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PSTN and VoIP. Chapter 1. PSTN-Public Switched Telephone Network. Ring Down Circuit-Direct connection between two phones with no signaling. Original phone system invented by Bell Pick up phone and talk –No ringing. Switched Telephone.

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pstn and voip


Chapter 1

pstn public switched telephone network
PSTN-Public Switched Telephone Network
  • Ring Down Circuit-Direct connection between two phones with no signaling. Original phone system invented by Bell
  • Pick up phone and talk –No ringing
switched telephone
Switched Telephone
  • Original switch was a telephone operator who manually connected the two voice paths.
  • Now switching is electronic
calling pennsylvania 6 5000
Calling Pennsylvania 6 5000
  • The telephone number identified a specific pair of wires
  • Circuit switched to complete a connection
  • VoIP phone numbers no longer tied to a specific set of wires but to a TCP/IP address.
  • Packet switched network
  • Original telephone signal was Analog –continuous voice wave.
  • Analog uses amplifiers resulting in accumulated noise caused by amplifying the signal and the noise.
  • Digital signals use a repeater which amplifies and cleans the signal.
  • Repeater does NOT amplify noise
pulse code modulation
Pulse Code Modulation
  • To convert an analog voice wave to digital we use PCM.
  • Filter the signal to eliminate frequencies over 4000 Hz
  • Sample the signal at 8000 Hz
  • Convert to digital – 8 bits
  • Nyquist criteria –Must sample at greater than twice the modulating frequency
local loop
Local Loop
  • Local Loop – Home telephone to central office switch
  • Trunk – path between central office switches
  • Switches connect trunks
  • User to Network Signaling – End user to PSTN
  • DTMF – Dual Tone Multi Frequency

Each digit is represented by TWO frequencies to eliminate errors

network to network signaling
Network to Network Signaling
  • Switches communicate with each other
  • Uses SS7
  • T1 lines over twisted pair
  • T3 line over coax
  • T3 over microwave
  • Signaling System 7 is out of band signaling.
  • SS7 is a method of sending messages between switches for basic call control.
  • Reduced post dialing delay
  • Increased call completion
  • Connection to the IN (Intelligent Network)
drawbacks to pstn
Drawbacks to PSTN
  • Data has overtaken voice as the primary traffic on networks built for voice
  • PSTN cannot create and deploy features quickly enough
  • Data/Voice/Video cannot converge on PSTN
  • Voice architecture not flexible enough to carry data
real time protocol
Real Time Protocol
  • RTP is used to carry voice data
  • RTP runs on UDP
  • RTP streams – RTP packet flows
  • TCP/IP – TCP causes retransmission of packet.
  • Not possible or desirable with RTP/UDP
  • RTP uses timestamps instead
  • Jitter – variation of interpacket arrival time or the difference between when a packet is supposed to arrive and when it is actually received
pstn numbering plans
PSTN Numbering Plans
  • Shortage of phone numbers
  • Must use all ten digits in most areas
  • Overlay- One area has multiple area codes
    • Existing area code has another “overlayed”
h 323
  • ITU-T specification for transmitting audio, data and video across an IP network
  • Translates between audio, video and data transmission formats
  • Media Gateway Control Protocol
  • Soft Switch
  • Control endpoints
  • Entire IP network acts as a virtual switch
  • P 28
session initiation protocol
Session Initiation Protocol
  • SIP – An Application layer protocol for creating, modifying and terminating sessions with one or more participants