05 fn42425en60gla1 ia interface doc

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Ia Interface and MEGACO / H.248 Protocol

FN42425EN60GLA1 © 2011 Nokia Siemens Networks

1

Content

1 Ia Interface and MEGACO & H248 protocol 3

1.1 Basic terms 4

1.2 MEGACO / H.248 Protocol 6

1.3 Terminations 8

1.4 Context 9

1.5 Commands 10

1.6 Transactions 14

1.7 Termination Properties and Descriptors 16

1.8 Service Change Reasons 20

1.9

Packages 22

1.10 Events 24

1.11 Tokens 26

1.12 Error descriptor 30

1.13 Examples 32

Ia Interface and MEGACO / H.248

Protocol




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© 2011 Nokia Siemens Networks2





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1 Ia Interface and MEGACO & H248 protocol




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1.1 Basic terms

Ia interface is reference point between AF and BGF, standardized by ETSI ES 283018. On this interface protocol H248 / MEGACO is used. ETSI specifies the H.248 Iaprofile.

SPDF

The Service Policy Decision Function (SPDF) is a functional element that coordinatesthe resource reservation request received from AF (e.g. P-CSCF). The SPDF makespolicy decisions using policy rules and forwards the session and media relatedinformation obtained from AF (Application Function) to the BGF (Border GatewayFunction).

The BGF has the "H.248 MGC" role.

BGF BGF is packet-to-packet gateway for user plane media traffic. The BGF performspolicy enforcement functions under the control of the SPDF.

A Border Gateway Function (BGF) provides the interface between two IP-transportdomains. It may reside at the boundary between an access network and a corenetwork or between two core networks. The BGF has the "H.248 MG" role.





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AF

PCS(SPDF)

BGF

Ia H248

Gq„+ Diameter

AccessTransportNetwork

CoreTransportNetwork

Reference architecture

Fig. 1




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1.2 MEGACO / H.248 Protocol

MEGACO - Media Gateway Control (IETF terminology) or H.248 (ITU terminology) isa protocol between Media Gateway Controller (MGC) and Media Gateways (MG) thatis identically standardized by ITU and IETF (H.248 from 06/2000 vs. MEGACO V1,RFC 3015).

PCS supports H.248 ABNF (Augmented BNF) coding (short and long text format).

ABNF is used for the syntax specification. It is specified in RFC4234

Media Gateway (MG): Provides a connection point between two dissimilar networks.It converts media provided in one type of network to the format required in anothertype of network. For example, a MG could terminate bearer channels from a switchedcircuit network (e.g., DS0s) and media streams from a packet network (e.g., RTP

streams in an IP network).

Media Gateway Controller (MGC): Provides the management of resources on theMG. It controls the parts of the call state that concern to connection control for mediachannels in a MG.

Stream: Bidirectional media or control flow received/sent by a media gateway as partof a call or conference.

Main tasks of MEGACO protocol are to:

control bearer connection setup, modification and release;

indicate of events concerning the bearer connections;

trigger and detect tones and signals on the bearer;

retrieve statistics about the bearer connection.

Call bearer control protocols are usually specific to the transport technology used, butare also dependent on the system. By standard is defined a basic framework forprotocol. A framework specifies the functional architecture, interfaces and abstractmodels.

The basic communication units exchanged between functional units defined in theframework is a message. With H.248 the set of messages is defined

Events describe what is detectable and reportable. Signals are various indicationsrunning on the bearer (e.g. busy tone). Parameters describe how bearers andrelated things can be described.

Procedures define rules and actions associated with messages, parameters, signalsand events.

A connection model for the protocol describes the logical entities, or objects, withinthe Media Gateway that can be controlled by the Media Gateway Controller. Themain abstractions used in the connection model are Terminations and Contexts.





