HK1030510B - Point-to-multipoint mobile radio transmission - Google Patents

Description

Point-to-multipoint mobile radio transmission

The present invention relates to point-to-multipoint wireless transmission and is particularly, although not necessarily, applicable to the General Packet Radio Service (GPRS) radio protocol recommended for mobile radio communications.

Popular digital cellular telephone systems, such as GSM (global system for mobile communications), are designed to emphasize voice communications. Data is typically transmitted over the air interface between a Mobile Station (MS) and a Base Station Subsystem (BSS) using a so-called circuit-switched transmission mode in which a physical channel, i.e. a series of evenly spaced time slots on one or more frequencies, is reserved for the duration of a call. For voice communications, the information streams to be transmitted are relatively coherent, and circuit switching transmission modes is physically efficient. However, during data calls, such as Internet access, the data stream is "bursty" and maintaining a physical channel for a long time in circuit switched mode appears to be an uneconomical use of an air interface.

The demand for data services for digital cellular telephone systems is growing rapidly and a new GSM-based service, known as General Packet Radio Service (GPRS), is currently being standardized by the European Telecommunications Standards Institute (ETSI) and specified in the general column of GSM 03.60. GPRS provides dynamically allocated physical channels for data transmission. That is, a physical channel is assigned to a particular MS-to-BSS communication line only when there is data to be transmitted. Unnecessary physical channel reservation when there is no data to transmit is avoided.

GPRS is expected to work with conventional GSM circuit switched transmissions in order for data and voice communications to efficiently utilize the air interface. GPRS thus utilizes the basic channel structure specified for GSM. In GSM, a given frequency band is divided into successive frames in the time domain, called TDMA (time division multiple access) frames. The length of the TDMA frame is 4.615 ms. Each TDMA frame is in turn divided into 8 successive time slots of equal duration. In conventional circuit switched transmission mode, when a call is initiated, a physical channel is defined for the call by reserving a given time slot (1 to 8) in each successive TDMA frame. A succession of 4 consecutive time slots on a physical channel is called a radio block and represents the shortest transmission unit for packet-switched data on a physical channel. Physical channels are likewise defined for conveying signal information. With the introduction of GPRS, physical channels will be dynamically assigned to switched circuit transmission modes or packet switched transmission modes. When the network requirements for switching circuit transmission modes are high, a large number of physical channels can be reserved for this mode. Otherwise, when the demand for GPRS transmission is high, a large number of physical channels may be reserved for this mode. In addition, a high speed packet switched transmission channel may be provided by allocating two or more time slots in each successive TDMA frame to a single MS.

For GSM Phase 2⁺The GPRS radio interface of (GSM 04.65) can be designed to have a hierarchy of functional specific logical layers as shown in fig. 1, where the Mobile Station (MS) and the network have the same hierarchy communicating over the MS/network interface Um. Each layer formats data received from adjacent layers, receives data from the bottom layer to the top layer, and transmits data from the top layer to the bottom layer.

On the top layer are many Packet Data Protocols (PDPs). Some of these PDPs are point-to-point protocols (PTP) adapted to transmit packet data from one MS to another MS or from one MS to a fixed terminal. Examples of PTP protocols are IP (Internet access protocol) and x.25. All PDPs utilize a common sub-network dependent convergence protocol (SNDCP), as its name suggests, to transform (or 'converge') the different PDPs into a common form (consisting of SNDCP units) suitable for further processing in a transparent manner. This architecture means that newly developed PDPs in the future can be easily incorporated into the existing GPRS architecture.

SNDCP specifies multiplexing and fragmentation of user data, data compression, TCP/TP header compression, and transmission according to the required quality of service. SNDCP units have approximately 1600 octets and comprise an address field containing a Network Service Access Point Identifier (NSAPI) for identifying the end connection, e.g. IP, x.25. Each MS may be assigned to a set of NSAPs that are not associated with other MSs.

There are also other GPRS end protocols at the top level, such as SMS and signaling (L3M). Each SNDCP (or other GPRS end protocol) unit is carried by one Logical Link Control (LLC) frame over the radio interface. The LLC frame system is set forth in the LLC layer (GSM 04.64) and includes a header frame with numbering and temporary addressing areas, variable length information areas, and a frame check sequence. More specifically, the addressing region includes a Service Access Point Identifier (SAPI) identifying the particular end of connection (and its associated priority and quality of service (QOS)) on the LLC interface network side and user side. One ligation end is SNDCP. Other endpoints include Short Message Service (SMS) and management layers (L3M). The LLC layer provides a convergence protocol for these different end protocols. The allocation of SAPI is constant and common to all MSs.

The Radio Link Control (RLC) layer specifies, among other things, the steps of segmenting and reassembling logical link control layer PDUs (LLC-PDUs) into RLC data blocks, and the steps of retransmitting unsuccessfully transmitted RLC blocks. The Medium Access Control (MAC) layer operates above the physical link layer (see below) and specifies the steps to enable multiple MSs to share a common transmission medium. The function of the MAC is to arbitrate between multiple MSs attempting to transmit simultaneously and to provide steps for collision avoidance, detection and recovery.

