Real-time data transmission on the ARPANET (RFC0508)
Original Publication Date: 1973-May-07
Included in the Prior Art Database: 2001-Nov-15
Internet Society Requests For Comment (RFCs)
L. Pfeifer: AUTHOR [+2]
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.
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
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, 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 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
Pfeifer & MacAfee [Page 1]
RFC 508 Real-Time Data Transmission On The Arpanet 7 May 1973
the number of hops), and conclusions were reached that with delay
characteristics similar to a lightly loaded ARPA Network speech
communications could be satisfactory from a human-factors standpoint.
Data for this experiment originated in an SEL 810-B computer located
in the Electrical Engineering Department at UCSB. This 70ns cycle
time computer is the heart of an interactive signal processing system
developed by Retz. It has associated hardware such as a card
reader, two IBM 1311 disk dri...