1. Modified from the talk by
MEGACO Protocol
Modified from the talk by
Wei-Kuo Chiang (江為國)
Internet Telecom. Dept. (K200)
CCL/ITRI
2019/2/23 1

2. H323, SIP & MGCP, MEGACO PSTN SS7 MGCP GK GW TN GK GW TN PSTN H.323CA
PSTN SG
MGCP
TGW
RGW GK GW TN GK GW TN CO
PSTN
H.323
RTP
MCU TN TN
MCU TN TN
GW : Gateway
GK : Gatekeeper
TN : Terminal
MCU : Multipoint Control Unit
CA : Call Agent
TGW : Trunking Gateway
RGW : Residential Gateway
SG : Singling Gateway
2019/2/23 2

3. H323, SIP & MGCP, MEGACO Concept H.323, SIP Problems MGCP, MEGACO
peer-to-peer
internet oriented
intelligent endpoint
optional GK
decentralized
Problems
maintenance
cost & scalability of large systems
signaling & media control are coupled
interoperability with SS7
MGCP, MEGACO
client-server
traditional telephony
intelligent server
“dumb” terminal
“stateless” terminal
centralized
Concept
gateway decomposed
separate call control from media ports
CA, MG, SG
interoperability with PSTN
2019/2/23 3

4. MEGACO Features Media Gateway Control Protocol
Change of Connection Model
Concept of Context
Change of Command Naming
Comparison of Commands
MEGACO Transaction & Descriptor
Call Flows: RGW to RGW
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5. MEGACO Connection Model
MGCP
MEGACO 1
Endpoint
Termination 2
Connection
Context 3
Call Agent (CA)
Media Gateway Controller (MGC)
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6. Text encoding and binary encoding
TransactionRequest
Consists of a number of commands
Usually requested by an MGC
TransacionReply
A number of responses
Text encoding and binary encoding
Augmented Backus-Naur Form, RFC 2234
Abstract Syntax Notation One (ASN.1), ITU-T R X.680
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7. MEGACO Connection Model
Termination
A logical entity on a MG
that sources / sinks media / control streams
Two kinds of Termination
Physical Interface
RTP stream (ephemeral termination)
Add/subtraction command
CreateConnection and DeleteConnection
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8. The properties of a termination
Grouped into a set of descriptors
Are included in MEGACO commands
Can be changed
Wild cards
*, all ; $, any
Termination ID
an arbitrary string assigned by the MG
at the time of their creation
Root Termination
the entire MG itself
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9. MEGACO Connection Model
Context
An association between a collection of terminations
Added to, removed from, moved from one to another
A termination can exist in only one context at any time
describe the topology (who hears/sees whom)
refer to Topology Descriptor
Context ID
a 32-bit integer chosen by the MG
 : ALL ; - : NULL; $ : Choosing one
Null Context
Containing all terminations that are not associated to any other termination
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10. MEGACO Contexts and Termination
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11. MEGACO Connection Model
Media Flow
Topology of a Context
describe the media flow between
Terminations within a Context
refer to Topology Descriptor
Mode of a Termination
describe the media flow at the ingress/egress of the media gateway
refer to Media Descriptor Stream Descriptor Local Control Descriptor (receive-only, send/receive, …)
Context
SCN 1
RTP
SCN 2
Contexts in GW
SCN 1
RTP
SCN 1
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12. Add 2 Terminations into 1 context
Concept of Context
MGCP
MEGACO
MGC CA
MGCP
MEGACO MG MG
RGW
RGW
RTP
RTP
Context IP  
Endpoint IP
Endpoint
Termination
SCN Channel
Termination
RTP Stream
Connection
established
after CRCX 
Add 2 Terminations into 1 context
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13. MEGACO Transactions The passing of commands and the responses
Commands are directed towards terminations within contexts
ContextID + TerminationIDs
Multiple commands can be grouped
TransactionRequest (TransactionID {
ContextID1 {Command, Command, … Command},
ContextID2 {Command, Command, … Command},
ContextID3 {Command, Command, … Command} } )
TransactionReplay (TransactionID {
ContextID1 {Response, Response, … Response},
ContextID2 {Response, Response, … Response},
ContextID3 {Response, Response, … Response} } )
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14. Messages Commands are executed in sequence An interim reply
If a command fails, the subsequent commands are not processed
Not the case for optional commands
O-”command-name”
An interim reply
TransactionPending (TransactionID {} )
Messages
Concatenate multiple transactions
The transactions are treated independently
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15. Action 2 for the Other Context Action 3 for Another Context
MEGACO Transactions
MGC to MG1:
MEGACO/1 [ ]:5555
Transaction = {
Context = $ {
Add = A4444,
Add = $ {
Media {
Stream = 1 {
LocalControl { …}
Local {
v=0
c=IN IP4 $
m= audio $ RTP/AVP 4
}}}}}
Transaction
Action 1 for One Context
Command 1
Command 2
Command 3
Action 2 for the Other Context
Command 1
Action 3 for Another Context
Command 1
Command 2
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16. Similar Commands in MGCP
MEGACO Command Naming
MEGACO
Similar Commands in MGCP
(1) Add (Termination to Context)
(1) CreateConnection (Endpoint)
