This paper was originally presented at the Second International Conference on Computer Communication, Stockholm, August 1974. It published in the proceedings as paper T 1226 pp 163-170.


Alarcia, G, C.T.N.E., Madrid, Spain
Herrera, S, C.T.N.E., Madrid, Spain

This paper has received some text editing. My intent was to improve readability by avoiding awkward English constructs and sometimes modifying a technical term to the commonly used English form e.g. "the teleprocess" to "teleprocessing". I have tried to indicate my alterations by using this font which is different from the original Courier font. Cases where a sentence was repaired by a pure deletion are thus invisible. Pure spelling errors are not marked. Unfortunately there are places where I was unable to ascertain the authors' intent. I have tried to minimize alteration to these passages. I have preserved the continental use of comma as decimal point and period as thousands seperator. Some of the drawings have been modified by replacing a dashed line with a coloured line. The intent is to improve readability. I have made these changes because I believe the content of this paper is worthwhile and should be read. I regret having made these changes without consultation with original authors. If any reader knows how to contact them, I would like to know. If there any scholars listening I could make the scanned page images available.
/Roger D Moore Etobicoke Canada Summer 2006


Many Telecommunications Organizations are planning or have installed Networks specially dedicated to data transmission in several ways. Some of these have found that Packet Switching Services may be the suitable solution to do it.

CTNE has implementeda Packet Switching Network, that has been in service since 1971. This paper contains a description of it.


In the decade of the sixties, when the need emerged for the transmission of data in Spain, CTNE tried to meet this need, in much the same manner as the rest of the world, by means of the classical methods:

Leased telegraphic as well as telephonic point to point circuits, and telephonic circuits via the Telephone Switching Network.

By that time and despite the multiple opinions expressed in articles and magazines to the effect that the planning and exclusive means of data transmission was not justified economically, the CTNE management, demonstrating great foresight, wanted to participate in the development of teleprocessing in this country. Thus, they took part in both an active and direct form, adopting the decision to develop the Network which we will describe below.

This Packet Switching Network is now in service with five hundred operational terminals, not to mention the corresponding trials being carried out at present with other subscribers. We can estimate a total installation of almost five thousand terminals in the next four years.


The Network is basically composed of the switching and retransmission computers (CCR), joined to one another, and connected to the Data Processing Centers of the subscriber (CCA), by means of one or several circuits of variable speed. The dimensions of the connections are relative to the traffic they must bear.

The subscriber's terminals (TA) are connected to the Network by concentrators and multiplexors.

In Figure number 1 we indicate the forecast number of terminals by this year (1974)in the services offered at present, which we will explain later in a separate paragraph, compared with those planned, for 1976


In accordance with these forecasts, the configuration of the Network can be seen in Figures 2, 3 and 4. The first refers to the present period, the second to the year 1978, and the third to the configuration planned for long term.

The number of connected terminalshas been kept in mind in these configurations, although studies are presently being carried out to determine the variations that needs and requirements will experience in accordance with the planned technological advances.

At the present time, five hundred terminals are connected to the Network with the configuration drawn in Figure number 2, in which two CCR's exist in Madrid and Barcelona and three concentrators in these same points.


It is hoped that digital facilities for the connections between CCR's and concentrators will be installed shortly.

Our forecasts indicate that, in the year 1978, the number of terminals will reach approximately five thousand with a distribution similar to that indicated in Figure number 3. The number of CCR's will be the same as at present but they will be complemented by a greater number of concentrators, distributed throughout the different regions,


The long-range configuration of the Network is shown in Figure number 4 and will feature six CCR's and the number of concentrators considered necessary (with at least one for each Spanish province).

Technological advances will be incorporated the Network in accordance with the studies of digital facilites, digital Switching, circuit facilities, etc.



At present, two distinct services are being used in the Packet Switching Network: Real Time and. Message Switching Services.

We will describe the characteristics of both below and they will appear graphically in Figure number 5.

3.1. Real Time Service

This service is directed to closed groups of users, in which the traffic transmitted by/towards a terminal is directed towards/from a determinate CCA, without the possibility of direct traffic between the terminals.

Two seconds have been fixed in the Network as the maximum allowable response time, defined as the time transpired (discounting the processing time of the subscriber's computer) from when the last character of the questioning message is transmitted from the terminal until the first character of the answering message is received.

It is interesting to point out in this case that a one-to-one correspondence does not exist. In fact, to one questioning message from the terminal, the user's computer can respond with more than one message destined for said terminal or for other terminals within the same closed group.

This service has a fixed address, which means that the message generated by one terminal will go to a determinate CCA every time. In the broad sense this is a virtual leased circuit service. The network functions in real time and with an aspect of concentration, for the CCR-CCA connection speed will be expanded in accordance with the studies of traffic generated by the group's set of terminals.

