Understanding, Integrating, and Troubleshooting Fax...
Transcript of Understanding, Integrating, and Troubleshooting Fax...
BRKUCC-2021
Understanding, Integrating, and Troubleshooting Fax Communications in Today's Enterprise Networks
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Fundamental Concepts
Fax Transport Methods
QoS Considerations
Design Best Practices
Troubleshooting
Fax Servers
Agenda
Fundamental Concepts
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Alexander Bain’s Facsimile Apparatus, British Patent 9745
Fax invented by Alexander Bain in 1843 (patented 30 years before the telephone)
Fax is a ubiquitous form of communication today and Fax over IP (FoIP) is often overlooked in IP Telephony
Group 3 (G3) is today’s fax standard (speeds up to 14.4 kbps)
Super G3 is an optional extension of G3 that allows for speeds up to 33.6 kbps
Fax Communications
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Modern fax communication is based on the Group 3 (G3) fax standard, which is composed of the ITU-T specifications of T.30, T.4, and T.6
T.30 details the signaling, T.4 specifies the Modified Huffman (MH) and Modified Read (MR) encoding methods, and T.6 covers Modified Modified Read (MMR)
G3 Fax Protocols
PSTN
T.30—Signaling and Negotiation Between Fax Machines
T.4/T.6—Specifies the Fax Page Encoding Algorithm
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Basic G3 Fax Transaction
The optional Called Subscriber Identification (CSI) and Non-Standard Facilities (NSF) messages may be transmitted along with the DIS message
The optional Transmitting Subscriber Identification (TSI) message may be transmitted along with the DCS message
Blue messages indicate low speed T.30 signaling and gold messages represent higher speed modulations used in training and page transmission
Sending Fax Receiving Fax
Digital Identification Signal (DIS)
Training Check (TCF)
Digital Command Signal (DCS)
Confirmation (CFR)
End of Procedure (EOP) Message Confirmation (MCF)
Disconnect (DCN)
Fax Page Transmission
Offhook, then dial Calling Tone (CNG)
Ringing, Offhook, Answer
Called Tone (CED)
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Super G3 uses the V.34 modulation to achieve page transmission speeds up to 33.6 Kbps compared to the maximum G3 speed of 14.4 Kbps
The signals used by SG3 are different than G3 and are only recently compatible with T.38.
Super G3 Fax G3 Fax Call Initiation
Super G3 Fax Call Initiation
CNG
CNG
CED
DIS
Originating Fax
Originating Fax
Terminating Fax
Terminating Fax
DIS
ANSam
Joint Menu (JM)
CJ Terminator
Calling Menu (CM)
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The default behavior for Cisco voice gateways is to initially handle all calls as voice calls so all fax calls start as voice calls
Compressed voice codecs are not designed to handle modulated data so alternative, real-time transport methods must be utilized
There are two principal methods of transporting fax communications
Transporting Fax Over IP
1. Passthrough 2. Relay
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Transporting G3/SG3 Fax Protocols Over IP
How Do Standard Fax Communications Integrate With IP Networks? T.30—Signaling and Negotiation Between Fax Machines
T.4/T.6—Specifies the Fax Page Encoding Algorithm
PSTN
Passthrough or Relay Transport
T.4/T.6—Specifies the Fax Page Encoding Algorithm
IP
T.30—Signaling and Negotiation Between Fax Machines
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Modulated fax data information is sampled and encoded as standard Pulse Code Modulation (PCM) (i.e., G.711) and encapsulated in Real Time Protocol (RTP) for transport over IP just like a voice codec does for human speech
From the gateway perspective, this is more or less a G.711 voice call
Commonly referred to as Voice Band Data (VBD)
Passthrough
IP
Analog Modulated Data
RTP Packet with PCM Payload
RTP RTP RTP
Analog Modulated Data
End-to-End Modulated Data
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The analog modulated fax data is demodulated by a Digital Signal Processor (DSP) on the gateway and the binary information is extracted
Binary information is passed over IP using one of several available relay protocols
A DSP on the destination gateway takes the binary information from the relay packets and re-modulates it into a fax data signal on the telephony side
Relay
Analog Modulated Data
Analog Modulated Data
Relay Relay Relay
Relay Protocol
01101010 11000111 10100101 00101011
Relay
IP
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All modulated calls begin as voice calls
The process that transitions a gateway from a VoIP call to the final fax media stream is known as the switchover
There are a variety of different switchover mechanisms that we will discuss throughout this presentation
The Switchover TGW OGW
Fax/Modem Media
VoIP Call
Fax Call
Switchover
Fax Transport Methods
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Fax Transport Methods
Fax over IP
Passthrough Relay
Fax Passthrough
(NSE)
Fax Pass-through
(Protocol)
T.38 Fax Relay (Protocol)
T.38 Fax Relay
(NSE) Cisco
Fax Relay
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Fax Passthrough Switchover Methods
Voice Mode
Fax Passthrough
NSE-Based Switchover
Fax Pass-through
Protocol-Based Switchover
Fax Passthrough
Passthrough Switchover: 1. Codec upspeed
2. VAD disabled on DSP 3. Jitter buffer transitions from
adaptive to a fixed optimum value
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“Fax Passthrough” generically refers to passing fax calls over the G.711 codec
“Fax Passthrough” can also refer to the more specific passthrough transport method using an NSE switchover to pass faxes over G.711
“Modem Passthrough” is a general term used to refer to an NSE-based switchover to passthrough for both fax and modem calls because it is the syntax used by the configuration command for this feature
The underlying operation of “modem passthrough” is different depending on whether it is a fax or a high speed modem call, so to avoid confusion we will refer to “modem passthrough” for a fax call as “fax passthrough” and “modem passthrough” for a modem call as “modem passthrough”
“Fax pass-through” details a switchover to passthrough using the call control protocol
Passthrough Terminology
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Passthrough Switchover Mechanisms
Terminating Gateway (TGW) initiates the switchover to passthrough upon detection of fax signals played by the answering fax device
IP
Tone Detection 1
1
RTP RTP RTP NSE NSE NSE NSE-Based Switchover 2
2 If NSE-based passthrough is configured, the TGW sends NSE switchover packets in the RTP media stream upon detection of CED (G3 fax) or ANSam (SG3 fax) tones
3 Protocol-Based Switchover H.323 SIP
3 If fax pass-through is configured, the TGW initiates the switchover in the H.323 or SIP call control protocols upon detection of fax message flags
OGW TGW
