective plurality of queues (Q_prl, ..., Q_prN) for storing data packets, only one of the queues being used for the storage of data packets of the traffic class associated with the transmitter-device physical interface.

5. A method according to Claim 4 in which the queues of the plurality of queues (Q_prl, ..., Q_prN) for storing data packets in each transmitter-device physical interface (TX_1, ..., TX_n) are managed in accordance with a shared buffer scheme.

6. A method according to Claim 4 or Claim 5 in which the receiver device (3) performs the role of master of the multi-physical interface (M-PHY) between the transmitter device and the receiver device.

7. A method according to Claim 6 in which the receiver device comprises a buffer (4) for receiving data packets from the queues (Q_prl, ..., Q_prN.) of the transmitter device, each of the transmitter-device physical interfaces (TX_1, ..., TX_n) indicating a state of fullness of the respective queue to a control unit (5) of the multi-physical interface (M-PHY) , and the control unit enabling (EN_1, ..., EN_n) the transmitter-device physical interface associated with the traffic class

(QoS_prl, ..., QoS_prN) with highest priority of those which, have indicated (CLAV_1, ..., CLAV__n) the presence of cells in the respective queues to transfer a data packet to the buffer of the receiver device.

8. A method according to Claim 7 in which the transmitter-device physical interfaces (TX_1, ..., TX_n) indicate the states of fullness of the respective queues (Q_prl, ..., Q_prN) by means of respective lines (TX CelL Available) of the multi-physical interface (M-PHY) .

9. A method according to Claim 4 or Claim 5 in which the transmitter device performs the role of master of the multi-physical interface (M-PHY) .

10. A method according to Claim 9 in which the receiver device comprises a plurality of buffers (RX_1, ... , RX_N) , each associated with a respective transmitter-device physical interface (TX_1, ..., TX_n) in order to receive data packets from the respective queue (Q_jorl, ..., Q_prN) , the receiving device also indicating states of fullness of the buffers to a control unit (5) of the multi-physical interface (M-PHY) , and the control unit consequently enabling data packets to be transferred from the queues of the transmitter device to the respective buffers of the receiver device.

11. A method according to Claim 10 in which the receiver device indicates the states of fullness of the respective buffers (RX_1, ..., RX_N) by means of respective lines (RX CelL Available) of the multi- physical interface (M-PHY) .

1 . A method according to any one of the preceding claims, in which the transmitter device and the receiver device form parts of a peripheral unit or station (UP) of a communication system with point-to-multipoint access in which a central unit (UC) determines the assignment of a communication means (MC) with time division to a plurality of the peripheral units.

13. A method according to Claim 12 in which the communication means (MC) is of the wireless type.

14. A method according to Claim 12 in which the communication means is a passive optical network.

15. A system for the transfer of ATM data packets carrying data belonging to traffic classes (QoS_prl, ... , QoS_prN) with different priorities between a first device (2, TX_1, ..., TX_n) or transmitter device comprising queues (Q_prl, ..., Q_prN) for storing the packets (CP1, ... , CPN) and a second device (3) or receiver device for receiving the packets from the transmitter device and for transmitting the packets on a communication means (MC) , in which the transfer is performed in accordance with the priority of the packets, characterized in that it comprises: in the first device, a plurality of transmitter- device physical interfaces (TX_1, ..., TX_n) each comprising at least one respective queue (Q_prl, ..., Q_prN) for storing data packets, means for establishing a one-to-one association (2) between each of the transmitter-device physical interfaces and a respective traffic class so that data packets belonging to the same traffic class are stored in the queue of the same device physical interface, and a standard multi-physical interface (M-PHY) between the first device and the second device so as to use lines (CLAV1, ..., CLAV_N) of the standard multi-physical interface for transferring information relating to the state of fullness of the queues of each transmitter device physical interface from the first device to the second device .

16. A system according to Claim 15, in which the standard multi-physical interface (M-PHY) is a UTOPIA Level 2 interface defined in the ATM Forum.

17. A system according to Claim 16, in which the means (2) comprise a demultiplexer which receives input data packets (CP1, .., CPN) from a flow and shunts the packets as outputs to respective transmitter-device physical interfaces, according to their priority.

18. A system according to Claim 17, in which the transmitter device and the receiver device form parts of a peripheral unit (UP) of a point-to-multipoint communication system.

19. A point-to-multipoint ATM communication system comprising a central unit (UC) , a plurality of peripheral units or stations (UP) , and a shared communication means (MC) which can be assigned to the peripheral units selectively, by a TDMA method, under the control of the central unit, characterized in that the peripheral units are formed in accordance with any one of Claims 14-17.