- Email: [email protected]
Computing from the Communication Point of View
Computing from the Communication Point of View E. E. DAVID, JR.* Bell Telephone Laboratories Murray Hill, New Jersey
1. 2. 3. 4. 5.
Introduction Transmission of Computer Information Coding of Information Computer-Communication Networks Conclusion References .
.
.
. .
.
.
.
. . .
109 112 119 122 127 128
1. Introduction
Computing and communication are natural allies. Both concern information. When computing, we manipulate and transform information ; in communication, we transport information. Though this distinction seems logical enough, computing and communication are so intertwined that it is difficult to determine where one stops and the other begins. Shannon recognized that information was a common element. So did von Neumann. Indeed the utility of both communication and computing resides in the information they supply to users. Computing operations invariably involve communication, if no more than transmission of information from a card-reader t o memory or to input registers, and transmission of output information to a printer in the same room. Similarly, communication usually involves computing and logic to code, decode, and address the information transmitted, or to make the connection between transmitter and receiver. This interweaving of computing and communication has led to information networks of great complexity and utility. Probably no one has attempted to detail the network of information paths in the United States or even any large segment of it. There are many elements: telephone lines, TV and radio networks, the postal system, libraries and archives, and so on. These are often considered as separate entities, but in fact information is flowing increasingly from one to the other, manually in many cases, but through electrical connections more and more. Overall, this information network can be thought of as a maze of pathways over which information flows between nodes. At each node, information is routed, transformed, or stored. Computers are appearing
* Present address: The President's Washington, D.C. 109
Science Advisor, The White House,
110
E. E. DAVID, JR.
increasingly often a t nodes in this network. For example, electronic switching systems for telephone communication now employ storedprogram control, that is, computer control. Commercial sale of computing services to remote customers via communication lines is becoming increasingly common. Thus the communication view of computing pictures computers as part of an information network. As such, computers perform communication functions, and communication facilities are intimately involved in computing services. This interdependence has been recognized recently by the FCC who are inquiring as t o the proper role for the government in encouraging computer-communications interplay [3]. This inquiry involves much beyond purely technical matters, of course, but it is a measure of the dynamic state of the computer-communication enterprise. One widespread use of computer-communications today is based upon TOUCH-TONEB telephones as computer terminals for credit authorization. For example, TOUCH-TONE telephones are installed at many stores and restauranEs accepting American Express credit cards. The cashier calls the computer by TOUCH-TONE telephone, receives a tone that indicates the computer has answered, and then keys in the customer's charge account number and the amount of the sale. The computer responds by recorded voice with the charge account number for verification and authorizes the credit or not. TOUCH-TONE signalling can also be used in placing orders quickly and accurately, so as to shorten delivery intervals and lower inventory costs. For example, the Standard Oil Company of Ohio (Sohio) furnishes each service station owner with a new kind of catalog with which he can order tires, batteries, and accessories. The catalog is made up entirely of cards, one for each different item he sells. When he has an order t o transmit, he inserts a telephone punched card into the TOUCH-TONE phone and automatically calls a teletypewriter station a t the regional office. Next, with another punched card, he identifies his station. Then, for each item he wants to order, he selects the appropriate card and inserts it into the TOUCH-TONE phone, adding the only variable, quantity, by using the buttons of the TOUCH-TONE telephone. Another example comes from a typical announcement in The New York Times as shown in Fig. 1. Of course, there are many other examples-airline reservation systems, stock and bond quotation systems, and of course remote access to computers for research, development, and education. As of the end of 1969, Bell Telephone Laboratories had over 250 consoles subscribing to various computer utilities. Uses include debugging of large programs, file sharing of data bases, and engineering
COMPUTING FROM THE COMMUNICATION POINT OF VIEW
111
Private Network Is Established
For Trading lnrtitutional Stock
By TERRY ROBARDS The establishment of an automated system of block trading by a private concern in competition with the New York Stock Exchange was announced yesterday. The new system, operated by the Institutional Networks Corporation, will compete for business from institutional investors with a comparable system being established by the exchange. However, it also will enable institutions to avoid paying commissions on transactions in listed securities. Basically, the new system, called Instinet, involves the computerized storage of information on blocks of stock that institutions either wish to sell or buy. Only blocks of at least 500 shares with a minimum value of $25,000 will be included in the system. Instinet subscribers will have terminals in their offices that will be linked to a time-shared computer in Watertown, Mass., and, through the computer, to all other institutional investors connected to the system in the United States. Tied to Central File Subscribers will be able to transmit the names of securities they wish to buy or sell to the computer’s central file, along with the quantity, price and time limit of their offer. Prices can be quoted in decimals to the nearest penny. Institutional investors also will be able to obtain a computer print-out on the amounts of shares being offered and bid for in any given security. At the same time, however, the identity of the prospective sellers or buyers will be kept secret. Every time a subscriber enters an offer to buy or sell into the system, he automatically will be assigned a different offer number to identify it, so that all negotiations may be conducted anonymously. Anonymity even extends through the completion of transactions. Although they may exchange money and securities, buyers and sellers will never know each other except as transaction numbers. According to Herbert R. Behrens, president of Instinet, subscribers will pay a minimum monthly fee of $1,740, plus communicationscharges and data processing unit charges based on their use of the system. The network will be open for transactions between 9:30 A.M. and 4:30 P.M. Monday through Friday. The monthly fee to be paid by subscribing institutions probably would be considerably less than the commissions they would pay for making large transactions on the stock exchange through brokerage houses. Institutional investors are believed to have accounted for more than $r-billion in brokerage-house commissions last year.
FIG.1. 0 1969 by The New York Times Company. Reprinted by permission.
112
E. E. DAVID, JR.
calculations. The amount of such use nationally is expanding at a rapid clip. A survey done in 1967 indicated that there were some 2100 computer installations in the United States having on-line remote terminals (not including the simple information retrieval services such as stock and bond quotations). The usage of such terminals was expected to triple by the end of 1968. As far as we can tell, it did. The number of terminals in use in 1967 was about 40,000; a t the end of 1968, 81,000, and a t the end of 1972, some 175,000 are expected. About half the terminals are located within 25 miles of the computer they are accessing. Links between computers for computer-computer communication are fewer in number, but again of increasing importance. Small computers particularly are playing a vital role in operating card readers, printers, and magnetic tapes as remote terminals as input-output for larger computers, Communication between computers of comparable size is less common still but is becoming important for load sharing where the demands on a local computer are likely t o exceed its capacityfor short periods of time. The technical underpinnings of all this activity cover a broad area of electronics and systems disciplines. Technical factors determine just what is feasible and what is not for the future of computer communications, but economics ultimately determines which possibilities will come into broad usage. This paper will review some of these fundamentals and point to areas where the economic factors are crucial. 2. Transmission of Computer Information
Computers deal with symbols (letters, numbers, words, and others)a finite number a t any time. For transmission to a distant destination, these symbols are represented by a collection of discrete electrical signals, often binary. This discrete form of information is fundamentally different from that represented by continuous signals such as those from TV cameras and microphones. These signals have a continuum of values both in time and intensity or amplitude in contrast t o discrete signals. I n transmitting discrete, or digital information, we select successively one from a finite set of preselected signals and send it to the receiver. There a decision-making circuit decides from the received waveform which of the possible signals was sent. The received waveform differs from the transmitted signal because inevitably noise is added and distortion occurs during transmission. It is these two factors -plus one other to be mentioned later-that determines the rate a t which symbols can be communicated and the number of errors which occur. For example, symbolic information is often coded into binary form ;
COMPUTING FROM THE COMMUNICATION POINT OF VIEW
113
FIG.2. Various standard pulses.
that is, there are only two signals which can be transmitted. One might be a pulse of standard amplitude and form, the other a zero or null signal, as illustrated in Fig. 2. To communicate, a sequence of such signals is transmitted one after another, each being a pulse or no pulse depending upon the information to be transmitted. If the spacing between successive signals is 7,then there are f = 117 elementary signals transmitted per second. According to the usual definition of the basic information unit as a choice between two equally likely alternatives, such a transmission channel can accommodate f bits per second if all elementary signals are received properly at the destination. The maximum number of bits per second that a channel can accommodate is, of course, related to its bandwidth and the signal-to-noise ratio (SNR) a t the receiver by Shannon’s famous result
c = w log(1+ SIN),
where C is the maximum number of bits per second, known as the channel capacity, W is the bandwidth, and SIN is the signal-to-noise ratio. I n actual transmission facilities, less than half the maximum theoretical rate is usually achieved because of the elaborate coding necessary to prevent errors in transmission as the transmission rate approaches the Shannon limit. Noise is one of the causes of transmission error. A typical digital signal is pictured in Fig. 3. The transmitted signal is shown at the top: the pulse-no pulse sequence is designated by ones and zeros corresponding to pulse-no pulse written above the numbered time slots. The received signal contaminated by noise is shown at the bottom. A rudimentary receiver might examine the received waveform a t times 1, 2, 3, . . . , and compare the value with a threshold one half the pulse amplitude. If the received signal is greater than the threshold, the receiver decides that ((one” (a pulse) was transmitted. If the received signal is less than the threshold, the receiver decides that a zero (no pulse) was
114
E. E. DAVID, JR. I I
I
0 2
1 T 3
I
4
O
O
, 6
5
I
7
O
I
, 9
8
O
1
,
0
t .
