MULTIPLE COMPUTER NETWORKS
AND INTERCOMPUTER COMMUNICATION
Dr. Lawrence G. Roberts
There are many reasons for establishing a network which allows many computers to communicate with
each other to interchange and execute programs or data. The definition of a network within this paper will
always be that of a network between computers, not including the network of typewriter consoles
surrounding each computer. Attempts at computer networks have been made in the past; however, the usual
motivation has been either load sharing or interpersonal message handling. Three other more important
reasons for computer networks exist, at least with respect to scientific computer applications. Definitions of
these reasons for a computer network follow.
1. Load Sharing: Both the program and data are transmitted to a remote computer to equalize the
load between the two facilities. This type of operation requires identical computers or languages.
When a given machine is loaded, consideration can be given to processing the program on another
machine. Many determinations must be made before an alternate machine is used (is there an
alternate machine for which appropriate software exists, is that alternate machine in a condition to
handle the program, will more time and dollars be spent on transmission than if thde user waits
until the original machine is available, etc.). Such determinations are very difficult and the gain
only moderate, hence load sharing is not a major consideration here. However, it is felt that some
load equalization will occur in any computer network.
2. Message Service: Ii~ addition to computational network activities, a network can be used to
handle interpersonal message transmissions. This type of service can also be used for educational
services and conference activities. However, it is not an important motivation for a network of
3. Data Sharing: The program is sent to a remote computer where a large data base exists. This type
of operation will be particularly useful where data files are too large to be duplicated economically.
Frequently gebgraphically discursed individuals need to access a common data base. Access to
this data base may be required simply to make an inquiry or may involve executing a complex
program using the data base. Use of a single data bank will save hardware required to store the
information and will eliminate theneed for maintaining multiple files. The term "single data bank"
does not necessarily mean the storing of all data at a single physical location, but rather the storing
of only one copy of each basic data file. This type of use is particularly important to the military
for command and control, information retrieval, logistics and war gaming applications. In these
cases, one command would send a program to be executed at another center where the data base
4. Program Sharing: Data is sent to a program located at a remote computer and the answer is
returned. Software of particular efficiency or capability exists on certain machines. For example, if
machine Y has a good LIST processor, it may be more efficient for users whose local machine is
X to use Y for LIST processing jobs. Even if a LIST processor exists for X, the time to execute the
program on Y may be sufficiently less than the time to execute on X that the total time (and/or
cost, including transmission) may be less. The use of specialized programs at remote facilities
makes possible large gains in performance. Perhaps even more important is the potential saving in
5. Remote Service: Just a query need be sent if both the program and the data exist at a remote
location. This will probably be the most common mode of operation until communication costs
come down. There will be a tendency for other cases to migrate toward this type of operation. For
example, in a graphics application, the program would be available or created on the remote
computer and it would generate the data structure in its own computer. It would modify and
update the data structure from network commands transmitting back display changes. This
category includes most of the advantages of program and data sharing but requires less
information to be transmitted between the computers.
The advantages which can be obtained when computers are interconnected in anetwork such that remote
running of programs is possible, include advantages due to specialized hardware and software at particular
nodes as well as increased scientific communication.
It is felt that new machine configurations can provide improvement factors of from 10 to 100 in the
problem area for which they were designed.
In some cases very large core and disk will substantially improve performance on existing machines. In
other cases the improvements will be brought about by introduction of new systems such as ILLIAC
1V1and macro modular machines.2A network is needed to make full use of machines with specialized
efficiency and with a network the development of such computers will be enhanced.
Specialized Systems Software
Handling jobs of widely varying sizes, particularly when initiated from many locations, presents an
extremely difficult scheduling problem for any single machine. A large machine serving a number of
smaller machines may provide significant improvements in efficiency by alleviating the scheduling
problem. Small time-sharing computers may be found to be efficiently utilized when employed as
communication equipment for relaying user requests to some larger remote machine on which substantive
work is done. What is envisioned is a system in which the local machine serves some limited needs of the
user while substantial requirements are satisfied by a remote computer particularly well adapted to handling
Once it is practical to utilize programs at remote locations, programmers will consider investigating what
exists elsewhere. The savings possible from non-duplication of effort are enormous. A network would
foster the "community" use of computers. Cooperative programming would be stimulated, and in particular
fields or disciplines it will be possible to achieve a "critical mass" of talent by allowing geographically
separated people to work effectively in interaction with a system.
The minimum requirement a system must meet to be eligible for membership in the network is a
time-sharing monitor which allows user programs to communicate with at leasttwo terminals. If
this requirement is uniformly met, the network can be implemented without major change to the
monitor at any installation, by the simple expedient of letting each computer in the network look
conceptually upon all the others as though they were its own remote user terminals.
Figuratively speaking, we may think of the computer-to-computer link in such a network as being
the result of removing a user terminal from its cable on computer A, removing a user terminal
from its cable on Computer B, and splicing the two computer cable ends together. Such a network
might operate as follows: (See Figure 1) The user dials up his home computer, CA, from a console.