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Payload Type Payload Encoding
Payload Types Used for Modulated Data
The Payload Type field identifies the type of data being carried in the RTP packet
0 8 18 90 96 97 100 119 122
G.711 µ-law
G.711 a-law G.729
RFC 2198 Passthrough Redundancy Cisco Fax Relay Switchover
Cisco Fax Relay Switchover ACK
Named Signaling Event (NSE) Cisco Text Relay
Cisco Fax Relay
Static
Dynamic And Unassigned
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NSE
RTP Payload
R Volume E Event ID Duration
Named Signaling Events (NSE)
Payload Type =100
RTP Header
PT
Cisco proprietary message sent as part of the RTP stream to signal an event, such as a switchover
This form of signaling is call control independent
The Event ID field uses Cisco-defined event numbers to signal a specific task in-band
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NSE Description
NSE Events Used for Modulated Data
The NSE Event ID field uses Cisco-defined values to signal in-band a variety of tasks
192 193 194 199 200 201 202 203
Triggered by 2100Hz tone. Signals switchover to passthrough
Triggered by ANSam tone detection. Signal to disable ECANs Triggered by 4s silence or carrier loss. Signals return to voice mode
Triggered by ANSam tone. Signals support of Cisco Modem Relay Triggered by fax V.21 Preamble. Signals switchover to T.38
ACK to NSE-200 confirming peer gateway switchover to T.38
NACK to NSE-200 implying peer gateway cannot switchover to T.38 Triggered by V.8 CM detection. Signals transition to Modem Relay
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Fax Passthrough Switchover (G3)
NSE-192
TGW
VoIP Call
OGW
OGW: Upspeed codec and switch to Passthrough mode
NSE-192 CED
TGW: Upspeed codec and switch to Passthrough mode Fax Passthrough Call
The underlying call control protocol is unaware of the transition to fax passthrough due to the protocol independent nature of the NSE switchover
The initial VoIP call can be setup by any of the common call control protocols (H.323, SIP, MGCP, or SCCP)
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Fax Passthrough Switchover (SG3)
NSE-192
NSE-193
TGW
VoIP Call
OGW
SG3 Fax Call
ANSam
TGW: Detect 2100 Hz tone. Upspeed codec and switch to Passthrough mode
NSE-192
OGW: Upspeed codec and switch to Passthrough mode
TGW: Detect phase reversal of ANSam. Disable ECAN
NSE-193
OGW: Disable echo cancellers
The initial VoIP call can be setup by any of the common call control protocols (H.323, SIP, MGCP, or SCCP)
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Confirming Fax Passthrough Switchover
Before Switchover: show call active voice brief
11F1 : 10 2924510ms.1 +10530 pid:1 Originate 200 active dur 00:00:07 tx:99/1903 rx:253/4993 IP 1.1.1.2:17932 SRTP: off rtt:0ms pl:4810/0ms lost:0/0/0 delay:60/60/70ms g729r8
VoIP Call
Fax Passthrough Call
After Switchover: show call active voice brief 11F1 : 10 2924510ms.1 +10530 pid:1 Originate 200 active dur 00:00:22 tx:877/125447 rx:1040/128809 IP 1.1.1.2:17932 SRTP: off rtt:1ms pl:40/0ms lost:0/0/0 delay:60/60/60ms g711ulaw TextRelay: off media inactive detected:n media contrl rcvd:n/a timestamp:n/a long duration call detected:n long duration call duration:n/a timestamp:n/a MODEMPASS nse buf:0/0 loss 0% 0/0 last 1359s dur:0/0s
Fax Passthrough Call
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Fax Passthrough Configuration
IP
SIP/H.323:
dial-peer voice 1 voip modem passthrough nse codec g711ulaw
(Note: Can be configured globally under voice service voip or the dialpeer for SIP and H.323. The configuration under the dialpeer takes precedence.)
SCCP:
voice service voip modem passthrough nse codec g711ulaw
(Note: Can only be configured globally under voice service voip for SCCP.)
RTP RTP RTP NSE NSE NSE Feature: fax passthrough
MGCP:
mgcp modem passthrough voip mode nse mgcp modem passthrough voip codec g711ulaw
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Fax Pass-through Switchover (G3)
100 Trying
ACK
200 OK
TGW
VoIP Call - SIP Signaling
OGW
Pass-through parameters for the upcoming session are established in the SIP INVITE and 200 OK messages
INVITE for G.711 Pass-through
Fax Pass-through Call
CED Tone
V.21 Preamble
The V.21 Preamble of the first T.30 message following the CED Tone (i.e. DIS) is the trigger for the transition to pass-through
An H.245 Request Mode for G.711 is used to signal the switchover in H.323 to fax pass-through mode
The initial VoIP call is setup using the SIP call control protocol
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Confirming Fax Pass-through Switchover
Before Switchover: show call active voice brief
126C : 58 11411290ms.1 +10520 pid:1 Originate 200 active dur 00:00:08 tx:112/2154 rx:307/6092 IP 1.1.1.2:17620 SRTP: off rtt:1ms pl:5930/0ms lost:0/0/0 delay:60/60/70ms g729r8
VoIP Call
Fax Passthrough Call
After Switchover: show call active voice brief 126C : 58 11411290ms.1 +10520 pid:1 Originate 200 active dur 00:00:41 tx:1677/252554 rx:1953/258112 IP 1.1.1.2:17620 SRTP: off rtt:4ms pl:40/0ms lost:1/0/0 delay:60/60/60ms g711ulaw TextRelay: off
media inactive detected:n media contrl rcvd:n/a timestamp:n/a long duration call detected:n long duration call duration:n/a timestamp:n/a
Fax Pass-through Call
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Fax Pass-through Configuration
IP
SIP/H.323:
dial-peer voice 1 voip fax protocol pass-through g711ulaw
Note: Can be configured globally under voice service voip or the dialpeer for SIP and H.323. The configuration under the dialpeer takes precedence.
SCCP:
Fax pass-through is NOT supported for SCCP
Feature: fax pass-through
MGCP:
Fax pass-through is NOT supported for MGCP
H.323 RM SIP INVITE
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Fax Transport Methods
Fax over IP
Passthrough Relay
Fax Passthrough
(NSE)
Fax Pass-through
(Protocol)
T.38 Fax Relay (Protocol)
T.38 Fax Relay
(NSE) Cisco
Fax Relay
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Fax Relay Switchover Methods
Voice Mode
T.38 Fax Relay
Protocol-Based Switchover (Standards-Based)
Cisco Fax Relay
RTP PT Switchover
Fax Relay
T.38 Fax Relay
NSE-Based Switchover
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Pre-standard and proprietary fax relay implementation for G3 fax transmissions (no SG3 support)
Default fax transport method on most Cisco voice gateways (Notable exception is ISR G2)
Uses a unique Payload Type switchover that is not used by any other modulated communication transport method
Cisco Fax Relay (CFR)
RTP Payload Type is set to 122 for Cisco Fax Relay data
UDP Header
IP Header
RTP Header
Cisco Proprietary Formatting
Demodulated fax HDLC data carried in the Cisco Fax Relay packet
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Cisco Fax Relay Switchover (G3)
PT-97 CFR Switchover ACK
PT-96 CFR Switchover
TGW
VoIP Call
OGW
Cisco Fax Relay Call
V.21 Preamble
The detection of fax flags (V.21 Preamble) at the TGW triggers the transition to CFR.
