Overview of H.225.0
Recommendation H.225.0 is drafted based on Q.931 and Q.932. Recommendation Q.931 is ISDN user-network interface layer-3 specification for basic call control. Recommendation Q.932 specifies the generic procedures for the control of ISDN supplementary services.
H.225.0 Messages
I. Overview of H.225.0 messages
H.225.0 basic call control messages are borrowed from Q.931 and Q.932. The former provides a greater share. As H.225.0 call signaling messages contain no call connection information, many Q.931 and Q.932 messages are thereby simplified here. Here, we will focus on H.225.0 call signaling messages only.
II. Call establishment messages
Table 4-10 Description of call establishment messages
| Message name | Meaning |
| Setup | To initiate call establishment. |
| Setup Acknowledge | To indicate that call establishment has been initiated, and request for subsequent address information. |
| Call Proceeding | To respond to Setup message, and indicate that requested access connection establishment has been initiated, and called number is complete. |
| Alerting | To indicate that the called subscriber alerting has been initiated. |
| Connect | To indicate acceptance of the access connection. |
| Progress | To indicate the progress of an access connection establishment in the event of interworking within a private network. |
III. Call clearing messages
Table 4-11 Description of call clearing messages
| Message name | Meaning |
| Release Complete | To indicate that the equipment sending the message has released the channel (if any) and call reference (CR). |
IV. Miscellaneous messages
Table 4-12 Description of call miscellaneous messages
| Message name | Meaning |
| Information | To provide additional information, such as subsequent called address. |
| Notify | To indicate information pertaining to a call, such as call suspension and resume. |
| Status Enquiry | To solicit a STATUS message from the peer layer-3 entity. |
| Status | To respond to the STATUS ENQUIRY message, or at any time during a call to report certain error conditions. |
| Facility | To convey supplementary service information to the network. |
| User Information | To transfer information to a remote subscriber, or to send to the subscriber to deliver information from the other subscriber. |
V. Q.932 messages
| Message name | Meaning |
| User Information | To transfer information to a remote subscriber, or to send to the subscriber to deliver information from the other subscriber. |
Message Format
I. Overview of message format
Figure 4-10 illustrates the generic format of a Q.931 message.
Figure 4-10 Generic architecture of Q.931 message
II. Protocol descriminator
It is set to Q.931.
III. Length of call reference
When the length of call reference is set to zero, the CRV is a virtual call reference, meaning that it is irrelevant to any call, and is used for supplementary services.
IV. Call reference value
This parameter is used to identify a call, and it is only valid in part of the call segment. For instance: The call model in gatekeeperRouted, CRV at the two signaling segments—source terminal-GK and GK-destination terminal—are different. GK establishes the association between two CRVs, and ensures accurate transmission of signaling messages. The CRV is generally used to associate multiple calls in three-party service or multi-party service.
V. Message type
Its value is coded by Q.931, as described in Section 4.3.2.1.
Information Elements
I. Bearer capability
It is a mandatory information element (IE) in H.225.0, though less significant as specified in Q.931. If this information element is received in a call between two H.323 terminals, it may be ignored by the receiver. If this information element appears in a Setup message for a call-independent signaling connection as defined in Recommendation H.450.1, the coding shall follow 7.2/H.450.1, which will not be elaborated here. H.225.0 specifies bearer capability as follows:
Information transfer capability
For calls originating from an ISDN endpoint the information indicated to the gateway shall be forwarded. This is to allow some advance information about the nature of the connection to be forwarded to the H.323 endpoint, for example, voice only vs. data vs. video; this would have an impact on the bandwidth required as well as on the ability/willingness to accept the call or not.
Calls that originate from an H.323 endpoint shall use this field to indicate their wish to place an audiovisual call. For audiovisual call, the field shall be set either to 'unrestricted digital information'. If a speech only call is to be placed, the H.323 terminal shall set the information transfer capability to either 'speech' or to '3.1 kHz audio'.
Rate multiplier
The segment shall be present if information transfer rate is set to 'multirate'. For a call originating from an ISDN endpoint, the gateway shall simply pass on the information that it receives from the ISDN. For a call originated from an H.323 endpoint, this shall be used to indicate the bandwidth to be used for this call on the SCN side. If a gateway is involved, this value shall reflect the number of external connections to be set up.
Layer 1 protocol
It is set to G.711 to indicate a voice-only call and H.221 and H.242 to indicate an H.323 videophone call.
II. Display
The network will send American Standard Code for Information Interchange (ASCII) to subscriber to be displayed.
III. Called party number and calling party number
The calling party number is used for charging and caller number display.
The called party number is used for routing.
If the numbering plan identification is set to “Private Numbering Plan” in a PBN originated call, this indicates that the E.164 address is not present in Setup message; and the call will be routed by an alias address in the user-to-user information.
IV. Cause
It indicates the generation of the message for future diagnosis. The Cause is mandatory in Release Complete, but optional elsewhere. The Cause information element and the ReleaseCompleteReason (a part of the Release Complete message) are mutually exclusive. The Cause is borrowed from Q.931, while ReleaseCompleteReason is for PBN.
