Method of and apparatus for establishing a wideband communication facility through a switched communications network having narrow bandwidth time division multiplexed channels

Method and apparatus are disclosed for establishing a wideband communication facility from a plurality of narrow bandwidth channels through a switched communications network from a first to a second terminal in response to a call from the first terminal indicating a customer-selected bandwidth for the wideband facility. Apparatus is also disclosed for establishing a wideband communication path through an illustrative switching system interconnecting groups of narrowband time division multiplexed channels established for the wideband facility. The path has a bandwidth greater than any of the TDM channels. Additional buffer memories and memory control arrangements are added to the initial and final stages of a time-space-time switching network to prevent any of the time slot data of a given time frame from being delayed and included in another time frame.

TECHNICAL FIELD 
This invention relates to communications networks having relatively narrow 
bandwidth time division multiplexed (TDM) channels and to switching 
systems for establishing therethrough a wideband communication facility 
wider than any of the TDM channels. The invention specifically pertains to 
arrangements for establishing a wideband communication facility through 
the network from a first to a second communications terminal without 
introducing time frame delay variaations between channels that are grouped 
together to form the wideband facility. 
BACKGROUND OF THE INVENTION 
While the existing public-switched telecommunications networks can offer 
access to a number of customer terminals requiring wideband data service, 
most digital communication within the switched network is limited to 64 
Kilobits per second (Kbps) due to the constraints imposed by existing 
switching systems and transmission facilities. 
In those specific applications requiring greater bandwidth, combining 
several narrowband channels to form a wideband facility between customer 
terminals via the network has been suggested in the prior art. One problem 
with these prior art arrangements is that each narrowband channel between 
customer terminals is established independently of the others, thereby 
establishing transmission paths with different transmission equipment, 
different lengths, and different propagation times. 
A second problem occurs when the data from grouped channels are not 
switched through a switching system in the same order as the data was 
received. This typically occurs when the timem slot interchanger of the 
switching system causes some, but not all, of the time slot data of a 
given time frame to be delayed and combined with the time slot data of 
another time frame. 
This second problem is aggravated when more than one stage of time 
multiplexed switching is utilized in a switching system. The variations in 
the length of the physical paths between two switching stages associated 
with the combined channels causes propagation time variations and time 
slot data misalignment. One prior art arrangement corrected these problems 
by sending a test signal at the beginning of a call to compute any delay 
of data from one time frame to another and by introducing delay in 
selected ones of the narrowband channels to correlate the ddata into its 
original patterns. In addition to introducing costly equipment to the 
network, this prior art arrangement does not address the problem of how to 
correct for any time slot data delay variations that may occur after an 
initial correction is made. 
SUMMARY OF THE INVENTION 
The foregoing problems are solved and a technical advance is achieved in an 
illustrative switched telecommunications network having narrow bandwidth 
time division multiplexed (TDM) channels by method of and apparatus for 
establishing a wideband communication facility through the switched 
network from a first to a second communications terminal in response to a 
call from the first terminal, the wideband facility having a 
customer-selected bandwidth greater than the bandwidth of any of the TDM 
channels. Advantageously, a customer may select the bandwidth of the 
wideband facility on a per call basis. Furthermore, TDM channels are 
selected and processed together through the illustrative network to reduce 
significantly differences in the path length of the channels through the 
network. 
In one illustrative embodiment, the wideband facility is comprised of two 
segments where the first segment is established between the calling 
terminal and a switching system in response to a call indicating the 
customer-selected bandwidth of the facility and the identity of the called 
terminal. The switching system of the network further responds to the call 
by establishing the second segment between the switching system and the 
called terminal to complete the wideband facility. 
The switching system receives a call service request message from the 
calling terminal on one of the TDM channels designated for out-of-band 
signaling and selects ones of the TDM channels for establishing the 
wideband facility with the customer-selected bandwidth indicaated in the 
call message. With the identity of the called terminal included in the 
call message, the processor also selects others of the TDM channels to 
establish the second segment of the facility. As a result, the switched 
telecommunications network is capable of providing Integrated 
In the final stage time slot interchanger where read and write cycles do 
not coincide due to switching network signal propagation delays, three 
buffer memories and a memory control arrangement are used for selectively 
writing the time slot data of three time frames into the three buffer 
memories during three consecutive periods of time. During a fourth period 
of time subsequent to the first period and overlapping the second and 
third periods, the time slot data of the first time frame is 
advantageously read out of the first buffer memory and interchanged after 
all the time slot data of the time frame has been written into the memory. 
