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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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 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.

II. CONFIGURATION

 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[3].  It has associated hardware such as a card
 reader, two IBM 1311 disk drives, a drum storage unit, A/D and D/A
 converters, Teletype, Tektronix 611 storage display unit, OLS
 keyboard, and a connection to an IBM 1800 computer.  This system is
 linked to the UCSB IBM 360/75 via a 500 kilobit line for high speed
 data transfers.  Software in both the SEL 810-B and the IBM 360
 enables the SEL to communicate with the ARPA Network.

 The hardware configuration of the data path between the SEL 810-B and
 UCLA is shown in Figure 1.  For simulating speech transmission, the
 SEL is thought of as a "speech processor", analyzing and encoding the
 one-way conversation of a person at UCSB talking to someone at UCLA.
 The fact that there was no "speech processor" at UCLA probably had
 little or no effect on the measurements that were made.  This is
 substantiated by noting that the SEL was a dedicated processor that
 did not introduce delays and if a similar dedicated processor was
 attached to the host computer at UCLA it probably would not have
 caused delays either.  However, the UCLA host merely discarded the
 data it received, thereby going through fewer steps than if an
 external processor was attached, and so our simulation was not exact.

 A configuration such as that of Figure 1 did yield information about
 host-to-host transmission, since the SEL was essentially a zero-delay
 data generator.  If real-time processors are to access the ARPA
 Network through large-scale time-shared host computers then host-to-
 host transmission rate and delay are important measurements.  In this
 configuration we can expect the host computers to be the primary
 bottlenecks in the data path.

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                   UCSB                                UCLA

|——————————————| |—————–| +——–+ +——-+ +——-+ | | | | | | | | 500 Kb/s | | | | |SEL810-B| +——+ | +——+ |IBM | |IBM | | |←|INTER-|↔|INTER-|→|360/75 | |360/91 | | |→|FACE | |FACE |←| | | +—-+|DISCARD | | +——+ +——+ | | | | NCP|–>+—-+ | | | | | +—-+| | | +——–+ +—^—+ +—-^–+ +—-+

     |                           |  |<--100 Kb/s-->  |  |
     V                           V  |                V  |
  +-----+                       +-----+            +-----+
  | D/A |                       | IMP |<---/  /<---| IMP |
  +-----+                       |     |--->/  /--->|     |
      |                         +-----+  \     /   +-----+
-|\   |                                   \   /
-| \<-+                                  50 Kb/s
-| /
-|/SPEAKER
              Figure 1.  Hardware configuration of data path used
                         for sending real-time data from the
                         SEL 810-B to the UCLA host discard socket.

 The host response time to requests from the external processor or the
 Network will be a function of type of host computer (IBM, DEC,
 UNIVAC, etc.), job load, and priorities given to both the Network and
 the external processor.  If host computers cannot provide the
 necessary throughput and necessary response times, then real-time
 devices may have to connect directly to IMPs (assuming the Network
 can properly support these devices).

III. SOFTWARE

 The standard NCP software was used in both host computers.  Several
 custom programs were required in the UCSB computers in order to
 transfer the data and make measurements.  These can be divided into
 three categories:

    1) I/O Programs.

    Routines were written for both the IBM 360 and the SEL to handle
    the transfer of data between the two computers and to enable the
    SEL to send an "attention interrupt" to the IBM 360.  These
    programs form the software part of the SEL/360 high-speed data
    link and are necessary for any communication between the two
    computers.

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    2) Network Communications Programs

    A protocol was developed which enabled the SEL to communicate to
    the 360 the desired Network connections to be made or broken, and
    the desired transfer of data across these connections.  This
    protocol was implemented for each computer using the above I/O
    routines.

    3) Measurement Control Program

    This assembly language program caused the SEL to push data towards
    the receiving host (UCLA) at a specified SEL process bit-rate.
    The program was also responsible for detecting and measuring the
    duration of any gaps introduced in the process.

IV. METHOD

 Within the SEL two buffers, each of 1 to 8 network packets in length,
 are first loaded with alternating bit patterns in consecutive 16-bit
 words.  A conversion process is then initiated on one of the buffers
 at a sampling frequency necessary to give the desired bit-rate.
 Since data is being sent out to a destination host we would expect
 the buffers to be filled by an analog-to-digital conversion process.
 However, in this experiment, the process of digital-to-analog
 conversion is used instead so that we can listen to the alternating
 bit patterns as a steady tone while still simulating an A-to-D
 process.