ERRORS 1
0
1
I
0
I
2
3
4
5
-T--i-T 6
I
7
8
0
9 1 0
FIG. 3. Top, transmitted signal ; bottom, received digital signal.
transmitted. If the noise perturbs the received waveform excessively, then the receiver decides incorrectly. This situation is illustrated in Fig. 3 a t pulse positions 6 and 8 (the transmitted and received sequences are written as sequences of ones and zeros above the transmitted and received waveforms). Actually, the signal-to-noise ratio required for nearly errorless transmission is surprisingly low. Calculations based upon gaussian noise interference and binary pulse transmission indicate the following [22].
S/N 13.3 17.4 19.6 21.0 22.0 23.0
Error probability 10-2 10-4 10-6 10-8 10-10
10-12
(Assuming lo5 pulses per second) This is about one error every 0.001 8 0 C 0.1 sec 10 6 0 C 20 min 1 day 3 months
Thus if the signal-to-noise ratio is kept above, say, 25 dB, transmission errors are practically nil from this source. The receiver can regenerate the transmitted signal exactly. This ability to regenerate or reconstruct the tranwmitted signal is one of the unique features of digital systems. On a long transmission line or path, it may not be possible to keep the signal-to-noise ratio high enough. One remedy is to place repeaters that duplicate the function of a decision-making receiver a t intervals along the line, regenerating the signal at each one. This is the basic property of a digital tvansmission system as contrasted to an analog syatem where each repeater merely amplifies its received signal before sending it along t o the next repeater. I n a n analog system, the noise
COMPUTING FROM THE COMMUNICATION POINT OF VIEW
115
added to the signal between repeaters is amplified at each repeater and so accumulates. In the digital system this accumulation does not occur since regeneration removes the noise completely a t each repeater, if the system is well designed. Another way of preventing noise-induced errors in transmission is to use digital codes which allow the receiver to detect and correct errors [ l o ] .Extra pulses are inserted in the transmitted digital stream so that the composite signal is guaranteed to have some fixed property that the receiver can confirm. One of the simplest such codes is the so-called parity check. Here an extra binary signal is inserted so that the number of ones in each block of say, ten pulse slots is even, regardless of the number of ones in the information signal. If the receiver makes an error because of noise ; the number of pulses will be odd, (in case of a single error) and the receiver will know that an error has been made in that block. It can then ask for retransmission of the block. There are many other error detection codes, some of which make it possible, for the receiver to correct errors without retransmission. Note that both analog and digital signals can be transmitted over either analog or digital systems. In sending analog signals over analog systems, the signal is transmitted ia a form and with a power that insures that the signal-to-noise ratio at the receiver is large enough to satisfy the required fidelity. I n the digital-digital case, the corresponding statement is that the system design must insure that the signal-tonoise ratio and the error detection and correction coding keeps the error rate below the required figure. The identical statement is appropriate for digital signaling over analog channels. In both cases the actual form of the digital signals put onto the transmission medium must be tailored to its characteristics [9].For example, many media do not transmit dc, such as those that employ transformer coupling at input and output. The pulse train of Fig. 3 has a dc component and so would not be appropriate for such media. This situation can be corrected by choosing elementary signals different from pulseno pulse, for example, positive pulse-negative pulse. I n Fig. 4, the same information as in Fig. 3 is shown coded using these signals. This waveform has no dc components. Of course, those media which carry radio frequencies only (microwave radio systems, for example) require modulators and demodulators to handle digital (or analog) signals. I
0
I
1
0
0
I
0
I
0 TIME
FIG.4. Digital signal using positive and negative pulses.
116
E. E. DAVID, 1R.
To transmit analog signals over digital systems, conversion of the continuous signal to discrete form is necessary. This “ analog-digital conversion ” is a well-developed technology in which an analog signal is “quantized” in both time and intensity. Pulse code modulation (PCM) is one such conversion. Here the signal is sampled in time and the amplitude of each sample is represented by a number, usually binary. It turns out that if the bandwidth of the antilog signal is W , then it must be sampled a t least 2 W times per second if it is to be recovered without distortion from its samples. If each sample is represented by b bits, then the required data rate is 2 Wb bits per second. There are coding methods other than PCM which yield lower data rates. These techniques generally hinge upon some predictable property of the analog signal, and are tailored for signals having that property. For example, speech signals have most of their energy below 1000 Hz, but for easy intelligibility, frequencies up to about 4000 Hz must be transmitted. A form of coding known as delta-modulation [2] is particularly well suited for digitizing such signals. After an analog signal is digitized, the digits, or bits, can be coded in various ways t o fit an available channel. For example, coaxial cable provides a channel where the attenuation per unit length increases with the square root of frequency. Thus it is advantageous to concentrate the digital signal energy at the low frequencies. This can be done by using multilevel, rather than binary, pulses. An example of an eight-level signal after transmission is shown in Fig. 5, where the traces of many successive pulses are superimposed. The eight levels are clearly visible.
Fro. 6. Eight-lev81 signal after transmission.
COMPUTING FROM THE COMMUNICATION POINT OF VIEW
117
The gaps, or “eyes” between them indicate the margin for the decision levels of a regenerative receiver. These decision levels determine which of the eight levels an individual pulse represents. The above discussion assumes that digital signals arrive at their receiver with noise added, but otherwise unaltered. Actually, the phase and frequency characteristics of a channel can alter the shape of signal drastically even in the absence of noise. Distortion of a rectangular pulse by a channel is shown in Fig. 6a. The effect of such distortions is to cause pulses to overlap into the time slots of adjacent pulses. This “intersymbol interference ’’ [9] can cause transmission errors just as noise can. TO reduce this effect to tolerable levels, the channel must be
FIG.6.- (a) Channel distortion of a rectangular pulse. (b) The same pulse equa-
lized.