Table 4-13 Mapping between the Cause IE and ReleaseCompleteReason
| ReleaseCompleteReason code | Corresponding Q.931 cause value |
| noBandwidth | 34 – No circuit/channel available |
| gatekeeperResources | 47 – Resource Unavailable |
| unreachableDestination | 3 – No route to destination |
| destinationRejection | 16 – Normal call clearing |
| invalidRevision | 88 – Incompatible destination |
| noPermission | 111 – Protocol error, unspecified |
| unreachableGatekeeper | 38 – Network out of order |
| gatewayResources | 42 – Switching equipment congestion |
| badFormatAddress | 28 – Invalid number format |
| adaptiveBusy | 41 – Temporary Failure |
| inConf | 17 – User busy |
| undefinedReason | 31 – |
The reverse mapping is not required as packet-based network entities are required to decode the Cause IE.
V. User-User Information Element
User-User Information Element (UUIE) is the most significant IE in H.225.0 call signaling. It shall be used by all H.323 entities to convey H.323-specific call control information in addition to normal E2E subscriber data. The call control information is the essence of H.323 call signaling system, and represents the call signaling capability of the entire H.323 system. UUIE is indispensable for messages such as Setup, Alerting, CallProceeding, Connect, Release Complete, Facility, and User Information.
Figure 4-11 shows the format of UUIE.
The protocol discriminator is changed to ASN.1. It means that the user information is changed from IA5 of Q.932 to a generic ASN.1.
The user information contains two parts. The body part is h323-UU-pdu, containing the UUIE contents of related messages, that is, the signaling information of H323. The optional part is the user data transmitted between terminals, and the data is IA5 character string, whose maximum length is 131 bytes. It is equivalent to user-user information defined in Q.932, but it is encapsulated in UUIE data structure defined by ASN.1. As an element in the data sequence, it is called user data.
H.225.0 defines the contents of h323-UU-pdu in UUIE of each related message. For example, the UUIE of Connect message contains the following contents:
l Protocol identifier It is set by the called endpoint as the version number of H.225 protocol supported by the endpoint.
l H.245 address: It is the transmission layer address of H.245 control channel of called endpoint or GK. According to this address, the calling endpoint can establish the H.245 control channel to the called endpoint or GK, and then establish the required media channel. This is the major purpose of H.225.0 call. This parameter can also be transmitted by UUIE of Alerting message or Call Processing message.
l Destination information: It is used to indicate the endpoint type, making the calling endpoint able to determine whether the call is related to gateway.
l Conference identifier: It is the conference identifier carried in Setup message.
l Call identifier: It is set by calling endpoint.
A typical example of Q.931 message
This is an example of Setup message.
Q.931
Protocol discriminator: Q.931
Call reference value length: 2
Call reference value: 6FD1
Message type: SETUP (0x05)
Bearer capability
Information element: Bearer capability
Length: 3
Coding standard: ITU-T standardized coding
Information transfer capability: Unrestricted digital information
Transfer mode: Packet mode
Information transfer rate: Packet mode
User information layer 1 protocol: Recommendation H.221 and H.242
Display
Information element: Display
Length: 7
Display information: 7670000
Calling party number
Information element: Calling party number
Length: 8
Type of number: Unknown
Numbering plan: E.164 ISDN/telephony numbering
Number: 7670000
Called party number
Information element: Called party number
Length: 8
Type of number: Unknown
Numbering plan: E.164 ISDN/telephony numbering
Number: 7670001
User-user
Information element: User-user
Length: 126
Protocol discriminator: X.208 and X.209 coded user information
ITU-T Recommendation H.225.0
h323_uu_pdu (H323-UU-PDU)
h323_message_body (setup)
setup
protocolIdentifier: 0.0.8.2250.0.3
sourceAddress (AliasAddress)
Item 0 (e164)
e164: 7670000
Item 1 (h323_ID)
h323_ID: 7670000
sourceInfo (EndpointType)
vendor (VendorIdentifier)
vendor (H221NonStandard)
t35CountryCode: 82
t35Extension: 0
manufacturerCode: 2290
productId: CnS H.323v2
versionId: 2.0
terminal (TerminalInfo)
mc: False
undefinedNode: False
destinationAddress (AliasAddress)
Item 0 (e164)
e164: 7670001
activeMC: False
conferenceID: 4E2C3030-DCBC-9839-3FB8-EB4A020D3C92
conferenceGoal (create)
create: create
callType (pointToPoint)
pointToPoint: pointToPoint
sourceCallSignalAddress (ipAddress)
ipAddress
ip: 191.169.150.171 (191.169.150.171)
port: 1074
callIdentifier (CallIdentifier)
guid: 3F503030-DCBC-9839-3FB9-D7A6F6B50BF4
mediaWaitForConnect: False
canOverlapSend: False
endpointIdentifier: 22-2
h245Tunneling: False
Line 1 indicates that the message is a Q.931 message.
Line 2 is protocol descriminator. Now, it is set to Q.931.
Line 3 defines the length of call reference value. It is set to 2 bytes.
Line 4 is call reference value. It is 6FD1.
