H.323
2
IP Telephony
Call Signaling
n For the establishment and tear-down of calls
n Q.931 modified by Rec. H.225.0
n
Reuse some messages with few modifications
n
A clever use of User-to-User information element
n
Convey all of the extra information needed in H.323
n
E.g., H.245 addresses to be used for logical channel
3
IP Telephony
Setup
n The first call-signaling message
n Bearer Capability
n
Most of the fields are not used.
n
Used in the circuit-switched world
n
It may be used when the call has originated from outside the H.323 network and has been received at a gateway.
n
A gateway needs to perform the mapping
n User-to-User information element
n
Mandatory: call id, call type, the caller information
n
Optional: source alias, destination alias, H.245 address
4
IP Telephony
Call Proceeding & Alerting
n Call Proceeding
n
Optional
n
call-establishment procedures are underway
n
Mandatory
n
User-to-user information element: destination information
n
Optional - H.245 address of the called party
n Alerting
n
The called user is being alerted
n
Indicating specific alerting tone to the calling party (optional)
n
The same parameters as Call Proceeding
5
IP Telephony
Progress & Connect
n Progress
n
Sent by a called gateway to indicate call progress in the case of inter-working with a CS network
n
Conveying in-band tones or announcements (optional)
n Connect
n
The called party has accepted the call.
n
Must be sent if the call is to be completed
n
Call Proceeding and Alerting are optional
n
User-to-User information
n
The same as Call Proceeding
6
IP Telephony
Release Complete & Facility
n Release Complete
n
Terminate a call
n
No Release message
n
In ISDN, Release and Release Complete
n
Cause information element, optional
n
Otherwise, a Release reason in User-to-User information element.
n Facility (Q.932)
n
A call should be redirected
n
Also be used for supplementary services
n
User-to-User contains reason parameter
n
E.g., routeCallToGatekeeper
7
IP Telephony
Interaction between Call Signaling and H.245 Control Signaling
n Call signaling: call establishment and tear- down
n H.245: the negotiation and establishment of media streams
n The two signaling protocols are closely tied together.
n When to begin the exchange of H.245 messages?
n
Between the Setup and Connect messages
n
Immediately after the Connect message
n
Equipment dependent
8
IP Telephony
PC to Phone Communication
(N) Logical Channels --- Audio
Call Signaling Channel (H.225 - Q931)
RAS Channel RAS Channel
H.323 Internet/PSTN
Gateway
H.323 Gatekeeper
H..323 Terminal PSTN
LAN T1 or
Analog
H.245 Control Channel
9
IP Telephony
Call Scenarios [1/5]
n Basic Call without GKs
Terminal Terminal
a b c
f e d
h g
Release Complete Setup
Alerting Call Proceeding
Connect H.245 Session Establishment
Media Exchange
H.245 Session Release
10
IP Telephony
Call Scenarios [2/5]
n A Basic Call with GKs and Direct Endpoint Call Signaling
Terminal Terminal
a b c
f e d
h g
Release Complete Setup
Alerting Call Proceeding
Connect H.245 Establishment
Media Exchange H.245 Release i
j k
n l m
ARQ ACF
ARQ
ACF
DRQ DRQ
DCF DCF
GateKeeper GateKeeper
n A Basic Call with
Gatekeeper-Routed Call Signaling
n
ARJ with a cause code of routeCallToGatekeeper
n
A Facility with a reason indicating the call be rerouted
Terminal GateKeeper GateKeeper Terminal
A B C D E F G H I J K L M N O P Q R S T U V W X y
ARQ Setup ACF
Call Proceeding
Setup
Call Proceeding ARQ ARJ Facility Release Complete
Setup
Call Proceeding
Call Proceeding Setup
ARQ ACF
Connect Connect Connect
Release Complete
Release Complete Release Complete
H.245 Establishment Media Exchange
H.245 Release
n Optional called-endpoint signaling
n
LRQ, LCF Terminal GateKeeper GateKeeper Terminal
A B C D E F G H I
J K L M N O P Q R S T U
ARQ
ACF Setup
Call Proceeding
ARQ ACF
Setup
Call Proceeding
Call Proceeding Setup
Connect
Connect
Connect
Release Complete
Release Complete Release
Complete
H.245 establishment Media Exchange
H.245 release LRQ
LCF
13
IP Telephony
H.245 Control Signaling
n Used between session participants to establish and control media streams
n
Agree on the media formats and bandwidth
n
Multiplexing multiple media streams
n
E.g., for lip synchronization between audio and video
n
Not carrying the actual media
n
A generic protocol for the control of media streams
n
Not dedicated for VoIP
n
How it works in an H.323 system?
