WO2006094087A2

WO2006094087A2 - A wireless communication system and method

Info

Publication number: WO2006094087A2
Application number: PCT/US2006/007354
Authority: WO
Inventors: Apostolis Salkintzis
Original assignee: Motorola, Inc.
Priority date: 2005-03-01
Filing date: 2006-02-28
Publication date: 2006-09-08
Also published as: GB2423887B; CA2599015A1; WO2006094087A3; EP1856927A4; EP1856927A2; KR20070112229A; CN101164353A; GB2423887A; GB0504146D0; KR100927321B1

Abstract

A wireless communication system (100, 200, 300) comprises a wireless local area network (WLAN) operably coupled to a cell-based radio communication system (160). The wireless local area network (WLAN) is adapted to support use of a control channel (360) for communication to or from the cell-based radio communication system (160). Preferably, a dual-mode wireless communication unit (112, 116) is configured as capable of communication with both the wireless local area network (WLAN) and the cell-based radio communication system (160), such that the a specific Multicast address (MCCH-mcast) and a Port Number (MCCH-port) are used on the control channel (360) for control channel communication by the dual-mode wireless communication unit (112, 116). In this manner, control channel communication is supported over a WLAN, thereby providing a wide range of new additional capabilities and benefits to end users of WLAN compatible units.

Description

WIRELESS COMMUNICATION SYSTEMS AND APPARATUS AND METHODS

AND PROTOCOLS FOR USE THEREIN

Field of the Invention

This invention relates to wireless communication systems and apparatus and methods for use therein. In particular, the invention relates to utilising a control channel over a wireless local area network (WLAN) . The invention is applicable to, but not limited to, facilitating and supporting control channel features, such as group calls over a WLAN.

Background of the Invention

Over recent years, the development in wireless communication has been dramatic. A number of wireless communications have been standardised, to facilitate inter-operability of communications between different manufacturers as well as ensure that all communication units offer the same level of performance in a particular communication field. One technology to undergo such rapid development and standardisation is Wireless Local Area Networks (WLANs) . WLANs have been targeted to provide wireless connectivity at bit rates higher than 100 Mbps . WLANs also offer the opportunity of enhanced security, enhanced mobility management, inter-working with cellular networks, etc. Thus, WLAN technology is anticipated as playing a key role in the wireless data market for many years to come .

Furthermore, WLANs are currently being enhanced to provide a guaranteed quality of service (QoS) , as can be seen in IEEE Std. 802. lle/D8.0, "Part 11: Wireless Medium

Access Control (MAC) and physical layer (PHY) specifications: Medium Access Control (MAC) Quality of Service (QoS) Enhancements", February 2004. This is a further reason as to why WLAN solutions for voice and video services are quickly emerging in the data marketplace .

Wireless communication^' systems, for example cellular telephony or private mobile radio communication systems, typically provide for radio telecommunication links to be arranged between a system infrastructure including a plurality of base transceiver stations (BTSs) and a plurality of subscriber units or terminals, often termed mobile stations (MSs) . An example of a zone/cell-based wireless communication system is a TETRA (TErrestrial Trunked Radio) system, which is a system operating according to TETRA standards and protocols as defined by the European Telecommunications Standards Institute (ETSI) . A primary focus for TETRA equipment is use by the emergency services, as TETRA provides dispatch and control services. The system infrastructure in a TETRA system is generally referred to as a switching and management infrastructure (SwMI) , which substantially contains all of the communication elements apart from the MSs. Similarly, cellular systems, such as the global system for mobile communications (GSM) , are also based on a cell-based methodology.

Communication over such wireless communication systems is typically performed over two types of logical channel - one or more traffic channels carrying data to/from the MS, and one or more control channels, carrying information relating to usage of the communication system, such as signalling information, available frequency information, timing information, etc. Different channels may be used for communication of the different forms of information.

In particular, information may represent speech, sound, data, picture or video information. Data information is usually digital information representing written words, numbers etc, i.e. the type of user information processed in a personal computer. In addition, signalling messages are communicated. These are messages relating to the communication system itself, e.g. to control the manner in which user information is communicated in compliance with the selected industry protocol such as TETRA.

For data to be transferred across communication networks via a data communication channel, a communication terminal addressing protocol is required. The communication units are generally allocated addresses that are read by a communication bridge, gateway and/or router, in order to determine how to transfer the data to the addressed destination communication unit. Such data transfer needs to be effectively and efficiently provided for, in order to optimise use of limited communication resources. Currently, the most popular protocol used to transfer data in communications systems is the Internet Protocol (IP) .

The Internet Protocol adds a data header on to the information passed from the transport layer. The resultant data packet is known as an Internet datagram. The header of the datagram contains information such as destination and source IP addresses, the version number of the IP protocol etc. Each exchange of data typically consists of sending one or more data packets on an uplink channel and one or more data packets on a downlink channel. A Packet Data CHannel (PDCH) in a TETRA network can serve several MSs at the same time. In this regard, the resources of the PDCH are then shared between the MSs on the channel on a statistical multiplex basis. This enables the air interface resources to be used in an optimal manner.

As an example, the TETRA known signalling procedure, used for accessing the PDCH, requires a MS firstly to request access to the PDCH via the main control channel (MCCH) on which control signalling messages are mainly sent. Following access approval by the SwMI and sending of an appropriate signalling message to the MS, the MS is then moved by receipt of the signalling message to the PDCH, where data packets are exchanged.

