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Description  |
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FIELD OF THE INVENTION
The invention relates to interfacing an ATM network with a STM network.
BACKGROUND OF THE INVENTION
In telecommunications systems, the vehicle that will most likely be used
for offering a wide range of different high-bandwidth services, e.g.,
multimedia services, will most likely be based on Asynchronous Transfer
Mode (ATM) protocols. These protocols define a particular data structure
called a "cell", which is a data packet of a fixed size (53 octets, each
octet comprising eight bits). A cell is formed by a header (five octets)
and payload (48 octets) for transporting routing and user information.
The cell-routing concept in ATM is based on two aspects comprising a
routing field in the cell header containing a Virtual Path Identifier
(VPI) and Virtual Channel Identifier (VCI). The VCI and VPI pair have only
local significance on the link between ATM switches (nodes). ATM switches
as well as so-called cross-connect apparatus use routing tables to map VCI
and VPI values received via an incoming link to outgoing values and an
outgoing link as a way of routing the associated cell through the ATM
switch (or cross-connect apparatus). A virtual Circuit Link (VCL) is a
logical link between two switches (or a cross-connect nodes) and is
identified by a VCI value. Similarly, a Virtual Path Link (VPL) is a
logical link between two switches (or cross-connect nodes) identified by a
respective VPI value. A virtual Circuit Connection (VCC) is an end-to-end
connection between two devices and is formed by the concatenation of VCLs,
and a Virtual Path Connection (VPC) is formed by the concatenation of
VPLs. If an appreciable number of VCCs follow the same route segment, then
it is likely that they will share the same VPC associated with that
segment. In such a case, intermediate switches do not change the VCI
values, and, therefore, are referred to as VP switches.
Current telephone networks as well as their associated transmission media,
routing and cross-connection devices are digital circuit switched
facilities, in which the routing of user information, e.g., voice and
voice band-data services, from a source to a destination is via an
end-to-end switched connection, which is dedicated for the duration of an
associated call using the connection. That is, the call is set-up by
assigning it to an idle time slot (one for each link) of a frame of time
slots that are transported over a digital link(s) interconnecting
origination and destination switches. As such, the connection is
semi-permanent--lasting only for the duration of the associated call.
In contrast, an ATM network does not use dedicated time slots. Instead,
VCCs and VPCs share the network resources asynchronously. An ATM network
thus has to ensure that its resources are sufficient to handle the traffic
that is transported via the VCCs and/or VPCs that it has established (set
up).
It is well-known that current circuit-switched voice and voice-band data
services use one of a number of different signaling and messaging
techniques for the purpose of establishing a circuit switched connection
between Synchronous Transfer Mode (STM) switches or accessing network
databases to process special telephone services. Such signaling techniques
include in-band signaling using so-called "borrowed bits" associated with
a data stream; in-band signaling using Multi-Frequency (MF) tones, and
out-of-band signaling using a separate packet network. In-band signaling
using "borrowed bits" is used by customer premises equipment (e.g., a
private branch exchange) to signal an STM network switch over a digital
transmission facility. MF in-band signaling is still used in some of the
switches associated with Local Exchange Carriers (LEC) to set up a call
connection, but such signaling is being replaced by out-of-band packet
signaling, for example, the packet signaling provided by the well-known
Signaling System 7 (SS7). SS7 signaling is used by Interexchange Carrier
(IXC) networks (e.g., AT&T) to establish call connections over their
associated intertoll digital networks and to access network databases.
Advantageously, most, but not all, LEC switches are now being provided
with the SS7 type of out-of-band signaling capability.
Network switches perform other functions in addition to signaling. These
other functions include, for example, Digital Signal Processing (DSP)
functions such as detecting special tones, playing recorded announcements,
canceling echoes, etc.
Presently, the designers of telecommunications networks are seriously
considering replacing the STM switching and associated transport
facilities with Broadband ISDN (B-ISDN) based on ATM as the underlying
technology. What this means is that the circuit switched structure,
associated signaling systems, databases, operations systems, etc., will be
replaced by systems using ATM based transport, signaling and messaging. At
this point in time, it appears that changing the STM switched transport to
ATM transport may be relatively easy and could be accomplished in the near
future. However, network signaling and messaging have been designed and
developed over many years to guarantee that critical network applications
will operate correctly. It is therefore unlikely that the entire signaling
network will be converted at once to broadband signaling. It is also
unlikely that a telecommunications carrier (LEC or IXC) will replace its
entire STM network at once with a B-ISDN/ATM network, but will more likely
migrate toward that end in stages such that during intermediate stages of
the conversion network may be composed of STM and ATM elements.
