This is a slightly expanded version of something I wrote a few years ago to give newcomers to the telephony side of things an insight into where the industry comes from but it goes on to explain it's self, I hope it is of interest.
I'm posting this here because, whilst it's about telephony, the idea is to introduce some of those moving into Voip and IP PABXs to why some of the terms we use in telephony are what they are.
The Basics Of Telephony
With more and more people moving into the telephony field from the data industry, I thought it might be a good idea to write an introduction to the telephony world so that people could understand where the terminology came from and how the industry developed! I will start by covering some of the terms in general useage and in expanding on them it should give an idea of how the industry grew to what it is now.
One of the points worth mentioning at the start is that when telecommunications first started out it grew from the original Telegraph network and as that was installed down the side of train tracks, so many railway terms were incorporated into Telecommunications, for example we speak of stations, the local loop, trunk line and even exchange. We even refer to signaling, however, we will start near the beginning in the days when calls were connected manually by a switchboard operator and telephones were elaborate wood and brass constructions that were wall mounted with the mouthpiece fixed to the front of the phone and the earpiece hanging on the side. It was necessary to have a way of signaling to the operator that you wanted to place a call, so in order to achieve that the hook that the earpiece hung from was fitted with a switch (Hookswitch or switchhook depending on where in the world you are from) which when the earpiece was lifted looped (from this we get the phrase local loop) the line causing current to flow through a relay in the switchboard which controlled an indicator which attracted the operators attention. In the early days an elaborate “Dolls eye” indicator was used where a dolls eye like indicator rotated down which was sign written with the extension number. In later versions this was replaced with lamps. I use the term lamp as, as an apprentice, I had it beaten into me that a globe is a map of the world and a bulb goes in the soil to grow flowers.
The operators switchboard was fitted with pairs of cords, the first of which would be used to connect to the socket associated with the calling party and the second was used to connect to the called party. These cords were fitted with plugs that had three connections on them, two were used for the line, the tip and the ring connector just behind the tip (it is from this that the two wires used to supply a phone line are known as the tip and ring). The third connection (the sleeve) was used for signaling, this became necessary because switchboards got so big you couldn’t keep track of which users were on the phone (an operator two or three boards away could have a call going to the party your caller wanted). So an operator would touch the tip of the plug to the edge of the socket she wanted to connect to, if she heard a click then she knew there was already a call on that line. This was known as a tip test
If a call was in progress between two parties and they needed to get in touch with the operator they would flick the hookswitch a couple of times, this would flash the indicator on the switch board and it is from this action that we now have a flash key to recall the operator (in some parts of the world it is known as a recall key because it is used to recall the operator or in modern systems it recalls dialtone) In some parts of the world a third wire was used for recalling the operator on which a local earth was placed when attention was required. In more recent times the flash became a specific duration and so the key is also sometimes known as a TLB key which stands for timed loop break (this is why the key has multiple names these days). It is also worth mentioning that, a standard telephone line (the likes of the one you have at home was called a ring downline because on the old switchboards they used to have little flaps which when the line rang would drop down revealing the phone number of the ringing line and when the call was answered by the operator they would manually reset the flap. So you rang down the flap! These have now become known as PLAR lines in Voip nonclamenture.
As more and more people subscribed to the developing telecommunications network a strange phenomenon was noticed. People were talking quieter when they were on the phone! This was due to a feedback system in the brain, what was happening was that you could hear your voice louder through the phone earpiece than you would normally so you spoke more softly. This prompted the invention of the ASTIC telephone on which most phones are based (ASTIC stands for Anti Side Tone Induction Coil and what it did was reduce the amount of feed back of your own voice in your handset). In the UK engineers developed over a thousand variants of the astic phone and for decades the models of phone were known by the variant number Eg 706 & 746. It is for similar reasons that voip phones now have a “silence” generator which fills in when VAD (voice activity detection) stops packets from being sent. This silence in Voip is actually not silent as true silence sounds dead and people associate true silence with being hung up on!
One final point on manual switchboards, we talked of plugs and cords earlier but a different version did exist and it was an early version of what was later to become the Crossbar system. The idea was you had a matrix of sockets and depending on where you inserted a plug it would cross connect the horizontal connection with the vertical connection. These switchboards suffered from the problem of scalability and tended to be used for small areas. The principal was also used for key and lamp switchboards of the 60s and 70s.
