WO1998027757A2

WO1998027757A2 - Method of passing a pbx station directory number over an air interface

Info

Publication number: WO1998027757A2
Application number: PCT/CA1997/000934
Authority: WO
Inventors: Joel Kao
Original assignee: Northern Telecom Limited
Priority date: 1996-12-18
Filing date: 1997-12-03
Publication date: 1998-06-25
Also published as: AU5219398A; WO1998027757A3

Abstract

A fixed wireless access system comprising a base station for bidirectional communication with a switching center, a private branch exchange (PBX) for establishing bidirectional communication with a plurality (K) of user stations, and a wireless trunk interface (WTI) for bidirectional wireless communication with the base station over an air interface and for bidirectional communication with the PBX over a trunk line is disclosed. The invention also proposes to deliver the station directory number (DN) over the fast associated control channel (FACCH) or the slow associated control channel (SACCH) of the IS-54B air interface. This is achieved using DTMF signalling or by providing a new set of messages in the extended protocol (EP) capability of the IS-54B air interface.

Description

METHOD OF PASSING A PBX STATION DIRECTORY NUMBER OVER AN AIR INTERFACE

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to fixed wireless access communication networks, and in particular to a method of passing directory numbers of a called party between a central office and a PBX over a wireless link.

Background Art

Wireless access technology was initially developed to provide phone services to remote communities. As technology improved and cost came down, this basic service has advanced from voice only transmissions in the rural areas, to voice, data, fax and other telecommunication service in the rural, sub-urban and urban areas. With the demand for wireless communication on a continuous rise, subscribers expect to receive the same full range of services such as data, video telephony, and video-on-demand, that are beginning to emerge in the wireline environment. Subscribers also increasingly expect voice and data quality that is equal to, or better than, in wireline telephone systems.

The main difference between wireless access, cellular, and PCS systems is that wireless access technology is not designed for mobility. Such systems do not allow for roaming and provide only limited cell site hand-off. Nevertheless, the existing cellular systems standards were adopted for wireless access, since the protocols for establishing the connection and communication between a customer and the service provider are similar.

An emerging system architecture called fixed wireless (radio) access, also known as wireless local loop or fixed radio, provides telephone, facsimile and data services to individual business and residential subscribers. A fixed wireless system comprises two main elements: a base station (or base), located at a cell site, and telephone stations located at the customer's premises, each equipped with a radio unit. If more than one telephone station is provided, all have the same destination address. The subscriber premise may also be equipped with a residential junction unit (RJU), which provides one or more independent telephone jacks for each subscriber. The base station is connected to standard switches in the public network through copper cables, optical fiber, or microwave links. A base station includes microwave radio units which supply a two-way radio link between the stations at subscribers premises and the base station.

A more recent architecture for fixed wireless radio access systems was designed for providing wireless communications between a cell site and a private branch exchange (PBX) installed at the customer premises, a plurality of on-premise subscriber stations being connected to the PBX over cabled connections. A PBX, which is a switching center, typically has a trunk connection to a central office, which is also a switching center. To transmit information between the PBX and the switching center over a wireless link, it is necessary for the wireless link to appear to the PBX as a central office trunk with direct inward /outward dial capabilities.

Such an architecture is disclosed in United States Patent Application No. 08/231,710 (Bannister et al, filed on April 22, 1994 and assigned to Northern Telecom Limited). For direct inward dial (DID) calls, the wireless system passes on information on the called party via an air interface to a radio unit (RU) and from there to the PBX, so that an incoming call can be presented to the proper phone in the PBX by making the interface between the RU and the PBX look like a central office trunk, as per the LSSGR FR-NWT-000064 BellCORE specification. The system disclosed in the Bannister et al. application can be configured with a plurality of RUs, each RU responding to a plurality of mobile identification numbers (MINs). A master radio unit controls a bank of slave RUs to route a call. To this end, an RU has a transceiver section which establishes a communication link between the PBX and the base station under the supervision of a programmable control unit, and a re-programmable memory for storing information such as MIN lists or MIN to station directory numbers (DN), or PBX address correlation information.

