Cisco VG200 Istruzioni per l'installazione e il funzionamento

CHAPTER

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Using the Cisco Voice Gateway 200

This chapter describes how to use the Cisco Voice Gateway 200 (VG200) with

Cisco CallManager 3.0 or later. It introduces the Media Gateway Control

Protocol (MGCP) and describes how MGCP supports redundant

Cisco CallManager servers. This chapter then provides an overview of the

configuration required to make this work. This chapter includes the following

topics:

•Voice over IP Overview, page 1-1

•Using the Cisco VG200 with Cisco CallManager, page 1-6

•Using the Cisco VG200 with MGCP, page 1-7

•Using a Cisco VG200 Gateway with H.323, page 1-8

•Where to Go Next, page 1-14

Voice over IP Overview

Voice over IP (VoIP) enables your gateway to carry live voice traffic (for example,

telephone calls and faxes) over an IP network. VoIP can consolidate voice and

data traffic for more efficient use of bandwidth, reduce toll charges, and enable

alternatives to expensive and proprietary PBX systems.

The Cisco VG200 is an important component of a VoIP solution, providing a

simple and inexpensive interface between an Ethernet data network and the Public

Switched Telephone Network (PSTN).

Chapter 1 Using the Cisco Voice Gateway 200

Voice over IP Overview

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This section covers the following topics, describing in a general way how VoIP

works and defining the standards supported by the Cisco VG200 used to

implement a VoIP network.

•Implementing a VoIP Network, page 1-2

•The H.323 Standard, page 1-3

•Media Gateway Control Protocol, page 1-5

Implementing a VoIP Network

To implement VoIP, you must first establish a working IP network and ensure that

the network is properly tuned to reduce congestion. Voice traffic is more sensitive

to congestion than data traffic, meaning that users will notice more congestion

when using VoIP telephones than when using IP for other purposes. For more

information about configuring IP, refer to the “Configuring IP” chapter in the

Cisco IOS 12.0 Network Protocols Configuration Guide.

You also need to thoroughly understand your company’s existing voice network

and dial plan, which is the pattern of dialed numbers that provides access to

different telephony endpoints. You can then begin the process of integrating your

voice and data networks, which should be done cautiously and systematically to

prevent disruption of existing telephone service.

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Chapter 1 Using the Cisco Voice Gateway 200 Voice over IP Overview

Figure 1-1 Typical VoIP Network

In a VoIP network, illustrated in Figure 1-1, it is possible for IP telephony devices

to interoperate directly. However, to connect to the PSTN, an intermediary device,

called a gateway, is required. A VoIP gateway, such as the Cisco VG200, allows

users of IP phones and PC-based soft phones to exchange calls with users of plain

old telephone service (POTS) phones on the PSTN. The gateway translates

between the signals used on the PSTN and the IP packets used to transmit data on

a TCP/IP network.

The H.323 Standard

The H.323 standard describes a method for converting between voice and data

transmission formats and for managing connections between telephony

endpoints. H.323 is actually a collection of protocols that define standard methods

for interconnecting H.323 endpoints (sometimes called terminals) and POTS

devices.

POTS phones

IP phones Soft phones

PSTN

Gateway

TCP/IP network

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Chapter 1 Using the Cisco Voice Gateway 200

Voice over IP Overview

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Figure 1-2 The H.323 Standard

H.323 endpoints can communicate among each other without the intervention of

any other devices, however to connect them to the PSTN, to manage call setup and

network bandwidth, and to simplify administration, the following devices are

often used:

•H.323 gateway—Connects H.323 devices to POTS devices on the PSTN

•H.323 gatekeeper—Manages network bandwidth and provides a central point

for call administration

•H.323 proxy—An H.323 endpoint that acts as an intermediary between H.323

endpoints and other IP telephony devices

To standardize communication among these devices and endpoints, H.323 utilizes

the following protocols:

•H.245—Used to negotiate channel usage and capabilities

•H.225—Used for call signaling and call setup

•Registration, Admission and Status (RAS)—Used to communicate with the

H.323 gatekeeper

•Real-Time Protocol/Real-Time Control Protocol (RTP/RTCP)—Used to

create and transmit audio packets on the IP data network

POTS phones

H.323 end point

PSTN

H.323 Gateway

H.323 Gatekeeper

Simple IP

telephony devices

H.323 Proxy

TCP/IP network

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Chapter 1 Using the Cisco Voice Gateway 200 Voice over IP Overview

H.323 is a peer-to-peer protocol, helping to establish and manage connections

among H.323 endpoints. As with other peer-to-peer protocols, the many-to-many

relationships become difficult to configure and maintain in large networks. MGCP

was developed specifically to overcome the problem of scalability inherent in the

H.323 protocol.