118
E. E. DAVID, JR.
“equalized ” by insertion of compensating networks a t the transmitter or receiver. Such networks correct principally phase distortion in the medium that arises from dispersion (different frequencies traveling different velocities). The same pulse of Fig. 6a is shown equalized in Fig. 6b. Equalization of channels is a well-developed art, but it is made more dificult by the changing nature of channels. Channel characteristics change with temperature and the aging of components, calling for readjusted equalization. Also, in a switched network, there are many channels available between points, and each requires a different equalization. Recently, adaptive equalizers have been developed that measure the intersymbol interference and compensate dynamically t o effect variations [ 6 ] . The combination of adaptive equalization and error-correction coding makes it economically feasible to transmit data over channels a t rates up to about 50% of the Shannon limit. For example, recent experiments [S] were carried out over a 10-MHz channel using 16 elementary signals which were merely pulses of 16 different levels. Thus each pulse transmitted was capable of carrying 4 bits. These were transmitted a t 11.65 megapulses per second giving a data rate of 46.60 megabits per second. The signal-to-noise ratio on this channel was about 50 dB, so the Shannon capacity was
C = 10 log, lo6%165 megabits per second The error rate was about lo-’’. A sample of the composite received signal (of 8 levels) is shown in Fig. 5 . Only 8 levels are shown since 16 levels are difficult t o resolve in a photograph of the sort used here. Another requisite for transmission of digital signals is time synchronization between transmitter and receiver. The receiver, in effect, samples the incoming wave-form a t the successive time slots where pulses are t o be found. Any displacement of the samples from these time slots results in degraded performance (increased error rates). One technique for maintaining synchronization involves deriving a timing signal a t the receiver by processing the incoming wave. For example, the receiver might incorporate a filter to isolate the fundamental component from the incoming wave. The timing for the sampling instants might then be derived from the zero-crossings of that sine wave. In some pulse sequences, however, the fundamental may be missing, SO special sgfnchronizing “pulse sequences may have t o be sent periodically. Another technique is to send timing information on a separate channel. This problem of ‘‘ end-to-end ” synchronization has been solved in a number of ways, none of them perfect [7]. Thus, jitter and uncertainty in receiver timing does add t o the error probability, but these effects can be held within bounds. ))
COMPUTING FROM THE COMMUNICATION POINT OF VIEW
119
Overall, digital transmission is a well-developed technology. For example, the usual telephone channel (250-3400 Hz nominal bandwidth) can handle 2000, 2400, 3600, or 7200 bits per second depending upon the modulation selected and the characteristics of the particular line. Much higher data rates are available using wideband circuits. Today, with the availability of increased data rates well established, interest is shifting toward how such facilities can be used economically and effectively. Two important factors are the coding of information into data form and the creation of data networks to use data links efficiently. 3. Coding of Information
A piece of information, such as that corresponding t o a printed page or a line drawing, can be represented in many different ways. Depending on the representation, the amount of data to be transmitted can vary by as much as 1000 to 1. It is generally true that the smaller data representations require more extensive terminal equipment for coding a t the transmitter and reproduction storage, or display a t the receiver. Thus there is inevitably a trade-off between total data to be transmitted and the complexity, or cost, of terminal equipment. A topical example of the above generalities is provided by graphical materials. Graphical information can be represented in a number of different ways, each with different communication requirements. For instance, a page of typing could be scanned line by line, just as a television picture is. This representation is the basis of facsimile transmission. It takes about 16 million bits to represent the page that way. Over a dialed telephone line (2000 bits per second) it would take about 10 minutes to transmit this number of bits. For example, the XeroxMagnavox telecopier operated a t about this speed (6 minutes per page). Over a 40-kilobit line, it takes less time, about 30 seconds per page. Another way t o represent a page of print is by assigning each letter a code, as in Morse. This takes only some 10,000 bits per page; in theory then only 5 seconds over a dialed connection is required. A similar disparity occurs in the case of a diagram (see Fig. 7). The scanned format which represents the diagram as a TV-like picture, requires about the same amount of data as for a page of printing in the format. Another way is to represent picture as a draftsman would produce it-by drawing one line of the figure after another in sequence (draftsman format). Of course, the number of lines (the amount of drawing) depends upon the complexity of the figure, but typically there is between 1000 and 100-to-1 reduction in number of bits required in the draftsman and scanned formats.
120
E. E. DAVID, JR.
-
l 1.25 MILLION BITS / PAGE
FACSIMILE MODE
SEND END POINTS ONLY ABOUT 30 THOUSAND BITS/ PAGE
DRAFTSMAN MODE
FIU.7.
This disparity is at the base of a graphics facility in which xerographic printers are used as computer output devices to supply immediate graphics. This system is called STARE[ I ] . The computer itself can generate diagrams in either the scanned or the draftsman form ; only the software is different in the two cases. The xerographic printer requires the scanned form since it normally functions as EL facsimile machine. It is driven usually over a 40-kilobit communication line. A different scheme is used in the STAREsystem, as illustrated in Fig. 8. The computer generates the diagram in the draftsman format and it is transmitted over a low capacity communication line--a 5400 bit per second facility, for example. This information is stored in a special memory a t the receiving point. Conversion of the diagram from the draftsman to the scanned format is done by special hardware attached to the memory. The converted data drives the xerographic printer, which then produces the copy.
COMPUTING FROM THE COMMUNICATION POINT OF VIEW
121
LONG HAUL TRANSMISSION IN DRAFTSMAN MODE
COMPUTER
AT 5400 EITS/SECOND (6 SECONDS / PICTURE
LOCAL TRANSMISSION IN FACSIMILE MODE 250,000 BITS/ SECOND GRAPHIC SCAN CONVERTER
XEROX L D X
FIG.8. STARE system communicstion.
The STAREgraphics system was designed for internal use in Bell Telephone Laboratories’ R & D operation. It was built by Xerox Corporation to specifications. The computer software was written at Bell Labs. STARE has been in operation for about 2 years. The STAREsystem gives the computer user, regardless of his location, the option of obtaining graphic output from the computer. The user may enter his program into the computer from a console in his office, say a teletypewriter. He then orders, by using simple instructions, graphs and diagrams to indicate the results. He walks a short distance down the hall and retrieves the graphs as they are produced by the printer, communicating over a low-speed facility. This proves t o be a great convenience for users. They no longer have to wait hours while a roll of 35 mm microfilm is developed, as they might with a microfilm output. This is one of the first adaptations of a copying machine for remote computer output. Needless to say, the STAREsystem has implications far beyond Bell Laboratories-for libraries, business
122
E.
E. DAVID, JR.
operations, education, and any other application where hard copy output is needed. I n the STARE system, the scan converter that drives the xerographic printer is the equipment which makes low data rate transmission possible. Thus the cost of the converter must be offset by a reduced cost of transmission and computing to generate the graphic. Just such considerations must be involved in the design of most computer-communication systems. I n the past, the costs of transmission and computing have been decreasing much more rapidly than the cost of terminal equipment. Thus there has been little motivation to code information in sophisticated ways to economize on transmission, except in extreme cases such as transatlantic circuits. With the coming of integrated circuitry and new memory technologies, the cost of terminals promises t o decrease by a substantial factor, and so coding for reduced data rates promises t o become more favorable economically.
4. Computer-Communication Networks Transmission lines can be used t o connect a computer to another computer, or a computer terminal to a computer. However, point-topoint connections do not provide the flexibility that a network of lines can. Networks have many functions ; prominent among them are (1) Multiple Connection. Connecting a computer or terminal t o any one of a number of other computers a t various locations. (2) Sharing. Permitting several terminals or computers to use the 8ame transmission line alternutely. (3) MuEtipZexing. Permitting several terminals or computers to transmit data over the same transmission line simultaneously. (4)Message Packing. Interleaving data onto the line so that idle periods of one transmitter can be used to send data from another. The act of connecting one point to another or routing information from one point t o another in a network known as “switching.” Many different switching schemes have been used or proposed, each with their own characteristics, which prove to be either an advantage or a disadvantage depending upon the situation. Two kinds of switching span the range of possibilities. I n circuit-switching, a point to-point connection is made and retained for the duration of the communication, regardless of whether data flow is continuous in time or not. Such a connection can be thought of as a metal circuit even though part of the actual circuit may be multiplexed with data from other sources and t o other destinations. I n message switching, communication is carried out by
COMPUTING FROM THE COMMUNICATION POINT OF VIEW
123
messages or packets (parts of messages) each addressed somewhat as a cable or telegram is, so that a circuit need be set up only when a message is originated. Here the circuit is in effect repeatedly established, but only when a message is to be delivered. Networks may use circuit or message switching or both. Three examples will be given to illustrate various features obtained by these techniques in combination. A hierarchical network using circuit Switching is illustrated in fig. 9. TO OTHER TOLL NODES
NALS
LCOMPUTER1 b FIG.9. A hierarchical network using circuit switching.
