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.

Note	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.

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.

1.    enable

2.    configure terminal

3.    voice service voip

4.    modem passthrough {nse| protocol}[payload-type number] codec {g711ulaw | g711alaw} [redundancy [maximum-sessions sessions] [sample-duration [10 | 20]]]

You must configure a VoIP dial peer on both the originating and terminating gateways to match the call--for example, using a destination pattern.

When the system keyword is entered, the following parameters are not available: nse, payload-type, codec, and redundancy. The configuration is taken from the modem passthrough nsecommand in voice-service configuration mode.

Note	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 dial peer.

1.    enable

2.    configure terminal

3.    dial-peer voice tag voip

4.    modem passthrough {system | nse [payload-type number] codec {g711ulaw | g711alaw} [redundancy]}

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:

• debug voip vtsp --Displays information about the voice telephony service provider (VTSP).
• debug vtsp --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 debug voip vtspcommand.
• show dial-peer voice --Used to verify that modem passthrough over VoIP is enabled.
• show call active voice --Displays the voice information for the active call table.
• show call history voice --Displays the voice information for the call history table.
• show dial-peer voice --Displays configuration information for dial peers.

To verify that modem passthrough is configured, you can use the show call active voice brief 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 1.1.1.2:16610 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

The following is sample configuration for the Modem Passthrough over VoIP feature for the Cisco AS5300 universal access servers:

version 12.2
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
voice service voip 
 modem passthrough nse codec g711ulaw redundancy maximum-session 5
!
resource-pool disable
!
ip subnet-zero
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
 framing esf
 clock source line primary
 linecode b8zs
 pri-group timeslots 1-24
!
controller T1 1
 shutdown
 clock source line secondary 1
!
interface Ethernet0
 ip address 10.10.2.2 255.0.0.0
 no ip route-cache
 no ip mroute-cache
!
interface Serial0:23
 no ip address
 encapsulation ppp
 ip mroute-cache
 no logging event link-status
 isdn switch-type primary-5ess
 isdn incoming-voice modem
 no peer default ip address
 no fair-queue
 no cdp enable
 no ppp lcp fast-start
!
interface FastEthernet0
 ip address 172.16.0.1 255.0.0.0
 no ip route-cache
 no ip mroute-cache
 load-interval 30
 duplex full
 speed auto
 no cdp enable
!
ip classless
ip route 192.168.0.0 255.255.0.0 10.10.1.1
no ip http server
!
voice-port 0:D
!
dial-peer voice 1 pots
 incoming called-number 55511..
 destination-pattern 020..
 direct-inward-dial
 port 0:D 
 prefix 020
!
dial-peer voice 2 voip
 incoming called-number 020..
 destination-pattern 55511..
 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
 login
!