PT-96 CFR Switchover
The TGW notifies the OGW of the impending switchover with a special RTP message using a PT value of 96
PT-97 CFR Switchover ACK
The CFR switchover packet is ACK’d with PT-97 message
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Confirming Cisco Fax Relay Switchover
Before Switchover: show call active voice brief
121C : 26 1027355900ms.1 +10510 pid:1 Originate 200 active dur 00:00:11 tx:117/2244 rx:424/8432 IP 1.1.1.2:18274 SRTP: off rtt:2ms pl:7810/0ms lost:0/0/0 delay:60/60/70ms g729r8
VoIP Call
Fax Passthrough Call
After Switchover: show call active voice brief 121C : 26 1027355900ms.1 +10510 pid:1 Originate 200 active dur 00:00:45 tx:1113/22164 rx:1001/19972 IP 1.1.1.2:18274 SRTP: off rtt:2ms pl:0/0ms lost:0/0/0 delay:0/0/0ms cisco TextRelay: off
media inactive detected:n media contrl rcvd:n/a timestamp:n/a long duration call detected:n long duration call duration:n/a timestamp:n/a!
Cisco Fax Relay Call
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Cisco Fax Relay Configuration
IP
SIP/H.323:
dial-peer voice 1 voip fax protocol cisco
Note: CFR is the default fax configuration on most IOS gateways.
SCCP:
voice service voip fax protocol cisco
Note: CFR can only be configured under voice service voip for SCCP starting in IOS Release 12.4(11)T.
RTP RTP PT-97 PT-96 Feature: Cisco Fax Relay
MGCP: ccm-manager fax protocol cisco mgcp fax t38 inhibit
Note: Like CFR, T.38 is also enabled by default for MGCP. However T.38 takes precedence over CFR and must be explicitly disabled with mgcp fax t38 inhibit.
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T.38 is the de facto standard for handling fax transmissions today
Cisco has always supported the 1998 version (known as version 0) of the ITU-T T.38 specification with UDP/UDPTL encapsulation
In IOS version 15.1(1)T Cisco started supporting SG3 fax over T.38 (version 3)
Cisco products can use either NSEs (proprietary) or the call control protocol (standards-based) to switch a call to T.38 mode
T.38 Fax Relay
UDP Header
IP Header
UDPTL Header
UDP Encapsulated T.38
UDP Transport Layer (UDPTL) header is simply a 2 byte sequence number
T.38 Internet Fax Packets (IFP) and optional redundancy packets are carried in the UDPTL payload
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T.38 Switchover—NSE (G3) TGW
VoIP Call
OGW
T.38 Fax Relay Call
NSE-200
OGW: Transition from voice mode to T.38
NSE-201
TGW: T.38 ACK received, start T.38 session
V.21 Preamble
The detection of fax flags (V.21 Preamble) at the TGW triggers the transition to T.38
The initial VoIP call can be setup by any of the common call control protocols (H.323, SIP, MGCP, or SCCP)
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T.38 Switchover—H.323 (G3) TGW
VoIP Call—H.323 Signaling
OGW
T.38 Fax Relay Call
V.21 Preamble
The detection of fax flags (V.21 Preamble) at the TGW triggers the transition to T.38
H.245 Request Mode T.38 ACK
H.245 Request Mode T.38
T.38 parameters for the upcoming fax relay session are negotiated in the H.245 Request Mode messages
Opening of T.38 Logical Channels
Closing of Voice Logical Channels Each gateway closes its voice media channel and opens a T.38 fax relay media channel while acknowledging the same events by the other side
The initial VoIP call is setup using the H.323 call control protocol
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T.38 Switchover—SIP (G3)
100 Trying
200 OK
TGW
VoIP Call - SIP Signaling
OGW
INVITE for T.38 mode
T.38 Fax Relay Call
V.21 Preamble
The detection of fax flags (V.21 Preamble) at the TGW triggers the transition to T.38
T.38 parameters for the upcoming fax relay session are negotiated in the SIP INVITE and 200 OK messages
ACK
T.38 fax relay switchover process using the SIP protocol stack is complete
The initial VoIP call is setup using the SIP call control protocol
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T.38 Switchover—MGCP (G3)
200 OK
200 OK
TGW
VoIP Call - MGCP Signaling
OGW
MGCP NTFY
T.38 Fax Relay Call
200 OK
Call Agent
V.21 Preamble
TGW notifies the CA that fax V.21 flags are detected MDCX - T.38
CA instructs TGW to switchover to T.38 with an MDCX message
MDCX- T.38
CA instructs OGW to switchover to T.38 with an MDCX message
The initial VoIP call is setup using the MGCP call control protocol
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T.38 Switchover—SIP (SG3)
100 Trying
200 OK
TGW
VoIP Call - SIP Signaling
OGW
INVITE for T.38 mode
T.38 SG3 Fax Relay Call
The detection of Calling Menu (CM) at the OGW triggers the transition to T.38
ANSam is sent across in the RTP media stream
ACK
T.38 fax relay switchover process using the SIP protocol stack is complete
ANSam (ANSam tunneled in voice codec)
CM
ANSam
Note: CM is squelched by the OGW DSP and sent over T.38. The CM/JM exchange is done entirely over T.38 prior to T.30 negotiation
The initial VoIP call is setup using the SIP call control protocol
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Confirming T.38 Fax Relay Switchover
Before Switchover: show call active voice brief
11E2 : 4 2956390ms.1 +10500 pid:1 Originate 200 active dur 00:00:11 tx:117/2244 rx:436/8643 IP 1.1.1.2:18496 SRTP: off rtt:3ms pl:4800/0ms lost:0/0/0 delay:60/60/70ms g729r8 !
VoIP Call
Fax Passthrough Call
After Switchover: show call active voice brief 11E2 : 4 2956390ms.1 +10500 pid:1 Originate 200 active dur 00:00:46 tx:707/13770 rx:914/12339 IP 1.1.1.2:18496 SRTP: off rtt:3ms pl:4800/0ms lost:0/0/0 delay:60/60/70ms t38 TextRelay: off
media inactive detected:n media contrl rcvd:n/a timestamp:n/a long duration call detected:n long duration call duration:n/a timestamp:n/a
T.38 Fax Relay Call
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NSE-Based T.38 Configuration
IP
SIP/H.323:
dial-peer voice 1 voip fax protocol t38 nse ls-redundancy 0 hs-redundancy 0 fallback cisco
Note: Configured globally under voice service voip or the dialpeer for SIP and H.323. The dialpeer configuration takes precedence.
Feature: NSE-Based T.38 RTP RTP RTP NSE NSE
SCCP: voice service voip fax protocol t38 nse ls-redundancy 0 hs-redundancy 0 fallback cisco
Note: Can only be configured under voice service voip for SCCP. Cisco IOS gateway support for NSE-based T.38 fax relay with the SCCP voice signaling protocol is introduced in Cisco IOS Release 12.4(11)T."
NSE
MGCP:
Enabled by default Note: no mgcp fax t38 inhibit is required but is default.
