TelecomWriting.com: Text of private line magazine. Volume 1, Number 1

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PRIVATE LINE: A JOURNAL OF INQUIRY INTO THE TELEPHONE SYSTEM

JUNE 1994: VOLUME 1, NUMBER 1

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TABLE OF CONTENTS

1. General Information on private line

2. The Front Cover and The Inside Front Cover

3. The Editorial Page

4. Telco Payphone Basics, Part 1

5. The Post Pay Coin Line

6. A Conversation With Motorola

7. The GTE RTSS Phone

8. California Toll Fraud Law

9. Ad rates and Miscellaneous Information

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1. General Info on private line: ISSN No. 1077-3487

B. Subscriptions: $24 a year for subscriber's in the U.S. $31 to Mexico and Canada. $44 overseas. Mailed first class or air rate . (1) Make checks or money orders payable to private line (2) A sample is four dollars. Back issues are five dollars apiece. The magazine is black and white. Double columns. Largely footnoted. (3) The mailing list is not available to anyone but me.

C. Mailing address: 5150 Fair Oaks Blvd. #101-348, Carmichael, CA 95608

D. e-mail address: privateline@delphi.com

E. Phone numbers: (916) 488-4231 (Voice) (916) 978-0810 (FAX)

F. You may put this file up at any internet site or bulletin board that you wish. All I ask is that you reproduce the file it in its entirety and that you not sell a hardcopy version of the output.

G. Comments and corrections are always welcome. I welcome submissions and I pay with subscriptions. You don't have to write in my style.

NB: I am now accepting electronic related advertisements for the January, 1995 issue. This will be the first newsstand edition of private line. Distributed by Fine Print Distributors, Austin Texas. Ads are $75 for a full page, $37.50 for a half page and $18.75 for a quarter page. No subscription required. Subscribers get free classifieds of 25 words or less.

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2. THE FRONT COVER AND THE INSIDE COVER

The front cover artwork of this issue is from a 1965 Western Electric advertisement. It is an edge on photograph of five circuit boards that were used in the Number 1 ESS. I included the text of that ad in the inside cover page. It reads:

"Electronic components by the thousands arrayed on circuit boards. These are at the heart of the Bell System's highly complex new Electronic Switching System. Now being built at Western Electric, a typical electronic system uses 160,000 diodes, 55,000 transistors, 226,000 resistors, capacitors and similar components. Over the next few years, millions of American telephone users will benefit from the new services ESS will offer. But for Western Electric the coming of ESS presents a technical challenge equal to any we have faced in the 83 years we have been a member of the Bell System. Not only do we stand behind the quality of the thousands of components, but we also make sure that each of these precision parts is assembled exactly. For the end requirement is that they work perfectly, each with each, and with every other of the billions of components in the nationwide Bell System communications network. We are able to do this job because, as members of the Bell System, we share its goals. Working together with people at Bell Telephone Laboratories, where ESS was developed, Western Electric people strive for perfection that enables your Bell telephone company to bring you the finest communications service in the world."

-- Western Electric, Manufacturing & Supply Unit of the Bell System

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3. THE EDITORIAL PAGE

private line a journal of inquiry into the telephone system

Setting the Agenda; A Rambling Mission Statement From Your Editor

The magazine 2600, The Hacker Quarterly, rekindled an interest in telephones that had laid dormant with me for over fifteen years. private line is an outlet for my interest in one the most marvelous, mysterious and elaborate inventions that man has ever invented: the telephone system. I hope that you find it a creative outlet for yourself as well. Let me tell you what I think are important goals for this magazine.

1. This magazine will write for the beginner. There is a lack of good, clear information for the beginner in telephony. Most texts and articles assume a working knowledge of the fundamentals. That won't be the case here. Books and magazines about telephony often read as though one electrical engineer was writing to another. That's because they usually are. But who writes for the beginner? This magazine will.

2. This magazine will turn articles into brochures. I want the information developed here to do more than sit in back issues. This material will go toward a series of beginner brochures on telephony. The magazine itself will be consecutively paged and indexed yearly. References will be given whenever possible.

3. This magazine will encourage questions about the information presented. I want to be corrected if I make a mistake. I want people to feel free to contribute and to question and to challenge anything that appears here. The articles that I write are not the Last Word, rather, they are my best attempt to explain some difficult subjects. They are a starting point for a discussion of the topics involved. I have an ego as far as presentation and layout go. But I have no ego as far as being corrected.

