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H.323 Introduction We have learned IP, UDP, RTP (RTCP) How voice - PowerPoint PPT Presentation

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H.323 Introduction We have learned IP, UDP, RTP (RTCP) How voice is carried in RTP packets between session participants How does one party indicate to another a desire to set up a call? How does the second party indicate a

  1. H.323

  2. Introduction � We have learned � IP, UDP, RTP (RTCP) � How voice is carried in RTP packets between session participants � How does one party indicate to another a desire to set up a call? � How does the second party indicate a willingness to accept the call? � The set-up and tear-down of the sessions � Signaling � In traditional telephony networks � ISUP, Integrated Services Digital Network User Part � A component of the Signaling System 7 (SS7) 2 IP Telephony

  3. H.323, ITU-T Recommendation � The 1 st version, 1996 � Visual Telephone Systems and Equipment for Local Area Network which Provides a Non-Guaranteed Quality of Service � Its scope was multimedia communications over LAN. � Version 2, 1998 � Packet-based Multimedia Communications Systems � Widely implemented in VoIP solutions � The most recent version is H.323 version 4. 3 IP Telephony

  4. The H.323 Architecture � Entities � Terminals � Gateways � Gatekeepers � Multipoint Control Unit (MCU) � Protocols � Registration, Admission and Status (RAS) Signaling � Call Signaling (Q.931) � H.245 � RTP/RTCP � Audio/video codecs � The objective of H.323 is to enable the exchange of media streams between H.323 endpoints (e.g., termianl, gateway, MCU) 4 IP Telephony

  5. H.323 Architecture Gateway H.323 Terminal Wireless H.323 Terminal Gatekeeper router router Gateway I SDN router MCU Gateway PSTN Enterprise Network Gateway 5 IP Telephony

  6. Terminals [1/2] � Offering real-time, two-way communications with other H.323 endpoints � Must support: � Voice - audio codecs � Signaling and setup - Q.931, H.245 � RTP/RTCP � Optional support: � Video � Data � RAS signaling 6 IP Telephony

  7. Terminals [2/2] Audio Codec G.711 Microphone/ Speaker G.723 G.729 RTP Video Codec H.261 Camera/ Display H.263 LAN Data Interface Data Equipment T.120 Interface System Control H245 Control System Control User Interface Q.931 Call Setup RAS Gatekeeper Interface 7 IP Telephony

  8. Gateways [1/2] � Interface between the LAN and the switched circuit networks (SCNs, e.g., ISDN, GSM, PSTN) � Mandatory Functions � Transmission Format Translation � Communication Procedure Translation � Call Setup and Clearing on Both Sides � Optional Function � Media Format Translation � Example: IP/PSTN gateway 8 IP Telephony

  9. Gateways [2/2] Gateway function H.323 Translation SCN LAN SCN Terminal (Transmission formats/ Terminal Function Communication procedures) Function I P/ PSTN Gateway H.323 Protocol Conversion SCN Terminal and Transmission Terminal Function Translation Function PSTNh I P Phone 9 IP Telephony

  10. Multipoint Control Unit [1/2] � MCU � Endpoint that supports conferences between 3 or more endpoints � Can be stand-alone device or integrated into a gateway, gatekeeper or terminal � Typically consists of multi-point controller (MC) and multi-point processor (MP) � MC - handles control and signaling for conference support (controls multipoint conference) � MP - receives streams from endpoints, processes them, and returns them to the endpoints in the conference (provides media switching or mixing) 10 IP Telephony

  11. Multipoint Control Unit [2/2] � MC and MP Terminal 1 Terminal 2 Gatekeeper 1 Gatekeeper 2 Gatekeeper 3 MC MC MC MP LAN MC MC MC MP MC MP Gateway 1 Gateway 2 Gateway 3 MCU 1 MCU 2 T1521250-96 NOTE ? Gateway, Gatekeeper and MCU can be a single device. 11 IP Telephony

  12. Multipoint Conference � A Conference Between Three or More Endpoints � Controlled by an MC � Types � Centralized � Decentralized � Mixed 12 IP Telephony

  13. Centralized Conference � MCU handles both signaling (MC) and stream processing (MP) MCU MCU (MC+MP) (MC+MP) media stream (unicast) control message Terminal Terminal Terminal Terminal Terminal Terminal 13 IP Telephony

  14. De-centralized Conference � MCU handles only signaling � streams go directly between endpoints � In this case MCU functions without MP MCU MCU (MC) (MC) control message control control message Terminal message Terminal media stream media stream (multicast) (multicast) media stream Terminal Terminal Terminal (multicast) Terminal 14 IP Telephony