(2) Modify (Termination-descriptor)
(2) ModifyConnection (Endpoint)
(3) Subtract (Termination)
(3) DeleteConnection (Endpoint)
(4) Move (Termination)
 CreateConnection + DeleteConnection
(5) AuditValue (Termination)
(4) AuditEndpoint
(6) AuditCapability (Termination)
(NONE)
(7) Notify (Termination)
(5) Notify (Endpoint)
(8) ServiceChange (Termination)
(6) RestartInProgress (Endpoint)
(NONE)
(7) AuditConnection
 Add, Modify, Move
(8) NotificationRequest (Endpoint)
 Add, Modify, Move
(9) EndpointConfiguration (Endpoint-encode)
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17. MEGACO Commands Add Modify Subtract Adds a termination to a context
Change the property values of a termination
E.g., to issue signals or to detect specific events
Subtract
Remove a termination from a context
The response provides statistics
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18. Move AuditValue AuditCapabilities From one context to another
Cannot be moved to the null context
AuditValue
Retrieve current values for properties, events, and signals
AuditCapabilities
Retrieve the possible values of properties, signals, and events
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19. Notify ServiceChange Inform the MGC of events that have occurred
Accompanied by a RequestID
ServiceChange
Inform an MGC that a group of termination will be taken out of service or will resume service
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20. Descriptors For use with commands and responses
To qualify a given command/response
Mandatory, forbidden or optional
Descriptorname=<someID>{parm=value, …}
Parameters can be fully specified,
under specified; the use of $,
over specified; a list of acceptable parameter value in order of preference
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21. Termination Descriptors
Transaction
Media Descriptors
Modem Descriptors
MUX Descriptors
Events Descriptors
Signals Descriptors
Digit Map Descriptors
Audit Dsp (unused in Reply)
Service Change Descriptors
Observed Events Descriptors
Event Buffer Descriptors
Statistics Descriptors
Package Descriptors
Action 1 for One Context
Topology Descriptor
Command 1
Command 2
Command 3
Action 3 for Another Context
Command 1
Command 2
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22. Modem Descriptor Multiplex Descriptor
Modem type and associated parameters
By default, a termination does not have a modem descriptor
They may be applied later in an Add or Modify
Multiplex Descriptor
Multimedia communications; different media streams on different channel channels
The association between media streams and bearers
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23. Media Descriptors Describes the various media streams Hierarchical
Termination State Descriptor
Stream Descriptor
Local Control Descriptor
Local Descriptor
Remote Descriptor
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24. Termination State Descriptor
ServiceStates
“test,” “out-of-service,” or “in service”
EventBufferControl
Off, the termination reports detected events immediately
Lockstep, the events will be buffered in a FIFO buffer, time-stamped
Examined upon receipt of a new Events Descriptor
Or when the control is set off
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25. Stream Descriptor
A stream is created by specifying a new StreamID on a particular termination in a context
A stream is deleted by setting empty Local and Remote Descriptors and by setting ReserveGroup and ReserveValue in LocalControl descriptor to “false”
Local Control Descriptor
Mode, ReserveGroup, and ReserveValue Properties
Mode: sendonly, receiveonly, sendreceive, inactive, and loopback
Send and receive directions are w.r.t. the exterior of a context
MGC specifies a set of choices for the session
ReserveGroup and ReserveValue indicate the resources should be reserved
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26. Events Descriptor Signal Descriptor
Local Descriptor and Remote Descriptor
Zero or more SDP sessions descriptions
It’s permissible to indicate several alternatives for a parameters
Events Descriptor
A RequestIdentifier and a list of events
Off-hook, fax-tone, etc.
EventControlBuffer property
Signal Descriptor
A list of signals that a termination is to apply
To just a single stream or to all streams
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27. Audit Descriptor Three type of signals
On/off: remains on until it is turned off
Timeout
Brief: only for a limited time
Audit Descriptor
A list of information to be returned from an MG to an MGC
A list of other descriptors such as
Modem, Multiplex, Events, Signals, ObserveEvents, DigitMap, Statistics, Packages, and Eventbuffer
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28. Service Change Descriptor
Used only in association with the ServiceChange command
The type of service change,
ServiceChangeMethod
Graceful, the removal of existing terminations w/o interrupting existing connections
Forced, an abrupt removal
Restart, after a specified delay
Disconnected, applied to the entire MG
Handoff, from the old MGC; a new MGC is taking over
or Failover, from MG to MGC
ServiceChangeDelay, a number of seconds
SericeChangeReason, Table 6-4, page 279.
and a new address
2019/2/23 28