I hope you contribute. I welcome the comments of hackers, futurists, telecom people and technology buffs. Anyone who is interested in the telephone system is welcome to participate. I am really a beginner to telephony myself; let's learn together.

Tom Farley

privateline@delphi.com

p.s. my handle is 'Sherman' and my callsign is KD6NSP

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4. Telco Payphone Basics, Part 1

A. Telephone Company Payphone Basics, Part 1

1. A telco payphone is one that is owned and operated by the phone company that provides local telephone service: a former Bell company, GTE, General Telephone or another independent. Ownership aside, however, the one thing that makes a telco payphone a telco payphone is the fact that the machine's decision making ability resides in the phone system and not in the machine itself. This is different than a COCOT (customer owned, coin operated telephone) which makes most decisions on its own. When people can choose their local carrier, the so called alternate dial tone, ownership will be a less important criteria. You might have MCI as your telco, for example, instead of Pacific Bell. Let's start at the beginning.

B. The Different Coin Lines

2. There are two classes of coin phone service and three kinds of coin lines. The first class is post-pay, in which coins are deposited after a connection is made. Post-pay provides a dial tone without a deposit. The second class is pre pay in which a deposit is needed before a connection is set up. The three kinds of coin lines are called post-pay, for its operating method; coin first, which means that a deposit is needed to get a dial tone; and dial tone first, the pre-pay service that provides a dial tone without a deposit. Coin first is probably a defunct operating system. Dial tone first is by far the most common kind of coin line.

C. What Is a Line?

3. A line can be two things in telephony: a wire that carries a phone call or a channel in a wire or cable that carries a call. In either case, a line connects a coin phone or a customer phone to the switching office that provides local service. It is distinguished from a trunk which connects switching offices to each other or switching equipment within an office to each other. A line is almost always used in conjunction with local service, whereas trunks are thought of as providing long distance or toll service.

4. In addition, a line can pass different voltages to signal different things. A trunk cannot. For example, a line can carry +48V DC to signal keypad inhibit, +130VDC to signal coin collect, and 75VAC to ring the phone. A trunk's voltage, however, remains the same. It must since the cable containing the trunk is usually carrying many calls at once; it is impossible to selectively control voltage within a channel in a common wire. Thus, lines and trunks often use different kinds of signals.

5. A coin line is the circuit that connects a payphone to a central office or an end office. The line uses two copper wires, collectively called the twisted pair. There is nothing special about the wires themselves. There is no set of common wires that runs to all the payphones served by a central office. The phrase coin line is a designation. It indicates that the line needs special equipment at the central office to work. And since the equipment at the CO can vary, so can the kind of coin line.

6. In addition, most telco coin lines are somewhat permanently connected to their switch. That is, the coin phone line is known to the central office to be a coin line. You would not, for example, have an unrestricted dial tone if you connected your lineman's handset to the wires. Instead, you would still be prompted for an initial deposit and you would still be asked by ACTS (1) to pay for long distance. At the very least, polarity would be reversed and long distance calling would be intercepted by an operator.

7. Post-pay, coin first and dial tone first refer to the kind of coin line service that exists at a particular central office. The switching equipment and its accessories determine the kind of coin line service. Let's look at the two classes of lines more closely.

D. The Metallic Line

8. A line was originally defined as a "(w)ire or wires connecting stations in a telephone or telegraph system."(2) It often used as shorthand for transmission line. It is also called a VF or voice frequency line. And sometimes it's called an analog line. Two wires called a twisted pair or paired cable connect most phones to an end office or a connecting point to the end office. This is 19 to 26 gauge insulated wire. Look inside a service terminal to see twisted pair. The terminal is the point where your house or office phone line and the telco wiring connects, usually on an outside wall.(3) The phrase trunk line is often heard. That's not a combination of trunk and line, but, again, a reference to the trunk as a transmission line.

9. A metallic line exists if there is a direct, physical connection with the end office and each subscriber's phone. Step by step offices, for example, may have each customer's twisted pair directly wired to a particular place in the switching frame. This metallic connection also exists with open wire, which uses copper wires strung from utility poles. In this case, twisted pair runs from the house or business to an aerial service terminal. Two uninsulated wires then go toward the end office, or more probably, a connecting point to the office. A metallic line, therefore, may change from one kind of wire to another. But it always keeps a copper connection of some sort for each subscriber phone or payphone.