Thus, all time slot data of a given time frame received from the initial 
stage time slot interchanger remains in that time frame for transmission 
on the outgoing channel group. 
The meory control arrangement associated with the three buffer memories 
includes an address arrangement for addressing a location in any of the 
three memories for each time slot of a time frame and a memory selector 
for selectively writing the data of a selected time slot of one time frame 
into an addressed location of one memory and reading the data of another 
selected time slot of another time frame out of an addressed location of 
another memory. The addressing arrangement includes a write time slot 
counter for indicating the three consecutive periods of time to the memory 
selector and a read time slot counter for indicating the fourth period of 
time to the selector in which data is to be read out of a memory.

Services Digital Network (ISDN) services with primary rate interface (23 
B+D) signaling on a single T-1 digital carrier transmission line. With two 
T-1 carrier lines, all 24 channels of one line are available to provide a 
1.536 Mbps bandwidth transmission facility for transmitting wideband data 
through a circuit-switched network. One channel of the other T-1 line is 
then utilized for common channel out-of-band signaling. 
To eliminate time slot delay variations within a switching system, this 
invention also includes novel apparatus for establishing a wideband 
communication path through the swiching system for communicating time slot 
data of a plurality of time frames between groups of narrow bandwidth time 
division multiplexed channels without any of the time slot data in one 
time frame being delayed to another time frame. 
In the illustrative embodiment of the invention, the wideband communication 
facility is established between a calling and a called terminal by 
establishing groups of narrow bandwidth time division multiplexed channels 
interconnected by one or more switching systems. Each group has a total 
bandwidth at least equal to the desired wideband facility. Each switching 
system establishes a wideband communication path to interconnect two 
groups of channels and advantageously assemblies all the time slot data of 
a given time frame from one channel group only into the same time frame 
for transmission on the other channel group. 
Buffer memory and control arrangements are incorporated in the initial and 
final switching stages of a specific time-space-time switching system 
network advantageously preventing any time slot data of a given time frame 
from being delayed to and included in another time frame. In an 
illustrative initial time slot interchanger where read and write cycles 
coincide, this is accomplished by two buffer memories and a memory control 
arrangement for selectively writing the data of a selected time slot of 
one time frame into one buffer memory and reading the time slot data of 
another time frame out of the buffer memory during the same period of 
time. In the subsequent period of time following the initial period, the 
time slot data of the first time frame is read out of the first buffer 
memory, and the time slot ddata of a third time frame is written into the 
second buffer memory. As a result, the time slot data of a given time 
frame is advantageously read out of a buffer memory and interchanged only 
after all the time slot data of the time frame has been written into the 
buffer memory. Thus, none of the time slot data of a given time frame is 
allowed to be delayed and included in another time frame. 
DETAILED DESCRIPTION 
Depicted in FIGS. 1 and 2 is an illustrative switched telecommunicaations 
network 100 including switching systems 101 and 102. This network includes 
illustrative apparaatus and utilizes an illustrative method for 
establishing a wideband communication facility between a calling customer 
interface terminal suchc as 103 and a called customer interface terminal 
suchc as 104 via a switching system such as 101 in response to a call from 
calling interface terminal 103. Network 100 serves customer interface 
terminals 103, 104, and 105 via pluralities of digital lines 150-151, 
152-153, and 154-155, respectively. Each digital line such as the 
well-known T-1 digital carrier line includes 24 time division multiplexed 
(TDM) channels. With well-known clear channel capability, each channel can 
serially transmit 64 Kilobits per second (Kbps) of data. To establish a 
wideband communication facility having a bandwidth wider than any of the 
narrow bandwidth TDM channels between interface terminals 103 and 104, 
groups of these TDM channels are formed and interconnected by switching 
system 101. The switching system includes illustrative apparatus for 
establishing a wideband communication path through the system for 
communicating time slot data between the formed groups without any time 
frame delay variations between the channels of the formed groups. In 
addition, a wideband communication facility may be established between a 
calling customer interface terminal 103 and a called customer interface 
terminal 105 via a plurality of switching systems such as 101 and 102. 
