I presume individual telephones are linked to exchanges
in the same way as computers in a network with a star topology
are linked to a hub. So if I make a call to another telephone on
the same exchange it is routed through this exchange?
Are exchanges linked to each other in one long line or do they all connect to a central point?
Is fiber optic cable used between exchanges or would microwave or satellite links be used?
If I call another phone which is engaged I get an engaged tone.
How does the phone know the other phone is engaged?
If a call is made from a landline to a cellphone,
how does this work?
A call can be made between a telephone on one network to a
telephone on another network. Are networks linked together?
Do exchanges send power to telephones?
Near where I live, there are rectangular covers
set into the ground on pavements. What do these do?
When the call is answered, I can talk to the other person
as if my phone was connected directly to theirs. Do all the
exchanges between us keep the connection open and what happens when we hang up?
I’m not an authority on telephony, so I’ll leave the other questions alone. But, here in the states, what you’ve described here seems to be a pad-mounted transformer that steps electrical voltage from a higher level down to a ‘household’ level for distribution. Nothing to do with the telephone system.
Tripler
An underground electrical distribution system, that is. . .
Tells the exchange what number you want to reach.
2) I presume individual telephones are linked to exchanges
in the same way as computers in a network with a star topology
are linked to a hub. So if I make a call to another telephone on
the same exchange it is routed through this exchange?
Yes
3) Are exchanges linked to each other in one long line or do they all connect to a central point?
none of the above. It is just like a network.
Is fiber optic cable used between exchanges or would microwave or satellite links be used?
Anything can be used depending on where you are calling but today it is mostly fiber optic as microwave and satelite links are disappearing for phone use.
If I call another phone which is engaged I get an engaged tone. How does the phone know the other phone is engaged?
The exchange of the called number knows the phone is engaged and signals the calling exchange.
If a call is made from a landline to a cellphone,
how does this work?
Landlines carry your call to the cell tower closest to the cell phone and the link is by radio from there.
A call can be made between a telephone on one network to a
telephone on another network. Are networks linked together?
Yes, obviously. You can pretty much reach any phone on earth to any other phone on earth (except for internal systems not connected to the outside).
Do exchanges send power to telephones?
Yes, very small amounts though. Your phone is powered by the phone line.
Near where I live, there are rectangular covers
set into the ground on pavements. What do these do?
When the call is answered, I can talk to the other person
as if my phone was connected directly to theirs. Do all the
exchanges between us keep the connection open and what happens when we hang up?
The connection is freed. Note that yhe connection is not a physical exclusive connection. Except for the two ends, the rest is information digitally transmitted over shared links.
I have absolutely no idea how it works in the UK, but here’s some stuff about the US PSTN:
Depends. End offices in the same local calling area are linked directly to each other. End offices are usually also connected by trunk to an entirely different sort of office called a “tandem.” The tandem has a different switch functionality and is pretty much the long-didtance gateway.
At least in this state, it’s just about 100% fiber in self-healing rings with diverse redundant routing.
There’s pretty much just one network. Different parts may be owned by different companies, but everything is interconnected.
Could be anything. Around here I’d suspect a remote terminal vault.
**5) If I call another phone which is engaged I get an engaged tone. How does the phone know the other phone is engaged? **
The phone is connected to a switch (which nowadays means a computer but they used to use electromechanical relays) at the exchange. The switch knows whether the phone is on-hook or off-hook, since the phone is always connected to the switch. When it’s off-hook and a request comes in to call that phone, the switch generates a “line engaged” (US: busy) signal to the calling switch, which is then heard by the caller on his phone. (Something similar happens when the phone is on-hook; in that case the switch generates a “ring” signal.) This is why if a US phone calls a UK phone, the line engaged signal sounds different than when calling a US phone; the UK switches use different signals.
6) If a call is made from a landline to a cellphone, how does this work?
The company that carries the landline calls (a LEC or CLEC in the US) has an agreement with the cellular company to do call handoffs. A call from a landline phone is carried in the LEC network, then handed off to the cellular company’s network for completion. (Then the cellular network figures out what cell is getting the strongest signal from the phone, and routes the signal to that cell. When everything works, as the destination phone moves between cells the cellular network switches the call to follow the phone around.) (A similar handoff agreement exists for making long-distance calls.)
If I call another phone which is engaged I get an engaged tone.
How does the phone know the other phone is engaged?
Your local exchange maintains a voltage across the pair of wires going to your house. When your phone is on the hook, the circuit is not completed (an open circuit) and no current flows. When you lift the receiver, the circuit is completed and current flows. This is how the exchange knows… it also explains question 8, this is the power that the phone uses. When you lift the phone the exchange detects a current and sends a dial tone. The dc voltage is out of band signalling, the dial tone is in-band signalling, the band being what is used to transmit the data in this case voice
I think the band is roughly 300Hz to 3kHz…
When the call is answered, I can talk to the other person
as if my phone was connected directly to theirs. Do all the
exchanges between us keep the connection open and what happens when we hang up?
There isn’t a physical wire reserved for your call (at least not these days), however capacity is reserved for the duration of your call, 64 kbits/s if i remember correctly. This is what is known as a circuit switched network as opposed to a a packet switched network, eg the internet.
