RLC/MAC PROTOCOL

Abstract: In the LAPDm a short message service is implemented by pre-emption of every second signalling message when there is a short message to be sent. Same pre-emption mechanism is also needed when the RLC/MAC protocol is used on the data link layer instead of the LAPDm. The mechanism is provided by defining at least two different identities to be used with frames transmitted on the signalling channel; and pre-empting frames on the MAC at least on the basis of the identity relating to a frame. The identities are preferably identified on the basis of the signalling radio bearer of the frame.

Description: RLC/MAC PROTOCOL FIELD OF THE INVENTION The present invention relates to implementing a message service in which user data may be sent between actual signalling, such as a short mes- sage service, with RLC/MAC protocol (Radio Link Control/Media Access Control) in a mobile communication system, and particularly to implementing the message service with RLC/MAC protocol on a SACCH (Slow Associated Control CHannel) with a DPSCH (Dedicated Physical Sub-CHannel). The SACCH is an associated control channel of which one frame is available for control signalling in the direction that the assigned channel is carrying traffic. BACKGROUND OF THE INVENTION The mobile communication system generally refers to any telecommunication system which enables wireless communication when a user is located within the service area of the system. Examples of such systems are cellular mobile communication systems, such as the GSM (Global System for Mobile communications), or corresponding systems, such as the PCS (Personal Communication System) or the DCS 1800 (Digital Cellular System for 1800 MHz), third generation mobile systems, such as the UMTS (Universal Mobile Communication System) and systems based on the above mentioned systems, such as GSM 2+ systems and the future 4 th generation systems. One typical example of a mobile communication system is the public land mobile network PLMN. In addition to normal speech and data connections set up on traffic channels in digital mobile communication systems, short digital data messages comprising user data can be transmitted between actual signalling on control and signalling channels of a system. The main reason for this is to avoid frame stealing from the traffic channel. These short digital data messages comprising user data are generally referred to as short messages. In the GSM, control channels used for short message transmissions are the SACCH used for transmission of measurements performed by a mobile station MS during the connection and a SDCCH (Stand alone Dedicated Control Channel) carrying out the actual signalling. If a mobile station has a connection on a traffic channel, short messages are forwarded on the SACCH, otherwise they are forwarded on the SDCCH. In the GSM, short message service has been implemented by using LAPDm (Link Access Protocol for the Dm channel) protocol in the data link layer, i.e. layer 2. The LAPDm provides services to upper layer, layer 3, via Service Access Points (SAP), which are identified by a Service Access Point Identifier (SAPI). The LAPDm has two different values for SAPI. Signalling is transferred in SAPI=0 frames using unacknowledged mode operation on the SACCH associated with a traffic channel. Short messages are transferred in SAPI=3 frames using acknowledged mode operation on the SACCH associated with a traffic channel. The LAPDm acknowledged mode operation priori- tizes messages from layer 3 entity so that if a SAPI=3 frame, i.e. a short message, is awaiting for transmission, two SAPI=0 frames are not sent in consecutive frames. It is also ensured that at least one SAPI=0 frame is transmitted after a SAPI=3 frame. In other words, short messages are transmitted on SACCH associated with a traffic channel by pre-emption of every second sig- nailing frame, i.e. SAPI=0 frame. However, in GSM 2+ -system called GERAN (GSM/EDGE (Enhanced Data rates for Global Evolution) Radio Access Network) Release 5, it has been suggested to use the RLC/MAC protocol instead of the LAPDm in order to get the protocol stacks less complicated. One of the problems associ- ated with the replacement of LAPDm with the RLC/MAC is, that there is no support for the above illustrated pre-emption mechanism in the RLC/MAC. BRIEF DESCRIPTION OF THE INVENTION An object of the present invention is thus to provide a method and a system for implementing the method so as to solve overcome the above problem. The object of the invention is achieved by a method and a system which are characterized by what is stated in the independent claims. The preferred embodiments of the invention are disclosed in the dependent claims. The invention is based on realizing the problem and providing the RLC/MAC protocol a mechanism for pre-empting the signalling