01-Chapter 1 H.248

Technical Manual – Signaling & ProtocolsTable of Contents

Table of Contents

Chapter 1 H.248........................................................................................................................... 1-1

1.1 Overview.......................................................................................................................... 1-1

1.1.1 Definition and Functions of the Mc Interface.........................................................1-1

1.1.2 H.248 Implementation in CSCN.............................................................................1-1

1.1.3 Structure of the Protocol Stack..............................................................................1-2

1.2 Introduction of H.248 Protocol..........................................................................................1-3

1.2.1 Overview...............................................................................................................1-3

1.2.2 Message Structure................................................................................................1-6

1.3 Signaling Procedures.....................................................................................................1-14

i

Technical Manual – Signaling & ProtocolsChapter 1 H.248

Chapter 1 H.248

1.1 Overview

H.248/MEGACO has been jointly developed within the ITU-T and the IETF. It is

named H.248 by the ITU-T and MEGACO by the IETF. Hereinafter, both are called

H.248 in this manual.

H.248 is a media gateway control protocol. In a decomposed gateway model, the

H.248 protocol is used for the communication between a media gateway controller

(MGC) and a media gateway (MG), implementing the function of the MGC controlling

MGs. In UMTS, the H.248 protocol is applied on the Mc interface.

1.1.1 Definition and Functions of the Mc Interface

I. Definition of the Mc Interface

The Mc interface is the standard interface between the MSC Server (GMSC Server)

and the MGW. It is H.248 protocol compliant. Aiming at special requirements of 3GPP,

H.248 extended Transaction and Package are defined. The Mc interface is an

additional interface for 3GPP R4. The physical interface mode may be ATM or IP.

Protocol messages through the Mc interface may be encoded in a binary format or in

a text format. The underlying transmission mechanism provides protocol bearer for it

by using MTP3b (ATM based signaling transfer) or SCTP (IP based signaling

transfer).

II. Functions Provided by the Mc Interface

The Mc interface provides the capabilities of the static and dynamic resources for the

MSC Server (GMSC Server) controlling the various transmission modes

(IP/ATM/TDM) in the MGW in the call processing procedure, such as terminal

property, terminal connection switching relationship, MGW borne media streams. The

Mc interface also provides the call independent MGW status maintenance and

management capability.

1


1.1.2 H.248 Implementation in CSCN

The H.248 protocol is utilized on the interface between MSOFTX3000 and UMG8900,

namely Mc interface defined in UMTS. See Figure 1-1.

Nc

MSC Server(MSOFTX3000)

Mc Mc

MWG(UMG8900)

MWG(UMG8900)

H.248 H.248

GMSC Server(MSOFTX3000)

Nc

MSC Server(MSOFTX3000)

Mc Mc

MWG(UMG8900)

MWG(UMG8900)

H.248 H.248

GMSC Server(MSOFTX3000)

Figure 1-1 H.248 protocol implementation in CSCN

1.1.3 Structure of the Protocol Stack

As shown in Figure 1-2, the H.248 protocol is applied to the Mc interface. Protocol

transmission may be based on IP (figure a) or based on ATM (figure b). IP based

transmission is typically used due to the current networking architecture.

H.248

SCTP

IP

MAC

L1

(G)MSC ServerMc

MGW

a) IP based

McMGW

b) ATM based

(G)MSC Server

H.248H.248

SCTP

IP

MAC

L1

STC

SAAL

AAL5

MTP3B

ATM

PL

H.248

STC

SAAL

AAL5

MTP3B

ATM

PL

H.248

SCTP

IP

MAC

L1

(G)MSC ServerMc

MGW

a) IP based

McMGW

b) ATM based

(G)MSC Server

H.248H.248

SCTP

IP

MAC

L1

STC

SAAL

AAL5

MTP3B

ATM

PL

H.248

STC

SAAL

AAL5

MTP3B

ATM

PL

Figure 1-2 Structure of H.248 protocol

2


1.2 Introduction of H.248 Protocol

1.2.1 Overview

I. Basic Concepts

Media Gateway (MG): The media gateway converts media provided in one type

of network to the format required in another type of network. For example, a MG

could terminate bearer channels from a switched circuit network (for example,,

PCM) and media streams from a packet network (for example,, media streams in

an IP network). This gateway may be capable of processing audio, video and

data alone, and will be capable of full duplex media translations. The MG may

also play some audio/video signals and perform a number of interactive voice

response (IVR) functions, or may perform media conferencing.

