Asynchronous transfer mode switching system

An asynchronous transfer mode switching network can be made to look like a synchronous tandem switch to end offices connected to the network by establishing a permanent virtual path through the network that carries information between the end offices. Individual channels to be switched are assigned ATM VCI addresses at both ends that correspond to the time slot channel being sent and a time slot channel being received.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to communication systems and 
communication networks. In particular, the present invention relates to 
asynchronous transfer mode communication networks and other communication 
networks that support virtual paths and virtual channels. 
2. Description of the Prior Art 
Many of the telecommunication networks in use today are synchronous digital 
networks. Digitized voice and other digital communications signals are 
transmitted synchronously (clocked) across these networks at a fixed rate, 
1.544 million bits per second for instance. When they are used to carry 
only voice signals, synchronous networks work quite well. Discrete time 
periods known as time slots are packed with the digital information for a 
particular call. Digital information for many calls is sequentially packed 
together, forming what is known in the art as a time division multiplexed 
(TDM) data stream, and transmitted serially, in the same sequence such 
that when the serial bit stream is recovered at a destination, the digital 
information in the discrete time intervals is extracted and used to 
reconstruct the original signal. 
Currently, voice, data and/or video are generally carried on separate 
networks each having capabilities that permit the transmission of these 
signals. In addition, high-rate digital signals are difficult to switch 
through current synchronous networks; they might require the concatenation 
of multiple 64 kilobit signal streams for example and be carried on 
separate communications networks that are capable of switching high-rate 
data. 
A relatively new technology known as asynchronous transfer mode switching, 
or ATM, provides a method by which voice, data, video, or other 
information, (hereafter "data") at differing data rates, can be carried on 
a single network. 
Asynchronous transfer mode is exactly what its name suggests: it's 
asynchronous. Packets of information, also known as ATM "cells" comprised 
of 53, 8-bit bytes that can represent video, audio, data or other 
information, are switched at high speeds through asynchronous transfer 
mode switches, asynchronously with respect to other cells. The first 5 
bytes of an ATM cell contain address and other information used to route 
the ATM cell to a destination and is known as a header. By convention, 
particular bytes of the 5-byte header are designated to contain 
predetermined types of information. FIG. 1 is a chart showing the 
structure of an ATM cell header block. In general, an ATM cell header 
block comprises at least two addresses: a VCI address and a VPI address. 
The address fields of the 5-byte header are used by ATM switches to route 
an ATM cell to a desired destination, i.e., the intended recipient of the 
remaining 48 bytes of the ATM cell. 
The 48 bytes of an ATM cell following the header carry information of 
interest, such as video and audio. While the individual bits that make up 
the 53 bytes of an ATM cell are synchronous with respect to each other, 
thereby permitting information recovery therefrom, ATM cells are 
asynchronous with respect to each other. ATM transmission rates are also 
typically much higher that most synchronous networks, thus an ATM network 
can accommodate a variety of different-rate data sources by blending ATM 
cells from a variety of data sources into an ATM network. 
Today, ATM switching networks consisting of ATM switches linked together 
through various transmission media such as fiber optic cable, coax, or 
twisted wire pairs, receive incoming ATM cells and re-send or re-transmit 
ATM cells to other ATM switches. ATM switches use the VPI and VCI 
addresses and other information in the header, (as shown in FIG. 1) to 
route incoming ATM cells to appropriate outbound channels that direct the 
ATM cell to its intended destination. In the process of switching an ATM 
cell, an ATM switch can rewrite the 5-byte header block with new VPI and 
VCI data. The newly written 5-byte header is used by subsequent ATM 
switches to route the cell through to other ATM switches or to an end 
office where an asynchronous-to-synchronous converter (also known as a 
SAC) coupling an end office to the ATM network can convert the information 
in the 48-byte information block to the signal or information of interest, 
e.g., a video conference image or voice of a telephone call. ATM switches 
coupled together can effectively route calls anywhere if the switches are 
programmed (provisioned) to know where incoming ATM cells that come in to 
a switch are supposed to go to and how the outgoing ATM cells are to be 
addressed for the next ATM switch. 
As set forth above, data transfer rates of ATM networks are typically much 
higher than on synchronous networks giving an advantage to an ATM network 
when the information to be switched is high rate, such as digitized video. 
While ATM networks can handle varying data rate information sources and 
have a speed advantage over synchronous networks, ATM switches in a 
network that are carrying a signal from a source to a destination must all 
be programmed to route cells correctly through the network. This 
programming creates an operating overhead that takes the form of 
programming to each switch. 
