Method and apparatus for establishing a wideband communication facility through a communication network having narrow bandwidth channels

A wideband communication facility is established via a switched network between calling and called terminals. The wideband facility comprises a number of segments, each segment including a plurality of narrowband time division multiplex channels having a total bandwidth at least equal to the bandwidth of the wideband facility. A switching system of the network interconnects the wideband facility segments and assembles all the data received in a given time frame from one segment into the single time frame for transmission to another facility segment. Additional buffer memories are added to the initial and final stages of a time-space-time switching network to insure that all the data received in one time frame from a given facility segment is assembled only into the same time farme for transmission on an outgoing facility segment.

TECHNICAL FIELD 
This invention relates to communication networks and particularly to a 
method and apparatus for establishing a wideband communication facility 
between customer terminals via a switched network. The invention further 
relates to apparatus for establishing a wideband communication facility 
utilizing a plurality of narrowband communication channels. The invention 
specifically pertains to apparatus for establishing the wideband 
communication facility without introducing time frame delay variations 
between the channels in which memory and control arrangements are provided 
for ensuring that a plurality of digital signals received synchronously or 
asynchronously are switched through a switching system in the same time 
frame and not disbursed throughout a plurality of time frames. 
BACKGROUND OF THE INVENTION 
With the advent of the information age, there is an increasing need to 
transmit large quantities of information among a multitude of customer 
terminals. The information, in the form of digital signals, customarily 
represents voice communication, video, facsimile, and/or bulk data 
transfers. 
While the existing public-switched telecommunication networks can offer 
access to a number of the customer terminals requiring wideband service, 
most digital communication within the switched network is limited to 64 
Kilobits per second (Kbps) due to the constraints imposed by existing 
switching and transmission facilities. 
In those specific applications requiring greater bandwidth, private line 
facilities can be provided between selected customer terminals. Such 
private line wideband facilities, however, are costly, and since they only 
serve a few terminals, they are frequently idle and not fully utilized. 
Combining several narrowband channels to form a wideband facility between 
customer terminals via the public-switched telecommunications network has 
been suggested in the prior art. However, correcting any time frame 
misalignment that may occur between the combined channels required the 
initial use of a test signal to detect time frame misalignment and a data 
recovery unit to compute any time frame misalignment and to introduce 
delay in selected ones of the narrowband channels to recorrelate the data 
into its original pattern. In addition to introducing costly equipment to 
the network, the prior art did not address the problem of how to correct 
for any time frame misalignment that may occur after an initial correction 
is made. Furthermore, significantly the art does not solve the problem of 
how to correct for time slot data delay from one time frame to another on 
the same channel. As a result, data is not received in the same order as 
it was transmitted. 
SUMMARY OF THE INVENTION 
The foregoing problems are solved and a technical advance is achieved by a 
novel method and apparatus for establishing a wideband communication 
facility through an illustrative switched telecommunications network 
having narrowband 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, a wideband facility is 
established between a calling and a called terminal through a switching 
system by establishing one segment of the facility between the calling 
terminal and a switching system and another segment between the switching 
system and the called terminal. Each segment is comprised of a group of a 
narrowband time multiplexed channels having a total bandwidth at least 
equal to the desired wideband facility. The switching system interconnects 
the two facility segments and advantageously assembles all the data 
received in a given time frame from the first segment only into the same 
time frame for transmission on the second segment. 
This method and apparatus is extendable to establish a wideband facility 
via any number of network switching systems. 
Illustratively, a segment of the wideband facility is established between 
each interconnecting switch serving the calling and called customer 
terminals. Each switch interconnecting two facility segments 
advantageously assembles all the data received in a given time frame from 
the incoming segment only into the same time frame for transmission on the 
outgoing segment. 
A calling customer terminal is enabled to specify the desired bandwidth of 
the wideband facility to meet the demands of that customer. Thus, any 
number of time division multiplex (TDM) channels are combinable to 
establish a desired wideband facility through a 
switched-telecommunications network. 
The bandwidth and destination of any wideband facility is controlled in 
response to a facility bandwidth request signal and a called customer 
identification signal sent from, for example, the calling customer 
terminal. 
