HK1093277B - Data flow control for multi-layered protocol stack - Google Patents

Description

Data flow control for multi-layer protocol stack

Technical Field

The present invention relates to a system and apparatus for controlling the flow of a number of Data Units (DUs) over a logical link. The invention further relates to a method and a computer program product for controlling the flow of a number of DUs over a logical link.

Background

With the advent of a mix of advanced multimedia applications and services, data transfer in cellular radio systems such as the global system for mobile communications (GSM) or universal mobile telecommunications system (UTMS) is becoming more and more complex. For optimal use and fair distribution of scarce radio resources, more sophisticated data flow control techniques are required, wherein "flow control" is understood to be the management of databases between nodes of a cellular radio system, such as mobile terminals and base stations, so that data can be processed at an efficient rate. The arrival of too much data before one of the nodes can process it can result in data overflow, meaning that the data is either lost or must be retransmitted. An example of a sophisticated data flow control technique is the introduction of priority based data flow control, where data is assigned a quality of service (QoS) parameter and/or a predefined class of service, and where the data flow can then be processed based on considerations of the QoS parameter and/or class of service, e.g. in order to optimize the delivery of data with the highest QoS requirements. The lack of proper priority-based data flow control can result in wasted resources on lower priority services, ultimately resulting in dissatisfaction of mobile users.

Figure 1 depicts a portion of the General Packet Radio Service (GPRS) protocol stack incorporated into the GSM protocol stack in the context of enhanced second generation GSM (GSM phase 2+) designs for more efficient use of GSM radio resources. The GPRS protocol stack controls communication between a mobile base station (MS)1 and a Serving GPRS Support Node (SGSN)2 via a Base Station Subsystem (BSS) 3.

As a result, the Logical Link Control (LLC) layer 4 of the GPRS protocol stack as shown in fig. 1 will have primary importance. This layer and its functionality is standardized by the European Telecommunications Standards Institute (ETSI) and described in technical material 3GPP TS 04.64 (release 8.7.0, month 12 2001).

LLC layer 4 is considered a sublayer of layer 2 in the ISO/OSI 7-layer reference model and operates above the Radio Link Control (RLC) layer 5 and the Base Station Subsystem GPRS Protocol (BSSGP) layer 6 to provide a logical link between a layer 3 entity in MS1 and its associated layer 3 peer entity in SGSN 2. Above the LLC layer 4, i.e. in the network layer (layer 3 of the ISO/OSI model) 7, there are provided a message Tunneling (TOM) protocol 7-1, a GPRS Mobility Management (GMM) protocol 7-2, a subnetwork dependent convergence (SNDC) layer 7-3, and an SMS protocol 7-4.

The RLC protocol 5 in combination with the Medium Access Control (MAC) protocol 8 operates on top of the GSM Radio Frequency (RF) channel (physical layer) 9 and enables reliable packet-based transmission between the MS1 and the BSS3 by statistically multiplexing several logical connections on the Packet Data Channel (PDCH) available for GPRS in one radio cell. Although RLC/MAC already contains an automatic repeat request (ARQ) arrangement to handle transmission errors over the air interface, LLC protocol 4 provides a reliable encrypted logical link between MS1 and SGSN2 by applying error handling mechanisms and flow control known from Integrated Services Digital Network (ISDN) -like link layer protocols. The ARQ arrangement in LLC is not designed to handle transmission errors over the air interface but to handle frame losses and errors in the fixed network or for link recovery after a cell change. BSSGP 6, running on top of network services 10 such as Frame Relay (FR) and underlay-1 (L1) protocol 11, is responsible for flow control between BSS3 and SGSN 2. The translation between the RLC protocol 5 and the BSSGP protocol 6 is done by the relay 12 in the BSS 3.

The TOM protocol 7-1 is a common protocol layer for the exchange of TOM protocol encapsulation between MS1 and SGSN 2. GMM protocol 7-2 uses LLC layer 4 services to convey messages between the MS1 and the SGSN 2. Which includes functions such as attachment, authentication, and transfer of session management messages for functions such as PDP context activation and deactivation. The network layer protocol is intended to be suitable for running services from many subnetworks and data links. GPRS supports several network layer protocols that provide protocol transparency for service users. Thus, all functions related to the transfer of network layer PDUs are performed in a transparent manner by the GPRS network entity. SNDCP7-3 adapts different network layers to logical links by mapping different packet data protocols (e.g., Internet Protocol (IP) data packets) onto services provided by the LLC layer. The SMS protocol 7-4 uses the LLC layer 4 service to transport short messages between the MS1 and the SGSN 2.

