Transcript H.323

H.323
Chapter 4
Introduction
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We have learned
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The set-up and tear-down of the sessions
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IP, UDP, RTP
Signaling
In traditional telephony networks
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ISUP, Integrated Services Digital Network User Part
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A component of the Signaling System 7 (SS7)
Internet Telephony
4-2
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H.323, ITU-T recommendation
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The 1st version, 1996
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Version 2, 1998
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Visual Telephone Systems and Equipment for Local Area
Network which Provide A Non-Guaranteed Quality of Service
Packet-based Multimedia Communications Systems
Widely implemented in VoIP solutions
Version 4
Internet Telephony
4-3
The H.323 Architecture
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Entities
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Terminals
Gateways
Gatekeepers
MCUs
Protocols
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Parts of H.225.0 - RAS, Q.931
H.245
RTP/RTCP
Audio/video codecs
Internet Telephony
4-4
H.323 Architecture
H.323 Network Architecture and Components
Internet Telephony
4-5
Internet Telephony
4-6
Terminals
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Endpoint on a LAN
Supports real-time, 2-way communications with
another H.323 entity
Must support:
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Voice - audio codecs
Signaling and setup - Q.931, H.245, RAS
Optional support:
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Video
Data
Internet Telephony
4-7
Internet Telephony
4-8
Gateways
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Interface between the LAN and the switched
circuit network
Mandatory Functions
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Optional Function
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Transmission Format Translation
Communication Procedure Translation
Call Setup and Clearing On Both Sides
Media Format Translation
Example: IP/PSTN gateway
Internet Telephony
4-9
Internet Telephony 4-10
Gatekeepers
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Optional, but must perform certain functions if
present
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Manage a zone (a collection of H.323 devices)
Usually one gatekeeper per zone
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e.g., Netmeeting does not use gatekeepers?
alternate gatekeeper might exist for backup and load
balancing
Typically a software application
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implemented on a PC, but can be integrated in a
gateway or terminal
Internet Telephony 4-11
 Mandatory functions:
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Address translation (routing)
Admission control
Minimal bandwidth control - request processing
Zone management
 Optional functions:
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Call control signaling - direct handling of Q.931
signaling between endpoints
Call authorization, bandwidth management, and call
management using some policy
Gatekeeper management information (MIB)
Directory services
Internet Telephony 4-12
MCUs
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MCU - Multipoint Control Unit
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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)
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MC - handles control and signaling for conference
support
MP - receives streams from endpoints, processes
them, and returns them to the endpoints in the
conference
Internet Telephony 4-13
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MC and MP
Terminal 1
Terminal 2
Gatekeeper 1
MC
Gatekeeper 2
MC
MC
Gatekeeper 3
MP
LAN
MC
Gateway 1
MC
MP
Gateway 2
MC
Gateway 3
MCU 1
MC
MP
MCU 2
T1521250-96
NOTE – Gateway, Gatekeeper and MCU can be a single device.
Internet Telephony 4-14
Centralized Conference
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MCU handles both signaling (MC) and stream
processing (MP)
MCU
(MC+MP)
media stream (unicast)
control message
Terminal
Terminal
Terminal
Internet Telephony 4-15
De-centralized Conference
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MCU handles only signaling
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streams go directly between endpoints
In this case MCU functions without MP
MCU
(MC)
media stream (multicast)
control message
Terminal
media stream
(multicast)
media stream (multicast)
control message
Terminal
Internet Telephony 4-16
Hybrid Multipoint Conference
Terminal
video stream
(multicast)
audio stream (unicast)
control message
MCU
(MC+MP)
video stream
(multicast)
audio stream (unicast)
audio stream (unicast)
control message
Terminal
video stream
(multicast)
control message
Terminal
Internet Telephony 4-17
Mixed Multipoint Conference
Decentralized side Centralized side
Terminal
Terminal
Terminal
MCU
(MC+MP)
Terminal
Terminal
Terminal
multicast audio and video
unicast audio and video
Internet Telephony 4-18
Overview of H.323 Singaling
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Audio codecs (G.711, G.723.1, G.728, etc.) and
video codecs (H.261, H.263)
Media streams transported on RTP/RTCP
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RTP carries actual media
RTCP carries status and control information
RTP/RTCP carried unreliably on UDP
Signaling is transported reliably over TCP
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RAS - registration, admission, status (over UDP)
Q.931 - call setup and termination
H.245 - capabilities exchange
Internet Telephony 4-19
H.323 Protocol Stack
Internet Telephony 4-20
Overview of H.323 Protocols
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H.225.0, a two-part protocol
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A variant of ITU-T recommendation Q.931, the ISDN
layer 3 spec.
