WO1997025809A1

WO1997025809A1 - Improved pstn delivery system and method

Info

Publication number: WO1997025809A1
Application number: PCT/US1997/000139
Authority: WO
Inventors: Mark Jacques; Bruce E. Thomas; David F. Held; Michael R. Carter; Keith R. Steffel
Original assignee: Sattel Communications Co.
Priority date: 1996-01-04
Filing date: 1997-01-03
Publication date: 1997-07-17
Also published as: AU1526797A

Abstract

A system and method for switching data calls using the public switched telecommunications network (PSTN) intercepts and demodulates analog PCM data calls at a local exchange or at another switch exchange to generate baseband data calls. The demodulated baseband calls are coupled to dedicated leased trunk lines servicing destination digital service providers. The leased trunk lines switch the baseband data calls using a framing protocol compatible with the conventional trunk protocol but with increased bandwidth utilization. Several baseband data calls are combined and share a conventional trunk time slot conventionally dedicated to a single modulated call.

Description

IMPROVED PSTN DELIVERY SYSTEM AND METHOD

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to switching methods and apparatus in public switched telecommunications networks (PSTN) and more specifically to switching methods and apparatus for use with dial-up data calls in a PSTN.

2. Description of the Related Art

In recent years the rapid spread of data communications has led to the use of public dial-up telephone lines for transfer of data in applications including electronic mail, file transfers, bulletin board serviςes, point of sale transactions and information retrieval services. A major portion of all traffic carried on the PSTN is attributable to these and other applications requiring data communications, and this portion of traffic is growing rapidly. This growth in the use of ordinary PSTN voice circuits to carry dial-up data calls to information service providers has contributed to overload of the long-distance network and has been a factor in requiring additional investment to expand overburdened segments of the network. This growth also has contributed to the present shortage of directory numbers in some regions of the United

States and the need for a concomitant nationwide network upgrade to create a new area code plan to establish more directory numbers. Enormous costs are involved in making such changes.

The existing PSTN was designed for voice service and at present does not support dial-up data calls efficiently. Most PSTN networks use digital technology, in which voice circuits occupy 64 kbit/second bandwidth in each direction of transmission. Dial-up data services require the transmission of data at "sub-rate" data speeds (typically between 300 bits per second and 28.8 kbit/second ). Because the conventional PSTN infrastructure does not have the ability to differentiate dial-up data calls from ordinary voice calls and to assign less than the standard 64 kbit/second bandwidth in each direction to data calls, bandwidth is under-utilized in the conventional transmission of sub-rate dial-up data calls. This bandwidth under- utilization for sub-rate dial-up data calls contributes to worldwide network blockage, the shortage of directory numbers, and inefficient use of telecommunications infrastructure. Further, as the use of dial-up data services expands more rapidly than ordinary voice services, the world telecommunications infrastructure becomes even less efficient.

The existing international and domestic PSTN infrastructure, which now predominately uses digital switching and transmission technology, handles all calls originated on common analog subscriber loops in basically the same way. The local exchange converts the analog signal present on the subscriber loop to a digital format using pulse code modulation (PCM) technology with a transmission bandwidth of 64 kbit/second in each direction of transmission. This is an efficient means of transporting voice signals through the PSTN worldwide, but is not efficient for the sup- rate dial-up data services in wide use today.

Dial-up data calls to information service providers typically operate in the following manner. A telephone network subscriber or "client" connects a personal computer or other data terminal equipment to a voice band data modem. The modem converts the digital data from the terminal equipment to an analog format suitable for transport through the PSTN in the same manner as voice communications through ordinary telephone circuits. The client's modem dials the desired service provider or "host" using a telephone number designated by the host as a "data line." The client's call is then answered automatically by the host computer or other data terminal equipment. The host typically is equipped to handle multiple data lines at one time.

