H.323

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H.323

• Chapter 4

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Introduction

• We have learned
• IP, UDP, RTP
• 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)

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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
• Version 4

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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
• Transmission Format Translation
• Communication Procedure Translation
• Call Setup and Clearing On Both Sides
• Optional Function
• Media Format Translation
• Example IP/PSTN gateway

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Gatekeepers

• Optional, but must perform certain functions if
  present
• e.g., Netmeeting does not use gatekeepers?
• Manage a zone (a collection of H.323 devices)
• Usually one gatekeeper per zone
• 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

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Centralized Conference

• MCU handles both signaling (MC) and stream
  processing (MP)

MCU (MCMP)
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)
media stream (multicast)
control message
control message
Terminal
Terminal
media stream (multicast)
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Hybrid Multipoint Conference
Terminal
audio stream (unicast)
control message
MCU (MCMP)
video stream (multicast)
video stream (multicast)
audio stream (unicast)
control message
audio stream (unicast)
control message
Terminal
Terminal
video stream (multicast)
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Mixed Multipoint Conference
Decentralized side
Centralized side
Terminal
Terminal
MCU (MCMP)
Terminal
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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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
• 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

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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_at_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
• H.261 QCIF
• G.711 (A-law and ?-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
• Several GKs, backup GK
• GRQ GK-request
• Known addresses, multicast 224.0.1.411718
• 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)
• To a GK or the GK discovery multicast address
• A GK can also send an LRQ
• LocationConfirm (LCF)
• 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 endpoints 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
• A firm order from the GK
• callModel
• 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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• P regranted Admission
• to minimize call setup delay
• A GK provide an endpoint with admission in
  advance
• the RCF includes the parameter preGrantedARQ

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Bandwidth Change

• Request an increase or decrease in allocated
  bandwidth
• Can change without request if within the limit in
  ACF
• BandwidthRequest (BRQ)
• The new bandwidth, the call id
• BCF
• BRJ
• The GK can also request an endpoint to change
  the bandwidth
• 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
• Call id, call reference value, call type, the
  bandwidth
• RTP session information (CNAME, RTP/RTCP address,
  etc.)
• IRQ
• or ACF with an irrFrequency parameter
• willRespondToIRR parameter
• IACK
• INACK
• An IRR message in error

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Disengage

• The end of the call
• DisengageRequest (DRQ)
• Call id, call reference value, a disengage reason
  (e.g., normalDrop)
• DCF
• DRJ
• 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)

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• Service Control
• SCI (ServiceControlIndication)
• SCR (ServiceControlResponse)
• to enable advanced features
• 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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• Progress
• sent by a called gateway
• the Cause information in-band tones
• the User-to-User info. same as Call-Proceeding
• 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
• In ISDN, Release and Release Complete
• Cause information element, optional
• 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 lt-gt GK lt-gt GK lt-gt EP
• ARJ with a cause code of routeCallToGatekeeper
• A Facility with a reason indicating the call be
  rerouted

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• Optional called-endpoint 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
• Require the recipient to perform some action and
  response
• Response
• In reply to Requests
• Commands
• 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 unidirectional media path
• An IP address and port number
• Has a number
• A two-party conversation
• Two logical channels exist
• Potentially in different formats
• Or a bidirectional channel consists of two
  logical channels
• An endpoint issues Open Logical Channel
• Logical channel number and media information (RTP
  payload type)
• Far endpoint responds with Open Logical Channel
  Ack
• 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
• A request message
• A sequence number plus the types of audio and
  video formats
• TerminalCapabilitySetAck
• 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

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• 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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• H.245 Message Encapsulation
• 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
• when no Q.931 message can be used
• to start a separate 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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The Decomposed Gateway

• Media gateway
• Media gateway controller