RJ-11/14: Telephone socket understood
The signal in the telephone cable is usually transmitted via two strands (RJ-11 6P2C - 6 pins 2 contacts). Theoretically, one strand would be enough, if it only were the right one; but then the modem could only run at half the speed. Most telephone cables do not only have two contacts on the left side of the plug, but also two more on the right side (RJ-14 6P4C). These are intended for looping the signal through. If, for example, a call comes in and the modem is currently on the Internet, it can loop the call back via the two right-hand pins and forward it to other devices.
A telephone socket usually has three connectors. These are connected in series. First comes the modem on the far left, then the right plug and finally the middle one. On the socket there are two input contacts (A1, B1, if that is correct) and two output contacts (a1, b1, as far as I know) that use to be covered by the jacket. You could connect another socket in series with a cable by use of the output contacts, which would give you a total of six serially connected plugs.
If one telephone is connected to each of the two serially connected sockets and the telephone connected first can loop through the signal (four contacts instead of only two), both telephones would ring simultaneously when a call is made. However, if one picks up with the first telephone, the signal for the second telephone is interrupted and the call can only be made with the first phone.
A socket has three connectors because they are devised for the use of a modem, a fax and a telephone. The fax normally comes on the right if it can forward calls to the telephone. In principle, you could also plug the telephone in on the right and the fax in the middle. However, if you then pick up the phone when a fax is being sent or received, the line is interrupted and the fax no longer goes through.
The socket is designed in such a way that if only one device is plugged in, it does not matter which of the three plugs it is connected to. The inner pins of a connector socket at the left and right side of the opening are in contact with each other in the unplugged state and conduct the current forth. However, if a plug is inserted into the socket, the pins on the left and right are spread apart and the conduction within the socket is interrupted. It is then up to the plugged-in device to pass on the signal itself if necessary.
When buying a telephone or fax, find out that it can loop through signals and make sure that it is connected with a cable that has two pins on both the left and right sides (6P4C). Some cables or plugs even have three contacts each. The extra contacts in the middle are, as far as I know, for an extra bell to announce an incoming call. I assume, however, that the third line can also be used for data, as is the case with some modems.
In principle, it is also possible to connect two sockets in parallel rather than serially. However, if a modem were to want to run in parallel with another device, it would register an error because the other device interfered with the line then, and thus the modem would stop transmission. Ringing two telephones in parallel should also be possible with this, although there would be interference here as well if two people picked up at the same time.
So much, if you still have an old analogue telephone connection. With ISDN (RJ-48/49/61, also known as ISDN-RJ-45 8P8C, different plug because it has 8 pins, 40 volts instead of LAN-RJ45 0.75 volts, but the plug is the same!), up to two or eight telephones and faxes can talk long-distance at the same time, which can also be achieved in a completely different way with extensions, each of which then has its own extension number. ISDN, in contrast to the age-old analogue connection, is a digital technology, although the bandwidth for telephone calls is guaranteed here. This means that a point-to-point connection is established and the telephone call cannot be disturbed unless the line has a fault.
It is different if you plug your telephone directly into the modem, as is the case with some internet providers. Then the continuous, analogue telephone signal is digitised, packed into IP packets and sent to the provider via the internet, similar to a VoIP telephone. If the line is overloaded, IP packets can be discarded. A hacker could also send with the same IP address as for their telephone connection and thus disrupt the signal. In both cases, you will notice a temporary hissing of the call tone, often to the point of unrecognisability. ISDN is a digital technology as well, but it establishes a point-to-point connection with guaranteed bandwidth.
Another problem arises if your provider’s IP/Internet access fails. Then you can no longer make telephone calls, unless you have ISDN or a good old analogue landline. The undisputed advantage of an analogue connection is that a telephone is also supplied with power via the connection, or at least that is provided for ordinary telephones that get by with it. In the event of a power failure, you can call the network operator’s fault hotline. Unfortunately, this is not possible with an ISDN connection. What if your mobile phone has just run out of battery or cannot connect to your provider’s access point?
If you have an old analogue landline, you should keep your contract, because when you get a new one, the formerly state-owned A1 (Austria) only sells a connection for the internet (i.e. no ISDN and no analogue connection), unless you are a business customer. Occasionally there are still local providers who can arrange such a connection, such as MCom-Systems or the Krainz GmbH in Carinthia mentioned below. However, even then the line is always run via A1, because alternative private Internet providers have never invested in analogue or ISDN-based telephone technology, as far as I know.
It used to be the other way round. You used to access the internet via a 64Kbit/s modem, which converted a digital signal into analogue coded tones. If you wanted to go on the internet, the modem had to call the telephone number of the internet provider. So while you were on the internet, you couldn’t make a phone call. Calls to the number of the Internet provider were possible with a special low tariff. You may also have been familiar with the possibility of war dialling via telephone booths.
However, if you still have an old 64K modem at home, you can use it to send a fax with your computer. External modems (i.e. a modem as a separate device to connect) are supported by Linux with drivers by default and you can use a programme like efax to send. Modem connections built into old notebooks, on the other hand, usually require their own driver, which was often only available for Windows. Such a modem can still be used to establish a point-to-point connection between two computers (see minicom, ppp, picocom, cu, tio, serial RS232 cable: /dev/ttyS0, stty & setserial under Linux).
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