Accordingly, there will be a need to develop technology that will
gracefully interface STM elements with ATM elements and allow ATM type
switches to interface with the different types of existing signaling
networks. The need for such interfacing has been recognized, but has been
limited to the transport of user information only. Accordingly, the
relevant technology has only advanced to the point of defining a Terminal
Adapter (TA) function to implement appropriate ATM Adaptation Layer (AAL)
protocols to interface conventional circuit-switched-transport protocols
(e.g., time slot protocols for voice and dedicated circuits) with ATM
based transport protocols.
SUMMARY OF THE INVENTION
Voice signals associated with a call and received from a STM switch may be
supplied, in accord with an aspect of the invention, to an associated ATM
switch for routing to an intended destination. Specifically, the voice
signals are accumulated as they are received from the STM switch via a STM
trunk to form the payload of an ATM data cell. When the payload of the
data cell is formed, then the payload and a header including a virtual
channel identifier determined, in accord with an aspect of the invention,
as a function of the identity of the STM trunk over which the voice
signals were received are supplied to the associated ATM switch for
routing.
In accord with another aspect of the invention, the foregoing determination
is made by translating the trunk identity on a one-to-one basis into the
virtual channel identifier. That is, the virtual channel identifier is
made to equal the identity of the trunk.
These and other aspects of the invention will be appreciated as they are
disclosed in the following detailed description and accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWING
In the drawing:
FIG. 1 is a broad block diagram of an illustrative ATM network in which the
principles of the invention may be practiced;
FIG. 2 illustrates a conventional busy/idle status table that is maintained
by a call processor of FIG. 1;
FIG. 3 illustrates a translation table that a call processor of FIG. 1 may
use to translate, in accord with the principles of the invention, the
address of an incoming trunk and associated trunk subgroup into an
incoming port, VCI and VPI;
FIG. 4 illustrates a translation table that a call processor of FIG. 1 may
use to translate between, in accord with the principles of the invention,
the address of an outgoing trunk and associated trunk subgroup into an
outgoing port, VCI and VPI;
FIG. 5 illustrates a table that an ATM input controller of FIG. 1 populates
with routing information relating to the routing of incoming VCI and VPI
to an outgoing port, VCI and VPI;
FIG. 6 is broad block diagram of an illustrative ATM network arranged to
interface with MF inband signaling in accord with an aspect of the
principles of the invention;
FIG. 7 is a simplified block diagram of an illustrative ATM network
arranged to interface, in accord with an aspect of the invention, a
signaling arrangement that is based on the "borrowed bit" scheme.
DETAILED DESCRIPTION
An exemplary embodiment of an ATM network arranged as an IntereXChange
Carrier network is shown in FIG. 1. ATM network 200 includes a plurality
of interconnected ATM switches. For the sake of clarity and simplicity
only two ATM switches are shown in the FIG., namely ATM switches 215 and
220. ATM switches 215 and 220 are connected to one another via an ATM
intertoll network 213 and are respectively connected to Local Exchange
Carrier (LEC) networks 100 and 300. Network 200 also includes a Signal
System 7 (SS7) network connected to the network 200 switches via
respective signaling links, e.g., links 154 and 155, and associated Signal
Transfer Points (STP), e.g., STPs 250-1 and 250-2. In accord with an
aspect of the invention, the network 200 ATM switches use the SS7 network
to communicate signaling information to each other for the purpose of
establishing respective virtual circuits, as will be discussed below in
detail. Thus, in accord with an aspect of the invention, the ATM switches
use the SS7 network to establish a virtual circuit connection, rather than
a circuit switched connection, as is the case in STM networks. The
signaling between the LEC CO switch and the IXC ATM switch may be in-band
or out-of-band using the SS7 signaling network. In accord with an
illustrative embodiment of the invention, a LEC CO switch may also use
out-of-band signaling.
As is well-known, a LEC network comprises a plurality of so-called local
Central Offices (CO) each of which may be, for example, the No. 5ESS
switch available from AT&T and described in the AT&T Technical Journal,
Vol. 64, No. 6, July/August 1985, pages 1303-1564. For the sake of clarity
and simplicity only one CO is shown in each LEC network 100 and 300. In an
illustrative embodiment of the invention, a CO switch operates in a
Synchronous Transfer Mode (STM) to transport speech signals associated
with a particular call over a circuit-switched connection in digital form.
