Can ABR Service Replace VBR Service in ATM Networks
Dr. Lawrence G. Roberts
The new Available Bit Rate (ABR) service with explicit rate flow control and a minimum rate guarantee (MCR) provides such a powerful service option that it may well be more effective for video and voice than the Variable Bit Rate (VBR) service currently defined for real time use. An MPEG compressed video signal can encode at any rate the network requests down to a defined minimum. If we set the Minimum Cell Rate (MCR) to that lowest rate, and adjust the coding rate dynamically to the network feedback, the resultant video is extremely high quality and the channel is used at over 95% utilization. On the other hand, if VBR service is used, the codec runs at its own rate and the network must allocate far more bandwidth than the average used due to the possibility of many such signals peaking at once. With VBR, the utilization for MPEG video is from 30-60%. Thus, ABR may be considerably less expensive for video with little or no loss in quality.
ATM CBR Service Class
The Constant Bit Rate (CBR) service class for ATM is only suitable for fixed rate signals. When voice is silence detected, it compresses by a factor of 3, but becomes variable rate and, therefore, is unsuitable for CBR service. Similarly, when using MPEG compression in video, the signal becomes variable rate with a rate of 4-5 times the average rate. In order to save this factor of 3 for voice and 4-5 times for video, it is important to switch to another service class like VBR or ABR where variable rate transmission is possible. Since these large compression factors are easily available today, it is my belief that CBR mode will only be used for backward compatibility and the majority of future applications will use a variable rate transmission mode.
ATM VBR Service Class
The primary usage of VBR mode in ATM networks will be for variable rate real time services like compressed voice and video. Voice presents practically no problem for ATM because of its relatively low bandwidth. It can be sent in VBR service with high efficiency because so many channels fit into an OC-3 or OC-12 ATM trunk. However, due to the vary small bandwidth for voice compression to video, voice will most likely constitute only a small part of overall ATM traffic. A much more important case is compression video. It is likely that a large part of ATM traffic in the future will be compressed video most likely using MPEG compression. Therefore, an important question is whether the VBR or the ABR service class would be more effective for this MPEG video.
MPEG Video Characteristics
For MPEG Video with 640 x 480 pixels and typical scenes, Simon Lam reports 1 that the video transmission consists of three types of frames: I frames with 275 K bits, P frames with 75 K bits and B frames with 20 K bits. The ratio of I frames and P frames to B frames is adjustable, but typically might be set to 1 I frame every 9 frames and 3 P frames every 9 frames. If we average together the B and P frames, we can produce a simplified characterization of the data stream as 11.1% I frames at 275 K bits and 88.9% BP frames at 33.75 K bits per frame. Thus, at 30 frames/sec the average bandwidth is 1.816 Megabits/sec and the peak rate (I frames) is 8.25 Megabits/sec. This is a peak to average ratio of 4.54: 1 and a peak rate to low rate ratio of 8.15:1.
VBR Efficiency with MPEG Video
First, we will examine the bandwidth utilization if this MPEG data stream is sent over an ATM network using the VBR service class without smoothing. (Alternatively, Simon Lam shows in 1how the MPEG stream can be smoothed using a large buffer and delaying the video.) The VBR channel would be set up with a PCR = 8.25 Mbps and SCR = 1.816 Mbps. The typical ATM network for the next few years will have a trunk size of OC-3 to OC-12 in size (155-622 Mbps). We will examine the case where many (N) MPEG users share the same trunk. By using the binomial distribution, we can determine the probability of K of these signals being at the high rate (8.25 mbps) at the same time. T hen, we can sum the probabilities of all cases where the total bandwidth (N-K at 1.0125 Mbps and K at 8.25 Mbps) exceeds the total available trunk bandwidth.
