Note: Descriptions are shown in the official language in which they were submitted.
CA 02244~70 1998 - 08 - 07 1 1 P253CA
IMPLEMENTING MINET ON LAYERS 1 AND 2
OF THE ISDN PROTOCOL STACK
FIELD OF THE INVENTION
This invention relates to communication systems
and in particular to a system for implementing
integrated services digital network (ISDN) services in a
proprietary communication system.
BACKGROUND TO THE INVENTION
ISDN is a collection of standards which is being
implemented in communication systems, which provides the
capability to transmit and receive voice and digital
signals in relatively broadband channels and to provide
enhanced services. A first standard is referred to as
ISDN-1, and describes basic rate interface (BRI)
services. ISDN-1 is described in Bellcore Special
Report SR-NWT-001937. A more-advanced standard,
referred to as ISDN-2 provides more advanced services,
which includes primary rate interface (PRI). ISDN-2 is
described in Bellcore Special Report SR-NWT-002120,
issue 1, May 1992.
To implement ISDN, special line circuits are
required to be used in switching systems, that can
communicate with peripherals such as special telephone
sets using the particular communication protocol of BRI
or PRI. The communication protocol is transmitted in
frames, and is comprised of three layers: a physical
layer 1, a second layer referred to as layer 2, and a
third layer referred to as layer 3. Layer 1 carries
control signals for network and terminal termination
equipment, the transmission capability of B and D
channels and the related timing and synchronization
functions. Layer 2 carries link control data and layer
3 carries network control data.
While it is understood that a person skilled in
the art has knowledge of the above, reference is made to
the publications ITU-T Recommendations 1.430 which
describes layer 1, Q.921 which describes layer 2, and
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Q.931 which describes layer 3, by the International
Telecommunication Union (ITU), dated March 1993, which
are incorporated herein by reference.
Switching system manufacturers had found that
they could offer services advanced from ISDN-1, using
proprietary switching systems, proprietary peripheral
equipment (e.g. telephone sets), and proprietary line
circuits linking the switching systems and peripheral
equipment. To conform such equipment to ISDN standards
in order to accommodate customers who wish to use
standard ISDN peripheral equipment would remove some
advanced services already offered, which is undesirable
since it would make the equipment less competitive.
SUMMARY OF THE INVENTION
The present invention is a communication control
system which allows a user to maintain the switching
system which can offer the advanced services, and use
either the ISDN peripherals to obtain the restricted
ISDN services conforming to ISDN standards, or use the
proprietary peripherals to provide the ISDN and more
advanced services.
The above is done by utilizing a service access
point identifier (SAPI) subfield in the address field of
the layer 2 protocol to identify whether an ISDN
protocol or a proprietary protocol should be used to
communicate with the peripheral, and thus control
operation of the ISDN or proprietary peripheral. In an
embodiment of the invention, a control field of the
layer 2 protocol contains functionally oriented messages
in the event the SAPI subfield defines an ISDN
peripheral protocol and contains stimulus oriented
messages in the event the SAPI subfield defines the
proprietary peripheral protocol.
In accordance with an embodiment of the
invention, a communication control system comprises: (a)
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a switching system peripheral controller, (b) plural
high level data link controllers (HDLCs) communicating
with the peripheral controller, (c) plural basic rate
interface (BRI) line cards each communicating with a
respective HDLC, (d) at least one proprietary peripheral
connected to a first BRI line card, (e) at lease one
integrated services digital network (ISDN) peripheral
connected to a second BRI line card, and (f)
communication channels between the peripherals and the
BRI line cards, comprising data channels carrying data
conforming to (i) a physical layer, (ii) a second layer
conforming to an operating system interface (OSI) layer
2 ISDN standard protocol, and (iii) which selectably
conforms to a layer 3 ISDN standard protocol for
operating an ISDN peripheral and a proprietary protocol
for operating a proprietary peripheral.
