31-07-2012, 04:09 PM
THE PUBLIC SWITCHED TELEPHONE NETWORK
[attachment=29754]
THE ANALOG TELEPHONE SYSTEM
• In use over wide areas until recently in some countries
Still widely used for connections to end users in North America
The local subscriber network reaches more users than any other network
Represents ∼ 90% of the net worth of the local Bell companies
• Precursor of the modern (mostly digital) PSTN
Services offered by long-established telephone companies are strongly
influenced by the history of the analog network
Regulatory environment
You have to understand the old analog network before you can understand
why we are where we are with digital networks
THE ANALOG SUBSCRIBER LOOP
• Transmission media
Single wire with earth return
◦ Vulnerable to currents induced by atmospheric electric-field variations
along the wire, ground loops, and variable ground resistance
A differential signal transmitted on a wire pair
◦ Common-mode rejection with proper matching and balanced detection
◦ Uninsulated open wire pair
◦ Twisted pair
No. 22 AWG copper wire (ρL ≈ 16.5 Ω/1000 feet)
Wires are called “tip” (+) and “ring” (−) (terminology comes from
phone plugs used in manual switching)
Closed, balanced loop not referenced to earth ground
Bandwidth: 180 Hz to 3.2 kHz (at 3 dB points)
Multipair cable
THE ANALOG SUBSCRIBER LOOP
• Signaling at the handset
Call alerting at receiving telephone
◦ 90 V RMS @ 20 Hz, superimposed on DC
Loop-start signaling
◦ Taking a telephone off-hook makes current flow in the subscriber loop
◦ When a telephone goes off-hook to initiate a call:
The central office propagates dial tone to the telephone
◦ When a telephone goes off-hook to receive a call:
The central office cancels the ring signal
Address signaling
◦ Pulse dialing
The loop is interrupted (at ∼.1 s intervals) a number of times equal
to the digit being dialed, or 10 times for 0 (in the US)
◦ DTMF
Introduced to permit signaling across microwave links
ANALOG INTEROFFICE TRUNKS
• Small cities have one central office; large cities have multiple CO’s
Signal attenuation (∼ 3 dB/3.5 mi) becomes an issue for total path lengths
greater than about 12 mi
◦ Amplification is required for long links
◦ Amplification is not feasible on a wire pair carrying full-duplex signals
◦ Conversion between a two-wire channel and a four-wire channel is
accomplished at the CO using a hybrid
Hierarchical network architecture:
◦ Design a combination of direct interoffice trunks and tandem (trunk-totrunk)
switches to minimize call blocking probability for a given total
cost and average traffic pattern
Interoffice trunks:
◦ Originally open-wire, then multipair cables (common until optical fiber)
◦ Later: 12 voice channels frequency-multiplexed onto one pair
◦ Still later: Coaxial cable, carrying subcarrier-multiplexed signals.
[attachment=29754]
THE ANALOG TELEPHONE SYSTEM
• In use over wide areas until recently in some countries
Still widely used for connections to end users in North America
The local subscriber network reaches more users than any other network
Represents ∼ 90% of the net worth of the local Bell companies
• Precursor of the modern (mostly digital) PSTN
Services offered by long-established telephone companies are strongly
influenced by the history of the analog network
Regulatory environment
You have to understand the old analog network before you can understand
why we are where we are with digital networks
THE ANALOG SUBSCRIBER LOOP
• Transmission media
Single wire with earth return
◦ Vulnerable to currents induced by atmospheric electric-field variations
along the wire, ground loops, and variable ground resistance
A differential signal transmitted on a wire pair
◦ Common-mode rejection with proper matching and balanced detection
◦ Uninsulated open wire pair
◦ Twisted pair
No. 22 AWG copper wire (ρL ≈ 16.5 Ω/1000 feet)
Wires are called “tip” (+) and “ring” (−) (terminology comes from
phone plugs used in manual switching)
Closed, balanced loop not referenced to earth ground
Bandwidth: 180 Hz to 3.2 kHz (at 3 dB points)
Multipair cable
THE ANALOG SUBSCRIBER LOOP
• Signaling at the handset
Call alerting at receiving telephone
◦ 90 V RMS @ 20 Hz, superimposed on DC
Loop-start signaling
◦ Taking a telephone off-hook makes current flow in the subscriber loop
◦ When a telephone goes off-hook to initiate a call:
The central office propagates dial tone to the telephone
◦ When a telephone goes off-hook to receive a call:
The central office cancels the ring signal
Address signaling
◦ Pulse dialing
The loop is interrupted (at ∼.1 s intervals) a number of times equal
to the digit being dialed, or 10 times for 0 (in the US)
◦ DTMF
Introduced to permit signaling across microwave links
ANALOG INTEROFFICE TRUNKS
• Small cities have one central office; large cities have multiple CO’s
Signal attenuation (∼ 3 dB/3.5 mi) becomes an issue for total path lengths
greater than about 12 mi
◦ Amplification is required for long links
◦ Amplification is not feasible on a wire pair carrying full-duplex signals
◦ Conversion between a two-wire channel and a four-wire channel is
accomplished at the CO using a hybrid
Hierarchical network architecture:
◦ Design a combination of direct interoffice trunks and tandem (trunk-totrunk)
switches to minimize call blocking probability for a given total
cost and average traffic pattern
Interoffice trunks:
◦ Originally open-wire, then multipair cables (common until optical fiber)
◦ Later: 12 voice channels frequency-multiplexed onto one pair
◦ Still later: Coaxial cable, carrying subcarrier-multiplexed signals.