According to the present invention, for an incoming call originating in the public switched telephone network (PSTN) and directed to a user station in the PBX, the DN of the calling party is passed on from the central office switch to a cell station, from the cell station to a wireless trunk interface (WTI) over a wireless link, and from there to the PBX, so that the connection is established with the proper station behind the PBX.

For calls originating from a user station behind the PBX to a called party in the PSTN, the DN of the calling party is passed on from the PBX to the WTI and from there, over a wireless link, to the cell station and to the central office switch, so that the calling party DN is presented to the switch. The DN of the calling party may be used, for example, for billing purposes. In other words, the invention is directed to a wireless radio access system provided with a direct inward dial (DID) and an automatic identification outward dialling (AIOD) function, for passing the directory number (DN) between the service provider and the customer. The DID and AIOD functions are implemented using the fast/slow associated control channel of the air interface.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a wireless radio access system for reliable, inexpensive wireless communication between a PBX installed at customer premises and a central office of a telephone service provider.

It is another object of this invention to provide a wireless to trunk interface for connecting a base station of a cellular wireless network to a PBX, which is adapted to pass the called DNs in both forward and reverse directions using the IS-54B air interface protocol. Accordingly, this invention comprises a fixed wireless access system comprising a base station for bidirectional communication with a switching center; a private branch exchange (PBX) for establishing bidirectional communication with a plurality (K) of user stations; a wireless trunk interface (WTI) for bidirectional wireless communication with the base station over an air interface, and for bidirectional communication with the PBX over a trunk line.

The invention further comprises a method for establishing communication between a user station in a public switched telephone network and a private branch exchange over a wireless link, comprising the steps of: (a) establishing a bidirectional communication link between a base station and a user station served by a switching center;

(b) establishing a bidirectional communication link between a private branch exchange (PBX) and a plurality (K) of user stations; and (c) establishing a bidirectional wireless communication link between a wireless trunk interface (WTI) and the base station using an air interface protocol, and establishing a trunk link between the WTI and the PBX.

Advantageously, the invention provides an economical alternative to wireless user stations while maintaining wireless transmission between the base station and the user stations. By providing a wireless trunk interface at the customer premises, and wireline connections on the premises, some steps of the current air interface protocols, such as the mobility hand-off function or roaming, are not necessary.

The system according to this invention provides low start-up costs and investment tailored subscriber growth. In addition, it provides a wide range of services over a wireless link, such as standard telephone service, pay phones, data and fax, on a common hardware platform.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of the preferred embodiments, as illustrated in the appended drawings, where:

Figure IA illustrates the structure of a time division multiple access (TDMA) frame;

Figure IB illustrates the TDMA slot format for the reverse channel (RECC);

Figure IC illustrates the TDMA slot format for the forward channel (FOCC);

Figure 2 is a block diagram of the wireless radio access system according to the invention; Figure 3A is a chart showing the mode of operation of the system of Figure 2 for an incoming call, with the called party DN provided in extended protocol messages; Figure 3B is a chart showing the mode of operation for an outgoing call, with the called party DN provided in EP messages;

Figure 4A illustrates the format for the fast associated control channel (FACCH); Figure 4B illustrates the format for the EP header word 1;

Figure 4C illustrates the format for the EP header word 2;

Figure 5A illustrates the EP data word format according to the current bit assignment in IS-54B standard, showing the field used for communicating the DN; Figure 5B illustrates the EP data word format according to another version of bit assignment in IS-54B standard, showing the field used for communicating the DN;

Figure 6A is a chart showing the mode of operation for an incoming call, with the called party DN provided through DTMF signalling;

Figure 6B is a chart showing the mode of operation for an outgoing call, with the called party DN provided through DTMF signalling; and

Figure 7 illustrates the DTMF message format.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A brief description of the specifications applicable to this invention follows.

The Advanced Mobile Phone System (AMPS) is implemented in over 40 countries with the main subscriber in the United States. This standard defines pairs of analog communication channels for establishing a radio link in both forward (base to station) and reverse (station to base) directions. A forward and a reverse control channel (FOCC and RECC) are assigned to exchange digital control information, and a forward and reverse analog voice channel (FVC and RVC) are assigned for user communication.