That is, a CO samples analog speech signals that it receives from a
telephone station set at an 8 khz rate and supplies the resulting digital
signals (samples) synchronously at a 64 kbs rate over the associated
connection, in which each such sample is formed by, for example, eight
bits (8-bit byte). Conversely, A CO converts digital signals that it
receives over a circuit switched connection into analog signals and then
supplies the resulting signals to a telephone line connected to a
respective telephone station set engaged in the call.
More particularly, a CO, e.g., CO 25, responsive to receipt of a telephone
call originated by an associated telephone station set, e.g., station set
S1, and responsive to a user thereat dialing a telephone number
identifying a called telephone station set, e.g., station set S2, collects
the digits as they are dialed by user. When CO 25 receives the last of the
dialed digits, it then routes the call towards its destination via a trunk
selected as a function of the dialed telephone number. The selected trunk,
in turn, connects CO 25 to a next switch that will form part of the
connection from the calling station to the called station set. As is well
known, and as discussed to some extent above, a CO alerts the next switch,
i.e., ATM switch 215, by sending a call set-up message thereto via a
signaling path. As mentioned above, such signaling may be inband
signaling, out-of-band signaling. etc. Assume at this point that the CO
uses out-of-band signaling which is sent over an associated SS7 network.
With this signaling mechanism, a call set-up message contains, inter alia,
(a) the dialed telephone number, (b) ANI identifying the calling station,
(c) identifiers respectively identifying the trunk and associated trunk
subgroup that will be used to route the call to switch 215, (d) a request
for an end-to-end connection and (e) the address of switch 215. It will be
assumed at this point that CO 25 transmits the message via link 26 and STP
28 of the LEC 100 SS7 network. (The transmitted call set-up message will
also be referred to herein as an Initial Address Message (IAM).)
Responsive to receipt of the IAM, STP 28 forwards the message to a
destination STP--STP 250-1--identified as a function of the address
contained in the received message. In a similar manner, STP 250-1 forwards
the message to switch 215 via data link 154, which supplies the message to
signal processor 215-1 interfacing ATM switch 215 with the SS7 network.
Signal processor 215-1, in turn, presents the received IAM to call
processor 215-2. Call processor 215-2 stores the IAM in associated memory
and, identifies, as a function of the dialed number, the next, or
destination, switch that may be used to establish the next or last segment
of the connection to the destination CO, e.g., CO 50. Since the called
station set is station S2, then call processor 215-2 identifies ATM switch
220 as the destination IXC switch and identifies an idle one of its
outgoing trunks and associated trunk subgroup that may be used to forward
the incoming call to switch 220 via transmission path 213. Call processor
215-2 does this by first translating the dialed number into the Network
Switch Number (NSN) assigned to switch 220. Call processor then selects an
idle outgoing trunk that may be used to forward the call to switch 220.
Call processor 215-2 selects such a trunk by translating the determined
NSN into one or more trunk subgroups. Call processor 215-2 then consults a
trunk subgroup status map which it maintains in its associated memory to
identify an outgoing trunk in one of the latter subgroups that is idle and
may be used to forward the call to the destination switch. (An example of
such status map is shown in FIG. 2, which is self explanatory.)
Assuming that the selected outgoing trunk and subgroup are, for example,
outgoing trunk 213-1 of subgroup 60 (not shown in the FIG.), then
controller 215-2 forms its own IAM message containing the call information
and identities (i.e., 213-1 and 60) of the trunk and subgroup that will be
used to forward the call to switch 220. Call processor 215-2 then sends
the latter IAM to STP 250-1 via signal processor 215-2 and data link 154
for forwarding to switch 220. STP 250-1, in turn and in a conventional
manner, sends the message to STP 250-2 for delivery to ATM switch 220 vial
link 155. Similarly, the switch 220 call processor (not shown) locates an
idle outgoing trunk that may be used to connect the call to CO 50 and
similarly creates its own IAM message for transmission to CO 50 via link
155, STP 250-2, LEC 300 STP 53 and data link 51.