In TETRA packet data communication, physical data channels carry both system control signalling and data payload (user communicated information) . These two types of traffic may be given different priorities, with control signalling usually being allocated a higher priority. TETRA packet data communication currently operates at a maximum of 28.8kbits/sec, which is significantly less than some other wireless communication technologies, such as WLAN.

Cell-based technologies, such as private mobile radio systems or cellular systems have not operated over WLAN. One reason for this is that WLAN systems do not support control channels.

The inventor has recognised therefore that a need exists for an improved mechanism and associated apparatuses, methods and communication protocols in order to facilitate cell-based communication over a WLAN system, wherein the abovementioned disadvantages/ limitations may be alleviated.

Summary of the invention

In accordance with a first aspect of the present invention, there is provided a wireless communication system. The wireless communication system comprises a wireless local area network (WLAN) operably coupled to a cell-based communication system and arranged such that the WLAN supports the use of one or more control channels .

In accordance with second, third and fourth aspects of the present invention, there is provided respectively a wireless local area network (WLAN) access gateway (WAG) , an InterWorking Function (IWF) and a wireless terminal adapted to facilitate control channel communication over a cell-based communication system such as a private mobile radio system or a wireless cellular communication system.

In accordance with a fifth aspect of the present invention, there is provided a method of communicating using a control channel between a wireless local area network (WLAN) and a wireless cell-based system such as a private mobile radio system or a cellular phone system.

In accordance with a sixth aspect of the present invention, there is provided a protocol for facilitating the aforementioned communications between a wireless local area network (WLAN) and a wireless cell-based system.

Further features of the present invention are defined in the dependent Claims .

Brief Description of the accompanying drawings

Embodiments of the present invention will now be described by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 illustrates a schematic block diagram of a WLAN inter-operating with a TETRA Switching and Management Infrastructure (SwMI) adapted in accordance with the preferred embodiment of the present invention;

FIG. 2 illustrates a further schematic block diagram of a WLAN inter-operating with a TETRA Switching and Management Infrastructure (SwMI) , which contains the various logical interfaces;

FIG. 3 illustrates a mechanism to support main control channel (MCCH) traffic in a WLAN, in accordance with the preferred embodiment of the present invention; FIG. 4 illustrates a protocol architecture of a ToW

(TETRA-over-WLAN) terminal according to the preferred embodiment of the present invention;

FIG. 5 illustrates a preferred packet structure employed in the TETRA over WLAN architecture;

FIG. 6 illustrates a preferred packet structure of a control-plane packet transmitted by an IWF that contains a D-SETUP message; and

FIG. 7 and FIG. 8 illustrate example signalling flows of TETRA over WLAN system, according to the preferred embodiments of the present invention.

Description of embodiments of the invention

In summary, the preferred embodiment of the present invention proposes to integrate WLAN technology with a wireless cell-based communication system and utilise a control channel to control communication therebetween. The preferred embodiment of the present invention is described with reference to utilising a control channel between a WLAN and a private mobile radio system, such as a TErrestrial Trunked RAdio system, as defined by the European Telecommunication Standards Institute (ETSI) . A proposed system configuration of both a WLAN inter- operating with a TETRA switching and management infrastructure (SwMI) is illustrated in the schematic block diagram of FIG. 1. A skilled artisan will appreciate that the inventive concept of the present invention applies equally to any wireless cell-based system that is able to utilise a control channel, such as a cellular communication system.

The preferred embodiment of the present invention proposes a dual-mode wireless communication unit. The dual-mode operation utilises a first private (or public) mobile radio technology, such as TETRA, and a second WLAN technology. The wireless communication terminals, hereinafter referred to as a TETRA over WLAN (ToW) terminal 112, 116, interface with the TETRA Switching and Management Infrastructure (SwMI) 160 over a WLAN radio interface 115. This is in contrast to a conventional TETRA terminal 132 interfacing with the TETRA SwMI 160 via a conventional TETRA enhanced base transceiver station (EBTS) 134 over a conventional TETRA radio interface and communication link 135. Thus, a dual-mode TETRA and WLAN supported terminal is described.

In the context of the present invention, a Tetra-over- WLAN (ToW) terminal 112, 116 is any WLAN terminal that is configured to be able to interface with the TETRA SwMI

160 and employ TETRA services by means of the protocols and functions specified herein. ToW terminals 112, 116 preferably associate with the WLAN by using a special Service Set IDentifier (SSID) . A preferred example of an

SSID is described in IEEE standard 802.11, edition 1999, titled "Wireless LAN Medium Access Control (MAC) and

Physical Layer (PHY) Specification". By means of this special SSID, the WLAN is able to differentiate between ToW terminals and non-ToW terminals and therefore apply different routeing and/or access control policies. The WLAN 110 preferably implements a special routeing enforcement policy for ToW terminals 112, 116. That is, the WLAN tunnels uplink packets from all ToW terminals 112, 116 to an Interworking function (IWF) 150 over a Ft tunnel. Thus, a Ft interface preferably operates between the WAG 142 and the IWF 150, and is used to implement a tunnelling scheme that tunnels IP packets through an IP network 140 between the WAG 142 and the IWF 150.

Although the preferred embodiment utilises a 'Ft Tunnel', it is envisaged that any possible tunnelling scheme could be used, e.g. IP encapsulation, GRE, etc. In a case when the IWF 150 interconnects with a WAG 142 over a leased line, tunnelling can be eliminated. Thus, packets originating from any ToW terminal 112, 116 are routed to the IWF 150 via the Ft tunnel.