14
IP Telephony
H.245 Message Groupings
n
Requests
n
Require the recipient to perform some action and to send an immediate response
n
Responses
n
In reply to Requests
n
Commands
n
Require the recipient to perform some action, but no explicit response is required.
n
Indications
n
Of an informational nature only
n
No action and response is expected.
15
IP Telephony
The Concept of Logical Channels
n
A Logical channel
n
A unidirectional media path
n
An IP address and port number supporting a particular type of media
n
Has a number that is specified by the sending entity
n
A two-party conversation
n
Two logical channels exist
n
Potentially in different formats
n
An endpoint issues Open Logical Channel
n
Logical channel number and media information (RTP payload type)
n
Far endpoint responds with Open Logical Channel Ack
n
An RTP port
n
Messages over H.245 Control Channel (channel number 0)
n
Permanently open as long as the endpoint is involved a call
16
IP Telephony
Capability Exchange [1/2]
n Share information regarding the endpoint’s reception and transmission capabilities
n Indicate a preference
n TerminalCabilitySet message
n
A request message
n
A sequence number plus the types of audio and video formats
n TerminalCapabilitySetAck
n
with a sequence number
n TerminalCapabilitySetReject
n
With a reason for rejection
17
IP Telephony
Capability Exchange [2/2]
n
TerminalCapabilitySetRelease
n
If no response within a timeout period
n
SendTerminalCapabilitySet
n
Request Terminal Capability information
n
A command message that does not requires a specific response
n
The endpoint that receives the messages should subsequently send a TerminalCapabilitySet message.
n
The SendTerminalCapabilitySet message contains two options.
n
The sender requests that the far endpoint indicates all its capabilities.
n
For confirmation
n
E.g., there has been a break in communication and one
endpoint wants to make sure that it still has the latest
information about the other.
18
IP Telephony
Master-Slave Determination
n
One of the endpoints needs to be the master
n
Of particular importance for the setup of a multi-party conference
n
Compare two pieces of information at each entity
n
A terminal type value
n
A terminal without an MC: 50
n
A gateway without an MC: 60
n
An MCU that supports audio, video and data conferencing: 190
n
An MCU that is currently managing a conference: 240 (the highest)
n
A random number (1..16,777,215)
n
Master-Slave Determination message
n
Master-Slave Determination Ack
n
A “master” or “slave” indication
19
IP Telephony
Open Unidirectional Logical Channel
Terminal Terminal
a
b
c
Open Logical Channel{
Forward Logical Channel Number Data Type
Forward Channel Parameters{
Session ID
RTP Payload Type. etc.}}
Open Logical Channel Ack{
Forward Logical Channel Number Logical Channel Ack Parameters{
Transport Address}}
Media Exchange
20
IP Telephony
Open Bidirectional Logical Channels
Terminal Terminal
a
*
b
c
Open Logical Channel{
Forward Logical Channel Number Forward Logical Channel Parameters{
Data Type Session ID
RTP Payload Type. etc.}
Reverse Logical Channel Parameters { Transport Address
Media type
RTP Payload Type, etc }}
Open Logical Channel Ack{
Forward Logical Channel Number Reverse Logical Channel Parameters{
Reverse Logical Channel Number Transport Address
RTP Payload Type, etc. }}
Media Exchange
*
Open Logical Channel Confirm { Forward Logical Channel Number }
d
21
IP Telephony
Closing Logic Channels and Ending a Session
n
CloseLogicalChannel, CloseLogicalChannelAck
n
Only the initiator can issue
n
Or the receiving end can humbly request
n