Fig. 2 gives a more detailed view of the LLC layer 4 structure seen from the MS1 side, wherein the signalling transfer between the parts of fig. 2 is given by dashed lines, and the joint signalling and data transfer is given by solid lines. The LLC layer 4 comprises a number of LLC entities (LLEs) 40-1 … 40-8, an LLC management entity 41, a multiplexer 42 and a number of LLC-SAPs 43-1 … 43-9. The services of LLE40-1 … 40-8 are provided to the protocols of network layer 7, i.e., SMS protocol 7-1, GMM protocol 7-2, SNDCP7-3, and TOM protocol 7-4, through SAP43-1 … 43-8. There is also a pure signaling link between GMM protocol 7-2 and the LLC management entity 41 via SAP 43-9. Under the LLC layer 4, an RLC layer 5 and a MAC layer 8 are placed. The LLC frames from the multiplexer 42 are passed to the RLC layer 5 via an RLC-SAP 50-1.

LLC layer 4 provides four LLC-SAPs 43-2 … 43-5 to SNDCP7-3, where SNDCP7-3 manages the actual data packet transmission. In GPRS, four radio priority levels are defined. Correspondingly, packet data transmission requires four LLC-SAPs, wherein the SAP identifier (SAPI) identifies the point at which the LLC entity provides LLC service to the network layer entity. The SAPIs for the four wireless priority levels are SAPI ═ 3, 5, 9, and 11, respectively. Services made available through LLC-SAP 43-2 … 43-5 are provided by LLE40-2 … 40-5. Each LLE controls the information flow of N-PDUs, respectively, that are conveyed as LLC service data units (LLC-SUDs) within LLC-PDUs over LLC links/connections between peer lls located in MS1 and SGSN 2. These N-PDUs are transported between LLEs and SNDCP7-3 above via corresponding LLC-SAPs. LLE provides the SNDCP7-3 with unacknowledged/acknowledged information transfer, flow control (in asynchronous balanced mode), and frame error detection. Each LLC connection is identified by a Data Link Connection Identifier (DLCI) that includes the SAPI and a Temporary Logical Link Identifier (TLLI). As already specified. SAPI is used to identify the SAP on the SGSN2 side and the MS1 side of the LLC interface, and is carried in the address field of each LLC frame. The TLLI is used to identify a particular MS 1. The allocation of TLLI is controlled by the GMM. The TLLI is not carried within the LLC frame.

The multiplexer 42 represents the entity that multiplexes the LLC connections of the individual LLE40-1 … 40-8 and outputs LLC frames (LLC-PDUs) that are transmitted to a peer multiplexer entity located at the remote SGSN 2LLC layer 4. On LLC frame transmission, multiplexer 42 generates and inserts a Frame Check Sequence (FCS), performs frame encryption functions, and provides a SAPI-based LLC layer contention scheme between individual LLE40-1 … 40-8, whose connections are multiplexed. On LLC frame reception, multiplexer 42 performs a frame decryption function and checks the FCS. If the frame passes the FCS check, the multiplexing process issues the LLC frame to the appropriate LLE. For cryptographic functions, a cryptographic algorithm is used to generate a key, for example starting from an identification key and an unpredictable random number generated by an authentication center. The FCS typically includes a 24-bit Cyclic Redundancy Check (CRC) code. The CRC-24 is used to detect bit errors in the frame header and information fields.

When passed to the RLC layer 5 by the RLC-SAP 50-1, the LLC frame from the multiplexer 42 is segmented into RLC data blocks. A selective ARQ between the MS1 and the network provides retransmission of erroneous RLC data blocks at the RLC layer 5 and MAC layer 8. When a complete LLC frame is successfully transferred over the RLC layer, it is forwarded to the peer LLC entity in the SGSN 2.

By multiplexing the connections of different LLEs 40-1 … 40-8 into LLC frames, the multiplexer 42 provides a common interface between the LLC layer 4 and the RLC layer 5 (through a single RLC-SAP 50-1) for all signaling and data transfers. However, the multiplexer also presents a bottleneck for smooth data flow for the different LLC connections controlled by the respective LLEs 40-1 … 40-8. This is due to the fact that for the radio resources available for the transmission of its RLC data blocks, which have a good expectation of the RLC layer, it is not possible to control the data flows of different LLC connections over a single RLC-SAP 50-1, respectively, due to the multiplexing procedure. It is therefore also not possible for the RLC layer 5 to adapt the flows of different LLC connections with different QoS requirements to the transmission state of the underlying radio resource, a fact that impairs the entire data flow of LLC-PDUs over the LLC link and thus the flow of N-PDUs contained in the LLC-PDUs as LLC-SUDs.

