Internet-Draft Coin-Compatible ATA January 2022
Albert Expires 2 August 2022 [Page]
Intended Status:
Standards Track
N. Albert

Coin Compatible Remote Operation


This document specifies a mechanism which allows coin phones to be supported over a VoIP connection.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on 5 July 2022.

Table of Contents

1. Introduction

1.1. The Problem

Many advances have been in Voice-over-Internet Protocol (VoIP) services in offering features and capabilities that rival or exceed those of traditional telephony infrastructure. Tremendous progress was made on this front in the first decade of the 21st century and continues into the present, though at significantly slower rates.

One area where VoIP is currently ill-poised to replace traditional telephony infrastructure is in servicing coin-operated telephones. Historically, coin phones have required special, dedicated equipment at the central office (CO) to handle them. With electromechanical switching equipment, coin lines required special equipment at the central office, separate from regular dial-subscriber equipment, to handle them. In modern electronic switches, coin lines are typically handled by special line cards that contain all the functionality needed to handle a coin line in them. This includes checking for ground, sending positive or negative voltage onto the line, and putting the line into positive or negative polarity states.

Many of today's payphones used integrated circuit boards to handle the logic of charging calls and executing the necessary coin dispositions for these. These are known as customer-owned coin operated telephones. These will not be covered in this proposal.

In this proposal, we will consider the requirements for providing coin service with authentic, accurate, and reliable Class 5 and Class 4 functionality, specifically in contexts where the coin phone is remote from the Class 5 switch.

2. Terminology

A coin-operated telephone shall refer to a telephone that is coin-operated which uses network-controlled coin signaling.

This is in contrast to a customer-owned coin-operated telephone (COCOT), which does not use network-controlled coin signaling.

COCOTs are most common today in public usage, but they are not relevant to this proposal and will not be discussed. Only network-controlled coin phones will be considered.

There are two major categories of coin phones in widespread existence. The earlier of these is the three-slot phone, which is furnished with three separate slots for nickels, dimes, and quarters. A separate slot is used for each distinct type of coin. Other denominations of coins are not permitted. A three-slot phone contains mechanical gongs which are struck by the coins when deposited, which produces an acoustically inconsistent but readily identifable signal that can be discerned by a human operator manually supervising deposits. A nickel corresponds to a high-pitched gong strike; a dime to two rapid high-pitched gong strikes; and a quarter to a low-pitched gong strike.

The newer type of coin phone is the single-slot phone, which is furnished with a single slot for receiving all deposits. Nickels, dimes, quarters (and dollar-coins, where used) are all deposited into this slot. Additionally, single-slot phones are equipped with totalizers which emit a standardized sequence of beeps in response to the type of coin deposited. These beeps have long been standardized as the dual-tone combination of 1700 and 2200 cycles per second. Some earlier models used only a single frequency (one of the two frequencies now used), and Canadian phones used a different frequency pair (including 1537 cycles per second). These will not be specifically considered in this proposal, but the ideas discussed could be applied to these variations as well.

The beeps emitted by single-slot coin phones are referred to as coin denomination tones (CDTs). A similar but slightly different pattern is used for indicating the type of coin deposited as with a three-slot phone: a nickel is represented by one long beep; a dime by two long beeps; and a quarter by five rapid beeps; the dollar, where supported, consists of twenty rapid beeps. Thus, each beep may be equated to a nickel's worth of deposits. A long beep consists of beeps of 66 ms on/off duration and rapid beeps of 33 ms on/off duration.

The initial deposit refers to the amount of money required to make a local phone call. This is the lowest amount of money that a call may cost, except for calls that are free. Calls may require deposits beyond the initial deposit.

A Class 5 switch refers to a telephone switch which directly serves end subscribers and telephone lines. These are the switches to which a coin phone would be directly connnected. In a traditional telephony environment, this comprises a direct electrical connection between the phone and the Class 5 switch.

A Class 4 switch refers to a toll switch to which other Class 5 switches are connected, which do not themselves directly serve any end subscribers or telephone lines. An operator services office operates at the Class 4 level.

Asterisk is a telephony toolkit that is widely used for designing private automated branch exchanges (PABXs or PBXs) and softswitches.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

3. Types of Coin Lines

This section discusses the five major types of local coin control and operation used in North America.