Every ToW terminal preferably implements the protocol architecture and the procedures specified below, in order to support TETRA services over WLAN. Physically, it is envisaged that a ToW 112, 116 may be any kind of wireless communication device with a WLAN interface, namely, a personal computer (PC) , laptop, PDA, dual-mode WLAN/TETRA terminal, etc.

From a SwMI perspective, ToW terminals 112, 116 may be considered as any TETRA terminal. That is, ToW terminals 112, 116 are preferably assigned a TETRA Individual Short Subscriber Identity (ISSI) , and thus able to initiate and participate in group calls, able to receive/send Short Data Service (SDS) messages, and, in general, able to utilise all authorized services provided by the TETRA SwMI 160. A ToW terminal 112, 116 is therefore able to communicate with other ToW terminals, with conventional

TETRA terminals, with dispatchers, PSTN users, and other TETRA entities in accordance with their subscription profile in the SwMI 160.

The ToW terminals 112, 116 preferably employ all of the known TETRA services, including group calls, short data service (SDS) messaging, packet services, etc. From a SwMI perspective, the ToW terminal 112, 116 is no different to any other conventional TETRA terminal 132.

Advantageously, the characteristics of the WLAN radio interface 115 enable extended capabilities and new features, such as high-speed data services, simultaneous voice and data, improved voice quality, reduced call setup and voice transmission delays, simultaneous reception of many group calls, monitoring of Main Control Channel (MCCH) traffic while receiving voice and/or data, etc. Thus, TETRA terminals (e.g. ToW terminals 112, 116) benefit from known advantages of WLAN technology.

The ToW terminals 112, 116 operate on a WLAN site, which can be considered as a geographical area wherein WLAN coverage is provided and it is controlled by a single WLAN Access Gateway 142. A WLAN site typically comprises one or more APs. The ToW terminals 112, 116 have a wireless interface to a WLAN access point 114. The WLAN Access Point 114 interfaces with WLAN terminals over any kind of WLAN interface, for example using IEEE 802.11 WLAN technology, as published by IEEE in the document titled "Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specification", IEEE standard 802.11, edition 1999. The WLAN access point has an interface 115 to the Internet Protocol (IP) network 140 via one or more WLAN Access Gateways 142. The IP network 140 is operably coupled to the TETRA SwMI 160 via key component of the proposed system, i.e. the Interworking Function (IWF) 150.

The IWF 150 is configured to interface 155 to the SwMI in a similar manner to a TETRA conventional base station 134. The IWF 150 interfaces also with one or more WLAN Access Gateways (WAGs) 142. In effect, the WAG 142 is a router, or a combination of router and Ethernet Switches to control a single WLAN site. The WAG interfaces with one or more APs 114 typically through an Ethernet 100BaseT medium. Preferably, one WAG is assigned for each WLAN site. The WAG 142 is preferably creating a Ft tunnel with the IWF 150 when there are ToW terminals in its WLAN site. It is envisaged that any known tunnel establishment protocol can be used, e.g. PPTP, L2TP, IPsec . The WAG 142 then applies the appropriate routeing enforcement policy. In addition, the WAG 142 is preferably releasing the Ft tunnel when there are no ToW terminals in its WLAN site, in order to free up capacity. It is envisaged that there may also be a static Ft tunnel that is not created/released dynamically.

Notably, both WAG and AP are off-the-shelf devices and their configuration is typical to known WAGs and APs, save for a signal processing function that has been adapted to support TETRA SSIDs, and route such TETRA communication according to a determination of the SSID. The IWF 150 uses known IP multicasting technology to transfer control packet data units (PDUs) and voice packets to the TETRA-over-WLAN (ToW) terminals 112, 116.

Thus, in this manner, mobile or fixed ToW terminals 112, 116 are able to access the typical services provided by the TETRA SwMI 160 by means of a WLAN network interface 115 and corresponding software drivers and applications, as will be appreciated by a skilled artisan. The ToW terminals 112, 116 re-use the majority of TETRA air interface protocols, as illustrated in the European Telecommunication Standards Institute's (ETSI) document - EN 300 392-2 v2.3.10, "Terrestrial Trunked Radio (TETRA); Voice plus Data (V+D) ; Part 2: Air Interface (AI)", ETSI, June 2003. Notably, the ToW terminals 112, 116 re-use the majority of TETRA air interface protocols on top of the WLAN radio interface. The TETRA SwMI 160 is preferably built on IP multicast and Voice-over-IP (VoIP) technologies, such that interfacing with IWF 150 is relatively straightforward, as will also be appreciated by a skilled artisan.

The InterWorking Function (IWF) 150 is a key functional element that interfaces with the TETRA SwMI 160 over any suitable interface, such as a proprietary interface, and also interfaces with one or more WAGs 142 over the Ft interface. Notably, one function of the IWF 150 is to hide from the SwMI 160 the peculiarities of the WLANs, and thus make it easier to integrate them with the SwMI 160.

As previously indicated, the preferred embodiment of the present invention proposes to integrate WLAN technology with a wireless cell-based coiπmunication system (e.g. a

PMR system such as TETRA) and utilise a control channel to control communication therebetween. The utilisation of a control channel between the WLAN and a TETRA system is described in greater detail below.

Referring now to FIG. 2, a further schematic block diagram of a WLAN inter-operating with a TETRA Switching and Management Infrastructure (SwMI) is illustrated, which contains the logical interfaces of the system of FIG. 1. As can be seen, a number of system/infrastructure elements are comparable to similar elements described with reference to FIG. 1. As such, they will not be described further here.

