Adaptor between

The invention is an adaptor which permits using a digital telephone device with an incompatible telephone network access. In its most preferred embodiment, the present invention is an adaptor which permits an ISDN telephone device to exchange digital data with two Switched-56 Access. An ISDN Basic Rate Access interface circuit interfaces with the ISDN telephone device, while two Switched-56 Access interface circuit interfaces with two Switched-56 Access. The adaptor also includes a clock recovery circuit for synchronizing digital data transfers occurring between the ISDN Basic Rate Access interface circuit and the ISDN telephone device with the digital data transfers occurring between the Switched-56 Access interface circuit and the Switched-56 Access. In an alternative embodiment, the clock recovery circuit synchronizes digital data transfers between the ISDN Basic Rate Access interface circuit and the Switched-56 Access interface circuits rather than conversely as in the preferred embodiments so a Switched-56 telephone device may exchange digital data with an ISDN Basic Rate Access.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to digital telecommunication 
services and, more particularly, to interoperability between two mutually 
incompatible digital telecommunication protocols. 
2. Description of the Prior Art 
Within the United States and Canada there exists two mutually incompatible 
protocols for exchanging digital data between computers using digital 
telecommunications. These two mutually incompatible conventions are 
respectively called Integrated Services Digital Network ("ISDN") Basic 
Rate Access and Switched-56 Access. The incompatibility between these two 
systems is of such a character that a telephone device which communicates 
using the ISDN protocol cannot exchange data with a Switched-56 Access. 
Conversely, a telephone device that communicates using the Switched-56 
protocol cannot exchange data with an ISDN Access. However, since the ISDN 
Basic Rate Access protocol is implemented more extensively throughout the 
world, e.g., Japan, Australia, Europe, etc., it would be advantageous if 
were possible for ISDN devices to transmit and receive digital data over a 
Switched-56 Access. 
While most local and long-distance telephone carriers in the United States 
offers Switched-56 digital telecommunications service, some Local Exchange 
Carriers do not offer ISDN Access. Switched-56 Access provides a user with 
a dial-up digital telecommunications capability for transmitting and 
receiving digital data either at 56 kilobits per second ("kbps") in a 
synchronous telecommunication mode, or 57.6 kbps in an asynchronous 
telecommunication mode. Switched-56 digital telecommunications service is 
widely used for low-bandwidth video conferencing to send and receive 
acceptable sound and pictures using only two Switched-56 telephone lines, 
i.e., transmitting and receiving at 112 kbps. Consequently, at present 
every major video conferencing equipment manufacturer incorporates a 
Switched-56 capability in its equipment. Analogous equipment is available 
for ISDN telecommunications throughout those regions of the world in which 
ISDN services are available. 
Physically, a Switched-56 Access in accordance with the American Telephone 
and Telegraph Company's ("AT&T's") standard consists of one or two twisted 
pairs of wires which carry time compressed multiplexed ("TCM") digital 
signals to provide one full-duplex synchronous transmission path at a data 
rate of 56 kbps. Conversely, in accordance with a recommendation of an 
International Telegraph and Telephone Consultative Committee ("CCITT"), an 
ISDN Basic Rate Access provides two (2) full-duplex 64 kbps digital data 
channels, called channel B1 and channel B2, plus another full-duplex 16 
kbps digital channel, called a D-channel. Under the CCITT recommendation, 
using time division multiplexing, all three of these digital data channels 
may be transmitted over a single pair of twisted wires, or over two pairs 
of twisted wires. The unrestricted data rate at which digital data may be 
transmitted over twisted pairs of wires in accordance with the ISDN 
recommendation for Basic Rate Access is 144 kbps, i.e. 64 kbps on each of 
the B1 and B2 channels plus 16 kbps on the D-channel. 
As described in an AT&T Technical Reference entitled "Special Access 
Connections To The AT&T Network," TR 41458, published April 1990, while a 
Switched-56 Access actually transmits and receives bidirectional TCM 
digital data at 128 kbps (64 kbps being alternatively first transmitted 
and then received), the eighth bit of each octet is not available for data 
transmission because it is subject to over-writing if data is transmitted 
between telephone company switches. Thus, with a Switched-56 Access user 
data may occupy only bits 1 through 7 of each octet thereby providing an 
effective data transfer rate of only 56 kbps. 
