Modem Passthrough over VoIP provides the transport of modem signals through a packet network by using pulse code modulation (PCM) encoded packets. This chapter describes the configuration for modem passthrough.
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Your software release
may not support all the features documented in this module. For the latest
caveats and feature information, see
Bug Search Tool and the
release notes for your platform and software release. To find information about
the features documented in this module, and to see a list of the releases in
which each feature is supported, see the feature information table.
Use Cisco Feature
Navigator to find information about platform support and Cisco software image
support. To access Cisco Feature Navigator, go to
An account on Cisco.com is not required.
Prerequisites for Configuring Modem Passthrough
Before configuring modem passthrough, perform the following tasks:
Establish a working VoIP-enabled network.
Verify network suitability to pass modem traffic. The key characteristics of the network are packet loss, delay, and jitter. These characteristics can be determined by using the Service Assurance Agent (SAA) feature of Cisco IOS software.
Configure clock sourcing on the T1 controller on the voice gateway that connects to the PSTN. For modem passthrough to operate correctly, the gateway clock must be synced with the PSTN clock. See the following example configuration:
Configure clock sourcing for all interfaces connected to the PSTN.
Restrictions for Configuring Modem Passthrough
Restrictions for configuring modem passthrough are as follows:
The modempassthroughprotocol and faxprotocolcommands cannot be configured at the same time. If you enter either one of these commands when the other is already configured, the command-line interface returns an error message.
The error message serves as a confirmation notice because the modempassthroughprotocol command is internally treated the same as the faxprotocolpass-throughcommand by the Cisco IOS software. For example, no other mode of fax protocol (for example, fax protocol T.38) can operate if themodempassthroughprotocol command is configured.
Even though the modempassthroughprotocol and faxprotocolpass-throughcommands are treated the same internally, be aware that if you change the configuration from the modempassthroughprotocolcommand to the modempassthroughnsecommand, the configured faxprotocolpass-through command is not automatically reset to the default. If default settings are required for the faxprotocolcommand, you have to specifically configure the faxprotocol command.
Modem passthrough over VoIP performs the following functions:
Represses processing functions like compression, echo cancellation, high-pass filter, and voice activity detection (VAD).
Issues redundant packets to protect against random packet drops.
Provides static jitter buffers of 200 milliseconds to protect against clock skew.
Discriminates modem signals from voice and fax signals, indicating the detection of the modem signal across the connection, and placing the connection in a state that transports the signal across the network with the least amount of distortion.
Reliably maintains a modem connection across the packet network for a long duration under normal network conditions.
When the gateway detects a data modem, both the originating gateway and the terminating gateway roll over to G.711. The roll over to G.711 disables the high-pass filter, disables echo cancellation, and disables VAD. At the end of the modem call, the voice ports revert to the prior configuration and the digital signal processor (DSP) goes back to the state before the rollover.
The gateway can detect modems at speeds up to V.90.
Payload redundancy enables the modem passthrough switchover and this causes the gateway to emit redundant packets. When redundancy is enabled, 10-ms sample-sized packets are sent. When redundancy is disabled, 20-ms sample-sized packets are sent.
Redundancy is enabled on one or both of the gateways. When only a single gateway is configured for redundancy, the second gateway receives the packets correctly but does not produce redundant packets.
Clock Slip Buffer Management
When the gateway detects a data modem, both the originating gateway and the terminating gateway switch from dynamic jitter buffers to static jitter buffers of 200-ms depth. The switch from dynamic to static is to compensate for PSTN clocking differences at the originating gateway and the terminating gateway. At the conclusion of a modem call, the voice ports revert to dynamic jitter buffers.
How to Configure Modem Passthrough
Modem passthrough can be configured at two levels:
The two configuration tasks can be used separately or together. If both are configured, the dial-peer configuration overrides the global configuration.
You must configure modem passthrough on both the originating and terminating gateways.
Configures modem passthrough for all dial peers on the gateway. The default behavior is nomodempassthrough.
nse--Specifies that named signaling events (NSEs) are used to communicate codec switchover between gateways.
protocol--Session Initiation Protocol (SIP)/H.323 protocol is used to signal modem pass-through.
payload-typenumber--(Optional) NSE payload type. Range varies, but is from 96 to 119 on most platforms. For details, refer to command-line interface (CLI) help. Default is 100.