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• H.248 - ITU terminology

• MEGACO - Media Gateway Control - IETF terminology

RFC 3015MGC

the management of resources

on the MG

MG

connection point between

two dissimilar networks

media media

H248 /

MEGACO

media media

H248 /MEGACO

Fig. 2




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1.3 Terminations

A Termination is the very basic element of connection model. It is logical entity on aMG that sources or sinks media and/or control streams. Termination has uniqueidentities -TerminationID. The Termination ID is assigned to the termination by theMG at the time of its creation.

Two types of terminations are distinguished:

physical terminations: Physical terminations are associated with a physicalresource (e.g. timeslot of PCM line) which cannot be automatically released orsetup.

ephemeral terminations: Ephemeral terminations do not represent physical

resource but logical transport resources. Examples for ephemeral terminations areRTP flows. They exist only for the duration of their use. Such resources can bedynamically established and released.

Terminations have a semi-permanent existence. Ephemeral Terminations arecreated by means of an Add command. They are destroyed by means of a Subtractcommand. In contrast, when a physical Termination is Added to or Subtracted from aContext, it is taken from or to the null Context, respectively.

Some examples:

Termination Id: "DS/S_17/STM1_01/[email protected]"hiG1200, timeslot(s) on PCM line(s)

Termination Id: ip/84/access/338port of RTP stream in an IP environment

TerminationID: "ROOT"means: ALL - the ROOT termination is used to indicate the entire MG

TerminationID "$" or ip/84/access/$means: CHOOSE. Signal to the MG to select an idle termination.

Each command specifies the TerminationId the command operates on.

A Termination is described by a number of characterizing Properties, which aregrouped in a set of Descriptors that are included in commands.

PCS supports up to 2 Terminations in one Context. Both must be IP Termination.





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1.4 Context

A Context is an association between numbers of Terminations. The Contextdescribes the topology (who hears/sees whom) and the media mixing and/orswitching parameters if more than two Terminations are involved in the association.

In general, an Add command is used to add Terminations to Contexts. If the MGCdoes not specify an existing Context to which the Termination is to be added, the MGcreates a new Context. A Termination may be removed from a Context with aSubtract command, and a Termination may be moved from one Context to anotherwith a Move command. A Termination exists in only one Context at a time.

The maximum number of Terminations in a Context is a MG property. Mediagateways that offer only point-to-point connectivity might allow at most two

Terminations per Context. Media gateways that support multipoint conferences mightallow three or more Terminations per Context.

Examples:

Context=170

Context = $

Connection model: termination and context

Fig. 3




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1.5 Commands

The protocol provides commands for manipulating Contexts and Terminations. Forexample to add Terminations to a Context, modify Terminations or subtractTerminations from a Context. Furthermore, commands make possible completecontrol of the properties of Contexts and Terminations. This includes specifying whichevents a Termination is to report, which signals/actions are to be applied to aTermination and specifying the topology of a Context (who hears/sees whom).

Below is an overview of the commands.

1. Add: The Add command adds a Termination to a Context. The Add command onthe first Termination in a Context is used to create a Context.

2. Modify: The Modify command modifies the properties, events and signals of a

Termination.

3. Subtract: The Subtract command disconnects a Termination from its Contextand returns statistics on the Termination's participation in the Context. TheSubtract command on the last Termination in a Context deletes the Context.

4. Move: The Move command atomically moves a Termination to another Context.

5. AuditValue: The AuditValue command returns the current state of properties,events, signals and statistics of Terminations.

6. Notify: The Notify command allows the Media Gateway to inform the MediaGateway Controller of the occurrence of events in the Media Gateway.

7. ServiceChange: The ServiceChange command allows the Media Gateway tonotify the Media Gateway Controller that a Termination or group of Terminationsis about to be taken out of service or has just been returned to service.ServiceChange is also used by the MG to announce its availability to a MGC(registration), and to notify the MGC of impending or completed restart of the MG.The MGC may announce a handover to the MG by sending it a ServiceChangecommand. The MGC may also use ServiceChange to instruct the MG to take aTermination or group of Terminations in or out of service.