The handling of terminals is effected by the Network TA ("polling, etc."), which eliminates the fruitless transmission of polling and response blocks. In this way the user's savings are considerable.

3.2. Message Switching.

The Message switching Service is also specifically designed in order to provide the required service to closed groups of users. It consists, fundamentally, of an exchange of messages between two stations (terminals or computers) of a closed group, specifying explicitly in said message, the conditions for sending forth the same.

The CCR, by means of the message's source, will be provided with the data regarding the message (e.g. code sequence of entry, etc.). In the Network, the heading of the message generated by the terminal will be analyzed; the destination (or destinations) will be identified, as well as priority, etc. The Network will be able, in addition, to adjust the message to the conditions required by the destination station (or stations) (e.g. code change, speed change, packet arrangement, etc.), transmitting it henceforth.

The fundamental difference from the previous service is that the fixed address does not exist, which means that each terminal can communicate with any other in the same closed user group.

3.3. Formats

The composition of the packets is indicated in Figure number 6 for the Real Time and Message Switching services.

The information transmitted is grouped into blocks of a maximum length of 255 octets (we understand a block to be a set of information octets sent from the terminal, excluding the headers, every octet of start or end, and the control of errors),

The Service Blocks possessing similar characteristics to those of the data, will be generated inside the Network, and will differ only in the heading.

3.3.1. Format in the Real Time Service

In the Real Time Service, the messages generated by a terminal always have a fixed address, for which reason, the packets do not have headings in the terminal - concentrator and vice versa. These packets are composed of a character of start of text, the block of information with a maximum size of 255 octets and an octet of end of text and a control character.

Once this packet reaches the concentrator a simplified heading of seven octets is placed, by the Network and the packet thus remains with a maximum length, in the concentrator-CCR line, of 268 octets:

Start of text conposed of two octets (DLE, STX), a heading of seven octets (ISP Identification service and priority (l). ISA Subscriber's identification service (l), ID Identification destination (2) NSM Serial number of message (l) NSB Serial number of block (l), and ESA subscriber extension service (l)), the block of information with a maximum length of 255 octets, the end of text composed of 2 octets (DLE, ETX) and. two characters of error control (CRC 1 and 2).

When the packet arrives at CCR, the heading of the packet reaches 11 octets, adding an additional two groups of two octets; the first IO, for identification of its origin in the network and the second NSC, the number of the packet transmitted by each circuit.

In the opposite sense CCA-TA, the heading is placed by the CCA, simplifying itself through the Network in a totally symmetrical form to that described previously.

3.3.2. Format in the Message Switching Service.

It is necessary to include explicitly in the Message Commutation Service, for the terminal, the conditions for the Address of the same, by means of the following format, which we wish to indicate succinctly below:

PRIORITY CODE (2 octets)

Once the message enters into the Network it is packaged and transmitted with one header of the same length as in the Real Tine Service, although with some small modifications in the sense of some of the octets making the heading.


Some Service Blocks are generated in the interior of the Network and they have similar composition to that of the data blocks, although with variations in the heading which indicates its nature and characteristics. Twelve different service blocks exist with the effect that we indicate below:

With these blocks the transmission protocols are effected forevery application existing in the Network.

4.1. Protocol in a Question/Answer System in the Real Time Service.

The following diagram has been reworked for HTML purposes. Message direction was originally indicated by horizontal lines with a half-arrowhead at the tip. I have replaced this with a colour scheme: Green indicates a message from CCA (host) towards TA (terminal adapter). Blue indicates a message from the terminal direction (CCR or TA) to the CCA (host). I have moved the definitions of Service Blocks to be adjacent to the diagram of message sequence.

We offer a drawing below of the direction and composition of the Data and Service Blocks in the indicated application.

BS1 - Open the TA or CCA to one type of service.

BS2 - Start admission of one message from TA.

BS3 - Stop admission of messages from the TA.

BS4 - Close the TA or CCA to one type of service.

BS5 - Status request of one TA.

BS6 - Status information of one TA.

BS7 - Change of the end of message condition of one TA.

BS8 - Transmission progress information of one block.

BS9 - Permanent failure of one TA.

BS10 -Affirmative response to one service block.

BS11 -Negative response to one service block.

BS12 -Notification that one or more blocks were not received, for transmission to one TA.


(1) Optional

(2) The ISA group of the last block of this message will indicate that the following operation is one input message (from TA to CCA)

(3) The CCR initialises the reception (e.g. start of polling).

(4) The ISA group of the last block of this message will indicate that the following operation is not an entry.

(5) This is necessary if the service was not opened previously.

(6) The CCA may send this message at any time.