Customer interface terminal 103 serves a plurality of customer terminal 
equipment 106-109, and customer interface terminal 104 serves a plurality 
of customer terminal equipment 110-113. Customer interface terminal 105 
serves a plurality of customer terminal equipment 114-117. As shown, a 
first plurality of digital lines such as 150-151 interconnects switching 
system 101 and customer interface terminal 103. A second plurality of 
digital lines such as 152-153 interconnects switching system 101 and 
customer interface terminal 104. A third plurality of digital lines such 
as 154-155 interconnects switchiing system 102 and customer interface 
terminal 105. Similarly, a fourth plurality of digital lines such as 
156-157 interconnects switching systems 101 and 102. 
In response to a call includiing a service request signal sent by a calling 
customer interface terminal, a wideband communication facility having a 
customer-selected bandwidth wider than any of the 64 Kbps TDM channels is 
established between calling and called customer interface terminals to 
transmit data at a much higher rate of, for example, 384 Kbps or 1536 
Kbps. The customer-selected bandwidth of the wideband facility is 
indicated by a facility bandwidth request signal sent by the customer 
interface terminal to the serving switching system. For example, this is 
accomplished by establishing a first segment of the wideband communication 
facility between calling customer interface terminal 103 and switching 
system 101 in response to a service request sent by the calling customer 
interface terminal via an out-of-band signaling channel. This first 
segment comprises a first group of the TDM channels between switching 
system 101 and customer interface terminal 103 having a total bandwidth at 
least equal to the customer-selected bandwidth of, for example, 384 or 
1536 Kbps as indicated by the facility bandwidth request signal. When a 
384 Kbps bandwidth request signal is sent by the calling customer 
interface terminal, a group of six 64 Kbps channels between the switching 
system and customer interface terminal are selected to form the first 
segment of the wideband facility. Alternatively, when a 1536 Kbps 
bandwidth request signal is sent by the calling customer interface 
terminal, a group of 24-64 Kbps channels are selected to form the first 
segment. 
A second segment of the wideband facility is established between switching 
system 101 and called customer interface terminal 104 in response to a 
called terminal identification signal sent by the calling interface 
terminal with the service request signal to the switching system. This 
second segment comprises a second group of TDM channels between switching 
system 101 and called interface terminal 104 having a total bandwidth at 
least equal to the bandwidth indicated by the calling interface terminal. 
With the two segments of the wideband facility established, the switchiing 
system establishes a wideband communication path having a bandwidth at 
least equal to the customer-selected bandwidth to interconnect the two 
segments. The switching system interchanges all the data of a given time 
frame received from the first channel group only within that time frame 
for transmission on the second channel group to the called interface 
terminal. As a result, none of the data of a given time frame is delayed 
to another time frame, and all the data leaves the switching system in the 
same order as it was received. 
Switching system 101 serves a plurality of customer terminal equipment such 
as computer 106, data terminals 107 and 108, and video equipment 109 via 
customer interface terminal 103. Similarly, switching system 101 also 
serves computer 110, data terminals 111 and 112, and video equipment 113 
via customer interface terminal 104. Switching system 102 serves a 
plurality of customer terminal equipment such as computer 114, data 
terminals 115 and 116, and video equipment 117 via customer interface 
terminal 105. The customer interface terminal equipment digitally 
multiplexes the data from a plurality of customer terminal equipment such 
as data terminal 107 and 108 and transmits the multiplexed data to a 
called interface terminal via a wideband communication facility. The 
called interface terminal demultiplexes the multiplexed data and sends the 
demultiplexed data to the indicated called terminal equipment such as data 
terminals 115 and 116. High bit rate customer terminal equipment such as 
computers 106 and 114 may also be interconnected by a wideband 
communication facility established between customer interface terminals 
103 and 105. A plurality of digital lines such as 156-157 interconnects 
the switchiing offices of the communication network. Thus, a wideband 
communication facility may be established between wideband terminal 
equipment by selectively grouping and interconnecting narrowband TDM 
channels via the switching systems and customer interface terminals. 
Control signaling between switching systems 101 and 102 is facilitated by a 
well-known common channel signaling (CCS) system transferring information 
between switching systems 101 and 102. For example, this CCS systems 
includes well-known signal transfer point 118 and data links 158 and 159 
for transferring information separate from the TDM channels. The CCS 
system transfers messages indicative of well-known billing, control, 
routing and supervisory information. The CCS messages are also used to 
transfer requests for service, the customer-selected bandwidth of the 
communication facility, and called terminal identification. A typical CCS 
system is described in the The Bell System Technical Journal, Vol. 57, No. 