Within North America, telephone network switches capable of ISDN Basic Rate 
Access telecommunications must also be capable of concurrent Switched-56 
telecommunications. To accommodate such concurrent operation, the CCITT 
Recommendations Q.930-Q.940 for ISDN Basic Rate Access permits a 
restricted mode of operation for each of the ISDN digital data channels B1 
and B2. In accordance with these CCITT Recommendations, if an ISDN device 
exchanges digital data with a Basic Rate Access operating in restricted 
mode, then the data channel B1 or B2 transmits and receives data at an 
effective data rate of 56 kbps rather than at the unrestricted rate of 64 
kbps. If an ISDN device operates in this restricted mode, it discards the 
eighth bit of every octet (eight bits) received from the Basic Rate 
Access. Analogously, an ISDN device transmitting digital data into a 
restricted ISDN Basic Rate Access uses only the first seven bits of each 
successive octet, and does not place any of the data being transmitted 
into the eighth bit of each octet. During set-up of an ISDN telephone 
call, signals transmitted from the telephone company switch notify the 
ISDN terminal equipment that digital data is to be transferred in 
restricted mode, or the terminal equipment placing a telephone call may 
notify the telephone company switch of that fact. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an adaptor by which an 
ISDN telephone device capable of operating in the restricted mode of the 
ISDN protocol may exchange digital data with another similarly capable 
telephone device using only Switched-56 Accesses. 
Another object of the present invention is to permit the operation of ISDN 
telephone devices in geographic regions in which the telephone company 
offers Switched-56 Access, but does not offer ISDN Basic Rate Access. 
Another object of the present invention is to permit the operation of 
Switched-56 telephone devices in geographic regions in which the telephone 
company offers ISDN Basic Rate Access, but does not offer Switched-56 
Access; or in which the telephone company charges more for two Switched-56 
Accesses than for a single ISDN Basic Rate Access. 
Briefly, the present invention in its preferred embodiment is an adaptor 
which permits an ISDN telephone device which receives and transmits 
digital data in accordance with CCITT Recommendations for ISDN Basic Rate 
Access to exchange digital data with a Switched-56 Access which transmits 
and receives the digital data in accordance with a Switched-56 protocol. 
The adaptor includes both an ISDN Basic Rate Access interface circuit, 
that provides an ISDN Basic Rate Access with which an ISDN telephone 
device may exchange the digital data in accordance with the CCITT 
Recommendations, and a Switched-56 Access interface circuit for 
interconnecting with a Switched-56 Access with which the adaptor may 
exchange digital data in accordance with the Switched-56 protocol. The 
adaptor also includes an adaptor bus for exchanging the digital data with 
the ISDN Basic Rate Access interface circuit, and with the Switched-56 
Access interface circuit. The adaptor includes a random access memory 
coupled to the adaptor bus for storing the digital data during an interval 
between its receipt by one of the interface circuits and its transmission 
from the other interface circuit. A programmable digital signal processing 
circuit, included in the adaptor and coupled to the adaptor bus, controls 
the operation of the interface circuits and of the random access memory. 
The adaptor also includes a clock recovery circuit for synchronizing 
digital data transfers occurring between the ISDN Basic Rate Access 
interface circuit and the ISDN telephone device with the digital data 
transfers occurring between the Switched-56 Access interface circuit and 
the Switched-56 Access. 
In its most preferred embodiment, the adaptor includes two Switched-56 
Access interface circuits each of which exchanges digital data with the 
adaptor bus. Thus, by connecting each Switched-56 Access interface circuit 
included in the preferred adaptor to a Switched-56 Access, the ISDN 
telephone device may concurrently exchange digital data with both 
Switched-56 Accesses thereby permitting the ISDN telephone device to send 
and receive digital data at 112 kbps. 
In an alternative embodiment, the clock recovery circuit synchronizes 
digital data transfers between the ISDN Basic Rate Access interface 
circuit and the Switched-56 Access interface circuit(s) rather than 
conversely as in the preferred embodiment. Accordingly, this alternative 
embodiment of the present invention permits a Switched-56 telephone device 
to transmit and receive digital data through an ISDN Basic Rate Access. 