Again, the WLAN preferably implements a special routeing enforcement policy for ToW terminals by tunnelling uplink packets from all ToW terminals to an Interworking function (IWF) 150 over, preferably, a Ft tunnel. Thus, a Ft interface preferably operates between the WAG 142, 225 and the IWF 150. A proprietary interface 155 is illustrated between the TETRA SwMI 160 and the IWF 150. It is envisaged that the IWF 150 may also employ a logical link to ToW terminals 112, 116 over an Ut interface 210, 215. In this regard, the Ut interface 210, 215 supports the protocols and procedures that govern the communication between a ToW 112, 116 and the IWF 150. As discussed later, a new protocol operates on this logical interface 210, 215.

Also, logical interface Wt is applied to the communication link between the WAG 142, 225 and the ToW terminals. The Wt interface supports the protocols and procedures that govern the coπimunication between a ToW terminal (fixed or mobile) and an AP. This interface is preferably compliant with the IEEE 802.11 basic specification.

Referring now to FIG. 3, the use of a control channel between the WLAN and the cell-based communication system is illustrated in greater detail. As shown in FIG. 3, in every conventional TETRA site 340, there is one main control channel (MCCH) 360, over which control signalling takes place between a conventional TETRA terminal 350 and its associated serving base transceiver station (EBTS) 330. More specifically, the MCCH 360 is preferably used to carry mobility management signalling, authentication and key management signalling, call control signalling, short message signalling, and part of the packet data control signalling. It is also preferably used to broadcast cell specific and network specific information, which helps the radio terminals identify the identity of the serving cell 330, the identity of neighbour cells, such as the WLAN site 110, the service level provided by the serving and neighbour cells, etc. This broadcast information is significant for cell re-selection purposes .

Therefore, for enabling TETRA services on the WLAN it is imperative to provide a standard mechanism for transporting all type of messages that are normally transmitted on the conventional MCCH. For this purpose, a specific Multicast address and the Port Number are used (denoted as MCCH-mcast and MCCH-port respectively) in communication 310, 320, which are pre-defined and a- priory known to all ToW terminals 112, 116. The Interworking Function (IWF) 150 ensures that all information that is typically transmitted on the downlink MCCH 310 is multicast on this pre-defined (MCCH-mcast , MCCH-port) socket. Once a ToW terminal 112, 116 is associated with an Access Point (AP) 114 and is assigned an IP address, it receives and processes the multicast traffic transmitted on this socket defined by the (MCCH- mcast, MCCH-port) pair. This processing is performed by a new protocol layer, referred to as the Adaptation Layer. In this manner, the ToW terminal 112, 116 receives the information normally broadcast on the downlink of the conventional MCCH channel 360.

This information helps the terminal recognize the identity of the WLAN site 110 that provides the cell- based service and determines whether it should register to this service or not. This information also helps the ToW terminal 112, 116 identify potential neighbour sites (implemented on either WLAN or conventional TETRA technologies) and monitor these sites in the context of its standard cell re-selection process.

For transporting signalling that normally uses the uplink MCCH in a conventional TETRA site, the same (MCCH-mcast, MCCH-port) are employed. In particular, every ToW terminal 112, 116 wishing to transmit uplink MCCH traffic 320 (e.g. a request for a new group call, or a request for a new short message) transmits a special message to the pre-defined (MCCH-mcast, MCCH-port) socket. This message (similar to all uplink TETRA specific messages) will be routed by the , WLAN infrastructure 114, 142, 140 to the IWF 150. The IWF 150 also listens on the (MCCH- mcast, MCCH-port) socket and processes incoming traffic on this socket. Therefore, with IP multicasting and a pre-defined multicast socket, the functionality of the conventional MCCH channel is supported over a WLAN network .

Note that for the example of a TETRA MCCH, the MCCH is implemented as a fixed TDMA channel. Thus, its capacity is limited (the overall bit rate is 7.2 kbps) . To provide increased control channel capacity in a TETRA cell, additional TDMA channels can be configured as Secondary Control Channels (SCCH) , which supplement the operation of the MCCH. However, in a WLAN environment, the capacity supported by the multicast channel (MCCH- mcast, MCCH-port) used to carry normal MCCH traffic can be very high and there is no need for provisioning of secondary multicast channels.

Referring now to FIG. 4, an overview of the communication layers associated with TETRA communication over a WLAN system is illustrated. The preferred communication architecture is comparable with the ETSI EN 300 392-2 TETRA specification. That is, control plane 410 information comprises SNDCP 415, mobility management (MM) 420 and a call management control entity (CMCE) 425. A mobile link entity (MLE) 430 and a logical link control (LLC) layer 435 are also supported.

In the user plane 440 the Real Time Protocol (RTP) or the Compressed RTP protocol 450 is used to transport TETRA adaptive code excited linear predicted (ACELP) encoded voice blocks 445 according to ETS 300 395, between the ToW and the IWF. Normally, one voice block is generated every 30 msec. On the Control Plane, all TETRA air interface protocols are re-used, except the TETRA MAC and Physical layer protocols, which are not applicable to a WLAN radio access. The LLC layer supports both the Basic Link services and the Advanced Link services, as described in ETSI, EN 300 392-2 v2.3.10, "Terrestrial Trunked Radio (TETRA); Voice plus Data (V+D) ; Part 2: Air Interface (AI)", ETSI, June 2003. The LLC layer runs in the ToW and in the IWF (or partially in the SwMI) .