4. Coin Dispositions

This section discusses the standard coin dispositions that must be able to be executed on demand, at the command of the Class 5 switch or, in some cases, Class 4 switch to which a coin phone may be indirectly connected.

Further details of these functions may be found in Telecordia Notes on the Network [SR2275].

The implementation, control, and integration of these functions are the key concern of this proposal, as these all concern physical properties of the phone line itself to which a coin phone is connected to and to which a remote Class 5 office would not have direct physical access (as would be the case in a traditional telephony environment). When a direct physical connection is possible, these functions do not generally pose a challenge, as the Class 5 switch may be directly connected to the coin equipment, such as via serial I/O. When this is impossible due to needing to transit over IP, other alternatives are required.

5. Software Considerations

This section addresses the problems discussed in Section 1.1 that exist largely in the software domain.

5.1. Audible Coin Signaling

The primary concerns with coin signaling can be separated by model: three-slot and single-slot coin phones.

Audible coin signaling was not used to convey information about deposits on local calls. Rather, these audible signals were used on long-distance calls or calls where the amount of money required to complete the call exceeded the initial deposit. These signals are detected manually (by a human) or automatically (by a computer) and are used to determine if enough money has been deposited to complete the call.

Put another way, audible signals are generally only used when the call is being processed at a Class 4 office. The signals are rarely or never used when a call is processed at the local Class 5 office.

5.1.1. Single-Slot Coin Detection

Detection of coins deposited into a single slot coin phone is essentially the task of accurately recognizing coin denomination tones. In conventional telephony switches, detection of coin denomination tones is done by dedicated hardware, similar to that used for detecting dual-tone multifrequency (DTMF) digits from subscribers.

A newer way to perform this detection is by using digital signaling processing (DSP). Softswitches use "soft" DSPs to perform this detection, so-called because the DSPs are implemented entirely in software. The Goertzel Algorithm is widely used for detecting DTMF digits. This algorithm has two advantages that are of particular importance:

  1. The algorithm can discern dual tones. Some DSP algorithms, such as that used for tone detection in Asterisk, are only capable of discerning a single-frequency tone. These algorithms have practical applications, but are not useful for detecting CDTs.
  2. The algorithm can accurately detect tones even in the presence of noise. Indeed, the importance of this cannot be understated. An algorithm that can accurately detect a dual-tone frequency combination in isolation may do so correctly, but fail to replicate this in environments that introduce noise, here referring to any audio that does not correspond with the tone we are trying to detect. Such an algorithm would not be useful for detecting CDTs.

Other factors contributing to the success of a DSP algorithm include its robustness to falsing, or false operation of the detector by an agent which is not the actual tone that we are trying to detect. This would be considered a false positive match.

While false positives are bad, equally bad, and arguably worse, are false negatives. An algorithm which fails to detect the presence of a CDT when one is, in fact, present, produces a false negative. These are particulary undesirable in the context of coin phones as these would correspond to deposits being made by a user that have not been detected, shortchanging the user. False positives and false negatives should be avoided, but false negatives must be kept to an absolute minimum and eliminated if possible.

The Goertzel Algorithm, for its part, has been shown to be fairly robust at detecting coin denomination tones, and performs with accuracy that, while perhaps subpar to that of a hardware DSP in a traditional telephony switch, may be generally considered "very good". Exact implementations may cause varying tendencies of success or failure. In general, nickels and dimes can be detected with very good accuracy. Quarters tend to prove slightly less reliable at detection. One possible reason for this is the short intervals that rapid CDT beeps use: a 33 ms on/off period. This is on the lower end of what most implementations of the Goertzel Algorithm are designed to accomodate. In any detection solution, rigorous testing of detection of all coin denominations and ensuring a satisfactory rate of false positives and negatives is essential.

Due to the nature of the tones and public knowledge of the frequencies, it is possible to artificially produce them using a device commonly referred to as a "red box". This allows a user to fradulently produce the same tones that would be emitted by the totalizer into the microphone of a coin phone to trick either the computer or a human into thinking the corresponding coins had been deposited. Some anti-fraud mechanisms existed to mitigate some of this impact, but there is no large-scale effective mitigation strategy for preventing this kind of fraud, especially when done by a computer which would not be capable of processing the context of the audio, which may reveal subtle clues as to whether the tones are being legitimately or fradulently produced.