The client's call enters the PSTN on local loops serving the client's premises which connects to a local exchange carrier located near the client. The local exchange carrier converts the analog modem signal to standard PCM digital format at 64 kbit/second transmission bandwidth, multiplexes the call with other traffic, and switches the call to an inter-exchange carrier. The inter-exchange carrier transports the call in PCM format to a local exchange carrier serving the host. The destination local exchange carrier then switches the call to the host's premises. Delivery to the host is performed using one of two methods. Using the first method, the call is converted from PCM format to an analog form, and is then switched to a local loop servicing the host's premises. At the host's premise, a voice band data modem connects to the local loop, recovering the data thereon. In a second method, the data call is sent to the host's premises by the local exchange carrier in PCM format using compressed digital facilities. With this method, the host converts the incoming PCM data signals to analog format in equipment referred to as "channel banks", and connects a voice band data modem to each local loop derived from the channel banks.

From the foregoing it is seen that the data call occupied 64 kbit/second bandwidth for the entire inter-exchange leg of the connection, as well as for the term nat ng eg to t e ost s prem ses n e case w ere a compresse g a transmission facility was employed. However, the data signal contained an information rate of only 300 bits per second to 28.8 kbit/second resulting in network inefficiency.

Dial-up data network efficiency also suffers as a result of the present practice of information service providers installing data modems at their premises or in their local exchange carriers' offices in quantities calculated solely to meet their own individual data traffic requirements during their service's busy hour. Busy hour traffic peakedness is statistically greater for smaller numbers of circuits, and therefore represents a disproportionate cost burden on facilities having a smaller numbers of circuits, and therefore represents a disproportionate cost burden on smaller information service providers.

Dial-up data network efficiency also suffers due to the lack of intelligent support within the PSTN for such calls. Dial-up calls from clients require validation, which at present is done by the host computer which maintains a user database and password information. It is therefore necessary to use the entire circuit between client and host for caller verification, which represents waste of significant PSTN resources particularly in the cases where unauthorized callers spend considerable time attempting to gain unauthorized access to a network.

Thus, there is a need for a PSTN method and system having improved efficiency in the transport of sub-rate data in order to reduce operating costs, postpone costly expansion of overburdened routes and area codes, and free existing equipment for use in traditional voice services.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a method and system for transporting sub-rate data in the PSTN environment, having increased bandwidth utilization and improving the average utilization of modems and related infrastructure equipment. The present invention increases efficiency by dedicating a portion of existing PSTN infrastructure to carrying sub-rate data calls from a switch exchange to a selected host premise. The present invention uses a data call subsystem at a switch exchange to intercept modulated data calls and switch the intercepted data calls to host destinations using leased trunk lines. The data call subsystem uses modems and data formatting and combining circuits located at the switch exchange to reduce the bandwidth of data calls and combine the reduced bandwidth (baseband) data calls to improve bandwidth utilization on the leased trunk lines. Preferably, the combined baseband data calls occupy a single timeslot conventionally occupied by a single modulated voice or data call and thus the frame formatting on the leased lines remains compatible with conventional trunks.

A data call subsystem is located at a switch exchange and is coupled to a conventional switching apparatus. The data call subsystem has an interface circuit for receiving and isolating individual circuits on conventional trunks at the switch exchange. The data call subsystem additionally has a switching matrix for routing circuits to a modem bank. The data call subsystem demodulates data calls on selected data circuits to generate baseband signals thereby reducing the signal bandwidth. The demodulated signal is combined with other similarly demodulated baseband data calls to form a combined signal having a bandwidth and format suitable for transmission in a conventional trunk timeslot. The combined signals are coupled to a leased trunk line dedicated to carrying baseband data. Because the baseband data is combined and converted to conform with a conventional trunk timeslot format, the baseband circuits on the leased trunk line are properly switched by any intervening conventional PSTN exchanges which may be routed between the data call subsystem and the host premise.

A data drop system is additionally located at the host premise. The data drop system has an interface circuit for isolating individual circuits and a drop and insert circuit coupled to a reformatting circuit to convert the data call to a desired format such as the RS -232 standard.

