Speaker : Hsuan-Ling Weng Advisor : Dr. Kai-Wei Ke Date: 2015/11/05 1

2  VoIP  SIP  Features  Network elements  Operation  Messages  Message flow of calling procedure  H.323  Typical H.323 Stack  Network Elements  Zone  RAS  H.225 Call Signaling  H.245  Message flow of calling procedure  References

 Voice over IP (VoIP) is a methodology and group of technologies for the delivery of voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet.  Other terms commonly associated with VoIP are IP telephony, Internet telephony, broadband telephony, and broadband phone service. 3

 Session Initiation Protocol (SIP) is the core protocol for initiating, managing and terminating sessions in the Internet.  These sessions may be text, voice, video or a combination of these.  SIP sessions involve one or more participants and can use unicast or multicast communication. 4

 Find out IP Address(Location) of users  Session Initiation  Session Management 5

 User agent:  A SIP user agent (UA) is a logical network end-point used to create or receive SIP messages and thereby manage a SIP session.  A SIP UA can perform the role of a user agent client (UAC), which sends SIP requests, and the user agent server (UAS), which receives the requests and returns a SIP response. 6

 Proxy server  The proxy server is an intermediary entity that acts as both a server and a client for the purpose of making requests on behalf of other clients.  A proxy server primarily plays the role of routing, meaning that its job is to ensure that a request is sent to another entity closer to the targeted user. 7

 Registrar  A registrar is a SIP endpoint that accepts REGISTER requests and places the information it receives in those requests into a location service for the domain it handles. 8

 Redirect server  A user agent server that generates 3xx (Redirection) responses to requests it receives, directing the client to contact an alternate set of URIs.  The redirect server allows proxy servers to direct SIP session invitations to external domains. 9

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 There are two different types of SIP messages: requests and responses.  SIP request  REGISTER: Used by a UA to register to the registrar.  INVITE: Used to establish a media session between user agents.  ACK: Confirms reliable message exchanges.  CANCEL: Terminates a pending request.  BYE: Terminates an existing session.  OPTIONS: Requests information about the capabilities of a caller without the need to set up a session. Often used as keepalive messages.  REFER: indicates that the recipient (identified by the Request-URI) should contact a third party using the contact information provided in the request. (call transfer) 11

 SIP response  Provisional (1xx): Request received and being processed.  Success (2xx): The action was successfully received, understood, and accepted.  Redirection (3xx): Further action needs to be taken (typically by sender) to complete the request.  Client Error (4xx): The request contains bad syntax or cannot be fulfilled at the server.  Server Error (5xx): The server failed to fulfill an apparently valid request.  Global Failure (6xx): The request cannot be fulfilled at any server 12

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 H.323 was first approved in February 1996, the same month that the first SIP draft was published.  Designed to operate over complex networks, such as the Internet.  First standards-based “Voice over IP”.  H.323 is a multimedia conferencing protocol, which includes voice, video, and data conferencing, for use over packet-switched networks. 14

 H.323 – “Umbrella” document that describes the usage of H.225.0, H.245, and other related documents for delivery of packet-based multimedia conferencing services.  H – Describes three signaling protocols (RAS, Call Signaling, and “Annex G”).  H.245 – Multimedia control protocol (common to H.310, H.323, and H.324). 15

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 Terminal  Terminals are the client endpoints on the LAN that provide real-time, two- way communications. All terminals must support voice communications; video and data are optional.  All H.323 terminals must also support H.245, which is used to negotiate channel usage and capabilities. Three other components are required: Q.931 for call signaling and call setup, a component called Registration/Admission/Status (RAS), which is a protocol used to communicate with a Gatekeeper; and support for RTP/RTCP for sequencing audio and video packets. T 17

 Gateway  The Gateway is composed of a “Media Gateway Controller” (MGC) and a “Media Gateway” (MG), which may co-exist or exist separately  The MGC handles call signaling and other non-media-related functions  The MG handles the media  Gateways interface H.323 to other networks, including the PSTN, H.320 systems, other H.323 networks (proxy), etc. GW 18

 MCU  Responsible for managing multipoint conferences (two or more endpoints engaged in a conference)  The MCU contains a Multipoint Controller (MC) that manages the call signaling and may optionally have Multipoint Processors (MPs) to handle media mixing, switching, or other media processing MCU 19

 Gatekeeper  The Gatekeeper is an optional component in the H.323 system which is used for admission control and address resolution  The gatekeeper may allow calls to be placed directly between endpoints or it may route the call signaling through itself to perform functions such as follow-me/find-me, forward on busy, etc. 20 GK

 A single Gatekeeper and all of the devices connected to it  There may be more than one physical Gatekeeper device that provides the logical Gatekeeper functionality for a zone GK GW MCU T There is no imposed limit on the number or types of devices in a zone 21

 Registration, Admission, and Status  Used between the endpoint and its Gatekeeper in order to  Allow the Gatekeeper to manage the endpoint  Allow the endpoint to request admission for a call  Allow the Gatekeeper to provide address resolution functionality for the endpoint  RAS signaling is required when a Gatekeeper is present in the network (i.e., the use of a Gatekeeper is conditionally mandatory) 22

 RAS messages generally have three types  Request (xRQ)  Reject (xRJ)  Confirm (xCF)  Exceptions are  Information Request / Response / Ack / Nak  The “nonStandardMessage”  The “unknownMessage” response  Request in Progress (RIP)  Resource Available Indicate / Confirm (RAI/RAC)  Service Control Indication / Response 23

