UNFRAMED ISOCHRONOUS SHAPING
METHOD TO REDUCE DELAY AND DELAY
VARIATION IN A CBR TRANSMISSION
CROSS REFERENCE TO RELATED
A claim of priority is made to U.S. Provisional Patent Application Ser. No. 60/029,176, entitled UNFRAMED ISOCHRONOUS SHAPING METHOD TO REDUCE 10 DELAY AND DELAY AND DELAY VARIATION IN A CBR TRANSMISSION SYSTEM, filed Oct. 28, 1996.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 15
BACKGROUND OF THE INVENTION
The present invention is generally related to telecommu- 20 nications apparatus, and more particularly to virtual circuits in telecommunications apparatus.
Voice carrying connection standards such as Tl and T3 multiplexed digital channels are well known. A Tl connection carries 24 standard voice channels and a T3 connection 25 carries 28 Tl connections. Tl and T3 connections can also carry video signals and computer data. Tl and T3 are synchronous systems in which each individual voice connection has periodic time slots within which to transmit voice carrying data. Asynchronous systems, such as Asynchronous Transfer Mode ("ATM") networks, are also known. ATM networks can carry different types of data such as voice, video and computer data. However, while ATM systems are capable of transmitting voice data, the voice data is transmitted asynchronously.
Under some circumstances it is desirable to integrate synchronous networks and ATM networks. For example, voice data could be received by a first ATM switch from a "circuit" in a first synchronous network, propagated asyn- 4Q chronously to a second ATM switch through a "virtual circuit," and subsequently transmitted over a circuit in another synchronous network. In order to prevent gaps from occurring in the second synchronous network the second ATM switch must maintain a sufficient reserve of voice data units in a "playout buffer" to fill each time slot allocated to the voice connection in the second synchronous network with voice data. However, if the voice data units must be transmitted through many ATM switches, the amount of memory required to maintain a sufficient reserve of voice 5Q data units in the playout buffer can become prohibitively large. More particularly, as the number of intermediate ATM switches increases, the effect on variable transmission delay at each subsequent downstream ATM switch in the virtual circuit is cumulative.
In accordance with the present invention, flow shaping is performed at each asynchronous device in a virtual circuit. More particularly, the flow of data units through the virtual go circuit is controlled at each asynchronous device such that the variable transmission delay remains substantially constant throughout the virtual circuit.
Flow shaping at each switch in the virtual circuit facilitates use of Asynchronous Transfer Mode ("ATM") net- 65 works in association with synchronous networks. Flow shaping causes variable transmission delay to remain sub
stantially constant throughout the virtual circuit. Further, because variable transmission delay remains substantially constant throughout the virtual circuit, the size of the playout buffer in the furthest downstream ATM switch need not be adjusted depending on the number of intermediate ATM switches in the virtual circuit. Hence, an ATM virtual circuit having an arbitrarily large number of intermediate ATM switch "hops" can be initiated without increasing the playout buffer size in the furthest downstream ATM switch.
BRIEF DESCRIPTION OF THE DRAWING
The invention will be more fully understood from the following Detailed Description of the Invention, in conjunction with the Drawing, of which:
FIG. 1 is a block diagram of a virtual circuit;
FIG. 2 is a diagram that illustrates virtual circuit end-toend delay;
FIG. 3 is a diagram which illustrates traffic shaping;
FIG. 4 is a diagram which illustrates initialization of a virtual clock;
FIG. 5 is a diagram of a virtual clock time ring;
FIG. 6 is a flow diagram which illustrates the method of cell reception processing;
FIGS. 7-14 are time-line diagrams that illustrate cell flow shaping in different scenarios; and
FIG. 15 is a flow diagram which illustrates the method of cell emission processing.
DETAILED DESCRIPTION OF THE
U.S. Provisional Patent Application Ser. No. 60/029,176, entitled UNFRAMED ISOCHRONOUS SHAPING METHOD TO REDUCE DELAY AND DELAY AND DELAY VARIATION IN A CBR
TRANSMISSION SYSTEM, filed Oct. 28,1996 is incorporated herein by reference.
FIG. 1 illustrates a virtual circuit in an Asynchronous Transfer Mode ("ATM") network 10. Voice data bits 12 enter the ATM network 10 through a synchronous connection, such as a Tl connection, associated with an ingress synchronous network 14. More particularly, the voice data bits 12 enter a segmentation device 16 in the ATM network. The segmentation device 16 translates the voice data bits into ATM cells 18. The ATM cells 18 are forwarded through a plurality of ATM switches 20 in the ATM network 10. Eventually, the ATM cells are transmitted to a reassembly device 22. The reassembly device 22 translates the ATM cells back into voice data bits. Reassembled voice data bits are queued in a FIFO-type playout buffer 24 and synchronously transmitted via a synchronous connection, such as a Tl connection, associated with an egress synchronous network 26. The flow of ATM cells 18 in the virtual circuit is controlled at each ATM switch 20 to approximate the behavior of the synchronous ingress network 14 and egress network 26.
Queuing of reassembled voice data bits in the playout buffer 24 reduces jitter. Although the flow of ATM cells 18 is controlled at each ATM switch 20 to approximate the behavior of the synchronous ingress network 14 and egress network 26, each switch in the ATM network 10 introduces a variable cell transmission delay. The variable delay causes the flow of cells in the ATM network to "jitter" in comparison with a perfectly synchronous connection. More particularly, jitter may cause data transmission rates inside