In mid to late 1980's it was recognized that an all-digital air interface would provide much better performance and security than the analog technology, and digital techniques began deployment in the early 1990's. In addition, as an increase in the reserved spectrum allocated in 1988 by the FCC to radio communications is not envisioned, optimization of the reserved spectrum allocation has been addressed by a number of techniques such as time division multiple access (TDMA). The TDMA technique allows multi-users to time- share one AMPS channel.

Figure IA illustrates the digital traffic frame structure according to the TDMA specification. The frame length of each TDMA channel consists of six equally sized time slots (slots 1 to 6), each exactly 162 symbols in length. The current TDMA system supports three full-rate users on a frequency channel, in which case slots 1 and 4 are allocated to user 1, slots 2 and 5 to user 2, and slots 3 and 6, to user 3, or six half- time users, in which case each slot is allocated to a user. An increase of this number to six full-rate users per frequency channel is planned. The structure of a TDMA control channel is identical to that of an AMPS control channel, but TDMA has twice as many control channels as AMPS. The technical requirements for TDMA are set in the IS-54B specification, which defines the communication protocols for digital, dual mode and analog wireless access.

Figure IB illustrates the format of a TDMA slot for the reverse channels connecting a user station to a base station. The guard time and the ramp time are used for preventing collision from different slots at the base station. The synchronization word 10 is used for slot synchronization, equalizer training and time slot identification. The coded digital verification color code (CDVCC) 16 indicates that the correct, rather than the co-channel, data is being decoded.

The user information /FACCH channels 12 and 18 are shared by a voice or data channel and a fast associated control channel (FACCH), which is a blank and burst channel used to send important control data. When transmitted, FACCH replaces speech or user data, and supports the transmission of DTMF, call release instruction, flash hook instructions and status requests. If FACCH is transmitted too frequently, speech suffers.

The slow associated channel 16 (SACCH) is a continuous channel used to communicate power level changes, hand-off requests, and also by the user's station to report the results of signal strength measurements of neighbouring base stations. SACCH uses a separate time slot, providing a single message over many consecutive time slots, and therefore does not affect speech quality. However, this is a low bit rate channel and therefore transmits a limited quantity of information.

Figure IC illustrates the format of a TDMA slot for the forward direction, from the base station to the user station. The channels in the forward direction are similar to the channels in the reverse direction, and were denoted with corresponding reference numerals.

Extended protocol (EP) is an optional expansion of the signalling messages between the base station and the user's station to allow addition of new system features and operational capabilities. These messages may be exchanged on the FACCH, or SACCH, or both. Figure 2 is a block diagram showing a fixed wireless system according to the invention. A switching centre, generally designated by 11, acts as an interface between the radio equipment at a base station 13 and the public switched telephone network (PSTN) 15. The switching center could be, for example, a DMS-lOOi, a class 5 local exchange, or a DMS MTX (a mobile switching center). The switching center 11 performs the switching for a plurality of base stations and the processing to manage the calls.

The radio system used at the base station 13 for the fixed wireless access system of the invention may be one originally designed for mobile cellular telephony. The base station 13 comprises a fixed wireless radio controller (FWRC) 17 which controls a plurality of dual mode radio units (DRUs) 19, each equipped with a transmitter and a receiver for processing analog and digital information. The term "dual mode" is used in this disclosure to specify that the respective unit is adapted for both analog and digital mode of operation. Special bit patterns in the bit fields "Order" and "Order Qualifier" of the FOCC and RECC indicate digital mode calls.

There are two types of DRUs used at the base station, depending upon the type of channel they serve. A control channel DRU is used to set-up calls. A backup is ready in case of prime failure. There are several voice and traffic channel DRUs at each base station.

An RF amplifier 21 and an antenna 23 are also present at the base station to provide conversion of the electrical/radio signals to radio /electrical signals.

