RFC 508 (rfc508) - Page 1 of 10

Real-time data transmission on the ARPANET

Network Working Group                                         L. Pfeifer
Request for Comments: 508                                      J. McAfee
NIC: 16159                            Computer Systems Laboratory / UCSB
                                                              7 May 1973

               REAL-TIME DATA TRANSMISSION ON THE ARPANET

I. INTRODUCTION

   The ARPA Network is rapidly proving to be a useful tool in computer
   communications and resource sharing.  It has been proposed that the
   same network might also be able to support real-time processes such
   as audio or video communications for conferencing purposes.  The
   degree of support of these types of processes will largely be
   determined by transmission bit-rates and delays.

   The IMP subnetwork throughput rates (one way) average about 37
   kilobits[1], therefore an external process must operate at a bit-rate
   below that level.  This would imply some form of data compression for
   both audio and video transmission.  Research in these areas is still
   in progress so these processes must be simulated at the present time.

   In addition to bit-rate, system response time (system delay) is an
   important factor since this has direct influence on the amount of
   data which must be buffered in order to keep a real-time process
   running without discontinuities or gaps.  Such delays may be caused
   by network loading, host loading, or an excessive number of IMP-to-
   IMP hops in the transmission path.

   In order to get a feel for the ability of the network to support a
   real-time process an experiment was conducted with real-time data
   being sent from the UCSB SEL810-B computer, by way of the UCSB IBM
   360 host, onto the ARPA Network and into a host discard socket in the
   UCLA IBM 360 computer.  This particular data path very nearly
   duplicates the path which might be taken if real-time devices were
   attached to large scale host computers operating in their normal mode
   (usually timesharing).  The experiment consisted of measuring the
   duration of gaps incurred at various process bit-rates, and buffer
   sizes ranging from one to eight network packets.

   Earlier experiments at MIT[2] simulated vocoded speech transmission
   over the ARPA Network using the TX-2 computer and "Fake host 3" in a
   destination IMP.  Speech was sampled by the TX-2 and simulated speech
   data blocks were sent to a particular fake host.  Receipt of an
   acknowledgment by TX-2 indicated that the corresponding blocks of
   speech data could be reconstituted.  Experiments were conducted with
   bit-rates from 2400-17000 bps and varying block sizes (depending on




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