Media Gateway Control Protocol

MGCP establishes a master-slave protocol which, when compared to H.323,

simplifies voice network administration and improves reliability and

performance. In particular, it makes configuration and administration of gateway

devices easy. With MGCP, gateways are defined by MGCP as slave devices in

relation to the master, referred to as a call agent, which manages connections

between endpoints and controls how gateways function.

Figure 1-3 MGCP

Because an MGCP gateway is a slave device, it derives most of the configuration

it requires from the call agent. To configure an MGCP gateway, all you need to do

is to identify the call agent on the gateway and identify the gateway to the call

agent. In addition, MGCP optionally supports multiple call agents, which can

eliminate a potential single point of failure in the voice network.

POTS phones

MGCP-managed endpoints

PSTN

MGCP Gateway

MGCP call agent

TCP/IP network

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Chapter 1 Using the Cisco Voice Gateway 200

Using the Cisco VG200 with Cisco CallManager

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Using the Cisco VG200 with Cisco CallManager

When used with Cisco CallManager 3.0 or later as a call agent, the Cisco VG200

functions as an MGCP gateway. The Cisco VG200 supports MGCP only on its

Foreign Exchange Station (FXS) and Foreign Exchange Office (FXO) analog

ports, while it supports H.323 on its T1 digital ports.

As shown in Figure 1-4, a Cisco VG200 gateway allows you to connect standard

POTS telephones (connected directly to the gateway or anywhere on the PSTN)

with Cisco IP SoftPhones, Cisco IP Phones, or any H.323-compliant telephony

devices. The Cisco VG200 provides a 10/100BaseT Ethernet port for connection

to the data network and a number of different telephony interfaces, depending on

the hardware and software configuration you choose.

Figure 1-4 Using the Cisco VG200 with Cisco CallManager

Currently, the Cisco VG200 supports the following types of analog ports and

connections:

•1 to 4 FXO ports for connecting to a central office or PBX

•1 to 4 FXS ports for connecting to POTS telephony devices

•1 or 2 BRI ports for connecting to ISDN

•1 or 2 E1/T1 digital ports:

–

For connecting to the PSTN using FXO emulation

–

For connecting to a T1 channel bank using FXS emulation

POTS

phones

Cisco

CallManager

Backup Cisco

CallManager

Cisco

Softphone

Cisco

IP phone

PSTN

Voice Gateway

10/100BaseT Ethernet

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FXO ports FXS ports

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Chapter 1 Using the Cisco Voice Gateway 200 Using the Cisco VG200 with MGCP

–

For connecting to a PBX by means of a trunk (tie) line using Ear and

Mouth (E&M) emulation

These ports can be used to integrate a VoIP network with POTS devices, PBXs,

ISDN, or the PSTN.

The key to integrating voice and data networks using a Cisco VG200 gateway is

Cisco CallManager, which handles call requests and establishes connections

between IP devices placing and receiving calls.

When you use a Cisco VG200 gateway with H.323, Cisco CallManager can be

used like an H.323 proxy. You can identify the Cisco CallManager server as the

only H.323 endpoint known to the Cisco VG200 gateway on its Ethernet (LAN)

port, so you do not have to configure and maintain a complex dialing plan on the

gateway. This also allows the use of less intelligent IP telephony devices within

an H.323 voice network, because not every device has to function as an H.323

endpoint.

You configure the Cisco VG200 gateway with information about

Cisco CallManager, using the Cisco IOS command line interface (CLI). The

procedures and commands required to perform this configuration are described in

Chapter 3, “Configuring Voice over IP.”

When used with MGCP, you must also configure the Cisco CallManager server

with information about each port on a Cisco VG200 gateway, using the

Cisco CallManager Administrator, a Web-based graphic user interface (GUI).

The Cisco CallManager Administration Guide describes the procedures for

performing this configuration.

Using the Cisco VG200 with MGCP

To use a Cisco VG200 gateway with MGCP, you must use Cisco CallManager as

the call agent, and you currently must use an analog voice network module and

FXS or FXO voice interface cards. You use the Cisco IOS CLI to enable MGCP

on the gateway and to identify the Cisco CallManager server. Cisco CallManager

then assumes control over establishing and tearing down connections between IP

endpoints on your network and endpoints connected through the PSTN.

When using MGCP with a Cisco VG200 gateway, all dial-plan related

configuration elements are controlled by Cisco CallManager, and should not be

configured in the Cisco VG200 gateway for MGCP-managed endpoints.

Chapter 1 Using the Cisco Voice Gateway 200

Using a Cisco VG200 Gateway with H.323

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When you use a Cisco VG200 gateway with Cisco CallManager, you can use its

redundant call agent feature to eliminate a potential single point of failure in the

VoIP network. This feature requires that you have twoor three Cisco CallManager

servers available on your network and that you identify each of these servers using

CLI commands. You identify the primary Cisco CallManager server as the MGCP

call agent, and identify the second and third Cisco CallManager servers as backup

servers.