CO 50, responsive to receipt of the IAM determines if the telephone
connection 70 to station S2 is busy. If so, then CO 50 returns a busy
message indicative thereof to switch 220 via the LEG 300 SS7 network. In
response to receipt of the message, the call processor of switch 220
releases its outgoing trunk to CO 50 and forwards the busy message to
switch 215 via the network 200 SS7 network. Similarly, call processor
215-2 releases outgoing trunk 213-1 of subgroup 60 and forwards the
message to CO 25 via the SS7 networks of network 200 and LEC 100. CO 25,
in turn, supplies busy tone to station S1 and releases its outgoing trunk
to switch 215.
If, on the other hand, telephone line 70 is not busy, then CO 50 returns a
call complete message indicative thereof to switch 220 via the
aforementioned SS7 network, and supplies ringing voltage to telephone line
70. Switch 220, responsive to receipt of the call complete message passes
the message to its associated call processor. The switch 220 call
processor then (a) forwards the call complete message to switch 215 via
its associated signal processor and the SS7 network of network 200, (b)
changes the status of its incoming trunk and outgoing trunk that will be
involved in routing the call through its associated switching fabric to
busy and (c) advises its associated translator circuit (not shown) of the
connection involving the switch 220 incoming and outgoing trunks. (Since
the architecture and operation of switch 220 is similar to that of switch
215, any discussion relating to switch 215 equally pertains to switch 220.
Therefore, the following discussion of the operation of controller 215-3
equally pertains to the switch 220 controller, translator, etc., (not
shown).)
In particular, signal processor 215-1 upon receipt of the call complete
message via data link 154 passes the message to call processor 215-2.
Similarly, processor 215-2 (a) forwards the call complete message to CO 25
via processor 215-1, STP 250-1 and the LEC 100 SS7 network, (b) changes
the status of incoming trunk 27-1 and outgoing trunk 213-1 that will be
involved in routing the call through its associated switching fabric to
busy and (c) advises its associated translator circuit 215-3 of the
connection that should be established between incoming trunk 1 of subgroup
27-1 and outgoing trunk 213-1 of subgroup 60. Translator 215-3, more
particularly, translates the incoming trunk and trunk subgroup identifiers
received from call processor 215-2 into a form that is "understood" by
conventional ATM switch controller 215-5. That is, in accord with an
aspect of the invention, translator 215-3 translates the identifiers--1,
and 27-1--associated with the incoming trunk into (a) respective
predetermined VC and VP identifiers and (b) an incoming port circuit,
e.g., port 27-1. Translator 215-3 does this using translation Table 400
shown in FIG. 3. Briefly, Table 400 comprises a plurality of entries in
which each entry comprises five fields 411 through 415 containing
associated translation data. Referring to entry 401, for example, a data
entry includes a trunk identifier (Ti) and associated trunk subgroup
identifier (TSGi) in fields 411 and 412, respectively, which are
translated into a predetermined incoming port (Pi), and VCIi and VPIi
contained in fields 413 through 415, respectively. In accord with an
aspect of the invention, such a translation is done on a one-to-one basis
as shown for entry 402, which translator 215-3 accesses to translate the
trunk and trunk subgroup identifiers that it receives from call processor
2 15-2. For example, trunk and trunk subgroup identifiers and 27-1,
respectively, are translated on a one-to-one basis into a VCI of 1 and VPI
of 27-1, respectively, as shown by the data inserted in fields 414 and 415
of entry 402. The trunk and TSG are also mapped into an identifier
identifying an incoming port, i.e., incoming port 27, as shown by the
contents of field 413 of entry 402. (Translator 215-3 uses a similar table
to translate the outgoing trunk and outgoing trunk subgroup identifiers
(213-1 and 60, respectively) into an outgoing port identifier, VCIo and
VPIo. An illustrative example of such a table is shown in FIG. 4, in which
entry 501 is used to do the latter translation.) Translator 215-3 then
supplies the results of the translation to controller 215-5.
Controller 215-5 activates the virtual circuit connection from input port 1
to output port 2 13-1 so that speech signals originating at station set S1
and destined for station S2 may be transported over switch fabric 215-4
during the associated virtual connection. Controller 215-5 does this by
supplying the input VCI/VPI (1/27) to output VCI/VPI (213-1/60) mapping to
input port 27. Port 27, in turn, enters the output VCI/VPI mapping data in
a routing map. An example of the latter map is shown in FIG. 5. In
particular, each of the switch 215 (220) port circuits stores a routing
map 600 in its associated port memory (not shown). The contents of fields
602 and 603 of each entry in the table, e.g., entry 601, respectively
contain a virtual channel and virtual path identifiers. That is, the
virtual channels associated with a particular virtual path are entered in
field 602 of sequential entries in the table, as shown for entry 601 and
the following entries.