These and other features, objects and advantages will be understood or 
apparent to those of ordinary skill in the art from the following detailed 
description of the preferred embodiment as illustrated in the drawing 
figure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The functional block diagram of FIG. 1 depicts an adaptor in accordance 
with the present invention enclosed within a dashed line 10. The adaptor 
10 includes an ISDN Basic Rate Access interface circuit 12 for exchanging 
digital data signals via an ISDN Basic Rate Access connector 14 with an 
ISDN telephone device (Not illustrated in FIG. 1.). Two pairs of 
electrical leads 16 couple the ISDN Basic Rate Access connector 14 to an 
ISDN isolation transformer circuit 18. The ISDN isolation transformer 
circuit 18 electrically isolates a PEB-2085 ISDN Subscriber Access 
Controller ("ISAC-S") integrated circuit 22, with which it exchanges 
digital data signals, from signals present on the electrical leads 16. The 
PEB-2085 22, which is marketed by Siemens Integrated Circuit Division, 
2191 Laurelwood Rd., Santa Clara, Calif., is more completely described in 
a Siemens publication entitled "ICs for Communications," Ordering No. 
M12T021. The PEB-2085 22 provides an electronic interface for ISDN data 
transmission and reception over the two (2) B-channels and the D-channel 
of a four-wire ISDN Basic Rate Access. The electronic interface of the 
adaptor 10 provided by the PEB-2085 22 corresponds to that of a standard 
ISDN S/T interface which operates in accordance with the CCITT ISDN Basic 
Rate Access Recommendation. Accordingly, the adaptor 10 exchanges digital 
data with an ISDN telephone device in a manner that is equivalent to that 
of a standard ISDN S/T interface. 
The PEB-2085 22 exchanges digital data with a digital signal processing 
circuit 32 via an adaptor bus 34. The digital signal processing circuit 32 
is preferably a TMS320C25 Digital Signal Processor ("DSP"), which is 
marketed by Texas Instruments Incorporated ("TI"), Post Office Box 809066, 
Dallas, Tex. 75380-9066. The TMS320C25 is more completely described in a 
TI publication entitled "TMS320C2x User's Guide" copyright 1990, TI 
publication no. 164907-9721 revision B December 1990. 
The adaptor bus 34 also connects the digital signal processing circuit 32 
to a memory 36, a programmable array logic ("") integrated circuit 38 
included in the ISDN Basic Rate Access interface circuit 12, and to two 
(2) identical Switched-56 Access interface circuits 40a and 40b. An ISDN 
access control and status signal bus 42 couples the 38 to the PEB-2085 
22. The 38 adapts various control signals transmitted by the digital 
signal processing circuit 32 for controlling the operation of the PEB-2085 
22, and adapts various status signals transmitted by the PEB-2085 22 for 
receipt by the digital signal processing circuit 32. The memory 36 
preferably includes 128 kilobytes of static RAM and 128 kilobytes of read 
only memory, electrically writable read only memory, or "Flash" memory 
("ROM"). The computer program executed by the digital signal processing 
circuit 32 is stored in the ROM portion of the memory 36. 
Each of the Switched-56 Access interface circuits 40a and 40bis a DSU/64 
DSU/CSU Module marketed by HT Communications of Simi Valley, Calif. An 
isolation transformer circuit 44a or 44b, which may be physically 
incorporated into the DSU/64 DSU/CSU Module, couples each Switched-56 
Access interface circuit 40a or 40b respectively to a Switched-56 access 
connector 46a or 46b via two pairs of electrical leads 48. Each isolation 
transformer circuit 44 electrically isolates the Switched-56 Access 
interface circuit 40a or 40b with which it exchanges digital data signals 
from signals present on a Switched-56 Access (Not illustrated in FIG. 1.). 
The DSU/64 is a synchronous/asynchronous DSU/CSU module which transmits 
and receives full-duplex digital data over leased-line Digital Data 
Service ("DDS") or over a dial-up Switched-56 Access at data rates up to 
64 kbps depending upon the type of access. Each Switched-56 Access 
interface circuit 40a or 40b includes a microprocessor bus interface which 
allows the digital signal processing circuit 32 to efficiently configure 
each Switched-56 Access interface circuit 40a or 40b and monitor its 
status. Furthermore, a parallel data interface and HDLC support provided 
by the Switched-56 Access interface circuit 40a or 40b facilitate its 
exchange of digital data with Switched-56 Accesses, with the digital 
signal processing circuit 32, and with the memory 36. 