Note that the trunk may be partially used for other services so that the trunk rate could be anything up to 622 Mbps. We would like the probability of the total required bandwidth exceeding the trunk capacity to be less than 10-12 because when this happens the buffers will quickly fill to the maximum delay allowed and cells will need to be thrown out. This paper only examines the basic probability of the bandwidth exceeding the trunk capacity and does not examine the improvements possible of the effect of adding delay with buffering. Figure 1 shows the trunk utilization for various available trunk capacities for the largest number of MPEG signals that can be placed on the trunk at one time without the conflict probability exceeding 10-12. With an OC-3 trunk, 25 MPEG signals can be on the trunk and with an OC-12 trunk the number increases to 195. Considering that the average signal is 1.81 Mbps, the average utilization with 25 signals on an OC-3 trunk is 30% and 57% with 195 on an OC-12 trunk.
Figure 1: Trunk Utilization For MPEG Video over CBR, VBR, and ABR ATM Services
The obvious conclusion that even with a standard resolution video stream, the size of ATM trunks currently available is far too low to achieve good utilization and the best we can expect with low delay is 30-60%.
ATM ABR Flow Control
In September, 1994, the ATM Forum voted to permit the use of a new type of flow control for Available Bit Rate (ABR) service. This flow control method, which was proposed by the author in August, 1994, is an explicit rate feedback technique with intelligent marking. Its operation is achieved by the source sending Resource Management (RM) cells every 32 cells or every 100 ms, whichever is sooner. These RM cells contain the current rate (CCR) the user is operating and at the rate the source wishes to operate, ER. The network switches examine the RM cells and determine if this virtual circuit (VC) is within the group of Vcs that are being constrained or bottlenecked by this node. If the VC is bottlenecked at this node, the switch will change the ER field to indicate the maximum bandwidth available for this VC. This is intelligent marking since only the node which is bottlenecking the VC will mark down the rate. The RM cell proceeds around the network from the source to the destination and is then returned toward the source. It is on the return path that the marking occurs. The delay from marking a VC and the source receiving the VC and changing its rate to the newly specified rate is determined by the speed of light primarily and is typically very short -- 100 microseconds in a local area network to 20 ms across country. Thus, the source can be kept operating at a rate that the network can support and the queries on the network remain very short. The buffer size can be set so that there is no cell loss in almost all cases. The network delays are typically so small (30 - 300 microseconds) that the ABR service could most likely be used for a real time service like compressed video. The main requirement is that the MPEG code respond to the RM cells requested rate so as to only feed the video to the network at the rate the network can support. Since MPEG uses a variable ratio of I frames anyway, its source rate can be varied from the BP rate of 1 Mbps to the I rate of 8 Mbps just by varying the ratio. Thus, so long as the network can always support 1 Mbps, the ration can be adjusted in response to the RM requested rate to achieve total compliance. The buffer size to achieve this is, at most, 1 I frame or 34 bytes.
The new Forum flow control includes a Minimum Cell Rate (MCR), which is negotiated at call setup time and then guaranteed by the network. Using this minimum guarantee, set at 1.25 Mbps, the MPEG coder is guaranteed of 1 I frame every 30 frames (1 sec) and normally could increase its rate to send I frames every 9 frames or more often, if desired.
Trunk Utilization with ABR vs VBR
As a result of the new flow control and the minimum rate, the new ABR service can support the same MPEG video stream previously examined for VBR service with no more than one frame of delay, and achieve the full 95% trunk utilization normally achieved under ABR service. The only requirement is that the coder be interactive, responding to the network specified rate. The impact on the video is that during high motion scenes, if the network also becomes congested the picture may blur slightly. However, the maximum blur acceptable may be set in advance when the MCR rate is reserved on call setup. If 1.25 Mbps creates too much blur, then the rate could be increased as necessary. The bandwidth savings, however, are major with the ABR service being 66% more efficient at OC-12 trunk rates and 217% more efficient at OC-3 trunk rates.
ABR is most likely going to prove to be the main ATM service utilized. CBR is clearly inefficient by 3:1 to 5:1. VBR for video can only achieve 60% utilization. ABR, on the other hand, can always achieve 95% utilization, even for video, so long as the coder is interactive. Thus, if along with all data using ABR video uses it also, the result will be a near total migration to ABR service.
1 S.S. Lam, S. Chow, and D.K.Y. Yau. An algorithm for lossless smoothing of MPEG video. In Proceedings of ACM SIGCOMM ‘94 (to appear), London, August 1994
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