The Automated Coin Toll System (or A.C.T.S.) was a widely used automated system, beginning in the late 1970s and into at least the 2000s, that automatically obtained required additional deposits from customers without operator intervention.

5.1.2. Three-Slot Coin Detection

Unlike single-slot phones, whose totalizers were designed with the chief objective of being able to produce signals would could readily be accurately decoded by a remote computer, the gongs used in three-slot phones were never designed to accomodate this capabilitiy. All detection of deposits signaled by the gongs was manually supervised by a human operator. Historically, the technology to accomodate automated detection of these gongs did not exist at the time when these were in widespread use. Secondly, a DSP similar to those used in detecting CDTs from single-slot totalizers would not be useful, as gongs can vary widely in their pitch, resonance, frequency, and other acoustic properties. Accordingly, it is much more difficult to codify what a computer program, if a suitable one were to exist, would need to listen for.

In present times, there are vast technological capabilities which provide an advantage, but none of these technologies has yet to be found to be a good match for three-slot phones. This may present an opportunity for the use of machine learning and artificial intelligence algorithms or programs. Present capabilities necessitate manual supervision of deposits, whenever they are to be detected remotely, by a human as opposed to automatically by machine.

5.1.3. Comments on Audible Signaling Usage

Audible coin signaling was never used insofar as satisfying the initial deposit was concerned. Instead, grounding of the line, by the payphone itself in response to the initial deposit being satisfied, would be done. In prepay operation, grounding the line signals the CO that dial tone may be provided (the line is essentially a ground start line). In semi dial tone first operation, grounding the line would allow the user to dial a number. In dial tone first operation, the initial rate test checks if the line is grounded and routes the call to an intercept recording if it is not. In semi-postpay operation, grounding the line would undo the mute caused by the polarity reversal on answer.

Postpay operation is the exception here. In postpay operation, no ground wire exists. Instead, 3650 ohms on tip and ring (using the bell coil) would signal to the CO that the initial deposit had been made, causing the CO to remove application of second dial tone [local-coin-control].

While historically CDTs have never been used to detect the initial deposit, CDTs are nonetheless emitted by the totalizers of single-slot phones when the initial deposit is satisfied (whether they are in local mode or toll mode is irrelevant). Accordingly, it is feasible to, rather than relying on a ground test or ground detection of any sort, use CDTs to detect the initial deposit. This does circumvent a security measure against "red boxing": the tones can be trivially spoofed, while an artificial ground on the line, less so. Use or nonuse of these tones will not cause any difference in user experience, but may make integration with a remote Class 5 switch significantly easier, at the slight expense of increased exposure to fradulent usage.

5.2. Coin Logic

The necessary logic to handle call routing and intercepts for a coin phone can be readily implemented in a stored program control (SPC) switch, including VoIP softswitches.

5.3. Coin Disposition Signaling

Because calls may be processed at Class 4 switches, such as on long-distance calls, which do not have any direct electrical connection to the calling phone, a method of signaling is required which allows a Class 4 switch to remotely control the coin phone. This kind of signaling requires a Feature Group C trunk, so furnished to allow this kind of remote coin control.

There are two methods that may be used for this purpose: multiwink signaling and Expanded In-Band Signaling. In this proposal, we will only consider the latter.

5.3.1. Multiwink Signaling

Multiwink signaling is the older of these two methods. The protocol consists of the Class 4 switch transmitting, in direct sequence, between one and five off-hook winks towards the Class 5 office, corresponding to the various coin disposition functions.

1 wink corresponds to Operator Released, 2 winks correspond to Operator Attached, 3 winks correspond to Coin Collect, 4 winks correspond to Coin Return, and 5 winks correspond to Ringback.

As may be surprised, functions which require more winks than others take longer to execute. The arrangement of the protocol is such that the most commonly and frequently executed functions require fewer number of winks.

Multiwink signaling was largely replaced by the faster Expanded In-Band Signaling and became obsolete. It will not be discussed further in this proposal. Further details about the protocol can be found in SR-2275.