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The set-up and tear-down of connections
Call signaling or Q.931 signaling
RAS signaling
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Registration, Admissions, and Status
Between endpoints and gatekeepers
Internet Telephony 4-21
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H.245, control protocols
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Used between two or more endpoints
Manage the media streams of a session
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Capability exchange
RAS, Q.931 and H.245
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RAS to obtain permission from a gatekeeper
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Q.931 to establish communication and set up the call
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Call-signaling channel
H.245 to negotiate media parameters
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RAS channel
H.245 control channel
Media streams over logical channels
Internet Telephony 4-22
H.323 Addressing
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An entity in the H.323 network
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A network address, an IP address
URL, Uniform Resource Locator
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E.g., ras://GK1@somedomain
A port number appended
RAS, a default port number = 1719
The TSAP, Transport Service Access Point
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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 Port =1720
Internet Telephony 4-23
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Terminal and gateways
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Have one or more aliases
Can take any number of forms
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Must be unique within a zone
E.164 number
Codecs
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Video codec is optional
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H.261 QCIF
G.711 (A-law and -law) is mandatory
Internet Telephony 4-24
RAS Signaling
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Used between a GK and endpoints in its zone
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Since a GK is optional
Defined in H.225.0
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GK Discovery
Registration
Unregistration
Admission
Bandwidth Change
Endpoint Location: GK (address translation)
Disengage
Status
Resource Availability: GW → GK
Non-standard
Internet Telephony 4-25
Gatekeeper Discovery
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Find a suitably accommodating GK
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GRQ – GK-request
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Several GKs, backup GK
Known addresses, multicast 224.0.1.41:1718
GK id: if empty, soliciting GKs
GCF – GK-Confirm
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Optionally, indicating one or more GKs to try
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I cannot help you, but try the GK next door
Only one GK can be choosed
GRJ – GK-Reject
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With a reason (e.g., a lack of resource)
Internet Telephony 4-26
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GK Discovery
Internet Telephony 4-27
Endpoint Reg and Reg Cancellation
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To become controlled by a GK
RegistrationRequest (RRQ)
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RAS signaling port is 1719
Includes
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An address for RAS messages
An address for call-signaling messages
An alias
One or more redundant addresses and aliases
Optional TTL, keepAlive parameters
RegistrationReject (RRJ)
RegistrationConfirm (RCF)
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May assign an alias
Internet Telephony 4-28
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UnregistrationRequest (URQ)
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Cancel registration
By endpoints
By GKs
UnregistrationConfirm (UCF)
UnregistraionReject (URJ)
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For an unregistered endpoint
Internet Telephony 4-29
Endpoint Location
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Request a real address of an alias
LocationRequest (LRQ)
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LocationConfirm (LCF)
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To a GK or the GK discovery multicast address
A GK can also send an LRQ
A call-signaling address
An RAS signaling address
LocationReject (LRJ)
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The endpoint is not registered
Internet Telephony 4-30
Admission
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Request permission from a GK to participate in a call
AdmissionRequest (ARQ)
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The type of the call (e.g., two-party or multi-party)
The endpoint’s own id
A call identifier (a unique string)
A call-reference value (an integer)
Information of the other party
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Aliases
Signaling address
Bandwidth (mandatory)
TransportQOS: endpoint or GK to reserve the resource
Internet Telephony 4-31
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AdmissionConfirm (ACF)
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Many of the same parameters as ARQ
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callModel
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A firm order from the GK
Optional in ARQ; mandtory in ACF
The endpoint sends call signaling directly or via the GK
AdmissionReject (ARJ)
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With a reason
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Bandwidth, address translation, unregistered endpoint
Internet Telephony 4-32
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Direct call signaling
Internet Telephony 4-33
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GK-routed call signaling
Internet Telephony 4-34
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Pregranted Admission
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to minimize call setup delay
A GK provide an endpoint with admission in advance
the RCF includes the parameter preGrantedARQ
Internet Telephony 4-35
Bandwidth Change
