The Public Switched Telephone Network (PSTN)
The Public Switched Telephone Network (PSTN) very simply, is the network of copper cables and switches over which landline calls are made. Circuit switching is used to connect calls from one phone (CPE = Customer Premise Equipment) to another CPE and is routed via switches residing in Central Offices (CO or Exchange), locally, nationally or internationally. The established call between the two CPE’s is called a circuit. Your phone at home, works on a dedicated pair of copper wires that runs through distribution points (DP’s), aerial cables, cabinets, underground cables, main distribution frame in the CO and connects back to the local switch and this is called a loop.
Copper is a good conductor of electricity but has some resistance in it, that increases with the length of the cable. Thicker the cable, lesser is the resistance and hence the length of the loop will be longer. For example, a 19 guage cable will stretch approx 22 KMs before the resistance in the cable builds to an extent that you cannot hear the other person on the call, whereas a thinner 26 guage cable will only go upto 5 KMs.
Calls between switches are connected via trunk circuits which unlike loops are not dedicated, but are shared by the CO’s. When you want to make a phone call to someone, you enter the phone number which is sent down the loop to your local switch. The local switch translates and understands the destination, will grab an unused trunk circuit heading in the destinations direction and connect you to the local switch in the called numbers local area where your signal is sent down the dedicated local loop to the called party’s phone. This process of connecting using switches and sharing trunks is called circuit switching.
In the early days, phone calls travelled as analog signals across copper wire. Every phone call needed its own dedicated copper wire connecting the two phones. That is why you needed operators assistance in making calls. The operators sat at a switchboard, literally connecting one piece of copper wire to another so that the call could travel across town or across the country. Long-distance calls were comparatively expensive, because you were renting the use of a very long piece of copper wire every time you made a call.
Beginning in the 1960s, voice calls began to be digitized and manual switching was replaced by automated electronic switching. Digital voice signals can share the same wire with many other phone calls. The advent of fiber optic cables now allows thousand of calls to share the same line. But fiber-optic and other high-bandwidth cables haven't changed the basic nature of circuit switching, which still requires a connection or circuit to remain open for the length of the phone call.
Connecting phone calls may require circuit switching between multiple switches and this would depend on the number being dialed. For example: 09 123 4567.
09 is the national area code (state code) and can be 1 to 3 digits long. 123 is the switch (Exchange) address and 4567 is the subscriber number or the number dedicated to a specific address or customer within that switch (Exchange).
To make international calls, the circuit needs to run via international carriers and your country’s exit / international access code, and the destination country’s codes are required in addition to the national number. Example 00 971 4 123 4567 where 00 is the country exit/international access code, 971 is the destination country code, 4 is the national code, 123 is the switch address and 4567 is the subscriber number. Country exit/international access code could be 2 or 3 digits long where as Country codes could be 1 to 3 digits long (USA = 1, New Zealand = 64, UAE = 971).
Copper is a good conductor of electricity but has some resistance in it, that increases with the length of the cable. Thicker the cable, lesser is the resistance and hence the length of the loop will be longer. For example, a 19 guage cable will stretch approx 22 KMs before the resistance in the cable builds to an extent that you cannot hear the other person on the call, whereas a thinner 26 guage cable will only go upto 5 KMs.
Calls between switches are connected via trunk circuits which unlike loops are not dedicated, but are shared by the CO’s. When you want to make a phone call to someone, you enter the phone number which is sent down the loop to your local switch. The local switch translates and understands the destination, will grab an unused trunk circuit heading in the destinations direction and connect you to the local switch in the called numbers local area where your signal is sent down the dedicated local loop to the called party’s phone. This process of connecting using switches and sharing trunks is called circuit switching.
In the early days, phone calls travelled as analog signals across copper wire. Every phone call needed its own dedicated copper wire connecting the two phones. That is why you needed operators assistance in making calls. The operators sat at a switchboard, literally connecting one piece of copper wire to another so that the call could travel across town or across the country. Long-distance calls were comparatively expensive, because you were renting the use of a very long piece of copper wire every time you made a call.
Beginning in the 1960s, voice calls began to be digitized and manual switching was replaced by automated electronic switching. Digital voice signals can share the same wire with many other phone calls. The advent of fiber optic cables now allows thousand of calls to share the same line. But fiber-optic and other high-bandwidth cables haven't changed the basic nature of circuit switching, which still requires a connection or circuit to remain open for the length of the phone call.
Connecting phone calls may require circuit switching between multiple switches and this would depend on the number being dialed. For example: 09 123 4567.
09 is the national area code (state code) and can be 1 to 3 digits long. 123 is the switch (Exchange) address and 4567 is the subscriber number or the number dedicated to a specific address or customer within that switch (Exchange).
To make international calls, the circuit needs to run via international carriers and your country’s exit / international access code, and the destination country’s codes are required in addition to the national number. Example 00 971 4 123 4567 where 00 is the country exit/international access code, 971 is the destination country code, 4 is the national code, 123 is the switch address and 4567 is the subscriber number. Country exit/international access code could be 2 or 3 digits long where as Country codes could be 1 to 3 digits long (USA = 1, New Zealand = 64, UAE = 971).