Media Gateway Controller (MGC): Controls the parts of the call state that pertain

to connection control for media channels in a MG.

Multipoint Control Unit: An entity that controls the setup and coordination of a

multi-user conference that typically includes processing of audio, video and data.

Stream: Bidirectional media or control flow received/sent by a media gateway as

part of a call or conference.

II. Connection Model

The connection model for the protocol describes the logical entities, or objects, within

the Media Gateway that can be controlled by the Media Gateway Controller. The main

abstractions used in the connection model are Terminations and Contexts. Figure 1-3

illustrates the connection model:

3


Termination

SCN Bearer Channel

Termination

SCN Bearer Channel

Termination

RTP Stream

Context

Context

Context

Media Gateway

Null Context

*

Termination

SCN Bearer Channel

Termination

SCN Bearer Channel

Termination

RTP Stream *

Termination

RTP Stream *

Context

Figure 1-3 Example of H.248/MEGACO connection model

In the connection model defined by the H.248/MEGACO, two entities, namely Context

and Termination, are included. A Context shall contain one or more Terminations;

otherwise, the Context will be deleted. A Termination shall exist in only one Context at

a single time point.

1) Context

A Context is an association between a number of Terminations. The Context

describes the topology and the media mixing/switching parameters if more than two

Terminations are involved in the association.

There is a special Context called the null Context. It contains Terminations that

Terminations that are not associated to any other Termination. Terminations in the null

Context can have their parameters examined or modified, and may have events

detected on them.

The maximum number of Terminations in a Context is a MG property.

The attributes of Contexts are:

ContextID: Context identifier, which is 32-bit and uniquely identifies a Context

within the scope of the MG.

Some special contextIDs are coded as shown in Table 1-1:

Table 1-1 Codes of special Contexts

Context Binary code Text code

NULL Context 0 ‘-’

4


CHOOSE

Context0xFFFFFFFE ‘$’

ALL Context 0xFFFFFFFF ‘*’

Topology: The topology of a Context describes the flow of media between the

Terminations within a Context. In contrast, the mode of a Termination describes

the flow of the media at the ingress/egress of the media gateway.

Priority: The priority is used for a Context in order to provide the MG with

information about a certain precedence handling for a Context. The value range

is 0~15. The less the value is, the higher the priority is.

Emergency indicator: An indicator for an emergency call is also provided to allow

a preference handling in the MG.

2) Termination

A Termination is a logical entity on a MG that sources and/or sinks media and/or

control streams. A Termination is described by a number of characterizing properties,

which are grouped in a set of descriptors that are included in commands.

Terminations have unique identifies (TerminationIDs), assigned by the MG at the time

of their creation.

Usually Terminations are grouped into two classes: semi-permanent Terminations and

ephemeral Terminations. Terminations representing physical entities have a semi-

permanent existence. For example, a Termination representing a TDM channel might

exist for as long as it is provisioned in the gateway. Only if the configuration

information is deleted, the corresponding Termination disappears. Ephemeral

Terminations represent ephemeral information flows, such as RTP flows, would

usually exist only for the duration of their use. Ephemeral Terminations are created by

means of an Add command. They are destroyed by means of a Subtract command. In

contrast, when a physical Termination is added to or subtracted from a Context, it is

taken from or to the null Context, respectively.

Termination dynamics: The protocol can be used to create new Terminations

and to modify the property values of existing Terminations.

TerminationIDs: Terminations are referenced by a TerminationID, which is an

arbitrary schema by the MG. A wildcarding mechanism using two types of

wildcards can be used with TerminationsIDs. The two wildcards are ALL and

CHOOSE.

Packages: Different types of gateways may implement Terminations that have

widely differing characteristics. In order to achieve MG/MGC interoperability,

optional properties of the Termination are grouped into Packages, and a

Termination realizes a set of such Packages.

5


Termination properties and descriptors: Terminations have properties. The

properties have unique PropertyID.

