Compaq W25 User Manual

Ericsson W25

Fixed Wireless Terminal for WCDMA/HSPA Mobile Networks

Application Note:

Fax over IP with Ericsson W25

Contents

1 INTRODUCTION .......................................................................3

1.1 BACKGROUND .............................................................................3

2 SYSTEM OVERVIEW................................................................4

2.1 THE FAX OVER IP SOLUTION.........................................................4

2.2 VOIP NETWORK...........................................................................5

2.2.1 SIP Server..................................................................................5

2.2.2 PSTN Gateway..........................................................................5

2.2.3 Session Border Controller (SBC)...............................................6

2.3 FAX CALL SETUP.........................................................................6

2.4 T.38 AND T30 PROTOCOL............................................................7

2.5 ACCESS NETWORK ......................................................................7

3 ERICSSON W25........................................................................8

3.1 SIGNALING...................................................................................8

3.1.1 Details over the SIP Implementation..........................................8

3.2 SECURITY....................................................................................8

3.3 MEDIA STREAM............................................................................8

3.3.1 Jitter Buffer.................................................................................8

3.3.2 Echo Canceller...........................................................................9

3.3.3 QoS............................................................................................9

3.4 INTEROPERABILITY.......................................................................9

4 W25 CONFIGURATION ..........................................................10

4.1 FAX SERVICE .............................................................................10

4.1.1 IMS User Agent........................................................................10

4.1.2 IMS Call Server........................................................................11

4.1.3 IMS Media Codec ....................................................................12

4.1.4 VoIP timer................................................................................14

5 REFERENCES ........................................................................15

6 ACRONYMS AND ABBREVIATIONS.....................................16

7 APPENDIX A...........................................................................17

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1 Introduction

This document gives a brief description of the Fax over IP and the
implementation in the Ericsson W25. It also includes a guidance of how to
configure the Ericsson W25.

1.1 Background

The Ericsson W25 is intended for residential and SOHO applications. The
focus for the Ericsson W25 is to enable both broadband data and voice
calls using existing 3G network. Additionally support for analog Fax
operation is essential when targeting business users such as SOHO users.

Fax support in GSM mobile networks is standardized, but not in 3G mobile
networks. The available options in 3G mobile networks are T.37, store and
forward mailbox, or T.38 Fax over IP. Since Fax communication is
considered legally binding when doing business, the demand on Fax
operation is that the sender of a Fax can get a receipt that the fax has been
delivered to the recipient in real-time. Therefore, Ericsson has chosen Fax
over IP (T.38) using packet switched connection in the Ericsson W25 to
enable Fax operation in 3G mobile networks. This, since it is the only option
that can guarantee that a fax has been delivered to the recipient in real­time. This can be done thanks to HS(D)PA introduced in 3G mobile
networks with features such as increased data bandwidth and reduced
delay.

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2 System Overview

Fax over IP is based on Voice over IP. The Voice over IP system utilizes IP
technology over broadband networks and uses SIP for control signaling
and RTP for media. The differences between Fax and Voice are the
codecs. For voice the normal voice codecs are G.711, G.729, AMR etc, but
T.38 is used for fax. The voice and fax media are both sent in RTP packets.

W25

Access Network

SIP ServerSIP Server

PSTN Networks

Fax GWFax GW

Fax over IP

T.38

Figure 1 - Typical network for VoIP running Fax (T.38)

The fax machines still use the analog interface and the T.30 fax protocol. In
the picture above one fax is connected to the dedicated fax port on the
Ericsson W25 and the other fax to the PSTN.

The Ericsson W25 has a SIP User Agent associated to the fax port. The
User Agent registers in the SIP server. The User Agent originates and
terminates the FoIP (Fax over IP) calls and use the SIP protocol to manage
and control the media streams (T.38). T.38 enables real-time facsimile
communication over an IP network.

W25 does NOT support fax in clear channel i.e. G.711. This is not feasible
due to, primarily, lack of phase-synchronization between the endpoints. It
may be possible to transmit shorter fax messages but the method would
not be proven robust enough to be useful.

2.1 The Fax over IP solution

To enable a fax call a SIP server and PSTN gateway is required. There is
actually no need for extra features more than a basic call, i.e. no
supplementary services are required.

