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IP Telephony (VoIP)
Introduction (1)
• 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.
Encoding, Transmission, & Playback (1)
• 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.
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.
• A Basic IP Telephone System
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
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.
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.
– H.323 Characteristics
• H.323 Layering
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:bob[at]somewhere.com
SIP Methods
• Six basic message types, known as methods:
• An Example SIP Session
Telephone Number Mapping & Routing (1)
• 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 è 4.3.2.1.5.5.5.0.0.8.e164.arpa
• 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)
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.
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
• 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.