2, February, 1974, and in U.S. Pat. No. 3,624,613 of W. B. Smith et al., 
issued Nov. 30, 1971. Substitution of the 2STP system commercially 
available from AT&T for the 1STP system described in the cited CCS system 
reference is recommended for high volume message applications. 
Switching systems 101 and 102 are typical stored program-controlled systems 
such as the 4 ESS.TM. digital switch which is manufactured by AT&T 
Technologies, Inc. This switching system is described in The Bell System 
Technical Journal, Vol. 56, No. 7, September, 1977 and Vol. 60, No. 6, 
Part 2, July-August, 1981, and need not be fully described herein for the 
reader to understand the present invention. Basically, switching system 
101 comprises switching network 119, central processor (CP) 120, and 
digital interface frames (DIF) 121-123 interconnected by peripheral unit 
bus 124. Also connected to central processor 120 is customer out-of-band 
signaling interface unit (CSIU) 125 and CCS terminal 126. Miscellaneous 
equipment units have not been shown to simplify the drawing. System 102 
similarly comprises switching network 127, central processor 128, and 
digital interface frames 129 and 130 interconnected by peripheral unit bus 
131. Also connected to central processor 128 are customer out-of-band 
signaling interface unit 132 and CCS terminal 133. 
Switching network 119 has a time-space-time switching configuration that 
utilizes time slot interchangers (TSI) 134-136 and time multiplexed switch 
(TMS) 137. Access to switching network 119 is via digital interface frames 
121-123 which perform time division multiplexing and demultiplexing 
between switching network 119 and digital lines 150-153, 156, and 157. 
Furthermore, the digital interface frames buffer and synchronize the data 
between the digital lines and time slot interchangers. Digital interface 
frames 121-123 also process peripheral control signals from central 
processor 120 via peripheral unit bus 124. 
Time slot interchangers 134-136 provide the initial time-space and final 
space-time stages of time-space-time switching network 119. The 
interchangers receive incoming pulse coded modulated (PCM) samples over 
digital facilities in well-known DS-120 format where 120, eight-bit PCM 
channels are time division multiplexed with eight maintenance channels to 
form a 128 time slot frame. The receiving portion of a TSI buffers the 
incoming lines to allow synchronization of the data with switching network 
timing and performs the initial time-space switching before transmitting 
the data to the TMS. After passing through the TMS, the data is returned 
to the same TSI or another TSI where the final space-time conversion is 
performed. The TSI then reloads the data onto outgoing DS-120 lines which 
is transmitted to the appropriate digital interface frame and digital 
line. 
Time multiplex switch 137 is a two-switch array comprised of solid state 
cross points which provide a multiplicity of unidirectional paths between 
its inputs and outputs. Each network connection through TMS 137 is made in 
terms of a pair of unidirectional paths in one of the 128 time slots 
sharing the paths on a repeating basis at an 8 Kilohertz (Khz) rate. This 
8 Khz rate corresponds to a 125 usec time frame period. The switches are 
controlled by information contained in time slot memories (not shown), and 
this information is placed in the memory by the central processor under 
the control of call processing programs. 
The majority of the logic, control, storage and translations functions 
required for the operation of the switching systems are performed by 
central processor 120. A typical central processor suitable for use 
illustrative switching system 101 is described in The Bell System 
Technical Journal, Vol. 56, No. 2, February, 1977. 
Control signaling between the switching systems is facilitated by 
well-known CCS terminal 126 that is connected to central processor 120 via 
peripheral unit bus 124. Customer out-of-band signaling between central 
processor 120 and customer interface terminal is facilitated by customer 
out-of-band signaling interface unit 125 connected to central processor 
120 via auxiliary unit bus 138. Each plurality of digital lines such as 
150-151 includes at least one channel devoted to customer out-of-band 
signaling. When only one T-1 digital line is utilized between a customer 
interface terminal and a switching system, one of the 24 TDM channels is 
utilized for customer out-of-band signaling. When more than one T-1 
digital line is utilized, one out of every 48 TDM channels is utilized for 
customer out-of-band signaling. This is commonly referred to as 23 B+D or 
47 B+DD signaling as described in AT&T Communications PUB 41459, 
"Integrated Services Digital Network (ISDN) Primary Rate Interface", June, 
1985, and AT&T Communications PUB 41460 "Special Access Data Channel 
Interface", October, 1984. With the customer out-of-band signaling 
arrangement, groups of six or 24 TDM channels may be formed to establish a 
384 or 1536 Kbps wideband facility segment. The switching network 
separates the individual TDM channels and connects the customer 
out-of-band signaling (D) channel through the network to customer 
out-of-band signaling interface signaling unit 125 via the digital 
interface frame and switching network as shown. The customer interface 
terminals and customer out-of-bnd signaling interface unit 125 utilize a 
multilayered signaling protocol such as the Q.931 protocol described in 
the aforementioned PUB references. 