Notably, a new layer of communication is specified in a TETRA over WLAN system, namely, an Adaptation Layer 460. The Adaptation Layer 460 provides the necessary adaptation functionality required to operate the TETRA air interface protocols over a WLAN. The Adaptation Layer 460 is implemented in a ToW and in an IWF and provides services that include a subset of the services provided by the TETRA MAC layer. In particular, it supports TETRA-compliant encryption and addressing using TETRA SSIs.

The Adaptation Layer interfaces with the UDP layer through a Control-Plane-Service Access Point (CP-SAP) 465 and one or more User-Plane-Service Access Points (UP- SAPs) 470. The CP-SAP 465 is always present and is used to carry control-plane traffic that is not associated with an ongoing call, i.e. control traffic normally transmitted on a TETRA MCCH or SCCH.

In accordance with the preferred embodiment of the present invention, an adaptation of multicast IP addresses and Port numbers, referred to as MCCH-multicast and MCCH-port and as described above with respect to FIG. 3, respectively, are used to transport such kind of traffic .

Conversely, user plane traffic is carried on a UP-SAP 470. At the beginning of a new call, a new UP-SAP 470 instance is created to support that particular call. A UP-SAP 470 instance is using a dynamically assigned multicast IP address and port number. As discussed below, this multicast IP address and port number are assigned by the IWF and are communicated to the ToW in the packet that signals the start of the new call . A UP- SAP 470 is dynamically created to support user data, traffic and call associated control traffic. Notably, the UP-SAP 470 also supports control-plane traffic 410 that is associated with an ongoing call (e.g. a D-TX- CEASED PDU) . The Adaptation Layer is used to differentiate between the user-plane traffic and the call-associate control traffic on the same UP-SAP.

The Adaptation Layer 460 in the ToW analyses every received packet and. identifies (based on the indicated SSI) if it should further be processed or be dropped. If it requires further processing, the Adaptation layer dictates whether decryption should be applied (i.e. if the received message is encrypted) and, if it was received over a UP-SAP, it forwards it either to an LLC entity or to an RTP entity.

FIG. 4 illustrates further aspects of the protocol architecture of a ToW terminal. The WLAN Physical layer 490 and WLAN MAC layer 485 are used for establishing wideband wireless connectivity with an AP. These layers are preferably compliant with the IEEE 802.11 specification, as described in the document titled:

"Wireless LAN Medium Access Control (MAC) and Physical

Layer (PHY) Specification", and published in the IEEE standard 802.11, edition 1999. However, it is envisaged that any WLAN can be used.

These layers are also compliant with the Quality of Service (QoS) enhancements specified in IEEE Std. 802. lle/D8.0, "Part 11: Wireless Medium Access Control (MAC) and physical layer (PHY) specifications: Medium Access Control (MAC) Quality of Service (QoS) Enhancements", February 2004.

It is envisaged that these layers may also support the security features specified in IEEE, "Port-Based Network

Access Control", IEEE standard 802. IX, edition 2001 and

IEEE Std 802.lli/DlO.O, "Part 11: Wireless Medium Access

Control (MAC) and physical layer (PHY) specifications:

Medium Access Control (MAC) Security Enhancements", April 2004.

Connectivity with the IWF is provided with the IP layer, by means of its routing and addressing services, as described in J. Postel's paper, titled "Internet Protocol", and published in RFC 791 in September 1981. The UDP layer provides error detection and multiplexing services, as specified in J. Postel's paper, titled "User Datagram Protocol", and published in RFC 0768, 1980.

The IP layer is implemented in the ToW terminal, the IWF and the intermediate IP Network (as shown FIG. 1) . The IP layer ensures that all IP datagrams from/to ToWs are routed to/from the IWF. If necessary, it may also enable different quality of service (QoS) routing to different kinds of IP datagrams, according to the Type of Service (ToS) field in their respective IP headers. Such QoS services may be required to provide preferential transportation services to IP datagrams carrying voice packets .

Referring now to FIG. 5, a preferred packet structure 500 employed in the TETRA over WLAN architecture is illustrated. In FIG. 5, the general format of control- plane 510 and user-plane 550 packets exchanged over the Ut interface (i.e. transmitted between the IWF and the ToW terminals) are illustrated.

The control-plane packets 510 carry normal TETRA LLC PDUs 530 encapsulated into IP/UDP. The structure of the Adaptation Layer header is a key component of the inventive concepts herein described. The structure of all other protocol fields (e.g. IP 515, UDP 520, RTP, LLC, CMCE, MLE, MM, SNDCP 535) is designed to comply with known protocol specifications.

That is, the LLC, CMCE, MLE, MM, and SNDCP protocol fields are described further in the document ETSI, EN 300 392-2 v2.3.10, "Terrestrial Trunked Radio (TETRA); Voice plus Data (V+D) ; Part 2: Air Interface (AI)", ETSI, June 2003. The IP protocol field is described further in the document by J. Postel, "Internet Protocol", RFC 791, Sep. 1981. The UDP protocol field is described further in the document by J. Postel, "User Datagram Protocol", and RFC 0768, 1980. It is envisaged that other kind of TETRA PDUs might also be encapsulated in the control-plane packets, e.g. for broadcasting security related information such as a Common Cipher Key (CCK) identifier or a Static Cipher Key (SCK) version number.