A wireless link between antenna 23 and antenna 27 connects the base station 13 with a wireless trunk interface (WTI) 29. WTI 29 is preferably installed at the customer premises which may be, for example, an office building, a shopping mall, apartment building, or the like. WTI 29 effects the re-conversion of the radio /electrical signal into an electrical/radio signal and communicates through trunk lines 31 with a PBX 33. Depending on the application, PBX 33 may be replaced by a key system unit (KSU).

A plurality of user stations (ST) 35 are connected to the PBX /KSU 33 in the known way, to obtain a multi-line application with an acceptable grade of service. Figure 2 illustrates telephone stations, but it to be understood that the stations may also be fax machines, data /video terminals, or any type of terminals supported by a telecommunication network.

WTI 29 is equipped with a plurality of transceivers (TX) 25 which are connected to antenna 27 through a duplex circuit 37. For example, 4-16 radio transceivers can provide 16 to 120 lines of telephone services. Circuit 37 is of a known type, designed to establish bidirectional communication between a TX and antenna 29. Antenna 27 may be an indoor (dipole) or outdoor (Yagi) antenna. Furthermore, a single antenna may be used for a group of TXs 25, or one antenna for each TX 25, depending on the application.

A TX 25 comprises a transmitter and a receiver for bidirectional transmission. For the forward direction, interface 39 extracts the messages provided by a TX receiver from the respective FOCC/FVC slot of the air interface and maps them into a digital signal block such as, for example, an incoming DS-30 block which is transmitted to PBX/KSU unit 33, and from there to the called station.

For the reverse direction, interface 39 converts the information from, for example, an outgoing DS-30 block, into IS-54B messages and provides the messages to a TX transmitter for mapping into the appropriate time slot.

Since a wireless link is established between a DRU at the base station 13 and a transceiver 25 at the customer premises, the slots normally allocated to the DN in the current air interface must be used to identify the DRU and the TX. Therefore, the DN of the called station behind PBX 33 must be transmitted to WTI 29 using other fields of the air interface, and presented to PBX 33 in a format acceptable for establishing a correct connection known as Direct Inward Dialling (DID) service. Similarly, the DN of the calling station ST 35 behind PBX 33, must be transmitted and presented to switch 11 using a format acceptable for use by the switching center 11, known as Automatic Identification of Outward Dialling (AIOD) service. According the present invention, DID and AIOD services are implemented over the FACCH or SACCH.

In one embodiment, these services are provided using EP capabilities on these channels and using two new messages defined as "Called_Station_DN" and "Called_Station_DN_ACK", in both the forward and reverse channels.

Figure 3A illustrates the mode of operation of the system of Figure 2 for connecting a call originating in PSTN 15 to a user station ST 35, without intervention of an operator.

A first stage includes the protocols for setting up a connection between the calling station in PSTN 15 and WTI 29, until a forward and a reverse voice channel are allocated by the system to the call. During this stage, a free TX^ 35 in the WTI 29 uses its unique mobile identification number (MIN) to establish the radio link with a voice channel DRU;, in a known way. Thus, in step 100, a connection between the switching center 11 and FWRC 17 is established in the known manner along a wireline link 12. In step 101, FWRC 17 looks for a free forward control channel and pages the DRUs to detect a free control channel DRUj which is allocated to the FOCC. In step 102, DRU_j broadcasts a page signal to WTI 29 on this FOCC. A wireless link is established between DRUi and WTI 29 if the user station 35 with the directory number of interest is recognised by the WTI. When a response to the page is received by FWRC 17, as determined in steps 110 and 111, controller 17 seizes a pair of voice/traffic channels (VCH), shown in steps 120 and 121. A free voice/traffic DRU is then allocated to the selected VCH and FWRC communicates the voice /traffic channel assignment to WTI 29 through the FOCC in steps 130 and 131.

The second stage comprises the protocols for establishing the connection between the calling and the called parties. During the second stage, the switching center 11 transmits a ring request to the FWRC, step 140. In step 150,WTI 29 tunes in the VCH allocated to the call. This implies that a transceiver Tx^, allocated to the call in progress, modulates the VCH carrier frequency with a SAT/DVCC message received from the base station on the VCH. DRU; transmits to the FWRC the SAT/DVCC signal in step 151. The supervisory audio tone (SAT) is used to confirm that the WTI is tuned on the voice channel, while the digital verification color code (DVCC) indicates that the WTI is tuned on a digital traffic channel.

