Multiplex communications – Pathfinding or routing – Switching a message which includes an address header
Reexamination Certificate
1998-06-12
2001-04-03
Olms, Douglas W. (Department: 2661)
Multiplex communications
Pathfinding or routing
Switching a message which includes an address header
C370S474000, C370S516000
Reexamination Certificate
active
06212189
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to packet-switching systems, such as asynchronous transfer mode (ATM) systems, and specifically to transmission-delay variations in such systems.
BACKGROUND OF THE INVENTION
Today's business communications environment consists of two separate network infrastructures: a voice network (such as a private branch exchange (PBX)) characterized by real-time, high-reliability, constant bit-rate (CBR) connections; and a data network (such as a packet network) characterized by high-bandwidth variable bit-rate (VBR) connections. Business needs for simplified maintenance, management, and access to information on diverse networks are forcing the convergence of these networks along with a new class of real-time multimedia networks. Asynchronous transfer mode (ATM) provides a single infrastructure that cost-effectively and flexibly handles both switching and transmission for the traffic types mentioned above (voice, video, and data) for both local-area networks and wide-area networks. The evolving network convergence requires the adaptation of the legacy PBX voice traffic to ATM. Voice telephony over ATM (VTOA) specifications allow adaptation of compressed or uncompressed voice pulse-code modulated (PCM) data streams into streams (virtual circuits) of CBR cells.
An ATM cell, regardless of the traffic it carries, is a packet 53 octets long: 48 octets of payload attached to a 5 octet header. The header contains addressing and management information used to direct the cell from source to destination and to ensure that the negotiated aspects of the traffic-flow through the ATM network are met. CBR traffic is assembled into cell payloads using ATM Adaptation Layer
1
(AAL1). The AAL1 cell constructor layer uses the first octet of the payload for its header and the remaining 47 octets to carry CBR information. ATM cell construction is then completed by attaching the ATM header to the payload.
The PBX provides multiple PCM streams (active channels, or existing calls) to multiple instances of the cell constructor, and the number of PCM streams and cell constructor instances changes as calls are added or removed. Assuming uncompressed-data, the PBX provides each instance of the cell constructor with one PCM octet every 125 microseconds (assuming a standard 8000 Hz sampling rate). It requires 5.875 milliseconds to fill a cell (47 octets * 125 microseconds/octet). Each fully-constructed (mature) cell is queued for transmission. Using a standard OC3c 155.52 MHz fiber optic interface, it takes 2.83 microseconds to transmit a cell. This creates a large variation in cell delay (jitter) through the transmit queue between a cell that enters the queue when it is the only cell to mature during the cell construction period and cells that enter the transmit queue when cells for many (a large fraction) of the channels (PCM streams) in the system mature at the same time. The variations in cell delay range from 2.83 microseconds for one cell maturing in a cell construction period to 682 microseconds for 241 cells maturing during a cell construction period (assuming a PBX that supports up to 241 simultaneous conversations). Such variation in delay is anathema to the concept of constant-bit-rate traffic.
SUMMARY OF THE INVENTION
This invention is directed to solving these and other problems and disadvantages of the prior art. According to the invention, variations in cell delay of existing streams (calls) are kept to a minimum by shifting the variations to the start times of processing of new streams, by manipulating the start times of processing of those new streams. Delays in starting up a call are much-less apparent and hence much-more tolerable than traffic jitter in the midst of a call. The invention therefore shifts the cell-delay variations from the midst of existing calls to the beginning of new calls, where those variations will not be noticed. Broadly according to the invention, traffic-delay variations of existing communications are limited by maintaining substantially constant traffic-delays in the traffic of existing communications, and shifting delay variations to traffic-start times of new communications. The shifting is preferably effected by manipulating (e.g., delaying) the start times of the new communications, in facilities shared by the existing and the new communications.
More specifically according to the invention, the transmission-delay variations between transmitted segments (e.g., packet payloads) of communications traffic of individual communications channels in a multi-channel transmission system are limited as follows. A determination is made of whether traffic segments from too many channels become available for transmission at a same time. This determination is preferably effected by anticipating whether the segments of traffic from too many channels will become available for transmission at the same time, e.g., during a same period of time such as a cell construction time, which is the period of time during which a segment of traffic (a cell) from each channel can become available for transmission. If it is determined that segments of too many channels do not become available at that same time, another (waiting) channel is allowed to commence making its segments of traffic available for transmission at that same time. But if it is determined that segments of too many channels do become available at that same time, the other channel is prevented from commencing to make its segments of traffic available for transmission at that same time. Preferably, the prevention continues until such time as it is determined that segments of too many channels do not become available at said such time, whereupon the other channel is allowed to commence making segments of its traffic available for transmission at said such time.
According to an illustrative embodiment of the invention, the transmission-delay variations between successive ATM cells of traffic of individual channels (preferably both narrowband and wideband) in a multi-channel ATM system is limited as follows. A determination is made of how many cells will mature for transmission in a next cell construction period—a period of time during which each channel could mature a cell. The determined number of maturing cells is then compared against a threshold, which is preferably proportional to the total number of presently-transmitting channels, and illustratively comprises the total number of presently-transmitting channels divided by the number of octets of traffic carried by each cell (47), plus one. If the number of maturing cells falls below the threshold, a channel waiting to begin transmission is permitted to begin maturing cells for transmission during that next cell construction period. But if the number of maturing cells does not fall below the threshold, the waiting channel is prevented from commencing to mature cells for transmission during that next cell construction period. Preferably, the determination and comparison are made at least during each cell construction period during which there is a channel waiting to begin transmission, and the waiting channel is permitted to commence maturing cells for transmission during the first cell construction period for which the number of maturing cells is determined to fall below the threshold.
Even more specifically, the invention provides a feedback path from the element that constructs cells (SAR—Segmentation And Reassembly) to the control element of the element that provides constant bit-rate (CBR) data (TSI—Time Slot Interchange). An anticipation circuit in the cell constructor provides the CBR data-provider control element with a count of the number of cells it will start constructing during the next construction period. The CBR data-provider control element waits to start a new CBR stream if the cell-start count is greater than the number of CBR streams divided by 47 (48 payload octets less 1 header octet in AAL1 cell payload). This function adds a new stream when the cell-start count is 0 and there are fewer than 47 CBR steams,
Avaya Technology Corp.
Olms Douglas W.
Vanderpuye Ken
Volejnicek David
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