BRIEF INTRODUCTION TO THE DRAWINGS
A better understanding of the invention will be
obtained by considering the detailed description below,
with reference to the following drawings, in which:
Figure l is a block diagram of a communication
control system in accordance with the prior art,
Figure 2 is a block diagram of a communication
control system in accordance with an embodiment of the
present invention,
Figure 3 is an illustration of a data packet
used in an ISDN layer 2 protocol,
Figure 4 is a block diagram of mixed types of
devices that can be used with the present invention, and
Figure 5 illustrates an arrangement of protocols
in accordance with another embodiment of the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
Figure l is a block diagram of pertinent parts
of an example proprietary communication switching
system, a type sold as SX2000tm PBX by Mitel
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Corporation. A description of details of this system
may be found in U.S. Patent 4,510,597 issued April 9,
1985 invented by Conrad Lewis, U.S. Patent 4,615,028
issued September 30, 1986, invented by Conrad Lewis and
Gino Totti, and in U.S. Patent 4,616,360 issued October
7, 1986, invented by Conrad Lewis and are incorporated
herein by reference.
In summary, a main control complex 1 controls a
peripheral control complex 3. The main control complex
is comprised of a main processor which communicates with
and controls a peripheral controller in the peripheral
control complex 3 via a message switch, and controls a
circuit switch which switches signals between
peripherals which signals pass through a peripheral
switch system in the peripheral control complex.
Peripherals such as proprietary digital
telephone sets 5 generate and receive signals via line
circuits 7, which interface the peripheral switch system
in the peripheral control complex. Each line circuit is
comprised of a microprocessor which communicates with
the peripheral switch system, and which interfaces a
high level data link controller (HDLC) 11. The HDLC 11
interfaces a multiplexer 13, which interfaces plural
digital network interface (DNI) circuits 15 to which the
peripherals 5 are connected.
The DNI circuits 15 can be type MT8972B digital
subscriber interface circuits sold by Mitel Corporation,
specification version MTS23 of which is incorporated
herein by reference.
In operation, the peripherals (below referred to
as telephone sets, for convenience) generate signals
(stimuli) which are received by the associated DNI
circuits. The DNI circuits are scanned via the
multiplexer 13 for control messages generated by the
telephone sets. These messages are routed, through the
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HDLC 11 and the microprocessor 9 to the peripheral
control complex, and subsequently to the main control
complex. Control messages are also routed in the
reverse direction.
In addition, the DNI circuits 15 are scanned for
the presence of connected telephone sets, and status
information is sent to the main control complex.
ScAnn;ng is needed in order to allow sharing of the DHLC
11 between several DNI circuits 15.
In order to provide specialized and advanced
services, the DNI circuits 15 communicate with the
telephone sets using proprietary protocols. Software or
firmware programs to implement these protocols are
stored in the DNI circuits, one referred to as MILAP,
which corresponds to the ISDN protocol layer 2, and
another referred to as MINET, which differs from the
ISDN layer 3 protocol in several ways. One of the
differences is that the MINET protocol is stimulus in
nature, to operate to the design of the proprietary
telephone sets 15. The MINET protocol has no
relationship to the state of a telephone call, and the
network layer concepts associated with call routing,
used in layer 3, are not implemented. The DNI circuit
does not support ISDN peripherals.
Reference is now made to Figure 2, which is a
block diagram of an embodiment of the present invention.
The proprietary communication system including the main
control complex 1 and the peripheral control complex, as
in the prior art, is maintained. However, the DNI
circuits 7 are replaced with basic rate interface (BRI)
line circuits 17.
The BRI line circuits 17 are comprised of a
microprocessor 9, but instead of an HDLC 11, plural
HDLCs 19 interface the microprocessor 9. Instead of DNI
circuits 15, BRI circuits 21 compatible with ISDN
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standards are used, each interfacing a corresponding
HDLC circuit 19. Scanning is not used. Having one HDLC
circuit 19 for each BRI circuit 21, and no scanning,
conforms to the ISDN standard.
S The proprietary system manufacturer's telephone
sets have their internal physical interfaces changed to
be compatible with the ISDN basic rate interface
standards (S interface). Now either or both the
proprietary telephone sets 5A and ISDN telephone sets 23
can be connected to corresponding BRI circuits 21.