The Adaptation Layer header 525, 565 is populated with critical information that is normally included in the TETRA MAC header. More specifically, the Adaptation Layer header 525, 565 preferably includes a TETRA SSI and an Encryption Mode field, which indicates whether the embedded LLC or RTP PDU is encrypted or not. In a downlink direction the TETRA SSI identifies the TETRA address of the packet recipient (s) , whereas in the uplink direction, it identifies the TETRA address of the packet originator.

In addition, the Adaptation Layer 525, 565 preferably includes more information in packets that signal the origination of a new call (of any kind) . In this case, the Adaptation Layer 525, 565 preferably includes also the Multicast address and the Port Number that are used to transport the voice packets of the upcoming call . Finally, the Adaptation Layer header 525, 565 preferably includes an information field that indicates if there is an LLC PDU or an RTP PDU encapsulated in the packet .

Referring now to FIG. 6, an example of how the various headers are populated in a packet 600 carrying a known TETRA D-SETUP message is illustrated.

A packet 610 is, for example, sent by the IWF. The packet 610 indicates that a terminal with SSI=46 635 originates a group call to GSSI=8388888. The IP address 615 of IWF is x.y.z.w and the multicast address and port corresponding to the well-known MCCH address and MCCH port 620, are designated as MCCH-mcast and MCCH-port, respectively. This packet is sent by the IWF to all WAGs that have previously established a Ft tunnel, or only to WAGs under which it is known that members of group GSSI=8388888 are present. Thus, the packet could ultimately be broadcast to all WLAN sites controlled by this IWF. Therefore, all ToW terminals in these sites will receive and decode the packet. The ToW terminals affiliated with the GSSI=8388888 will thus be configured to receive the upcoming user-plane information for this group call by creating a new UP-SAP instance. The new UP-SAP of the adaptation layer header 620 is bound to the multicast address gl.g2.g3.g4 and port Gp.

Also a packet 650 is, for example, sent by a ToW. The packet 650 indicates that a ToW terminal has a SSI=46, in order to affiliate to group with GSSI=8388888. The IP address 655 of the ToW is designated as a.b.c.d. It is noteworthy that in this packet 650, the Adaptation Layer 665 does not include a multicast IP address and port pair, since the packet does not originate a new call.

Referring now to FIG. 7 and FIG. 8, example signalling flows of TETRA over _ WLAN system are illustrated, according to the preferred embodiments of the present invention. In particular, FIG. 7 illustrates a signalling flow 700 comprising a WLAN association and location update. FIG. 7 illustrates the typical message sequence 700 that takes place between a ToW terminal 710, an AP and WAG of a WLAN 715 and an IWF 720. The message in step 725 is sent when a WLAN terminal requests an association with the WLAN using the SSID="TETRA" . This happens preferably either when the WLAN terminal powers up or when it chooses to change radio access technology, i.e. to leave a TETRA site and join a WLAN site. Notably, all the control signalling takes place over a WLAN radio network, instead of the conventional TDMA channel that carries MCCH signalling.

The WLAN preferably acknowledges the request in step 730.

At point 1 735, the WAG 715 creates an Ft tunnel with the predefined address of the IWF 720, if there is no such tunnel already in place. The WAG 715 also sets up its forwarding function in order to forward subsequent packets from the ToW 710 to the IWF 720 via the Ft tunnel. Next, the ToW 710 initiates a DCHP procedure to obtain an IP address, as shown in step 740 and step 745. This IP address is typically assigned in step 745 by the IWF 720, using an internal DCHP server or possibly an external DHCP server.

At point 2 750, the ToW 710 starts receiving packets with destination IP equal to MCCH-multleast and UPD port equal to MCCH-port. The values of MCCH-multicast and MCCH-port are assumed to be pre-configured in the ToW 710. However, it is envisaged that other means could also be developed for sending these parameters to the ToW 710, if necessary. After point 2 750, the Adaptation Layer in the ToW 710 starts receiving and transmitting packets 755, 760, 765, 770, 772, 775 that include TETRA traffic normally transmitted on the MCCH channel.

All packets received by ToW terminal 710 on socket (MCCH- mcast, MCCH-port) are transmitted by IWF 720 (i.e. IPsrc=x.y . z .w) and contain information normally transmitted on the TETRA downlink MCCH, such as group call initiate messages, system information messages, short messages, etc. Also, all these packets contain the TETRA short subscriber identity (SSI) of the intended recipient or recipients, as well as a flag specifying if the embedded LLC PDU is encrypted or not. Likewise, any packet received by IWF 720 on socket (MCCH-mcast, MCCH- port) is transmitted by an individual ToW 710 and contains information normally transmitted on the TETRA uplink MCCH, such as location registration requests, call control messages, etc.

In FIG. 7, after the ToW 710 receives the first D-MLE- SYSINFO message 755, the ToW 710 determines the identity of the TETRA network and the cell identity corresponding to the WLAN 715, and decides to register with this cell. Consequently, the ToW 710 sends to the IWF 720 a U- LOCATION-UPDATE-DEMAND PDU 760 in order to request the SwMI to register/update its new location and to update its affiliation to a specific TETRA talkgroup . As mentioned previously, this PDU 760 is transmitted with destination IP address equal to MCCH-mcast and destination UDP port equal to MCCH-port. The IWF 720 determines the TETRA identity of the ToW 710 by inspecting the SSI field. The subsequent messages D-AUTHENTICATION-DEMAND 765, U- AUTHENTICATION-RESPONSE 770, and D-AUTHENTICATION-RESULT 772, are used for authenticating the ToW 710 and deriving dynamic encryption keys in a way that conforms to the TETRA security specification. Assuming the authentication is successful all messages following U- AUTHENTICATION-RESPONSE 770 are transmitted with encryption enabled.

