ZyXEL Communications VOP1372G-61 Handbuch

VOP1372G-61

Support Notes

December 2011

Edition 1.0

Content

Introduction ...............................................................................................................3

H.248 Signaling Procedures .....................................................................................11

VOP1372G-61...........................................................................................................17

H.248 application .....................................................................................................29

Debug guide .............................................................................................................37

Introduction

What is VoIP?

Traditional voice switching business has long been the mainstay for a relatively small

group of global equipment suppliers. But times has changes, carriers are adopting a

standardized IP Multimedia Subsystem architecture for next-generation networks

with distributed softswitch call control and independent Media Gateways. In addition

to the legacy circuit switch equipment suppliers, the equipment market reshuffles

and opens to new vendors.

Media Gateway is a network element which packetizes voice traffic from POTS signals

to IP packets, and performs critical voice network features such as echo cancellation,

silence detection and suppression, etc. Softswitch is the one which provides call

control to Media Gateways in a packet voice network.

Through VOP1372G-61, ZyXEL transforms its IP DSLAM portfolio into multi-service

DSLAM and starts to enter the Media Gateway market worldwide.

VoIP is the sending of voice signals over the Internet Protocol. This allows you to

make phone calls and send faxes over the Internet at a fraction of the cost of using

the traditional circuit-switched telephone network.

What is H.248?

H.248 protocol is a media gateway control protocol that enables Switched Circuit

Network (SCN) to transmit voice traffic over IP. H.248 protocol specifies

master/slave architecture for decomposed gateways. In the master/slave

architecture, Media Gateway Controller (MGC) is the master server and MGs are

the slave clients that behave as simple switches. One MGC can serve multiple MGs.

Network Diagram

H.248 protocol enables the creation, modification and deletion of media streams

across a media gateway, including the capability to negotiate the media formats to

be used.

Terminations and Contexts

The connection model (the figure below) for the H.248 protocol describes

the logical entities, or objects, within the MG that can be controlled by

the MGC. The main abstractions used in the connection model are

Terminations and Contexts.

Example of H.248 connection model

Terminations

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 identities (TerminationIDs), assigned by the MG

at the time of their creation.

There are two types of terminations: semi-permanent terminations and

ephemeral terminations. Terminations representing physical entities have

a semi-permanent existence. Terminations representing ephemeral

information flows, such as RTP flows, would usually exist only for the

duration of their use.

Contexts

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

Context describes the topology and the media mixing and/or 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 are not associated to any other Termination.

Contexts are identified by a Context_Id which is assigned by the MG and is

unique within the MG.

H.248 Message Structure

A message is an information unit sent or received by the H.248 protocol.

In the H.248 protocol, one or more commands are encapsulated in a

message.

The H.248 message mechanism is shown below.

H.248 Message Structure

Message

Messages start with a header which is followed by several transactions.

The header contains a Message Identifier (MID) and a Version Number.

The MID identifies the sender of the message which may be a domain

address, domain name or device name. Domain name is a suggested

default. The Version Number identifies the version of the protocol the

message conforms to.

Transaction

A message contains one or more transactions. The transactions in a

message are treated independently. There is no order implied.

Transactions include requests and responses, and responses are

divided into two types: TransactionReply and TransactionPending.

TransactionRequest

The TransactionRequest is invoked by the sender. There is one

Transaction per request invocation. A request contains one or more

Actions, each of which specifies its target Context and one or more

Commands per Context.

The structure of Transaction Request is as

follows: TransactionRequest(TransactionId {

ContextID {Command … Command},

. . .

ContextID {Command … Command } })

The TransactionID parameter must specify a value for later

correlation with the TransactionReply or TransactionPending response

from the receiver.

TransactionReply

Transaction Reply is a response of the transaction receiver to the

Transaction Request. Every transaction should have its Reply. A

TransactionRequest stops being executed either if all commands in

the TransactionRequest have been carried out successfully or a

failure is encountered during the execution of a non-optional

command in the TransactionRequest.

The structure of Transaction Reply is as

follows: TransactionReply(TransactionID {

ContextID { Response …Response },

. . .

ContextID { Response …Response } })

The TransactionID parameter must be the same as that of the

corresponding TransactionRequest.

TranctionPending

The receiver invokes the Transaction Pending. A Transaction Pending

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 command will take some

time to complete.

The structure of Transaction Pending is as

follows:

TransactionPending(TransactionID{ } )

The TransactionID parameter must be the same as that of the

corresponding TransactionRequest.

Actions

Actions are related to Contexts. Actions are identified by a ContextID. In

an action, commands should be processed in order.

An action begins with the Context header 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 to that Context.

Context header is followed by several commands, and these commands

are related to the Context identified by the ContextID.

Commands

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

the Context and Termination attributes including specifying the topology

structure of the Context and specifying the event reported by the

Termination, for example, what signals and actions can be imposed on

the Termination. A command is composed of the command header and

command parameters.

All commands as described in para 7.2 of ITU-T recommendation

H.248.1 version 2 shall be supported:

Add: To add adds a Termination to a Context. This shall also be used to a

Context when the first termination is added to a

Context.

Modify: To modify the properties, events, signals of an existing

termination.

Subtract: To remove a termination from its context. This command is

also used to delete a context when the last termination is removed

from the context.

Move: To atomically move a Termination to another

Context.

AuditValue: The AuditValue command returns the current state of

properties, events, signals and statistics of Terminations.

AuditCapabilities: The AuditCapabilities command returns all the

possible values for Termination properties, events and signals allowed

by the Media Gateway.

Notify: This command allows the MG to inform the MGC of the

occurrence of events in the MG.

ServiceChange: To allows the Media Gateway to notify the Media

Gateway Controller that a Termination or group of Terminations is about

to be taken out of service or has just been returned to service.

ServiceChange shall also be 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 ServiceChange command. The MGC may also use

ServiceChange to instruct the MG to take a Termination or group of

Terminations in or out of service.

Descriptors

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. Descriptors may be returned as

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

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

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

DescriptorName=<someID>{parm=value,

parm=value…}.

H.248 Packages

All the packages as per annex E of ITU-T recommendation H.248.1 version 2

are required to be supported by both MG and MGC.

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