Buffer memory and control arrangements are incorporated in initial and 
final switching stages of a specific time-space-time switching system 
network advantageously to correct time slot data delay variations from one 
time frame to another. This is accomplished by the switching network 
interconnecting the compositional wideband channels to transmit only 
narrow-band channel data in the same time frame as received. In an 
illustrative initial time slot interchanger where read and write cycles 
coincide, two such buffer memories are alternatively written into and read 
out of to prevent any time slot data time frame variations and to keep all 
the data received in a time frame in the same outgoing time frame. In an 
outgoing time slot interchanger where read and write cycles do not 
coincide due to switching network signal propagation delays, three buffer 
memories are successively written into and read out of to keep all the 
data received in a given time frame from the initial time slot 
interchanger for transmission in the same time frame to the outgong 
facility segment.

DETAILED DESCRIPTION 
Depicted in FIGS. 1 and 2 is an illustrative telecommunications network 100 
including switching systems 101 and 102. This network includes 
illustrative apparatus and utilizes an illustrative method for 
establishing a wideband communication facility between a calling customer 
interface terminal such as 103 and a called customer interface terminal 
such as 104 via a switching system such as 101. 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. In addition, a wideband communication facility 
may be, for example, 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 105 
serves a plurality of customer terminal equipment 114-117. The 
telecommunications network and customer interface terminals are 
interconnected by a plurality of digital lines 150-157 such as the 
well-known T-1 digital carrier line that serially transmits 64 Kilobits 
per second (Kbps) of data via 24 time division multiplex (TDM) channels 
having well-known clear channel capability. As shown, a first plurality of 
digital lines such as 150 and 151 interconnect switching system 101 and 
customer interface terminal 103. A second plurality of digital lines such 
as 152 and 153 interconnects switching system 101 and customer interface 
terminal 104. A third plurality of digital lines such as 154 and 155 
interconnects switching system 102 and customer interface terminal 105. 
Similarly, a fourth plurality of digital lines such as 156 and 157 
interconnect switching systems 101 and 102. Each TDM channel has a 
relatively narrow bandwidth for exchanging data at a maximum rate of, for 
example, 64 Kbps. 
In response to 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 TDM channels may be 
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 384 or 1536 Kbps as 
indicated by a facility bandwidth request signal. When a 384 Kbps 
bandwidth request signal is sent by the calling customer interface 
terminal, 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, 24-64 Kbps channels between 
switching system 101 and customer interface terminal 103 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. 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 switching system 
interconnects the two segments with a plurality of switching system 
network paths having a bandwidth at least equal to the customer-selected 
bandwidth and assembles all the data received in a given time frame from 
the first channel group established in the first facility segment only 
into the same time frame for transmission on the second channel group in 
the second facility segment to the called interface terminal. 
Telecommunications network 100 comprises a plurality of switching systems 
such as 101 and 102. Switching system 101 serves a plurality of customer 
terminal equipment such as computer 106, data terminals 107 and 108, 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 113 via customer interface 
terminal 105. The customer interface terminal equipment digitally 
multiplexes the data from a plurality of customer terminal equipment such 
as data terminals 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 and 157 
interconnects the switching 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 system 
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 Bell System Technical Journal, vol. 57, No. 2, 
February, 1978, 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 4ESS.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 interchanger (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 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, 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 system are performed by 
central processor 120. A typical central processor suitable for use in 
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 125 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 TDM channels on digital 
lines 150 and 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+D 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-band signaling interface unit 125 utilize a 
multilayered signaling protocol such as the Q.931 protocol described in 
the aforementioned PUB references. 
Customer interface terminals 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 a commercially available 
digital private branch exchange. 
One of the problems associated with grouping a number of TDM channels to 
establish a wideband communication facility is transferring the data in 
one time slot to another in the same time frame. For example, when the 
data in time slot 2 from a first group of TDM channels in a given time 
frame is to be written into time slot 17 of the same time frame, the data 
can be easily written into a buffer memory, read out of time slot 2, and 
written into time slot 17 of the same time frame. However, when the data, 
for example, from time slot 23 is to be read into time slot 7 of the same 
time frame, the data in time slot 23 cannot be buffered, read out of time 
slot 23, and written into time slot 7 of the same time frame along with 
the 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 written 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 overlap. 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 received in a given time frame in the 
same outgoing time frame, several buffer memories and memory control 
selectors were added to the receive and transmit time slot interchangers 
in switching networks 119 and 127. 