Customer interface terminal 103-105 are digital multiplexers for 
multiplexing and demultiplexing data transferred between the T-1 digital 
lines and the customer terminal equipment. The customer interface terminal 
also interfaces the customer out-of-band control signaling between the 
customer terminal equipment and the out-of-band signaling D channel. For 
example, a customer interface terminal may be commercially available 
digital private branch exchange. 
As previously suggested, one of the problems associated with grouping a 
number of TDM channels to establish a wideband communication facility is 
interchanging all the time slot data of a time frame without any time slot 
data being delayed to and included in another time frame. For example, 
when the data in time slot 2 from a first group of TDM channels of a given 
time frame is to be inserted intoo time slot 17 of the same time frame, 
the data can be easily written into a buffer memory during time slot 2, 
and read out of the buffer memory during time slot 17 of the same time 
frame. However, when the data, for example, from time slot 23 is to be 
inserted into time slot 7 of the same time frame, the data in time slot 23 
cannot be written into a buffer memory during time slot 23 and then read 
out during time slot 7 of the same time frame along withe he other data 
from the same group of TDM channels in the same time frame. Accordingly, 
the data in various time slots from a first group of TDM channels in a 
given time frame would be inserted into the time slots of another time 
frame, thus interchanging the order of the data associated with a given 
wideband communication facility. In addition, the read and write cycles of 
the final time slot interchanger typically overlap to compensate for 
switching network delay variations. This also causes time slot data to be 
delayed from one time frame to another. To solve this delay problem and 
keep all the data of a given time frame in the same time frame, several 
buffer memories and memory control selectors were added to the receive and 
transmit time slot interchanges in switching networks 119 and 127. 
Depicted in FIG. 3 is a detailed block diagram of receive time slot 
interchanger 135, transmit time slot interchanger 136, and time multiplex 
space switch 137 of switching network 119. Receive time slot interchanger 
135 includes well-known buffer memory 200, time slot counter 201, time 
slot memory 202, and address multiplexer 203 interconnected as shown. The 
functions of these units are well known and more fully described in the 
aforementioned switching system references. Since the read and write 
cycles of receive time slot interchanger 135 coincide, only one additional 
buffer memory 205 and memory control selector 206 were added and connected 
as showon in interchanger 135. In addition, the read enable (RE) and write 
enable (WE) control signal leads were connected to memory control selector 
206 instead of buffer memory 200. Time slot counter 201, time slot memory 
202, time slot multiplexer 203, and memory selector 206 form control unit 
204. Each of buffer memories 200 and 205 include a plurality of respective 
locations such as 220 and 225 addressable by control unit 204 for storing 
data associated with each time slot of a time frame. This two buffer 
memory arrangement utilizes a flip-flop or alternating read-write cycle in 
which all the data of a given frame received from a group of TDM channels 
is written into only one buffer memory during a 125 microsecond period of 
time and then read out of the same buffer memory during a subsequent 125 
microsecond time period. For example as depicted in FIG. 4, when data of a 
given time frame TF1 is being written into a first buffer memory 200 
during a 125 microsecond time period T1, the data of the previous time 
frame TF0 is read out of a second buffer memory 205. During the next 125 
microsecond period T2, the read/write proces is reversed. For example, 
during 125 usec time period T2, the data of time frame TF2 is written into 
second buffer memory 205, and the data of time frame TF1 is read out of 
first buffer memory 200. Accordingly, the data of a given time frame from 
a first group of TDM channels is buffered for a full time frame period to 
ensure that all of the data of the time frame remains in the same time 
frame when the data in the various time slots is interchanged. Well-known 
memory control selector 206 under the control of time slot counter 201 and 
read and write enable control signals from the TSI controller (not shown) 
alternates the read/write operation between buffer memories 200 and 205. 