Finally, in message 775, the IWF 720 acknowledges the acceptance of the ToW 710 previous request to update its location and its affiliation to a specific TETRA talkgroup 760. After this point, the SwMI network will forward all further traffic for this ToW 710 to the WLAN 715 through the IWF 720. This traffic includes both control-plane messages, such as requests for private or group calls, requests for short messages, etc., as well as user-plane messages, such as voice blocks.

It is noteworthy that the above message sequence conforms to the message sequence specified in TETRA specifications for authentication and location management. However, it is envisaged that the above message sequence can be adapted to any message sequence adopted by a cell-based communication system, such as a cellular communication system or another PMR network, such as Project 25 (see TIA/EIA-102.BAAA, "Project 25 FDMA Common Air Interface", May 1998) .

A skilled artisan will appreciate that FIG. 7 represents only a simple example of an association request and location update message, and does not aim to show every possible communication.

Referring now to FIG. 8, a signalling flow 800 comprising a message sequence for group call initiation and participation is illustrated. The signalling flow 800 illustrates communication between a ToW terminal 805, an AP and WAG of a WLAN 810 and an IWF 815.

An indication of a new Group call 820 is received at IWF 815, for ToW terminals affiliated to group ^X8388888'. The destination multicast address of a packet 825 and the destination UDP port are the well-known MCCH-mcast and MCCH-port, respectively. The Adaptation Layer header in this packet indicates that the new call will use an IP multicast address gl.g2.g3.g4 and the UDP port Gp. This is illustrated in the MCCH D-SETUP message 825 transmitted from the IWF 815 to the ToW terminal 805. All ToW terminals in the WLAN area of the IWF receive this packet, irrespective of whether they are engaged in a call or not. ToW terminals affiliated to group '8388888' and willing to participate in this group call, will create a new UP-SAP instance and will bind it to the designated multicast address and UDP port (i.e. gl.g2.g3.g4/Gp) . ToW terminal 805, with SSI='9O', receives this group call .

After receiving the MCCH D-SETUP message 825, a series of IP multicast datagrams are transmitted from the IWF 815 to the ToW 805. Each datagram 830, 835 carries a voice packet from the originator. The datagrams 830, 835, and possibly subsequent datagrams, indicate in the Adaptation Layer header that they carry encrypted RTP PDUs for the group with GSSI=8388888.

Datagram message 840 is a call-associated control packet, carrying a D-TX Ceased PDU. The Adaptation Layer in the ToW 805 understands that this carries an LLC PDU (as opposed to an RTP PDU) and thus forwards it to the LLC layer as indicated in the Info field. After datagram message 840, the considered ToW 805 decides to take control of the group call and thus sends an uplink call- associated control packet 845. The uplink call- associated control packet 845 carries a U-TX Request PDU that requests from the SwMI permission to transmit.

In response, the SwMI grants transmit permission to ToW 805 with a D-Tx Granted message 850. Thus, starting from message 855, the ToW 805 transmits a series of user-plane packet that contain encrypted RTP PDUs .

In an enhanced embodiment of the present invention, security and authentication procedures can be readily incorporated into the TETRA over WLAN system. The authentication procedure can be readily supported by exchanging the appropriate layer-3 messages between the ToW and the IWF, e.g. a D-AUTHENTICATION DEMAND and a U- /D-AUTHENTICATION RESPONSE, as described in ETSI, EN 300 392-7 v2.1.1, "Terrestrial Trunked Radio (TETRA); Voice plus Data (V+D) ; Part 7: Security", Feb. 2001.

During this procedure, the Adaptation Layer in the ToW is responsible to run the appropriate security algorithms and create a Derived Ciphering Key (DCK) . This key is subsequently used by the Adaptation Layer to encrypt and decrypt all LLC and RTP PDUs over the Ut interface.

Support of Static and Common Cipher Keys (SCK, CCK) is also possible. The CCK is generated by the SwMI to protect group addressed signalling and traffic, as well as also protecting SSI identities.

In addition, it is envisaged that a yet further enhanced embodiment of the present invention will support individual and telephone interconnect calls in a similar manner to the signalling flow for group calls described above. A skilled artisan will appreciate that other procedures and corresponding MCCH signalling flows, e.g. for SDS and packet data transmission/reception, can be easily adapted and incorporated using the principles and the protocols hereinbefore described. In particular, for packet data transmission, a specific set of multicast IP address and port number (designated as PDCH-mcast and PDCH-port, respectively) could be assigned by the IWF and communicated over the control channel when a ToW requests access to a packet data channel.

Thus, the inventive concept has proposed a mechanism for utilising a control channel when communicating between a WLAN and a cell-based communication system such as a TETRA network.

It will be understood that the aforementioned architecture, devices, functional elements, protocol and signalling flows, embodying the inventive concepts described above, tend to provide at least one or more of the following advantages : (i) The integration of WLANs with cell-based networks provides a wide range of new capabilities and benefits, which will result to product differentiation and competitive advantages. (ii) Officers and other business users are able to use dual-mode cell-based/WLAN terminals in the field as well as in the office environment and/or over a private WLAN.

(iii) Over the WLAN, enhanced services can be provided to the end user. For example:

(a) Enhanced voice encoding schemes to provide much improved voice quality (i.e. no need to restrict to the limited capacity offered by the cell-based traffic channels) ; (b) Wideband data services could be supported;

(c) Voice and data services could be simultaneously provided.