5.3.2. Expanded In-Band Signaling

Expanded In-Band Signaling uses the same multifrequency (MF) tone pairs commonly used for internal network routing. This allows the system to function using standard MF sender and receiver equipment.

The general protocol consists of a Class 4 switch transmitting an off-hook wink towards the Class 5 office, whereupon the Class 5 switch disconnects the audio path and attaches an MF receiver to the call. A single MF digit is transmitted to indicate the function requested. Upon receiving this digit, the Class 5 switch then carries out the requested operation (e.g. a coin collect or return). Upon completion, the talk path is restored.

Coin Collect corresponds to 700 + 1100 Hz (digit "2"), Coin Return corresponds to 1100 + 1700 Hz (digit "KP"), Operator Attached corresponds to 1300 + 1500 Hz (digit "0"), Operator Released corresponds to 900 + 1500 Hz (digit "8"), and Ringback corresponds to to 700 + 1700 Hz (digit "ST3P"). Additionally, a combined Coin Collect + Operator Released function is available and corresponds to 1500 + 1700 Hz (digit "ST").

Because Expanded In-Band Signaling requires nothing more than being able to send and receive winks and MF digits, this is readily supported in many VoIP softswitching environments today, including Asterisk. Feature Group C compatible trunks are also readily achievable in IP environments.

6. Required Hardware

This section addresses the problems discussed in Section 1.1 that exist largely in the hardware domain.

The chief challenge posed today is that there does not currently exist a widely available technology that allows a Class 5 softswitch to fully control and communicate with an embedded system at the location of the payphone. This section explores options for achieving this end.

6.1. PCB

The simplest approach, and most successfully implemented thus far, is to use a custom printed circuit board (PCB) or other simple circuit in conjunction with some other technology for, at minimum unidirectional (from the Class 5 switch towards the coin phone), but possible bidirectional, communication.

A few projects to this end do exist [coin-ctrl]. However, these types of designs have several shortcomings which make them presently unsuitable, or at least, insufficient, primarily because these tools, while useful, without exception, expect a physical connection between the Class 5 switch (e.g. Asterisk server) and the coin phone. This expressly violates one of the key constraints of the problem at hand.

An additional layer of abstraction is thus needed to interface with such a circuit: this would be some device physically connected to the board, or part of the board, capable of speaking IP. However, this immediately adds a slew of other requirements which suddenly greatly complicate the project.

There are a few considerations to consider in the type of communications that will be needed. First, it must be NAT-friendly, such that the Class 5 switch, may at any time, send a command to the circuit, unsolicited. This requires maintaining a port open in the firewall indefinitely, in a manner not unlike a SIP registration. A technology like websockets would be useful in enabling this kind of connectivity; however, the overhead in doing so is too large (requires too much processing power and capabilities) for even commonly available low-power embedded systems such as the Arduino.

The following capabilities would be the minimum required:

This allows for Coin Collect, Coin Return, Operator Attached, and Operator Released to function and be remotely controlled by the Class 5 switch (and by extension, anything else connected to the Class 5 switch). These features are the bare minimum required for a minimally viable product.

Ringback does not require any special functionality as the phone can simply be rung back as if it were receiving an incoming call; this functionality can be handled entirely by the Class 5 switch.

The ability to send +25VDC and -25VDC for the stuck coin test and initial rate test are nice but not absolutely required for an MVP. The intitial rate test functionality is not required if and only if used with single-slot phones only. This is because CDTs can be used to ascertain the initial deposit. This is not how initial deposit detection has historically been done, but it does not introduce any difference in user experience or functionality. Likewise, ground detection is not absolutely required for this reason.

The stuck coin test is more important since it is typically invoked whenever coins are collected or returned to make sure that coins have not gotten stuck. Given that this occurs whenever a coin collect or return occurs, a separate command from the Class 5 switch may not be required; this logic could merely be coupled to the other functions.

