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�� 5H.323 (II)

Calling a public phone from the Internet

Conferencing

Telephone Number Mapping (ENUM)

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� Complex case for calling4 calling a public phone from the Internet, using a gatekeeper

� Call across multiple zones

� Conferencing4 MC and MP

4 Create and join the conference

4 H.332

� ENUM

� Future Work


Complex case for callingComplex case for calling


Complex case for calling

� The situation is:4 IP address is dynamic� in most ISPs

4 Using the Gatekeeper

4 Registration -- ability to know that someone has logged on and can bereached at a particular terminal, sometimes called IpresenceJ

4 Admission -- checking the right to access resources

4 Status -- monitoring the availability of telephone-related networkresources, such as gateways and terminals

� Zone �� set of all H.323 endpoints, conference servers (MCUs) or

gateways managed by a single gatekeeper



� Locating the Gatekeeper4 The endpoint will contain the information about the address of the gatekeeper.

4 If not, it will contain the TFTP server address whereit can download the

configuration file.

� Find the gatekeeper4 a H.323 terminal should send a multicast Gatekeeper Request (GRQ) to thegroup address 224.0.1.41 on UDP port 1718

4 GRQ message will specify whether it is willing to contact a particular

gatekeeper

4 GRQ message will contain its aliases, allowing a gatekeeper to reply only to

specific groups of terminals



� Locating the Gatekeeper4 GRQ has very low TTL (time to live), for finding the local first and then

expand the ring.

4 One or more gatekeeper will reply with Gatekeeper Confirm (GCF)

4 Name of the gatekeeper

4 Unicast address and port that are used for RAS

4 Or names and trnasport information about other gatekeepers.



� Locating the Gatekeeper



� Registration4 A terminal sends the Registration Request (RRQ) message (UDP 1719)

4 A gatekeeper replies with a Registration Confirm (RCF) message

4 Gatekeeper can send keepAlive RRQs to specify a maximum time to live

of a terminal



� Requesting permission to make a new call4 Found the gatekeeper and registered

4 Needs permission to call +33 123456789

4 Sends an Admission Request (ARQ) message to the gatekeeper

� ARQ message includes:4 A sequential number.

4 The GK-assigned terminal identifier.

4 The type of call (point to point).

4 The call model that the terminal is willing to use (direct or gatekeeper-routed)



� ARQ message includes:4 The destination information (in this case the E.164 address +33 123456789, but

it could also have been email alias).

4 A Call Reference Value (CRV), which should be copied in the SETUP

message.

4 A globally unique CallID.

4 An estimation of the bidirectional bandwidth that will be used for this call for

media streams. This includes audio and video that will be sent from the called

party and is measured excluding network overhead.

� If gatekeeper decided to receive the call4 Send the AdmissionConfirm (ACF) message



� 2 different calling establishments:4 Direct model

4 The calling endpoint can establish the channels directly with the called

endpoint

4 Gatekeeper routed model

4 The calling endpoint establish the channels with the gatekeeper which will

relay the call-signaling and call control information to the called endpoint



� Direct model - Call signaling



� Direct model T Termination phrase



� Direct model T Advantages4 Very simple implementation, very few messages must be supported.

4 The implementation can be made almost stateless if the accounting functions

are external.

4 The established calls are not affected if the gatekeeper fails.

4 More importantly for marketing purposes, since the gatekeeper really does not

do much, the manufacturer can claim the great performance figure of several

hundred calls per second!



� gatekeeper-routed model4 Handles the call-signaling information

4 Does not let the endpoint establish calls directly.

4 Can act as multiprotocol softswitches acting both as an H.323 routed mode

gatekeeper and as a SIP proxy with access to enough information to convert

between signaling protocols

4 e.g., H.323 and SIP



� gatekeeper-routed model


Calls across multiple zones



� Direct call model T Call setup4 Using LRQ (Location Request) and LFC (Location Confirm)



� Direct call model T Call tear-down



� Gatekeeper-routed modelT Call setup



� Gatekeeper-routed modelT Call tear-down


Conferencing


Conferencing

� MC (Multipoint Controller)4 Provides the Control Functions

4 decides who is allowed to participate

4 how new participants are introduced in existing conferences

4 who is allowed to broadcast media, etc.

� MP (Multipoint Processor)4 receives audio, video, and/or data streams and distributes them to endpoints

participating in a multipoint conference

4 mixing or switching of incoming media streams

4 send only the resulting processed outgoing stream to each terminal


Conferencing

� Setup4 Someone wants to establish the conference

4 Sends SETUP to the MCU with conferenceGoal=Create and CID

4 If MCU decides to accept e replies with CONNECT

4 Exchanges the TerminalCapabilitySets

4 The master/slave procedure begins e MCU will always win

4 Invites the others to join the conference.


Conferencing

� Inviting new participants


Conferencing

� Joining an existing conference4 Sending a SETUP message to the MCU with the CID at the conference and conferenceGoal=join.

4 If the terminal only knows the Alias of the conference, leave the CID at 0.

4 Most commercial MCUs use this simpler model, where all participants calling the same number are automatically bridged into the same conference.

4 This model does not suppose any support for the H.323 conferencing features at the endpoint.

4 In order to secure the conference and prevent any random user from calling the bridge directly, the call can be routed though a routed mode gatekeeper, which decides on the fly if the participant is allowed and may translate the initial called party number of the SETUP message into a conference number


Conferencing

� Browsing existing conferences

4 The MCU can in theory provide a list of existing conferences that a

terminal could join by sending a conferenceListChoice H.245

message to a terminal.

4 This can be used, for instance, when an alias that has been used in the

SETUP message is in fact the name of a group of conferences

4 e.g., [email protected] might be a group name for

Q931support, H245support, and [email protected]

4 Again most commercial MCUs use a simpler Web-based

administrator interface to browse for ongoing conferences.


Conferencing

� H.3324 describes the electronic equivalent of a panel conference

4 loosely coupled conference

4 is designed to scale to thousands of participants

� Mix of a usual tightly coupled conference (used by permanent speakers)

and a multicast RTP/RTCP conference (as known on the mBone) for

passive listeners

4 The difficult part is to allow the panel to invite a listener to talk, to let listeners request and be granted the right to ask a question


Conferencing

� H.332


ENUM


ENUM

� ENUM4 Resolves the telephone number into a call control resource information

4 Refined and expanded by the Telephone Number Mapping working group of

the IETF, resulting in RFC 2916

4 Decomposes any telephone number into a pseudo host name

4 +46-8-9761234

4 all non-digit characters (−, .) are removed

4 the number is written in reverse order (right to left)

4 E.g. the new domain name is E164.arpa

4 4.3.2.1.6.7.9.8.6.4.e164.arpa


Future Work


Future work

� Pre-connect announcements. -- still not flexible enough to allow multiple

media servers to stream media to the calling endpoint� H.323v5 solves

this problem with H.460.6.

� Call release scenarios using Q.931 messages IforgottenJ by the H.323

standard (e.g., the DISCONNECT message).

� Precise rules on how to transport and use the Iprogress indicatorJ, which

specifies whether in-band information is present or not (it should be

ignored when present)


Future work

� Precise rules on how to interwork with the ISDN network, and in

particular the handling of media-type information (3.1-kHz audio, fax)

� H.323v4 does not handle call loop detection in a robust way (i.e., with

hop counters in every message and mapping rules with SS7 ISUP

messages). H.323v5 solves this problem.

� H.323 is, with MGCP, the most mature of VoIP protocols today

� H.323 has also entered the 3G space: the H.324-M standard used for

videoconferencing of recent 3G handsets also uses H.245 for session

control

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