Baseband data calls are routed from the host premise to the data call subsystem located at the switch exchange with increased bandwidth utilization as well. The data drop subsystem combines a plurality of digital baseband data calls to create a signal which occupies a single conventional trunk timeslot. At the switch exchange, the data call subsystem receives and converts the baseband calls to conventional analog call circuits. The conventional call circuits are then routed to the client premises using conventional trunks.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a block diagram of a PSTN system in accordance with the present invention;

Figure 2 is a block diagram of a data call subsystem and a data drop subsystem in accordance with the present invention; and gure s a ow agram o a sw c ng me o n accor ance w e present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

There is illustrated in Figure 1, a public switched telecommunications network (PSTN) having a PSTN subsystem in accordance with the present invention. The PSTN illustrated has a client premises 101 including data terminal equipment 102 and a voice band modem 103. Voice band modem 103 modulates a digital data signal received from data terminal equipment 102 to generate an analog PCM signal suitable for transmission over a local analog loop 104. Local analog loop 104 is a conventional analog link coupling client premises 101 to local exchange 105. Local exchange 105 has a conventional switch system 106 and a data call subsystem 107 in accordance with the present invention. Data call subsystem 107 is coupled to conventional switch 106 by conventional trunk lines 108. Data call subsystem 107 is configured to handle data calls bound for selected destination telephone numbers. These destination telephone numbers are the telephone numbers of selected digital service providers, such as digital network service providers, internet access providers and the like. Conventional switch 106 switches calls (circuits) having specific destination telephone numbers to data call subsystem 107 via trunks 108 using a conventional trunk framing format such as the Tl or El format. Subsystem 107 switches data calls to a destination data trunk line 109 which services the corresponding digital service provider's host premise 110. In accordance with the present invention, data trunk line 109 is a leased trunk line dedicated to transferring data calls (as opposed to voice calls). Data trunk line 109 uses a framing protocol with efficient bandwidth utilization in the switching of data calls which have lower bandwidth requirements than conventional voice calls. Host premise 110 has a data drop subsystem 111 for retrieving and handling each data call circuit. Thus, subsystem 107 intercepts data calls at the local exchange and switches the intercepted data calls to the corresponding digital service provider using bandwidth efficient data trunk lines 109.

Data trunk line 109 conforms with conventional Tl or El formats but uses a modified framing protocol to increase bandwidth utilization for data calls which typically transfer data at rate less than required for voice calls. Because data trunk 109 conforms with conventional El and Tl formats, the circuits on data trunks 109 are switched by conventional exchanges in the same manner that conventional Tl and El trunks are switched. Thus, the use of data call subsystem 107 at an exchange is transparent to any intervening exchanges between data call subsystem 107 and host premises 110.

In the illustrated embodiment, data call subsystem 107 is coupled to the PSTN at the local exchange. Subsystem 107 can be alternatively or additionally be located at any other PSTN switch exchange including at an inter -exchange or a tandem exchange 112. Whether located at a local exchange or at another level, data call subsystem 107 intercepts selected data calls and switches the selected data calls to the destination digital service provider at host premises 110 using data trunk lines 109 for efficient bandwidth utilization.

There is illustrated in Figure 2, a data call subsystem 107 in accordance with the present invention. Data call subsystem 107 has a trunk interface 201 for isolating individual circuits (typically 64 kbit/s PCM circuits) on conventional trunks and for coupling the individual circuits to a switch matrix 203. Trunk interface 201 additionally receives individual modulated circuits (data calls) from switch matrix 203. Data call subsystem 107 converts the individual circuits to a format compatible with conventional trunk framing formats. Thus trunk interface 201 is an interface between conventional PSTN trunks and switch matrix 203. Switch matrix 203 routes selected circuits to a bank of modems 204.

Modems 204 demodulate the data call on each coupled circuit to generate a digital baseband signal. Modems 204 appear to the client premise 101 to be the terminating modem at the host premise 110 (destination digital service provider) in a conventional PSTN system. Modems 204, however are located at a switch exchange rather than at the host premise 110. Demodulation converts the analog PCM signal to a baseband digital signal having a reduced bandwidth. For example, an analog data call prior to demodulation is a 64 kbit/second circuit. After demodulation, the bandwidth of the circuit is reduced for example to 28.8 kBit/second or less.

Baseband formatter 205 receives the outputs of modem bank 204, serializes the data and couples the serialized baseband signals to a second switch matrix 206. Switch matrix 206 is coupled to data trunk line 109 using a data trunk interface circuit 207. Data trunk interface circuit 207 combines the data calls in conformance with a customized framing format for coupling to data trunk lines 109.