Disclosure of Invention

It is an object of the present invention to provide an improved control of the flow of data units over logical links in a communication system running a multi-layer protocol stack.

The proposed system for controlling the flows of a number of Data Units (DUs) over logical links comprises at least two Link Control (LC) means of a first type, wherein each first-type LC means converts a first-type DU into a second-type DU and vice versa and provides a first interface to said first-type DU and a second interface to said second-type DU, and a second-type LC means having an interface to said second-type DUs, wherein the second interface of each said first-type LC means is directly connected to a respective interface of said second-type LC means, and wherein said second-type LC means comprises means for separately controlling the individual flows of second-type DUs, which are transferred over their respective interfaces.

Compared to the prior art, the present invention omits the use of a multiplexer that multiplexes the second-type DUs that are transmitted over the second interfaces of the respective first-type LC means, and transmits the multiplexed second-type DUs over a single interface of the second LC means. Instead, the invention proposes a second type of LC means having several interfaces, wherein each of said interfaces is directly connected to a respective second interface of each of said at least two first type of LC means. The second-type LC has means to control the flow of second-type DUs over its respective interface. As a result, the present invention allows the flow of the respective second-type DUs converted from the respective first-type DUs of the respective first-type LC means to be controlled separately. Due to the one-to-one relationship between the first-type DUs and the second-type DUs that are converted by the same first-type LC means, the second-type LC means can then also control the flows of the respective first-type DUs that are transferred over the respective first interfaces of said respective first-type LC means. In other words, according to the present invention, the flow of the respective first-type DUs that are passed to the respective first interfaces of the respective first-type LC means can be controlled by said second-type LC means.

According to the system of the present invention, it is preferred that said first-type LC means converts said first-type DUs into second-type DUs and vice versa by performing at least one frame check function and a ciphering function. The omission of the prior art multiplexer requires that part of the multiplexer functions, at least the insertion and verification of the frame check sequence, and the encryption and decryption of the frames, be incorporated into each of said first type of LC means. Thus the conversion of the first-type DUs into the second-type DUs involves the insertion of a frame check sequence and encryption, whereas the conversion of the second-type DUs into the first-type DUs involves at least decryption and verification of the frame check sequence. It is noted that according to the system of the present invention it is still possible to have a prior art set-up of first-type DUs that do not perform a frame check function and an encryption function, and that the DUs that are transmitted over the respective second interfaces are multiplexed by a multiplexer that performs a frame check function and an encryption function and that is transmitted over a separate interface of said second-type LC means. The invention only requires the presence of at least two first-type LC means having a frame check function and an encryption function, and said second-type LC means has an interface which is uniquely connected to the second interfaces of said at least two first-type LC means. One interface of the multiplexer can thus be connected to another interface of said second-type LC means, but only separate flow control is possible for DUs converted by the first-type LC means without multiplexing thereafter, whereas the flow of multiplexed DUs can only be controlled jointly.

According to the system of the present invention, it is further preferred that at least two first-type LC means are distinguishable by the characteristics of the first-type DUs that are transferred over their respective first interfaces, and that said means for separately controlling the respective flows of the second-type DUs that are transferred over said respective interfaces of said second-type LC means are controlling the respective flows of said second-type DUs based on a consideration of the characteristics of the first-type DUs to or from which said second-type DUs are converted. The at least two distinguishable first-type LC means are then adapted for the transfer and conversion of at least two different types of first-type DUs, and the flow of first-type DUs transferred over the first interfaces of the at least two distinguishable first-type LC means can be controlled by the second LC means due to the direct connection between the second interface of each first-type LC means and the respective interface of said second-type LC means, and the ability of the second-type LC means to control the flow of second-type DUs transferred over their respective interfaces, respectively. Likewise, it is also possible to have as many distinguishable first-type LC devices (and corresponding first-type DUs) as interfaces of second-type LC devices.

According to the system of the present invention it is preferred that said second-type LC means comprises at least one DU buffer for buffering second-type DUs that are transferred over their respective interfaces. The DU buffer stores second-type DUs to be transmitted over the joint transmission link, which link may be at least partially controlled by the second-type LC means.

According to the system of the present invention it is further preferred that said means for separately controlling respective flows of second-type DUs that are transferred over said respective interfaces of said second-type LC means controls the respective flows of said second-type DUs based on considerations of characteristics of first-type DUs to or from which said second-type DUs are converted and on considerations of the state of said at least one DU buffer. Then the respective flows of said second-type DUs over the respective interfaces of said second-type DUs can be adapted to the state of the buffer. The state of the buffer may describe the number of stored DUs, the number of remaining memory locations, information about the rate or storing and flushing of the buffer, etc. For example, if the buffer is slowly flushed, the second-type LC means may reduce the flow of all second-type DUs, or may reduce the flow of only the second-type DUs over one of their respective interfaces.