frames by de- fining at least two different identifiers, preferably signalling radio bearer identifiers, the first indicating a signalling message and the second indicating a possible short message and modifying the protocol to pre-empt every second frame with the first identifier. An advantage of the invention is that it allows transmission of short messages as if the LAPDm would be used and thus there is no need to steal speech frames for the short message transmission, for example. Another advantage is that the physical layer of SACCH can be kept as it is. BRIEF DESCRIPTION OF THE DRAWINGS In the following the invention will be described in greater detail by means of preferred embodiments with reference to the attached drawings, in which Figure 1 illustrates basic parts of a communication system; Figure 2 shows a protocol architecture according to a first preferred embodiment of the invention; and Figure 3 is a flow chart illustrating the operations of the MAC according to the first preferred embodiment of the invention. DETAILED DESCRIPTION OF THE INVENTION The present invention can be applied to any communication system where short message service, or corresponding user data message service transmitted between the signalling on signalling channels, is provided by the RLC/MAC protocol. Such systems include the above mentioned systems, for example. In the following the invention will be described by using a GERAN system as an example without restricting the invention thereto. Figure 1 shows a very simplified network architecture illustrating only basic parts of the communication system 1. It is obvious to a person skilled in the art that the system 1 comprises network nodes, functions and structures, which need not be described in greater detail here. A mobile station MS comprises the actual terminal and a detachably connected identification card SIM, also called a subscriber identity module. In this context, the mobile station generally means the entity formed by the subscriber identity module and the actual terminal. The MS can be any equipment capable of communicating in a mobile communication system and supporting short message service or a corresponding service. In the example of Figure 1 , the system 1 comprises a core network CN and a radio access network GERAN. The GERAN is formed of a group of radio network subsystems (not shown in Figure 1), which are connected to the core network CN via a so-called lu-interface 2. The GERAN may be a GSM/EDGE Radio Access Network and the CN may be a GSM/UMTS core network. For short message service the system 1 comprises a short mes- sage service gateway SMSGW having a connection to a short message service centre SMSC. The short message service gateway SMSGW is the common term for a Gateway Mobile Switching Centre for Short Message Service SMS-GMSC and an Interworking Mobile Switching Centre for Short Message Service SMS-IWMSC. The SMS-GMSC receives a short message from the SMSC, requests routing information and delivers the short message through the CN and the GERAN to the mobile station MS. Correspondingly the SMS- IWMSC can receive short messages from a mobile station to be forwarded to the short message service centre SMSC. Figure 2 shows the lu-interface protocol architecture of the SACCH with a DPSCH according to the first preferred embodiment of the invention. The protocol stack shown in Figure 2 is a protocol stack of a control plane. The corresponding layers and sublayers implemented according to the invention may be used in other interfaces, too. The thick lines between layers and sub-layers illustrate possible data flows and circles illustrate service access points for peer-to-peer communication. The lu-interface protocol architecture illustrated in Figure 2 comprises three protocol layers: a physical layer L1 , the data link layer L2 and a network layer L3. The data link layer L2 comprises following sublayers below a RRC (Radio Resource Control) of the layer L3: the radio link control RLC and the medium access control MAC. The RRC has a control interface (not shown in Figure 2) between the RRC and the RLC, between the RRC and the MAC and between the RRC and the L1. These interfaces allow the RRC to control the configuration of the lower layers. The RRC according to the invention will provide in the control plane at least two different signalling radio bearers 0, 3, one used among other things for short message service or corresponding user data message service and the other for signalling. Each signalling radio bearer is identified by a sig- nailing radio bearer identity SRBid, or a corresponding identity which can be mapped one-to-one with an associated signalling radio bearer. In the first and second preferred embodiment of the invention SRBid 0 corresponds to the SAP 0 of the LAPDm and SRBid 3 corresponds to the SAPI=3 of the LAPDm. The