29. DigitMap Descriptor A dialing plan A string or a list of strings
[0-9] and [A-K]
x: a wild card
.: zero of more repetitions of the immediately proceeding digit
A start timer, to start
A short timer, when more digits are needed
A long timer, to differentiate different routing
(0|00|[1-7]xxx|8xxxxxxx|Fxxxxxxx|Exx|91xxxxxxxxxx|9011x.)
The timers are applied by default
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30. Statistics Descriptor
Statistical information regarding the usage of a termination
For the response of a Subtract command
In response to the AuditValue command
Observed Events Descriptor
Mandatory in the Notify command
RequestIdentifier
Optional time-stamp for each observed event
yyyymmddThhmmssss
For the response of a ServiceChange command
In a response to the AuditValue command
Events stored in the event buffer
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31. Topology Descriptor Relevant to a context only
How media streams should flow within a context
By default, all terminations in a context can send and receive media to and from each other
Isolate, oneway and bothway
A useful tool for implementing services such as call waiting
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32. Action 3 for Another Context
Context Descriptors
Context 1
Context 2
Context 3
Transaction T2 T2 T2
Action 1 for One Context
Topology Descriptor
Command 1
Command 2 T1 T3 T1 T3 T1 T3
1. No topology
descriptor
2. T1, T2
isolate
3. T3, T2
oneway
Context 4
Context 5
Context 6 T2 T2 T2
Action 3 for Another Context
Command 1 T1 T3 T1 T3 T1 T3
4. T2, T3
oneway
5. T2, T3
bothway
6. T1, T2
bothway
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33. Packages Groups of properties, signals, events and statistics
For different types of terminations
Items are defined and given identifiers
Tone Detection Package, DTMF Generator Package, Analog Line Supervision Package, …
Package Descriptor
Provides the MGC with a list of the packages implemented in a termination
2019/2/23 33

34. MEGAO Command and Response
Command Syntax
MEGACO/1 [ ]:3456
Transaction = {
Context = - {
Modify = * {
Events = 7777 {al/of},
DigitMap = Map1 {
(0|00|[1-7]xxx|8xxxxxxx|Fxxxxxxx|Exxx|9011x.)} }
2019/2/23 34

35. Response Syntax MEGACO/1 {111.111.333.333]:5678 Reply = 12345 {
context = - { Modify = * } }
context = - { Modify = * {
Error = 500 {Internal Gateway Error}
}}}
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36. Call Setup Using MEGACOpp
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37. Based on Detected Events
pp. 295-
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38. 2019/2/23 38

39. Call Flow: RGW to RGW Internet MGC 1 3 2 4 RGW A4444  A5555 
MGC 1 3 2 4
RGW
RGW
Internet
A4444 
A5555 
NULL Context
NULL Context
1.(3) ServiceChange(ROOT): MG register with MGC
2.(4) Modify(A4444): MGC set Mode(SendReceive) & Events(al/of)
2019/2/23 39

40. Call Flow: RGW to RGW Internet MGC 5 6 RGW A4444  A5555 
MGC 5 6
RGW
RGW
Internet
A4444 
A5555 
NULL Context
NULL Context
DTMF detection/
Digit Map Complete Event
5. Notify(A4444): MG report an off hook event(time)
6. Modify(A4444): MGC set Events(al/on, dd/ce, {DigitMap = Dialplan0})
Signals(cg/dt)
DigitMap = Dialplan0{…}
Call Progress Tone Generator/Dial Tone
2019/2/23 40

41. 8. Add(A4444,$:Mode(ReceiveOnly), Local(RTP))
Call Flow: RGW to RGW
MGC 7 8
RGW
RGW
Internet
A4444 
A4445 
A5555
Context ID = 2000
Full Match
Digit String
7. Notify(A4444): MG report a Digit Map Complete event(ds= ,Meth=FM)
8. Add(A4444,$:Mode(ReceiveOnly), Local(RTP))
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42. 10. Modify(A4444: Signals(cg/rt)); Modify(A4445: Remote(…))
Call Flow: RGW to RGW
MGC 10 9
RGW
RGW
Internet
A4444
A5556
A5555 
A4445 
Context ID = 2000
Context ID = 5003
9. Add(A5555: Mode(SendReceive), Events(al/of), Signals(al/ri),
$: Mode(SendReceive), Local(RTP), Remote(...))
10. Modify(A4444: Signals(cg/rt)); Modify(A4445: Remote(…))
2019/2/23 42

43. Call Flow: RGW to RGW Internet MGC 12 13 11 RGW A4444 A4445 A5556 
MGC 12 13 11
RGW
RGW
Internet
A4444
A4445
A5556 
A5555 
Context ID = 2000
Context ID = 5003
11. Notify(A5555: Observed(al/of))
12. Modify(A5555: Events(al/on), Signals()); to turn off ringing
13. Modify(A4445: Mode(SendReceive)); Modify(A4444: Signals())
2019/2/23 43

44. PSTN Originating Call to IMS
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45. IMS UE termination, CS Domain roaming
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46. PSTN termination call
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47. PSTN initiated session release
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48. Mobile-initiated Hold and Resume of a Mobile-PSTN Session
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