A bidirectional channel can be closed by either end
n
Once all logical channels are closed
n
EndSession, EndSession commands
Terminal Terminal
Request Channel Close
Forward Logical Channel Number Reason
Request Channel Close Ack
Forward Logical Channel Number Close Logical Channel
Forward Logical Channel Number source
reason
Close Logical Channel Ack
Forward Logical Channel Number End Session
End Session a
b
c
d
e
f
22
IP Telephony
A Slow Start
Terminal Terminal
Gatekeeper Gatekeeper
Plus
Capability exchange
Master-slave determination
ARQ
ACF Setup
Call Proceeding ARQ Alerting ACF
Connect OLC (bidirectional) OLC Ack OLC Confirm
Media Exchange CLC
CLC Ack End Session End Session
Release Complete
DRQ
DCF DCF
DRQ
OLC : Open Logical Channel CLC : Close Logical Channel
s r
q p
o n
m l
k
j i
h g
f e
d c
b a
23
IP Telephony
Fast-connect Procedure
n Q.931
n
A faststart element : OpenLogicalChannel requests
n
No H.245 control channel
Terminal Terminal
Gatekeeper Gatekeeper
ARQ ACF
ARQ
ACF
DRQ DRQ
DCF DCF
Setup ( faststart [ logical channel infol ] ) Call Proceeding
Alerting Connect ( faststart [ logical channel infol ] )
Media Exchange Release Complete
a b
l k
i
j
h
g f
e d
c
24
IP Telephony
H.245 Message Encapsulation
n H.245 message encapsulated with Q.931 messages as octet strings
n
Set the element h245Tunneling to true
n
The encapsulated data is contained within the h245Control element.
n A conflict exists between encapsulation and fast- start.
n What happens if an endpoint needs to send an
H.245 message at a time when it does not have
a need to send a Q.931 message?
25
IP Telephony
Conference Calls
n MC manages multi-point conference
n Pre-Arranged Conference
n
Participants call in to a separate MCU through Q.931 call signaling.
n The MCU specifies the conference mode (centralized or de-centralized).
n
Communication Mode command (H.245)
26
IP Telephony
An Ad-Hoc Conference
n Expand an existing two-party call to a conference call
n
One of the endpoints (or one of the endpoints’ gatekeepers) must contain an MC (master).
Endpoint 3 Endpoint 2
Setup ( CID = N ) Connect ( CID = N )
Capability exchange Master-slave determination
Setup ( CID = N )
Connect ( CID = N )
a b
l k i j h g f e d c
Endpoint 1 (with MC)
m
Logical Channel Establishment Media Exchange
Master-slave determination Logical Channel Establishment
Capability exchange
Multipoint Conference Multipoint Conference Media Exchange Media Exchange
27
IP Telephony
The Decomposed Gateway
n
The H.323 gateway can comprise
n
Media Gateway (MG): handles media stream
n
Media Gateway Controller (MGC): performs call control
n
Signaling Gateway (SG): interfaces with external signaling network (e.g., SS7)
n
The function of MGC is separate from that of GK.
Media
Gateway Media
Gateway Signaling
Gateway Signaling
Gateway
Control
Messaging Control
Messaging
MGC MGC
External Bearer Network
External Bearer Network
H.323
iGSM: VoIP Service for Mobile
Network
29
IP Telephony
Introduction to iGSM [1/2]
n A VoIP value-added service for mobile network
n iGSM supports user mobility for GSM subscribers to access VoIP services.
n
With the same identity, a GSM subscriber can receive the service when he changes the terminal from the GSM MS to the H.323 terminal.
n
The GSM network remains the same.
n Terminal Mobility, Session Mobility, Service
Mobility
30
IP Telephony
Introduction to iGSM [2/2]
n iGSM Gateway performs the conversion for
communication protocols and transmission formats between GSM and H.323 networks.
n
GSM MAP and RAS protocol translation
n
GSM/PSTN/IP call setup and release
n iGSM Gatekeeper records location information as the
VLR in a GSM network.