The modern telephone network is of this kind and has been used extensively for data transmission. The network contains many switching nodes used as illustrated. A local switching node can connect local terminals to a local computer, thus there need not be as many computer ports as terminals unless all terminals must be connected simultaneously. For longer distances, connections (called trunks) between switching centers can be used. Again, trunks may be used by many different terminals on different occasions. Transmission facilities between centers may be multiplexed so that they can carry many simultaneous communications. Multiplexing is accomplished by time or frequency
124
E. E. DAVID, JR.
ILLINOIS
FY 1969-
FY 1970----FY 1971 ............. ALL COMMUNICATION LINES ARE LEASED AND OF 50 KILOBIT CAPPCITY
FIG.10. ARPA computer network. All oommuniation lines are leased and of 50-kilobit oapacity.
sharing. A typical connection in a large metropolitan area may involve 2-3 switching nodes. A hierarchical, circuit-switched network provides the functions of multiple connection, sharing, and multiplexing, but not message packing. Note that switching a t nodes in networks of this kind is controlled by a computer or a computerlike device. The Advanced Research Project Agency (ARPA) of the Department of Defense has recently begun to set up an experimental one-level message-switched network [ I l l . Some of the nodes and lines are shown in Fig. 10. At each node there is a communications computer. It accepts messages coming from other nodes, examines their address, and routes them onward if they are not destined for that node. If they are, the computer reformats them for transmission to the local computer after, perhaps, assembling several into larger units. The communication computer also accepts data from local computers, reformats it, and sends it onto the network in addressed packets. This type network (which has been called a bufleered network) performs multiple connection, sharing, multiplexing, and message packing. It carries out all these functions simultaneously at the expense of inserting memory and logic at the switching nodes. This memory and processing implies delay between origination and receipt of a message, just how much depending upon the intensity and statistics of the traffic. I n the ARPA net, the delay is estimated a t 1.5 seconds maximum for any message packet. Still another network configuration is the “round robin” messageswitched network [a] shown in Fig. 11. Here a number of stations are
COMPUTING FROM THE COMMUNICATION POINT OF VIEW
9
125
rTRANSMISSION L I N E
POTHER
D I S C 4
CALCOMP
FORMAT
WHERE TO 6
WHERE FR 6
OP CODE
DATA
END
4
FIG.11. Round robin message-switched network.
connected to a loop which transmits data a t a high rate. The stations may be computers or terminals or both, and they share the transmission capacity of the loop. A “supervisory computer” is present to seize control of the loop in case of failure a t one of the stations. Each station is connected to the loop by special interface logic which can examine incoming messages and originate outgoing messages. Each message is headed by an address to some station on the loop, and contains an “operation code ” which specifies how the subsequent portion of the message is to be handled a t its destination. This scheme permits a station t o originate and carry on communication with another station on the loop. Each station is allowed access t o the loop in sequential order; that is, control is passed around the loop from station t o station. The supervisory computer may be used to connect the loop t o a larger network. This loop system performs multiple connection, sharing, multiplexing, and message packing but a t the cost of transmitting a substantial amount of “ protocol ” data in addition t o addresses. There are many possible network arrangements. The one most appropriate depends upon the statistics of the data traffic. Two extreme situations span the range. Some applications create nearly continuous streams of data. For example, in some business situations, sales data is accumulated a t outlying stores during the day and transmitted later in the evening as a batch to a central data processing center. Typically a circuit switched system is appropriate, for the circuit is used continuously for periods of many minutes or even hours. On the other hand, in on-line usage of a remote computer from a teletypewriter, traffic in both directions tends to come in short bursts (particularly from the computer
126
E.
E. DAVID, JR.
to the TTY). Also different sources generate vastly different data rates; for example teletypewriters can generate data a t 100 bits per second while some disk transfers take place a t 4 x lo6 bits per second. If the transmission medium does not handle the source rate normally, then circuitry arid memory for buffering i t to the line rate is necessary. The packets so created tend to come in bursts also. An appropriate message-switched system can accommodate burst-type traffic, multiplexing, and packing to achieve high loading on wideband (costly) transmission facilities. Of course, multiple connection and sharing are also achieved. The examples quoted earlier illustrate this point. However, as in the case of coding, it is possible to trade equipment complexity for average data rate. Minimum cost may involve more than deciding that the traffic is burstlike or not. An interesting example is provided by PICTUREPHONEQ terminals which will nominally be used for face-to-face conversation (see Fig. 12). They display their
FIG.12. Face-to-faceconverslttion via the PICTUREPHONE.
COMPUTING FROM THE COMMUNICATION POINT
OF VIEW
127
picture using a full raster scan 30 times per second which requires a 1-MHz line. PICTUREPHONE terminals can be used also for computer communication. The computer prints messages on the screen, and the user may communicate t o the computer using his TOUCH-TONE keys. The point is that the PICTUREPHONE display must be transmitted continuously even though the actual data from the computer comes in short infrequent bursts. PICTUREPHONE terminals may well become the most numerous visual display actually in the field in the next 5-10 years. PICTUREPHONE service will operate through a circuit-switched network. Even so, it will likely be an important computer terminal because the incremental cost for using it in that way will be low. However, should such usage be contemplated over, say, a costly transcontinental route, then it may well be wise to use message-switching on that portion, reverting t o circuit switching a t the ends. For example, the computer burst might be sent as an addressed message to a display buffer located near the PICTUREPHONE set. The display buffer would store the picture and supply it as a standard PICTUREPHONE signal to the local set. There is one actual instance, however, in which a switching plan in this spirit is actually in use. On some transatlantic telephone circuits, conversations are broken into “speech spurts ” which are preceded by an address (or identifier) so that other speech spurts can be interleaved in time on the same circuit; that is, the spurts are message-switched. This system is known as TASI (Time-Assignment Speech Interpolation) and has been in use for over 10 years. Basically it capitalizes on the single talker nature of conversations. Since only one speaker is talking a t a time usually, another conversation can use the reverse channel. TASI is used on groups of 36 transatlantic channels; the statistics of conversations makes it possible t o accommodate 72 simultaneously on these 36 channels with very little impairment. It would not be surprising to see similar arrangements for data in the future. 5. Conclusion
Computer communication is a subject with many facets. Broad, sweeping conclusions require serious system studies in which the advantages of various trade-offs and possibilities are examined critically. Even such studies may not establish any general principles but only ad hoc solutions to specific problems. Nevertheless, one trend seems clear. As the cost of terminals and switching computers decreases, message switching will become increasingly competitive with pure circuit switching. Thus, computers and communication will become even more strongly intermixed.
128
E. E. DAVID, JR.
REFERENCES 1 . Christensen, C., and Pinson, E. N., Multifiinction graphics for a large computer system. Proc. AFIPS Pall Joint Computer Conf., Los Angeles, 1967 31, 697-71 1. 2. deJager, F., Delta modulation, a method of PCM transmission using the oneunit code. Philips Res. Rept. 7, 442--466 (1952). 3. Dunn, D. A., Policy h u e s presented by the Interdependence of Computer and Communications Services, Report 73798-1 dated Feb. 1969. Stanford Research Institute, Menlo Park, California (FCC Docket 16979). 4 . Farmer, W. D., and Newhall, E. E., An experimental distributed switching system to handle bursty computer traffic. Proc. ACM CorLf., Pine Mountain, Ueorgiu, 1969, pp. 1-35. 5 . Freeny, 6. L., King, B. G., Pedersen, T. J., and Young, J. A., High spced hybrid digital transmission. Proc. Intern. Conf. Commun., Boulder, Colorado, 1969 pp. 38-7-38-12. 6. Gersho, A., Adaptive equalization of highly dispersive channels for data transmission. Bell System Tech. J . 48, 85-70 (1969). 7. Manley, J. M., The generation and accurnulation of timing noise in PCM systems and experimental and theoretical study. Bell System Tech. J . 48, 541-613 (1969). 8. Pierce, 5.R., The transmission of computer data. Sci. Am. 215, 144-150, 152, 154, 156 (1966). 9. Pierce, J. R., Some practical aspects of useful digital transmission. IEEE Spectrum 5, 63-70 (1968). 10. Peterson, W . W., Error Correcting Codes. MIT Press, Cambridge, Massachusetts (1961) ; see Berlckamp, E. R., Algebraic Coding Theory. McGraw-Hill, New York, 1968; Mussey, J. L., Threshold Decoding. MIT Press, Cambridge, Massachusetts, 1963. 11. Roberts, L., Resource Bharing computer network. I E E E Intern. Conf. Digest 15, 326-327 (lQ69). 1 2 . Shannon, C. E., Pierce, J. R.. Oliver, D., The philosophy of PMC. Proc. I R E 36, 1324-1331 (1948).