The payload-type must match on the originating and terminating gateways.
codec--Codec selections for upspeed.
g711ulaw--Codec G.711 u-law, 64000 bits per second for T1.
g711alaw--Codec G.711 a-law, 64000 bits per second for E1.
redundancy--(Optional) Enables a single repetition of packets (using RFC 2198) to improve reliability by protecting against packet loss.
maximum-sessionsvalue--(Optional) Maximum number of simultaneous pass-through sessions. Ranges and defaults vary by platform.
Configuring Modem Passthrough for a Specific Dial Peer
You must configure a VoIP dial peer on both the originating and
terminating gateways to match the call--for example, using a destination
system keyword is entered, the following
parameters are not available:
redundancy. The configuration is taken from
themodempassthroughnsecommand in voice-service configuration mode.
When modem passthrough is configured for a specific dial peer, the
dial-peer configuration takes precedence over the global configuration.
Use the following steps to configure modem passthrough for a specific
Configures modem passthrough for a specific dial peer. The
default behavior for modem passthrough in dial-peerconfiguration mode is
to the global configuration.
that named signaling events (NSEs) are used to communicate codec switchover
payload type. Range varies by platform, but is from 96 to 119 on most
platforms. The default is 100.
selections for upspeeding.
G.711 u-law 64000 bits per second for T1.
G.711 a-law 64000 bits per second for E1.
Enables a single repetition of packets (using RFC 2198) to improve reliability
by protecting against packet loss.
Troubleshooting Tips for Modem Passthrough
Use the following steps to troubleshoot modem passthrough:
Ensure that you can make a voice call.
Ensure that modem passthrough over VoIP is configured on both the originating gateway and the terminating gateway.
Ensure that the originating and terminating gateways have the same NSE payload-type number.
When two gateways are configured in voice-service configuration mode, ensure that the originating and terminating gateways have the same maximum-sessions value.
Use the following commands to troubleshoot modem passthrough:
debugvoipvtsp--Displays information about the voice telephony service provider (VTSP).
debugvtsp--Used to trace how the router interacts with the digital signal processor (DSP) based on the signaling indications from the signaling stack and requests from the application. Effective with Cisco IOS Release 12.3(8)T, this command was replaced by the debugvoipvtspcommand.
showdial-peervoice--Used to verify that modem passthrough over VoIP is enabled.
showcallactivevoice--Displays the voice information for the active call table.
showcallhistoryvoice--Displays the voice information for the call history table.
showdial-peervoice--Displays configuration information for dial peers.
To verify that modem passthrough is configured, you can use the showcallactivevoicebrief command. In the following sample output, the IP call leg shows the keyword MODEMPASS to signify that the call is in modem passthrough mode:
11DD : 1 1565860ms.1 +15340 pid:2 Answer 100 active
dur 00:00:19 tx:864/110008 rx:858/102929
Tele 0/0/0 (1) [0/0/0] tx:12270/12270/0ms g711ulaw noise:-11 acom:6 i/0:-14/-59 dBm
11DD : 2 1570100ms.1 +11090 pid:1 Originate 200 active
dur 00:00:19 tx:858/102929 rx:864/103096
IP 188.8.131.52:16610 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 1031s dur:0/0s
Modem Passthrough Configuration for Cisco AS5300 Example
The following is sample configuration for the Modem Passthrough over
VoIP feature for the Cisco AS5300 universal access servers:
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
voice service voip
modem passthrough nse codec g711ulaw redundancy maximum-session 5
ip ftp source-interface Ethernet0
ip ftp username lab
ip ftp password lab
no ip domain-lookup
isdn switch-type primary-5ess
cns event-service server
mta receive maximum-recipients 0
controller T1 0
clock source line primary
pri-group timeslots 1-24
controller T1 1
clock source line secondary 1
ip address 10.10.2.2 255.0.0.0
no ip route-cache
no ip mroute-cache
no ip address
no logging event link-status
isdn switch-type primary-5ess
isdn incoming-voice modem
no peer default ip address
no cdp enable
no ppp lcp fast-start
ip address 172.16.0.1 255.0.0.0
no ip route-cache
no ip mroute-cache
no cdp enable
ip route 192.168.0.0 255.255.0.0 10.10.1.1
no ip http server
dial-peer voice 1 pots
incoming called-number 55511..
dial-peer voice 2 voip
incoming called-number 020..
modem passthrough nse codec g711ulaw redundancy
session target ipv4:10.10.0.2
line con 0
exec-timeout 0 0
transport input none
line aux 0
line vty 0 4