Thereafter, when a port receives routing information from its associated
controller 215-5, it enters the routing information in appropriate one of
the table 600 entries. For example, it is seen that routing information
has been entered in fields 604 through 606 of entry 601. Thereafter, when
input port 1 receives an ATM cell bearing a VCI and VPI of 1 and 27,
respectively, then it processes the cell in accord with the contents of
entry 601 of routing table 600, as will be explained below in detail.
Controller 215-5 also activates another, but opposite, virtual connection
from port 213-1 to port 27 to transport speech or data signals that
originate at station S2 and received via switch 220 and destined for
station S1. Accordingly, an opposite virtual connection may be so
activated when a cell carrying samples of station S2 speech samples (or
voice-band data) are received via switch 220. (It is noted that switch 220
performs similar routing functions in response to receipt of the call
complete message.)
When CO 25 receives the call complete message, it supplies an alerting tone
to telephone line 26 to notify the user thereat that the call connection
has been completed and that a ringing signal is being supplied to station
S2. When the user at station S2 answers the call, then he/she may
communicate with the station S1 user in which the ensuing speech (or
voice-band data) will be transported via ATM network 200. Specifically,
first considering speech signals received at CO 25 from station S1, CO 25
digitizes such signals in the manner described above and outputs the
result to its associated trunk 1 of TSG 27. (It is noted that the latter
trunk and TSG respectively correspond to a channel (channel 1) and group
of channels (group 27) of a time frame during which CO transmits a digital
sample of a station S1 speech signal over path 27-1. Echo Canceler 205
receives the digital sample and, in a conventional manner, cancels the
sample if it represents an echo of a digital speech sample originating at
station S2. If not, then the sample is presented to STM/ATM Terminal
Adapter 210.
TA 210, more particularly, is arranged to pack samples of voice signals as
they are received from STM switch 25 via trunk (channel) 1 of trunk group
27 into an ATM cell. TA 210 maintains a predetermined table which it uses
to map between trunks and VCIs and between trunk subgroups and VPIs
transported over link 211. When a payload of 47 or 48 octets (depending on
the particular ATM adaptation layer) have been so collected, then TA 210,
in accord with an aspect of the invention, translates the trunk address
and trunk group address over which the samples were received into a VCI,
VPI and incoming port address. In accord with another aspect of the
invention, such a translation is done at TA 210 (similarly so at TA 225)
on a one-to-one basis. Accordingly, TA 210 translates a trunk address of 1
and a TSG address of 27 into a VCI of 1 and VPI of 27, respectively. TA
210 then forms a cell header of five octets including the translated VCI
and VPI values and prepends (prefixes) the header to the 48 octet payload
to form an ATM cell. TA 210 then supplies the resulting ATM cell to
originating port 27-1 of switch 215. Port 27-1, responsive to receipt of
the cell, checks its associated routing table 600 to determine if routing
translation information has been stored therein for the VCI and VPI
contained in the received cell. If not, port 27-1 discards the cell.
Otherwise, port 27-1 translates the VCI and VPI contained in the cell into
an outgoing address. In the instant case, port 27-1 translates the VCI and
VPI of 1 and 27, respectively, into an outgoing port address of 213-1, VCI
of 213-1 and VPI of 60 based on the contents of entry 601 of table 600
(FIG. 5). Port 27-1 then substitutes the latter VC and VP identifiers for
the VCI and VPI identifiers contained in the received cell and presents
the result to switch fabric 215-4 for routing, in a conventional manner.
That is, switch fabric 215-4 routes the cell to port 213-1 via a virtual
circuit connection identified by the VC and VP identifiers attached to the
routed cell. Upon receipt of the cell from switch fabric 215-4, output
port 213-1 stores the cell in a queue (e.g., a First-In, First-Out memory)
associated with high-speed transmission path 213. When the data cell
reaches the top of queue, it is then unloaded from the queue and
transmitted, either by itself or part of a so-called super frame, over
path 213 to destination ATM switch 220. ATM switch 220 then, using its own
table 500, similarly translates the VC and VP identifiers in the received
cell into output VC and VP identifiers and then routes the cell via its
associated switch fabric and virtual circuit identified as a function of
the latter identifiers. Upon receipt of the cell via the associated switch
fabric, the switch 220 output port stores the cell in an associated queue.