10. Both twisted pair or open wire help complete an electrical connection or circuit between the phone and the office. Circuit is often used interchangeably with line, creating more confusion than it should. Since this pair forms a circuit in the shape of a loop, it is often called the loop. The wires are also occasionally called conductors. That's because they conduct the electricity that operates the phone as well as carrying the conversation itself. The wires themselves are called tip and ring. Some assume that one wire is negatively charged and the other positively charged. Not so. Tip and ring do not refer to a pre-designated electrical state. As mentioned before, both tip and ring will have negative or positive voltages placed on them to signal different things.

11. Not all lines, however, are based on a physical, metallic contact with their local switch. This is especially true with long distances between a phone and its end office. Resistance builds in a line as length increases. Signal strength goes down at the same time. Many means have been used to extend the length of the coin line or the subscriber line beyond, say, six miles.(4) At some point though, the conventional metallic line becomes unfeasible. Amplifiers or repeaters are needed to take the signal further. And a different operating system is needed to go along with this equipment. In these cases, the metallic pair may terminate at a connecting point to the central office. Special equipment then puts many, many subscriber lines on a single cable or a group of wires. The CO then provides a channel within the transmission line only when it detects that a phone has gone off hook. There is no longer a physical connection between each customer's twisted pair and the central office equipment. Let's look at the non-metallic line.

E. The Non-Metallic Line

12. The second definition of a line is that it is the communication channel connecting the subscriber to the local office. A line in this case represents an individual radio frequency that a phone call is placed on. These are called carrier frequencies. This differs from the voice frequency that carries the call in a normal line. This kind of line is also called carrier. Many, many conversations can be placed on a single wire through a process called multiplexing. (5)The most familiar example of multiplexing might be a cable TV line. A single wire or coaxial cable can carry dozens of television channels. The simplest form of multiplexing in telephony is called split carrier.

Split carrier

13. Split carrier uses a single twisted pair to carry more than one phone call. It is also called subscriber carrier. Some include this in a larger category called pair gain systems. It is not usually used to overcome long distances but rather to provide another line when there is no free twisted pair. In older buildings, for example, spare lines are frequently not available. In fact, you may be using split carrier now and not know it. "Whining sounds, echoes and slow dial tone response"(6) may indicate that your telephone line is being split and that your line is on the carrier side. Another sign might be if your modem does not work on your telephone line but does on your neighbor's. That's because the modem is using a carrier of its own to transmit information. The two are rarely compatible. A line on the carrier side is only a voice grade line.

14. The voice frequency or VF channel is the normal, background path that carries a conversation on a twisted pair. You'll also hear the terms base band, voice channel and voice path. This is the first channel of two on split carrier. The second channel is created by transmitting a radio frequency at, say, a constant 100 kilohertz. That's a hundred thousand cycles per second. By comparison, the AM radio band begins at about 540 kilohertz. The signal of the second line is impressed on the steady carrier frequency. This causes the carrier signal to move up and down or modulate according to the changes in speech. So, two channels are now on one line. One conversation doesn't affect the other because you can't hear radio frequencies unaided. You now have a carrier line and not a voice frequency line. Subscriber carrier needs special equipment. I doubt that any coin phone uses this technique because of the number of voltage driven signals that must pass down the line. Never-the-less, some telcos may use split carrier for a public phone instead of a party line in rural areas. Perhaps. Let's look at more complicated multiplexing schemes. Again, these are examples of non- metallic lines.

Analog multiplexing

15. Both split carrier and voice frequency lines use analog signals. That's because normal speech, music and tones are all analog signals, once they're on the phone line. They are analogs, electrical representations of speech.(7) They are not altered or converted to a digital form. In other words, routine traffic in the local loop. Analog multiplex systems are used primarily for trunk traffic, that is, handling calls between switching offices. Analog carrier or N carrier is rarely used in the local loop. So, I'll discuss it more in the section on trunks. Some multi-channel analog systems do tie a customer's phone to its local switch but I have not found much information on them. Specialized equipment would be needed for coin phones; installed at the point where the multiplexer connects to the twisted pairs. This is needed to translate payphone signals from the central office to the voltages that control the phone. As I mentioned before, a channel in a cable cannot handle different kinds of direct current signaling. But twisted pair can. Hence, a need for an interface.

16. It seems that most telcos decided that if they were going to install a carrier system for the local loop, they were going to use digital techniques. Both digital and analog multiplex systems use amplifiers or repeaters to keep signal strength up over long distances. Even so, analog signals degrade with distance. But digital signals remain stable for the length of their trip. That's because they are not an electrical representation of speech but a mathematical or numerical representation.

