European Telecommunications Standard Institute
TD-00
ETSI STC SPS3
Sophia Antipolis, France, 25-29 January 1999
Source:
FRANCE TELECOM
Title:
IN and H.323
1. Introduction
In a previous contribution it was proposed that SPS3 studies the mapping of IN models and protocols
to H.323 Telephony. This document further elaborates on this topic.
2. H.323 call control
Recommendation H.323 covers multimedia communications on IP networks. In the H.323 architecture
a Gatekeeper is an (optional) entity which controls H.323 terminals in a given zone. Figure 1 describes
the protocol stacks for call control.
H.245
media
channel
control
H225.0
call control
(Q.931 based)
TCP
RAS
channel
UDP
IP
Figure 1 - H.323 Call control protocol stacks
H.225.0 specifies the call control protocol which is based on Q.931 and the RAS (Registration
Admission Status) protocol which is used between an H.323 terminal and a Gatekeeper for registration
admission, bandwitdh messages and address translation.
H.245 specifies the media channel control protocol used between terminals for the negociation of
transmitting capabilities and the control of logical channels.
H.323 allows for the signaling messages (H.225.0 and H.245) to be relayed by the Gatekeeper instead
of being sent from one endpoint to the other (Gatekeeper Routed Call Signaling). Media streams
however are not routed through the Gatekeeper and are established directly between endpoints
(Figure 2).
INAP
GK
RAS
Q.931
H.245
RAS
Q.931
H.245
H.323
Terminal
Media streams
H.323
Terminal
Figure 2 - Signaling and media paths for Gatekeeper Routed Call Signaling
3. BCSM for H.323 telephony calls
In the rest of this document we consider the case of telephony calls and make the assumption that
Gatekeeper Routed Call Signaling is used so that the Gatekeeper can monitor signaling messages
between H.323 endpoints.
The use of Q.931 for call signaling suggests that a Gatekeeper could model a BCSM and provide the
functionality of a SSF/CCF. Figure 3 shows the message flows for an ordinary H.323 voice call.
Different arrow styles are used to indicate different protocols. A possible sequence of state transitions
for an Originating BCSM is indicated on the right of the figure. Mapping of a Terminating BCSM would
be similar. In this example we have mapped H.225.0/Q.931 messages to BCSM transitions. No use
was made of H.245 messages because a telephony call was assumed. Support for multimedia
communications will require mapping of H.245 to a state model. This should be done in the context of
the work on the separation of call and connection. Support for the triggering of value added services on
registration or access requests will require mapping of RAS messages to the BUCSM.
4. Mapping of INAP operations
If a Gatekeeper is to implement an SSF functionality so that it can be controlled by a SCF, INAP SSFSCF operations need to be transposed to the H.323 context.
For event related operations and operations which do not modify the sequence of a basic call
(InitialDP, Continue, ReleaseCall,...), the mapping of parameter values would be needed, but the
actions to be executed by the Gatekeeper would be straightforward.
Operations which modify the connection view such as SplitLeg or MergeCallSegments are not so
easily transposed in a H.323 context. This is because the media streams are established directly
between endpoints and are not relayed by the Gatekeeper.
A possible way to solve this problem is to force the media streams to go through an relaying entity
which could be controlled by the Gatekeeper. Such a configuration could be natural if the call is
established from a local network to the outside world and a proxy is used to filter traffic leaving this
network. It would also be relevant in the case of calls established between an H.323 terminal and a
terminal on the PSTN as these calls must be routed through a gateway. The gateway architecture
currently favored by the IETF uses a gateway control protocol between the media gateway and the
controlling entity (this would be in the Gatekeeper which would play the rôle of Media Gateway
Controller). Candidate protocols are for example MGCP and MDCP. Figure 4 shows the different
entities involved. In order to emulate a SSF, INAP operations would have to be translated by the
Gatekeeper in terms of the gateway control protocol and the signaling protocol on the PSTN side.
Figures 5-7 show how some typical INAP sequences could be translated into the different protocols.
H.323 messages have not been detailed and use of MGCP is assumed between Media Gateway and
Media Gateway Controller.
The constraint of forcing the media streams to be relayed though an intermediate point may not be
convenient for all calls inside the local network. Another possibility could be to require support in H.323
terminals of supplementary services such as Call Hold and Call Transfer. These services would serve
as building blocks for the operations manipulating the connection view states.
Term A
Term B
GK
ARQ
ACF
Setup
O_NULL
Call Proceeding
Auth_Orig_Att
Collect Info
Analyse Info
Select Route
Auth_Call_Setup
Setup
Send_Call
ARQ
ACF
Call Proceeding
Alerting
O_Alerting
Term Cap Set
Term Cap Set Ack
Open Logical Channel
Open Logical Channel Ack
Media Channel
Term Cap Set
Term Cap Set Ack
Open Logical Channel
Open Logical Channel Ack
Media Channel
Connect
O_Active
Conversation
End Session Command
End Session Command
Release Complete
DRQ
DRQ
DCF
DCF
RAS messages
ARQ/ACF Admission Request/Confirm
H.225.0/Q.931 messages
O_Null
H.245 messages
DRQ/DCF Disengage Request/Confirm
Figure 3 - H.323 message flows and correspondig O_BCSM transitions
INAP
IP network
PSTN
GK
Media
Gateway
Controller
RAS
Q.931
H.245
MGCP ?
H.323
terminal
Media streams
Media
Gateway
Figure 4
1) Connect(@PSTN)
IP network
PSTN
GK
RAS
Q.931
H.245
3) IAM
Media
Gateway
Controller
2) CreateConnection
H.323
terminal
Media
Gateway
Figure 5 - Connect
1) InitiateCallAttempt(@PSTN)
3) Connect(@IP)
IP network
PSTN
GK
5)
2) IAM
Media
Gateway
Controller
RAS
Q.931
H.245
4) CreateConnection
6) ModifyConnection
H.323
terminal
Media
Gateway
Figure 6 - InitiateCallAttempt followed by Connect
1) DisconnectLeg
4) Connect(@IP)
H.323
terminal
PSTN
GK
RAS
Q.931
H.245
2)
H.323
terminal
IP network
Media
Gateway
Controller
5)
3) ModifyConnection
6) ModifyConnection
Media
Gateway
Figure 7 - DisconnectLeg followed by Connect