When the cell is thereafter unloaded from the queue it is transmitted over
path 226 connected to TA 225. TA 225, in turn, translates the VC and VP
identifiers contained in the received cell into trunk and trunk subgroup
identifiers, in accord with an aspect of the invention. In accord with
above mentioned aspect of the invention, such translation is done on a
one-to-one basis. TA 225 then unpacks the payload of 48 octets of the
received cell and supplies them to the so-called ATM Adaptation Layer
(AAL) implemented in TA 225. The AAL (a) buffers the received octets, (b)
removes the AAL header, if any, (c) performs AAL functions with respect to
the received octets, and (d) then sends each octet in sequence to CO 50
via EC 230 and translated trunk and subgroup of path 52. As mentioned
above, the latter trunk and trunk subgroup may be a time slot of a group
of time slots, in which the such transmission of octets over path 231-1
occurs during the identified time slot.
As mentioned above, an STM switch may employ in-band MF signaling to
communicate signaling information to an IXC. We have recognized that ATM
network 200 may be readily adapted to receive such information via in-band
signaling and then, in accord with an aspect of the invention, present
such information to the originating ATM switch, e.g., switch 215, via
another signaling network, e.g., the SS7 network. Thus, the architecture
of network 200 does not have to change to interface with a signaling
technique different from the signaling technique employed by the SS7
network. Advantageously, then, ATM network 200 may interface with central
offices using different signaling techniques to communicate signaling
information to a next switch, wherein the next switch may be an ATM switch
rather than an STM switch.
Referring then to FIG. 6, assume that the user at station set S3 places a
call to station set S4 by going off-hook and dialing the telephone number
associated with the latter station set. When CO 175 has collected the last
of the dialed digits and has determined that the call is to be routed via
network 200, it selects an idle trunk connecting to network 200 and
transmits an off-hook signal thereto over the selected trunk and path 176.
Signal processor 240 of module 245 monitors the signals received via the
selected trunk of port 241 and returns a signal over the trunk to CO 175
if the call can be accepted by TA 255 (referred to as TA 210 in FIG. 1).
(It is noted that this will generally be the case for the illustrative
embodiment of the invention. However, it is understood that embodiments
may allow call blocking to increase the gain call (channel) multiplexing.)
Assuming that TA 255 accepts the call, then CO 175 begins to transmit the
dialed telephone number and caller's ANI via the selected trunk (digital
channel of path 176). Interface port 241 of module 245 multiplexes the
contents (eight bit byte) of each trunk (channel) to a respective signal
path 242 extending to an associated EC 244, which then presents the byte
to Terminal Adapter (TA) 255. TA 255, in turn, accumulates such bytes as
they are received from the source trunk to form a cell and then presents
the cell to an associated input port of switch 215, as described above.
However, the input port discards the cell since a virtual circuit
connection for the call has not yet been activated. If, on the other hand,
the data byte contains signaling information (e.g., dialed digits), then
port 241 extracts the signaling information and sends it to signal
processor 240 via path 242. Signal processor 240, responsive to a data
indicative of a MF signal appearing on path 242, collects the data and
succeeding such data until it has accumulated the signals indicative of at
least the called telephone number. Signal processor 240 then, as described
above, forms an SS7 IAM message containing inter alia, (a) the dialed
telephone number, (b) ANI identifying the calling station, if acquired (c)
identifiers respectively identifying the trunk and associated trunk
subgroup over which the calling information was received, (d) a request
for an end-to-end connection and (e) the address of switch 215. Signal
processor 245 then transmits the message via data link 157 and STP 250-4.