Transmit time slot interchanger 136 as depicted in FIG. 3 comprises buffer 
memory 207, write time slot counter 208, read time slot counter 209, time 
slot memory 210, and address multiplexer 211 interconnected as shown. 
Write time slot counter 208 and read time slot counter 209 are included in 
the transmit time slot interchanger to provide a predetermined time slot 
delay between the read and write cycles of the buffer memories to 
compensate for time delays caused by the switching network components. The 
predetermined offset between write time slot counter 208 and read time 
slot couner 209 could also be generated by using one counter and one 
address circuit. Accordingly, the read and write cycles of a time frame do 
not coincide as in receive time slot interchanger 135. Control unit 215 
includes write time slot counter 208, read time slot counter 209, time 
slot memory 210, address multiplexer 211, and memory control selector 214 
for addressing and selectively reading and writing locations 227, 232, and 
233 in respective memories 207, 212, and 213. To once again ensure that 
the time slot data of a given time frame remains in that time frame, 
buffer memories 212 andd 213 under the control of memory control selector 
214 interconnected as shown have been added to transmit time slot 
interchanger 136. As a result, the time slot data of three time frame is 
consecutively written into buffer memories 207, 212 and 213, respectively, 
under the control of unit 215. Similarly, interchanged time slot data is 
read out of buffer memories 207, 212, and 213 under the control of unit 
215 during three consecutive time frame periods such that the time slot 
data of any given time frame is not read out of the same memory when data 
is being written into the memory. 
FIG. 5 is a timing diaagram depicting the read/write cycle of transmit time 
slot interchanger buffer memories 207, 212, and 213. As shown, time slot 
data of time frame TF1, TF2, and TF3 is consecutively written into first, 
second, and third buffer memories 207, 212, and 213 during three 
consecutive 125 usec time periods T1, T2, and T3, respectively, As 
previously indicated, a multiple time slot delay between the initial read 
and write cycles of 125 usec time periods T1-T'1, T2-T'2, and T3-T'3 is 
inserted by write and read time slot counters 208 and 209 such that 125 
microsecond time period T'1, for example, starts 17 time slots after time 
period T1. However, once initiated, the time slot data in third, first and 
second buffer memories 213, 207, and 212 is consecutively read out during 
time periods T'1, T'2, and T'3 such that the time slot data of a given 
time frame is never read out of a buffer memory when data of another time 
frame is being written into the same buffer memory. 
Depicted in FIGS. 6-11 is a flow chart illustrating an illustrative method 
for establishing a wideband communication facility between a calling and a 
called customer terminal via at lease one switching system in response to 
a call from the calling terminal. For purposes of illustration, let it be 
assumed that a customer served by customer interface terminal 103 desires 
to establish a wideband communication facility to called customer 
interface terminal 104 via switching system 101. In this illustrative 
embodiment, the TDM channels of digital lines 150-157 are segregated into 
groups of six TDM channels with the first channel in each group being 
designated as as master channel. All the TDM channels have at least a busy 
and an idle state. The state of each channel is indicaated, for example, 
in memory of each customer interface terminal and the switching system 
serving the channel. In the idle state, the TDM channel is available for 
use. In the busy state, the TDM channel has been selected for use or is 
being used. Specifically, it is desired that a wideband communication 
facility be established between computer 106 and 110 via customer 
interface terminals 103 and 104 and switchcing system 101. A request for 
service message signal included in the call is sent from a calling 
customer interface terminal for a master channel associated with a 
particular group of TDM channels. Calling customer interface terminal 103 
sends the request for service message signal on the out-of-band signaling 
D channel of digital line 150 for a master channel in an idle state (block 
601 of FIG. 6). The request for service message signal typically includes 
a called terminal identification signal and a facility bandwidth request 
signal indicating the customer selected bandwidth for the wideband 
communication facility. Central processor 120 receives the request for 
service signal from calling customer interface terminal 103 via customer 
out-of-band signaling interface unit 125 (block 602). In this example, a 
calling customer can select either a 384 or a 1536 Kbps bandwidth. When a 
384 Kbps rate bandwidth is customer selected (block 603), central 
processor 120 selects six 64 Kbps TDM channels to form a first group of 
TDM channels for the first segment of the wideband facility between 
calling interface terminal 103 and switching system 101 (block 604). 
