Transcript h.323
H.323
Chapter 4
Introduction
We have learned
The set-up and tear-down of the sessions
IP, UDP, RTP
Signaling
In traditional telephony networks
ISUP, Integrated Services Digital Network User Part
A component of the Signaling System 7 (SS7)
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H.323, ITU-T recommendation
The 1st version, 1996
Visual Telephone Systems and Equipment for Local Area
Network which Provide A Non-Guaranteed Quality of Service
Version 2, 1998
Packet-based Multimedia Communications Systems
Widely implemented in VoIP solutions
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The H.323 Architecture
Entities
Terminals
Gateways
Gatekeepers
MCUs
Protocols
Parts of H.225.0 - RAS, Q.931
H.245
RTP/RTCP
Audio/video codecs
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H.323 Architecture
H.323 Network Architecture and Components
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Terminals
Endpoint on a LAN
Supports real-time, 2-way communications with
another H.323 entity
Must support:
Voice - audio codecs
Signaling and setup - Q.931, H.245, RAS
Optional support:
Video
Data
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Gateways
Interface between the LAN and the switched
circuit network
Mandatory Functions
Optional Function
Transmission Format Translation
Communication Procedure Translation
Call Setup and Clearing On Both Sides
Media Format Translation
Example: IP/PSTN gateway
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Gatekeepers
Optional, but must perform certain functions if
present
Manage a zone (a collection of H.323 devices)
Usually one gatekeeper per zone
e.g., Netmeeting does not use gatekeepers?
alternate gatekeeper might exist for backup and load
balancing
Typically a software application
implemented on a PC, but can be integrated in a
gateway or terminal
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Mandatory functions:
Address translation (routing)
Admission control
Minimal bandwidth control - request processing
Zone management
Optional functions:
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
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MCUs
MCU - Multipoint Control Unit
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
MP - receives streams from endpoints, processes
them, and returns them to the endpoints in the
conference
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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
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NOTE – Gateway, Gatekeeper and MCU can be a single device.
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Centralized Conference
MCU handles both signaling (MC) and stream
processing (MP)
MCU
(MC+MP)
media stream (unicast)
control message
Terminal
Terminal
Terminal
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De-centralized Conference
MCU handles only signaling
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
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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
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Mixed Multipoint Conference
Decentralized side Centralized side
Terminal
Terminal
Terminal
MCU
(MC+MP)
Terminal
Terminal
Terminal
multicast audio and video
unicast audio and video
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Overview of H.323 Singaling
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 is transported reliably over TCP
RAS - registration, admission, status (over UDP)
Q.931 - call setup and termination
H.245 - capabilities exchange
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H.323 Protocol Stack
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H.323 Protocol Stack
Control
Data
Audio
Video
G.7xx
H.225.0 H.245
A/V Cntl
Gatekeeper
H.26x
RTCP
T.120
RTP
TCP
Control
Reg,
Adm,
Status
(RAS)
UDP
IP
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Overview of H.323 Protocols
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
Call signaling or Q.931 signaling
RAS signaling
Registration, Admissions, and Status
Between endpoints and gatekeepers
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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
Q.931 to establish communication and set up the call
Call-signaling channel
H.245 to negotiate media parameters
RAS channel
H.245 control channel
Media streams over logical channels
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H.323 Addressing
An entity in the H.323 network
A network address, an IP address
URL, Uniform Resource Locator
E.g., ras://GK1@somedomain
A port number appended
RAS, a default port number = 1719
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 Port =1720
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Terminal and gateways
Have one or more aliases
Can take any number of forms
Must be unique within a zone
E.164 number
Codecs
Video codec is optional
G.711 (A-law and mu-law) is mandatory
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RAS Signaling
Used between a GK and endpoints in its zone
Since a GK is optional
Defined in H.225.0
GK Discovery
Registration
Unregistration
Admission
Bandwidth Change
Endpoint Location: GK (address translation)
Disengage
Status
Resource Availability: GW → GK
Non-standard
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Gatekeeper Discovery
Find a suitably accommodating GK
GRQ – GK-request
Several GKs, backup GK
Known addresses, multicast 224.0.1.41:1718
GK id: if empty, soliciting GKs
GCF – GK-Confirm
Optionally, indicating one or more GKs to try
I cannot help you, but try the GK next door
Only one GK can be choosed
GRJ – GK-Reject
With a reason (e.g., a lack of resource)
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GK Discovery
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Endpoint Reg and Reg Cancellation
To become controlled by a GK
RegistrationRequest (RRQ)
RAS signaling port is 1719
Includes
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)
May assign an alias
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UnregistrationRequest (URQ)
Cancel registration
By endpoints
By GKs
UnregistrationConfirm (UCF)
UnregistraionReject (URJ)
For an unregistered endpoint
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Endpoint Location
Request a real address of an alias
LocationRequest (LRQ)
LocationConfirm (LCF)
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)
The endpoint is not registered
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Admission
Request permission from a GK to participate in a call
AdmissionRequest (ARQ)
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
Aliases
Signaling address
Bandwidth (mandatory)
TransportQOS: endpoint or GK to reserve the resource
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AdmissionConfirm (ACF)
Many of the same parameters as ARQ
callModel
A firm order from the GK
Optional in ARQ; mandtory in ACF
The endpoint sends call signaling directly or via the GK
AdmissionReject (ARJ)
With a reason
Bandwidth, address translation, unregistered endpoint
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Direct call signaling
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GK-routed call signaling
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Bandwidth Change
Request an increase or decrease in allocated
bandwidth
BandwidthRequest (BRQ)
The new bandwidth, the call id
BCF
BRJ
The GK can also request an endpoint to change the
bandwidth
Can change without request if within the limit in ACF
The endpoint must comply
Closely tied to H.245 signaling (for logical channels)
A reduction in bandwidth
Require an existing logical channel to be closed and reopened
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An example bandwidth change
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Status
A GK be informed of the status of an endpoint
InformationRequestResponse (IRR)
Endpoint information
The active call information
IRQ
or ACF with an irrFrequency parameter
Call id, call reference value, call type, the bandwidth
RTP session information (CNAME, RTP/RTCP address, etc.)