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NNotify

AuditValueReply

AVAuditValue

ServiceChangeReply

SCServiceChange

NotifyReply

ModifyReply

MFModify(Event=….)

SubstractReply

SSubstract

ModifyReply

MFModify

AddReply

AAdd

tokenCommand

MEGACO commands

Fig. 4




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Commands and Terminations between PCS and BGF:

Following table shows in which direction commands are sent, which terminations theycan be associated with, and which wildcard options are supported for the specific

command.

Command SentBy

Used on Termination Type Wildcard Support

IP ROOT W- O-

Add SPDF Yes No No No

AuditCapabilities

- - - - -

AuditValue SPDF No Yes No No

Modify SPDF Yes Yes No NoNotify BGF Yes No No No

ServiceChange BGF Yes Yes No No

Subtract SPDF Yes No Yes No

The PCS does not perform any state synchronization via audit commands. The auditcommand is just used to evaluate the BGF supported packages.




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1.6 Transactions

Commands between the Media Gateway Controller and the Media Gateway aregrouped into Transactions, each of which is identified by a TransactionID.

Transactions are presented as TransactionRequests. A request contains one ormore Actions, each of which specifies its target Context and one or more Commandsper Context. Each action typically specifies a ContextID. However, there are twocircumstances where a specific ContextID is not provided with an action. One is thecase of modification of a termination outside of a context. The other is where thecontroller requests the gateway to create a new context.

Corresponding responses to a TransactionRequest are received in a single reply,possibly preceded by a number of TransactionPending messages. A

TransactionReply includes the results for all of the Commands in the correspondingTransactionRequest. The TransactionID parameter must be the same as that of thecorresponding TransactionRequest. A TransactionPending indicates that theTransaction is actively being processed, but has not been completed. It is used toprevent the sender from assuming the TransactionRequest was lost where theTransaction will take some time to complete.

The TransactionResponse Acknowledgement parameter can be found in anymessage. It carries a set of "confirmed transaction-id ranges". Entities may choose todelete the copies of the responses to transactions whose id is included in "confirmedtransaction-id ranges" received in the transaction response messages.





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Fig. 5




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1.7 Termination Properties and Descriptors

Terminations have properties. These properties are described by Descriptors.There are a number of common properties for Terminations and properties specific tomedia streams. The common properties are also called the termination stateproperties. For each media stream, there are local properties and properties of thereceived and transmitted flows.

The following table lists all of the possible Descriptors and their use. Not alldescriptors are legal as input or output parameters to every command.

Command parameters are structured into a number of descriptors. In general, thetext format of descriptors is:

DescriptorName=<someID>{parm=value, parm=value_.}.Parameters may be fully specified, over-specified or under-specified:

Fully specified parameters have a single, unambiguous value that the commandinitiator is instructing the command responder to use for the specified parameter.

Under-specified parameters, using the CHOOSE value, allow the commandresponder to choose any value it can support.

Over-specified parameters have a list of potential values. The list order specifiesthe command initiator's order of preference of selection. The command responderchooses one value from the offered list and returns that value to the commandinitiator.





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Descriptorname

Description

Media A list of media stream specifications.

TerminationState Properties of a termination (which can be defined in

packages) that are not stream specific.

Stream A list of Remote/Local/LocalControl Descriptors for a singlestream.

Local Contains properties that specify the media flows that the MGreceives from the remote entity.

Remote Contains properties that specify the media flows that the MGsends to the remote entity.

LocalControl Contains properties (which can be defined in packages) that

are of interest between the MG and the MGC.

Events Describes events to be detected by the MG and what to do whenan event is detected.

Signals Describes signals applied to terminations.

Audit In Audit commands, identifies which information is desired.

Packages In AuditValue, returns a list of packages realized by thetermination.

ServiceChange In ServiceChange, what, why ServiceChange occurred, etc.

ObservedEvents In Notify or AuditValue, report of events observed.