Then DRUj exchanges with TX_k the "Alert_With_Info" and

"Alert_With_ Info_Ack" messages during steps 160 and 161. The "Alert_With_Info" is sent by the base station to the WTI to seek confirmation that the TX^ is prepared to receive information on the voice /traffic channel. The WTI confirms the order by a change in the SAT/DVCC.

Before entering the conversation task, FWRC places the called party DN into an EP message in step 170, and DRU; transmits the EP message within the called station DN to the transceiver TX_k over the FACCH /SACCH, as indicated in step 171. TX^ receives the EP message

"Called_Station_DN" and maps the DN to a trunk 31 for the called stations ST_m behind PBX 33 , in step 172. On receipt of the acknowledgement from ST_m in step 173, WTI 29 returns to DRU the EP message "Called_Station_DN_Ack", as illustrated in step 174, over FACCH/SACCH.

The system enters next in the conversation task, to exchange information between the called and calling stations. During this task, PBX 33 informs TX^, and TX_k informs DRU; that the respective connection has been established in steps 180, and 181. In turn, DRU informs FWRC over the air interface that the station ST_m is ready to exchange information with the calling number in step 182, and FWRC issues a ring trip to switching center II in step 183. The calling station in PSTN 15 can now communicate with ST_m.

Figure 3B illustrates the mode of operation of the system of Figure 2 for transmitting the calling party DN to the switching center II, for a call originating at a station ST_m , without intervention of an operator.

The first stage includes the protocols for setting-up a connection between WTI and switching center 11, until a pair of reverse and a forward voice channel are seized. Interface 39 at WTI 29 receives the "off-hook", "Dialled String" and "Send" signals from PBX/KSU 33 over a trunk line 31 and forwards same to a selected TX_k as shown in step 200. Next, WΗ 29 selects a free RECC and a free control channel DRU in step 201 and transmits the dialled string to DRU} over a wireless link on the RECC. DRU} communicates the dialled string to the FWRC in step 202. When switching center 11 detects the off-hook signal issued by St_m, it transmits a supervision report to FWRC 17 in step 203.

FWRC 17 seizes a forward and reverse voice /traffic channel and a free voice/traffic DRU in steps 210 and 211, which is followed by the switching center 11 making the connection between FWRC 17 and the called station behind the switch in step 220. FWRC 17 also informs DRU} and WTI 29 of the voice/traffic channel assignment in steps 221 and 222.

During the second stage, transceiver TX_k tunes to the voice/traffic channel, step 230, and broadcasts the SAT/DVCC to

FWRC, step 231 indicating that the voice/traffic channels are ready for transmission. WTI 29 extracts the called station number from the block received from PBX 33 in step 240, maps same into the EP message

"Called_Station_DN" and transmits this message to DRU; in step 241 over the reverse FACCH/SACCH of the air interface. DRU; acknowledges receipt of this message by sending back to WΗ 29 the message "Called_Station_DN_Ack" over the forward FACCH/SACCH in step 242. The called station number is then forwarded to FWRC in step 250.

During the conversation task, FWRC 17 establishes the connection with the called number in PSTN 15 in the known manner, and information may be exchanged between the called station in the

PSTN 15 and the calling station ST_m.

Figure 4A illustrates the format for the fast associated control channel (FACCH). The EP structure and signalling on this channel have been designed to allow various combinations of systems and mobile stations to be simultaneously operational. A message illustrated at 20 in the forward extended protocol begins with a two word header, the format of header word 1 and header word 2 being shown in Figures 4B and 4C. The EP header word 1 comprises the type field (T1T2), digital color code (DCC), the last significant digits (24 bits) of the mobile identification number (MIN), and a cyclic redundant code (P). EP header word 2 comprises T1T2, DCC, the most significant digits (10 bits) of the MIN, the EP indicator (EF), the message length (MSL), message type (MST) and P. Field 24 for the message type, shown in bold on Figure 4C, is used to indicate that the message refers to the called station DN.

