EFCI scheme with early congestion detection

Abstract

An adjustable bit rate (ABR) feedback control scheme is provided where the effects of multiloop delays and high priority traffic transmission are built into the control model. The data traffic is filtered by a low pass filter. Then, the low frequency bandwidth of the filtered traffic is measured and compared to a predetermined threshold. If the measured value exceeds the threshold, the ABR traffic flow is reduced. If the measured value is less than the threshold, the ABR traffic flow is increased. In addition, a General Prediction Control (GPC) method may be applied to the control model for optimal performance. An object of the invention is to minimize the unused link capacity subject to no congestion, where the ABR traffic is adapted to the low frequency variation of high priority traffic flow for high efficiency.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the field of communications, and to an apparatus and method for congestion control in high speed networks. More particularly, the present invention relates to flow control techniques and the flow control of Available Bit Rate (ABR) traffic in, for example, Asynchronous Transfer Mode (ATM) networks. 2. Background and Material Information With advanced ATM technologies, future high speed networks will be able to support a wide spectrum of applications with diverse traffic characteristics and service requirements. Using ATM Forum terminologies, traffic is classified into three classes: Constant Bit Rate (CBR), Variable Bit Rate (VBR) and Available Bit Rate (ABR). The CBR and VBR traffic are transmitted in high priority with sufficient reservation of buffer space and link capacity; their arrival rate is normally not adaptable to the changing network environment. In contrast, the ABR traffic is tra...

AI Technical Summary

Benefits of technology

imize the unused link capacity. An advantage of the present invention is it eliminates unnecessary highly frequent adaptations of ABR traffic rate as found in most existing control schemes, which are caused either by the high frequency variation of CBR / VBR traffic or by the rapid change of queue congestion status. Because of the filtering, the ABR traffic varies very smoothly along with the slow time variation of the low frequency CBR / VBR traffic.

Another object of the invention is to provide an explicit rate ABR traffic control scheme by using a process control design method, referred to herein as Generated Prediction Control (GPC), to provide a closed loop stable controller with linear quadratic optimal performance. An advantage of the present invention is that multiloop delays of ABR connections are built into the design of the control model. The low frequency high magnitude oscillations of ABR traffic flow, as usually found in most proposed ABR control schemes, are therefore eliminated. As a result, not only is the buffer capacity requirement for ABR traffic substantially reduced, but the control stability condition is also significantly improved.

In a preferred embodiment, a new explicit rate control scheme based on the results of frequency domain analysis of multimedia traffic is provided. Examples of frequency domain analysis of multimedia traffic can be found in S. Q. LI and C. HWANG, “Queue Response to Input Correlation Functions: Continuous Spectral Analysis,”IEEE / ACM Trans. Networking, Vol. 1, No. 6, December 1993, pp. 678-692 (LI (I) et al. hereinafter), and S. Q. LI, S. CHONG, and C. HWANG, “Link Capacity Allocation and Network Control by Filtered Input Rate in High Speed Networks,”IEEE / ACM Trans. Networking, Vol. 3, No. 1, February 1995, pp. 10-15 (LI (II) et al. hereinafter), the disclosures of which are expressly incorporated herein by reference in their entireties. The inventors have found that the link capacity required by input traffic at each node is essentially captured by the traffic's low frequency characteristics. In other words, no congestion occurs at the node if the control design guarantees that the aggregate CBR / VBR / ABR traffic rate, filtered in a low frequency band, never exceeds the link capacity. All the high frequency traffic is absorbed and smoothed out via limited buffering. Further, the filtered traffic rate is significantly less than its original non-filtered rate.

Another object of the present invention is to provide significant improvement of ABR traffic performance by replacing the queue threshold detection with the bandwidth threshold detection, using an Explicit Forward Congestion Indication (EFCI) scheme. According to an aspect of the invention, a modified EFCI scheme is provided, which is called EFCI-ECD scheme, where ECD refers to early congestion detection. A simulation study has shown about a 60% savings in buffer capacity for the EFCI-ECD scheme to achieve the same throughput as the original EFCI scheme, providing the same set of source control parameters. The amplitude of the ABR traffic oscillation is reduced by about 50%. Note that the traffic filtering operation can easily by implemented by digital signal processing (DSP) chips. Using today's technology, a common DSP chip only costs a few U.S. dollars whereas the high speed SRAM chips for buffer capacity are relatively much more expensive. Moreover, a single DSP chip can be shared by many links for multiple traffic measurement purposes.

Thus, there are at least three major advantages for the EFCI-ECD scheme of the present invention: (1) none of the existing EFCI protocols at source and destination needs to be changed; (2) the buffer capacity required at each switching node is substantially reduced; and (3) the ABR traffic oscillation is much reduced.

The present invention is described as a traffic congestion control apparatus for use in a network having a plurality of types of data traffic. The data traffic comprises high priority traffic and low priority traffic. The network has a plurality of links through which the data traffic flows, each link being susceptible to data traffic congestion. The apparatus has a processor which comprises a filter through which said data traffic flows, and a sampler which periodically measures a characteristic of the data traffic filtered by said filter. The measured characteristic indicates a present link capacity requirement of the filtered traffic. A flow control system for adjusting a flow rate of the low priority traffic is also provided. The apparatus also comprises a comparator which compares the measured characteristic with a predetermined threshold. The comparator sends a first signal to the flow control system when the measured characteristic exceeds the predetermined threshold to indicate link congestion. The comparator sends a second signal to the flow control system when the measured characteristic is below the predetermined threshold to indicate unused link capacity. The flow control system reduces the transmission rate of the low priority traffic in response to receipt of the first signal and increases the transmission rate of the low priority traffic in response to receipt of the second signal.

Problems solved by technology

However, because of multiloop delay and the CBR / VBR traffic variation, the link capacity will not be fully utilized.

Images