  15. Mixed Conference Decentralized side Centralized side Terminal Terminal Terminal Terminal MCU MCU Terminal Terminal Terminal (MC+MP) Terminal (MC+MP) Terminal Terminal Terminal Terminal multicast audio and video unicast audio and video 15 IP Telephony

  16. Gatekeepers � Optional, but must perform certain functions if present � e.g., Netmeeting does not use gatekeepers? � Authorize network access � Manage a zone (a collection of H.323 endpoints) � Terminals, gateways, multipoint controllers (MCs) � Ensure QoS if used in conjunction with bandwidth and/or resource management techniques � Usually one gatekeeper per zone � Alternate gatekeeper might exist for backup and load balancing. � Mandatory functions: � Address translation (routing) � Admission control � Bandwidth control � Zone management 16 IP Telephony

  17. H.323 Zone Terminal Terminal Terminal Gatekeeper IP-based IP-based Network Network Router Gateway MCU Gateway Router Terminal Terminal 17 IP Telephony

  18. Overview of H.323 Signaling [1/2] � Audio codecs (G.711, G.723.1, G.728, etc.) and video codecs (H.261, H.263) � Media streams transported on RTP/RTCP � RTP carries actual media � RTCP carries status and control information � RTP/RTCP carried unreliably on UDP � Signaling � RAS - registration, admission, status (over UDP) � Q.931 - call setup and termination (over TCP or UDP) � H.245 - capabilities exchange (over TCP) 18 IP Telephony

  19. Q.931 Over TCP or UDP? � The establishment of a TCP connection takes a little time, which can lead to a delay in call setup. � Both TCP and UDP can be used in parallel. � The sending entity sends the first message using UDP and simultaneously establishes a TCP connection. � If no response has been received, the TCP connection is used. 19 IP Telephony

  20. H.323 Protocol Stack Audio/Video Terminal/Application Application Control Audio/Video H.225.0 H.255.0 H.245 Codes RTCP RAS Call Control Signaling Signaling Signaling RTP Unreliable Transport (UDP) Reliable Transport (TCP) Network Layer (IP) Data Link Layer Physical Layer 20 IP Telephony

  21. Overview of H.323 Protocols [1/2] � H.225.0, a two-part protocol � A variant of ITU-T recommendation Q.931, the ISDN layer 3 spec. � The set-up and tear-down of connections between H.323 endpoints � Call signaling or Q.931 signaling � RAS signaling � Registration, Admissions, and Status � Between endpoints and gatekeepers � Used by a gatekeeper to manage the endpoints within its zone 21 IP Telephony

  22. Overview of H.323 Protocols [2/2] � H.245, control protocols � Used between two or more endpoints � Manage the media streams of a session � Capability exchange � RAS, Q.931 and H.245 � RAS to obtain permission from a gatekeeper � RAS channel � Q.931 to establish communication and set up the call � Call-signaling channel � H.245 to negotiate media parameters � H.245 control channel � Media streams over logical channels 22 IP Telephony

  23. H.323 Addressing � An entity in the H.323 network has � A network address (e.g., an IP address) � URL, Uniform Resource Locator (if DNS is available) � E.g., ras://GK1@somedomain � The TSAP, Transport Service Access Point � An id for a particular logical channel at a given entity � GK UDP Discovery Port = 1718 � GK UDP Reg. and Status Port = 1719 � Call-signaling TCP or UDP Port = 1720 � Registered with IANA � Terminals and gateways � Have one or more aliases � Can take any number of forms � Must be unique within a zone � E.164 number � It can correspond to the telephone numbers that are reachable at the PBX (private branch exchange). 23 IP Telephony

  24. RAS Signaling [1/2] � Used between a GK and endpoints in its zone � Functions � GK Discovery enables an endpoint to determine which gatekeeper is available to control it. � Registration/Unregistration enables an endpoint to register/unregister with a particular gatekeeper. � Admission is used by an endpoint to request access to the network for the purpose of participating in a session. � Bandwidth Change � Used by an endpoint to request the gatekeeper to allocate extra bandwidth to the endpoint � Used by a gatekeeper to instruct an endpoint to reduce the amount of bandwidth consumed. 24 IP Telephony

  25. RAS Signaling [2/2] � Endpoint Location � The gatekeeper translates an alias to a network address. � Disengage is used by an endpoint to inform a gatekeeper that it is disconnecting from a particular call. � Status is used between the gatekeeper and endpoint to inform the gatekeeper � About the health of an endpoint � About certain call-related data, such as current bandwidth usage � Resource Availability � Used to inform the gatekeeper of an endpoint ’ s currently available capacity � Non-standard 25 IP Telephony

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