The chief advantage of a solution of this kind is that it would be low-power (ideally). To this end, a line-powered device of some kind is preferred, since the circuit would only be needed during a call or immediately after one, when the phone is off-hook and thus receiving a constant supply of -48VDC. However, it will need more power than can be supplied instantaneously by the phone line (which may provide on the order of 30 mA). To this end, a capacitor could be used; the capacitor would charge up when the phone is on-hook, essentially charging off the idle phone line. This would allow it to charge up to 48V. A suitable technique for this is to charge two capacitors to 48VDC in parallel and then discharge them in series, which would provide 96VDC, which should be adequate voltage to operate the coin relays on such a short local loop (which may be only a few feet, as opposed to the several miles that 130 +/- VDC would accomodate).

6.2. Supporting Analog Telephone Adapter

It so happen that there already exists an embedded device of fairly high processing capabilities designed to be interface with a telephone and connected to a remote Class 5 switch over IP: an analog telephone adapter, or ATA.

Modifications to an ATA would allow for adequate remote Class 5 support of a coin phone. Support would need to be added to the hardware and the firmware to accomodate this, as there are no current ATAs which would have the capabilities required.

Principally, the required capabilities consist of being able to tell the ATA to put positive or negative voltage on the line (therby inducing a collect or return, or initial rate or stuck coin test) and reverse polarity (to either positive or negative).

A standardized suite of SIP NOTIFY messages would expose the additional functionality to the Class 5 switch and allow such functionality to be used in a manner that is agnostic of the specific properties of the ATA itself. The following capabilities would be required, at minimum:

These capabilities would allow for the four required functions for a minimally viable integrated ATA solution (MVP): Coin Collect, Coin Return, Operator Attached, and Operator Released. Additionally, the X-DC-Voltage-Disposition message could be used to perform Stuck Coin Test and request Initial Rate Checks. The latter, however, is not necessary if CDTs are used as satisfaction of the initial deposit as opposed to a local ground.

Note that X-DC-Voltage-Disposition must be capable of being received and processed by the ATA even when it is idle. This allows the switch to determine what action should be taken when a caller hangs up and to instruct the ATA to collect or return coins depending on the answer disposition of the call. This capability is mandatory, since the ATA has no way of knowing this on its own. This was true historically as well, as loop start lines do not receive answer supervision.

Ground detection is an optional, but highly useful, enhancement that allows for initial deposits to be satisfied without needing to rely on CDTs. This aligns with how traditional telephony switches ensure the initial deposit has been satisfied and eliminates the possibility of "red box" fraud for the initial deposit (but not any further deposits). The following capabilities would be required, at minimum, to allow for adequate ground detection:

Technically, it may seem that only the X-Line-Grounded header capability is required. This is because if this header is always sent when the line is grounded, the question posed by sending the X-Is-Line-Grounded message can be answered simply by the switch keeping track of whether or not it has received a X-Line-Grounded: 1 during the call so far. However, in dial tone first operation, a call may complete and return to dial tone; thus, the line would presumably no longer be grounded. Ideally, an X-Line-Grounded: 0 would be sent at this point, but this complicates the bookkeeping enough that being able to request, on demand, whether the line is grounded or not is a highly desirable capability to have.

It is worth mentioning that detecting CDTs for the local deposit as opposed to checking the local ground status of a line (or being informed of it) result in similar functionality, but they are not completely identical. If listening for CDTs, the Class 5 switch effectively determines what constitutes the initial deposit. However, if the Class 5 switch is looking for a purported ground on the coin line, then the payphone itself is determining what constitutes the initial deposit. This means that using a ground shifts some of this responsibility to the location of the phone itself, as with traditional coin lines on the PSTN. If using CDTs, the actual initial deposit the payphone is configured for is irrelevant; the Class 5 switch determines what the initial deposit is and whether or not that has been satisfied.

Each of these mechanisms of initial deposit satisfaction has advantages and disadvantages which are likely to depend on particular applications. However, in practice, the two should be synchronized regardless; that is, the Class 5 switch's notion of the initial deposit should correspond to what payphones in the field are physically programmed for. Otherwise, regardless of which method is used, discrepancies in accounting and operation will occur, such as if a customer hears a recording to deposit 10 cents, but 25 cents is actually required to make a local call. In fact, there is no reason both methods could not be used in conjunction: the Class 5 switch can use CDTs to count coins as they are deposited, and verify this assertion using the ground status of the line. This provides even more robust security beyond that which existed in traditional coin line operation, as neither a fradulent ground on the line nor fradulent CDTs alone would be capable of bypassing the initial deposit.