Statistics In Subtract and Audit, report of statistics kept on a termination orstream.

Error Contains an Error Code and optionally error text; it may occur incommand replies and in Notify requests.




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Example of some Descriptor in Add and AddReply:

To add a new termination to context, the Add command is used. The MGC must

issue an Add.request to the MG. In the Add.request typically contains:

context ID: It specifies the context a new termination shall be added to. If a newcontext is to be created, the context ID is set to a wildcard ($).

termination ID: The termination ID indicates which termination shall be added.Typically the MGC will set this to CHOOSE, which means, that the MGW canfreely select one of its terminations.

termination descriptors: The termination can be specified by Media, Stream, Local,LocalControl, Events, Signals ….descriptors.

The MGW adds the termination, either by creating it (ephemeral termination) or takeit from the NULL context (physical termination).

The AddReply gives the acknowledgement and can contain as additional parametersan Observed Events descriptor and a Statistics descriptor.

When text encoding the protocol, the descriptors consist of session descriptions asdefined in SDP (RFC2327).





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Transaction = 222{Context = $ {

Priority = 0,

Add =ip/83/access/${Media {Stream = 1 {Local {v=0\r\nc=IN IP4 $\r\nm=audio $ RTP/AVP 0\r\n} ,Remote {v=0\r\nc=IN IP4 192.168.7.141\r\nm=audio 5134 RTP/AVP 0\r\n} ,LocalControl {Mode: Inactivegm/rsb=ON

} } } },Add =ip/83/core/${Media {Stream = 1 {Local {v=0\r\nc=IN IP4 $\r\nm=audio $ RTP/AVP 0\r\n} ,LocalControl {Mode: Inactivegm/rsb =ON}

Termination Id

Termination Id

Reply=222{

Context=168{Add=ip/83/access/334 {Media{Stream=1{Local{v=0\r\nc=IN IP4 192.168.7.250\r\nm=audio 16568 RTP/AVP 0\r\n}}\n\t\t\t}\n\t\t},Add=ip/83/core/336{Media{Stream=1{Local{v=0\r\nc=IN IP4 192.168.8.80\r\nm=audio 16568 RTP/AVP 0\r\n}

Termination Id

Termination Id

Fig. 6




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1.8 Service Change Reasons

There may be events which must always be reported without any activationmechanism. Such events are indicated by a SERVICE CHANGE command. Typicalexamples for events triggering a SERVICE CHANGE are

termination out of service / termination in service

MGW restart

MGW registration (initial startup).

MSC Server out of service,

etc.

The following ServiceChangeReasons are defined. This list may be extended by anIANA registration.





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Service Change Reasons

915 State Loss

914 Event Capability Failure

913 Signal Capability Failure

912 Mux Capability Failure

911 Modem Capability Failure

910 Media Capability Failure

909 MGC Impending Failure

908 MG Impending Failure

907 Transmission Failure

906 Loss of lower layer connectivity (e.g. downstream sync)

905 Termination taken out of service

904 Termination malfunctioning

903 MGC Directed Change

902 Warm Boot

901 Cold Boot

900 Service Restored

Fig. 7




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1.9 Packages

Protocol defines basic set of parameters, events and signals and statistics. Butbecause different technologies and systems require different parameters, events,signals and procedures it is possible to define packages, which are specific.

Packages may also define new error codes.

Packages defined by IETF will appear in separate RFCs.

Packages defined by ITU-T may appear in the relevant recommendations (e.g. asannexes).