Switch 215 and then switch 220 process the IAM message in the manner
discussed above. That is, the switch 220 call processor locates an idle
outgoing trunk that may be used to route the call to destination CO and
then creates its own IAM message for transmission via link 155 and STP
250-2 to the network 200 signal transfer point that interfaces with that
CO. The latter STP, in turn, retransmits the message to STP 250-3 for
delivery to signal processor 240 of module 235. Responsive to receipt of
the IAM message via link 156, signal processor 240 of module 235 selects
the idle trunk to CO 180 (associated with the trunk from switch 250 to TA
225) and sends an off-hook signal thereto via port 241 of module 235 and
the selected trunk. If CO 180 can accept the call, then it returns an
off-hook signal via the latter trunk. Signal processor 240 of module 235
responds to the off-hook by transmitting the called number contained in
the received IAM message over the selected trunk to CO 180. In addition,
signal processor 240 of module 235 returns a call complete message to
switch 220 via the network 200 SS7 network, in which the latter message
contains the trunk and TSG of the trunk selected by the latter signals
processor. ATM switch 220 processes the call complete message in the
manner described above and transmits a call complete message to switch
215, which similarly processes the message in the manner described above.
As also mentioned above, switch 215 returns an SS7 call complete message
to the originating CO. However, in the instance case, the latter message
is sent via signal processor 240 of module 245. Signal processor, in turn,
sends an off-hook (wink) signal to CO 175 via the trunk that CO 175
selected to route the station set S3 call to network 200. As is
well-known, the latter wink signal is a functional equivalent of the SS7
call complete message. When the station S4 user answers the call, then the
S3 user may begin to communicate with the station S4 user via the virtual
connections respectively that are established by switches 215 and 220 as
they are needed.
When either the station set S3 or S4 user terminates the call--"hangs up",
then CO 175 or 180, as the case may be, sends an on-hook signal to network
200. Assuming that the on-hook signal is sent by CO 175 over the selected
trunk connecting to module 245, then signal processor 240 of module 245,
responsive to receipt of the on-hook signal (sent by port 241 of module
245), forms an SS7 network call termination message containing, inter
alia, the identity of the latter trunk and its associated TSG and then
sends the message to switch 215 via data link 157 and the SS7 network.
Upon receipt of the termination message, the switch 215 call processor (a)
directs the input port associated with the call to clear the entry that it
made in its translation Table 600 for the call, (FIG. 5), (b) sets the
status of the trunk to idle in the status table (FIG. 2) associated with
switch 215 and (c) sends a call termination message to switch 220, in
which the latter message identifies the trunk and TSG identifiers that
translate to VCI and VPI that are used to route the call from switch 215
to switch 220. The switch 220 call processor responds similarly to the
receipt of the latter message and sends a call termination message to the
destination CO via link 155 and STP 250-2 such that the message is instead
delivered to signal processor 240 of module 135 via STP 250-3 and link
156. The latter signal processor, in turn, transmits an on-hook signal to
CO 180. CO 180 sets the status of the return path of the trunk connecting
to module 235 to idle and then waits for the station set S4 user to
"hang-up".
As mentioned above, an incoming port of an ATM switch discards a data cell
if a virtual circuit connection for the associated call has not been
activated. Alternatively, such discarding may be done at the Terminal
Adapter, e.g., TA 255, 210. etc., at the direction of the call processor
of the associated ATM switch, e.g., switch 215. Specifically, TA 255 (210,
etc.,) may be arranged so that it receives control instructions from the
associated call processor via a communications path connecting the TA to
the call processor. Such a path may comprise a virtual circuit connection
from the call processor through the switch fabric to control port
connection to the TA via path 216. In this way, the call processor may
instruct the TA not to accumulate data received via a particular trunk,
i.e., an idle trunk. Thereafter, when the trunk become busy and a virtual
circuit has been assigned thereto, then the call processor instructs the
TA to begin forming data cells from the data received via the trunk.
As also mentioned above, the architecture of network 200 does not change to
interface with a method of signaling different than Signaling System 7.
ATM network 200 may thus interface with central offices, or other
entities, that use different signaling techniques, as discussed above in
connection with in-band signaling. One such entity that is commonly
referred to as a nodal, for example, a Private Branch Exchange (PBX), uses
a "bit borrowing" scheme to transmit signaling information. Turning then
to FIG. 7, there is shown nodal (PBX) 190 connected to network 200 via
communications path 195, in which the latter path 195 may be a so-called
T1 carrier transmission line. As is well-known, the transmission protocol
that is used in a T1 carrier system is a 125 microsecond frame composed of
24 channels in which each channel comprises eight bits. A telephone call
is routed via the T1 carrier system via a channel assigned to the call.
For example, PBX 190 routes long distance calls originating at PBX 190 to
network (or IXC) 200 by assigning each such call to a respective one of
the aforementioned channels for the duration of the call. In this sense a
channel is either busy (off-hook) or idl | | |
