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1 IP Telephony (VoIP) CSI4118 Fall 2005. 2 Introduction (1) A recent application of Internet...

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1 IP Telephony (VoIP) CSI4118 Fall 2005
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Page 1: 1 IP Telephony (VoIP) CSI4118 Fall 2005. 2 Introduction (1) A recent application of Internet technology – Voice over IP (VoIP): Transmission of voice.

1

IP Telephony (VoIP)

CSI4118

Fall 2005

Page 2: 1 IP Telephony (VoIP) CSI4118 Fall 2005. 2 Introduction (1) A recent application of Internet technology – Voice over IP (VoIP): Transmission of voice.

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Introduction (1)

• A recent application of Internet technology – Voice over IP (VoIP): Transmission of voice over Internet

• How VoIP works

– Continuously sample audio– Convert each sample to digital form– Send digitized stream across Internet in packets– Convert the stream back to analog for playback

• Why VoIP– IP telephony is economic; High costs for traditional telephone

switching equipments.

Page 3: 1 IP Telephony (VoIP) CSI4118 Fall 2005. 2 Introduction (1) A recent application of Internet technology – Voice over IP (VoIP): Transmission of voice.

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Introduction (2)

• Challenge– Voice transmission delay– Call setup: call establishment, call termination, etc. – Backward compatibility with existing PSTN (Public

Switched Telephone Network)

• IP Telephony Standards:– ITU (International Telecommunication Union) controls

telephony standards.– IETF (Internet Engineering Task Force) controls

TCP/IP standards.

Page 4: 1 IP Telephony (VoIP) CSI4118 Fall 2005. 2 Introduction (1) A recent application of Internet technology – Voice over IP (VoIP): Transmission of voice.

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Encoding, Transmission, & Playback (1)

• Both groups agree on the basics for encoding and transmission of audio:

– Audio is encoded using a well-known standard such as Pulse Code Modulation (PCM).

– Audio is transferred using the Real-time Transport Protocol (RTP).

– RTP message is encapsulated in a UDP datagram that is further encapsulated in an IP datagram for transmission.

Page 5: 1 IP Telephony (VoIP) CSI4118 Fall 2005. 2 Introduction (1) A recent application of Internet technology – Voice over IP (VoIP): Transmission of voice.

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Encoding, Transmission, & Playback (2)

• UDP is used for transport because– lower overhead: audio must be played as it arrives.– Playback cannot be stopped to wait for a

retransmitted packet.

• Two independent RTP sessions exist, because an IP phone call involves transfer in two directions– IP phone acts as sender for outgoing data, and – IP phone acts as receiver for incoming data.

Page 6: 1 IP Telephony (VoIP) CSI4118 Fall 2005. 2 Introduction (1) A recent application of Internet technology – Voice over IP (VoIP): Transmission of voice.

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Signaling Systems & Protocols

• Main complexity of VoIP: Call setup and call management.

• The process of establishing and terminating a call is called Signaling.

– In traditional telephone system, signaling protocol is SS7 (signaling System 7).

– In VoIP, signaling protocols are:• SIP (Session Initiation Protocol), by IETF• H.323, by ITU• Megaco & MGCP, jointly by IETF and IUT.

– VoIP signaling protocols should be able to interact with SS7.

Page 7: 1 IP Telephony (VoIP) CSI4118 Fall 2005. 2 Introduction (1) A recent application of Internet technology – Voice over IP (VoIP): Transmission of voice.

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A Basic IP Telephone System

• The simplest IP telephone system uses two basic components:

- IP telephone: end device allowing humans to place and receive calls.- Media Gateway Controller: providing overall control and coordination between IP phones; allowing a caller to locate a callee (e.g. call forwarding)

Page 8: 1 IP Telephony (VoIP) CSI4118 Fall 2005. 2 Introduction (1) A recent application of Internet technology – Voice over IP (VoIP): Transmission of voice.

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Interconnection with Others (1)

• IP telephone system needs to interoperate with PSTN or another IP telephone system.

• Two additional components needed for such interconnection:– Media Gateway– Signaling Gateway

Page 9: 1 IP Telephony (VoIP) CSI4118 Fall 2005. 2 Introduction (1) A recent application of Internet technology – Voice over IP (VoIP): Transmission of voice.

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Interconnection with Others (2)

• Media gateway: translates audio between IP network and PSTN.

• Signaling Gateway: translates signaling operations.

Page 10: 1 IP Telephony (VoIP) CSI4118 Fall 2005. 2 Introduction (1) A recent application of Internet technology – Voice over IP (VoIP): Transmission of voice.

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Signaling Protocols

• Two major protocols: H.323, SIP

• H.323, invented by ITU, defines four elements that comprising a signaling system:– Terminal: IP phone– Gatekeeper: provides location and signaling functions;

coordinates operation of Gateway.– Gateway: used to interconnect IP telephone system

with PSTN, handling both signaling and media translation.

– Multipoint Control Unit: provides services such as multipoint conferencing.

Page 11: 1 IP Telephony (VoIP) CSI4118 Fall 2005. 2 Introduction (1) A recent application of Internet technology – Voice over IP (VoIP): Transmission of voice.

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Signaling Protocols

• SIP: Session Initiation Protocol. Invented by IETF.• SIP defines three main elements that comprise a

signaling system:– User Agent: IP phone or applications– Location servers: stores information about user’s

location or IP address– Support servers:

• Proxy Server: forwards requests from user agents to another location.

• Redirect Server: provides an alternate called party’s location for the user agent to contact.