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Request an increase or decrease in allocated
bandwidth
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BandwidthRequest (BRQ)
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The new bandwidth, the call id
BCF
BRJ
The GK can also request an endpoint to change the
bandwidth
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Can change without request if within the limit in ACF
The endpoint must comply
Closely tied to H.245 signaling (for logical channels)
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A reduction in bandwidth
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Require an existing logical channel to be closed and reopened
Internet Telephony 4-36
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An example bandwidth change
Internet Telephony 4-37
Status
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A GK be informed of the status of an endpoint
InformationRequestResponse (IRR)
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Endpoint information
The active call information
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IRQ
or ACF with an irrFrequency parameter
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Call id, call reference value, call type, the bandwidth
RTP session information (CNAME, RTP/RTCP address, etc.)
willRespondToIRR parameter
IACK
INACK
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An IRR message in error
Internet Telephony 4-38
Disengage
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The end of the call
DisengageRequest (DRQ)
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DCF
DRJ
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Call id, call reference value, a disengage reason (e.g.,
normalDrop)
For an unregistered endpoint
not a call party
The GK might issue DRQ to an endpoint
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The endpoint must
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Close the session
Respond to the GK with a DCF message
Internet Telephony 4-39
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Resource Availability
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ResourceAvailableIndicate (RAI)
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An gateway sends to a GK
The available call capacity and bandwidth for each protocol
almostOutofResource parameter
RseourceAvailableConfirm (RAC)
Internet Telephony 4-40
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Service Control
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SCI (ServiceControlIndication)
SCR (ServiceControlResponse)
to enable advanced features
Request in Progress
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RIP
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A given request takes longer the the timeout period
The expected delay and the reason
After timeout, the RAS message can be retransmitted
Internet Telephony 4-41
Call Signaling
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For the establishment and tear-down of calls
Q.931 modified by rec. H.225.0
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Reuse some messages with few modifications
Specify a number of rules regarding the usage of
information elements defined in Q.931
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E.g., no Transit Network Selection, …
Certain Q.931 mandatory information forbidden or optional
A clever use of User-to-User information element
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Convey all of the extra information needed in H.323
E.g., H.245 addresses to be used for logical channel
Internet Telephony 4-42
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Setup
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The first call-signaling message
Q.931 Protocol Discriminator
A call reference
Bearer Capability
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RTP information, such as payload type
A gateway needs to perform the mapping
User-to-User information element
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Mandatory: Call id, call type, conference id, the caller
information
Optional: source alias, destination alias, H.245 address,
destination call-signaling address
Internet Telephony 4-43
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Call Proceeding
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Optional
call-establishment procedures are underway
Mandatory
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Protocol discriminator, call reference, and message type,
user-to-user
User-to-user: similar to the setup message
Alerting
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The called user is being alerted
The same parameters as Call Proceeding
Internet Telephony 4-44
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Progress
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sent by a called gateway
the Cause information – in-band tones
the User-to-User info. – same as Call-Proceeding
Connect
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The called party has accepted the call
Must be sent if the call is to be completed
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Call Proceeding and Alerting are optional
User-to-User information
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The same as Call Proceeding
Plus Conference Identifier
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The same as Setup
Internet Telephony 4-45
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Release Complete
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Terminate a call
No Release message
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Cause information element, optional
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In ISDN, Release and Release Complete
Otherwise, a Release reason in User-to-User
Facility (Q.932)
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A call should be redirected
Also be used for supplementary services
User-to-User contains reason parameter
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E.g., routeCallToGatekeeper
Internet Telephony 4-46
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Interaction between Call Signaling and H.245
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Call signaling: call establishment and tear-down
H.245: the negotiation and establishment of media
streams
When to begin the exchange of H.245 messages?