 When a buffer is filled (played out) a "write" operation is initiated
 to send that buffer to UCLA.  The next buffer is then tested to see
 if the previous "write" has been completed, i.e. the buffer is empty.
 If the next buffer is empty the process continues normally.  If the
 next buffer is not empty it means that one of the computers on the
 Network has not taken the data fast enough, therefore a gap has been
 introduced in the real-time process.  At this point the D-to-A
 converter is shut off resulting in an audible break in the tone that
 is being played out.  A timer is also started to test for the empty
 buffer every one millisecond and to measure the duration of the gap.
 When the next buffer is finally emptied the D-to-A process is resumed
 and the gap data recorded in a table.

V. PROCEDURE

 A connection to the UCLA host discard socket was first established
 using the network communications programs.  Every test from this
 point on required a repetition of the following steps.

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    1) Initialize the UCSB IBM 360 for double buffered data transfers
       using specified buffer sizes.

    2) Initialize the SEL measurement control program with the proper
       buffer size and process bit-rate.

    3) Start the test.  A constant tone from the speaker indicates
       that the process is being properly maintained.  Gaps in the
       tone indicate gaps in the process.

    4) After 30 seconds, stop the test.

    5) Examine the gap table to determine the number of gaps, the
       duration of each gap, and the average duration.

 The entire procedure was carried out from the SEL end using the
 interactive On-Line System.  The timing interval of 30 seconds was
 measured with a sweep second hand of a watch and the test was started
 and stopped manually.  All tests were conducted during prime time to
 obtain typical loading conditions.

VI. RESULTS

 A total of 179 tests were conducted.  Of these, 176 were 30 second
 tests and three were long duration tests.  Table I contains the
 results of the 30 second tests.  Buffer sizes were varied from one to
 eight Network packets and for each buffer setting 22 different
 process bit-rates (usually in increments of 1200 bps) were attempted.
 These measurements were performed over a period of three days during
 prime time.

 Those test conditions which were successful contain only two items of
 information in Table I: time of day and number of buffers
 transmitted.  All but seven of the tests were successful.  The tests
 which were unsuccessful, i.e. experienced gaps, are those entries in
 Table I which contain additional information such as number of gaps,
 and maximum, average and minimum gap duration.

 An examination of those tests which failed shows that the longest gap
 which occurred was 8 seconds in duration.  There were three other
 significant failures between 9:52 A.M. and 9:59 A.M. on 2/7/73.
 There are strong indications that it was the UCSB 360 that caused
 these gaps to occur.  This conclusion is based upon the fact that the
 Electrical Engineering On-Line classroom (16 interactive graphics
 terminals) was in full use that day until 10:00 A.M. and the SEL
 connection to the IBM 360 has lower priority in the 360 than the UCSB

Pfeifer & MacAfee [Page 5]  RFC 508 Real-Time Data Transmission On The Arpanet 7 May 1973

 On-Line System.  The remaining three tests which failed did not do so
 at any regular time, bit-rate, or buffer size so no definite
 statements can be made about their source of delay.

 The overall picture presented by Table I is very promising.  In 96
 percent of the trials a communications link of the two host computers
 and a portion of the ARPA Network was able to take data from a real-
 time process operating as high as 30,000 bits/second.  Further
 encouragement is given by three additional tests which were carried
 out at 30,000 bps and a buffer size of 2,016 bits.  On 2/5/73 at 2:20
 P.M. a 5-minute test was executed with no gaps.  On 2/6/73 at 11:58
 A.M. the same test was executed for 8 minutes with no gaps.  The
 third test was conducted for 18 minutes on 2/7/73 at 11:53 A.M. with
 no gaps in the process.

 The tests were not carried out often enough or over a long enough
 period of time to obtain any statistical results or predictions. The
 measurement task is made somewhat difficult by the fact that the
 state of the overall communications link is never repeatable from one
 test to the next. For example, it was found that a test which failed
 could usually be repeated successfully, even when it was carried out
 within 15 seconds of the previous test.