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Protocol-Based T.38 Configuration
IP
SIP/H.323:
dial-peer voice 1 voip fax protocol t38 version [0|3] ls-redundancy 0 hs-redundancy 0 fallback cisco
Note: version 0 configures legacy T.38 G3 fax support, whereas version 3 enables the newly supported SG3 over T.38 feature [as of 15.1(1)T]
SCCP:
SCCP does not support protocol-based (version 0 or version 3) T.38
MGCP NTFY Feature: Protocol-Based T.38
MGCP:
mgcp package-capability fxr-package mgcp default-package fxr-package
Note: These two commands are needed to go from the default configuration of NSE-based T.38 to protocol-based T.38. The command no mgcp fax t38 inhibit is required for both NSE and protocol-based T.38, but it is the default. MGCP does not offer any version 3 support.
H.323 RM SIP INVITE
QoS Considerations
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In many cases, fax devices are located where VoIP is already installed
You can “piggyback” fax traffic onto the same QoS policy implemented for VoIP
As a rule of thumb, if good voice quality exists between locations, then fax communications should also work
Are VoIP QoS Policies Sufficient for Fax?
Branch 1 Branch 2
IP
Branch 3
Headquarters
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Delay or latency: the amount of time it takes a packet to travel from source to destination
Packet loss: the amount of packets that are unsuccessful in arriving at the destination
Jitter: the measure of the variability over time of the latency across a network
QoS Network Factors
Terminating Gateway
Originating Gateway
T.38 T.38 T.38 T.38 T.38
Delay
Packet Loss Jitter
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Fax over IP (FoIP) is generally more affected by packet loss than VoIP
Ideally no packet loss should occur for a fax call
If packet loss is present then use T.38 with redundancy
Enabling T.38 redundancy requires more bandwidth
FoIP and Packet Loss
T.38 Fax Relay With Redundancy Level Set to 1
Originating Gateway
Terminating Gateway
IFP 2 (Primary)
IFP 1 (Secondary)
IFP 3 (Primary)
IFP 2 (Secondary)
IFP 1 (Primary)
T.38 Fax Packets
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Delay is not as impacting to FoIP compared to VoIP
FoIP calls have been known to handle network delays of 1 second or more
However, as a best practice it is still recommended to minimize network delays as much as reasonably possible because too much delay will cause FoIP calls to fail
Watch out for multiple IP and PSTN hops and satellite links
FoIP and Delay
PSTN
PSTN IP IP
Multiple IP and PSTN hops are prime sources of additional delay
Satellite links cause large amounts of delay
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T.38 and Cisco fax relay use 300 ms fixed jitter or playout buffers Fax passthrough uses the last voice mode setting before the
switchover With large playout buffers, FoIP can handle larger amounts of jitter
than VoIP but as a best practice it is still recommended to keep jitter to a minimum
FoIP and Jitter
IP
Variably spaced T.38 packets arrive at the playout buffer and some may even be out of sequence
Packets are re-sequenced if necessary and placed in the required order for playout
Evenly spaced packets are played out to the DSP for transmission on the PSTN
300 ms Fixed Playout Buffer
11
Fax
8
Fax
10
Fax
9
Fax
7
Fax
6
Fax
5
Fax
4
Fax
3
Fax
2
Fax
1
Fax
DSP Codec (T.38)
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QoS Design Parameters for Fax
Delay Jitter Packet Loss
Voice < 150 ms (one-way, (mouth to ear)
< 30 ms (average, one-way)
< 1%
Fax
< 1000 ms < 300 ms for fax relay, < 30 ms for passthrough
None*, unless using T.38 with redundancy
*Fax passthrough is very sensitive to packet loss and may be able to handle 0.1%–0.2% loss depending on when in the fax transaction the loss occurs and if it is consecutive packets. Cisco fax relay can handle more loss than passthrough but T.38 with redundancy is still the best choice for fax calls when packet loss is occurring.
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 50
Fax, modem, and text QoS marking via DSCP can be simply added at the dial-peer level for H.323 and SIP or globally for MGCP with the command “mgcp ip qos dscp”
Dial-peer QoS markings can be viewed with the command show dial-peer voice
Default dial-peer QoS markings are cs3 for signaling and ef for media
QoS Marking Configuration on Gateways
IP
dial-peer voice 100 voip destination-pattern 100 session target ipv4:10.1.1.1 fax protocol t38 ls-redundancy 0 hs-redundancy 0 fallback cisco
cae-dh-3660#show dial-peer voice 100 <snip> type = voip, session-target = `ipv4:10.1.1.1', technology prefix: settle-call = disabled ip media DSCP = ef, ip signaling DSCP = cs3, <snip>
T.38 T.38
Fax Gateway
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 51
QoS Marking and Shaping Example
dial-peer voice 919 voip destination-pattern 919....... session target ipv4:10.0.0.19 fax protocol t38 ls-redundancy 0 hs-redundancy 0 fallback cisco no vad
class-map match-all fax_voice match ip dscp ef class-map match-any call_signaling match ip dscp cs3 match ip dscp af31 ! policy-map WAN class fax_voice priority percent 33 class call_signaling bandwidth percent 5 class class-default fair-queue
Fax Fax OGW
PC
TGW
Phone Phone PC
IP
CLASSIFICATION & MARKING QUEUEING & DROPPING POST-QUEUING OPERATIONS
Voice and T.38 fax traffic can share the same QoS marking and shaping.
