| Internet-Draft | VoIP Magneto Support | June 2022 |
| Albert | Expires 1 January 2023 | [Page] |
This document specifies a proposed mechanism by which magneto phones could interoperate with VoIP networks.¶
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Voice-over-Internet Protocol (VoIP) technologies have allowed for many advances in communications that have largely (though not completely) captured the feature set and capabilities of traditional wireline networks and technologies.¶
However, these developments are limited to "modern" traditional telephony. In particular, they only encompass common battery technologies. Prior to common battery being widely used, magneto phones with local battery were widely used to place and receive calls.¶
A magneto typically takes the form of a crank which is operated manually by its user. When rotated, the magneto produces alternating current (AC). Like in later topologies, magneto phones are connected as stations to a manual switchboard at the central office (dial services were exclusively in the domain of common battery technologies).¶
The magneto played an important role in signaling. When the magneto is operated at the station, this produces current which operates a light at the switchboard. In dial service, common battery allows this to occur automatically when the local loop is interrupted by simply removing the receiver. Likewise, operating the magneto at the switchboard would produce the alternating current that rang the ringer at the telephone.¶
Unlike with common battery technologies, there is no automatic mechanism by which the opening of the local loop (a hangup) can be detected. This is what "ringing off", signaling the central office that a call is through by cranking the magneto at the end of the call, is for. In contrast to common battery environments where going on-hook would interrupt the local loop and signal the operator, this must, as with originating requests for service, be done manually. Thus, ringing off informs the operator that the call is over and the connection can be disconnected at the switchboard.¶
Current VoIP technologies do not satisfactorily allow for the authentic interoperation of magneto and local battery technologies. This document proposes a setup that could be used to allow for authentic operation and use of local battery devices, such as magneto phones, with VoIP technologies.¶
Local battery shall refer to the furnishment of batteries to supply operating power for telephone instruments at the customer premises for telephone station apparatus. This is in contrast to common battery, in which this power is furnished by the central office to any served instruments via the local loop itself.¶
Magneto shall refer to a class of local battery telephone station apparatus operated with a magneto crank.¶
ATA shall refer to an analog telephone adapter, a VoIP device used to connect analog station equipment with VoIP equipment.¶
FXS shall refer to Foreign Exchange Station or Subscriber. Telephone sets are connected to FXS ports, which "provide" service.¶
FXO shall refer to Foreign Exchange Office. FXO ports can be used to "request" service from a switch, e.g. at the central office.¶
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.¶
This section addresses the problems discussed in Section 1.1.¶
Most analog telephone adapters (ATAs) typically furnish FXS ports that are used to connect telephone station equipment with a VoIP server. However, some ATAs have one or more FXO ports. A common type of ATA provides 1 FXS port and 1 FXO port (2 ports total).¶
At a high level, the FXO port can be used for AC signaling while the FXS port can be used for the actual call path itself. In other words, a magneto telephone would be connected to both an FXS port and an FXO port. These roles are discussed individually below.¶
The FXS port would be the primary port providing general telephone functionality, and this port would provide the actual voice interface for telephone calls. This is similar to how FXS ATAs are intended to be used, and this approach is not dissimilar to other intended uses. However, ommitted from usage would by an AC ringing from the central office (upstream VoIP telephony server) as well as any signaling.¶
Incoming calls to the magneto subscriber will be indicated by AC ringing on the FXS port, as normal.¶
The FXO port would be the primary port providing telephone signaling that is out-of-band from the perspective of the FXS channel. As opposed to using the normal on and off-hook events in conjunction with ringing on the FXS interface for telephone signaling as is done with common battery systems, these signals would be ignored on the FXS interface. Instead, the FXO interface will be used for these purposes.¶
To originate a call, the magneto will be operated by the subscriber, producing alternating current. Since FXO ports are designed to be connected to central office lines, they are used to receiving alternating current so this is compatible with the constraints of the equipment as it was designed. Cranking the magneto will thus provide the FXO port with AC current that will cause it to originate a request to the VoIP server, thinking it has received a call. This event can then be interpreted through context by the upstream server.¶
Similarly, to ring off, the same activity can be repeated at any point during the call. This will be interpreted as a ring off through context by the VoIP server. This has no impact to the call since the call itself is taking place on the FXS port.¶
Ringing cannot occur on the FXO port since ringing must be provided by FXS interface.¶
In some ways, this is similar to the way that Common Channel Interoffice Signaling (CCIS), e.g. Signaling System 7 (SS7) has allowed for the conveyance of key signaling information out-of-band, where it is inaccessible on voice channels. In a similar manner, the FXO channel would be primarily responsible for conveying signaling information relevant to the call itself, such as requests for service (operation of the magneto crank), notice of incoming calls (AC ringing from the FXO interface), and notice of call termination from the subscriber (operation of the magneto crank to ring off).¶
While operating the magneto on an FXO port is within its intended scope, it is not within the scope of what an FXS port is designed to handle. Therefore, the FXO and FXS ports must be electrically isolated. Furthermore, the magneto crank should be electrically disconnected from the rest of the telephone, including the ringer, and connected only to the FXO port.¶
Safety is a concern that should be adequately assessed, given the potential dangers of faulty wiring with the AC current produced by a magneto crank or delivered to a ringer. Generally cranking a magneto phone will also ring the local ringer due to the nature of the circuitry. If this cannot be effected with this arrangement, it will be a minor detraction in authenticity of operation. If doing so is feasible, adequate thought must be given to the safety of any electrical coupling between the magneto crank and the ringer. This is because the magneto crank must be connected to the FXO port to generate AC ringing, and the ringer must be connected to the FXS port to receive AC ringing. If the FXS and FXO ports are not to be electrically connected, then any arrangement which connects the magneto crank and the ringer must be done so in a way that ensures the sufficient isolation of different parts of the telephone to avoid endangering the instrument and/or the user.¶
The author is aware of no security risks of this proposal.¶