1. 2. 3. 4. 5.
Introduction Transmission of Computer Information Coding of Information Computer-Communication Networks Conclusion References .
.
.
. .
.
.
.
. . .
109 112 119 122 127 128
1. Introduction
Computing and communication are natural allies. Both concern information. When computing, we manipulate and transform information ; in communication, we transport information. Though this distinction seems logical enough, computing and communication are so intertwined that it is difficult to determine where one stops and the other begins. Shannon recognized that information was a common element. So did von Neumann. Indeed the utility of both communication and computing resides in the information they supply to users. Computing operations invariably involve communication, if no more than transmission of information from a card-reader t o memory or to input registers, and transmission of output information to a printer in the same room. Similarly, communication usually involves computing and logic to code, decode, and address the information transmitted, or to make the connection between transmitter and receiver. This interweaving of computing and communication has led to information networks of great complexity and utility. Probably no one has attempted to detail the network of information paths in the United States or even any large segment of it. There are many elements: telephone lines, TV and radio networks, the postal system, libraries and archives, and so on. These are often considered as separate entities, but in fact information is flowing increasingly from one to the other, manually in many cases, but through electrical connections more and more. Overall, this information network can be thought of as a maze of pathways over which information flows between nodes. At each node, information is routed, transformed, or stored. Computers are appearing
* Present address: The President's Washington, D.C. 109
Science Advisor, The White House,
110
E. E. DAVID, JR.
increasingly often a t nodes in this network. For example, electronic switching systems for telephone communication now employ storedprogram control, that is, computer control. Commercial sale of computing services to remote customers via communication lines is becoming increasingly common. Thus the communication view of computing pictures computers as part of an information network. As such, computers perform communication functions, and communication facilities are intimately involved in computing services. This interdependence has been recognized recently by the FCC who are inquiring as t o the proper role for the government in encouraging computer-communications interplay [3]. This inquiry involves much beyond purely technical matters, of course, but it is a measure of the dynamic state of the computer-communication enterprise. One widespread use of computer-communications today is based upon TOUCH-TONEB telephones as computer terminals for credit authorization. For example, TOUCH-TONE telephones are installed at many stores and restauranEs accepting American Express credit cards. The cashier calls the computer by TOUCH-TONE telephone, receives a tone that indicates the computer has answered, and then keys in the customer's charge account number and the amount of the sale. The computer responds by recorded voice with the charge account number for verification and authorizes the credit or not. TOUCH-TONE signalling can also be used in placing orders quickly and accurately, so as to shorten delivery intervals and lower inventory costs. For example, the Standard Oil Company of Ohio (Sohio) furnishes each service station owner with a new kind of catalog with which he can order tires, batteries, and accessories. The catalog is made up entirely of cards, one for each different item he sells. When he has an order t o transmit, he inserts a telephone punched card into the TOUCH-TONE phone and automatically calls a teletypewriter station a t the regional office. Next, with another punched card, he identifies his station. Then, for each item he wants to order, he selects the appropriate card and inserts it into the TOUCH-TONE phone, adding the only variable, quantity, by using the buttons of the TOUCH-TONE telephone. Another example comes from a typical announcement in The New York Times as shown in Fig. 1. Of course, there are many other examples-airline reservation systems, stock and bond quotation systems, and of course remote access to computers for research, development, and education. As of the end of 1969, Bell Telephone Laboratories had over 250 consoles subscribing to various computer utilities. Uses include debugging of large programs, file sharing of data bases, and engineering
COMPUTING FROM THE COMMUNICATION POINT OF VIEW
111
Private Network Is Established
For Trading lnrtitutional Stock
By TERRY ROBARDS The establishment of an automated system of block trading by a private concern in competition with the New York Stock Exchange was announced yesterday. The new system, operated by the Institutional Networks Corporation, will compete for business from institutional investors with a comparable system being established by the exchange. However, it also will enable institutions to avoid paying commissions on transactions in listed securities. Basically, the new system, called Instinet, involves the computerized storage of information on blocks of stock that institutions either wish to sell or buy. Only blocks of at least 500 shares with a minimum value of $25,000 will be included in the system. Instinet subscribers will have terminals in their offices that will be linked to a time-shared computer in Watertown, Mass., and, through the computer, to all other institutional investors connected to the system in the United States. Tied to Central File Subscribers will be able to transmit the names of securities they wish to buy or sell to the computer’s central file, along with the quantity, price and time limit of their offer. Prices can be quoted in decimals to the nearest penny. Institutional investors also will be able to obtain a computer print-out on the amounts of shares being offered and bid for in any given security. At the same time, however, the identity of the prospective sellers or buyers will be kept secret. Every time a subscriber enters an offer to buy or sell into the system, he automatically will be assigned a different offer number to identify it, so that all negotiations may be conducted anonymously. Anonymity even extends through the completion of transactions. Although they may exchange money and securities, buyers and sellers will never know each other except as transaction numbers. According to Herbert R. Behrens, president of Instinet, subscribers will pay a minimum monthly fee of $1,740, plus communicationscharges and data processing unit charges based on their use of the system. The network will be open for transactions between 9:30 A.M. and 4:30 P.M. Monday through Friday. The monthly fee to be paid by subscribing institutions probably would be considerably less than the commissions they would pay for making large transactions on the stock exchange through brokerage houses. Institutional investors are believed to have accounted for more than $r-billion in brokerage-house commissions last year.
FIG.1. 0 1969 by The New York Times Company. Reprinted by permission.
112
E. E. DAVID, JR.
calculations. The amount of such use nationally is expanding at a rapid clip. A survey done in 1967 indicated that there were some 2100 computer installations in the United States having on-line remote terminals (not including the simple information retrieval services such as stock and bond quotations). The usage of such terminals was expected to triple by the end of 1968. As far as we can tell, it did. The number of terminals in use in 1967 was about 40,000; a t the end of 1968, 81,000, and a t the end of 1972, some 175,000 are expected. About half the terminals are located within 25 miles of the computer they are accessing. Links between computers for computer-computer communication are fewer in number, but again of increasing importance. Small computers particularly are playing a vital role in operating card readers, printers, and magnetic tapes as remote terminals as input-output for larger computers, Communication between computers of comparable size is less common still but is becoming important for load sharing where the demands on a local computer are likely t o exceed its capacityfor short periods of time. The technical underpinnings of all this activity cover a broad area of electronics and systems disciplines. Technical factors determine just what is feasible and what is not for the future of computer communications, but economics ultimately determines which possibilities will come into broad usage. This paper will review some of these fundamentals and point to areas where the economic factors are crucial. 2. Transmission of Computer Information
Computers deal with symbols (letters, numbers, words, and others)a finite number a t any time. For transmission to a distant destination, these symbols are represented by a collection of discrete electrical signals, often binary. This discrete form of information is fundamentally different from that represented by continuous signals such as those from TV cameras and microphones. These signals have a continuum of values both in time and intensity or amplitude in contrast t o discrete signals. I n transmitting discrete, or digital information, we select successively one from a finite set of preselected signals and send it to the receiver. There a decision-making circuit decides from the received waveform which of the possible signals was sent. The received waveform differs from the transmitted signal because inevitably noise is added and distortion occurs during transmission. It is these two factors -plus one other to be mentioned later-that determines the rate a t which symbols can be communicated and the number of errors which occur. For example, symbolic information is often coded into binary form ;
COMPUTING FROM THE COMMUNICATION POINT OF VIEW
113
FIG.2. Various standard pulses.
that is, there are only two signals which can be transmitted. One might be a pulse of standard amplitude and form, the other a zero or null signal, as illustrated in Fig. 2. To communicate, a sequence of such signals is transmitted one after another, each being a pulse or no pulse depending upon the information to be transmitted. If the spacing between successive signals is 7,then there are f = 117 elementary signals transmitted per second. According to the usual definition of the basic information unit as a choice between two equally likely alternatives, such a transmission channel can accommodate f bits per second if all elementary signals are received properly at the destination. The maximum number of bits per second that a channel can accommodate is, of course, related to its bandwidth and the signal-to-noise ratio (SNR) a t the receiver by Shannon’s famous result
c = w log(1+ SIN),
where C is the maximum number of bits per second, known as the channel capacity, W is the bandwidth, and SIN is the signal-to-noise ratio. I n actual transmission facilities, less than half the maximum theoretical rate is usually achieved because of the elaborate coding necessary to prevent errors in transmission as the transmission rate approaches the Shannon limit. Noise is one of the causes of transmission error. A typical digital signal is pictured in Fig. 3. The transmitted signal is shown at the top: the pulse-no pulse sequence is designated by ones and zeros corresponding to pulse-no pulse written above the numbered time slots. The received signal contaminated by noise is shown at the bottom. A rudimentary receiver might examine the received waveform a t times 1, 2, 3, . . . , and compare the value with a threshold one half the pulse amplitude. If the received signal is greater than the threshold, the receiver decides that ((one” (a pulse) was transmitted. If the received signal is less than the threshold, the receiver decides that a zero (no pulse) was
114
E. E. DAVID, JR. I I
I
0 2
1 T 3
I
4
O
O
, 6
5
I
7
O
I
, 9
8
O
1
,
0
t .