Figure 5A illustrates the EP data word format according to the current bit assignment in the IS-54B standard. The EP date word comprises field T1T2, DCC, a 24-bit data field and P. The four digits of the station DN are inserted in a data word field 26, using 16 bits (4 bits /digit).

Figure 5B illustrates a new EP data word format for the IS-136.2 digital control channel (DCCH), defined herein as "BSMC Message Delivery", and shows how field 26 of the EP message on forward FACCH may be used for a proprietary message type (MST) field of 5 bits, a 16-bit data field for the called station DN, and a 3-bit field for a proprietary version protocol identification. Three message data words are necessary in this case for transmitting the DN.

The communication of the DN in the EP for the reverse channel message is effected in a similar way.

Figures 6A and 6B are charts showing the mode of operation for an incoming call (base station to WTI) and an outgoing call (WTI to base station), respectively, according to another embodiment of the invention where the called party DN is transmitted in the "Send_Burst_DTMF" message, over the FACCH channel. The message comprises a string of characters beginning with a special character, such as for example sign "#", followed by the last four digits of the called station DN. For an incoming call, this message is sent from base station 13 to WTI 29 after the base station receives the Alert_With_Info_Ack. For an outgoing call, this message is sent from the WTI to the base station after moving to the voice channel.

Figure 6A shows the DTMF mode of operation for an incoming call. As in the case of the EP mode of operation, the first stage includes the protocols for setting up a connection between the calling station in PSTN 15 and WTI 29, until a forward and a reverse voice channel are allocated to the call by the system. Steps 300 to 331 of the first stage correspond to steps 101 to 131 described above, resulting in assignment of a voice channel, selection of a voice channel DRU, and a TXj, based on the unique MIN of the WTI, as shown in steps 330 and 331. During the second stage, the switching center 11 transmits a ring request to the FWRC in step 340. In step 350, WTI 29 runes on the VCH allocated to the call. DRUj transmits to the FWRC the SAT/DVCC signal in step 351. In steps 360 and 361, DRU- exchanges the "Alert_With_Info" and "Alert_With_Info_Ack" messages with TX_k. Before entering the conversation task, the DRU; transmits the called station DN to the transceiver TX_k in a burst DTMF over the

FACCH/SACCH, as indicated in step 370. The DN is mapped to a trunk line 31 to connect the called ST_m with TX^, as shown in step 371. Connection between the ST_m and the switching center is established as shown in step 380 to 383, and the system enters into the conversation task.

Figure 6B is a chart showing the mode of operation according to the DTMF mode of operation for an outgoing call. Steps 400 to 422 are similar to the respective steps 200 to 222 of the EP mode of operation described in connection to Figure 3B. The DN of the calling party, ST_m, received by WTI 29 over a trunk line 31, is extracted and sent to DR at the base station in a burst DTMF over the FACCH channel, as illustrated in steps 440 and 441. Figure 7 illustrates the format of the "Send_Btιrst_DTMF" message, showing in bold the digits field 28 for transporting the DN. While the invention has been described with reference to particular example embodiments, further modifications and improvements which will occur to those skilled in the art, may be made within the purview of the appended claims, without departing from the scope of the invention in its broader aspect.

Claims

WHAT IS CLAIMED IS:

1. A fixed wireless access system comprising: a base station for bidirectional communication with a switching center; a private branch exchange (PBX) for establishing bidirectional communication with a plurality (K) of user stations; and a wireless trunk interface (WTI) for bidirectional wireless communication with said base station over an air interface, and for bidirectional communication with said PBX over a trunk line.

2. A system as claimed in claim 1, wherein said WTI comprises: a transceiver for converting an air interface (AIF)-compatible incoming signal into an input electrical signal, and for converting an output electrical signal into an AIF-compatible outgoing signal; and means for re-formatting said input electrical signal into an trunk-compatible outgoing signal, and for receiving a trunk-compatible outgoing signal and re-formatting same into said output electrical signal.