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ALERT (H.248.23)Enhanced Alerting

ANDISP (H.248.23) Analog Line Display

XAL (H.248.26)Extended Analog Line Supervision Package

AMET (H.248.26) Automatic Metering

IPFAXIP Fax package

FAXFax package

TXPText Telephone package

TXCText Conversation package

FTMDFAX/Textphone/Modem Tones Detection package

CTYPCall Type Discrimination package

NBNotification Behavior

SEGSegmentation Package

TDMCTDM Circuit Package

RTPRTP Package

NTNetwork Package

CTBasic Continuity Package

AL Analog Line Supervision Package

CDCall progress tones detection package

CGCall Progress Tones Generator Package

DDDTMF Detection Package

DGBasic DTMF Generator Package

TONEDETTone Detection Package

TONEGENTone Generator Package

ROOTBase Root PackageGGeneric

PackageIDPackage Name

Fig. 8




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1.10 Events

The EventsDescriptor parameter contains a RequestIdentifier and a list of events thatthe Media Gateway is requested to detect and report. The RequestIdentifier is usedto correlate the request with the notifications that it may trigger. Requested eventsinclude, for example, fax tones, continuity test results, and on-hook and off-hooktransitions.

Each event in the descriptor contains the Event name and optional parameters. TheEvent name consists of a Package Name (where the event is defined) and anEventID.





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H.248.2CONNCHNGConnection Mode Changed

H.248.2CONNCHANGEConnection State ChangeH.248.2FAXCONNCHANGEFax Connection State Change

H.248.2CALLDISRESCall Type Discrimination Result

H.248.2DTONEDiscriminating Tone Detected

H.248.2CONNCHANGEConnection State Change

H.248.1PLTRANSPayload Transition

H.248.1QUALERTQuality Alert

H.248.1NETFAILNetwork Failure

H.248.1CMPCompletion of a continuity test

H.248.1FLFlashhook

H.248.1OFOff-hook

H.248.1ONOn-hook

H.248.1CEDigitMap Completion Event

H.248.1DA…DDDTMF character A … D

H.248.1DODTMF character #

H.248.1DSDTMF character *

H.248.1D0 ... D9DTMF character 0 ... 9

H.248.1LTDLong Tone Detected

H.248.1ETDEnd Tone Detected

H.248.1STDStart Tone Detected

H.248.1SCSignal Completion

H.248.1CAUSEGeneric error event

StandardEvent IDEvent name

Fig. 9




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1.11 Tokens

Token Coding for Token

Add "A"

Audit "AT"

AuditCap "AC"

AuditValue "AV"

Auth "AU"

Bothway "BW"

Brief "BR"

Buffer "BF"

Ctx "C"

ContextAudit "CA"

DigitMap "DM"

Disconnected "DC"

Delay "DL"

Duration "DR"

Embed "EM"

Emergency "EG"

Error "ER"

EventBuffer "EB"

Events "E"

Failover "FL"

Forced "FO"

Graceful "GR"

H221 "H221"

H223 "H223"

H226 "H226"

HandOff "HO"

ImmAckRequired "IA"

Inactive "IN"





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Isolate "IS"InSvc "IV"

InterruptByEvent "IBE"

InterruptByNewSignalsDescr "IBS"

KeepActive "KA"

Local "L"

LocalControl "O"

LockStep "SP"

Loopback "LB"

Media "M"

Megacop "!"

Method "MT"

MgcId "MG"

Mode "MO"

Modify "MF"

Modem "MD"

Move "MV"

MTP "MTP"

Mux "MX"

Notify "N"

NotifyCompletion "NC"

Nx64k "N64"

ObservedEvents "OE"

Oneway "OW"

OnOff "OO"

OtherReason "OR"

OutOfSvc "OS"

Packages "PG"

Pending "PN"

Priority "PR"




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Profile "PF"Reason "RE"

Recvonly "RC"

Reply "P"

Restart "RS"

Remote "R"

ReservedGroup "RG"

ReservedValue "RV"

Sendonly "SO"

Sendrecv "SR"

Services "SV"

ServiceStates "SI"

ServiceChange "SC"

ServiceChangeAddress "AD"

SignalList "SL"

Signals "SG"

SignalType "SY"

Stats "SA"

Stream "ST"

Subtract "S"

SynchISDN "SN"

TerminationState "TS"

Test "TE"