ERRORS 1
0
1
I
0
I
2
3
4
5
-T--i-T 6
I
7
8
0
9 1 0
FIG. 3. Top, transmitted signal ; bottom, received digital signal.
transmitted. If the noise perturbs the received waveform excessively, then the receiver decides incorrectly. This situation is illustrated in Fig. 3 a t pulse positions 6 and 8 (the transmitted and received sequences are written as sequences of ones and zeros above the transmitted and received waveforms). Actually, the signal-to-noise ratio required for nearly errorless transmission is surprisingly low. Calculations based upon gaussian noise interference and binary pulse transmission indicate the following [22].
S/N 13.3 17.4 19.6 21.0 22.0 23.0
Error probability 10-2 10-4 10-6 10-8 10-10
10-12
(Assuming lo5 pulses per second) This is about one error every 0.001 8 0 C 0.1 sec 10 6 0 C 20 min 1 day 3 months
Thus if the signal-to-noise ratio is kept above, say, 25 dB, transmission errors are practically nil from this source. The receiver can regenerate the transmitted signal exactly. This ability to regenerate or reconstruct the tranwmitted signal is one of the unique features of digital systems. On a long transmission line or path, it may not be possible to keep the signal-to-noise ratio high enough. One remedy is to place repeaters that duplicate the function of a decision-making receiver a t intervals along the line, regenerating the signal at each one. This is the basic property of a digital tvansmission system as contrasted to an analog syatem where each repeater merely amplifies its received signal before sending it along t o the next repeater. I n a n analog system, the noise
COMPUTING FROM THE COMMUNICATION POINT OF VIEW
115
added to the signal between repeaters is amplified at each repeater and so accumulates. In the digital system this accumulation does not occur since regeneration removes the noise completely a t each repeater, if the system is well designed. Another way of preventing noise-induced errors in transmission is to use digital codes which allow the receiver to detect and correct errors [ l o ] .Extra pulses are inserted in the transmitted digital stream so that the composite signal is guaranteed to have some fixed property that the receiver can confirm. One of the simplest such codes is the so-called parity check. Here an extra binary signal is inserted so that the number of ones in each block of say, ten pulse slots is even, regardless of the number of ones in the information signal. If the receiver makes an error because of noise ; the number of pulses will be odd, (in case of a single error) and the receiver will know that an error has been made in that block. It can then ask for retransmission of the block. There are many other error detection codes, some of which make it possible, for the receiver to correct errors without retransmission. Note that both analog and digital signals can be transmitted over either analog or digital systems. In sending analog signals over analog systems, the signal is transmitted ia a form and with a power that insures that the signal-to-noise ratio at the receiver is large enough to satisfy the required fidelity. I n the digital-digital case, the corresponding statement is that the system design must insure that the signal-tonoise ratio and the error detection and correction coding keeps the error rate below the required figure. The identical statement is appropriate for digital signaling over analog channels. In both cases the actual form of the digital signals put onto the transmission medium must be tailored to its characteristics [9].For example, many media do not transmit dc, such as those that employ transformer coupling at input and output. The pulse train of Fig. 3 has a dc component and so would not be appropriate for such media. This situation can be corrected by choosing elementary signals different from pulseno pulse, for example, positive pulse-negative pulse. I n Fig. 4, the same information as in Fig. 3 is shown coded using these signals. This waveform has no dc components. Of course, those media which carry radio frequencies only (microwave radio systems, for example) require modulators and demodulators to handle digital (or analog) signals. I
0
I
1
0
0
I
0
I
0 TIME
FIG.4. Digital signal using positive and negative pulses.
116
E. E. DAVID, 1R.
To transmit analog signals over digital systems, conversion of the continuous signal to discrete form is necessary. This “ analog-digital conversion ” is a well-developed technology in which an analog signal is “quantized” in both time and intensity. Pulse code modulation (PCM) is one such conversion. Here the signal is sampled in time and the amplitude of each sample is represented by a number, usually binary. It turns out that if the bandwidth of the antilog signal is W , then it must be sampled a t least 2 W times per second if it is to be recovered without distortion from its samples. If each sample is represented by b bits, then the required data rate is 2 Wb bits per second. There are coding methods other than PCM which yield lower data rates. These techniques generally hinge upon some predictable property of the analog signal, and are tailored for signals having that property. For example, speech signals have most of their energy below 1000 Hz, but for easy intelligibility, frequencies up to about 4000 Hz must be transmitted. A form of coding known as delta-modulation [2] is particularly well suited for digitizing such signals. After an analog signal is digitized, the digits, or bits, can be coded in various ways t o fit an available channel. For example, coaxial cable provides a channel where the attenuation per unit length increases with the square root of frequency. Thus it is advantageous to concentrate the digital signal energy at the low frequencies. This can be done by using multilevel, rather than binary, pulses. An example of an eight-level signal after transmission is shown in Fig. 5, where the traces of many successive pulses are superimposed. The eight levels are clearly visible.
Fro. 6. Eight-lev81 signal after transmission.
COMPUTING FROM THE COMMUNICATION POINT OF VIEW
117
The gaps, or “eyes” between them indicate the margin for the decision levels of a regenerative receiver. These decision levels determine which of the eight levels an individual pulse represents. The above discussion assumes that digital signals arrive at their receiver with noise added, but otherwise unaltered. Actually, the phase and frequency characteristics of a channel can alter the shape of signal drastically even in the absence of noise. Distortion of a rectangular pulse by a channel is shown in Fig. 6a. The effect of such distortions is to cause pulses to overlap into the time slots of adjacent pulses. This “intersymbol interference ’’ [9] can cause transmission errors just as noise can. TO reduce this effect to tolerable levels, the channel must be
FIG.6.- (a) Channel distortion of a rectangular pulse. (b) The same pulse equa-
lized.