Stored program-controlled central processor 120 examines the indicated 
state of each TDM channel in the selected group to determine whether each 
channel is in an idle state (block 605). When any channel in the selected 
group is not in an idle state, the central processor sends an out-of-band 
message to the customer interface terminal 103 denying the service request 
and discontinues establishing the wideband facility (block 606). When all 
of the channels in the selected group are in an idle state, the central 
processor reserves the selected group for the first facility segment by 
advancing the indicaated state of all the channels in the selected group 
to a busy state (block 607). 
Having reserved the first group of channels for the first facility segment, 
central processor searches for a group of idle channels to the called 
terminal as indicated by the called terminal identification signal sent by 
the calling interface terminal (block 608). The bandwidh of the idle 
channel group to the called terminal must be at least equal to the 
customer selected bandwidth as indicated by the facility bandwidth request 
signal. When an idle channel group cannot be found, the central processor 
denies the service request from the calling customer terminal and 
discontinues establishing the wideband facility (blocks 609 and 610). When 
an idle group of channels has been found, central processor 120 reserves 
the selected group of idle channels for the second segment of the called 
terminal by advancing the indicated state of the channels in this second 
group to a busy state (block 611). 
After all the channels in the second group have been reserved for the 
second facility segment, the central processor selects six network paths 
through switching network 119 to interconnect the first and second channel 
groups (block 701 of FIG. 7). The central processor via customer 
out-of-band signaling interface unit 125 sends the request for service and 
facility bandwidth request signals to called customer interface terminal 
104 (block 702). In response, the called customer interface terminal 
returns an acknowledgement message signal to central processor 120 of 
switching system 101 via the out-of-band D signaling channel (block 703). 
As a consequence, switching network 119 interconnects the first and second 
channel groups via the selected network paths (block 704). Switching 
system 101 then forwards the acknowledgement signal to the calling 
terminal 103 via the out-of-band D signaling channel (block 705). 
When the wideband communication facility is established, the calling and 
called terminals exchange data on the facility via switching network 119 
(block 706). A previously described, switching network 119 assembles all 
the time slot data received in a given time frame from the first channel 
group only into the same time frame for transmission to the second channel 
group. This ensures that the data communication between calling and called 
terminals 103 and 104 is received in the samae order as it was sent. 
Upon receiving a disconnect signal from either of the calling and called 
terminals (block 707), central processor 120 disconnects the first and 
second channel groups at the switching network and changes the indicated 
state of the first and second facility group channels to the idle state 
(block 708). When the indicataed state of all channels in the first and 
second groups is changed to the idle state, the channels are available for 
subsequent use by the customer terminal equipment served by customer 
interface terminals 103 and 104. 
A previously suggested, a wideband communication facility may interconnect 
two customer interface terminals through two or more switching systems. To 
illustrate this example, let it be assumed that computers 106 and 114 
desire to be interconnected with a wideband communication facility via 
first and second switching systems 101 and 102 in response to a call from 
the calling terminal. As previously illustrated in FIG. 6, calling 
customer interface terminal 103 sends a request for service message signal 
on the out-of-band D signaling channel to switching system 101 for a 
master channel in an idle state (block 601). Switching system 101 receives 
the request for service message signal along with the called terminal 
identification and the facility bandwidth request signals (block 602). 
This customer signaling information is received via the customer 
out-of-ban signaling D channel and customer out-of-band signaling 
interface unit 125. Upon receipt of the service requst, central processor 
120 analyzes the service request and determines the customer-selected 
bandwidth for the wideband facility (block 603). When a 1536 Kbps 
bandwidth is customer-selected, the central processor selects a first 
group of 24 channels in a digital line for the first segment of the 
wideband facility (block 612). The central processor then determines 
whether the indicated state of all of the first group channels are in the 
idle state (block 613). When the indicated state of any of the first group 
channels is not in the idle state, the central processor denies the 
service request and dicontinues establishing the facility (block 606). On 
the other hand, when all of the channels in the first group are in the 
idle state, the central processor reserves all the second group channels 
by advancing the indicated state of all the channels in the selected group 
to the busy state (block 614). 
After all the first group channels have been reserved, central processor 
120 searches for a second group of 24 idle channels to second switching 
system 102 that serves customer interface terminal 105 (block 615). The 
second channel group is selected in response to the called terminal 
identification signal sent by the calling customer interface terminal 103. 
When an idle channel group to switching system 102 cannot be found, the 
central processor denies the service request and discontinues establishing 
the facility (block 610). However, when all ofo the channels in the 
selected group are indicated in the idle state (block 616), the central 
procesor reserves the second group channels by advancing their indicated 
state to busy (block 617). 
