Transcript IP Telephony

IP Telephony
Voice-Data Convergence
What is IP Telephony?
• Transmit telephone conversations in IP
packets sent over the Internet or another
network, such as Ethernet, Frame Relay, or
ATM carrying IP packets
IP Packet
IP Telephony
• Digitize the outgoing voice signal
• Packetize (place in packets) and send over
IP packet-switched networks
• Reverse at other end
Digitize
Packetize
Why IP Telephony?
• Save Money
Digitize
64 kbps
• Digitizing Voice More Efficiently
– Current telephone system also digitizes voice for
internal digital communication within the telephone
network
– Current telephone system generates a stream of 64
kbps, then steals 8 kbps for signaling, leaving 56
kbps
– This is a lot of bits to move per second on expensive
long-distance and international lines
Why IP Telephony?
• Save Money
• Digitizing Voice More Efficiently
– With newer technology, IP telephony
digitizes voice to between 12 kbps and 16
kbps with good quality
– Fewer bits to send means lower transmission
cost
Digitize
12 kbps16 kbps
Why IP Telephony?
Packetize
• Save Money
• Packet-Switched Network Delivery
– Traditional telephony is circuit-switched
– Charged for 56 kbps channels whether use
them or not
– Packet switching multiplexes transmissions
– Only pay for capacity actually used
Why IP Telephony?
• Save Money
• Voice-Data Convergence
– Corporations now have separate networks for voice
and data
– This is expensive in terms of staffing labor and
technical charges
– Voice-data convergence: use one network (IP) for
both
– Reduces staff and technical costs
Why IP Telephony?
• Save Money: Recap
– Efficient digitization to send less than 8 kbps
– Multiplexing on packet-switched networks
Internet connection is already in place
– Voice-data convergence reduces staff, other
costs
Problem of Latency
• Packet-Switched Networks Often Have Latency
(Delay)
• Latency is Bad for Voice Conversations
– At latency of 200 milliseconds (ms), conversation is
difficult because of turn-taking awkwardness
– At latency of 500 ms, conversation is impossible
– Variable latency from one packet to the next makes
voice sound jittery (jitter)
Reducing Latency
• Problem is the Internet Backbone
– Often has high latency
• ISPs Have Lower Latency Internally
– May offer service level agreements (SLAs) for
latency
Site
ISP
Internet
Backbone
Usually
Low Latency
Often
High Latency
ISP
Site
Reducing Latency
• Solution
– Connect all corporate sites to a single ISP
– Possible because some ISPs have access points
in many places around the country or even
around the world
Site
Single ISP
SLA for
Low Latency
Site
Reducing Latency
• Solution
– Connect all corporate sites to a single packetswitched PSDN
– Packet-switched PSDNs also have low latency,
SLAs
Site
PSDN
SLA for
Low Latency
Site
Evolution of IP Telephony
• Initially, Only Use Between Sites
– Sites already have PBXs (Private Branch
Exchanges) to handle internal site telephony
– Add IP telephony modules to PBXs
– Communicate over the Internet in IP
– Use traditional telephony within sites
– Saves on long-distance, which is expensive
Site
Site
Evolution of IP Telephony
• Initially, Only Use Between Sites
– No need to change system within sites
– Such changes would be expensive and would
not reduce long-distance and international
calling charges
– Little agreement about what technology to
use in internal telephone systems
Site
Site
Evolution of IP Telephony
• Eventually, Service to the Desktop
– Combine with data service to the desktop
– Computer-Telephony Integration (CTI) applications
• When caller calls in, automatically linked with relevant data
• To improve applications, not necessarily to save money
– Problem: technology is not readily available or
standardized
IP Telephony Standards
• Based on H.323 Videoconferencing
Standard from ITU-T
– For videoconferencing over the Internet or
other IP networks
• IETF, ITU-T have agreed to work together
on IP telephony standards
Questions about IP Telephony
• Will it Really Save Money?
– Was very promising when long-distance and
international costs were very high
– But these costs are falling rapidly and will
continue to do so
– Will it save enough money to be worthwhile?
• Will it be Sufficiently Reliable?
– Ordinary telephony is super reliable
– Can IP telephony offer sufficient reliability?