PHYSICAL LAYER : Public Switched Telephone Network(PSTN)

Physical layer:
Public Switched Telephone Network (PSTN)

Short for Public Switched Telephone Network, which refers to the international telephone system based on copper wires carrying analog voice data. This is in contrast to newer telephone networks base on digital technologies, such as ISDN and FDDI.Telephone service carried by the PSTN is often called Plain Old Telephone Service(POTS). 

The history of the PSTN is a history of monopoly, regulation, antitrust, deregulation, patents and right-of-way issues. Mergers permitted by the deregulation in 1996 have produced large regional carriers with near-monopolistic control over certain markets.Here is where the regional 'Baby Bells' ended up.

Manual routing at local exchange office (CO)

The PSTN circa 1900

PSTN  A domestic telecommunications network usually accessed by telephones, key telephone systems, private branch exchange trunks, and data arrangements. Note: Completion of the circuit between the call originator and call receiver in a PSTN requires network signaling in the form of dial pulses or multifrequency tones. (As defined in FS-1037-C)

• PSTN switching is based on circuit switching by duplex connections
• Temporary bidirectional connections
• Originally for speech (voice) only at 300-3400 Hz
• Earlier two subscribers connected by purely physical
• connection (physical switch contacts)Nowadays by time slots ~ ISDN is integrated to PSDN
• PCM is the TDMA standard for the digital transmission
• PCM time slots consist of 8 bit samples
• For voice digital exchange sets up 64 kbit/s connections
• Data connections by (1) modems, (2) ISDN interface (3) leased lines via X.25 / Frame relay, or (4) ADSL

• Structure
• The Local Loop
• Trunks And Multiplexing
• Switching

   Network Structure

• Minimize Number Of Wires
• Add Multiple Levels

Typical Circuit

The Local Loop

• MODEMS
• (A)DSL
• WIRELESS

Modems – Sine Wave Carrier

TECHNIQUES

•  POTS (Plain Old Telephone Service) – Frequency Multiplexing

•  DMT (Discrete MultiTone)

  

   DSL Equipment

•  ADSL  Modem: 250 QAM modems

•  DSLAM: Digital Subscriber Line Access Multiplexer

Trunks And Multiplexing

•  Frequency division multiplexing (FDM)

•  Wavelength division multiplexing (WDM)

•  Time division multiplexing (TDM)

 aside on compression

•  SONET

Frequency Division Multiplexing

    

     Wavelength Division Multiplexing

•   FDM for optical

Time Division Multiplexing (T1)

•  codec 8000 samples / second (4000 Hz signals)

•  125 µsec / sample

•  PCM (Pulse Coded Modulation)

•  codec is multiplexed between 24 analog lines

•  each analog line inserts 8 bits every 125 µsec

•  7*8000=56000 bps / channel

•  1.544 Mbps aggregate

Framing 

•  Framing bit is used to recover the sender’s clock

•  The framing bit is 010101010101010....

•  I.e., 4000 Hz

•  Filtered for analog customer (3100 Hz filter)

•  Not likely for digital customers

Multiple T1's

•  Bitwise multiplexing

•  4 to 1; 7 to 1; and 6 to 1

•  Overhead added at each step for framing

•  Recovery

•  T2 and T4 are only used inside the phone company

Compression

•  Differential modulation (send change to value, rather than value)

•  Delta modulation (shown) +1 or -1

•  Predictive encoding: extrapolate from earlier values and then send the change to this extrapolation.

SONET (Synchronous Optical NETwork)

•  STS-1 Synchronous Transport Signal-1

•  SPE – Synchronous Payload Envelope

SONET Rates

•  Gross – signalling rate

•  SPE – excludes line and selection overhead

•  User – excludes path overhead

SWITCHING

PSTN has successfully concluded as

There are potential services for the future...

 

•    Multimedia conferencing

•    Free calls (paid by advertisers)

– with the profiling feature

•    Speed Call list provisioning

•    Service Subscription

•    Follow-me service provisioning

•    Access to communications bills

•    Account information changes

•    VPN Provisioning