May 2004
Fax and Modem Training
1
Modem Types
2
T.30 FAX protocol
• T.30 is control protocol between fax machines. It is
used for call establishment, capabilities exchange,
end-of page and multipage signaling and call
release
• The T.30 control signals are send at speed of
300bps and are modulated according to ITU V.21
standard
• Image data is send at speed of 2,400 – 14,400bps
3
T.30 FAX Tones
• CNG – Originating FAX calling tone, short
periodic tone of 1100Hz, 0.5sec every 3.5
seconds
• CED – FAX answering tone, continuous
2100Hz , 2.6 to 4.0 seconds duration
• V.21 Preamble – series of HDLC flags send
for 1.5seconds, modulated on 1650Hz
4
Fax basic call
CNG
Answer
CED
Preamble followed by DIS
Preamble followed by DCS
Training (TCF)
Preamble followed by CFR
Training (TCF)
Transmit Fax page)
End of procedure (EOP)
5
T.30 FAX signal messages
• DIS – Digital Identification signal indicating terminating fax capabilities
(such as data rate)
• DCS – Digital Command signal indicating mtransmission mode (such
as rate) that will be used by originating fax.
• TCF – Training check sequence sent for 1.5 seconds
• CFR – Confirm to receive message, indicating that the fax is ready to
receive the document.
• MPS – MultiPage signal, send after each page (if more than one page
is send)
• MCF – Message Confirmation that the page was received
• EOP – End of procedure message, indicating that there are no more
page to be sent.
• DCN – Disconnect message
6
Fax T.30 Diagram
CNG
DCS/TCF
Fax Image P1/MPS
Fax Image P2/EOP
DCN
PCM In ORG
L
CED/DIS
CFR
MCF
MCF
PCM In ANS
R
8
7
T.38 messages
T.38 messages are divided into three groups:
1)
T.30 Indicator messages – 1 byte packets indicating No signal, CNG, CED,
V.21preamble,…) – such as
"00 00 01 06“
“00 00” – are the T.38 packet sequence number
“01” – number of bytes, 1 byte in this example
“06” – code of V.21 preamble
“04” – code of CED signal
2) Low speed data control messages (V.21 data) – usually 6 bytes packets carrying T30
control messages such as DIS, DCS, EOP,DCN (used for FAX capabilities exchange, end of
page and call release)…. – such as
"00 02 06 c0 01 80 00 00 c0" (packets starting with c0 01 80 .. are the V.21 data)
3) High speed data type message– usually 50bytes (or more) packets containing fax image.
8
T.38 Packets
The following parameters define the T.38 packet redundancy.
a) FaxRelayEnhancedRedundancyDepth = 0 to 4
b) FaxRelayRedundancyDepth = 0 to 2
The "'FaxRelayEnhancedRedundancyDepth" parameter defines the redundancy level (or number of repetitions) of T.30
Indicator messages and redundancy level of low speed data control messages.
For example if "FaxRelayEnhancedRedundancyDepth = 2" each indicator message will be send three times:
00 00 01 06
00 00 01 06
00 00 01 06
The Low speed data control messages with FaxRelayEnhancedRedundancyDepth =4
00 09 06 c0 01 80 00 00 8c 00 04 06 c0 01 80 00 00 c2
06 c0 01 80 00 00 c0
06 c0 01 80 00 00 ff
06 c0 01 80 00 00 c8
In the example above the first six bytes are current data (c0 01 80 00 00 8c), while the other 4 groups of six bytes contain
data of earlier four T.38 packets.
The "FaxRelayRedundancyDepth" parameter defines the redundancy of high speed data packets.
For example if "FaxRelayRedundancyDepth = 2" each T.38 packet will carry current data and the data of previous two
packets.
9
Annex D
Real-time facsimile over H.323 systems
• Annex defines H.323 procedure to transfer
T.38 packets.