3. A system as claimed in claim 2, wherein said transceiver comprises: a dual mode receiver for receiving said AIF-compatible incoming signal and providing said input electrical signal; and a dual mode transmitter for receiving said output electrical signal and providing said AIF-compatible outgoing signal.

4. A method for establishing communication between a user station in a public switched telephone network and a private branch exchange over a wireless link, comprising the steps of:

(a) establishing a bidirectional communication link between a base station and a user station served by a switching center;

(b) establishing a bidirectional communication link between a private branch exchange (PBX) and a plurality (K) of user stations; (c) establishing a bidirectional wireless communication link between a wireless trunk interface (WTI) and said base station using an air interface protocol, and establishing a trunk link between said WTI and said PBX.

5. A method as claimed in claim 4, wherein said air interface protocol is a cellular system dual mode mobile station-base station compatibility standard IS-54B.

6. A method as claimed in claim 5, wherein, for an incoming call, said step of establishing a bidirectional wireless communication link comprises the substeps of: receiving a page request from a calling station in the public switched telephone network to a called station in said PBX; setting-up a connection between a control channel dual radio unit (DRU) at said base station and a transceiver at said WTI, using a mobile identification number (MIN) of said transceiver contained in said page signal; selecting a voice channel DRU and establishing connection between said voice channel DRU and said transceiver over a forward voice channel (FVC) and a reverse voice channel (RVC); at said WTI, tuning said transceiver on said RVC and FVC and confirming that said transceiver and said voice channel DRU are ready to communicate; communicating the directory number (DN) of a called station (ST) of said plurality (K) of user stations from said voice channel DRU to said transceiver using a called party DN message transmitted over a forward traffic channel; and at said WTI, mapping the DN of said called station to a trunk block, and transmitting said trunk block to said PBX for establishing connection with said called station.

7. A method as claimed in claim 6, wherein said step of confirming that said transceiver and said voice channel DRU are ready to communicate comprises exchanging the "Alert_With_Info" and "Alert_With_Info_Ack" control signals.

8. A method as claimed in claim 6, wherein said forward traffic channel is one of the fast associated control channel (FACCH) and the slow associated control channel (SACCH) defined in IS-54B standard.

9. A method as claimed in claim 6, wherein said forward traffic channel is one of the forward fast associated control channel (FACCH) and slow associated control channel (SACCH), and said called party DN message is provided in an extended protocol message word.

10. A method as claimed in claim 6, wherein said forward traffic channel is the forward fast associated control channel (FACCH), and said called party DN message word is provided in a dual tone multi- frequency (DTMF) data burst.

11. A method as claimed in claim 5, wherein, for an outgoing call, said step of establishing a bidirectional wireless communication link, comprises the substeps of: receiving a page request from a calling station in said PBX to a called station in the public switched telephone network; setting-up a connection between a control channel dual radio unit (DRU) at said base station and a transceiver at said WTI, using a mobile identification number (MIN) of said transceiver contained in said page signal; selecting a voice channel DRU and establishing connection between said voice channel DRU and said transceiver over a reverse voice channel (RVC) and a forward voice channel (FVC); at said WTI, tuning said transceiver on said RVC and FVC; receiving at said WTI the directory number (DN) of a calling station (ST) of said plurality (K) of user stations, from said PBX over said trunk link and communicating said DN from said transceiver to said voice channel DRU using a calling party DN message transmitted over a reverse traffic channel; at said base station, forwarding the DN of said called station to a switching center in said PSTN.

12. A method as claimed in claim 11, wherein said reverse traffic channel is one of the fast associated control channel (FACCH) and the slow associated control channel (SACCH).

13. A method as claimed in claim 6, wherein said reverse traffic channel is one of the forward fast associated control channel (FACCH) and slow associated control channel (SACCH), and said called party DN message is provided in an extended protocol message word.

14. A method as claimed in claim 6, wherein said forward traffic channel is the forward fast associated control channel (FACCH), and said called party DN message word is provided in a dual tone multi- frequency (DTMF) data burst.