Once the redundant call agent feature is configured, if the primary

Cisco CallManager server becomes unavailable, the Cisco VG200 gateway will

monitors MGCP packets sent by the Cisco CallManager and when no such traffic

is detected, it sends keepalive packets to which the Cisco CallManager server

should respond. If the Cisco VG200 gateway does not detect any packets from the

Cisco CallManager for a specified period, it will try to establish a new connection

with the backup Cisco CallManager server.

Another feature of the Cisco VG200 is called switchback, which refers to the way

that the gateway reestablishes a connection with the primary Cisco CallManager

server when the primary server becomes available again. You can configure the

Cisco VG200 to reestablish connection immediately, or to wait for a specified

length of time to ensure greater stability in the voice network.

Using a Cisco VG200 Gateway with H.323

The Cisco VG200 functions as an H.323 gateway with the following voice

interface types, which are currently not supported with MCGP:

•E1 (R2 and E&M analog emulation)

•T1-CAS (FXO, FXS, and E&M emulation)

•E1-PRI

•T1-PRI

•ISDN/BRI

•E&M

Note that H.323 configuration is more complicated compared to MGCP and does

not provide for redundant call agents. The following sections describe how to use

the Cisco VG200 with H.323 with various interface and signalling types:

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Chapter 1 Using the Cisco Voice Gateway 200 Using a Cisco VG200 Gateway with H.323

•T1-CAS Analog Port Emulation, page 1-9

•FXO Emulation, page 1-10

•FXS Emulation, page 1-11

•E&M Emulation, page 1-11

•Linking PBX Users with E&M Trunk Lines, page 1-12

•Direct-Inward Dialing on a BRI Port, page 1-13

T1-CAS Analog Port Emulation

T1 is a digital signaling standard that uses time-division multiplexing (TDM) to

divide the available bandwidth of 1.544 Mbps into twenty-four 64-kbps timeslots.

E1 is a similar standard, commonly used in Europe and many parts of Asia, which

provides 2.048 Mbps bandwidth, divided into 32 time slots, with 30 time slots

available for voice conversations.

A DS-0 group is a group or collection of DS-0 timeslots. Each of these DS-0

groups can be used to emulate an FXO analog connection to the PSTN, an FXS

analog connection to a T1 channel bank, or an E&M connection for enabling a

trunk (tie) line.

When you configure your DS-0 groups, you can configure DS-0 hunt groups,

which allow any available voice port to be used for a range of phone numbers.

This eliminates the chance that a single DS-0 could fail, or that a DS-0 is busy at

the time of a call.

To use a Cisco VG200 gateway to interconnect a VoIP network and a T1

connection, you configure from 1 to 24 DS-0 groups for each T1 connection, and

then define a voice port for each DS-0 group. The Cisco VG200 currently supports

T1 connections using channel associated signaling (CAS).

You can use DS-0 groups with FXO analog port emulation to connect to a PSTN

central office (CO). When connecting T1-CAS to the PSTN, either the

Cisco VG200 gateway or the Cisco CallManager must be able to identify the

dialed number (DNIS) of each call and route it to the appropriate endpoint on the

IP network.

Chapter 1 Using the Cisco Voice Gateway 200

Using a Cisco VG200 Gateway with H.323

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FXO Emulation

Figure 1-5 Using FXO Analog Emulation with T1-CAS

To make this work you must obtain Direct Inward Dialing (DID) service from

your local exchange carrier (LEC) on the DS-0 groups used for incoming calls.

DID service is provided only on incoming connections, so to use it, you must

divide your T1 channels into incoming and outgoing DS-0 groups.

You can configure your dial plan on a Cisco VG200 gateway, using Cisco IOS CLI

commands to identify the destination endpoint and coder-decoder (CODEC) for

each incoming call on each DS-0 group. This is a straightforward task, but this

method of configuration does not take full advantage of the Cisco CallManager

and can only be used for H.323 endpoints.

You can configure a Cisco VG200 gateway to direct all incoming calls to the

Cisco CallManager, and configure your dial plan using the Cisco CallManager

Administrator. This simplifies administration and lets you connect simple IP

telephony devices (such as the Cisco IP Phone 7960 and Cisco IP SoftPhone) to

the PSTN. This configuration is described in Chapter 3, “Configuring Voice over

IP.”

Cisco CallManager

H.323 endpoint

Simple IP

telephony devices

Voice Gateway

10/100BaseT Ethernet

33345

PSTN

T1-CAS

DS-0 groups 0-23

FXO emulation

DID incoming lines

DS-0 groups 0-23

Port 0

Port 1

Gateway

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