Depicted in FIG. 2 is a detailed block diagram of receive time slot 
interchanger 135, transmit time slot interchanger 136, and time multiplex 
switch 137 of switching network 119. Receive time slot interchanger 135 
comprises 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 connected as shown 
in interchanger 135. In addition, the read enable (RE) and write enable 
(WE) control signal leads where connected to memory control selector 206 
instead of buffer memory 200. This two buffer memory arrangement utilizes 
a flip-flop or alternating read-write cycle in which all the data in a 
given time 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 the subsequent 125 microsecond 
time period. For example as depicted in FIG. 4, when data from a given 
time frame TF1 is being written into a first buffer memory 200 during a 
125 microsecond time period T1, the data from the previous time frame TF0 
is read out of a second buffer memory 205. During the next 125 microsecond 
period T2, the read/write process is reversed. For example, during 125 
usec time period T2, the data from time frame TF2 is written into second 
buffer memory 205, and the data from time frame TF1 is read out of first 
buffer memory 200. Accordingly, the data in 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 received in the given time frame is assembled 
only 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 alternates the read/write operation between buffer memories 
200 and 205. 
Transmit time slot interchanger 136 as depicted in FIG. 2 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 
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. Accordingly, the 
read and write cycles of a time frame do not coincide as in receive time 
slot interchanger 135. To once again ensure time slot data of a given time 
frame remains in that time frame, buffer memories 212 and 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 consecutive time frames is consecutively written into buffer 
memories 207, 212 and 213, respectively. Similarly, interchanged time slot 
data is read out of buffer memories 207, 212, and 213 during three 
consecutive time frame periods such that the time slot data in 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 diagram depicting the read/write cycle of transmit time 
slot interchanger buffer memories 207, 212, and 213. As shown, time frame 
data 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 least one switching system. 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 a master channel. All the TDM 
channels have at least a busy and an idle state. The state of each channel 
is indicated, for example, in memory of each customer interface terminal 
and 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 switching system 101. 
A request for service message signal 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 indicated 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 bandwidth 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 acknowledgment 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 acknowledgment 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). As 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 communicated between calling and called 
terminals 103 and 104 is received in the same 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 indicated state of all the 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. 
As 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. 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-band signaling D channel and customer out-of-band 
signaling interface unit 125. Upon receipt of the service request, 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 discontinues 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 of the channels in the 
selected group are indicated in the idle state (block 616), the central 
processor 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 systems 
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 response 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 by advancing their indicated 
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 
acknowledgment 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 acknowledgment 
signal received from the called customer interface terminal (block 902). 
In addition, the acknowledgment signal is forwarded to first switching 
system 101 (block 903). 
Upon receipt of the acknowledgment 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 acknowledgment signal to calling customer interface signal 
terminal 103 (block 905), and calling and called terminals exchange data 
over the wideband 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 
acknowledgment signal to calling customer 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 
channels to the idle state (block 1005), returns a disconnect 
acknowledgment 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 acknowledgment 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 acknowledgment 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 apparatus for and method of 
establishing a wideband communication facility between a called and a 
calling customer terminal is merely an illustrative embodiment of the 
principles of this invention and that other apparatus and methods may be 
devised by those skilled in the art without departing from the spirit and 
scope of this invention. In particular, this method and apparatus may be 
utilized to establish a wideband communication facility across an entire 
telecommunications network by interconnecting any number of wideband 
facility segments with a plurality of network switching systems. In 
addition, each interconnecting switching system must be adapted to 
assemble all the data received in a given time frame from an incoming 
segment into the same time frame for transmission on the outgoing facility 
segment. Furthermore, the bandwidth of facility is selectable in response 
to a facility bandwidth request signal indicating the customer-selected 
bandwidth.