FIG. 2 shows a block diagram of a prior art asynchronous transfer mode 
switching network (200) known as a "permanent virtual circuit" network 
(200) or "PVC" the purpose of which is to route a call from a first end 
office (202) to a second end office (204). Such a connection would be 
required for a typical phone call. End office switch 202 is a telephone 
end office into which telephone calls are received from end users or 
customers of the phone company and from which time-division-multiplexed 
trunk groups (218) carry synchronously encoded digital information 
destined to another end office 204. End office 204 typically has other 
customers or telephones connected to it. 
In the PVC network (200) shown in FIG. 2, a path or route is set up, 
maintained, and exists for every possible end office 204 that end office 
202 can be connected to by programming intervening ATM switches (214, 215, 
216 and 217) with information needed by each switch to enable it to "know" 
where incoming information is coming from and where to route it to. A 
problem with the PVC networks (200) shown in FIG. 2 is that they require 
infrastructures in the form of physical trunks to exist between each and 
every end office and an ATM switch. Those skilled in the art will 
recognize that to an end office (202) switch, a PVC network (200) looks 
like direct connections to other switches. Each trunk that originates from 
an end office is routed through ATM switches (214 and 216) using paths 
that are set up (provisioned) and which are infrequently changed. In 
implementation, reference numeral 218 identifies multiple trunk groups, 
each one of which typically carries synchronous communication signals 
destined to a different end office switch. 
In order to be switched through an ATM network, synchronous signals on the 
trunk groups 218 must be converted to an asynchronous transfer mode format 
which is the function of the synchronous to asynchronous converter, or 
SAC, (210). The synchronous-to-asynchronous converter (210) receives 
synchronous information and assembles blocks of asynchronous transfer mode 
information, each of which has a header block appended to it that is an 
address that routes the cell across at least one ATM switch (214) toward a 
destination ATM switch (216). It is also well-known in the art that the 
information in the 5-byte ATM cell header block typically gets the cell 
across only one switch (214), the traversal of which by the cell is 
accompanied by new data being written into the header. In the permanent 
virtual circuit shown in FIG. 2, a permanent path is set up through 
particular switches and channels thereof. The synchronous-to-asynchronous 
converter (210) converts the synchronous information to an ATM cell and in 
the process, effectively maps the destination of the formerly synchronous 
information of each trunk of trunk groups (218) to a particular end office 
on a trunk-by-trunk and switch-by-switch basis by way of the information 
it programs or writes into the 5-byte header it creates for ATM cells it 
formats. 
The output of the SAC (210) is a plurality of asynchronous transfer mode 
cells or packets each of which is 53 bytes in length. As set forth above, 
the first five bytes being effectively the address of the destination, the 
following 48 bytes being the information from the customer or source 
coupled to the originating end office switch (202). ATM cells or packets 
from a particular trunk in trunk group 218 destined to a particular 
office, are addressed with the effective address of an ATM switch (216) to 
which the packets are sent. This second ATM switch (216) routes the ATM 
cell to the appropriate end office through an asynchronous-to-synchronous 
converter (212) that is coupled to the destination end office (204) for 
the information originating at the sending end office (202). 
FIG. 1 shows the format of an ATM header block. It contains an address that 
effectively identifies an ATM switch to which the packet is to be sent as 
well as other routing information necessary to route the cell through the 
network (221) shown in FIG. 2. Each ATM switch (214 and 216) in the 
network (221) includes tables that are each preprogrammed with information 
necessary for the switches (214 and 216) to resend ATM cells rewritten 
with address data required to route the cells through the next switch in 
the network (221). 
The ATM switches shown in FIG. 2 (214 and 216) are well-known ATM permanent 
virtual circuit switches (ATM-PVC). Each of these switches themselves have 
addresses for ATM cells addressed to them; decode address information in 
the ATM header cells and route ATM cells to an asynchronous-to-synchronous 
converter (212) the function of which is to recreate a time division 
multiplexed signal, i.e., a synchronous signal from the asynchronous ATM 
cell. The terminating end SAC (212) reconstructs the synchronous time 
division multiplexed (TDM) data stream originally output from the end 
office switch 202. End office switch 204 receives synchronous TDM 
information from the asynchronous-to-synchronous converter (212) (also 
known as a synchronous-to-asynchronous converter or SAC) and routes the 
call to the appropriate end user using signaling information (228) it 
receives from the originating end office signaling transfer point or STP 
(206) and its own local STP (208) when the call is first set up. 
The embodiment shown in FIG. 2 generally switches calls through the 
asynchronous transfer mode network 221 at a relatively high speed because 
each route from an end office switch 202 to an end office switch 204 is 
established and, as is known in the art, is pre-provisioned. The 
connection between an end office switch 202 and 204 is established with 
signaling information for the call between the two end offices (202 and 
204) being carried on an interoffice signaling network comprising in part 
signaling transfer points or STPs 206 and 208. 