Both Switched-56 and ISDN Accesses employ synchronous data transmission in 
which a timing signal is distributed from a central location outward 
throughout an entire telephone network to individual telephone devices. 
Accordingly, digital data transmitted from a telephone company switch 
located at its central office supplies the centralized timing signal to 
each of the Switched-56 Access interface circuits 40a and 40b included in 
the adaptor 10. Since data transfers between the adaptor 10 and the ISDN 
telephone device that is connected to the ISDN Basic Rate Access connector 
14 must be synchronized with the timing signal that the adaptor 10 
receives from a telephone company switch, the adaptor 10 includes a clock 
recovery circuit 52. The clock recovery circuit 52 receives a timing 
signal from one of the Switched-56 Access interface circuit 40a or 40b via 
a clock input signal line 54, and retransmits that timing signal via a 
clock output signal line 56 to the PEB-2085 22. Depending upon precise 
details of the electronic circuits included in the Switched-56 Access 
interface circuit 40a or 40b and in the ISDN Basic Rate Access interface 
circuit 12, the clock recovery circuit 52 may be a separate circuit as 
depicted in FIG. 1, or the clock recovery circuit 52 may be incorporated 
into either the Switched-56 Access interface circuit 40a or 40b or into 
the ISDN Basic Rate Access interface circuit 12, or the clock recovery 
circuit 52 may be incorporated partially in both the Switched-56 Access 
interface circuit 40a or 40b and the ISDN Basic Rate Access interface 
circuit 12. Once again depending upon precise details of the electronic 
circuits included in the Switched-56 Access interface circuit 40a or 40b 
and in the ISDN Basic Rate Access interface circuit 12, the clock recovery 
circuit 52 may included a phase locked oscillator to insure proper 
synchronization with the timing signal received by either the Switched-56 
access connector 46a or 46b. 
The PEB-2085 22 uses the timing signal that it receives from the clock 
recovery circuit 52 via the clock output signal line 56 to synchronize 
signals between the PEB-2085 22 and the ISDN telephone device connected to 
the ISDN Basic Rate Access connector 14 with signals concurrently being 
received by the Switched-56 Access interface circuit 40a or 40b. The 
Switched-56 Access interface circuit 40a or 40b recovers this timing 
signal from the signal which it receives continuously from a telephone 
company switch via the Switched-56 access connector 46a or 46b. Since the 
Switched-56 Access interface circuit 40a or 40b receives the signal 
continuously from a telephone company switch even when the Switched-56 
Access interface circuit 40a or 40b is not actively involved in exchanging 
digital data with a Switched-56 Access, e.g. while the ISDN telephone 
device is placing a telephone call, the clock recovery circuit 52 operates 
properly for providing a timing signal to the PEB-2085 22 so long as the 
Switched-56 Access interface circuit 40a or 40b from which the clock 
recovery circuit 52 receives the timing signal is physically connected to 
a Switched-56 Access. 
Electrical power for circuits included in the adaptor 10 is provided by a 
separate power supply not illustrated in FIG. 1. Depending upon the 
particular type of ISDN telephone device connected to the ISDN Basic Rate 
Access connector 14, it may or may not be necessary for the ISDN S/T 
interface to supply an electrical current to the ISDN telephone device in 
addition to the digital data signals. If the ISDN S/T interface must 
supply an electrical current to the ISDN telephone device, then a 
commercially available power adaptor capable of supplying such an 
electrical current must be interposed into an electric cable which 
interconnects the ISDN Basic Rate Access connector 14 with the ISDN 
telephone device. 
The adaptor 10 is facilely adapted for interconnecting a Switched-56 
telephone device with an ISDN Basic Rate Access by interconnecting the 
timing signal in a direction opposite to that described above. For such a 
reversed operation of the adaptor 10 from that described thus far, the 
ISDN Basic Rate Access interface circuit 12 would supply the timing signal 
to the clock recovery circuit 52, which in turn would provide the timing 
signal to the Switched-56 Access interface circuit 40a or 40b, or to both 
the Switched-56 Access interface circuits 40a and 40b. 