Central processor 202 controls operation of data call subsystem 107 by sending and receiving line signals to and from trunk interface 201 and by controlling the switching of switch matrices 203, 206. Central processor 202 decodes the line signaling data to detect the originating and destination telephone numbers. Central processor 202 additionally performs system enhancement functions such as originating (source num er ver cat on us ng a mu t - requency co e reg ster . ecause data call subsystem 107 interfaces directly with conventional PSTN trunks in local and transit exchanges, trunk signaling and supervisory information conventionally associated with a call is available to data call subsystem 107.

In the preferred embodiment, data call subsystem 107 detects the call initiation on each circuit and decodes both the dialed destination number and the caller's automatic number information ("ANT, or originating number). Both the ANI and the destination numbers are stored in a multi-frequency code (MFC) register 208. MFC register 208 is an expandable memory space for storing signaling information associated with each circuit. Preferably, central processor 202 performs call authorization and selects a destination trunk appropriate to service the destination number. Central processor 202 performs call authorization and routing by comparing the ANI with information stored in an updateable database 211. Database 211 is coupled to central processor 202 and is a memory storage device for storing source and destination numbers and associated information. Central processor 202, next assigns a destination data trunk line 109 to each circuit.

In the preferred embodiment, data trunk line 109 is a leased trunk line serving the destination number and has a modified framing convention having increased efficiency in switching data calls. The modified framing convention multiplexes up to four data calls in a single 56 or 64 kbit/s timeslot on data trunk 109 and thus increases trunk line efficiency. Conventional Tl and El call framing formats allocate a fixed bandwidth for each call, regardless of whether the call is a voice call or a data call. Thus, improved efficiency is obtained by time division multiplexing several data calls (each call requiring less bandwidth than the allocated amount) in a single conventional Tl or El call timeslot. Although the illustrated embodiment uses a modified framing convention to combine up to four data calls in a single conventional timeslot, the principles of the invention apply to other network systems having different bandwidth ratios and thus a greater or lesser number of data calls can be multiplexed into a single timeslot in such systems.

There is also illustrated in Figure 2, a data drop subsystem 111 in accordance with the present invention. Data drop subsystem 111 is located at the host premises to send and receive the El or Tl formatted signals to and from the PSTN. Data drop subsystem 111 has a data trunk interface 210, a drop /insert circuit 212 and a data reformatter 213. In the sending direction, data drop subsystem 111 combines up to four digital dial up data calls using data reformatter 213. Data reformatter 213 generates a single circuit (signal) carrying several multiplexed data calls. Drop /insert circuit 212 is coupled to data reformatter 213 and to data trunk interface 210 for interfacing the two subsystem components.

There is illustrated in Figure 3, a flow diagram of a method of switching data calls in accordance with the present invention. In accordance with the present invention, a call is received on a 64 kbit/s PCM circuit. Subsystem 107 detects the signaling present on the individual 64 kbit/s PCM circuits in trunks 108. Call initiation is detected 301 by decoding and recognizing conventional call initiation signaling. After call initiation is detected 301, subsystem 107 assigns 302 a multi-code frequency register or common channel signaling unit to the circuit and decodes 303 both the destination number and the caller's automatic number information ("ANI", or originating number). Subsystem 107 then optionally performs call authorization. Call authorization compares the originating number against a previously stored database of numbers to determine an authorization status. Subsystem 107 additionally selects an appropriate outgoing destination trunk to link the initiated call to the dialed destination number. Call subsystem 107 then routes the PCM circuit to a modem which demodulates 306 the call signal to generate a digital baseband signal. Additionally, any associated signaling information present on the conventional PCM trunk span is converted to digital form as is done by standard dial-up data modems and included with the digital baseband signal The demodulated digital baseband signal is combined 307 with as many as three other digital baseband signals in a single conventional 56 or 64 kbit/s D4 timeslot. The combined signals are switched 308 to a data trunk serving the dialed destination number (the digital service provider).

At the information service provider's premises 110 the digital baseband signals contained in the data trunks are separated and reformatted 310 into standard RS-232 or equivalent digital interfaces for coupling 311 to the service provider's data terminal equipment.