TimeOut "TO"

Topology "TP"

Trans "T"

ResponseAck "K"

V18 "V18"

V22 "V22"

V22bis "V22b"





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V32 "V32"V32bis "V32b"

V34 "V34"

V76 "V76"

V90 "V90"

V91 "V91"

Version "V"




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1.12 Error descriptor

Error Codes sent by MGC:

400 Syntax error in message Yes

401 Protocol Error No

402 Unauthorized Yes

403 Syntax error in transaction request No

406 Version Not Supported Yes

410 Incorrect identifier No

411 The transaction refers to an unknown ContextId No

413 Number of transactions in message exceeds maximum No

421 Unknown action or illegal combination of actions Yes

422 Syntax Error in Action No

430 Unknown TerminationID Yes

431 No TerminationID matched a wildcard Yes

433 TerminationID is already in a Context Yes

434 Max number of Terminations in a Context exceeded Yes

435 Termination ID is not in specified Context Yes

441 Missing Remote or Local Descriptor Yes442 Syntax Error in Command No

443 Unsupported or Unknown Command Yes

444 Unsupported or Unknown Descriptor Yes

445 Unsupported or Unknown Property Yes

446 Unsupported or Unknown Parameter Yes

447 Descriptor not legal in this command Yes

448 Descriptor appears twice in a command Yes

449 Unsupported or Unknown Parameter or Property Value Yes

454 No such parameter value in this package Yes457 Missing parameter in signal or event Yes

458 Unexpected Event/Request ID Yes

501 Not Implemented Yes

503 Service Unavailable Yes

533 Response exceeds maximum transport PDU size Yes

The PCS sends out the error codes listed above. The errors, which are marked in thethird column with “No”, will not be sent out, but instead an error code 400 will be

generated.





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Error Codes sent by MG:

Supported H.248.8 Error Codes: ALL, with exception of:

#514 "Media Gateway cannot send thespecified announcement"

#518 "Event buffer full"

#519 "Out of space to store digit map"

#520 "Digit Map undefined in the MG"

#522 "Functionality Requested inTopology Triple Not Supported"

Supported Error Codes defined inpackages:

All error codes defined in supportedpackages need to be supported.

The PCS reacts on all error codes not mentioned below in the same way: Thesession will either be rejected (ADD error command) or removed (MODIFY error) onthe Application Function interface.

The following error codes have a special meaning:

412 No ContextIDs available432 Out of TerminationIDs or No TerminationID available506 Number of Transaction Pendings Exceeded510 Insufficient resources526 Insufficient bandwidth529 Internal hardware failure in MG530 Temporary Network failure531 Permanent Network failure

In all these cases the PCS will mark the BGF as overloaded and wait a configurabletime before it retry to allocate resources on the BGF.




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1.13 Examples

ConnectionConnection Set UpSet Up – – Virtual TrunkingVirtual Trunking

4: Context = $Add = 10,Add = $

5: Reply Context = abc Add = 10, Add = 11

{local parameter (SDP)}

8: Context = abcModify = 11

{remote param. (SDP)}

9: Reply Context = abcModify = 11

Create Transaction,Create RTP-Term.,Choose localsession parameter

Add remotesession parameter

to transaction data

6: Context = $Add = 20,Add = $

{remote parameter (SDP)

7: Reply Context = xyz Add = 20, Add = 21{local parameter (SDP)}

Create Transaction,Create RTP Term.,choose localsession parameter

hiG 1100 / 1200hiG 1100 / 1200

Fig. 10





33

ConnectionConnection SetSet Up:Up: PCS 5000PCS 5000 -- BGFBGF

P-CSCF SPDFClient 1 SBC Client 2

1.SIP: invite

2.Diam.:AAR

3.H248: Add

4.H248: AddReply

5.Diam.:AAA

First SessionFirst Context(Client 1- Remote partner)

Fig. 11

05 fn42425en60gla1 ia interface doc

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