It is further preferred according to the system of the present invention that said second-type LC means comprise means for evaluating the status of the physical transmission link over which said second-type DUs are transmitted. Said second-type DUs may be transferred directly or indirectly over said physical transmission link, i.e. further LC means may be provided between said second-type LC means and said physical transmission link. Accordingly, the second-type DUs may or may not be further converted before being transmitted over the physical transport channel. The state of the physical transmission link can be distinguished by any parameter or combination of parameters related to the data transmission, such as the available transmission bandwidth, error characteristics, delay characteristics, signal-to-noise ratio, carrier-to-interference ratio, etc.

According to the system of the present invention it is further preferred that said means for separately controlling the respective flows of second-type DUs that are transferred over said respective interfaces of said second-type LC means controls the respective flows of said second-type DUs based on considerations of characteristics of first-type DUs to or from which said second-type DUs are transferred and on considerations of the state of the physical link over which said second-type DUs are transferred. The characteristics of the distinguishable DUs of the first type together with the state of the physical transmission link then determine the flow control.

It is further preferred according to the system of the present invention that the means for separately controlling the respective flows of the second-type DUs, which are transferred over said respective interfaces of said second-type LC means, comprise means for running a flow control protocol. This flow control protocol controls the respective flows of the second-type DUs over said respective interfaces of said second-type LC means and may communicate with the respective first-type LC means via control commands to request further second-type DUs that may be stored in the buffer of said first-type LC means or to block further transmissions of the second-type DUs from the buffer of the first-type LC means.

According to the system of the present invention, it is further preferred that the flow control protocol is an X on/X off flow control protocol. Wherein the receive data buffer sends an X-off command to the transmit data buffer when the receive buffer is full, and sends an X-on command to the transmit data buffer when it is ready for further data buffering. Other flow control protocols are possible.

According to the system of the present invention it is further preferred that said characteristic is a quality of service (QoS) requirement of said first-type DUs. The first-type DUs that are converted by said first-type LC means to second-type LC means can then be prioritized differently by said second-type LC means. For example, as the buffer of the second-type LC means is filled more and more, the second-type LC means may reduce the flow of second-type DUs corresponding to the first-type DUs with the lowest QoS requirements, so that at least second-type DUs with higher QoS requirements may be further processed by means following the buffer in the second-type LC means, which may be e.g. a transmitter/receiver of the physical link or a further LC means controlling the physical transmission link. The QoS requirements may include information about network availability, throughput (effective data rate), packet loss rate, delay, and jitter.

According to the system of the present invention, it is preferable that the system further comprises a link management means controlling said first type LC means. The control includes, for example, parameter initialization, error handling, and connection flow control calls.

According to the system of the present invention, it is preferred that the logical link is a logical link between a Mobile Station (MS) and a Serving GPRS Support Node (SGSN) in a mobile radio system operating according to the General Packet Radio Service (GPRS) standard or a derivative thereof, said first type of LC means is a Logical Link Control (LLC) entity of the GPRS LLC protocol with an additional sequence-based frame check function and ciphering function, the first interface is a service access point (LLC-SAP) of an LLC entity, the second-type DUs are LLC frames, the interface of the second-type LC device is Radio Link Control (RLC) -SAP, the first-type DU is an N-PDU of a GPRS network layer protocol, and said second type LC means implements the functionality of the gprs RLC protocol and further comprises said means for separately controlling respective flows of LLC frames transmitted over said respective RLC-SAPs.

N-PDUs with different QoS requirements, e.g. representing SNDCP, have to be transferred between peer entities located at the MS network layer and the SGSN network layer, respectively, by encapsulating them as LLC-SDUs in LLC-PDUs transferred over logical links between peer LLEs. According to the invention, the connections between the LLE that are available to the network layer over the LLC-SAPI can now be controlled separately for each LLC-SAPI, because, according to the system of the invention, the LLC connections are no longer multiplexed before using the RLC connection services over a single RLC-SAP, but instead use the respective RLC connections that are available over several respective RLC-SAPs, respectively. With the omission of the prior art LLC multiplexer, there is practically no flow impedance between the RLC entity and the network layer entity, e.g. the SNDCP entity. The RLC layer can thus have full data flow control for the data to be transferred on each LLC-SAP. The RLC layer communicates directly with the LLE, which have their respective small buffers, for uplink and downlink data flows, e.g., via an X on/X off flow control protocol. If a problem occurs in the RLC buffer or on the radio link, flow control can suspend the transmission of low priority SNDSP PDUs and instead retain the transmission of high priority SNDCP PDUs. Apart from the fact that the number of primitives between the entities is greatly reduced and results in a reduced MCU load and the use of buffers, the multiplexer itself and its expensive buffers can be omitted.