A drawback of the architecture of the embodiment shown in FIG. 2 is the 
need to have a network connection for each possible connection that an end 
office (202) may have to other end offices (204). In urban environments, 
for example, where there may be numerous end office switches, there must 
be a trunk group (218), a synchronous-to-asynchronous converter (210), an 
asynchronous-to-synchronous converter (212) for every conceivable end 
office to end office connection that is possible. 
An alternate embodiment for asynchronous transfer mode switching is shown 
in FIG. 3 known as switched virtual circuit or SVC (300). In FIG. 3 a 
network (301) of asynchronous transfer mode switches (306, 308, 309) 
perform the switching and routing of ATM cells that originate from the 
synchronous to asynchronous converters (314) coupled to the end office 
switches 302. Stated alternatively, the end office switch 302 has a single 
trunk group (310) coupled to a synchronous-to-asynchronous converter 
(314). 
The synchronous-to-asynchronous converter, i.e., SAC (314) assigns VPI and 
VCI addresses to ATM cells it assembles based upon signaling information 
received from the narrow-band to broad-band signaling interface circuit 
(324), which receives signaling information from the originating end 
office (302). These VPI and VCI addresses are used to route the cells to 
only the next ATM switch (306) in the ATM network (301) that receives the 
ATM cells from the SAC (314). The identity of the particular ATM switch in 
the ATM network (301) to receive ATM cells carrying the call from the SAC 
(314) is also established by the narrow-band to broad-band signaling 
interface circuit (324). The ATM switch in the ATM network that receives 
ATM cells from the SAC (314) decodes the VPI and VCI addresses and 
switches the cells out to another ATM switch after rewriting the VPI and 
VCI addresses again. VPI and VCI addresses assigned by subsequent ATM 
switches in the network (301) that route the ATM cells through the 
network, are established using signaling information from the originating 
end office that is sent to the narrow-band to broad-band signaling 
interface circuit (324), when the call is first set up. The narrow-band to 
broad-band signaling interface circuit (324) sends a broad-band signaling 
message to the first switch (306) of the network (301) to set up the call 
across the network (301). The last switch (308) in the network (301) to 
route the call then sends another broad-band signaling message to the 
narrow-band to broad-band signaling interface circuit, NB-BB interface 
circuit (326) for the destination end office (304). Upon the reception of 
the broad band signaling message from this last switch (308) the NB-BB 
interface circuit (326) formats a narrow band signaling message and picks 
an available time slot that can be constructed by the local SAC (316) and 
sends an appropriate narrow band signaling message to the destination end 
office (304) that identifies the time slot from the SAC (316) where the 
call from the originating end office (302) can be located. 
In operation, the ATM cells are assigned new addresses by switches they are 
routed through. This routing information with the signaling information on 
the interoffice signaling channel (326, 324, 328) allows ATM cells to be 
properly routed through the SVC network of FIG. 3. 
A problem with the embodiment shown in FIG. 3 is that for a particular call 
to be routed between end office switch 302 and end office switch 304, a 
path must be established through each ATM switch as the call set up 
progresses through the network. Accordingly, the call setup time from end 
to end for the embodiment shown in FIG. 3 exceeds that for the embodiment 
shown in FIG. 2 and varies as a function of the number of ATM switches 
traversed from one end of the network shown in FIG. 3 (300) to the other. 
On the other hand, for the embodiment shown in FIG. 3, the infrastructure 
overhead comprising the trunk groups 310 and 312 is reduced because each 
end office must be coupled only to a synchronous-to-asynchronous and an 
asynchronous-to-synchronous converter which are then coupled to the 
asynchronous transfer mode switching network. 
An advantage of the embodiment shown in FIG. 2 is that the call setup time 
is reduced at the expense of communication media or infrastructure. An 
advantage of the embodiment shown in FIG. 3 is that the infrastructure is 
reduced but the call setup time is increased. 
An asynchronous transfer mode communication network that combines the 
advantages of the architectures shown in FIG. 2 and FIG. 3 without the 
disadvantages of either would be an improvement over the prior art. 
An object of the invention is to provide an asynchronous transfer mode 
communications network having minimized call setup time and minimized 
telecommunications infrastructure. 
SUMMARY OF THE INVENTION 
Once an ATM PVC path is set up through an ATM switching network, signaling 
information from an originating end office coupled to the ATM switching 
network is used to map time-slot-based synchronous information for a call 
that comes into an end office into ATM cells that are programmed with VCI 
addresses chosen to identify the particular time slot carrying the call. 