ROOT Termination: The ROOT Termination is typically used to refer to the entire

gateway. Packages may be defined on ROOT. Root thus may have properties,

events, signals, statistics and parameters. The ROOT Termination may appear

in a Modify, Notify, AuditValue, AuditCapability and ServiceChange commands.

Any other use of the ROOT TerminationID is an error.

Commands: The protocol provides commands for manipulating the logical

entities of the connection model, Contexts and Terminations. Most commands

are for the specific use of the Media Gateway Controller as command initiator in

controlling Media Gateways as command responders. The exceptions are the

Notify and ServiceChange commands: Notify is sent from Media Gateway to

Media Gateway Controller, and ServiceChange may be sent by either entity. For

the meanings of the commands, please reference the following section about

command explanation.

Descriptors: The parameters to a command are termed Descriptors. A Descriptor

consists of a name and a list of items. Descriptors may be returned in the

response as output from a command. In any such return of descriptor contents,

an empty descriptor is represented by its name unaccompanied by any list.

1.2.2 Message Structure

A message is an information unit sent by the H.248 protocol. A message may be

encoded in a binary format or in a text format.

In the case of binary codes, specifications defined in ITU-T X.680 (ASN.1) are

used for description, and BER rules defined in X.690 for encoding;

In the case of text format, RFC 2234 ABNF specifications are followed.

MGCs should support both encoding formats. MGs may support one of or both

formats. Any H.248 message shares the same structure as shown in Figure 1-1.

6


Figure 1-1 H.248 message structure

A message contains multiple transactions that have nothing to do with each other and

can be handled separately; a transaction is composed of several actions and actions

correspond to Contexts; an action constitutes a series of commands restricted by a

Context. In this way, H.248 message mechanism is shown in Figure 1-2.

H.248 message

Transaction1

ContextID1

Command1

Des-1 Des-n

Commandn

ContextIDn

TransactionIDn

H.248 message

Transaction1

ContextID1

Command1

Des-1 Des-n

Commandn

ContextIDn

TransactionIDn

Figure 1-2 Message mechanism

7


I. Message

Information units transmitted or accepted by the H.248 protocol are called messages.

A message begins with the Header followed by several transactions.

The message Header contains the Message Identifier (MID) and the Version Number:

The MID identifies the message sender, and may be set to a provisioned name

(for example, domain address/domain name/device name). Domain name is a

suggested default.

The Version Number identifies the version of the protocol the message conforms

to. Versions consist of one or two digits, beginning with version 1 for the present

version of the protocol.

The transactions in a message are treated independently. There is no order implied.

I. Transaction

Commands between the Media Gateway Controller and the Media Gateway are

grouped into Transactions, each of which is identified by a TransactionID.

Transactions consist of one or more Actions. An Action consists of a series of

Commands that are limited to operating within a single Context.

A Transaction begins with a Transaction Header (TransHdr), in which TransactionID is

contained. TransactionID is assigned by the sender of the Transaction, and it is

unique within the scope of the sender.

TransHdr is followed by the Actions of the Transaction. The Actions must be executed

in order. At the first failing command in an Action, processing of the remaining

commands in that Transaction stops except Optional Command. Transactions

guarantee ordered commands processing, which is one significant function to

introduce Transactions.

Commands may be marked as “Optional” which can override this behavior. If a

command marked as Optional results in an error, subsequent commands in the

Transaction will be executed.

Transactions include requests and responses, and responses are divided into two

types: TransactionReply and TransactionPending.

TransactionRequest

Each TransactionRequest requests to activate one Transaction. One Transaction

contains one or more Actions and each Action includes one or more commands

related to one single Context.

The structure of TransactionRequest is as follows:

8


TransactionRequest(TransactionId { ContextID {Command ... Command},

. . . ContextID {Command ... Command } })

TransactionReply

TransactionReply is a response of the Transaction receiver to the

TransactionRequest, indicating that the receiver completes the executing of the

TransactionRequest command. Every transaction should have its Reply. The

following cases indicate the completion of the executing of a TransactionRequest:

3) All the commands in the TransactionRequest are successfully executed;

4) A non-optional command in the TransactionRequest fails to be executed.