A further proposed arrangement for controlling flows of a number of Data Units (DUs) over logical links of a communication system comprises at least two first-type Link Control (LC) means, wherein each first-type LC means converts a first-type DU into a second-type DU and vice versa and provides a first interface to said first-type DU and a second interface to said second-type DU, and second-type LC means having an interface to the second-type DUs, wherein the second interface of each of said first-type LC means is directly connected to a respective interface of said second-type LC means, and wherein said second-type LC means comprises means for controlling separately respective flows of the second-type DUs, which flows are communicated over the respective interfaces. The device may represent, for example, a mobile terminal, in particular a mobile phone, a Personal Digital Assistant (PDA) or a computer equipped with a network card, operating in a communication system according to the GPRS standard or a derivative thereof, or may form part of such a terminal.

A further proposed method for controlling the flow of a number of Data Units (DUs) over a logical link comprises the steps of: transmitting first-type Data Units (DUs) over respective first interfaces of at least two first-type Link Control (LC) devices; converting the first-type DUs into second-type DUs and vice versa in the respective first-type LC devices; transmitting the second-type DUs over the respective second interfaces of the first-type LC means; -transferring said second-type DUs over respective interfaces of second-type LC means, wherein said second interface of each of said first-type LC means is directly connected to a respective interface of said second-type LC means; and respectively control the respective flows of the second-type DUs that are communicated over the respective interfaces of said second-type LC means.

According to the method of the present invention, it is preferred that said step of converting said first-type DUs into second-type DUs and vice versa comprises at least one frame check function and a ciphering function.

According to the method of the present invention, it is preferred that at least two first-type LC means are distinguishable by the characteristics of the first-type DUs that are transferred over their respective first interfaces, and that said characteristics of the first-type DUs to or from which said second-type DUs are converted are taken into account in said step of separately controlling the respective flows of the second-type DUs over said respective interfaces.

According to the method of the present invention, it is preferred that the method further comprises the step of buffering second-type DUs transferred over said respective interfaces of said second-type LC means in at least one DU buffer.

According to the method of the present invention, it is preferred that in said step of separately controlling respective flows of second-type DUs over said respective interfaces, said characteristics of first-type DUs to or from which said second-type DUs are converted are taken into account, and the state of said at least one DU buffer is taken into account.

According to the method of the present invention, it is preferred that the method further comprises the step of evaluating the status of the physical transmission link on which the second-type DUs are transmitted.

According to the method of the present invention, it is preferred that in said step of separately controlling said respective flows of second-type DUs over said respective interfaces, said characteristics of first-type DUs to or from which said second-type DUs are converted are taken into account, and said evaluation state of said transmission link is taken into account.

According to the method of the present invention, it is preferred that said step of separately controlling said respective flows of the second-type DUs over said respective interfaces comprises the steps required for running a flow control protocol.

According to the method of the present invention, it is preferred that said flow control protocol is an X on/X off flow control protocol.

According to the method of the present invention, it is preferred that said characteristic is a quality of service (QoS) requirement of said first-type DUs.

According to the method of the present invention, it is preferred that the method further comprises the step of controlling said first type LC means by means of a link management means.

According to the method of the present invention, it is preferred that said logical link is a logical link between a Mobile Station (MS) and a Serving GPRS Support Node (SGSN) in a mobile radio system operating according to the General Packet Radio Service (GPRS) standard or a derivative thereof, said first type LC means is a Logical Link Control (LLC) entity of the GPRS LLC protocol with additional sequence-based frame check and ciphering functions, said first interface is a serving access point (LLC-SAP) of the LLC entity, said second type DU is an LLC frame, said interface of said second type LC means is a Radio Link Control (RLC) -SAP, said first type DU is an N-PDU of the GPRS network layer protocol, and said second type LC means implements the functions of the RLC protocol and further performs the step of controlling the respective flows of the GPRS frames LLC frame, respectively, based on a consideration of the characteristics of the N-PDU from which said LLC frame is converted or converted, the respective flows of LLC frames are transmitted over at least two RLC-SAPs.

Furthermore, a computer program product is proposed, wherein said computer program product is directly loadable into the internal memory of a digital computer and comprises software code portions for performing the method steps of the above-mentioned method claims when said product is run on a computer.