The MILAP protocol used to correspond between
the line circuit and the telephone set, which
corresponds to layer 2 of the ISDN standard, is changed
to comply with the aforenoted Q.921 standard for layer
2, except in one regard, to be described below. In
addition, the BRI line circuit stores both the
proprietary MINET protocol as well as the ISDN Q.931 for
layer 3.
Figure 3 illustrates an example of a format of a
frame of data 24 transmitted in accordance with layer 2
protocol. The frame is comprised of first and last flag
fields each comprised of beginning and end 0 bits with
six 1 bits between, at least one address field, at least
1 control field, and frame check sequences (FCS). Part
of the address field is a service access point
identifier (SAPI) subfield 25.
In accordance with the ISDN standard, the SAPI
value identifies a point at which data link layer
services are provided by data link layer entity type to
a layer 3 of management entity. Thus the SAPI specifies
a data link layer entity type that should process a
datalink layer frame and also a layer 3 or management
entity which is to receive information carried by the
data link layer frame. The following SAPI values were
allocated by Q.921:
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SAPI VALUE Related Layer 3 or
Management Entity
0 Call control procedures
1 - 15 Reserved for future
standardization
16 Packet communication in X.25
level 3 procedures
17 - 31 Reserved for future
standardization
63 Layer 2 management
procedures
All others Not available for level 2
procedures
In accordance with an embodiment of the present
invention, one of the SAPI values in this layer 2
protocol identifies conformance with the ISDN Q.931
S protocol at layer 3, and value of the same bit or group
of bits identifies conformance with the proprietary
MINET protocol at layer 3. These bits in the layer 2
protocol can be established at the time of installation
of whichever sets 5A or 23, by the installer programming
the data stored in the BRI line circuits 21, or in the
factory, with the correct line circuit being plugged
into the line card 17 at a connector which corresponds
to the correct telephone set 5A, 23 at the time of
installation of the telephone sets.
The control messages supported by the above
should include both the stimulus oriented messages used
with the proprietary telephone sets 5A, and the
functionally oriented messages as required in layer 3
and described in the Q.931 standard. Some examples of
the latter type of message are call setup messages,
disconnect messages, etc.
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Figure 4 illustrates peripherals connected via a
single ISDN circuit terminating at a PBX, in accordance
with the present invention. A computer 26, requiring a
wide bandwidth e.g. to communicate video and/or data,
operates using the ISDN protocol. The protocol stack
used to communicate via the communication control system
is shown adjacent the computer, and is comprised of the
physical layer, layer 2 conforming to ISDN Q.921, and
layer 3 conforming to ISDN Q.931.
The communication control system can also
communicate with proprietary telephone set 5A, using the
protocol stack shown next to the telephone set 5A,
comprised of the physical layer, layer 2 conforming to
ISDN Q.921, and layer 3 conforming to the proprietary
protocol MINET.
It may be seen that devices that conform to ISDN
layers 1 and 2 can be mixed, since the SAPI field
identifies what protocol to use for layer 3. Services
such as enhanced telephone services can thus be provided
using proprietary peripheral equipment to a proprietary
communication control system that can at the same time
control standard ISDN peripherals.
Figure 5 illustrates the protocol stacks shown
in Figure 4, and in addition that used by the line cards
in the PBX. In the latter, it is shown that the
physical layer and layer 2 (Q.921) of the PBX
communicate with the physical layers and layers 2 in the
proprietary telephone set and in the ISDN terminal. The
PBX communicates in layer 3 with the proprietary
telephone set using the proprietary MINET protocol, for
example, and with the ISDN terminal in the Q.931
protocol.
Thus advanced call features implemented on
previous proprietary telephone sets using proprietary
protocols now will work identically with those sets that
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support layers 1 and 2 and the proprietary protocol on
layer 3. Because layers 1 and 2 that conform to the BRI
ISDN protocol are used, any stAn~Ard ISDN terminal, such
as a digital modem, may be connected to the same circuit
and operated normally assuming that the ISDN layer 3
call control has been implemented as described above.
A person understanding this invention may now
conceive of alternative structures and embodiments or
variations of the above. All those which fall within
lo the scope of the claims appended hereto are considered
to be part of the present invention.