RRC of the invention will configure the RLC and the MAC ac- cordingly via the interfaces. The signalling radio bearers may be configured at RRC Connection Setup and remain until the RRC connection is released, for example. Another possibility is that the signalling radio bearers use a predefined default configuration. In other words the RRC of the invention differs from the RRC of prior art so that it reserves at least two signalling radio bearers for a connection. The RLC provides the upper layer L3 among other things acknowledged and unacknowledged data transfer. The RLC of the invention will provide acknowledged data transfer for those messages the frame header of which comprise the signalling radio bearer identity used also for short messages, i.e. SRBid 3 frames in the first and second preferred embodiment of the invention. The unacknowledged data transfer is provided for signalling messages, i.e. SRBid 0 frames in the first and second preferred embodiment of the invention. Preferably for each signalling radio bearer, i.e. for each identity, an RLC entity 4 is established as soon as there is a frame to be sent and the entity exists preferably until the corresponding DSPCH is released. The MAC handles the access to and multiplexing onto the physical sub-channels thus defining among other things logical channels to be used for each radio bearer service. The MAC is also responsible for the MS measurement reporting. For a mobile station MS there is one MAC entity in the mobile station side and one in the network side. The MAC according to the first pre- ferred embodiment of the invention will pre-empt every second frame having signalling radio bearer identity indicating a signalling message in case there is a frame having the other signalling radio bearer identity. In other words, the MAC provides in lu-mode priority handling between traffic flows on the basis of the signalling radio bearer identities. The operation of the MAC protocol in the first preferred embodiment of the invention is illustrated in Figure 3. For clarity's sake, the figure does not mention the header information and the like added by the MAC to the received information. The functionality described in the Figure 2 is performed in each cycle , i.e. in the GERAN in every 480 ms something is sent on the SACCH. If there is no measurement reports to send, a fill frame is sent. In the first preferred embodiment of the invention, it is assumed that the fill frame is generated at the MAC layer. In some other embodiments of the invention the fill frame may be generated in the RLC or in the RRC level. In the first preferred embodiment of the invention there is two inputs in the MAC, an input 0 for SRBid 0 frames and an input 3 for SRBid 3 frames, and three buffers: one input buffer called InBuffer 0 for SRBid 0 frames, an- other input buffer called InBuffer 3 for SRBid 3 frames and an output buffer called OutBuffer. The InBuffer 3 is a FIFO type buffer. Referring to Figure 3, the MAC reads the input 3 for SRBid 3 frames in step 301 , and if the input 3 is not empty (step 302), adds, in step 303, the content of the input 3 to the InBuffer 3, and reads, in step 304, the input 0 for SRBid 0 frames. The MAC will proceed to the same step 304, if the input 3 was empty (step 302). If the input 0 is not empty (step 305), the MAC adds, in step 306, the content of the input 0 to the InBuffer 0. After that the MAC checks, in step 307, if the InBuffer 3 is empty. If not, the MAC checks, in step 308, was the frame sent in the previous cycle a SRBid 0 frame. If it was, the MAC removes, in step 309, the first element in the InBuffer 3 from the InBuffer 3 to the OutBuffer, empties, in step 310, the InBuffer 0 and sends, in step 311 , the content of the OutBuffer on the SACCH. In other words the pre-emption of a SRBid 0 frame in the first preferred embodiment of the invention occurs in step 310. It is performed by a pre-emption function which can be called a pre- emptor. If the frame sent in the previous cycle was not a SRBid 0 frame (step 308), or if the InBuffer 3 is empty (step 307), the MAC removes, in step 312, the element in the InBuffer 0 from the InBuffer 0 to the OutBufferO and sends, in step 311 , the content of the OutBuffer on the SACCH. If the input 0 was empty (step 305), the MAC adds, in step 313, a fill frame to the InBuffer 0 and continues to step 307 to check, if the InBuffer 3 is empty. The fill frame is added (step 313) for continuous transmission on SACCH. In another embodiment of the invention the SRBid 3 frames are not buffered in the MAC. In the embodiment the MAC will acknowledge the corresponding RLC entity, when a SRBid 3 frame was sent and the RLC will send another SRBid 3 frame only after it received the acknowledgement. The acknowledgement may be performed between steps 309 and 310 of Figure 3, for example. Thus the MAC