(iv) Control traffic normally transmitted on the cell-based MCCH can now be received while there are voice and/or data sessions active. In conventional cell-based communication, a terminal in a voice session cannot also receive the control traffic on a main control channel (MCCH) , because voice and MCCH traffic are transmitted on different channels. However, in accordance with the preferred embodiment of the present invention with cell- based communication over a WLAN, this is now feasible.

(v) A skilled artisan will appreciate that, by means of IP multicast, many Group Calls can be monitored simultaneously by a single user, etc.

(vi) WLANs feature large air interface capacity and can therefore support many simultaneous cell-based voice/data calls in an efficient and cost effective manner .

(vii) A skilled artisan will also appreciate that call set up delays and voice transmission delays can be considerably reduced. Thus, since the WLAN supports higher bit rates, the control messages and the voice datagrams are transmitted faster.

(viii) Ability to support seamless roaming between WLAN access and conventional cell-based access: i.e. mobiles entering a WLAN area are treated in a similar manner to them entering a new cellular location area in that they use the typical cell-based mobility management procedures to update the SwMI with their new location.

(ix) Supplementary cell-based features (e.g. Late Entry, Dynamic Regrouping, etc) can be readily supported - some of them with less SwMI processing or intervention (e.g. Priority Monitoring) .

(x) Cell-based terminals can participate simultaneously in group call(s), data session(s) and also receive information normally sent on MCCH. This creates new capabilities not available on conventional cell-based radio systems .

(xi) The preferred architecture has minimum impact on the TETRA SwMI. The IWF can be considered as a special kind of cell-based Base Station, which can easily interface with the SwMI core.

(xii) Local Site Trunking (or fallback mode) in a WLAN area can be easily supported by proper engineering of the IWF (i.e. support the control channel and call processing even without connection with the SwMI) .

(xiii) Full compatibility between ToW terminals and conventional cell-based terminals is maintained. Whilst specific implementations of the present invention have been described, it is clear that one skilled in the art could readily apply further variations and modifications of such implementations within the scope of the accompanying claims .

Thus, a wireless communication system, a wireless local area network (WLAN) access gateway (WAG) , an InterWorking Function (IWF) and a wireless terminal adapted to facilitate control channel communication between a cell- based communication system and the wireless local area network (WLAN) have been described. Furthermore, a method of communicating between a wireless local area network (WLAN) , a cell-based communication system and a protocol therefor, have been described.

Claims

1. . A wireless communication system (100, 200, 300) comprising a wireless local area network (WLAN) operably coupled to a cell-based radio communication system (160), the wireless communication system (100, 200, 300) characterised in that the wireless local area network (WLAN) is adapted to support use of a control channel (360) for communication to or from the cell-based radio communication system (160) .

2. A wireless communication system (100, 200, 300) according to Claim 1 further characterised by a dual-mode wireless communication unit (112, 116) being configured as capable of communication with both the wireless local area network (WLAN) and the cell-based radio communication system (160), such that a specific

Multicast address (MCCH-mcast) and a Port . Number (MCCH- port) are used on the control channel (360) for control channel communication by the dual-mode wireless communication unit (112, 116) .

3. A wireless communication system (100, 200, 300) according to Claim 2 wherein the dual-mode wireless communication unit (112, 116) associates with the WLAN over the control channel (360) by using a special Service Set IDentifier (SSID) to enable the WLAN to differentiate between a terminal (116) capable of communication over a WLAN and a terminal not capable of communication over a WLAN .

4. A wireless communication system (100, 200, 300) according to any preceding Claim further characterised in that the WLAN comprises or is operably coupled to a WLAN access gateway (WAG) (142) and an Interworking function (IWF) (150) arranged to interface communication between the WLAN and the cell-based communication system (160).

5. A wireless communication system (100, 200, 300) according to any of preceding Claims 2 to 4 wherein a plurality of dual-mode wireless communication unit (112, 116) are configured to use the control channel (360) is configured to perform one or more of the following:

(i) Set up group calls;

(ii) Location registration procedures;

(iii) Cell re-selection operation;

(iv) Support call control messages; ^■ (v) Carry mobility management signalling;

(vi) Support authentication and key management signalling,

(vii) Support short message signalling,

(viii) Broadcast cell specific and/or network specific information.

6. A method of communicating between a wireless local area network (WLAN) and a cell-based communication system (160) by a dual-mode terminal (112, 116) capable of operating over the WLAN and the cell-based communication system (160), the method characterised by the step of: providing a control channel to facilitate control channel communications between the wireless local area network (WLAN) and the cell-based communication system

(160) .

7. A method of communicating according to Claim 6 further characterised in that the step of providing a control channel is performed by an adaptation layer in a protocol employed by the cell-based communication system (160) .

8. A method of communicating according to Claim 6 or Claim 7 further characterised by the steps of: assigning the dual-mode terminal (112, 116) a WLAN association identifier; and using, by the dual-mode terminal (112, 116), a specific Multicast address (MCCH-mcast) and a Port Number (MCCH-port) on the control channel (360) for control channel communication.

9. A method of communicating according to any of preceding Claims 6 to 8 further characterised by the step of: differentiating, by the WLAN, between a terminal (112, 116) capable of cell-based communication over a WLAN and a terminal not capable of cell-based communication over a WLAN using the identifier.

10. A method of communicating according to any of preceding Claims 6 to 9 further characterised by the step of authenticating the dual-mode terminal (112, 116) prior to allowing the dual-mode terminal (112, 116) to use the control channel .