Drawings

These and other features of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the drawings:

FIG. 1 is a generalized view of the lower three protocol layers of a protocol stack of a prior art General Packet Radio Service (GPRS) system;

fig. 2 is a detailed view of the structure of the LLC layer and the RLC layer of a GPRS protocol stack of the prior art, as seen from the Mobile Station (MS) side;

fig. 3 is a detailed view of the structure of the LLC layer and the RLC layer of the GPRS protocol stack as seen from the MS side according to the present invention;

fig. 4 is a detailed view of the structure of the RLC layer as seen from the MS side according to the present invention.

Detailed Description

Fig. 3 depicts the structure of the Logical Link Control (LLC)4a and Radio Link Control (RLC)5a layers of the GPRS protocol stack, as seen from the MS1 side, including the modifications according to the invention. Modifications to the prior art protocol stack as described in fig. 1 and 2 only affect the LLC layer 4a and the RLC layer 5 a. For a description of the network layer 7 above the LLC layer 4a and the Medium Access Control (MAC) layer 8 below the RLC layer 5a, reference is made to the corresponding description of the prior art in fig. 1 and 2.

Compared to the prior art protocol stack of fig. 2, the protocol stack according to the present invention comprises a modified LLC layer 4a, which removes the multiplexer 42. The function of the multiplexer is transferred to modified LLC entities (LLE)40-1a … 40-8a, each of which is connected to one LLC service access point (LLC-SAP)43-1 … 43-8, respectively. The modified LLE40-1a … 40-8a is no longer connected to the prior art multiplexer, but is instead connected to the MAC-SAP50-1a … 50-8a, respectively. The modified LLE40-1a … 40-8a is still controlled by the LLC management entity 41.

User data packets, such as Internet Protocol (IP) data packets, are adapted to the services provided by the LLC layer 4a via the subnet independent convergence protocol (SDNCP). The SDNCP uses network protocol data units (N-PDUs) to communicate information of user data packets between network layer peer entities. The transfer of N-PDUs between network entities uses a QoS dependent LLC connection provided by the modified LLE40-2a … 40-5a and made available to the network layer through LLC-SAP 43-2 … 43-5. In the LLC layer 4a, the N-PDUs are transmitted as LLC service data units (LLC-SDUs) as part of LLC-PDUs (LLC frames). The LLC PDUs are transferred between peer LLC entities located at MS1 and SGSN2, respectively, using a connection provided by the bottom RLC layer. Again, LLC-PDU is transmitted as LLC-SDU as part of RLC-PDU.

Since the prior art multiplexer 42 is omitted, the LLC frames are no longer multiplexed, so there is one RLC-SAP 50-1a … 50-8a for each modified LLE40-1a … 40-8 a. Frame check and encryption, which is the task of the multiplexer 42 in the prior art, is now performed by the modified LLE40-1a … 40-8a, i.e., inserting and encrypting a frame check sequence at frame transmission and decrypting and checking a frame check sequence at frame reception.

The modified RLC layer 5a now advantageously controls the flow of LLC frames, respectively, corresponding to the different QoS requirements of the N-PDUs associated therewith. This is achieved by running the X on/X off (Xon/Xoff) protocol between the modified RLC layer 5a and the modified LLE40-1a … 40-8 a. The modified RLC layer 5a can thus select from which modified LLE buffer to receive further LLC frames by sending an X on ("send more LLC frames") or an X off ("no more LLC frames are sent") message to the modified LLE40-1a … 40-8a accordingly. The modified RLC layer 5a comprises at least one buffer for buffering RLC frames to be delivered to the MAC layer 8 for transmission on the physical propagation channel 9, and dominating means for evaluating the current state of said physical transmission channel 9. Knowing the status of the physical transport channel 9, the status of its buffers and the QoS requirements of the respective LLC frames that can be polled separately from the respective buffers in the respective modified LLE40-1a 3540-8 a, the modified RLC layer 5a can adapt the transmission of LLC frames over said physical channel 9 to the QoS requirements of LLC frames. When the channel is bad and the buffer starts to fill more and more, it is suggested to suspend the transmission of LLC frames with lower QoS requirements and concentrate on the transmission of LLC frames with higher QoS requirements. This flow control for individual LLC frames is performed on the uplink and downlink. This reduces the number of service primitives required for communication between the protocol entities and thus helps to reduce MCU load. Further, the use of buffers is also reduced. Due to the improved flow control between the LLC 4a and the RLC layer 5a, the flow control of N-PDUs transmitted between network entities located in the MS1 and the SGSN2 using a special QoS requirement LLC connection becomes more granular and more suitable for the state of the physical transmission link 9.