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Between the Setup and Connect messages
Equipment dependent
Internet Telephony 4-47
Call Scenarios
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Basic Call without GKs
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Reliable transport
Internet Telephony 4-48
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Call with GKs and Direct-Endpoint Call Signaling
Internet Telephony 4-49
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Gatekeeper routed/direct-endpoint call signaling
Internet Telephony 4-50
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Gatekeeper-routed call signaling
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EP <-> GK <-> GK <-> EP
ARJ with a cause code of routeCallToGatekeeper
A Facility with a reason indicating the call be rerouted
Internet Telephony 4-51
Internet Telephony 4-52
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Optional
calledendpoint
signaling
Internet Telephony 4-53
H.245 Control Signaling
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To establish and control media streams
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Agree on the media formats and bandwidth
Multiplexing multiple media streams
No actual media
A generic protocol for the control of media streams
How it works in an H.323?
Internet Telephony 4-54
H.245 Message Groupings
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Requests
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Response
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In reply to Requests
Commands
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Require the recipient to perform some action and response
Require the recipient to perform some action, but no
response is necessary
Indications
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Of an informational nature only
Internet Telephony 4-55
The Concept of Logical Channels
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A Logical channel
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A two-party conversation
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Logical channel number and media information (RTP payload type)
Far endpoint responds with Open Logical Channel Ack
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Two logical channels exist
Potentially in different formats
Or a bidirectional channel consists of two logical channels
An endpoint issues Open Logical Channel
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A unidirectional media path
An IP address and port number
Has a number
An RTP port
Messages over H.245 Control Channel (channel number 0)
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Permanently open
Internet Telephony 4-56
H.245 Procedures
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Capabilities Exchange
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Share information regarding receive and transmit
capabilities
Indicate a preference
TerminalCabilitySet message
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TerminalCapabilitySetAck
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A request message
A sequence number plus the types of audio and video
formats
Empty with a sequence number
TerminalCapabilitySetReject
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With a reason for rejection
Internet Telephony 4-57
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TerminalCapabilitySetRelease
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If no response within a timeout period
SendTerminalCapabilitySet
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Request terminal Capability information
Request to indicate all of its capabilities
Or request confirmation about specific capabilities
A command message that is replied
Internet Telephony 4-58
Master-Slave Determination
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One of the endpoints needs to be the master
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Of particular importance for the setup of a multi-party
conference
Compare two pieces of information at each entity
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A terminal type value
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A terminal without an MC: 50
A gateway without an MC: 60
An MCU for audio, vedio: 190
An MCU managing a conference: 240 (the highest)
A random number (1..16,777,215)
Master-Slave Determination message
Master-Slave Determination Ack
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A “master” or “slave” indication
Internet Telephony 4-59
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Establishing and Releasing Media Streams
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For media exchange
Internet Telephony 4-60
Open Unidirectional Logical Channel
Internet Telephony 4-61
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Open Bidirectional Logicl Channels
Internet Telephony 4-62
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Closing Logic Channels and Ending a session
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CloseLogicalChannel, CloseLogicalChannelAck
Only the initiator can issue
Or the receiving end can humbly request
A bidirectional channel can be closed by either end
Once all logical channels are closed
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EndSession, EndSession commands
Internet Telephony 4-63
Close a Logical Channel
Internet Telephony 4-64
A Slow Start
Plus
Capability exchange
Master-slave determination
Internet Telephony 4-65
Fast-connect Procedure
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The Setup
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A faststart element
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OpenLogicalChannel
requests
No H.245 control
channel
Internet Telephony 4-66
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H.245 Message Encapsulation
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is encapsulated with Q.931 messages
Set the element h245Tunneling true in the first Q.931
message
The h245Control element contains the encapsulated
data
Conflict with the faststart element
Use Facility
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when no Q.931 message can be used
to start a separate H.245 control channel
Internet Telephony 4-67
Conference Calls
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MC manage multi-point conference
A Pre-Arranged Conference
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Establish a call with the MCU
The MCU specifies the conference mode
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Communication Mode command (H.245)
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Specify all the sessions in the conference
The transmit requirements of each endpoint, not the receive
requirements
Internet Telephony 4-68
An Ad-Hoc Conference
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Expand an existing two-party call
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One the the endpoints must contain an MC
Internet Telephony 4-69
New Features in H.323 Version 2
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H.235 - security and authentication, i.e.
