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

DigitizeDigitize PacketizePacketize

Why IP Telephony?

• Save Money

• 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

DigitizeDigitize 64 kbps

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

DigitizeDigitize12 kbps-16 kbps

Why IP Telephony?

• 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

PacketizePacketize

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

SiteSite ISPISP InternetBackbone

InternetBackbone ISPISP SiteSite

OftenHigh Latency

UsuallyLow Latency

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

SiteSite Single ISPSingle ISP SiteSite

SLA forLow Latency

Reducing Latency

• Solution– Connect all corporate sites to a single packet-

switched PSDN– Packet-switched PSDNs also have low latency,

SLAs

SiteSite PSDNPSDN SiteSite

SLA forLow Latency

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

SiteSite SiteSite

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

SiteSite SiteSite

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?