Figure 4 gives a detailed view of one possible implementation of the RLC layer 5a of the GPRS protocol stack according to the present invention. In the modified RLC layer 5a, an RLC buffer 51-2a … 51-5a is provided for buffering RLC frames (containing LLC frames) to be delivered to the MAC layer 8. In this exemplary implementation, the RLC buffers 51-2a … 51-5a are provided to respective RLC-SAPs 50-2a … 50-5a, the respective RLC-SAPs 50-2a … 50-5a provide service to respective modified LLEs 40-2a … 40-5a, and the respective modified LLEs 40-2a … 40-5a in turn provide service to SNDCP7-3 via respective LLC-SAPs 43-2 … 43-5. RLC buffer 51-2a … 51-5a is controlled by flow control instance 52 a. This flow control instance runs an X on/X off protocol between the RLC buffer 51-2a … 51-5a and the respective modified LLE40-2a … 40-5a, so that the modified RLC layer 5a can choose from which modified LLE a further LLC frame is received based on knowledge of the state of the physical transport channel 9, which is provided to the flow control instance by the channel state instance 53 a. The channel state instance 53a may determine the state of the physical transmission itself or receive information about the state of the physical transmission channel from higher or lower protocol layers.

The invention has been described above by means of only one of a number of possible embodiments. It should be noted, however, that there are alternative methods that will be apparent to those skilled in the art and that may be implemented without departing from the scope and spirit of the appended claims, e.g., the present invention may be used to improve flow control in other, but not necessarily wireless, communication systems to overcome the bottleneck of multiplexers in a multi-layer protocol stack. Various flow control mechanisms between the protocol layers are possible with advantage. In particular, it is also possible to have a hybrid system, wherein a flow control of bundles of multiplexed LLC frames and non-multiplexed LLC frames is performed.

Claims (24)

1. An apparatus for controlling a flow of a number of data units, DUs, over logical links, comprising:

at least two link-control LC means of a first type, wherein each first-type LC means is adapted to convert a first-type DU into a second-type DU and to provide a first interface to said first-type DU and a second interface to said second-type DU, and

-second-type LC means having interfaces to said second-type DUs, wherein the second interface of each of said first-type LC means is directly connected to a respective interface of said second-type LC means, and wherein said second-type LC means comprises means for separately controlling individual flows of the second-type DUs that are transferred over their respective interfaces, wherein the means for separately controlling individual flows of the second-type DUs comprises means for running a flow control protocol adapted to communicate with individual first-type LC means for requesting further second-type DUs from said first-type LC means or preventing further transmission of second-type DUs from said first-type LC means.

2. The apparatus according to claim 1, wherein said first-type LC means is adapted to convert said first-type DUs into second-type DUs by performing at least a frame check function and a ciphering function.

3. Apparatus according to claim 1 or 2, wherein

At least two first-type LC means are distinguished by the characteristics of the first-type DUs transmitted through their respective first interfaces, and

said means for separately controlling respective flows of second-type DUs that are communicated over said respective interfaces of said second-type LC means is adapted to control the respective flows of said second-type DUs based on characteristics of first-type DUs to which said second-type DUs are converted.

4. An arrangement according to claim 3, wherein said second-type LC means comprises at least one DU buffer for buffering second-type DUs transmitted over their respective interfaces.

5. The apparatus according to claim 4, wherein said means for separately controlling respective flows of second-type DUs that are communicated over said respective interfaces of said second-type LC means is adapted to control respective flows of said second-type DUs based on characteristics of first-type DUs to which said second-type DUs are converted, and based on a state of said at least one DU buffer.

6. An arrangement according to claim 3, wherein said second-type LC means comprises means for evaluating the state of a physical transmission link over which said second-type DUs are transmitted.

7. The arrangement according to claim 6, wherein said means for separately controlling respective flows of second-type DUs that are transferred over said respective interfaces of said second-type LC means is adapted to control the respective flows of said second-type DUs based on characteristics of first-type DUs to which said second-type DUs are transferred, and based on a state of a physical link over which said second-type DUs are transferred.

8. The apparatus of claim 1, wherein the flow control protocol is an X on/X off flow control protocol.

9. An arrangement according to claim 3, wherein said characteristic is a quality of service, QoS, requirement of said first-type DUs.

10. An arrangement according to any of claims 1-2, wherein further comprising link management means adapted to control said first type LC means.