willRespondToIRR parameter
IACK
INACK
An IRR message in error
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Disengage
The end of the call
DisengageRequest (DRQ)
DCF
DRJ
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
The endpoint must
Close the session
Respond to the GK with a DCF message
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Resource Availability
ResourceAvailableIndicate (RAI)
An gateway sends to a GK
The available call capacity and bandwidth for each protocol
almostOutofResource parameter
RseourceAvailableConfirm (RAC)
Request in Progress
RIP
A given request takes longer the the timeout period
The expected delay and the reason
After timeout, the RAS message can be retransmitted
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Call Signaling
For the establishment and tear-down of calls
Q.931 modified by rec. H.225.0
Reuse some messages with few modifications
Specify a number of rules regarding the usage of
information elements defined in Q.931
E.g., no Transit Network Selection, …
Certain Q.931 mandatory information forbidden or optional
A clever use of User-to-User information element
Convey all of the extra information needed in H.323
E.g., H.245 addresses to be used for logical channel
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Setup
The first call-signaling message
Q.931 Protocol Discriminator
A call reference
Bearer Capability
RTP information, such as payload type
A gateway needs to perform the mapping
User-to-User information element
Mandatory: Call id, call type, conference id, the caller
information
Optional: source alias, destination alias, H.245 address,
destination call-signaling address
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Call Proceeding
Optional
call-establishment procedures are underway
Mandatory
Protocol discriminator, call reference, and message type,
user-to-user
User-to-user: similar to the setup message
Alerting
The called user is being alerted
The same parameters as Call Proceeding
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Connect
The called party has accepted the call
Must be sent if the call is to be completed
Call Proceeding and Alerting are optional
User-to-User information
The same as Call Proceeding
Plus Conference Identifier
The same as Setup
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Release Complete
Terminate a call
No Release message
Cause information element, optional
In ISDN, Release and Release Complete
Otherwise, a Release reason in User-to-User
Facility (Q.932)
A call should be redirected
Also be used for supplementary services
User-to-User contains reason parameter
E.g., routeCallToGatekeeper
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Interaction between Call Signaling and H.245
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?
Between the Setup and Connect messages
Equipment dependent
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Call Scenarios
Basic Call without GKs
Reliable transport
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Call with GKs and Direct-Endpoint Call Signaling
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Gatekeeper routed/direct-endpoint call signaling
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Gatekeeper-routed call signaling
EP <-> GK <-> GK <-> EP
ARJ with a cause code of routeCallToGatekeeper
A Facility with a reason indicating the call be rerouted
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Optional
calledendpoint
signaling
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H.245 Control Signaling
To establish and control media streams
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?
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H.245 Message Groupings
Requests
Response
In reply to Requests
Commands
Require the recipient to perform some action and response
Require the recipient to perform some action, but no
response is necessary
Indications
Of an informational nature only
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The Concept of Logical Channels
A Logical channel
A two-party conversation
Logical channel number and media information (RTP payload type)
Far endpoint responds with Open Logical Channel Ack
Two logical channels exist
Potentially in different formats
Or a bidirectional channel consists of two logical channels
An endpoint issues Open Logical Channel
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)
Permanently open
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H.245 Procedures
Capabilities Exchange
Share information regarding receive and transmit
capabilities
Indicate a preference
TerminalCabilitySet message
TerminalCapabilitySetAck
A request message
A sequence number plus the types of audio and video
formats
Empty with a sequence number
TerminalCapabilitySetReject
With a reason for rejection
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TerminalCapabilitySetRelease
If no response within a timeout period
SendTerminalCapabilitySet
Request terminal Capability information
Request to indicate all of its capabilities
Or request confirmation about specific capabilities
A command message that is replied
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Master-Slave Determination
One of the endpoints needs to be the master
Of particular importance for the setup of a multi-party
conference
Compare two pieces of information at each entity
A terminal type value
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
A “master” or “slave” indication
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Establishing and Releasing Media Streams
For media exchange
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Open Unidirectional Logical Channel
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Open Bidirectional Logicl Channels
Fig. 4-15
Closing Logic Channels and Ending a session
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
EndSession, EndSession commands
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Close a Logical Channel
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A Slow Start
Plus
Capability exchange
Master-slave determination
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Fast-connect Procedure
The Setup
A faststart element
OpenLogicalChannel
requests
No H.245 control
channel
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Conference Calls
MC manage multi-point conference
A Pre-Arranged Conference
Establish a call with the MCU
The MCU specifies the conference mode
Communication Mode command (H.245)
Specify all the sessions in the conference
The transmit requirements of each endpoint, not the receive
requirements
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An Ad-Hoc Conference
Expand an existing two-party call
One the the endpoints must contain an MC
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New Features in H.323 Version 2
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:
Bypasses some setup messages
Triggered by Q.931 Fast Start message that contains
basic capabilities
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New Features in H.323 Version 2 (cont.)
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)
T.120 channel opened like any H.323 channel
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T.120 Data Channel
Defined in T.120
A Channel Between Endpoints
A Reliable Channel
Functions
Transport Data
Multipoint Delivery
Internetworking
PSTN, ISDN
LAN (TCP/IP, IPX)
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The Future of H.323
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
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H.323 Version 3
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
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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
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H.323 Version 4
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
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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
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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
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