Design Best Practices
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 53
Unified Communications Integration Methods Cisco Unified Communications Manager, Gateway-Controlled
Cisco Unified Communications Manager, CA-Controlled
Design considerations Bandwidth utilization
Super G3
Protocol and product support
SIP Trunking and FoIP Support
T.38: de facto standard
Design Best Practices
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 54
All signaling and media packets are exchanged between the voice gateways
NSE-based switchovers (fax passthrough and NSE-based T.38) and payload type switchovers (Cisco fax relay) happen within the voice media stream while protocol-based switchovers (fax pass-through and protocol-based T.38) happen within the call control protocol
Gateway to Gateway
IP Fax Gateway Fax Gateway
Voice Media
Fax Media
Voice Media NSE/PT Switchover
H.323/SIP Voice Signaling
H.323/SIP Fax Signaling
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 55
Cisco Unified Communications Manager (Unified CM) controls the setup of the initial voice call between the gateways using standard call control signaling such as H.323, SIP, MGCP, and SCCP
The switchover to a fax media stream is handled by the gateways themselves without the knowledge of Unified CM
Cisco Unified Communications Manager, Gateway-Controlled
IP
H.323/SIP/MGCP/SCCP H.323/SIP/MGCP/SCCP
Voice Media NSE/PT Switchover
Fax Media
Switchover for fax passthrough, NSE-based T.38, and Cisco fax relay occurs within the voice media stream
Voice media is established by Unified CM using the H.323, SIP, MGCP, or SCCP protocols
Fax Gateway
Unified CM
Fax Gateway
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 56
Use Cisco Unified Communications Manager, Gateway-Controlled, when a Cisco Unified Communications Manager version does not support a certain fax media negotiation in the call control protocol (like T.38)
NSE-based T.38 allows for T.38 to occur even if a Cisco Unified Communications Manager version does not offer T.38 support within the call control protocol
This integration method must also be used with non-standard switchover mechanisms such as fax passthrough and Cisco fax relay
Remember that NSE and PT switchovers are Cisco proprietary and this integration method only works with Cisco devices and not third-party equipment
Cisco Unified Communications Manager, Gateway Controlled
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 57
Cisco Unified Communications Manager controls the setup of the initial voice call between the voice gateways and controls the switchover to T.38 fax relay
T.38 support for H.323, SIP, and MGCP is available in Cisco Unified Communications Manager version 6.0(1) and later
Cisco Unified Communications Manager, CA-Controlled
IP H.323/SIP/MGCP
Voice Media
T.38 Media
Switchover for protocol-based T.38 is handled by Unified CM within the call control protocol
Voice media is established by Unified CM using the H.323, SIP, or MGCP protocols
Fax Gateway
Unified CM
Fax Gateway T.38 Switchover
Cisco Best Practice
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 58
Recommended as a best practice! Only protocol-based T.38 (using H.323, SIP, or MGCP) is
capable of this integration method This integration method is standards-based and allows for
interoperability with third-party devices such as gateways and fax servers
If Cisco Unified Communications Manager and voice gateways support this integration method then this is recommended compared to the Cisco Unified Communications Manager, Gateway-Controlled method which depends on proprietary switchover mechanisms
T.38 support within Cisco Unified Communications Manager varies depending on software version
Cisco Unified Communications Manager, CA-Controlled
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 59
Only release 6.0(1) and later offers support for T.38 within the call control protocols of H.323, SIP, and MGCP
The SCCP protocol only handles T.38 using an NSE-based switchover
Cisco Unified Communications Manager and T.38 Support
Call Control Protocol Support for T.38
Cisco Unified Communications Manager Software Release
H.323 4.1(1), 4.2(3), 5.0(1), and 6.0(1)
H.323 and MGCP 4.2(3) and 6.0(1) H.323 and SIP 5.0(1) and 6.0(1) H.323, SIP, and MGCP 6.0(1) and later
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 60
Unified Communications Integration Methods Cisco Unified Communications Manager, Gateway-Controlled
Cisco Unified Communications Manager, CA-Controlled
Design considerations Bandwidth utilization
Super G3
Protocol and product support
SIP Trunking and FoIP Support
T.38: de facto standard
Design Best Practices
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 61
Different FoIP transports use varying amounts of bandwidth
On links where saving bandwidth is a priority then relay is a better choice
T.38 redundancy handles packet loss much better than fax passthrough/ pass-through with redundancy
FoIP Bandwidth Utilization
Codec Bandwidth1
G.711 (64 Kbps) 83 Kbps
G.729 (8 Kbps) 27 Kbps
G.723 (6.3 Kbps) 19 Kbps
Fax passthrough/ pass-through (G.711) 83 Kbps
Fax passthrough (G.711) with redundancy 170 Kbps
T.38 (no redundancy) 25 Kbps2
T.38 (redundancy level 1) 41 Kbps2
T.38 (redundancy level 2) 57 Kbps2
Cisco fax relay 48 Kbps2
1Values are approximate with Ethernet or Frame Relay headers
2Values are peak and only occur during the sending of a page at 14.4 Kbps; gateways can force lower fax speeds for additional bandwidth savings
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 62
The sg3-to-g3 spoofing feature controlled by the command fax-relay sg3-to-g3 forces Super G3 fax calls to negotiate down to G3 speeds for compatibility with T.38 or Cisco fax relay
The command fax-relay ans-disable helps further with forcing SG3 fax transmissions to G3 speeds by suppressing SG3 ANSam tones
Modem passthrough will handle SG3 calls at their native speeds
Starting in IOS 15.1(1)T SG3 over T.38 is supported and these other methods for handling SG3 faxes are not necessary
Methods for Handling SG3 Fax Calls Sending Fax Receiving Fax
OGW TGW
CNG
ANSam
CM
DIS
Sending fax initiates SG3 call by sending CM
Sg3-to-g3 spoofing feature on OGW blocks CM message
Absence of SG3 CM causes receiving fax to initiate G3 fax procedures (DIS message)
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 63
Endpoints must support SG3 and gateways must support the SG3 over T.38 feature (IOS 15.1(1)T and higher)
Only SIP and H.323 call control protocols are supported
Use the same call control protocol (SIP or H.323) end to end
As this is a relatively new implementation industry wide, interoperability problems may be present when this feature is used between different vendors
SG3 over T.38 Design Best Practices
Super G3 Fax Super G3 Fax Voice Gateway (with SG3 support)
Voice Gateway (with SG3 support)
Unified CM
SIP SIP
H.323 H.323
SG3 over T.38
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 64
DCS (ECM disabled)
ECM ensures error-free transmission of fax pages When ECM is disabled, fax pages can proceed with minor errors that may
or may not be noticeable Cisco gateways can force ECM to be disabled when configured for T.38 or
Cisco fax relay Disabling ECM may be useful for scenarios where network impairments