Link provides a physical channel between the MS and the network. The physical radio frequency layer (phys. rf) specifies, among other things, the carrier frequency and GSM radio channel structure, the modulation of the GSM channels, and the transmitter/receiver characteristics.

For GPRS transmission, three different changeable management states are specified: IDLE, STANDBY, and READY. An IDLE-state MS does not "attach" to GPRS, so the network is not aware of this MS. However, the MS is listening for broadcast control messages at all times, e.g., to determine the selection of a network cell. The MS in STANDBY state is GPRS-attached, with its location (routing area) tracked by the network. However, no data is transmitted. The MS is in READY state while data is being sent and for a while thereafter. So that MSs in READY state are also tracked by the network. As currently proposed, there are 16 unique NSAPI codes available to identify a PDP. The NSAPI codes are dynamically allocated by the network, so the MS must be either in STANDBY state or in READY state in order to know the allocated codes. As currently proposed, an MS in IDLE state cannot receive transmissions in any PDP. For PDPs, such as IP and x.25, this is not a problem, since when such a transmission occurs, the MS will always be in either STANDBY or READY state.

In addition to PTP, it is possible that other PDPs, in particular point-to-multipoint (PTM) transmissions, are specified in the future when GSM is derived, wherein data is transmitted to a group of MSs (PTM-G, point-to-multipoint-group call) or to all mobile stations within an area (PTM-M, point-to-multipoint-multicast). The use of such PDPs includes operator announcements, advertising, and specific information transfer, such as the results of football games, news, etc. PTP-G is similar to PTP in the sense that an MS must be in either STANDBY or READY state to receive a transmission. However, as it is required (as specified in GSM 03.60 chapters 6.1-6.2 and 14.2) that the MS receive PTM-M transmissions in all states, including IDLE state, a hitherto unrecognized problem arises with PTM-M. Because no PDP context is valid when the MS is in the IDLE state and the NSAPI code assigned by the network is dynamic, it is impossible for the IDLE MS to assign the correct NSAPI code to the PTM-M and therefore cannot receive the PTM-M.

Although GPRS has been discussed above in relation to GSM, it should be noted that GPRS has broader applicability. For example, GPRS is applicable to the proposed third generation standard umts (universal Mobile telecommunications system) by changing only the low level radio protocols.

It is an object of the present invention to overcome the above mentioned problems. In particular, it is an object of the present invention to enable a mobile station to receive PTM-M even when the MS is in an IDLE state.

According to a first aspect of the present invention there is provided a method of operating a mobile communications system which supports radio data transmissions between a Mobile Station (MS) and a network in a plurality of different Packet Data Protocols (PDPs), including a point-to-multipoint-multicast (PTM-M) protocol, wherein the protocol is identified by protocol identifiers communicated between the network and the mobile station, the method comprising constantly allocating a unique protocol identifier to PTM-M transmissions and dynamically allocating other identifiers to other packet data protocols.

Preferably, the data is formatted for transmission in a sub-network dependent convergence protocol (SNDCP). SNDCP formats data into one of a number of different Packet Data Protocols (PDPs) for transmission through the system, and vice versa for receiving data. SNDCP processes data into SNDCP units, each of which contains a Network Service Access Point Identifier (NSAPI) for the SNDCP, identifying the PDP NSAPI in use that can provide the protocol identifier. Typically, the NSAPI has one of values 0 to 15, and one of these values is constantly assigned to the PTM-M.

Data for transmission and reception may be formatted by a Logical Link Control (LLC) layer below the SNDCP layer. LLC formatting includes using a Service Access Point Identifier (SAPI) to identify service access points on the network side and user side of the LLC layer. The SAPI may provide the protocol identifier.

The invention is particularly applicable to GPRS as specified for GSM networks. However, it can also be used in other systems such as GPRS for UMTS.

According to a second aspect of the present invention there is provided apparatus for carrying out the method of the first aspect of the present invention above.

According to a third aspect of the present invention there is provided a mobile communications device arranged to support the method of the first aspect of the present invention above, the device comprising a memory in which is stored a constantly allocated PTM-M protocol identifier, and signal processing means for determining when a transmission from the network contains said PTM-M protocol identifier and for sequentially receiving and processing said transmissions.

The above embodiments of the third aspect of the invention include a mobile cellular telephone and a combined mobile telephone/personal digital assistant device.

For a better understanding of the present invention and to show how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:

figure 1 shows the protocol layers of a GPRS radio link of the network of figure 1;

FIG. 2 schematically illustrates the architecture of a GSM/GPRS digital cellular telephone network;

FIG. 3 illustrates the upper layers of the protocol of FIG. 1 in more detail; and

FIG. 4 illustrates a modification to the architecture shown in FIG. 3.

The basic 'architecture' of a GSM cellular network supporting GPRS is shown in figure 2. The terms used in fig. 2 are by conventional definition in the list given below. Other items used in this description are also defined.