Digital multiplexing

17. You've probably seen a sine wave of an analog signal. It's a rise and fall pattern. By plotting its coordinates on graph paper, you know, C-3, B-4, A-2 and so on, we can record its shape in a numerical or digital form. And the more points we plot the more accurate the record becomes. Digitizing produces its plots by instantaneously measuring the ups and downs of signal strength. In T1, a signal's strength is measured or sampled two things: 1), the strength level itself and 2), the time at which it occurs. These two measurements or electrical plots are converted to binary numbers or bits. An eight bit group makes up a byte. Blocks and blocks of these fast moving digits then represent speech.

18. Sampling takes a lot of measurements. But it is not continuous, even at eight thousand times a second. There are always small gaps. These breaks and blocks differ an analog signal from a digital one. A digital signal is made up of discrete units whereas an analog signal is a continuous unit. Built in error checking and uniform rules for encoding and decoding enables digitizing to faithfully reproduce a signal over thousands of miles. Fike gives some good examples in "Understanding Telephone Electronics." A digital carrier system makes the most sense when it ties into a digital central office. This saves the step of converting digital signals back to the analog ones that a simple end office can deal with.

19. T1 or T carrier is the most common form of digital transmission used in the local loop. T1 is used primarily for trunks but it also provides tens of thousands of local lines to central offices and remote switches. This system converts the normal analog signal of a subscriber pair into a digital signal The signal is abbreviated as DS. A typical digital multiplex system might be Western Electric's SLC-96. (Subscriber loop carrier, version 96) It can accept 96 local subscriber lines. But only five wires may run to the distant office since the signals are multiplexed. We'll look at how it interfaces with the twisted pairs of the local loop in the discussion of the local switch.

F. The Local Switch

20. The kind of coin line service provided usually depends on the equipment installed at the local switch. The type of switch itself is often less of a concern than the options that go with it. Post-pay operation, for example, usually depends on an end office with step by step switching equipment. But step by step can be converted to pre-pay. On the other hand, most crossbar switches and all electronic switches have been configured for pre-pay service already.

21. Most central offices controlling payphones need the hardware that enables automated coin toll service (ACTS).This is a system wide program that handles most long distance calls from payphones. It's what you get when you dial a 1+ call from most of the country. The Bell System designed this program in the late 1970's for use by all the regional Bell companies as well as subscribing independents.(9) Calling card service was developed a few years later.(10) This required additional equipment. Not having this equipment means that a particular CO may not provide coin line service. This is why you'll often see payphones in a town grouped to a certain prefix. It's a sign that that exchange has had certain hardware installed. In addition, the kind of trunk lines and local lines that the CO connects to will also influence the way that an office is configured.

22. I'm not sure if it's profitable for me to spend much time discussing individual switches. Many, many books have been written on them and their variants.(11) Comparatively little has been spent on discussing step by step offices or switches below the central office. So, I'll do that. The discussion of the individual coin line may give more information an a particular switch. The post-pay section, for example, deals with the community dial office in detail. We'll look at it in general here and then mention other end offices.

G. The End Office

23. The end office is your local switch, the one that your subscriber line or coin line is first tied to. It is at the bottom of the switching hierarchy, a so called class five office. This is usually a central office but not always. Many, many rural communities are served instead by a community dial office or CDO. These are mostly step by switches, serving far fewer lines than a normal central office handles. Slightly closer communities may be served by a digital switch called a remote. The CDO depends on a central office that can be quite a distance away. They are usually connected by an analog carrier or T1 to the central office. Most CDO's don't have trunks to the outside world. Long distance service needs to go out through the central office. A CDO may not generate its own dial tone. But it does generate the power necessary for the local phones to work. Some CDO's are called package offices.

24. Package offices seem to refer to a particular switching arrangement, particularly the No.5 Crossbar package community dial office.(12) This was a system of trunks and hardware that retrofitted certain CDO's. The dial office had to use the Number 5 crossbar as its central office switch. This package brought many features of the number 5 to rural areas. This was an expensive arrangement. These offices had to have enough traffic and revenue to justify it. I expect that they have probably been replaced in former Bell System country, since greater revenue drives quicker upgrading. I would welcome hearing about any crossbars that are still in operation. So, what kind of CDO took its place?