make it difficult for ECM to be efficient
Error Correction Mode (ECM) Sending Fax Receiving Fax
OGW TGW
CNG
DIS (ECM enabled)
CED
DIS (ECM disabled)
ECM support indicated in DIS message
TGW overwrites ECM setting to disabled
Sending fax believes receiving fax does not support ECM
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 65
For increased interoperability, T.38 fax relay can fall back to Cisco fax relay or fax pass-through if the initial T.38 negotiation fails
T.38 also has an automatic fallback mechanism that tries a protocol-based T.38 fax relay switchover if the configured NSE-based switchover fails
T.38 Fallback
Sending Fax
Receiving Fax
OGW
6608 TGW
IP
3845 TGW
Receiving Fax
Cisco fax relay
T.38 supported by 3845 and call is successful
T.38 call fails to 6608 but fallback to Cisco fax relay is successful
dial-peer voice 100 voip fax protocol t38 ls-redundancy 0 hs-redundancy 0 fallback cisco
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 66
FoIP Call Control Protocol Support Transport Method
H.323 SIP MGCP SCCP
Cisco fax relay Yes* Yes* Yes* Yes*
NSE-based T.38
Yes Yes Yes Yes
Protocol-based T.38
Yes Yes Yes No
Fax pass-through (protocol-based)
Yes Yes No No
Fax/Modem passthrough (NSE-based)
Yes Yes Yes Yes
*IOS platforms such as the 5350, 5400, and 5850 utilizing the Nextport DSP cards do not support Cisco fax relay
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 67
Cisco ATA only supports NSE-based passthrough (modem passthrough)
SCCP voice gateways (IOS or VG248) do not support any protocol-based switchovers(like fax pass-through and protocol-based T.38), so NSE-based T.38 and modem passthrough must be used
MGCP gateways do not support fax pass-through, only modem passthrough can be utilized
Legacy Nextport DSP platforms (AS5350, AS5400, and AS5850) do not support Cisco fax relay
Legacy 6608 and 6624 cards only support Cisco fax relay and modem passthrough
FoIP Product Support Caveats
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 68
Fax Method T.38 Fax Relay G.711 Voice Pass-through
Pros Highest fax success rates can be achieved
Cleanest solution from signaling and media point of view
Use less bandwidth than G.711
Most widely deployed Simplest solution
Provides bandwidth savings as G.729 voice call only upspeeds to G.711 if call is fax
Cons Interoperability between different vendors
Not offered by some Service Providers
Consumes a large amount of bandwidth for all calls
Sensitive to impairments, no redundancy
Interoperability between different vendors
Not supported by Cisco Unified Communications Manager
Sensitive to impairments, no redundancy
FoIP Transport Options for SIP Trunks SIP Trunks typically use three different methods to transport
standard G3 fax calls G.711 Voice - All calls (voice and fax) are always sent using the G.711 voice
codec Pass-through - A SIP RE-INVITE message is used to up-speed to G.711 when a
fax tone is detected T.38 Fax Relay – Call is switched to T.38 fax relay using a SIP RE-INVITE when
a fax tone is detected
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 69
T.38 should be your first choice, followed by G.711 voice or passthrough
Use SIP call control end-to-end if possible (required by SG3 over T.38)
SCCP (Skinny) endpoints do not support standards-based T.38
Test, Test, Test! Not all SIP trunk providers support T.38 and interoperability issues are common
FoIP Best Practices for SIP Trunks
SIP Service Provider
1. T.38 2. G.711 Voice or Passthrough
FoIP
SIP SIP
H.323 H.323/MGCP CUCM
CUBE
Voice Gateway
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 70
Before version 8.5, CUCM only sends SIP delayed offer for outbound calls
Many SIP trunk providers only support SIP early offer
For T.38 fax calls to work with most SIP Service Providers an IOS MTP configured with codec passthrough is needed
Or the command early offer forced under voice service voip on CUBE can spoof an early offer towards the provider and an IOS MTP is not needed
SIP Trunk Early Offer/Delay Offer Issue
SIP Service Provider
FoIP
SIP delay offer SIP early offer
Call #2
SIP early offer
“early offer forced” on CUBE spoofs delay offer from CUCM to early offer
Call #1
SIP early offer
SIP delay offer
SIP delay offer SIP service provider does not support SIP delay offer and call fails to setup
CUCM
CUBE
Voice Gateway
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 71
T.38: De Facto Standard for FoIP
T.38 Should Always Be Your First Choice When Handling FoIP Traffic!
T.38 CFR Fax Pass-through
Fax/Modem Passthrough
Standards-Based, Third-Party Integration Low Bandwidth
Redundancy
SG3 Support
SIP Trunk Support
SRTP and CRTP Support Legacy Platform Support
Troubleshooting
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 73
Capture media and call control
Packet Captures
Packet captures are one of the best tools for troubleshooting fax problems
Ideally captures should be made at both endpoints to ensure that both signaling and media are seen
Cisco Unified Communications Manager only participates in the call control signaling
Voice Gateway
IP
CUCM
Capture call control
Fax Voice Gateway
Capture media and call control
Fax
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 74
Packet capture software such as Wireshark or the traffic export feature on the IOS voice gateways are good tools for obtaining packet captures
Packet Captures
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 75
For protocol-based switchovers like fax pass-through and T.38, the “VoIP Call Analysis” feature in Wireshark offers a graphical means for analyzing the switchover
“VoIP Call Analysis” also breaks down the T.38 media stream, showing the T.30 signaling and page transmission
Select “Statistics” from top of Wireshark window, “VoIP Calls”, click on a call, and then click “Graph”
Packet Captures: VoIP Call Analysis
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 76
Any fax call can be broken down into three troubleshooting stages 1. Initial Voice Call
2. Switchover
3. Fax Media
Once the problem has been narrowed down to a specific stage, then the appropriate troubleshooting strategy can be implemented
Call Troubleshooting Stages Gateway
H.323/SIP/MGCP/SCCP Call Setup
Voice Media
NSE or Protocol-Based Switchover
Fax Media
Gateway
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 77
Use the command debug voip rtp session named-event to view NSE-related switchovers and events on IOS gateways, such as fax passthrough and NSE-based T.38
Most common cause of problems with NSE-based switchovers are an incorrect configuration or a transcoder or proxy in the call path
Stage 2: Troubleshooting NSE-Based Switchovers
debug voip rtp session named-event Jan 10 22:01:58.471: s=DSP d=VoIP payload 0x64 ssrc 0x1EF2 sequence 0x1FBD timestamp 0x20631C26 Jan 10 22:01:58.471: Pt:100 Evt:192 Pkt:00 00 00 <Snd>>>
Jan 10 22:01:58.483: s=VoIP d=DSP payload 0x64 ssrc 0x9A20101 sequence 0x18C2 timestamp 0xADA80F0A Jan 10 22:01:58.483: <<<Rcv> Pt:100 Evt:192 Pkt:00 00 00
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 78
Stage 2: Packet Captures: NSE Switchover
The default RTP payload type for NSE messages is 100