Signaling information sent to the destination end, that a call is to be 
routed to the destination end office, triggers an allocation of an unused 
time slot coming into the destination end office. The destination end 
office recovers information from the time slot. Signaling information from 
the originating end office is used to route the call to the appropriate 
destination.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 4, ATM switches of a network (400) are pre-provisioned 
(programmed) to establish permanent virtual paths through the network. The 
virtual path through an ATM network minimizes call set-up time. 
When a call comes into an end office (402) to be routed through the ATM 
network (401) to a destination end office (404), time division multiplexed 
synchronous data is formatted into an ATM cell and assigned a VPI address 
to route the cell through the ATM network (401). Signaling information, 
including the called number, is routed to a signaling transfer point or 
STP (426) for delivery to a synchronous-to-asynchronous converter circuit, 
SAC (410) and NB-BB signaling interface circuit (423) for the originating 
end office (402). 
The NB-BB interface circuit (423) analyzes the called number and determines 
the end point for ATM cells switched through the network (401). The NB-BB 
interface circuit (423) determines a PVC path through the ATM network 
(401), identifying a VPI address that will route cells through the network 
(401). The VPI address can be thought of as a destination address in that 
it routes cells through the network to a destination effectively 
identified by the VPI address. 
After having picked an appropriate VPI, the NB-BB interface circuit (423) 
picks a VCI value to be inserted into the ATM header block of ATM cells 
being formatted by the originating end SAC (410). The VCI is chosen using 
a busy/idle list maintained by the NB-BB interface circuit (423). The VCI 
effectively identifies the ATM cells carrying the synchronous time 
division multiplexed information which comprises the call. 
Once a VCI is identified, a new signaling message is sent to a signaling 
transfer point (428) for the NB-BB interface circuit (434) and SAC (412) 
for the destination end office (404). The NB-BB interface circuit (434) 
acknowledges the VCI address assignment by the sending end NB-BB signaling 
interface circuit (423). 
The NB-BB signaling interface circuit (434) picks a time slot on trunk 
group 408 that is not being used. A narrow band signaling message is sent 
by the NB-BB interface circuit (434) to the destination end office (404) 
that identifies the time slot on trunk group 408 that will carry the call 
coming into the originating end office (402). End office 404 acknowledges 
the time slot assignment by the NB-BB signaling interface circuit (434) 
and thereafter begins mapping incoming data in that time slot and routes 
recovered information therefrom to an appropriate destination. 
Signaling information exchanged between the two end offices and 
corresponding NB-BB signaling interface circuits and SACs allows the 
sending office to transfer synchronous data to a SAC, as if the SAC were a 
tandem switch. A SAC at the receiving, or destination end, recovers the 
ATM cells, and reconstructs a synchronous signal assigning the data of 
interest a channel time slot which the receiving end office knows how to 
route to the proper destination by virtue of the signaling information 
from the originating end. 
VCI addresses are assigned by the NB-BB interface circuit (423) on a 
call-by-call basis. The NB-BB signaling interface (423) instructs the SAC 
(410) to use particular VPI and VCI values necessary to route the ATM 
cells carrying information from a particular time slot, through the ATM 
network to the destination SAC (412). Instructions from the NB-BB 
interface circuit (423) to the SAC (410) occur via any appropriate 
communication channel (411). While the NB-BB interface circuit 423 is 
sending instructions to its SAC (410), analogous instructions are being 
sent between the NB-BB interface circuit 434 and SAC 412 via any 
appropriate communication channel (411). 
VCI addresses may be rewritten as cells progress through the network 
subject to the requirement that when the ATM cells arrive at the intended 
destination and are to be converted back to synchronous data, the VCI 
address assigned by the sending-end SAC are reassigned to the cells by the 
time they arrive at the receiving end. The asynchronous-to-synchronous 
converter (412) reconstructs the originally synchronous signal and assigns 
it to a channel on trunk group 408 that was established at call set up. 
The ATM switches that could be used to practice the invention include any 
ATM switch that provides PVC switching. One such switch is the Lucent 
Technologies Globeview switch. Signaling transfer points that could be 
used to practice the invention are generally available in the public 
switched network and include Lucent Techologies STPs. 
Synchronous-to-asynchronous converters that could be used to practice the 
invention need to be able to accept VPI and VCI address programming in 
real time. The NB-BB interface circuit needs to convert a narrow-band 
signaling signal, e.g. ISUP, to for example BISUP and to program the SAC 
in real time. The NB-BB interface circuit also needs to be able to 
determine ATM cell routing through an ATM network and to be able to 
determine an end office destination from ATM cell addresses and signaling 
information from the originating end office.