H. The remote switching system

25. The RSS No. 10 or Remote Switching System was the Bell System's answer to improve rural service in about 10% of their outstate CDO's.(13) A subscriber' s line connected to the RSS. The RSS uses T1 to connect with an electronic office or ESS as far as 175 miles away. They were originally configured to work with the No.1ESS and then the 1AESS. Most but not all of these older CO's have been retired. An electronic switch, the RSS No.10 shares much of the same architecture as its bigger brothers. Even, so, the CO controlling the switch has to have certain hardware installed in order to work with it.

26. Wire pairs from the local loop would terminate inside a small building containing a remote switch and the T1 carrier facility. The RSS provides power to the loop and the T1 equipment sends the subscriber traffic to the ESS office. A payphone would be enabled by a special circuit board inside the T1 service cabinet. This plug in module provides the proper interface to the switch.(14) The RSS would provide the power necessary to implement all the voltages needed for signaling the coin phone. One interesting aspect is that a TSPS operator could handle a coin call from as much as thousand miles away, since it is the distance from the CO to the operator that is now a controlling factor, and not the distance from the payphone to the central office. Another remote switch seems to be the DMS-10.

27. The DMS-10 switching system is a Northern Telecom product designed to Bell System standards. It handles 200 to 6000 lines. Why did Bell use an NT product? Cost. The DMS-10 is a small digital switch. It can provide some custom calling services that may generate a little more revenue than a normal rural switch. This may help the telco generate a faster return on its money in a low traffic area. The ultimate remote switch is probably the No. 5A Remote Switching Module.

28. The No. 5A Remote Switching Module or RSM, is, as you've guessed, the specific remote switch for the No.5ESS. T-1 or fiber optic takes the local traffic to the No. 5. The big difference here is that this switch can pass long distance calls to the network without going through the central office first. A CDO doesn't normally have trunks to the outside world. With this CDO, however, the trunks are so arranged that long distance traffic may go directly to a toll office and not first to the CO. The term CDO is applied less and less as the years go on. People often just call these switches remotes or modules.

29. It's impossible in an introduction to cover all the possible configurations of the end office. There are many, many kinds of arrangements. The most important thing to remember is the dependence of the CDO or remote switch on the central office. Microwave radio may be used in some areas to connect to a central office. A cellular phone site is also an end office. It provides dial tone. I know that Ericksson digital switches have been installed in many Motorola built cell sites.

I. A few thoughts on step by step

30. Step by step switching is still with us. And probably for a little longer. In fact, step by step may outlast crossbar, a different kind of switching system deemed superior to step by step, or SXS as it's sometimes abbreviated. Apparently, the Bell System choice for SXS was Western Electric's No.355A. In 1974, step by step was used by the Bell System for 22 million phone lines, one half million of which were coin lines.(15) By 1980, 15 million lines were still in service. Step by step was to be phased out by 1990.(16) That would have eliminated the 800 central offices with SXS in ten years. Does anyone have an updated census of the regional holding companies, the former baby bells? The story outside of the former Bell System is very different.

31. Telephony magazine used to publish a directory and buyers' guide that was invaluable. It was a roster of the non-Bell operating companies, a state by state guide to the independents, including GTE. It list thousands and thousands of exchanges with step by step. The last one I have is from 1987. Very few crossbars are noted in the West. I understand that Automatic Electric did not make a crossbar. GTE supposedly relied on makers like International Telephone and Telegraph (ITT) to supply one when needed. It would have made sense for A.E. to concentrate making step by step equipment. It's well suited to the small towns that independent phone companies catered to. Step by step offices probably have more add on equipment than any other. They need it to fit in with the increasingly digital world.

32. A good example are touch tones. An SXS office couldn't process them before, say, the mid 1960's. Now each office must. But step by step manufactured after this time would have the right circuitry built in. Coin service is another problem. Converting an office to dial tone first was costly. And as coin phone signaling changes so must the CO. More add on equipment needed. Want to implement ACTS? 911? Getting an electro-mechanical office to implement these features is quite a task. And while the telcos may want to put in custom calling everywhere, they have many problems with step by step. Trunking is another matter, too. Common channel signaling is seemingly bypassed, ignored or badly implemented throughout thousands of miles of step by step country. Not all exchanges, after all, have the enabling hardware to do System 7. I'll cover this more in the next issue.

It's my experience that the most fun with the phone system comes at the outer edges of it. There are as many hidden doors and gates there as there are in Alice's Wonderland. But where do they lead? In the next issue I will continue this discussion on basics. I'll try to cover trunks in general, some terms on signaling, and the role of the operator and TSPS.