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+—–+—————————————————————+

+—–+——-+——-+——-+——-+——-+——-+——-+——-+

+—–+——-+——-+——-+——-+——-+———————–+

+—–+——-+——-+——-+——-+——-+——-+——-+——-+

+—–+——-+——-+——-+——-+——-+——-+——-+——-+

+—–+——-+——-+——-+——-+——-+——-+——-+——-+

+—–+——-+——-+——-+——-+——-+——-+——-+——-+

+—–+——-+——-+——-+——-+——-+——-+——-+——-+

+—–+——-+——-+——-+——-+——-+——-+——-+——-+

+—–+——-+——-+——-+——-+——-+——-+——-+——-+

Pfeifer & MacAfee [Page 7]  RFC 508 Real-Time Data Transmission On The Arpanet 7 May 1973

+—–+——-+——-+——-+——-+——-+——-+——-+——-+

+—–+——-+——-+——-+——-+——-+——-+——-+——-+

+—–+——-+——-+——-+——-+——-+——-+——-+——-+

+—–+——-+——-+——-+——-+——-+——-+——-+——-+

+—–+——-+——-+——-+——-+——-+——-+——-+——-+

+—–+——-+——-+——-+——-+——-+——-+——-+——-+

+—–+——-+——-+——-+——-+——-+——-+——-+——-+

+—–+——-+——-+——-+——-+——-+——-+——-+——-+

+—–+——-+——-+——-+——-+——-+——-+——-+——-+

+—–+——-+——-+——-+——-+——-+——-+——-+——-+

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+—–+——-+——-+——-+——-+——-+——-+——-+——-+

+—–+——-+——-+——-+——-+——-+——-+——-+——-+

+—–+——-+——-+——-+——-+——-+——-+——-+——-+

+—–+——-+——|+——-+——-+——-+——-+——-+——-+

    | 2/7/73 (a.m.)||     2/6/73 (a.m.)     |     2/5/73 (p.m.)     |
    +--------------|+-----------------------+-----------------------+
                   V
      2/5/73            |  2/6/73           |  2/7/73
      5 min. test       |  8 min. test      |  10 min. test
      @ 2:20 pm         |  @ 11:53 am       |  @ 11:53 am
      no gaps           |  no gaps          |  no gaps
      4669 buffers sent | 7141 buffers sent | 16071 buffers sent

   +--              +--------+
   | time of day----|  9:35  |    Results of a test for transmitting
   | # buffers sent-|   76   |    data from a continuous external
   |                |--------|    process at UCSB (SEL 810B computer)

KEY-| max. gap time–| 119 ms | through the UCSB Host computer, over

   | avg. gap time--|  50 ms |    the ARPA network, and into a UCLA
   | min. gap time--|   2 ms |    (site 65) Host discard socket
   | # gaps (discon-|   3    |    (socket 9).  Each test (approx.) 30
   |  tinuity in    +--------+    sec.
   |  process)
   +--

VII. CONCLUSIONS

 Based upon the results of this experiment the following conclusions
 can be drawn:

    1) High bit-rate real-time processes can use the ARPA Network to
       transmit data for relatively long periods of time.

    2) Real-time processes accessing the Network through large-scale
       timesharing host computers can expect arbitrary delays or gaps,
       probably attributable to the host computers and not the
       Network.

Pfeifer & MacAfee [Page 9]  RFC 508 Real-Time Data Transmission On The Arpanet 7 May 1973

    3) Techniques for handling gaps of 1/2 to 1 second may be possible
       but 8 second gaps, as measured in this experiment, will cause
       extreme hardship on any real-time process.

 This experiment has pointed up the need to conduct additional tests
 using a complete transmission link with actual data and with
 monitoring equipment at both the sending and receiving ends. Our
 current and future efforts are directed toward carrying out such
 experiments.

REFERENCES

[1] "Interface Message Processors for the ARPA Computer Network",

    Quarterly Technical Report No. 16, 1 Oct 1972 to 31 Dec 1972,
    Bolt, Beraneck and Newman, Inc.

[2] Semiannual Technical Summary on Graphics, Lincoln Laboratory,

    Massachusetts Institute of Technology, Nov 1971.

[3] D.L. Retz, "An Interactive System for Signal Analysis: Design,

    Implementation, and Applications", CSL Report No 25, Computer
    Systems Lab, University of California, Santa Barbara, CA, 1972.

        [ This RFC was put into machine readable form for entry ]
              [ into the online RFC archives by Via Genie ]

Pfeifer & MacAfee [Page 10]