(7) TA error

(8) The CCA takes note of this circumstance.


In this paragraph, we will describe these services that have been studied for the short-term implementation in the Network, once requirements have been determined.

The characteristics of each one is indicated in Figure number 7 (a) and (b).

5.1. Communication between CCA's

In this service, each Data Processing Center of the subscriber (CCA) can have its own private Network of TA's which will work in its own service, in the normal manner.

The set of CCA's which can communicate among themselves is considered a closed group, for the purpose of the Network. The CCA's belonging to other existing private Networks do not have access to this closed group.

The CCA's of a closed group do not have any reason "to be homogeneous" but rather a CCA can be any model from any manufacturer on the market.

5.2. Massive Information

High-speed sporadic transmissions are included in this service. They only require brief periods of time which do not justify the use of a leased circuit. In addition, they require a low rate of transmission error for which the Telephone Switching Network is not adequate.

This service is used for closed user groups and always features a fixed address in the TA-CCA sense. The format used is the same as that of the Real Time Service and differs only in that this service is of a lower priority.

5.3- Terminal linked to several CCA's service

This service will support an expansion of the Real Time Service for closed users groups that are interested in the possible connection of a terminal with several computers that belong to the same closed users group.

The format used inside the Network will be the same as that of the Real Time Service. The Central Terminal of the subscriber will provide additional information to anyone wishing the communication.

5.4. File Service

This service supposes an expansion of the philosophy of the utilization of the Network, for it is not really a pure communication service, but rather the keeping of stored information up-to-date and permitting its use by the closed user group.


Although we have said that the Packet Switching Network of the CTNE tries to meet a specific transmission service demand for data, it is possible to study comparatively some applications of the users effected by means of rented circuits and those that operate via the Network.

Figure number 8 shows the curves for the occupation of the subscriber's computer in accordance with the number of terminals, for banking applications of checking accounts. In the graph in which the number of terminals for point-to-point circuits are expressed we have considered that there are no concentration facilities.

The handler used by the subscriber's computer in the same application, using the Remote Switching Network is dimensioned in accordance with the figure 9.

The number of transactions for this application in both cases is shown in Figure number 10. The relative decrease in the number of transactions per terminal noted in the curve can be explained by the fact that the terminals which have been connected first to the Network are those which will later handle the greater number of transactions and others of less relative importance are connected later.

The number of transactions by terminal in the point to point service are greater than in the Packet Switching Network, this fact may be explained because of the billing procedure: In the Packet Switching Network the tariff depends on the packets transmitted, and the customer intends to save as far as he can, refining the program so the number of transaction are reduced.

Finally, and as a very important data, in the Figure number 11, we indicate the percentage of total time lost due to breakdown of the system, regarding the CCA's as well as the terminals and the links.

This percentage, in the case of the Packet Switching Network, is much less than those offered by the point to-point services. This can be explained easily if we keep in mind the duplication of the switching computers and connections in the Network. From the communication point of view only the local circuits cause interruptions. In all the other cases, except in the event of a catastrophic failure, all the services of the Network are redundant.

[Note: The solid line provides a reliability estimate for a point-to-point (leased line) network. The dashed line is an estimate for RETD. /RDM]


The implementation of the Network has been a great investment for the CTNE which can be justified in any case as a public service rendered to all those who are attended by the Company.

In accordance with our forecasts, the Network will begin to be profitable in 1976, in accordance with the studies carried out and whose graphic representation we are including in the Figure number 12.

In order to obtain this graph, the following premises had to be considered:

1) A concentrator for each 150 terminals (on the average) is considered necessary.

2) A line of 4,800 bit/sec can normally bear the traffic of 75 terminals (at the average estimated speeds).

3) At present, the traffic obtained in the terminals connected to the Network produce a daily average of 50,500 characters and 731 messages.

Bearing in mind the development of the applications, the average traffic to be reached is 80,000 characters per day and 1,200 messages per day.

4) In any case, the development of the study has been carried out for the conditions of normality to be reached in a brief period of time.


The most favorable acceptance received by the Network in Spain, as well as the news of the existence of firm plans for the establishment of public Packet Switching Networks in other countries, makes the Company feel highly in favor of an international connection of Networks of this type or perhaps the establishment of a International Packet Switching Network. This has already been considered in several international organisms.

We think that this service, complemented accordingly with the studies of the demand with an open public Packet Switching Service, can meet a significant need for the growth of some applications. The Packet Switching can meet this need in a completely satisfactory manner and with a great saving and economy for the user which cannot be achieved with any other type of service.

To close, we wish to emphasize the fact that all the development have been made with the complete collaboration of the users and the specifications of the Network have been the result of the labor of many manyears by both users and CTNE's personnel.