Central processor 120 then selects 24 network paths through switching 
network 119 to interconnect the first and second channel groups (block 801 
of FIG. 8). The request for service, bandwidth, and called terminal 
identification signals are sent to second switching system 102 via the 
common channel signaling system (block 802). As shown in FIG. 2, 
out-of-band signaling information is transferred between switching 
sysstems 101 and 102 via signal transfer point (STP) 118 and data links 
158 and 159. In response, switching system 102 reserves all the second 
group channels from switching system 101 by advancing their indicated 
state to the busy state (block 803). 
Following a process similar to that performed by switching system 101, 
central processor 128 of switching system 102 searches for a group of idle 
channels to the called customer interface terminal in responses to the 
receipt of the called terminal identification and facility bandwidth 
request signals (block 804). When an idle group of channels to the called 
customer interface terminal can not be found (block 805), central 
processor 128 denies the service request and discontinues establishing the 
facility (block 806). When a third group of idle channels between 
switching system 102 and called terminal can be found, central processor 
128 reserves all the third group channels be advncing their indicaated 
state to the busy state (block 807). Central processor 128 then selects 24 
network paths through switching network 127 to interconnect the second and 
third channel groups (block 808). The service request and bandwidth 
request signals are then sent to the called customer interface terminal 
105 (block 809). 
Upon receipt of the service request and bandwidth request signals from 
switching system 102, called customer interface terminal 105 returns an 
acknowledgement signal to second switching system 102 (block 901 of FIG. 
9). Switching network 127 interconnects the first and second channel 
groups with the selected network paths in response to the acknowledgement 
signal received from the called customer interface terminal (block 902). 
In addition, the acknowledgement signal is forwarded to first switching 
system 101 (block 903). 
Upon receipt of the acknowledgement signal from switching system 102, 
switching network 119 interconnects the first and second channel groups 
with the selected network paths (block 904). Switching system 101 then 
forwards the acknowledgement signal to calling customer interfac signal 
terminal 103 (block 905), and calling and called terminals exchange data 
over the widebnd communication facility consisting of the interconnected 
first, second and third channel groups (block 906). 
Upon completion of the data exchange between computers 106 and 114, calling 
customer interface terminal 103 sends a disconnect signal to first 
switching system 101 (block 907). In response, switching system 101 
forwards the disconnect signal to second switching system 102 (block 908). 
Switching system 101 also disconnects the first and second channel groups 
(block 1001 of FIG. 10), changes the indicated state of the first group of 
channels to the idle state (block 1002), and returns a disconnect 
acknowledgement signal to calling cusomer interface terminal 103 (block 
1003). 
Similarly, in response to the receipt of the forwarded disconnect signal 
second switching system 102 disconnects the second and third channel 
groups (block 1004), changes the indicated state of the second group of 
the channels to the idle state (block 1005), returns a disconnect 
acknowledgement signal to first switching system 101 (block 1006) and 
forwards the disconnect signal to the called customer interface terminal 
(block 1007). 
In response to the receipt of the disconnect acknowledgement signal, first 
switching system 101 changes the indicated state of the second group of 
channels to the idle state (block 1101 of FIG. 11). 
In response to the receipt of the disconnect signal, called customer 
interface terminal 105 sends a disconnect acknowledgement signal to second 
switching system 102 (block 1102), and switching system 102 responds by 
changing the indicated state of the third group of channels to the idle 
state (block 1103). 
It is to be understood that the above-described method of and apparatus for 
establishing a wideband communication facility through a switched 
communications network between a first and second terminal in response to 
a call from the first terminal and apparatus for establishing a wideband 
communication path for communicating wideband data beween groups of 
narrowband TDM channels without time frame delay variations between the 
channels is merely an illustrative embodiment of the principles of this 
invention and that other apparatus may be devised by those skilled in the 
art without departing from the spirit andd scope of this invention. In 
particular, this apparatus may be utilized to establish a wideband 
communication path through any time division switching system serving 
narrowband TDM channels without causing time frame delay variations 
between groups of channels that are formed to establish a wideband 
communication facility. Furthermore, the method of and apparatus for 
establishing a wideband facility through a switched network between first 
and second terminals in resposne to a call from the first terminals may be 
utilized to provide "bandwidth on demand".