• Registrar Server: receives user’s registration requests and updates the database that location server consults.

Page 12: 1 IP Telephony (VoIP) CSI4118 Fall 2005. 2 Introduction (1) A recent application of Internet technology – Voice over IP (VoIP): Transmission of voice.

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H.323 Characteristics• H.323 consists of a set of protocols that work

together to handle all aspects of communication, including:– Transmission of a digital audio phone call– Signaling to set up and manage phone call– Allows transmission of video and data while a phone

call is in progress– Sends binary message– Incorporates protocols for security– Uses a special hardware Multipoint Control Unit for

conferencing calls– Defines servers for address resolution, authentication,

accounting, features, etc.

Page 13: 1 IP Telephony (VoIP) CSI4118 Fall 2005. 2 Introduction (1) A recent application of Internet technology – Voice over IP (VoIP): Transmission of voice.

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H.323 Layering

• H.323 uses both UDP and TCP over IP.– Audio travels over UDP– Data travels over TCP

Page 14: 1 IP Telephony (VoIP) CSI4118 Fall 2005. 2 Introduction (1) A recent application of Internet technology – Voice over IP (VoIP): Transmission of voice.

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SIP Characteristics• Operates at the application layer.

• Encompasses all aspects of signaling, e.g. location of called party, ringing a phone, accepting a call, and terminating a call.

• Provides services such as call forwarding.

• Relies on multicast for conference calls.

• Allows two sides to negotiate capabilities and choose the media and parameters to be used.

• SIP URI is similar to email address. (with prefix “sip:”) E.g. sip:[email protected]

Page 15: 1 IP Telephony (VoIP) CSI4118 Fall 2005. 2 Introduction (1) A recent application of Internet technology – Voice over IP (VoIP): Transmission of voice.

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SIP Methods

• Six basic message types, known as methods:

Page 16: 1 IP Telephony (VoIP) CSI4118 Fall 2005. 2 Introduction (1) A recent application of Internet technology – Voice over IP (VoIP): Transmission of voice.

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An Example SIP Session

• User agent A contacts DNS server to map domain name in SIP request to IP address.

• User agent A sends a INVITE message to proxy server that uses location server to find the location of user agent B.

• Call is established between A and B. Then media session begins.

• Finally, B terminates the call by sending a BYE request.

Page 17: 1 IP Telephony (VoIP) CSI4118 Fall 2005. 2 Introduction (1) A recent application of Internet technology – Voice over IP (VoIP): Transmission of voice.

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Telephone Number Mapping & Routing (1)

• How should users be named?– PSTN follows ITU standard E.164 for phone numbers. E.g. 1-

613-123-4567– SIP uses IP addresses. E.g. sip:[email protected]

• In an integrated network (PSTN + IP), two problems defined:– Locate a user– Find a efficient route to the user

• IETF proposed two protocols: – ENUM: E.164 NUMbers– TRIP: Telephone Routing over IP

Page 18: 1 IP Telephony (VoIP) CSI4118 Fall 2005. 2 Introduction (1) A recent application of Internet technology – Voice over IP (VoIP): Transmission of voice.

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Telephone Number Mapping & Routing (2)

• ENUM

– Converting E.164 phone number into a Uniform Resource Identifier (URI)

– Using Domain Name System to store mapping

– A phone number is converted into a special domain name: e164.arpa

• E.g. 1-800-555-1234 4.3.2.1.5.5.5.0.0.8.e164.arpa

Page 19: 1 IP Telephony (VoIP) CSI4118 Fall 2005. 2 Introduction (1) A recent application of Internet technology – Voice over IP (VoIP): Transmission of voice.

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Telephone Number Mapping & Routing (3)

• TRIP

– Finding a user in an integrated network

– Used by location server or other NEs to advertise routes

– Independent of signaling protocols

– Dividing the world into a set of IP Telephone Administrative Domains (ITADs)

Page 20: 1 IP Telephony (VoIP) CSI4118 Fall 2005. 2 Introduction (1) A recent application of Internet technology – Voice over IP (VoIP): Transmission of voice.

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IP Telephones and Electrical Power

• Analog telephone system continues to work when electrical power are unavailable

– The wires that connect a telephone to the central office supply the power

• Currently, IP telephones have to depend on an external source of power

– IP phones must have both network connection and power connection.

– Several mechanism proposed to integrate power with network connections.

Page 21: 1 IP Telephony (VoIP) CSI4118 Fall 2005. 2 Introduction (1) A recent application of Internet technology – Voice over IP (VoIP): Transmission of voice.

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Summary (1)• IP telephony or VoIP refers to the transmission of voice telephone

calls over IP networks.

• Hot area both in research and market because of low cost

• Challenge in backward compatibility with PSTN

• The complexity of IP telephony is on signaling. Both ITU and IETF propose signaling standards.

– H.323, by IUT – SIP, by IETF, offering similar functions to H.323, but simpler than H.323.– Both are competing to be recognized as #1 signaling protocol

Page 22: 1 IP Telephony (VoIP) CSI4118 Fall 2005. 2 Introduction (1) A recent application of Internet technology – Voice over IP (VoIP): Transmission of voice.

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Summary (2)• H.323 uses a set of protocols for call setup and management

• SIP uses a set of servers to handle various aspects of signaling

• ENUM maps an E.164 telephone number into a URI (usually SIP URI)

• TRIP provides routing among IP telephone administrative domains

• IP telephones depends on external power, while analog phones don’t.


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