passwords for registration with gatekeeper
H.450.x - supplementary services such as call
transfer and forwarding
Fast call setup:
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Bypasses some setup messages
Triggered by Q.931 Fast Start message that contains
basic capabilities
Internet Telephony 4-70
New Features in H.323 Version 2 (cont.)
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Mechanism to specify alternative gatekeepers to
endpoints
Gatekeeper can request forwarding of Q.931 information
on direct routed calls; only RADCOM can play back
H.323 streams off a network: a true differentiation
Smoother integration of T.120 (optional standard for
data)
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T.120 channel opened like any H.323 channel
Internet Telephony 4-71
T.120 Data Channel
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Defined in T.120
A Channel Between Endpoints
A Reliable Channel
Functions
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Transport Data
Multipoint Delivery
Internetworking
 PSTN, ISDN
 LAN (TCP/IP, IPX)
Internet Telephony 4-72
The Future of H.323
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Inter-Gatekeeper Communication:
Current H.323 standards do not provide an
inter-zone model that scales well for large networks
Inter-gatekeeper protocols being discussed to enable
gatekeepers to efficiently locate one another to route
calls to non-local address
Hierarchical arrangements with “clearing house”
gatekeepers have been proposed
This is critical for widespread interoperability between
VoIP service providers
Internet Telephony 4-73
H.323 Version 3
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Modest improvements to H.323 version 2
Maintaining and Reusing Connections
Conference out of Consultation
Caller ID
Language Preference
Usage of Annex E/H.323
Remote Device Control
Generic Capabilities
Annex G/H.225.0 - Communication between
Administrative Domains
Internet Telephony 4-74
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Annex E/H.323 - Protocol for Multiplexed Call
Signaling Transport
Annex F/H.323 - Simple Endpoint Type
H.341 - H.323 Series MIB
Supplementary Services
Internet Telephony 4-75
H.323 Version 4
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Many new enhancements
Gateway Decomposition
Multiplexed Stream Transmission
Supplementary Services
Annex K/H.323
Annex L/H.323
H.450.8 - Name Identification Service
H.450.9 - Call Completion
H.450.10 - Call Offer
H.450.11 - Call Intrusion
Internet Telephony 4-76
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Additive Registrations
Alternate Gatekeepers
Usage Information Reporting
Endpoint Capacity
Caller Identification Service
Tones and Announcements
Mapping Aliases
Indicating Desired Protocols
Bandwidth Management
Reporting Call Status
Internet Telephony 4-77
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Enhancements to Annex D (Real-Time Fax)
Call Linkage
Tunneling
QoS
H.245 in Parallel with Fast Connect
Generic Extensibility Framework
H.323 URL
Call Credit-Related Capabilities
DTMF Relay via RTP
Internet Telephony 4-78
The Decomposed Gateway
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Media gateway
Media gateway controller
Internet Telephony 4-79