Thus, using the present system and method, an incoming call request is represented in the standard data modem "AT command set" message format or with hardware flow control. Additionally, in accordance with the present invention, the service provider's data terminal equipment answers the call in the same "AT command set" message format or with hardware flow control, which is sent back to the client premises 101 using data call subsystem 107. In this mode of operation, modem 204 converts the received digital baseband signal to an analog PCM format suitable for transmission in a conventional PSTN.

Data call subsystem 107 monitors the call during the time it remains in progress, and when it receives a call terminating signal from either end of the circuit it converts t at signa to t e necessary ormat to s gna ca term nat on an sen s it to t e other end of the circuit, simultaneously recording call duration or other call information. At any time the information service provider may download call records and changes in its user database and password information to the PSTN subsystem.

Therefore, the method and system of the present invention provides increased bandwidth utilization and improving the average utilization of modems and related infrastructure equipment.

Claims

CLAIMSWhat is claimed is:

1. A method of switching data in a public switched telephone network (PSTN) adapted to switch a plurality of modulated calls including voice calls and data calls and having a plurality of trunk lines using a frame protocol allocating a predetermined bandwidth slot to each modulated call, comprising the steps of: selecting a plurality of modulated data calls on one or more trunk lines; demodulating the selected data calls to generate a plurality of baseband data calls; combining the plurality of baseband data calls to occupy the predetermined bandwidth slot; and coupling the combined baseband data calls to a destination trunk line.

2. The method of claim 1 wherein the step of selecting a plurality of modulated data calls comprises the steps of: performing call authorization; and selecting a plurality of calls having a common destination umber.

3. The method of claim 2 wherein the step of performing call authorization comprises the steps of: detecting an originating and destination number; comparing the originating and destination numbers to an authorization list to determine a call authorization status;

4. The method of claim 1 wherein the step of demodulating the selected data calls to generate a plurality of baseband data calls comprises the steps of: selecting a plurality of modems from a modem bank; and coupling the selected plurality of data calls to the selected plurality of modems.

5. The method of claim 1 further comprising the steps of: receiving a plurality of data calls from the destination trunk; and converting each data call from the destination trunk format to an interface format compatible with a destination computer interface.

6. A metho of switc ing data calls in a public switc e te ep one networ (PSTN) having trunk lines coupling switch exchanges and each having a plurality of timeslots for accommodating modulated data calls, comprising the steps of: demodulating a plurality of data calls at a first switch exchange to generate a plurality of baseband data calls; combining the plurality of baseband data calls to occupy a single trunk time slot; and switching the combined baseband data calls to a second switch exchange using a single trunk timeslot.

7. The method of claim 6 further comprising the steps of: at the first switch exchange, receiving from the second switch exchange a plurality of baseband data calls occupying a single trunk timeslot; modulating each of the received plurality of baseband data calls to generate a modulated data call; and coupling each modulated data call to a single trunk timeslot.

8. A system for coupling a plurality of selected data circuits on a first set of trunks, the first set of trunks including one or more trunks each having a framing convention for ordering a plurality of data circuits, to a second trunk, the apparatus comprising: a first trunk interface having a plurality of inputs coupled the first set of trunks for selectively isolating individual circuits on the first set of trunks; a plurality of demodulators coupled to the switch matrix outputs for downconverting the data signals on the data circuits to generate a plurality of baseband signals; and a reformatter coupled to the plurality of demodulators for generating an output comprising a plurality of the baseband signals conformed to a framing convention, the output coupled to a trunk line.

9. The system of claim 8 wherein the first trunk interface comprises: a de-framing circuit for recovering individual call circuit on the first set of trunks; a switch matrix coupled to the de-framing circuit for routing individual data circuits to switch matrix outputs responsive to the switch matrix configuration; and a controller coupled to the de-framing circuit for decoding line signal information associated with the individual circuits and for configuring the switch matrix configuration responsive to the line signaling information.

10. The system of claim 9 wherein the controller comprises: a processor; a multi-code frequency register coupled to the processor for storing signaling information; and a memory couple to the processor for storing call authorization data.