118
E. E. DAVID, JR.
“equalized ” by insertion of compensating networks a t the transmitter or receiver. Such networks correct principally phase distortion in the medium that arises from dispersion (different frequencies traveling different velocities). The same pulse of Fig. 6a is shown equalized in Fig. 6b. Equalization of channels is a well-developed art, but it is made more dificult by the changing nature of channels. Channel characteristics change with temperature and the aging of components, calling for readjusted equalization. Also, in a switched network, there are many channels available between points, and each requires a different equalization. Recently, adaptive equalizers have been developed that measure the intersymbol interference and compensate dynamically t o effect variations [ 6 ] . The combination of adaptive equalization and error-correction coding makes it economically feasible to transmit data over channels a t rates up to about 50% of the Shannon limit. For example, recent experiments [S] were carried out over a 10-MHz channel using 16 elementary signals which were merely pulses of 16 different levels. Thus each pulse transmitted was capable of carrying 4 bits. These were transmitted a t 11.65 megapulses per second giving a data rate of 46.60 megabits per second. The signal-to-noise ratio on this channel was about 50 dB, so the Shannon capacity was
C = 10 log, lo6%165 megabits per second The error rate was about lo-’’. A sample of the composite received signal (of 8 levels) is shown in Fig. 5 . Only 8 levels are shown since 16 levels are difficult t o resolve in a photograph of the sort used here. Another requisite for transmission of digital signals is time synchronization between transmitter and receiver. The receiver, in effect, samples the incoming wave-form a t the successive time slots where pulses are t o be found. Any displacement of the samples from these time slots results in degraded performance (increased error rates). One technique for maintaining synchronization involves deriving a timing signal a t the receiver by processing the incoming wave. For example, the receiver might incorporate a filter to isolate the fundamental component from the incoming wave. The timing for the sampling instants might then be derived from the zero-crossings of that sine wave. In some pulse sequences, however, the fundamental may be missing, SO special sgfnchronizing “pulse sequences may have t o be sent periodically. Another technique is to send timing information on a separate channel. This problem of ‘‘ end-to-end ” synchronization has been solved in a number of ways, none of them perfect [7]. Thus, jitter and uncertainty in receiver timing does add t o the error probability, but these effects can be held within bounds. ))
COMPUTING FROM THE COMMUNICATION POINT OF VIEW
119
Overall, digital transmission is a well-developed technology. For example, the usual telephone channel (250-3400 Hz nominal bandwidth) can handle 2000, 2400, 3600, or 7200 bits per second depending upon the modulation selected and the characteristics of the particular line. Much higher data rates are available using wideband circuits. Today, with the availability of increased data rates well established, interest is shifting toward how such facilities can be used economically and effectively. Two important factors are the coding of information into data form and the creation of data networks to use data links efficiently. 3. Coding of Information
A piece of information, such as that corresponding t o a printed page or a line drawing, can be represented in many different ways. Depending on the representation, the amount of data to be transmitted can vary by as much as 1000 to 1. It is generally true that the smaller data representations require more extensive terminal equipment for coding a t the transmitter and reproduction storage, or display a t the receiver. Thus there is inevitably a trade-off between total data to be transmitted and the complexity, or cost, of terminal equipment. A topical example of the above generalities is provided by graphical materials. Graphical information can be represented in a number of different ways, each with different communication requirements. For instance, a page of typing could be scanned line by line, just as a television picture is. This representation is the basis of facsimile transmission. It takes about 16 million bits to represent the page that way. Over a dialed telephone line (2000 bits per second) it would take about 10 minutes to transmit this number of bits. For example, the XeroxMagnavox telecopier operated a t about this speed (6 minutes per page). Over a 40-kilobit line, it takes less time, about 30 seconds per page. Another way t o represent a page of print is by assigning each letter a code, as in Morse. This takes only some 10,000 bits per page; in theory then only 5 seconds over a dialed connection is required. A similar disparity occurs in the case of a diagram (see Fig. 7). The scanned format which represents the diagram as a TV-like picture, requires about the same amount of data as for a page of printing in the format. Another way is to represent picture as a draftsman would produce it-by drawing one line of the figure after another in sequence (draftsman format). Of course, the number of lines (the amount of drawing) depends upon the complexity of the figure, but typically there is between 1000 and 100-to-1 reduction in number of bits required in the draftsman and scanned formats.
120
E. E. DAVID, JR.
-
l 1.25 MILLION BITS / PAGE
FACSIMILE MODE
SEND END POINTS ONLY ABOUT 30 THOUSAND BITS/ PAGE
DRAFTSMAN MODE
FIU.7.
This disparity is at the base of a graphics facility in which xerographic printers are used as computer output devices to supply immediate graphics. This system is called STARE[ I ] . The computer itself can generate diagrams in either the scanned or the draftsman form ; only the software is different in the two cases. The xerographic printer requires the scanned form since it normally functions as EL facsimile machine. It is driven usually over a 40-kilobit communication line. A different scheme is used in the STAREsystem, as illustrated in Fig. 8. The computer generates the diagram in the draftsman format and it is transmitted over a low capacity communication line--a 5400 bit per second facility, for example. This information is stored in a special memory a t the receiving point. Conversion of the diagram from the draftsman to the scanned format is done by special hardware attached to the memory. The converted data drives the xerographic printer, which then produces the copy.
COMPUTING FROM THE COMMUNICATION POINT OF VIEW
121
LONG HAUL TRANSMISSION IN DRAFTSMAN MODE
COMPUTER
AT 5400 EITS/SECOND (6 SECONDS / PICTURE
LOCAL TRANSMISSION IN FACSIMILE MODE 250,000 BITS/ SECOND GRAPHIC SCAN CONVERTER
XEROX L D X
FIG.8. STARE system communicstion.
The STAREgraphics system was designed for internal use in Bell Telephone Laboratories’ R & D operation. It was built by Xerox Corporation to specifications. The computer software was written at Bell Labs. STARE has been in operation for about 2 years. The STAREsystem gives the computer user, regardless of his location, the option of obtaining graphic output from the computer. The user may enter his program into the computer from a console in his office, say a teletypewriter. He then orders, by using simple instructions, graphs and diagrams to indicate the results. He walks a short distance down the hall and retrieves the graphs as they are produced by the printer, communicating over a low-speed facility. This proves t o be a great convenience for users. They no longer have to wait hours while a roll of 35 mm microfilm is developed, as they might with a microfilm output. This is one of the first adaptations of a copying machine for remote computer output. Needless to say, the STAREsystem has implications far beyond Bell Laboratories-for libraries, business
122
E.
E. DAVID, JR.
operations, education, and any other application where hard copy output is needed. I n the STARE system, the scan converter that drives the xerographic printer is the equipment which makes low data rate transmission possible. Thus the cost of the converter must be offset by a reduced cost of transmission and computing to generate the graphic. Just such considerations must be involved in the design of most computer-communication systems. I n the past, the costs of transmission and computing have been decreasing much more rapidly than the cost of terminal equipment. Thus there has been little motivation to code information in sophisticated ways to economize on transmission, except in extreme cases such as transatlantic circuits. With the coming of integrated circuitry and new memory technologies, the cost of terminals promises t o decrease by a substantial factor, and so coding for reduced data rates promises t o become more favorable economically.
4. Computer-Communication Networks Transmission lines can be used t o connect a computer to another computer, or a computer terminal to a computer. However, point-topoint connections do not provide the flexibility that a network of lines can. Networks have many functions ; prominent among them are (1) Multiple Connection. Connecting a computer or terminal t o any one of a number of other computers a t various locations. (2) Sharing. Permitting several terminals or computers to use the 8ame transmission line alternutely. (3) MuEtipZexing. Permitting several terminals or computers to transmit data over the same transmission line simultaneously. (4)Message Packing. Interleaving data onto the line so that idle periods of one transmitter can be used to send data from another. The act of connecting one point to another or routing information from one point t o another in a network known as “switching.” Many different switching schemes have been used or proposed, each with their own characteristics, which prove to be either an advantage or a disadvantage depending upon the situation. Two kinds of switching span the range of possibilities. I n circuit-switching, a point to-point connection is made and retained for the duration of the communication, regardless of whether data flow is continuous in time or not. Such a connection can be thought of as a metal circuit even though part of the actual circuit may be multiplexed with data from other sources and t o other destinations. I n message switching, communication is carried out by
COMPUTING FROM THE COMMUNICATION POINT OF VIEW
123
messages or packets (parts of messages) each addressed somewhat as a cable or telegram is, so that a circuit need be set up only when a message is originated. Here the circuit is in effect repeatedly established, but only when a message is to be delivered. Networks may use circuit or message switching or both. Three examples will be given to illustrate various features obtained by these techniques in combination. A hierarchical network using circuit Switching is illustrated in fig. 9. TO OTHER TOLL NODES
NALS
LCOMPUTER1 b FIG.9. A hierarchical network using circuit switching.