 Typically, RAS communications is carried out via UDP through port 1719 (unicast) and 1718 (multicast)  For backward compatibility sake, an endpoint should be prepared to receive a unicast message on port 1718 or 1719  Only UDP is defined for RAS communications  GRQ and LRQ may be send multicast, but are generally sent unicast  All other RAS messages are sent unicast 24

 Gatekeeper Request - GRQ  When an endpoint comes to life, it should try to “discover” a gatekeeper by sending a GRQ message to a Gatekeeper  Address of a Gatekeeper may be provisioned  The endpoint may send a multicast GRQ  Address of a Gatekeeper may be found through DNS queries (Annex O/H.323)  There may be multiple Gatekeepers that could service an endpoint, thus an endpoint should look through potentially several GCF/GRJ messages for a reply 25

 Gatekeeper Registration – RRQ  Once a Gatekeeper has been “discovered”, the endpoint will then register with the Gatekeeper in order to receive services  Communication is exclusively via port 1719 (unicast)  Endpoint will send an RRQ and expect to receive either an RCF or RRJ 26

 Admission Request - ARQ  Once registered with a Gatekeeper, the endpoint may only initiate or accept a call after first requesting “admission” to the Gatekeeper via the ARQ message  The Gatekeeper may accept (ACF) or reject (ARJ) the request to place or accept a call 27

 Location Request - LRQ  The LRQ message is sent by either an endpoint or a Gatekeeper to a Gatekeeper in order to resolve the address of an alias address (e.g., to turn a telephone number into an IP address)  While LRQs may be sent by endpoints, they are almost exclusively sent by Gatekeepers 28

 Bandwidth Request - BRQ  Subsequent to initial call setup, the endpoint may wish to use more or less bandwidth than previously indicated via the BRQ  Note that, while it is syntactically legal for the GK to send a BRJ to a request asking for less bandwidth, this makes no sense and should not be done  An endpoint must send a BRQ subsequent to initial call establishment if the actual bandwidth utilized is less than initially requested 29

 Disengage Request - DRQ  Once a call completes, the endpoint sends a DRQ message to the Gatekeeper  The Gatekeeper may send a DRJ, but this is strongly discouraged… if an endpoint is sending a DRQ, it means the call is over and cannot be “rejected”!  The Gatekeeper may also send a DRQ to force the endpoint to disconnect the call 30

 Information Request - IRQ  The IRQ is sent by the Gatekeeper to the endpoint to request information about one or all calls  There are many details about each call that are reported to the Gatekeeper in the Information Response (IRR) message 31

 H Call Signaling is used to establish calls between two H.323 entities  It was derived from Q.931 (ISDN call signaling), but was modified to be suitable for use on a packet based network  H also borrows messages from Q

 H Call Signaling Messages  Setup  Call Proceeding  Alerting  Information  Release Complete  Facility  Progress  Status  Status Inquiry  Setup Acknowledge  Notify  Connect 33

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 H.245 provides “control” to the multimedia session that has been established  Terminal capability exchange  Master/Slave determinations  Logical channel signaling  Closing the H.245 Control Channel 35

 Four H.245 Message Types(and examples of each)  Request  masterSlaveDetermination  terminalCapabilitySet  Response  masterSlaveDeterminationAck  terminalCapabilitySetAck  Command  sendTerminalCapabilitySet  Indication  userInput 36

 Capabilities Exchange  The capability exchange (or “caps exchange”) allows two endpoints to exchange information about what media capabilities they possess, such as G.711, G.723, H.261, and H.263  Along with the type of media, specific details about the maximum number of audio frames or samples per packet is exchanged, information about support for silence suppression, etc. are exchanged  Using this capability information, endpoints can select preferred codes that are suitable to both parties 37

 Master Slave Determination  Once capabilities are exchanged, the endpoints negotiate master and slave roles  The master in a point to point conference really only has the power to indicate when channels are in conflict (e.g., when one the other terminal tries to open a channel that is not compatible)  The slave device must yield to the requests of the master device and reconfigure channels appropriately 38

 Logical Channel Signaling  Channels are opened by exchanging “openLogicalChannel” (OLC) messages  The OLC will contain one of the capabilities that was previously advertised by the other endpoint  Within the OLC, a “session ID” is assigned  Session 1 is the default audio session, 2 is the default video session, and 3 is the default data session 39

 Closing the H.245 Control Channel  H.323 specifies that, in order to close the H.245 Control Channel, the endpoint must:  Close all open logical channels  Wait for all acknowledgement messages  Send an “endSession” command  Wait for an “endSession” from the other side  In reality, most endpoint vendors don’t bother– they just use the H Release Complete command to terminate the call and close the H.245 Control Channel, as that is much more efficient 40

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 [1] Voice over IP, URL:  [2] Session Initiation Protocol, URL:  [3] H.323 Papers and Presentations, URL:  [4] 黃威穎著，「 H.323 網路電話音訊監控與錄製系統之研製」，碩士論文，國 立台北科技大學資訊工程系碩士班，台北， 2008 。  [5] 蔡家瑞著，「客製化 H.323 協定之至慧型網路電話監控語錄音系統」，碩士 論文，國立台北科技大學資訊工程系碩士班，台北， 2009 。  [6] 張以磊著，「分散式網路事件分析紀錄系統之研製」，碩士論文，國立台北 科技大學資訊工程系碩士班，台北， 2013 。 42

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