The general architecture of the GPRS protocol layers has been described above with reference to figure 1. The invention is mainly related to the upper layers of the architecture, such as RLC, LLC and layer 3 entities, which are shown separately in fig. 2. The layer 3 entities shown are signaling, SMS, and packet data protocols IP and X.25 (two PTPs), PTM-G, and PTM-M.

The LLC layer formats the data into LLC frames, each containing a Data Link Connection Identifier (DLCI), and one SAPI (with values from 0 to 15) at a time. As already described above, SAPI identifies service access points on the network side and the user side of the LLC layer. The SAPI has a predefined value, known to the network and the MS listening, so the LLC layer can correctly route received transmissions even in IDLE state. Consider, for example, the case where one transmission is received by one MS. The LLC layer selects the correct service access point, i.e., signaling, SMS, or SNDCP, based on SAPI.

In the case where the SAPI identifies SNDCP, the data is processed according to SNDCP. Each SNDCP data unit in turn contains a NSAPI identifying the particular PDP, i.e. IP, x.25, PTM-G, or PTM-M, being used. The NSAPI may have a value from 0 to 15, represented by a four-bit binary code. Unlike SAPIs, which are constantly assigned, NSAPIs for IP, x.25, and PTM-G are dynamically assigned by the network. The MS is informed of this dynamic allocation by a signaling message. However, they are only received by MSs in either STANDBY or READY state.

One NSAPI is constantly assigned to the PTM-M PDP, which is known to the MS and the network. Since this PTM-M NSAPI is pre-stored in the memory of the MS with SAPI. In the case where the MS is in the IDLE state, the received SNDCP unit is routed from the LLC layer to the SNDCP, and the NSAPI of this unit is read to determine whether it corresponds to the PTM-MNSAPI. If so, SNDCP processes the unit accordingly and uses the PTM-MPDP. If the NSAPI does not correspond to the PTM-M NSAPI, no further processing is performed, since the PDP used cannot be identified.

Fig. 4 shows a modification of the protocol structure shown in fig. 3. This is based on PTM-M transmissions that are not transported over the SNDCP layer. Rather, these transmissions are passed directly from the LLC layer to the PT-M layer. In this case, a PTM-M transmission may be identified by constantly assigning a SAPI to the PTM-M transmission.

BSC base station controller

BSS base station subsystem

BTS basic transceiver station

GGSN entrance GPRS support node

GPRS general packet radio service

GSM global mobile communication system

HLR internal location register

IP internet protocol

L3M layer 3 management

LLC logical link control

MAC medium access control

MS mobile station

MSC mobile switching center

NSAPI web service access point identifier

PC/PDA personal computer/personal data assistant

PDP packet data protocol

PDU data unit

PSTN public switched telephone network

PTM-G point-to-multipoint group

PTM-M point-to-multipoint multi-cast

PTP point-to-point

RLC radio link control

SAPI service access point identifier

SGSN serving GPRS support node

SMS short message service

SNDCP sub-network dependent convergence protocol

SS7 Signaling System No. 7

TCP/IP

TDMA time division multiple access

Um mobile station to network interface

UMTS universal mobile telecommunications service

X.25 network layer protocol specification

Claims (5)

1. A method for operating a mobile communication system supporting radio data transmission between a Mobile Station (MS) and a network under a plurality of different Packet Data Protocols (PDPs) including a point-to-multipoint-multicast (PTM-M) protocol, wherein the protocol is identified by a protocol identifier (SAPI, NSAPI) communicated between the network and the mobile station, characterized in that the method comprises constantly assigning a unique protocol identifier to the PTM-M transmission and dynamically assigning other identifiers to other protocols.

2. A method as claimed in claim 1, characterized in that the method forms part of a General Packet Radio Service (GPRS).

3. A method as claimed in claim 2, characterized in that the data is formatted for transmission in accordance with a sub-network dependent convergence protocol (SNDCP), the data being formatted for transmission in one of a plurality of different Packet Data Protocols (PDPs) for transmission through the system and vice versa for reception of the data, the SNDCP processing the data in SNDCP units, each unit containing a Network Service Access Point Identifier (NSAPI), the SNDCP being identified by the PDP in use, and the NSAPI providing said protocol identifier.

4. A method according to claim 2, characterized in that the data for transmission and reception is formatted by a Logical Link Control (LLC) layer below the SNDCP layer, the LLC formatting including identifying service access points on the network side and the user side of the LLC layer using a Service Access Point Identifier (SAPI) and the SAPI providing said protocol identifier.

5. A mobile communication device for use in a system supporting radio data transmission between a Mobile Station (MS) and a network under a plurality of different Packet Data Protocols (PDPs), including a point-to-multipoint-multicast (PTM-M) protocol, wherein said protocol is identified by a protocol identifier (SAPI, NSAPI) transmitted between the network and the mobile station, characterised in that the device comprises a memory for storing constantly allocated PTM-M protocol identifiers (SAPI, NSAPI), and signal processing means for determining when a transmission from the network contains said PTM-M protocol identifier and for sequentially receiving and processing said transmission.