31
IP Telephony
iGSM Architecture
VLR
HLR MSC
iGSM Gateway
Gatekeeper iGSM VLR MS
BTS/BSC
H.323 Terminal
IP Network
GSM
32
IP Telephony
iGSM Gatekeeper
n The iGSM gatekeeper maintains an database that stores information for all iGSM subscribers.
n
MSISDN of the MS
n
Transport address of the H.323 terminal (MS resides)
n
Password of the iGSM subscriber
n
HLR address of the iGSM subscriber
n
IMSI of the MS
n
User profile that indicates the service features and restrictions of the iGSM subscriber
n
Presence indication
33
IP Telephony
Registration
HLR MSC
Gateway iGSM
Gatekeeper iGSM VLR MS
H.323 Terminal
IP Network GSM
1
8
7 6 5
4
3 2
6 5
4 3
VLR
34
IP Telephony
Message Flow (Registration)
H.323
Terminal Gatekeeper
(new VLR) Gateway HLR old VLR
RRQ ( MSISDN , passwd )
IRQ ( UpdateLoc, IMSI, VLR, MSC num )
UPDATE_LOCATE ( IMSI, VLR, MSC number ) INSERT_SUBS_DATA ( user profile )
IRR ( InsSubsData, user profile )
IRQ ( InsSubDataAck )
CANCEL_LOCATION ( IMSI )
CANCEL_LOCATION_ack INSERT_SUBS_DATA_ack
INSERT_SUBS_DATA_ack ( HLR number ) IRR ( UpdateLocAck, HLR number )
RCF
2
3
5 5
4
8 7 4
3
6 6
6
35
IP Telephony
Message Flow (De-registration)
H.323 Terminal Gatekeeper
(old VLR) Gateway
HLR
CANCEL_LOCATION ( IMSI )
CANCEL_LOCATION_ack
1
URQ UCF IRR ( CanLoc, IMSI )
IACK ( CanLocAck )
2
2
2
1
1
36
IP Telephony
Call Delivery
HLR
iGSM Gateway
Gatekeeper (iGSM VLR)
H.323 terminal
Gateway MSC
Originating Switch
GSM
PSTN
IP Network 12
3 6
1
1 2
4 2
2 5 3
3 11
7 8 10
9
37
IP Telephony
Message Flow (Call Delivery)
Originating
Switch GMSC HLR Gateway Gatekeeper
VLR H.323 Terminal
IAM SEND_ROUTING_INFORMATION ( MSISDN )
PROVIDE_ROAMING_NO ( IMSI )
LRQ ( RoamNo, IMSI ) LCF ( RoamNoAck, MSRN ) PROVIDE_ROAMING_NO_ack ( MSRN ) SEND_ROUTING_INFORMATION_ack ( MSRN )
IAM
ARQ ACF Set-up
Call Proceeding
ARQ Alerting ACF
ACM ACM Connect
ANM ANM
Conversation
1 2
3 2
2
3 3
4
5 6 7 8
9 11 10
11 11
12 12 12
38
IP Telephony
Mis-routing Scenario [1/2]
VLR MSC
A HLR
VoIP GSM Gateway
GSM MS
VLR B
IP Phone
GSM VoIP Network
1
(a) Step I
p P A
VLR MSC
A HLR
GSM Gateway VoIP
GSM MS
VLR B
GSM VoIP Network
(b) Step II
P B
VLR MSC A
HLR
VoIP GSM Gateway
GSM MS
VLR B
GSM VoIP Network
4
(c) Step III
P B
2
p
3
p
39
IP Telephony
Summary
n
The incoming calls to the user are misrouted until one of the following events occurs.
n
The subscriber originates a call.
n
The subscriber moves to another location area (LA).
n
We proposed an analytic model to study mis-routing due to user mobility in iGSM.
n
The results indicate that the probability of one mis-routed call delivery is about 1~20%.
n
To avoid the mis-routing problem, the iGSM subscriber should turn off their MS when moving to the IP network.
n
The “turn-off” action results in a detach GSM message to de-register the MS.
n
When the subscriber turns on the MS in the GSM coverage area, an explicit registration is performed.
n