10
Replacing an existing audio stream with a
T.38 fax stream
Originating
Endpoint
Terminating
Endpoint
Established voice call
Fax tone is detected
RequestMode (T.38)
RequestModeAck (T.38)
CloseLogicalChannel (audio)
OpenLogicalChannel (T.38)
CloseLogicalChannel (audio)
OpenLogicalChannel (T.38)
CloseLogicalChannelAck (audio)
OpenLogicalChannelAck (T.38)
CloseLogicalChannelAck (audio)
OpenLogicalChannelAck (T.38)
Fax transmission and reception
11
SIP T.38 Call Flow
12
T.38 REINVITE Message (F11)
INVITE sip:+1-650-555-2222@obelix.banyuls.com;user=phone SIP/2.0
Via: SIP/2.0/UDP obelix.banyuls.com:5060; branch=z9hG4bK2d008.1
Via: SIP/2.0/UDP iftgw.there.com:5060;branch=z9hG4bK2d017.1
Max-Forwards: 69
Record-Route: <sip:+1-650-555-2222@obelix.banyuls.com;
maddr=obelix.banyuls.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.banyuls.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 18 INVITE
Contact: <sip:+1-650-555-2222@iftgw.there.com>
Content-Type: application/sdp
Content-Length: 320
v=0
o=faxgw1 2890844527 171090 IN IP4 iftgw.there.com
s=Session SDP
c=IN IP4 iftmg.there.com
t=0 0
m=image 49172 udptl t38
a=T38FaxVersion:0
a=T38MaxBitRate:9600
a=T38FaxRateManagement:transferredTCF
a=T38FaxMaxBuffer:72
a=T38FaxMaxDatagram:316
a=T38FaxUdpEC:t38UDPFEC
a=T38FaxUdpEC:t38UDPRedundancy
13
SIP 200OK to T38 REINVITE (F14)
SIP/2.0 200 OK
Via: SIP/2.0/UDP obelix.banyuls.com:5060; branch=z9hG4bK2d008.1
Via: SIP/2.0/UDP iftgw.there.com:5060;branch=z9hG4bK2d017.1
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.banyuls.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 18 INVITE
Contact: <sip:+1-303-555-1111@ift.here.com>
Content-Type: application/sdp
Content-Length: 320
o=faxgw1 2890846527 2890846528 IN IP4 ift.here.com
s=Session SDP
c=IN IP4 iftmg.here.com
t=0 0
m=image 15002 udptl t38
a=T38FaxVersion:0
a=T38MaxBitRate:9600
a=T38FaxRateManagement:transferredTCF
a=T38FaxMaxBuffer:72
a=T38FaxMaxDatagram:316
a=T38FaxUdpEC:t38UDPRedundancy
14
H323/SIP FAX Configuration Parameters
• IsFaxUsed
0 - No
1 - Fax is send/received using SIP or H323 Annex D. T38 procedure
• DetFaxOnAnswerTone
0 - Receiving Gateway will initiate T38 on receiving V.21 preamble
signal from FAX (default)
1- Receiving GW will initiate T38 upon receiving of CED answer tone
from Fax (not recommended)
• CNGDetectorMode
0 – don’t use (default)
2 - T.38 will start after CNG detection (applicable only for SIP and
not recommended)
15
Channel Parameters (1)
•
DJBufMinDelay
0 to 150 msec (default = 70)
Dynamic Jitter Buffer Minimum Delay.
•
DJBufOptFactor
0 to 13 (default = 7)
Dynamic jitter buffer frame error/delay optimization.
13 is used for fax and modem calls
•
FaxTransportMode Sets the Fax transport
0 = disable (transparent mode)
1 = relay, (default, to be used for T.38)
2 = bypass.
16
Channel Parameters (2)
•
FaxRelayEnhancedRedundancyDepth
0 to 4 (default =0)
Number of repetitions to be applied to each fax control packet
•
FaxRelayRedundancyDepth
0 to 2 (default =0)
Number of repetitions to be applied to each fax relay payload when transmitting to network
•
FaxRelayMaxRate
Limits the maximum rate at which fax messages are transmitted.
0 = 2.4 kbps
1 = 4.8 kbps
2 = 7.2 kbps
3 = 9.6 kbps
4 = 12.0 kbps
5 = 14.4 kbps, (default)
•
FaxRelayECMEnable
0 = Disable using ECM mode during Fax Relay
1 = Enable using ECM mode during Fax Relay. (default)
17
Channel Parameters (3)
•
FaxModemBypassBasicRTPPacketInterval
0 – set internally, default (20msec)
1 – 5msec (not recommended)
2 – 10msec
3 – 20msec
•
FaxModemBypassDJBufMinDelay
0 to 150 msec (default 40msec)
Jitter Buffer during Modem/Fax bypass ,
•
NSEMode
Cisco compatible modem bypass mode
0 = NSE Disabled (default)
1 = NSE Enabled
Note: if used with SIP, the SDP contains “a=rtpmap:100 X-NSE/8000” for 100 as NSE payload type
•
NSEPayloadType
Range 96-127, default NSE payload type is 105. Cisco are using 100
18
Channel Parameters (4)
•
FaxModemBypassCoderType - Coder to be used while performing fax/modem bypass.
Refer to acTCoders enumeration. Usually, high bit rate coders such as G.711 and
G.726/G.727 should be used.
0 = G711 A-law =0, (default)
1 = G711 -law=1,
4 = G726_32
11 = G727_32.
•
FaxModemBypassM Number of 20 msec payloads to be used for generating one RTP
fax/modem bypass packet.
1, 2 (default = 1)
•
FaxBypassPayloadType - Fax Bypass RTP dynamic payload type, default 102
•
ModemBypassPayloadType Modem Bypass RTP dynamic payload type, default 103
19
Channel Parameters (5)
•
V21ModemTransportType
0 = Transparent, (default)
2 = ModemBypass.