The modern telephone network is of this kind and has been used extensively for data transmission. The network contains many switching nodes used as illustrated. A local switching node can connect local terminals to a local computer, thus there need not be as many computer ports as terminals unless all terminals must be connected simultaneously. For longer distances, connections (called trunks) between switching centers can be used. Again, trunks may be used by many different terminals on different occasions. Transmission facilities between centers may be multiplexed so that they can carry many simultaneous communications. Multiplexing is accomplished by time or frequency
124
E. E. DAVID, JR.
ILLINOIS
FY 1969-
FY 1970----FY 1971 ............. ALL COMMUNICATION LINES ARE LEASED AND OF 50 KILOBIT CAPPCITY
FIG.10. ARPA computer network. All oommuniation lines are leased and of 50-kilobit oapacity.
sharing. A typical connection in a large metropolitan area may involve 2-3 switching nodes. A hierarchical, circuit-switched network provides the functions of multiple connection, sharing, and multiplexing, but not message packing. Note that switching a t nodes in networks of this kind is controlled by a computer or a computerlike device. The Advanced Research Project Agency (ARPA) of the Department of Defense has recently begun to set up an experimental one-level message-switched network [ I l l . Some of the nodes and lines are shown in Fig. 10. At each node there is a communications computer. It accepts messages coming from other nodes, examines their address, and routes them onward if they are not destined for that node. If they are, the computer reformats them for transmission to the local computer after, perhaps, assembling several into larger units. The communication computer also accepts data from local computers, reformats it, and sends it onto the network in addressed packets. This type network (which has been called a bufleered network) performs multiple connection, sharing, multiplexing, and message packing. It carries out all these functions simultaneously at the expense of inserting memory and logic at the switching nodes. This memory and processing implies delay between origination and receipt of a message, just how much depending upon the intensity and statistics of the traffic. I n the ARPA net, the delay is estimated a t 1.5 seconds maximum for any message packet. Still another network configuration is the “round robin” messageswitched network [a] shown in Fig. 11. Here a number of stations are
COMPUTING FROM THE COMMUNICATION POINT OF VIEW
9
125
rTRANSMISSION L I N E
POTHER
D I S C 4
CALCOMP
FORMAT
WHERE TO 6
WHERE FR 6
OP CODE
DATA
END
4
FIG.11. Round robin message-switched network.
connected to a loop which transmits data a t a high rate. The stations may be computers or terminals or both, and they share the transmission capacity of the loop. A “supervisory computer” is present to seize control of the loop in case of failure a t one of the stations. Each station is connected to the loop by special interface logic which can examine incoming messages and originate outgoing messages. Each message is headed by an address to some station on the loop, and contains an “operation code ” which specifies how the subsequent portion of the message is to be handled a t its destination. This scheme permits a station t o originate and carry on communication with another station on the loop. Each station is allowed access t o the loop in sequential order; that is, control is passed around the loop from station t o station. The supervisory computer may be used to connect the loop t o a larger network. This loop system performs multiple connection, sharing, multiplexing, and message packing but a t the cost of transmitting a substantial amount of “ protocol ” data in addition t o addresses. There are many possible network arrangements. The one most appropriate depends upon the statistics of the data traffic. Two extreme situations span the range. Some applications create nearly continuous streams of data. For example, in some business situations, sales data is accumulated a t outlying stores during the day and transmitted later in the evening as a batch to a central data processing center. Typically a circuit switched system is appropriate, for the circuit is used continuously for periods of many minutes or even hours. On the other hand, in on-line usage of a remote computer from a teletypewriter, traffic in both directions tends to come in short bursts (particularly from the computer
126
E.
E. DAVID, JR.
to the TTY). Also different sources generate vastly different data rates; for example teletypewriters can generate data a t 100 bits per second while some disk transfers take place a t 4 x lo6 bits per second. If the transmission medium does not handle the source rate normally, then circuitry arid memory for buffering i t to the line rate is necessary. The packets so created tend to come in bursts also. An appropriate message-switched system can accommodate burst-type traffic, multiplexing, and packing to achieve high loading on wideband (costly) transmission facilities. Of course, multiple connection and sharing are also achieved. The examples quoted earlier illustrate this point. However, as in the case of coding, it is possible to trade equipment complexity for average data rate. Minimum cost may involve more than deciding that the traffic is burstlike or not. An interesting example is provided by PICTUREPHONEQ terminals which will nominally be used for face-to-face conversation (see Fig. 12). They display their
FIG.12. Face-to-faceconverslttion via the PICTUREPHONE.
COMPUTING FROM THE COMMUNICATION POINT
OF VIEW
127
picture using a full raster scan 30 times per second which requires a 1-MHz line. PICTUREPHONE terminals can be used also for computer communication. The computer prints messages on the screen, and the user may communicate t o the computer using his TOUCH-TONE keys. The point is that the PICTUREPHONE display must be transmitted continuously even though the actual data from the computer comes in short infrequent bursts. PICTUREPHONE terminals may well become the most numerous visual display actually in the field in the next 5-10 years. PICTUREPHONE service will operate through a circuit-switched network. Even so, it will likely be an important computer terminal because the incremental cost for using it in that way will be low. However, should such usage be contemplated over, say, a costly transcontinental route, then it may well be wise to use message-switching on that portion, reverting t o circuit switching a t the ends. For example, the computer burst might be sent as an addressed message to a display buffer located near the PICTUREPHONE set. The display buffer would store the picture and supply it as a standard PICTUREPHONE signal to the local set. There is one actual instance, however, in which a switching plan in this spirit is actually in use. On some transatlantic telephone circuits, conversations are broken into “speech spurts ” which are preceded by an address (or identifier) so that other speech spurts can be interleaved in time on the same circuit; that is, the spurts are message-switched. This system is known as TASI (Time-Assignment Speech Interpolation) and has been in use for over 10 years. Basically it capitalizes on the single talker nature of conversations. Since only one speaker is talking a t a time usually, another conversation can use the reverse channel. TASI is used on groups of 36 transatlantic channels; the statistics of conversations makes it possible t o accommodate 72 simultaneously on these 36 channels with very little impairment. It would not be surprising to see similar arrangements for data in the future. 5. Conclusion
Computer communication is a subject with many facets. Broad, sweeping conclusions require serious system studies in which the advantages of various trade-offs and possibilities are examined critically. Even such studies may not establish any general principles but only ad hoc solutions to specific problems. Nevertheless, one trend seems clear. As the cost of terminals and switching computers decreases, message switching will become increasingly competitive with pure circuit switching. Thus, computers and communication will become even more strongly intermixed.
128
E. E. DAVID, JR.
REFERENCES 1 . Christensen, C., and Pinson, E. N., Multifiinction graphics for a large computer system. Proc. AFIPS Pall Joint Computer Conf., Los Angeles, 1967 31, 697-71 1. 2. deJager, F., Delta modulation, a method of PCM transmission using the oneunit code. Philips Res. Rept. 7, 442--466 (1952). 3. Dunn, D. A., Policy h u e s presented by the Interdependence of Computer and Communications Services, Report 73798-1 dated Feb. 1969. Stanford Research Institute, Menlo Park, California (FCC Docket 16979). 4 . Farmer, W. D., and Newhall, E. E., An experimental distributed switching system to handle bursty computer traffic. Proc. ACM CorLf., Pine Mountain, Ueorgiu, 1969, pp. 1-35. 5 . Freeny, 6. L., King, B. G., Pedersen, T. J., and Young, J. A., High spced hybrid digital transmission. Proc. Intern. Conf. Commun., Boulder, Colorado, 1969 pp. 38-7-38-12. 6. Gersho, A., Adaptive equalization of highly dispersive channels for data transmission. Bell System Tech. J . 48, 85-70 (1969). 7. Manley, J. M., The generation and accurnulation of timing noise in PCM systems and experimental and theoretical study. Bell System Tech. J . 48, 541-613 (1969). 8. Pierce, 5.R., The transmission of computer data. Sci. Am. 215, 144-150, 152, 154, 156 (1966). 9. Pierce, J. R., Some practical aspects of useful digital transmission. IEEE Spectrum 5, 63-70 (1968). 10. Peterson, W . W., Error Correcting Codes. MIT Press, Cambridge, Massachusetts (1961) ; see Berlckamp, E. R., Algebraic Coding Theory. McGraw-Hill, New York, 1968; Mussey, J. L., Threshold Decoding. MIT Press, Cambridge, Massachusetts, 1963. 11. Roberts, L., Resource Bharing computer network. I E E E Intern. Conf. Digest 15, 326-327 (lQ69). 1 2 . Shannon, C. E., Pierce, J. R.. Oliver, D., The philosophy of PMC. Proc. I R E 36, 1324-1331 (1948).