The structure of TransactionReply is as follows:

TransactionReply(TransactionID { ContextID { Response ...Response },. . . ContextID { Response ...Response } })

TransactionPending

The receiver invokes the TransactionPending. A TransactionPending indicates that

the Transaction is actively being processed, but has not been completed. It is used to

prevent the sender from assuming the TransactionRequest was lost where the

transaction will take some time to complete.

The structure of TransactionPending is as follows:

TransactionPending (TransactionID { } )

Transactions are presented as TransactionRequests. Corresponding response to a

TransactionRequest is received in a single reply, possibly preceded by a number of

TransactionPending messages.

The H.248 protocol supports the transactions as shown in Table 1-1:

Table 1-1 H.248 transactions

Transaction Description

MGW

Communication Up

Message reported by MGW after resumption of MGC-MGW

communication.

MGW Out Of

Service

Reported to MGC when MGW becomes faulty, to indicate

MGW to get out of service.

9



MGW RestorationRestoration message reported by MGW after its recovery

from fault.

MGW Register

This function actively sends the Register message to the

MGC to request for registration when the whole system is

powered up. Only after successful registration of MGW can

the MGC use the resources on the MGW.

MGW Re-RegisterIn some cases, such as MGC handover, MGC may request

the MGW to register again.

(G)MSC Server

Ordered Re-

Register

(G)MSC SERVER requests the MGW to register again, and

the MGW initiates the transaction after it receives the

command.

(G)MSC Server

Restoration

After recovery of (G)MSC SERVER from fault, (G)MSC

SERVER sends this message to the MGW.

Termination Out Of

Service

After a Termination fails, the MGW sends this message to

the MGC so that the MGC will no longer use this resource.

Termination

Restoration

When the Termination recovers from failure, the MGW

sends this message to notify the MGC to update the

resource status.

Audit ValueTo audit the current values of the various attributes

requesting for Termination resources.

Audit CapabilityTo audit the capability set of the various attributes

requesting for Termination resources.

MGW Capability

Change

In case of change of MGW due to fault or OMC

configuration, the MGW uses this transaction to notify the

MGC, so that the MGC will update the capability status of

the MGW.

(G)MSC Server Out

Of ServiceTo notify MGW when (G)MSC SERVER becomes faulty.

Change Through

Connection

To change the MODE attribute of Termination. This

operation can be used to control the directions of media

flows, including forward, backward, bi-directional, isolated.

10



Change Flow

Direction

To control the direction of the media flow between

Terminations by modifying the topology parameter between

Terminations.

Isolate Bearer

Termination

To isolate one Termination from its media flow relation with

other Terminations, so that it has no media flow relation with

any Termination.

Join Bearer

TerminationTo add a Termination in an existing CONTEXT.

Establish Bearer

To establish bearer between MGWs. This operation includes

applying for a Termination resource and establishing the

bearer to the destination MGW.

Prepare Bearer

To apply for Termination resource from MGW. This is an

operation prior to bearer. It may result in the generation of a

new CONTEXT.

Activate

Interworking

Function

To activate the IWF on an MGW.

Release Bearer To release bearer between MGWs. This operation does not

release the Termination resource.

Release

Termination To release Termination resource.

Bearer Released Bearer release completion event reported by MGW. This

event is requested by the MGC.

Bearer EstablishedBearer establishment completion event reported by MGW.

This event is requested by the MGC.

Send Tone

Send-tone operation. During a call, the MGC can request

the Termination to send a certain tone to a certain direction,

such as ring-back tone, busy tone, and so on.

Play

Announcement

To play announcement in intelligent or supplementary

services, and so on.

Send DTMF To send DTMF tone.

Detect DTMF To request MGW to detect DTMF tones.

11



Report DTMFMGW reports to the MGC about the completion of DTMF

tone detection.

Announcement

Completed

Announcement playing completion message reported by

MGW.

Activate Voice

Processing

Function

To activate the voice processing function, including EC,

Reserve Circuit.

Tunnel Information

Up

MGW reports IPBCP frame to MGC, and the MGC sends it

to the peer MGW by means of tunnel.

Tunnel Information

Down

MGC sends the IPBCP message from another MGC to

MGW.

Tone Completed Tone playing completion event reported by MGW.

Stop

AnnouncementMGC requests MGW to stop sending ANNOUNCEMENT.