J. References

(1) Automated coin toll service, or a derivative thereof, is the automated operator that you get when dialing a 1+ call from most telco payphones. For instance, if you dial, say, 1+(916) 213-9999 (an imaginary number), a computer generated voice will come on the line to tell you how much to deposit. You then hang up. You'll get a good insight into the rates charged and the kind of coin service an area provides by dialing the same number from different payphones in different areas. Do the same with COCOTs. Listen for switch sounds in the background. You may even be connected to a billable, long distance number without being charged. That shouldn't happen. But it does sometimes. As Goldstein says, "Anything is possible." My advice? Go rural. And go GTE.

(2) Douglas-Young, John. "Illustrated Encyclopedic Dictionary of Electronics. "West Nyak, Parker. 1981. 335 Out of print but worth looking for. This hardback is a good, one volume dictionary of electronics. The section on electro-acoustics is great. His wife is Carol Young who is the author of the readily available "New Penguin Dictionary." This book, unfortunately, is much harder to understand and less complete.

(3) Martin, John T."Chilton's Guide to Telephone Installation and Repair." Radnor, Chilton Book Company. 1985. 5 A great how-to book. I'm not sure if this edition is still in print but a revised version should be.

(4) Among others, Schillio, Robert F. 'A Circuit That Stretches Coin Telephone Service' Bell Laboratories Record. 51:4 (April 1973) 120. Don't write off these early articles. They provide many clues as to why things were done in a certain way, even if a particular piece of equipment is no longer in service or a practice discontinued.

(5) Fike, John L. and George E. Friend. "Understanding Telephone Electronics". 2d ed. Carmel, SAMS. 1990 16. Now in its third edition, this book is widely available. You should buy this book. It assumes a working knowledge of electronics. A beginner can push through most of it with dedication. The second edition, however, has only a two page index for a 284 page technical work.

(6) Martin, 53

(7) An AC signal is an alternating current signal. Tones are often called AC signals. The alternating part refers not to alternating power current but to the shape of an alternating waveform, above and below a median point, the rise and fall of any normal analog signal.

(8) Rey, R.F., ed. "Engineering and Operations in the Bell System". 2d ed. Murray Hills, N.J. AT&T Bell Laboratories. 1983 373

(9) Staehler, R.E. and W.S. Hayward. Jr. 'Traffic Service Position System No. 1, Recent Developments: An Overview' The Bell System Technical Journal. 58:6 (July --August 1979) 111 Tough article but lots of interesting details. Find a place you can check this out for a week; it's really too long for photocopying but too essential to let go.

(10) Confalone, B.E., B.W. Rogers and R.J. Thornberry, Jr. 'Calling Card Service--TSPS Hardware, Software, and Signaling Implementation' The Bell System Technical Journal. 61:7 (September, 1982) 1676 Another essential. Find a corresponding article in the Bell System Record if you find the B.S.T.J. too intimidating.

(11) Fike gives a good, basic description of switches. If you want to bury yourself in the subject then check out G.E Schindler,ed. "Engineering and Operations in the Bell System: Switching Technology: 1925 -- 1975." Murray Hills, Bell Laboratories. 1982. Or, if you want something practical, read Agent Steal 'Central Office Operations' 2600: The Hacker Quarterly. 7:4 (Winter, 1990) 12--21

(12) Schluttenhofer, R.A.'Two-Way Trunks For Package Offices' Bell Laboratories Record (November, 1965) 402

(13) Sevcik, Richard W. and D. Paul Smith. 'Custom calling comes to Clarksville (upstate New York)' Bell Laboratories Record. 58:2 (February, 1980) 63. Fascinating article about a little known subject, the Remote Switching System.

(14) Some may contend that the T1 line is a trunk in this situation and not a collection of subscriber lines. A trunk, after all, is a circuit between switches. They are partially correct. A remote unit is not a fully functional switch. It cannot operate without the central office. It may not generate its own dial tone. It can be viewed as an extension of the CO and not as an independent office. A PBX is also a switch. But its lines to the CO are treated as lines and not trunks. The PBX is not functional without certain central office features. It is not able , for example, to pass long distance traffic to the world without the CO. Line and trunk are often used interchangeably in a discussion of traffic between the CDO and the central office.

(15) Peterson, Gerald H. "Improving Coin Service For Step-by-Step". Bell Laboratories Record (February 1974) 41

(16)Rey, 735