Now telecommunications took a turn for the morbid and underhanded. An undertaker was losing business to his competitor because the local switchboard operator was the wife of his competitor. This led him to the conclusion that the caller should be able to control his own call and to facilitate that he devised a switch that could be controlled by briefly disconnecting the loop across the wires and eventually the rotary dial was developed to give controlled make and break timings (this is where we get the term loop disconnect dialing from). The man who invented this switch was called Almon Brown Strowger and for decades The telephone network consisted solely of “Strowger” exchanges (or if you live in the US Central office)
Strowger switches were a two motion switch which were also known as step by step so when you dialed say 5 the loop would be disconnected and reconnected creating 5 pulses (hence pulse dialing) and by means of a ratchet the switch would step up 5 levels then it would automatically hunt across the bank of contacts for a free connection. The way it knew if a connection was in use was by a third wire in the connection known as the privacy wire. So we now have three connections again, the tip, the ring and now the sleeve has become the privacy wire, this was later abbreviated to the “P” wire (in some areas you may come across the term “P wire link” and it refers to this method of ensuring privacy over multiplexed links) The final switch in a telephone call however is stepped in both directions by the dial pulses to end up on the wire that ultimately connects to your phone. It was soon realized that these switches were too expensive to dedicate an individual switch to one line so they came up with a cheaper option called a uniselector. When you pick up the phone it hunts for a free switch and if it finds one returns dial tone. The dial tone is there to say yes your call can be processed unlike in mobile phones where you dial the entire number before seeing if the system can handle your call. In the days of the strowger switch a telephone exchange (or Central Office which by the way is where the term CO line comes from) was a noisy place and if it went quiet you knew there was a problem. These days it is the other way round!
We should look at some of the problems involved in distributing the calls to the phones on the end of the line. The network uses 48V as that was considered in most countries to be a safe level and utilizes a positive ground. The reason for the positive ground is to overcome problems caused by electrolytic action, metal ions travel to the wire from the ground rather than your wire just corroding away. The voltage is provided from a power supply but constantly floating on that supply is a large bank of batteries. The batteries are one of the reasons 48V is used as getting it to a round 50V would have been difficult. Because of these large battery banks in the exchange, the system is referred to as a central battery system. The PBXs (we now just use PBX but it used to be PMBX or PABX standing for Private Manual Branch eXchange or Private Automatic Branch eXchange) (yes telephone people can’t spell) had their own power systems and these were referred to as Local battery systems.
One of the drawbacks of using 48V is the distance you can send a call before the impedance of the line is too great for it to work. It has to be remembered that a telephone line is a full duplex medium which actually allows the collision of information (you can both speak at the same time). This causes a problem because in order to go further distances you have to amplify the signal and an amplifier is a one direction device. So enter the hybrid transformer, this device was designed to take a two wire bidirectional connection and split it to two, two wire omni directional connections giving a transmit and receive pair each of which could be amplified at repeater stations along the line (it should be noted that the most common reason for “echo” on calls is an imbalanced hybrid transformer somewhere along the line). A common use for this was when two PBXs needed to be connected to each other independently of the PSTN which is a circuit known as a 4 wire E & M connection (there were/are 2 wire E & M circuits for use over shorter distances) The E stood for earth and the M for Magneto (some call it ear and mouth standing for transmit and receive the reason magneto is the term is that it used AC and on old manual boards, the AC used to ring the handset was generated by a small hand cranked Magneto) The E & M are signaling wires. The need for these types of circuit has been virtually wiped out these days because with the signals being transmitted digitally there is no need for amplifiers and the equipment that does convert it to the 48V system can be stationed so close to the PABX.
Sending phone calls over longer distances by amplifying the calls presets a problem as you can’t amplify dial pulses so this is where different signaling systems came along. The most common being DTMF (dual tone multi frequency), which uses a combination of two frequencies for each digit. It is commonly accepted that there are four tones for the rows across the dial pad and three for the columns down the dial pad however there is a fourth column where the keys are labeled A,B,C & D. The reason for these is that the keypad was intended to be useable for entering information into computers (think hexadecimal). These tone combinations are now used in some network control roles. There are other systems of tone signaling like MFC (which stands for Multi Frequency, Compelled) which became the signaling system used for international dialing, in this system each “digit" has to be acknowledged by the receiving end before the next will be sent and whilst uncommon it was also used for indial services to some PABXs but I believe it has recently been discontinued in Australia. In the UK there was also a Tie line service known as AC15 which utilized 2280Hz signaling but I personally have not come across that in Australia.
Now the network took a leap forward with the advent of DTMF as in the old Strowger exchanges, the dial pulses directly controlled the movement of the switch. DTMF needed a “Register” to interpret the number dialed and direct the switches to establish the call and with this method Crossbar switches made a brief return to popularity but were soon ousted by SPC (stored program controlled) electronic switches which is where we are today with most exchanges. However with the advent of Voip the next transformation of the network is under way.