The public switched telephone network (PSTN) is the network consisting of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites and undersea telephone cables, all inter-connected by switching centers and single global address space for telephone numbers based on the E.163 and E.164 standards, thus allowing any telephone in the world to communicate with any other.
Originally a network of fixed-line analog telephone systems, the PSTN is now almost entirely digital in its core and includes mobile as well as fixed telephones.
Technology in the PSTN
• Network topology
The telephone exchanges are arranged into hierarchies, so that if a call cannot be handled in a local cluster, it is passed to one higher up for onward routing, reducing the number of connecting trunks required between operators over long distances and keeping local traffic separate.
In modern networks the cost of transmission and equipment is lower and, although hierarchies still exist, they are much flatter, with perhaps only two layers.
• Digital channels
Most automated telephone exchanges now use digital switching rather than mechanical or analog switching. However, analog two-wire circuits are still used to connect the last mile from the exchange to the telephone in the home (local loop). To carry a typical phone call from a calling party to a called party, the analog audio signal is digitized at an 8 kHz sample rate with 8-bit resolution using a special type of nonlinear pulse code modulation known as G.711. The call is then transmitted from one end to another via telephone exchanges.
The call is carried over the PSTN using a 64 kbit/s channel called Digital Signal 0 (DS0). A Digital Signal 1 (DS1) circuit carries 24 DS0s on a T-carrier (T1) line, or 32 DS0s (30 for calls, 2 for framing and signaling) on an E-carrier (E1) line. In modern networks, the multiplexing function is moved as close to the end user as possible.
The following list includes a few of the popular custom calling features commonly found in the PSTN today:
• Call waiting
• Call forwarding
• Three-way calling (Enables conference calling)
• Display calling party's directory number
• Call blocking
• Calling line ID blocking
• Automatic callback
• Call return
References:
http://fengnet.com/book/voip/ch01lev1sec3.html
http://en.wikipedia.org/wiki/Public_switched_telephone_network
Originally a network of fixed-line analog telephone systems, the PSTN is now almost entirely digital in its core and includes mobile as well as fixed telephones.
Technology in the PSTN
• Network topology
The telephone exchanges are arranged into hierarchies, so that if a call cannot be handled in a local cluster, it is passed to one higher up for onward routing, reducing the number of connecting trunks required between operators over long distances and keeping local traffic separate.
In modern networks the cost of transmission and equipment is lower and, although hierarchies still exist, they are much flatter, with perhaps only two layers.
• Digital channels
Most automated telephone exchanges now use digital switching rather than mechanical or analog switching. However, analog two-wire circuits are still used to connect the last mile from the exchange to the telephone in the home (local loop). To carry a typical phone call from a calling party to a called party, the analog audio signal is digitized at an 8 kHz sample rate with 8-bit resolution using a special type of nonlinear pulse code modulation known as G.711. The call is then transmitted from one end to another via telephone exchanges.
The call is carried over the PSTN using a 64 kbit/s channel called Digital Signal 0 (DS0). A Digital Signal 1 (DS1) circuit carries 24 DS0s on a T-carrier (T1) line, or 32 DS0s (30 for calls, 2 for framing and signaling) on an E-carrier (E1) line. In modern networks, the multiplexing function is moved as close to the end user as possible.
The following list includes a few of the popular custom calling features commonly found in the PSTN today:
• Call waiting
• Call forwarding
• Three-way calling (Enables conference calling)
• Display calling party's directory number
• Call blocking
• Calling line ID blocking
• Automatic callback
• Call return
References:
http://fengnet.com/book/voip/ch01lev1sec3.html
http://en.wikipedia.org/wiki/Public_switched_telephone_network
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