Line 5 is message type. Now, it is Setup.
Line 6 to line 15 means that the caller is an H.323 endpoint, and the information transfer capability is “Unrestricted digital information”. It implies that the caller is about to make a video call. Layer 1 protocol is set to H.221/H.242, which means that the call is an H.323 video call.
Line 16 to line 19 The network will send American Standard Code for Information Interchange (ASCII) to subscriber to be displayed.
Line 20 to 25 The calling party number: used for charging and caller number display.
Line 26 to 31 The called party number: used for routing.
Line 32 to 33 indicates the following is UUIE.
Line 34 indicates that the UUIE length is 126 bytes.
Line 35 protocol discriminator.
Line 36 to end The UUIE contents in the Setup message. The UUIE contains the following contents:
sourceAddress (AliasAddress): It can be an E.164 address or an H.323 identifier. Here, the E.164 address is a telephone number 7670000, and the H.323 identifier is also 7670000.
sourceInfo (EndpointType): Here, the country code is 82, extension is 0 and manufacturer code is 2290. productId and versionId are text strings that can provide product information.
destinationAddress (AliasAddress): It can be an E.164 address or an H.323 identifier. Here, the E.164 address is a telephone number 7670001, and there is no H.323 identifier.
callType (pointToPoint): Using this value, called party's GK can attempt to determine 'real' bandwidth usage. Here, it is an end-to-end call.
sourceCallSignalAddress (ipAddress): It is the transmission layer address (IP address plus TCP port number) of cal signaling channel of local endpoint. Here, the IP address is 191.169.200.31, and the port number is 1074.
callIdentifier: the call identifier is 3F503030-DCBC-9839-3FB9-D7A6F6B50BF4.
Basic Procedures
I. Overview of basic procedures
This section takes the example that two endpoints register with the same GK, and describes the signaling procedures in two message transmission modes, for the purpose of detailing the call control procedure of H.323 system.
II. Basic call setup procedure (direct routing mode)
Figure 4-12 shows the signaling procedure, and follows brief description.
Figure 4-12 Signaling procedure (direct routing) of public GK
1) Scenario 1: Endpoint 1 (calling party) sends ARQ message to its GK through RAS channel, to request the GK to originate a call to endpoint 2.
2) Scenario 2: The GK agrees to accept the call. It translates the transmission layer address (IP address plus TCP port number) of call signaling channel of endpoint 2, and the sends the address in ACF message to endpoint 1.
3) Scenario 3: Endpoint 1 establishes call signaling channel to endpoint 2, and sends Setup message through the channel. If the ARQ has CRV, the Setup message and following signaling messages have the same CRV.
4) Scenario 4: Endpoint 2 sends back Call Proceeding message, indicating that the call has been processed. For the call between two H.323 terminals, the messages except UUIE do not carry other information element. If the call is between H.323 terminal and ISDN terminal, that is, if endpoint 2 is a gateway, endpoint 2 will transparently transmit the information elements from SCN side, such as bearing capability and proceeding indicator, to endpoint 1. If endpoint 1 is an H.323 terminal, there is no explanation information. If endpoint 1 is also a gateway, such information elements will be transmitted to the calling party at SCN side.
5) Scenario 5: Endpoint 2 sends ARQ to the GK through RAS channel to accept the call.
6) Scenario 6: The GK agrees to accept and sends back ACF.
7) Scenario 7: Endpoint 2 sends back Alerting message to endpoint 1, and waits for answer by subscriber.
8) Scenario 8: The subscriber answers the call. Endpoint 2 sends Connect message to endpoint 1, and the message carries H.245 control channel TCP port number of endpoint 2. Now , the call is set up.
If the GK does not allow endpoint 2 to accept the call, it will send back ARJ. In this case, endpoint 2 will send Release Complete message to endpoint 1.
III. Basic call setup procedure (gatekeeper routed mode)
Figure 4-13 Signaling procedure (gatekeeper routed) of public GK
Figure 4-13 shows the signaling procedure. The differences from the signaling procedure in direct routing mode are as follows:
1) The ACF message sent by the GK to endpoint 1 does not carry transmission layer address of call signaling channel of endpoint 1. Instead, the ACF message carries the transmission layer address of call signaling channel of the GK. Meanwhile, the GK establishes the call signaling channel to endpoint 2.
2) Afterward, the call signaling messages from endpoint 1 can only be transmitted to the GK, which transfers the messages to endpoint 2. Because endpoint 2 establishes signaling channel only with the GK, it can only send signaling messages to the GK, which transfers the messages to endpoint 1.
3) After the call is set up, endpoint 2 tells the GK the H.245 control channel transmission layer address in Connect message, but the information in Connect message sent by the GK to endpoint 1 is up to the transmission mode of H.245 control message. If the GK uses direct routing mode to transmit media control messages, the messages contain the H.245 control channel address of endpoint 2; if the GK uses transfer mode, the messages contain the H.245 control channel address of the GK. In this case, the GK has MC functions.
When either calling party or called party hooks on, a Release Complete message is sent to the GK, which then sends a Release Complete message to the peer end. Then TCP connection is disconnected
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