Analog lineAnalog line

T.30T.30

The SIP server needs to support SIP according to RFC 3261 and the Fax
gateway needs to support SIP and T.38. In addition to this the operator
might have requirements on charging etc, which is not taken into account in
this document. There are several products on the market which fulfill the
basic requirements.

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2.2 VoIP network

The basic building blocks for Voice and Fax over IP are a SIP server and a
PSTN gateway with T.38 support. Sometimes the PSTN gateway might
consist of two units; Media Controller and Media Gateway. Normally a
Session Border Controller (SBC) is used to handle security.

W25

2.2.1 SIP Server

The SIP server is the heart of VoIP system. It keeps track of the location of
each User Agent (UA). Calls are routed through the server. Each User
Agent has to authenticate to the server.

2.2.2 PSTN Gateway

The PSTN Gateway can be one single unit or divided in a Media Gateway
Controller (MGC), including the Signaling Gateway (for SS7/ISUP
connectivity), and the Media Gateway (MGW). Divided or not the
functionality is the same.

To be able to send fax between FWT and Fax machines on PSTN, each
User Agent in a FWT has an associated E.164 number in the PSTN. The
interworking between PSTN and VoIP is done in the PSTN gateway. Note
that the Fax E.164 number is separate from the E.164 number associated
with the SIM card in the FWT. So there is one E.164 number for fax and
another for voice.

SIP/RTP

SBCSBC

SIP

SIP ServerSIP Server

SIP

RTP

MGC-SGMGC-SG

MGWMGW

ISUP

TDM

PSTN Networks

2.2.2.1 Media Gateway Controller (MGC)

The Media Gateway Controller (MGC) provides interworking between the
SIP session control signaling and ISUP call control signaling to/from
external PSTN/PLMN networks. Furthermore, it controls the Media
Gateway resources.

The MGC normally provides the following capabilities:

• Handles multimedia session establishment, modification, and
termination using the SIP protocol in the IP Multimedia domain
and appropriate ISUP protocol in the circuit switched domain.

• Supports addressing and routing of multimedia sessions to and
from SIP server and interconnected PSTN nodes.

• Controls one or more Media Gateways.

• Performs mapping of application level signaling (SIP/ISUP).

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2.2.2.2 Media Gateway (MGW)

The Media Gateway (MGW) provides interworking between PSTN and IP
media streams, i.e. conversion between circuit-switched TDM (Time
Division Multiplexing) bearer circuits and packet-switched media streams
(RTP).

2.2.3 Session Border Controller (SBC)

The SBC can also be called outbound proxy, which normally is a SIP aware
proxy/stateful firewall. If an SBC is used in the network, the User Agent has
to be configured to use the SBC for all SIP sessions. Usually the SBC is
placed together with the firewall so the only way in to the SIP server is
through the SBC. An SBC can also overcome problems with NAT in the
network.

2.3 Fax Call Setup

The picture below shows the SIP signaling during a fax call.
When sending a fax from a fax-machine connected to the W25, the off-

hook status as well as the called number (B-subscriber)is detected by the
W25. This is included in a INVITE-message which is sent to the SIP server.

The SIP server forwards the INVITE to the appropriate fax gateway, which
then calls subscriber B on the PSTN. When the B subscriber answers, a
voice call is setup using the voice codec G.711. At the same time as the B
Fax answers, it starts sending out CED (called station identification) tones,
which is detected by the Fax gateway. Now the Fax gateway sends a re­invite to use T.38 protocol instead of voice codec G.711. The normal fax
training, e.g. negotiating fax transfer speed, is done using the T.38 protocol.
After the training, the pages are transferred and then the faxes go on-hook,
which then results in a SIP BYE message.

W25

Invite

200 OK

ACK

Re-Invite (T.38)

200 OK

ACK

IP

Fax GWFax GW

Ring signal

Detects off-hook

Fax tones

detected

Fax answer

Dialing

IP

SIP ServerSIP Server

Invite

200 OK

Audio (G.711)

Re-Invite (T.38)

200 OK

T.38

Fax call

completed

Detects
on-hook

Bye

200 OK

Bye

200 OK

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