Multiplex communications – Data flow congestion prevention or control – Flow control of data transmission through a network
Reexamination Certificate
2000-05-19
2004-09-28
Hsu, Alpus H. (Department: 2665)
Multiplex communications
Data flow congestion prevention or control
Flow control of data transmission through a network
C370S395400
Reexamination Certificate
active
06798744
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related to the field of digital communications. More specifically, the present invention is directed to methods and/or systems and/or apparatuses for providing enhanced flow control in digital signals.
BACKGROUND OF THE INVENTION
A number of prior art techniques have been proposed and developed for managing traffic in computer networks using flow control. Some references to known flow control implementations include:
1. ATM Generic Flow Control (GFC)—For UNI connections the first four bits of each ATM cell have been reserved for flow control. As in the present invention, the GFC were to communicate transmit-on/transmit-off (XON/XOFF) information, but unlike the present invention, this prior art flow control is applied to the entire link as opposed to a single virtual connection (VC).
2. BECN (Backward Explicit Congestion Notification)—This technique is per-VC flow control employed in ATM networks. The feedback is very slow (<10 updates per second); therefore, large cell buffers must be used in conjunction to avoid cell loss.
3. ATM Forum Available Bit Rate (ABR) service—Is based on specifying the ATM cell rate, as opposed to a simple XON/XOFF indication. ABR is more suited for end-to-end flow control within networks with large latencies. It lacks the simplicity of the present invention.
4. ATM Forum QFC—This is a credit based system. It lacks the simplicity of the present invention.
5. Transmission Control Protocol (TCP)—This is a Layer 4 end-to-end flow control (amongst other things) protocol.
6. T1 systems—The timing is extracted from the raw frame rate of the link. This imposes the burden that the clock for link itself be very well controlled. With the present invention, a suitable line rate clock can generated with a simple 3rd overtone crystal oscillator that free runs.
7. SONET systems—Again, synchronization is based on extracting timing from the frame rate.
8. SRTS—Synchronous Residual Time Stamp is a method for timing reference carriage across an ATM network. It generates a 4-bit remainder of the running phase difference between the source end clock and the network Stratum timing reference, transports this value to the receiving end, which then regenerates the source end clock from this 4-bit SRTS value and the Stratum reference.
The possibility of congestion is inherent in an access multiplexer, such as a DSLAM. In the downstream direction, the WAN link can generate a burst of cells for a particular modem at a rate exceeding the modem's bandwidth capacity. Therefore, feedback to the traffic scheduler is required to cause it to buffer and smooth cell bursts to prevent downstream buffer overflow.
In the upstream direction, the aggregate bandwidth of all subscribers can exceed that accommodated by the WAN uplink. Flow control is generally required in a multiple access system to ensure fair access to the up-link, to minimize cell or data loss, and to minimize the impact of greedy users on others.
In DSLAM systems, such as described in greater detail in the references cited above, flow control has been adapted for a variety of prior architectures. One class of known prior DSLAM solutions uses packet or cell switch architectures. This requires signaling and traffic management functionality on the access port line cards and on the WAN uplink port card. Additionally, intercard switching is typically required in these solutions. Some examples of such solutions are Transwitch-Cubit-based switch architectures, Motorola-MPC860SAR-based architectures, and IgT-WAC-185/186/187/188-based switch architectures.
An alternative prior solution centralizes signaling and traffic management functionality on the WAN uplink port card by applying a shaping function on a port basis to all traffic in the downstream direction. This per port shaping function shapes the aggregate traffic to a port (such as an xDSL modem) to handling rate of that port. This solution thereby attempts to eliminate the need for further traffic management functionality on the access port line cards.
Various of these prior DSLAM flow control techniques suffer from a number of disadvantages, such as:
1. Significant increased complexity results from providing the signaling and traffic management functionality on both the access port line cards and the WAN uplink port card. This occurs due to the large number of access port line cards in a typical DSLAM, therefore requiring a large number of physical instances of this complex and costly functionality.
2. Placing the signaling and traffic management functionality on each access port and WAN uplink card additionally adds the requirement to provide intercard switching capability. The intercard switching solution is generally complex due to the large number of access port cards.
3. Placing the signaling and traffic management functionality on each access port and WAN uplink card additionally forces a distributed software control and provisioning requirement, thus adding significant complexity to the software layer.
4. Traffic latency and delay variation is increased due to the traffic transiting two traffic management structures—one on the access port card and one on the WAN uplink card.
5. Solutions using per port traffic shaping to eliminate the need for traffic management functionality on the access line card, must adjust the shaping rate in real time, each time the access port changes rates. This will happen frequently when using rate adaptive splitterless xDSL technology.
6. Solutions using per port traffic shaping to eliminate the need for traffic management functionality on the access line card must ensure PHY buffer overflow is avoided. To do this, the shape rate must be less then the actual PHY rate, since the two rates are not synchronized. This rate difference represents a loss in throughput bandwidth.
SUMMARY OF THE INVENTION
The present invention is directed to providing improved flow control in certain digital communication environments. In various embodiments, the present invention may be embodied in devices, systems, and methods relating to digital communication.
In particular embodiments, the present invention may be most easily understood in the context of a Digital Subscriber Loop Access Multiplexer (DSLAM) architecture, such as the architecture described in the provisional applications referenced above. In particular embodiments, the present invention addresses issues that can arise in DSLAM architectures where the complexity of the system is concentrated in a few common cards, so that the multitude of line cards each are simple. In a particular example architecture, each XDSL signal has a relatively low bit rate and it is therefore technically feasible to perform ATM layer functions, such as traffic management, on a single entity. The invention, in particular embodiments, addresses a side-effect of removing traffic management queuing from individual line cards. This side effect is the need to pace the transfer of cells to the line card to avoid cell loss. The invention in specific embodiments uses a per-PHY flow-control mechanism to achieve this.
In specific aspects of specific embodiments, the invention provides a method of flow control in a digital communications system wherein data flows in one direction in a combined channel and in an opposite direction in multiple channels. In the combined channel, according to the invention, data units (such as cells or packets) have included in them a portion of data indicating available
ot-available status of channels in the return direction. Before a channel in the return direction is selected for transmitting, the available
ot-available status provided in the data portions is checked.
In further aspects, every data unit flowing in the combined channel contains such a portion. In a further aspect, each data unit only provides status of a subset of return channels and therefore multiple data units are needed to update the status of all return channels.
In a further aspect, there is a delay of one or more data units between the status provided in the portion a
Bradshaw John Richard
Brown Jeffery John
Loewen Jonathan David
Hsu Alpus H.
LeBlanc Stephen J.
Nguyen Toan D.
PMC-Sierra Inc.
Quine Intellectual Property Law Group P.C.
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