TRAFFIC SHAPER FOR VBR VIDEO SOURCE

by Anders Johansson, Dragos Ilie and  Ola Nilsson

Abstract

The evolution of data communications towards ATM networks has forced the development of new techniques aimed to improve the flows of data (i.e. ATM cells) throughout the network. Traffic policing, traffic shaping and statistical multiplexing are some techniques worth mentioning.

The requirement to keep these techniques up to date and nevertheless stable and highly effective is one of the main reasons for repeatedly simulating the events occurring inside a network or simulating the network as a whole. Simulation is also a way to test new ideas without causing damage upon the physical network, something well appreciated by the network users.
 

Why shaping the traffic

One part of the ATM specification is the "Traffic Management Specification" found on the web at http://www.atmforum.org. The document specifies how two ATM networks negotiate the parameters for one or more connections and the methods used to ensure and/or enforce the terms of the negotiation (also called traffic contract). Some of the parameters used for the traffic contract include Peak Cell Rate (PCR), Sustainable Cell Rate (SCR), Maximum Burst Size (MBS), Minimum Cell Rate (MCR) and Cell Delay Variation Tolerance (CDVT).

After the networks set up the traffic contract they start monitoring the traffic flow to check if the terms negotiated are being respected. Sources found to disrespect the contract are punished by the means of cell deletion, often without any notification. These actions are part of the traffic policing. The Generic Cell Rate Algorithm (GCRA) is used to define conformance with respect to the traffic contract.

A good way to avoid punishment for non-conforming cells is to shape the traffic before sending it to the other side. Results of traffic shaping can be burst length limitation, peak cell rate reduction, cell scheduling policy and cell delay variation limitation. These results can occur by themselves or in some combination depending mostly on the traffic shaping algorithm implementation.

The assignment

Part of the course "Modern Techniques of Networking" was to simulate some traffic and to analyze different aspects attached to it. In our case we had to simulate a traffic shaper for a VBR (Variable Bit Rate) video source.

The source is sending data as frames, at a rate of f frames per second where f usually is 25 when a flicker-free image is desired. The frame size is variable, an effect of the MPEG-coding. When a frame enters the shaper it is divided into ATM cells that are placed into an internal buffer. The buffer size is B ATM cells and when the buffer is full the cells that that cannot be stored are lost. This brings up the parameter of Cell Loss Rate (CLR).

The shaper outputs cells from the buffer at a peak rate, PCR, of r cells per second. In other words a cell is outputted each 1/r seconds and the rest of the cells in the buffer age by 1/r. The age of each cell is a measure for the delay between the arrival and departure of that cell. This second parameter is the Cell Delay Rate (CDR).

vbr_video.pdf - Documentation for the traffic shaper as PDF
vbr_video.ps - Documentation for the traffic shaper as PostScript

GCRA.pdf - Paper on GCRA traffic policing algorithm as PDF
GCRA.ps - Paper on GCRA traffic policing algorithm as PostScript

simtime.tar.gz  - simulate one set of parameters at one time
sim3d.tar.gz - simulate a whole interval of parameters with desired step length
simtime.zip - packed with Winzip 6.3
sim3d.zip - packed with Winzip 6.3
 

Updated 98.05.26, 21:16