The NSE Event ID is shown in the payload as a hex value. In this case 0xC8 is 200 in decimal (NSE-200), which is a T.38 fax relay switchover request
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 79
Like NSE-based switchovers, confirm RTP payload type (PT) switchovers as utilized by Cisco fax relay using the command debug voip rtp session named-event
Troubleshooting PT-Based Switchovers debug voip rtp session named-event Jan 10 16:03:32.838: s=VoIP d=DSP payload 0x60 ssrc 0x0 sequence 0x0 timestamp 0x0 Jan 10 16:03:32.838: <<<Rcv> Pt:96 Evt:0 Pkt:03 00 02 Jan 10 16:03:32.838: s=DSP d=VoIP payload 0x61 ssrc 0xFFEB sequence 0x0 timestamp 0x0 Jan 10 16:03:32.838: Pt:97 Evt:0 Pkt:07 00 02 <Snd>>> Jan 10 16:03:32.842: s=DSP d=VoIP payload 0x60 ssrc 0xFFE2 sequence 0x0 timestamp 0x0 Jan 10 16:03:32.842: Pt:96 Evt:0 Pkt:07 00 02 <Snd>>> Jan 10 16:03:32.842: s=VoIP d=DSP payload 0x61 ssrc 0x0 sequence 0x0 timestamp 0x0 Jan 10 16:03:32.842: <<<Rcv> Pt:97 Evt:1 Pkt:B8 01 E0
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 80
Troubleshooting Protocol-Based Switchovers—H.323 and T.38
H.245 Request Mode message (from the terminating side) initiates the T.38 switchover
Voice Gateway
Voice Media
T.38
*Feb 3 04:34:53.070: H245 MSC OUTGOING PDU ::= value MultimediaSystemControlMessage ::= request : requestMode : <SNIP> type dataMode : { application t38fax : { t38FaxProtocol udp : NULL t38FaxProfile { fillBitRemoval FALSE transcodingJBIG FALSE transcodingMMR FALSE version 0 t38FaxRateManagement transferredTCF : NULL t38FaxUdpOptions { t38FaxMaxBuffer 200 t38FaxMaxDatagram 72 t38FaxUdpEC t38UDPRedundancy : NULL <SNIP>
debu
g h2
45 a
sn1
H.245 Request Mode and ACK
Closing of G.711 Logical Channels
Opening of T.38 Logical Channels
Voice Gateway
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 81
Troubleshooting Protocol-Based Switchover—SIP and T.38
re-INVITE
ACK
200 OK
SIP re-INVITE message (from the terminating side) initiates the switchover to T.38
*Feb 3 22:32:18.514: //-1/xxxxxxxxxxxx/SIP/Msg/ccsipDisplayMsg: Sent: INVITE sip:172.18.110.86:5060 SIP/2.0 Via: SIP/2.0/UDP 14.1.97.15:5060;branch=z9hG4bK618FF From: <sip:[email protected]>;tag=4320068C-1F6B <SNIP> v=0 o=CiscoSystemsSIP-GW-UserAgent 8190 1299 IN IP4 14.1.97.15 s=SIP Call c=IN IP4 14.1.97.15 t=0 0 m=image 16582 udptl t38 c=IN IP4 14.1.97.15 a=T38FaxVersion:0 a=T38MaxBitRate:14400 a=T38FaxFillBitRemoval:0 a=T38FaxTranscodingMMR:0 a=T38FaxTranscodingJBIG:0 a=T38FaxRateManagement:transferredTCF a=T38FaxMaxBuffer:200 a=T38FaxMaxDatagram:72 a=T38FaxUdpEC:t38UDPRedundancy
debu
g cc
sip
mes
sage
s
Voice Gateway
Voice Gateway
Voice Media
T.38
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 82
Stage 2: Troubleshooting Protocol-Based Switchover—MGCP and T.38
A NTFY containing FXR/t38(start) initiates the MGCP T.38 switchover
Unified CM
CAE-DH-3845-2#show ccm-manager MGCP Domain Name: CAE-DH-3845-2 Priority Status Host ============================================== Primary Registered CAE-DH-7845 (172.18.110.85) First Backup None Second Backup None
Current active Call Manager: 172.18.110.85 <SNIP>
TIP: Omission of the configuration command mgcp package-capability fxr-package is the most common cause of MGCP T.38 failing
*Feb 4 17:01:36.597: MGCP Packet sent to 172.18.110.85:2427---> NTFY 935850485 S2/SU0/DS1-0/23@CAE-DH-3845-2 MGCP 0.1 N: [email protected]:2427 X: 17 O: FXR/t38(start) <---
Voice Gateway (Sending)
Voice Gateway (Receiving)
Voice Media
T.38
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 83
Cisco Real Time Monitoring Tool (RTMT) log viewer can show protocol-based switchover information
Stage 2: Troubleshooting Protocol-Based Switchovers With Cisco RTMT
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 84
Fax Server
IP
Stage 2: SIP Trunk Troubleshooting
SIP Service Provider
CUCM
CUBE
Voice Gateway
If initial voice call is not established and CUCM is present check for early/delay offer issue
If T.38 re-INVITE is not occurring, check terminating side support, confirm MTP configuration, review proper dial peer config on CUBE
Use debug ccsip messages for further call control troubleshooting
Confirm receive side sends T.38 SIP re-INVITE
For T.38, MTP needs codec passthrough
Check dial-peer config on CUBE
debug ccsip messages for deeper look at SIP signaling
If SIP service provider does not support SIP delay offer and CUCM is present without MTP, then early offer forced needs to be configured in CUBE
MTP
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 85
DSP packet statistics on Cisco voice gateways provide IP network information about the active, incoming media stream for fax/modem passthrough, Cisco fax relay, and T.38
Errors in these stats typically indicate packet loss and/or jitter problems
The command “show call active voice brief” also provides some DSP statistics and information about the media stream
Stage 3: Troubleshooting the Fax Media - Gateway DSP Statistics
CAE-DH-3845-2#terminal monitor CAE-DH-3845-2#show voice call 0/1/1 0/1/1 vtsp level 0 state = S_CONNECTvpm level 1 state = FXSLS_CONNECT vpm level 0 state = S_UP calling number , calling name unavailable, calling time 02/03 04:20 CAE-DH-3845-2# ***DSP FAX RELAY STATISTICS*** Max Jit Depth: 36, Max Nwk RxQ Depth 3, Jitter Overflow Pkt Drops: 0 Nwk RxQ Overflow: 0, Tx Pkts: 247, Tx Pkts Drops(Nwk Busy): 0 Rx Pkts: 677, Rx Pkts Loss: 0, Rx Invalid Pkts: 0, Rx Pkts Out Of Seq: 0 Recent Hi-Speed Modulation: V.17/long/14400TX Pages: 2 Max SendInQ Depth 2, Max RecvOutQ Depth 2 Max Hi-Speed Buf Usage 54, SendInQ Overflow 0, RecvOutQ Overflow 0
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 86
The command debug fax relay t30 all-level-1 displays T.30 messages for T.38 and Cisco fax relay from the DSP’s perspective on the POTS/PSTN side of the gateway
Inbound messages are flagged fr-msg-det and messages sent by the DSP out the telephony interface are indicated by fr-msg-tx
Stage 3: Fax Relay Message Debugging Voice Gateway
fr-msg-det Message received by DSP
fr-msg-tx Message transmitted by DSP
IP
Fax
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 87
This debug is only valid for T.38 and Cisco fax relay as the gateway plays an active role in modulating and demodulating the T.30 fax messages
Stage 3: Sample Output From “debug fax relay t30 all-level-1”
Nov 2 17:19:22.928: 0/0/0 (42) 626612205 fr-entered=10(ms)
timestamp=626613325 fr-msg-det NSF
timestamp=626614305 fr-msg-det CSI
timestamp=626615005 fr-msg-det DIS
timestamp=626617385 fr-msg-tx TSI
timestamp=626618185 fr-msg-tx DCS
timestamp=626623295 fr-msg-det CFR
timestamp=626648365 fr-msg-tx EOP
timestamp=626649985 fr-msg-det MCF
timestamp=626652165 fr-msg-tx DCN
Nov 2 17:20:03.208: 0/0/0 (42) 626652485 fr-end-dcn
Output from the Cisco voice gateway command: debug fax relay t30 all-level-1
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 88
Primary debug used for troubleshooting T.38 and Cisco fax relay problems on a Cisco voice gateway
Only low-speed T.30 messages are displayed so high speed communications such as trainings and page transmissions are not shown
If there are multiple fax calls occurring at once you can specify a called or calling number to only view the debug messages for a specific call
The “level-2” option for this debug is only readable by the DSP vendor and it should not be enabled