11. The apparatus according to claim 3, wherein said logical link is a logical link between a mobile station MS and a serving GPRS support node SGSN in a mobile radio system operating according to the general packet radio service GPRS standard or a derivative thereof, wherein said first type LC means is a logical link control LLC entity of the GPRSLCs protocol with additional sequence based frame checking and ciphering functions, wherein said first interface is a serving access point LLC-SAP of the LLC entity, wherein said second type DU is an LLC frame, wherein said interface of said second type LC means is a radio link control RLC-SAP, wherein said first type DU is an N-PDU of the GPRS network layer protocol, and wherein said second type LC means implements the functions of the GPRS RLC protocol and further comprises said means for separately controlling the respective flows of the LLC frames, the respective flow of LLC frames is transmitted via said respective RLC-SAP.

12. A communication system comprising an apparatus according to any one of claims 1-7.

13. A method for controlling the flow of a number of data units, DUs, over logical links, comprising the steps of:

controlling respective first interfaces of the LC means to transmit first type data units DU over at least two first type links;

-converting said first-type DUs into second-type DUs in respective first-type LC means;

transmitting the second-type DUs over the respective second interfaces of the first-type LC means; and

-transferring said second-type DUs over respective interfaces of second-type LC means, wherein said second interface of each of said first-type LC means is directly connected to a respective interface of said second-type LC means; and is

Respective flows of second-type DUs are controlled by running a flow control protocol adapted to communicate with respective first-type LC means for requesting further second-type DUs from said first-type LC means or preventing further transmission of second-type DUs from said first-type LC means, respectively, which respective flows are communicated over respective interfaces of said second-type LC means.

14. The method of claim 13, wherein said step of converting said first-type DUs to second-type DUs comprises at least one of a frame check function and a ciphering function.

15. A method according to claim 13 or 14, wherein

At least two first-type LC devices are distinguished by characteristics of first-type DUs transmitted through their respective first interfaces, an

The step of controlling respective flows of second-type DUs over said respective interfaces is based on said characteristics of first-type DUs to which said second-type DUs are converted.

16. The method according to claim 15, wherein said method further comprises the step of buffering second-type DUs transferred over said respective interfaces of said second-type LC means in at least one DU buffer.

17. The method of claim 16, wherein the step of separately controlling respective flows of second-type DUs over said respective interfaces is based on said characteristics of first-type DUs to which said second-type DUs are converted, and on a state of said at least one DU buffer.

18. The method according to claim 15, wherein said method further comprises the step of evaluating the status of the physical transmission link on which the second-type DUs are transmitted.

19. A method according to claim 18, wherein the step of separately controlling said respective flows of second-type DUs over said respective interfaces is based on said characteristics of first-type DUs to which said second-type DUs are converted and on said evaluation state of said transmission link.

20. The method of claim 14, wherein the flow control protocol is an X on/X off flow control protocol.

21. The method according to claim 15, wherein said characteristic is a quality of service, QoS, requirement of said first-type DUs.

22. A method according to any of claims 13-14, further comprising the step of controlling said first type LC means by link management means.

23. The method according to claim 15, wherein said logical link is a logical link between a mobile station MS and a serving GPRS support node SGSN in a mobile radio system operating according to the general packet radio service GPRS standard or a derivative thereof, wherein said first type LC means is a logical link control, LLC, entity of the GPRS LLC protocol with additional sequence-based frame checking and ciphering functions, wherein said first interface is a serving access point, LLC-SAP, of the LLC entity, wherein said second type DU is an LLC frame, wherein said interface of said second type LC means is a radio link control, RLC-SAP, wherein said first type DU is an N-PDU of the GPRS network layer protocol, and said second type LC means implements the functions of the GPRS RLC protocol and further performs the step of separately controlling the respective flows of the LLC frame based on the characteristics of the N-PDU into which said LLC frame is converted or derived, the respective flows of LLC frames are transmitted over at least two RLC-SAPs.

24. An apparatus for controlling a flow of a number of data units, DUs, over logical links, comprising:

means for controlling the respective first interfaces of the LC means to transfer first type data units DU over at least two first type links;

means for converting said first-type DUs to second-type DUs in respective first-type LC means;

means for transmitting said second-type DUs over respective second interfaces of said first-type LC means; and

means for transferring said second-type DUs over respective interfaces of second-type LC means, wherein said second interface of each of said first-type LC means is directly connected to a respective interface of said second-type LC means; and is

Means for separately controlling respective flows of second-type DUs, which are communicated over respective interfaces of said second-type LC means, wherein the means for separately controlling respective flows of second-type DUs comprises means for running a flow control protocol adapted to communicate with respective first-type LC means for requesting further second-type DUs from said first-type LC means or preventing further transmission of second-type DUs from said first-type LC means.