Rivier College, CS699 Professional Seminar
IP Telephony
(Article Presentation)
by Samir Goswami
Source: http://www.dialogic.com
IP Telephony
• IP Telephony uses the Internet to send audio between two or more
computer users in real time, so that users can converse.
– Vocal Tec Introduced the first IP telephony software in 1995
• IP telephony technology has caught the attention of the world and
virtually every major telecommunication company has launched
research to better understand the latest threat to their market.
• The original Internet telephone products based on multimedia PCs
offer ability to combine voice and data on one network.
How Does It Work?
• In one side gateway connects to the telephone word. It can
communicate with any phone in the world.
• On the other side, the gateway connects to the Internet world. It can
communicate with any computer in the world.
• The gateway takes the standard telephone signal, digitizes it,
compresses it, packetizes it for the Internet using IP protocol and
routes it the destination over the Internet.
• The gateway reverses the operation for packets coming in from the
network and going out the telephone.
• Both the operations take place at the same time allowing a full duplex
conversation.
How Does It Work? (Continued)
Internet Telephony Gateway
Phone
Network
Connection
Translation
Function
Internet
Connection
Internet
PSTN
How Well Does It Work?
• There are two main factors contributing to quality
– Voice Quality
– Turnaround Time (Latency)
• Improve technologies for voice coding and lost packet reconstruction
have yielded products where speech is easy to understand
• Latency affects the pace of conversation. Humans can tolerate about
250 ms of latency. Today’s IP telephony products exceed this latency
• Latency will continue to improve driven by three factors
– Improved Gateways
– Deployment over private networks
– Internet development (Today’s Internet was not designed with Real
Time Communication in mind)
• Today connection sound like traditional calls, routed over a satellite
circuit which are usable but require some getting used to.
How Well Does It Work? (Continued)
• Echo Cancellation
– The Internet telephony gateways must perform echo cancellation.
Echo is not a problem in local calls because latency is not long
enough for echo to come back as separate transmission. But when
long distance calls are being made,echo will disrupt conversation if
it is not canceled. Since telephone system does not do echo
cancellation, IP telephony gateway must do it.
•
Full Duplex
– Phone calls are full duplex, so are IP telephony products
– Not all voice cards can support full duplex operation, so computer
must have full duplex voice card installed.
Telephone Interface
• The telephone connection of the gateway needs to exhibit
two critical features
– Approved version in all major countries
• Cost saving for IP telephony is on international call
– Must be scalable
• System might range from 2-several thousand lines
Call Control Protocol
• H323 is clearly emerging as the standard call control
protocol
• H323 specification defines
– Packet standards for terminal
– Equipment and services for multimedia
communications over LAN communicating to systems
connected to telephony networks such as ISDN
Voice Coders
• Key technical requirements for coders
– Low Bandwidth 8kp/s or less
– High quality for voice (3.5 mean opinion score)
– Low Latency
– Ability to reconstruct lost packets
• G.723.1 is emerging as popular coding choice
– G.723.1 is an algorithm for compressed digital audio
over telephone lines