He logs in normally by transmitting characters from his console to the monitor. He sets up a user
program and this program, through the second channel, calls the remote computer, logs in, sets up
the desired user program on the remote computer, submits parameter data to it and receives the
results. Note that neither system was required to behave in an unusual fashion. The monitors did
what they always do. The only requirement, as stated earlier, was that the user program be allowed
to communicate with two terminals, its own user terminal and the remote computer. Most present-
day monitors provide for such a capability.
A computer-computer network link as described above was established in 1966 experimentally
between MIT Lincoln Lab's TX-2 computer and System Development Corporation's Q-323. Both
nodes are general purpose, time-shared computers. This link allows programs on either computer
to utilize programs such as compilers and graphics systems which exist only at the other node. The
basic motivation was to test an initial network protocol, determine how well automatic dial up
communications service worked, and determine the extent of the time-sharing monitor changes
necessary. This has been done and the link is now utilized by users to increase their capability,
thus providing more evaluation data.
Interface Message Processor
One way to make the implementation of a network between a set of time-shared computers more
straightforward and unified is to build a message switching network with digital interfaces at each
node. This would imply that a small computer, an interface message processor (IMP), would be
located with each main computer to handle a communications interface. It would perform the
functions of dial up, error checking, retransmission, routing and verification. Thus the set of IMP's,
plus the telephone lines and data sets would constitute a message switching network (See Figure
The major advantage of this plan is that a unified, straightforward design of the network can be
made and implemented without undue consideration of the main computer's buffer space, interpret
speed and other machine requirements. The interface to each computer would be a much simpler
digital connection with an additional flexibility provided by programming the IMP. The network
section of the IMP's program would be completely standard and provide guaranteed buffer space
and uniform characteristics, thus the entire planning job is substantially simplified. The data sets
and transmission lines utilized between the IMP's would most likely be standardized upon, but as
changes occurred in the communication tariffs or data rates, it would be more straightforward just
to modify the program for the IMP's rather than twenty different computers. As soon as the need
became apparent, additional small computers could be located at strategic connection points
within the network to concentrate messages over cross-country lines. Finally, the modifications
required to currently operating systems would be substantially less utilizing these small computers
since there would be no requirement to find buffer spaces, hold messages for retransmission,
verify reception of messages and dial up telephone lines.
ARPA supports a number of computer research groups throughout the country most of which have their
own time-shared computer facility. These researchers have agreed to accept a single network protocol so
that they may all participate in an experimental network. The communication protocol is currently being
developed. It will conform to ASCII conventions as a basic format and include provisions for specifying
the conventions as a basic format and include provisions for specifying the origin, destination, routing,
block size, and sum check of a message. Messages will be character strings or binary blocks but the
communication protocol does not specify the internal form of such blocks. it is expected that these
conventions will be distributed in final form during July 1967.
Figure 3 shows a tentative layout of the network nodes and communication paths. However, since most of
the communications will be dial-up, the paths are just hypothetical. It is hoped that concentration and store
and forward capability will be available through the use of Interface Message Processors. The development
of the IMP's and the use of them at each node would allow store and forward operation as well as speeding
the realization of a unified network.
There are 35 computers shown in Figure 3 at 16 locations, there being several computers at most locations.
A rough estimate would place the number of consoles attached to the 35 computers by the end of 1967 at
1500 and the number of displays at 150. Assuming four characters per second for typewriters and 20
characters per second for scopes, the total I/O rate to the computers is 9000 char/sec. Estimating that 10%
of this I/O communication rate will be forwarded to another computer in the network leads us to an average
transmission rate of 60 char/sec per location. Thus, given console type activity on the network (messages of
from 10 to 1000 characters) the normal 2000 bits/second type communication should be sufficient at first.
The common carriers currently provide 2 or 4 wire, 2 kc lines between two points either dialed or leased, as
well as higher band width leased lines and lower band width teletype service. Considering the 2 kc offering,
since it is the best dial up service, the use of 2 wire service appears to be very inefficient for the type of
traffic predicted for .the network. In the Lincoln-SDC experimental link the average message length
appears to be 20 characters. Each message must be acknowledged so that the originator may retransmit o;
free the buffer. Thus the line must be reversed so often that the reversal time will effectively half the
transmission rate. Therefore, full duplex, four-wire service is more economic and simpler to use.
Current automatic dialing equipment requires about 20 seconds to obtain a connection and a similar time to
disconnect. Thus the response time is much too long assuming a call is made only after a message arrives
and that the line is disconnected if no other messages arrive soon. It has proven necessary to hold a line
which is being used intermittently to obtain the one-tenth to one second response time required for
interactive work. This is very wasteful of the line and unless faster dial up times become available, message
switching and concentration will be very important to network participants.
1. Slotnick, D. L., "Achieving Large Computing Capabilities Through An Array Computer,"
presented at SJCC (April 1967).
2. Clark, W. A., "Macromodular Computer Systems," Proc. SJCC (April 1967).
3. Marill, T. and Roberts, L. G. , "Toward a Cooperative Network of Time-Shared Computers," Proc.
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Copyright © 2001 Dr. Lawrence G. Roberts