I work with a couple of weirdos… i’d like to know what the heck they were talking about to even get to this point… but i thought i’d come here for an official response…
*If you are talking over a landline telephone, don’t the two phones have to be connected by wire to make a connection? How is the connection made if you are in the US, and calling Europe? Is there telephone wire running below the Atlantic ocean or something?
Yes, there are communication cables under the Atlantic, and under other oceans. However, a lot of intercontinental communication is via satellites these days.
When I was a kid, I asked a telephone repair guy what the voltage of the phone line was (it had shocked me once when I was messing around with the wires). IIRC, he said 48 volts. Is this still the case?
Most telephone calls go via fibre optics, not by satellite. People prefer to not have the satellite delay. The only satellite link-ups I’ve found regularly are when phoning Bermuda via cheap third-party companies. (Of course, phoning less developed places will be a different matter)
TAT-9 & TAT 10 (TAT being the designation for TransAtlantic optical Telephone cable) were laid in the early 1990’s. TAT-9 & TAT 10 provided 565 Mbps of bandwidth each and the repeaters were spaced at 60 miles. Since then we’ve come a long way. TAT-12 & TAT-13 were laid between the USA, England & France in the mid-90’s and provided 10Gbps of bandwidth and employing EDFA (Erbium Doped Fiber Amplifier) technology. Most recently, TAT-14 was laid by a consortium of 48 telecom carriers and completed in August 2001. TAT-14 links Denmark, France, Germany, Holland, the U.K. and the U.S.A. Total price of this monster was 1.4 billion USD. TAT-14’s capacity is a whopping 640 gigabits per second.
More info can be found in this tangential thread which is primarily about light amplification techniques used in data tramsmission.
Thank for fighting my ignorance, GorillaMan and UncleBeer. I talk by phone to my family in Australia a lot, and I had thought the response time was a bit too fast for the signal to have gone through satellites. So I’d now guess that there are similar fibre-optic cables linking North America with East Asia and with Australia.
from the phone in the house the dropwire goes to a Distribution Point (DP), usually up a pole. These DPs are gathered together in the (usually green) cabinet somewhere in the street from where Local Cables carry the lines to the exchange. In the exchange the Local Cables are wired to the Main Distribution Frame (MDF) where they are presented on numbered blocks. The Frames Technician connects your local pair to another pair of terminals on the MDF, usually on the other side, using a length of 2-pair wire known as a ‘jumper’. These terminals are permanently wired to a card within the digital exchange unit (often referred to as the ‘switch’). If it’s in a small town or a rural area the switch will be a Remote Concentrator Unit (RCU)which is one of a number of slave stations parented on a processor site in a bigger exchange. Large numbers of phone calls are aggregated together into a single signal which in turn will probably be aggregated with other signals into an even larger one to be passed to another large exchange. A fair number of alternative paths are provided, with a certain amount of duplication so that if someone digs a hole in the road and breaks a cable it doesn’t isolate the exchange. Increasingly rings of fibre cable are provided so that a circuit has an alternative path it can automatically switch to.
The zero in the dialling code tells the switch this is going to be a trunk call. You can usually omit the dialling code if the called party is on one of the other RCUs parented on the processor site, e.g. Hatfield, Cuffley & Welwyn Garden City are all parented on Potters Bar and share the 01707 code with it.
When the called party answers, his phone sends a Called Subscriber Answer (CSA) signal back down the line to his exchange to remove ringing current from his phone, remove the ringing tone from your phone, set up the speech path and start metering. When you replace your handset (or ‘clear down’ as we call it) your phone sends the Calling Subscriber Cleardown (CSC) signal into your exchange which breaks the connection, removes the ‘busy’ signal from the line that stops the system trying to route any other calls to it, and ends metering.
-50V is the norm for powering an ordinary telephone, and indeed most equipment within the exchange. Normally this won’t give you a shock unless you are digging around in the mass of wires on the MDF and touch a sweaty armpit onto a terminal :eek: Telephone Repeater Stations used to have -24V to power the equipment for some reason. ISDN2 lines have -90V, and can be dangerous.
-48 is the US normal on-hook voltage. Close enough to 50, really.
Off-hook, the voltage drops to 6-8 volts, and standard ringing is 90 volts AC at 20 Hz.
It’s all at pretty low available current and high impedance (it’s signalling, rather than power distribution), so it’s rather rare to be seriously injured by a phone line, but that 90 volts will give you quite a sting.
As for the OP’s question 9 - “Near where I live, there are rectangular covers
set into the ground on pavements. What do these do?” - they cover rectangular holes.
I don’t know what they’re called now, but when I worked for AT&T they were SLCs (Subscriber Loop Concentrators.) Fundamentally there is a line from your house to the central office, but it is inefficient to have one for each phone - both in wiring, and because the switch is limited in the number of phone lines it can handle. Since not everyone in an area calls at the same time, the SLC multiplexes several subscriber lines to one.
I left the books from my two week telephone class when I left, but IIRC there is a hierarchy of offices, from the central office up to long line offices for long distance. That was before all the long distance companies, though.
I suspect fiber cable is pretty much universal now, but for a long time microwave was more effective, especially in more rural areas. And yes, almost all intercontinental traffic is carried by undersea cable. AT&T used to own a couple of ships just for this. A lot of the capacity improvement was in ways of pushing more bandwidth down existing cable. There were some amazing reliability and performance requirements .