•
V22ModemTransportType
0 = Transparent
2 = ModemBypass, (default).
•
V23ModemTransportType
0 = Transparent
2 = ModemBypass, (default).
•
V32ModemTransportType (For V.32 & V.32bis modems)
0 = Transparent
2 = ModemBypass, (default).
•
V34ModemTransportType (For V.34 & V.90 modems)
0 = Transparent
2 = ModemBypass, (default).
20
T.38 FAX parameters – example 1
IsFaxUsed = 1
/* Should be “1” for H323/Annex D
FaxTransportMode = 1
(default)
DetFaxOnAnswerTone = 0
/* Fax Relay, shall be set automatically if IsFaxUsed=1
/*
or SIP/T38
Rx GW will start T38 on V.21 Preamble – (default)
;Optional but recommended
FaxRelayEnhancedRedundancyDepth = 4
FaxRelayRedundancyDepth = 2
FaxRelayECMEnable = 1
FaxRelayMaxRate = 5 (or lower)
/*
/*
/*
/*
Repetitions of control packets
Repetitions of T.38 payload packets
Enable ECM mode
14.4 kbps (default), 3 for 9,6,…
21
Trasparent FAX parameters – example
2
IsFaxUsed = 0
/* Don’t use Annex D
FaxTransportMode = 0
CoderName = g711Ulaw64k
V21ModemTransportType =
V22ModemTransportType =
V23ModemTransportType =
V32ModemTransportType =
V34ModemTransportType =
DJBufOptFactor = 13
SCE = 0
ECE = 1
/*
/*
/*
/*
/*
/*
/*
/*
/*
/*
0
0
0
0
0
Transparent mode, fax is handled same as voice
G.711 A_law or U_law
Transparent
Transparent
Transparent
Transparent
Transparent
optimized jitter buffer for data
No silence suppression
Echo canceller enabled
22
Fax/Modem Bypass Mode – example 3
IsFaxUsed = 0
/* Don’t use T38 H323/SIP procedures
FaxTransportMode = 2
FaxModemBypassCoderType = 0
/* Bypass mode, fallback to G.711 if fax
/* g711Ulaw64k/A_law default bypass coder
FaxBypassPayloadType = 102 /* FAX Bypass dynamic payload type
ModemBypassPayloadType = 103 /* Modem Bypass dynamic payload type
FaxModemBypassBasicRTPPacketInterval= 3
V21ModemTransportType
V22ModemTransportType
V23ModemTransportType
V32ModemTransportType
V34ModemTransportType
=
=
=
=
=
2
2
2
2
2
/*
/*
/*
/*
/*
FaxModemBypassDJBufMinDelay = 40
/* 2 for 10msec, 3 for 20msec
Bypass
Bypass
Bypass
Bypass
Bypass
/* 0-150msec, minimal delay for bypass
23
Modem Bypass CISCO NSE Mode
parameters – example 4
IsFaxUsed = 1
/* Use T38 H323/SIP procedures
NSEMode = 1
NSEPayloadType = 100
FaxModemBypassBasicRTPPacketInterval= 2
/* 2 for 10msec, 3 for 20msec
ModemRTPBypassPayloadType = 103 /* Modem Bypass dynamic payload type
V21ModemTransportType
V22ModemTransportType
V23ModemTransportType
V32ModemTransportType
=
=
=
=
2
2
2
2
V34ModemTransportType = 2
/*
/*
/*
/*
Bypass
Bypass
Bypass
Bypass
/* Bypass
;
24
V.34 Faxes
• Up to 33,600 bit/s compared to 14400 with T.30 faxes
• At the start looks exactly same as modem
• Can’t be transmitted using T.38 relay, unless fallback to
T.30
• Always works with ECM enabled
• Answering fax tone is called ANSam (2100hz with
amplitude-modulation.
• The ANSam tone is followed by V.8 signal
• V.34 capable terminal responds with Call Menu (CM) signal
to the detection of ANSam.
25
Supporting V.34 Faxes
V.34 fax machine full rate is available only in Bypass mode ,
when the channel is configured in one of the configurations described below:
1)
FaxTransportMode = 2 (Bypass)
VxxModemTransportType = 2 (Modem bypass)
In this configuration, both T.30 and V.34 faxes (and all other Modems) work in Bypass mode
2)
FaxTransportMode = 1 (Relay)
VxxModemTransportType = 2 (Modem bypass)
In this configuration, T.30 faxes use Relay mode (T.38) while V.34 Fax uses
Bypass mode.
In order to use V.34 fax in Relay mode (fallback to T.38), you must configure:
FaxTransportMode = 1 (Relay)
VxxModemTransportType = 0 (Transparent)
This configuration forces the V.34 fax machine to work in T.30 mode.
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