Debug does not currently work for SG3 over T.38 calls This debug (as well as packet captures) is useful for viewing unique
T.30 problem signatures
Tips for Using “debug fax relay t30 all-level-1”
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 89
NSF/CSI/DIS
Unique T.30 Debug Signature: Delay
IP PSTN
T.30 Negotiation Failure With a Likely Cause of Too Much Delay Between the Fax Endpoints
fr-entered (10ms) fr-msg-det NSF fr-msg-det CSI fr-msg-det DIS fr-msg-tx DCS fr-msg-det CSI fr-msg-det DIS fr-msg-tx DCS fr-msg-det NSF fr-msg-det CSI fr-msg-det DIS fr-msg-tx DCS
Voice Gateway
NSF/CSI/DIS
NSF/CSI/DIS
DCS (train)
DCS (train)
DCS (train)
debug fax relay t30 all-level-1
T.30 messages are not being received before previous messages are retransmitted
Common causes of delay are satellite links or multiple VoIP hops
Adjusting the playout buffer on Cisco voice gateways may help
For example, configure playout-delay fax 100 under the appropriate dial-peer to lower the playout buffer to 100 ms
Voice Gateway
© 2011 Cisco and/or its affiliates. All rights reserved. Cisco Public BRKUCC-2021 90
Stage 3: Unique T.30 Debug Signature: Corruption
IP PSTN
T.30 Negotiation Failure With a Likely Cause of a Corrupted Training Signal
NSF/DIS
FTT
FTT
TSI/DCS (train)
TSI/DCS (train)
TSI/DCS (train)
Fax machine is receiving a corrupted training signal and sending an FTT message
Most common cause of a corrupted training is PSTN impairments, such as clock slips
First step is to always confirm that any digital circuits are free of errors
fr-entered=10(ms) fr-msg-det NSF fr-msg-det DIS fr-msg-tx TSI fr-msg-tx DCS fr-msg-det FTT fr-msg-tx TSI fr-msg-tx DCS fr-msg-det FTT fr-msg-tx TSI fr-msg-tx DCS fr-msg-det FTT
debug fax relay t30 all-level1
FTT
Voice Gateway
Voice Gateway
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Stage 3: Viewing Digital Circuit Errors
T1 0/1/0 is up. Applique type is Channelized T1 Cablelength is long gain36 0db No alarms detected. alarm-trigger is not set Soaking time: 3, Clearance time: 10 AIS State:Clear LOS State:Clear LOF State:Clear Version info Firmware: 20060711, FPGA: 13, spm_count = 0 Framing is ESF, Line Code is B8ZS, Clock Source is Line. CRC Threshold is 320. Reported from firmware is 320. Data in current interval (729 seconds elapsed): 0 Line Code Violations, 0 Path Code Violations 24 Slip Secs, 0 Fr Loss Secs, 0 Line Err Secs, 0 Degraded Mins 24 Errored Secs, 0 Bursty Err Secs, 0 Severely Err Secs, 0 Unavail Secs Total Data (last 24 hours) 0 Line Code Violations, 0 Path Code Violations, 2989 Slip Secs, 0 Fr Loss Secs, 0 Line Err Secs, 0 Degraded Mins, 2989 Errored Secs, 0 Bursty Err Secs, 0 Severely Err Secs, 0 Unavail Secs
Output from the Cisco voice gateway command: show controller t1
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The TCF or training signal is a pattern of all 0s
Other hex values shown in the TCF decode can cause the training to fail
This training pattern can be checked within Wireshark to confirm its integrity
Stage 3: Confirming the Training in T.38
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In addition to training problems, PSTN errors and impairments can also cause scan lines to become corrupted
Problem will not be viewable with Error Correction Mode (ECM) faxes but instead faxes will take a long time to complete or fail completely
Stage 3: Scan Line Corruption Snippet from a corrupted fax page
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Stage 3: Audio Trace of Scan Line and Training Corruption
DIS CFR
DCS-train
MCF
Page EOP
DCN
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Ensure that all digital circuits in the fax path are error free
Configure the clocking correctly on Cisco voice gateways to avoid slips on digital PSTN interfaces, which are notorious for causing training and scan line corruption
Make sure that the IP path is free from jitter and packet loss
Major corruption problems prevent the fax endpoints from training while less severe problems may allow training to occur but fax page data is affected
Preventing Corruption of the Training and Scan Lines
Fax Servers
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A software application running on a server that processes fax calls over a PSTN or IP connection
A “Cisco Unified Communications Manager” for handling fax traffic
Provides secure, automated, and efficient handling of an organizations fax traffic
What Is a Fax Server?
Fax Server
Cisco Voice Gateway
H.323/SIP Call Setup
G.711 Voice
T.38 Fax
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Server hardware requirements are defined by fax server vendors
Application software is produced by a variety of vendors and it handles administration, configuration, and integration functions
Fax engine is either hardware (typically a PCI fax board installed into the server) or software-based
Fax Server Components
Server Hardware
Application Software
Fax Engine
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Why Migrate to a Fax Server?
Security Faxes can be directed to a specific user or storage system with limited access
Workflow Integration
Fax servers can help automate process management procedures and specific workflow applications
Archiving Faxes can efficiently stored and indexed to comply with industry mandates such as Sarbox or SOx and HIPAA
Accountability All faxes sent or received along with confirmation of their success or failure can be handled with a fax server solution
MFP Integration MFPs integrate directly with fax servers for increased cost savings and flexibility
Email Integration Users have the ability to send/receive faxes directly via email
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Users can access a fax server through interfaces such as a web browser, email, an MFP, and client software
Other devices and automated processes can communicate directly with a fax server through different APIs
Fax Server Access Methods
IP
Fax Server
Web Browser
Mail Server
MFP
Client Software Application
API
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Fax servers communicate with Cisco Unified Communications Manager using H.323 or SIP
Regardless of the call control protocol between the fax server and Cisco Unified Communications Manager, H.323, SIP, or MGCP can be used between Cisco Unified Communications Manager and the gateway
T.38 media flows between the fax server and the voice gateways
Fax Server Integration With Cisco Unified Communications Manager
Fax Server
Cisco Voice Gateways
Unified CM
H.323/SIP
H.323/SIP H.323/SIP
MG
CP
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Most fax servers only support T.38 so the SIP trunk provider must support T.38 as well
No special modifications or configurations are typically necessary on a fax server to utilize SIP trunks
Use SIP call control throughout call flow if possible
CUBE cannot transcode between T.38 and passthrough
SIP Trunk Integrations with a Fax Server
SIP Service Provider
Passthrough
SIP SIP
Fax Server
T.38
T.38
CUBE is unable to receive T.38 from a fax server and transcode it to G.711 passthrough to be compatible with a SIP trunk provider
CUCM CUBE
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