These four SIP NOTIFY messages (and the hardware capabilities to which they would correspond) would allow for adequate and robust operation of coin phones, regardless of the type of coin control that is being used for the line. Each case will be reviewed below.

6.2.1. Mechanisms of Operation

Prepay. Dial tone is not returned to the coin phone until the initial deposit has been made, corresponding either to a local ground being effected and the receipt of an X-Line-Grounded: 1 and/or the receipt of CDTs corresponding to at least the initial deposit.

Semi-Dial Tone First. Dial tone is returned immediately, but nothing may be dialed until the initial deposit has been made. Thus, the caller will not be able to break dial tone until the receipt of an X-Line-Grounded:1 and/or the receipt of CDTs corresponding to at least the initial deposit.

Dial Tone First. Dial tone is returned immediately and free calls may be made without no further requirements. However, on non-free calls, the initial deposit must be satisfied. Thus, either the switch must have previously received an X-Line-Grounded: 1 and/or have previously received CDTs corresponding to at least the initial deposit.

Semi-Postpay. The calling party is muted on local calls when they are answered until the initial deposit is satisfied. Traditionally, this was effected through a polarity reversal, and the phone itself would mute the caller. If no polarity reversal occurs, the switch must either receive an X-Line-Grounded: 1 and/or receive CDTs corresponding to at least the initial deposit to undo a switch-side mute. Alternately, most ATAs have the capability of polarity reversal, though not on demand. Making this available as a SIP NOTIFY capability, e.g. X-Polarity-Reversal would allow this to be accomplished using local phone muting.

Postpay. Postpay operation is unique in that postpay lines do not have a ground wire and do not use ground status at all. A momentary application of 3650 ohms across tip and ring would signal to the CO that the initial deposit has been satisfied. Either the ATA would need to be able to detect this and communicate this unsolicited, e.g. X-Ohms: 3650,200, or CDTs should be used exclusively to satisfy the deposit on local calls.

6.2.2. Challenges In Implementation

While this would likely be a more comprehensive and integrated solution, several challenges are posed by this kind of solution. For one, there are no widely available open source ATAs; the hardware and firmware are generally proprietary. Even if the firmware could somehow be modified, potential legal issues might ensue. This opens up a ripe opportunity for an open source ATA that would at least be generally on par with current basic capabilities. This would allow support for the capabilities discussed in this section to easily be added by any interested party, and any party, generally, would be able to modify the firmware at will. This would be helpful in dealing with this and other features that large companies may typically consider too niche to pursue. However, the challenge remains that additional hardware capabilities are required in such an ATA; that is, custom firmware alone would be insufficient, at least for this problem.

Additional energy consumption can be discounted, since the functionality would be integrated into an existing device that would likely be already being used in another design. A circuit-based design requires a device such as an ATA as it is, so even if a low-energy design were used in the design of such a circuit, it would not provide any comprable energy savings or an integrated ATA solution.

6.3. Other Considerations

7. Security Considerations

Existing risks of toll fraud due to "red boxing" remain the same when a coin phone is controlled by a Class 4 switch.

Risks of fraud may be heightened on local calls if CDTs are used as satisfaction of the initial deposit as opposed to an actual ground test or ground detection being performed.

Additionally, being able to send arbitrary voltage dispositions unsolicited to an ATA may pose safety or system integrity risks. These are not considered for the moment, given that the Class 5 switch is considered to be a "trusted entity", but there may be significant potential for abuse or damage if this capability was used maliciously. As such, receipt of these SIP NOTIFY messages should be restricted only to authenticated SIP proxies, i.e. the SIP proxy to which the ATA is currently registered. Unsolicited SIP NOTIFY messages from other sources should be ignored, regardless of whether these kinds of messages are generally allowed (e.g. unsolicited SIP INVITEs).

8. References

8.1. Normative References

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.

8.2. Informative References

Harte, H., "Western Electric 1D/2D Payphone Controller Interface for Asterisk", , <>.
North, J., "Local Coin Control in the 1970s", , <>.
Technologies, T., "SR-2275", .

Authors' Addresses

Naveen Albert
United States