Stop Tone MGC requests MGW to stop sending Tone.

Stop DTMF MGC requests MGW to stop sending DTMF tone.

Stop DTMF

Detection MGC requests MGW to stop DTMF detection.

Confirm Char MGC requests MGW to confirm the reserved resources.

Modify Char MGC modifies resources previously reserved on MGW.

Reserve Char MGC reserves resources on MGW.

Bearer Modified Bearer modification completion event.

Bearer Modification

Failed Bearer modification failure event.

TFO Activation MGC activates the TFO function of MGW.

Optimal Codec and

Distant List Notify MGW reports Codec List of Codec negotiation during TFO.

Codec Modify MGW reports Codec modification result.

Distant Codec List MGW reports remote Codec negotiation result.

12



Command

Rejected

When MGW detects an illegal or inexecutable command

from the MGC, it returns Command Rejected.

Modify Bearer

Characteristics MGC requests to modify bearer resource.

II. Action

Actions are related to Contexts. An action consists of a series of commands that are

limited to operate within one Context. Actions are identified by a ContextID. In an

action, commands should be processed in order.

An action begins with the Context header (CtxHdr) in which ContextID is contained for

identifying the Context this action corresponds to. ContextID is assigned by the MG

and is unique within the scope of the MG. The MGC shall use the ContextID in all

subsequent transactions relating that Context.

CtxHdr is followed by several commands, and these commands are related to the

Context identified by the ContextID.

III. Command (CMD)

Commands are the major contents in an H.248 message. They control the Context

and Termination attributes including to specify the event reported by the Termination

what signals and actions can be imposed on the Termination, as well as specifying

the topology structure of the Context. A command is composed of the command

header (CMDHdr) and command parameters. In H.248 protocol, command

parameters are grouped into “Descriptors”.

H.248 protocol defines eight commands, all of which are sent to MG by MGC

except the command “Notify”, which is sent to MGC by MG. The command

“ServiceChange” can be sent by either the MG or the MGC. The meanings of

H.248 commands are as follows:

13


Table 1-2 H.248 commands

CommandSending

directionMeaning

Add MGC?MG

The Add command adds a Termination to a

Context. The Add command on the first

Termination in a Context is used to create a

Context.

Modify MGC?MGThe Modify command modifies the properties,

events and signals of a Termination.

Subtract MGC?MG

The Subtract command disconnects a

Termination from its Context and returns

statistics on the Termination’s participation in

the Context. The Subtract command on the

last Termination in a Context deletes the

Context.

Move MGC?MGThe Move command atomically moves a

Termination to another Context.

AuditValue MGC?MG

The AuditValue command returns the current

state of properties, events, signals and

statistics of Terminations.

AuditCapabilities MGC?MG

The AuditCapabilities command returns all the

possible values for Termination properties,

events and signals allowed by the media

gateway.

Notify MG?MGC

The Notify command allows the MG to inform

the MGC of the occurrence of events in the

MG.

14


CommandSending

directionMeaning

ServiceChange MGC?MG

The ServiceChange command allows the MG

to notify the MGC that a Termination or group

of Terminations is 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 an 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 ServiceChange command.

IV. Descriptor

The parameters to a command are termed Descriptors. A descriptor consists of a

name and a list of items. Some items may have values. Many commands share

common descriptors. In general, the text format of descriptors is as follows:

DescriptorName=<someID> { parm = value, parm = value ...... }

H.248 protocol defines 18 types of descriptors, as shown in Table 1-3.

Table 1-3 Descriptors

Descriptor

NameDescription

Modem Identifies modem type and properties.

Mux

Describes multiplex type for multimedia Terminations (for

example, H.221, H.223, H.225.0) and Terminations forming the

input mux.

Media A list of media stream specifications.

TerminationStat

e

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 single stream.

LocalContains properties that specify the media flows that the MG

receives from the remote entity.

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Descriptor

NameDescription

RemoteContains properties that specify the media flows that the MG

sends to the remote entity.

LocalcontrolContains properties that are of interest between the MGC and

the MG.

Events A list of events that the MG is requested to detect and report.

EventBuffer

A list of events, with their parameters if any, that the MG is

requested to detect and buffer when EventBufferControl equals

LockStep.