A multi-tasking computer program stored in the memory 36 and executed by 
the digital signal processing circuit 32 controls the overall operation of 
the ISDN Basic Rate Access interface circuit 12 and of the Switched-56 
Access interface circuits 40a and 40b to effect exchanges of digital data 
between the ISDN Basic Rate Access connector 14 and the Switched-56 access 
connectors 46a and 46b. In particular, the computer program executed by 
the digital signal processing circuit 32 causes digital data to be 
temporarily stored into the RAM of the memory 36 during the interval 
between the instant at which it is supplied to the adaptor bus 34 
respectively by the interface circuits 12, 40a and 40b and the instant at 
which it is received from the adaptor bus 34 respectively by the interface 
circuits 40a, 40b and 12. 
Since both ISDN B-channels share the ISDN D-channel for telephone call 
control data such as call "set-up" and "tear-down" and for other call 
control data, for telephone call control data received from the 
Switched-56 Access the multi-tasking computer program executed by the 
digital signal processing circuit 32 must multiplex two streams of call 
control data into the single ISDN D-channel. Conversely, the multi-tasking 
computer program executed by the digital signal processing circuit 32 must 
de-multiplex telephone call control data received from the ISDN D-channel 
for separate transmission to the respective Switched-56 Accesses. 
While the adaptor 10 must imitate a standard ISDN S/T interface, a 
multi-tasking computer program for execution by the digital signal 
processing circuit 32 may be readily implemented. Because the CCITT ISDN 
S/T interface is highly symmetric, the protocol for transmitting digital 
data signals from the ISDN S/T interface to an ISDN telephone device is 
substantially the same as the protocol for an ISDN telephone device's 
transmission of digital data signals to the ISDN S/T interface. Therefore, 
standard software available commercially from several different companies 
for use in ISDN telephone devices may be easily and simply modified for 
execution by the digital signal processing circuit 32 so the digital 
signal processing circuit 32 appropriately controls the operation of the 
ISDN Basic Rate Access interface circuit 12 in accordance with the 
restricted mode of operation of the ISDN Basic Rate Access protocol. 
Since the Switched-56 Access interface circuits 40a and 40b are self 
contained units capable of independently interfacing with Switched-56 
Accesses, the computer program executed by the digital signal processing 
circuit 32 only provides high level call control data to the Switched-56 
Access interface circuits 40a and 40b. For example, the computer program 
executed by the digital signal processing circuit 32 need only send the 
Switched-56 Access interface circuit 40a or 40b data directing it to dial 
a telephone number and the telephone number, and the Switched-56 Access 
interface circuit 40a or 40b will thereafter properly perform that entire 
function without further intervention by the computer program executed by 
the digital signal processing circuit 32. 
Since ISDN digital data occupies separate channels B1 and B2, there is a 
one-to-one correspondence between digital data exchanged between the 
Switched-56 access connector 46a or 46b of the adaptor 10 and a 
Switched-56 Access, and digital data exchanged over a single ISDN 
B-channel operating in restricted mode and the ISDN telephone device 
connected to the ISDN Basic Rate Access connector 14. Accordingly, the 
computer program executed by the digital signal processing circuit 32 need 
not multiplex or de-multiplex the digital data as it must for the call 
control data. 
Although the present invention has been described in terms of the presently 
preferred embodiments, it is to be understood that such disclosure is 
purely illustrative and is not to be interpreted as limiting. For example, 
while an AT&T Switched-56 physical interface and a Northern Telecom 
Switched-56 physical interface employs a four (4) wire connection such as 
that depicted in FIG. 1, an alternative AT&T Switched-56 physical 
interface employs only a two (2) wire connection. Thus an adaptor 10 in 
accordance with the present invention may employ a two (2) wire interface 
rather than the four (4) wire interface depicted in FIG. 1. 
Correspondingly, while the preferred embodiment of the adaptor 10 has been 
described as employing a pair of Switched-56 Access interface circuits 40a 
and 40b, an adaptor 10 in accordance with the present invention may employ 
only a single Switched-56 Access interface circuit 40a or 40b. However, an 
adaptor 10 having only a single Switched-56 Access interface circuit 40a 
or 40b would be capable of exchanging digital data at a rate that is 
one-half of that for the preferred embodiment of the adaptor 10, i.e., 56 
kbps rather than 112 kbps. Consequently, without departing from the spirit 
and scope of the invention, various alterations, modifications, and/or 
alternative applications of the invention will, no doubt, be suggested to 
those skilled in the art after having read the preceding disclosure. 
Accordingly, it is intended that the following claims be interpreted as 
encompassing all alterations, modifications, or alternative applications 
as fall within the true spirit and scope of the invention.