according to the first preferred embodiment of the invention will discard half of the SRBid 0 frames whenever there is a SRBid 3 frame to be transferred simply by replacing one SRBid 0 frame with a SRBid 3 frame. In other words, the SRBid 3 frame steals bandwidth from SRBid 0 frames on the SACCH. The MAC according to the second preferred embodiment of the invention utilizes the round robin principles, which are well known by a person skilled in the art. Thus in the second preferred embodiment of the invention the MAC sends a frame from an InBuffer 0, then from an InBuffer 3 if there exists a frame, then again a frame from the InBuffer 0, etc. In the InBuffer 0 there are SRBid 0 frames. In the InBuffer 3 there are SRBid 3 frames. The inBuffers are emptied according to the FIFO principle. Although the invention has been described above by assuming that there exist buffers in the MAC, it is obvious to one skilled in the art that the invention can be applied without buffers. Although the invention has been described by assuming that there exists only two signalling radio bearers (and signalling radio bearer identities), the invention can be applied to any pre-emption mechanisms between several radio bearers by pre-empting the low priority frame by one with higher priority. In addition to prior art means, the system implementing the functions of the present invention and the network nodes of this system comprise means for providing a pre-emption of user data send in signalling channels. More precisely, the network nodes may comprise means for implementing the functions described above. The current network nodes comprise processors and memory, which can be utilized in the functions according to the invention. All changes necessary for implementing the invention can be made as added or updated software routines, by means of application-specific integrated circuits (ASIC) and/or programmable circuits, such as EPLD, FPGA. It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.

Claims: CLAIMS 1. A method of prioritizing frames transmitted on a signalling channel in a communication system using a MAC protocol in a data link layer, characterized by the method comprising the steps of: defining at least two different identities to be used with frames transmitted on the signalling channel; and pre-empting frames on the MAC at least on the basis of the identity relating to a frame. 2. A method according to claim ^characterized by indicating by a first identity a signalling frame; indicating by a second identity a frame that may contain user data; and pre-empting every second frame relating to the first identity whenever there is a frame relating to the second identity to be transmitted. 3. A method according to claim 1 or 2, characterized by reserving the same amount of radio signalling bearers for a connection as there are defined identities; mapping each identity to a signalling radio bearer; and identifying the identity of a frame in the MAC on the basis of the signalling radio bearer of the frame. 4. A method of prioritizing frames transmitted on a signalling channel in a communication system using a MAC protocol in a data link layer, characterized by the method comprising the steps of: reserving at least two different radio bearers for a connection with the signalling channel; and pre-empting frames on the MAC at least on the basis of the radio bearer of a frame. 5. A method according to claim 4, characterized by the radio bearers being signalling radio bearers with a SACCH, and the method further comprising the step of defining at least one signalling radio bearer to be an acknowledged signalling radio bearer, which can be used to transmit user data between signalling. 6. A communication system providing a service transmitting user data on a signalling channel, the system comprising an interface in which a MAC protocol is used in a data layer characterized in that the system further comprises at least two different radio bearers (0,3) for a connection, and a pre-emptor (MAC) to pre-empt frames on the MAC at least on the basis of the signalling radio bearer of a frame. 7. A communication system according to claim 6, characterized in that at least one of the radio bearers is used to transmit also the user data and at least one of the radio bearers is used to transmit only signalling and the latter one is pre-empted. 8. A communication system according to claim 6 or 7, charac- terized in that the radio bearers are signalling radio bearers.

Inventors: LINTULAMPI RAINO, SEBIRE BENOIST, SEBIRE GUILLAUME, LINTULAMPI, RAINO, SEBIRE, BENOIST, SEBIRE, GUILLAUME
Applicants: LINTULAMPI RAINO, NOKIA CORP, SEBIRE BENOIST, SEBIRE GUILLAUME, LINTULAMPI, RAINO, NOKIA CORPORATION, SEBIRE, BENOIST, SEBIRE, GUILLAUME
Application year: 2001
Main classification: H04Q7
IPC codes: H04L12, H04L29, H04W4, H04W28, H04W48, H04W72