SignalsDescribes signals and/or actions to be applied (for example,

ringback tone) to the Terminations.

Audit Specifies what information is to be audited.

ServiceChangeIn ServiceChange, what, why service change occurred, and so

on.

DigitMapA dialing plan resident in the MG used for detecting and

reporting digit events received on a Termination.

Statistics In Subtract and Audit, Report of Statistics kept on a Termination.

Packages A AuditValue, returns a list of packages realized by Termination.

ObservedEvent

sIn Notify or AuditValue, report of events observed.

TopologySpecifies flow directions between Terminations in a Context,

and applies to a Context instead of a Termination.

1.3 Signaling Procedures

The following part uses an example to illustrate a typical implementation of the H.248

protocol. The diagrams of the call procedure abstractly display the interaction

between a media gateway and the media gateway controller instead of taking issues

like time graduation into account.

The example is about a call set up between two residential gateways. User A and

User B are respectively connected to two residential gateways RGW1 and RGW2,

16


and the gateways are under the control of the same media gateway controller. The

example only describes a successful call, and it is assumed that the gateways have

registered on the media gateway controller.

The procedure is divided into two processes, namely call setup process and call

backout process.

I. Call Setup Process

H.248 call setup process is shown in Figure 1-2.

17


18


Figure 1-2 Call setup process

1) MGC sends a Modify message to both gateways to detect the offhook event on

the terminations.

2) It is assumed that User A hooks off first. After RGW1 detects the event, it sends

to MGC a Notify message in which the corresponding event information and

detected timestamp are contained. MGC returns a response message.

3) MGC sends a Modify command to RGW1, indicating to RGW1 to send dial tone

to User A. RGW sends dial tone to the user and meanwhile returns a response

message.

4) User A hears the dial tone and begins to dial.

5) MGC receives the Notify message from RGW1 and begins to analyze the digits.

It is assumed that the called party is connected to RGW2 which is managed by

the same MGC. MGC creates a new context for RGW1 and adds a physical

termination TermA in it. If User B is idle, ringback tone is sent to User A. At the

same time, an ephemeral termination is created and then added in the preceding

context. The connection domain IP address and the media domain port number

of the ephemeral termination are not specified. RGW1 creates a context whose

ID is 1. The physical termination TermA is added in the context. Meanwhile, an

ephemeral termination EphA is created and its IP address and port number are

assigned. Then, RGW1 returns a corresponding response in which the IP

address and the port number used are indicated.

6) MGC sends a similar transaction to RGW2. RGW2 creates a context whose ID is

2, then it adds the physical termination TermB in the context; meanwhile, RGW2

creates an ephemeral termination EphB and returns a response message.

7) User B hooks off. RGW2 uses a Notify command requesting to report this event

to MGC. MGC also returns a Notify response.

8) MGC sends to RGW1 a message to stop sending ringback tone to User A, and

sets the remote SDP information of EphA. The mode of both terminations is

modified to SendRecv (previously both created as RecvOnly mode). RGW1

returns a response message indicating the success of the operation.

9) MGC sends a transaction to RGW2, indicating to stop ringing tone on TermB.

After the completion of the processing, RGW2 returns a response.

10) The users can have a conversation. Once the call is terminated by either party,

the other party will hear busy tone.

II. Call Backout Process

Call backout process is shown in Figure 1-1.

19


Figure 1-1 Call backout process

1) It is assumed that the calling party User A will terminate the call. RGW1 sends a

Notify message to MGC to report this event. MGC returns a response message

of the Notify command.

2) MGC generates a Modify command, indicating to RGW2 to play busy tone to

User B. The mode of both terminations is set to RecvOnly. RGW2 returns a

response indicating the success of the operation.

3) MGC directs RGW1 to remove both terminations from the context 1 and return

the statistics information of the ephemeral termination as the response.

4) User B hears busy tone and then hooks on. RGW2 reports a Notify message to

MGC. MGC returns a corresponding response message.

20


5) The gateway sends a Subtract command to delete TermB and EphB from the

context 2. RGW2 also deletes the terminations from the context 2 and then

returns a response in which the statistics information of the ephemeral

termination is contained. Here, a call procedure ends. The terminations return to

the initial status and await a new call.

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