Follow modem relay QoS best practices
Use CAC
Disable call waiting on all dedicated modem and fax ports
Use G.711 for all calls involving a modem
Use a single signaling protocol and gateway family
Observe the following recommended best practices to ensure optimum performance of modem traffic transported over an IP infrastructure:
Ensure that the IP network is enabled for QoS following the same QoS best practices for modem relay.
Use CAC to ensure that calls are not admitted if they exceed the specified total bandwidth limit.
Use G.711 for all calls involving a modem. If one of the gateways does not support modem relay, modem pass-through is negotiated (G.711 only). If modems are used, the best-practice recommendation is to use G.711 for all calls.
Where possible, use a single signaling protocol and gateway family to minimize interoperability issues.
Disable call waiting on all dedicated modem and fax ports.
Caution Do not use the IP network to connect modems that will be used to troubleshoot or diagnose problems with the IP network. In this case, the modems used to troubleshoot the devices that compose the IP infrastructure should be connected to a POTS.
Note Use Cisco Feature Navigator to find information about platform support and Cisco IOS software image support.
Summary
This topic summarizes the key points discussed in this lesson.
© 2005 Cisco Systems, Inc. All rights reserved. GWGK v1.01-33
Summary
Cisco fax relay and T.38 fax relay protocols terminate modulated fax signals, extract digital information, and relay digital information through a data network using data packets. At the terminating side, digital information is extracted from the packet, modulated, and played out.
Faxes can be transmitted successfully when codecs such as G.726 and G.711, with no echo cancellation or VAD, are used.
This method of sending faxes through the voice codec is usually referred to as in-band faxing or fax pass-through.
Fax pass-through occurs when incoming fax data is not demodulated or compressed for its transit through the packet network. Using fax pass-through, two fax machines communicate directly with each other over a transparent IP connection.
© 2005 Cisco Systems, Inc. All rights reserved. GWGK v1.01-34
Summary (Cont.)
Modem relay occurs when modem signals are demodulated at one gateway, converted to digital form, and carried in SPRT packets to another gateway where the modem signal is recreated, remodulated, and passed to the receiving modem.
With modem passthrough, the modem traffic is carried between the two gateways in RTP packets using an uncompressed voice codec
Fax and modem relay and passthrough are all configured at the dial-peer prompt.
Lesson Self-Check
Use the questions here to review what you learned in this lesson. The correct answers and solutions are found in the Lesson Self-Check Answer Key.
Q1) Describe the similarities between Cisco fax relay and T.38 fax relay. (Source: Fax Relay)
Q2) When is fax pass-through used? (Source: Fax Pass-Through)
A) Fax pass-through is used when incoming fax data is not demodulated or compressed for its transit through the packet network.
B) Fax pass-through is always used; it is the default mode for passing faxes through a VoIP network, and Cisco fax through is the default fax pass-through type.
C) Fax pass-through is used when incoming fax data needs to be hairpinned
back out to the PSTN without passing through the network.
D) Fax pass-through is used when the gateway needs to break down T.30 fax tones into their specific HDLC frames for transmission across the VoIP network.
Q3) What type of connection is needed for two fax machines using fax pass-through to communicate? (Source: Fax Pass-Through)
A) an IPSec-based connection B) a VoIP connection
C) a transparent IP connection
Q4) What two things must be in place for gateway support of modem relay? (Choose two.) (Source: Modem Relay)
A) Both modems must be high-speed modems using at least V.42bis bidirectional compression.
B) Cisco IOS Software Release 12.2(11)T must be running on the gateways.
C) The gateway must support channel-associated signaling to allow the fax tones to be encoded on the link.
D) Modem relay must be enabled on both gateways. However, the default configuration is sufficient.
E) The gateway must have sufficient DSP resources on the multiflex trunk card to encode the modem-relay tones into the appropriate codec.
F) Both modems must be high-speed modems capable of encoding and decoding V.32bis symbols.
G) Modem relay must be configured and enabled on both gateways.
Q5) The debugs from a gateway reveal that T.38 negotiation fails and the call reverts to an audio codec. What is the reason? (Source: Fax Relay)
A) SIP T.38 fax relay is not supported by both gateways.
B) SIP T.38 has been configured at each end with conflicting parameters.
C) T.38 is not a supported fax protocol.
D) One gateway is running T.38v3, so the other must be upgraded to Cisco IOS Software Release 12.2(11)T or later.
Q6) How much bandwidth does T.38 fax relay require? (Source: Fax Relay) A) 56-kbps transmission rate
B) 32-kbps transmission rate C) 128-kbps transmission rate D) 64-kbps transmission rate
Lesson Self-Check Answer Key
Q1) Cisco fax relay and T.38 fax relay terminate the modulated fax signal, extract the digital information, and then relay the digital information through the data network using data packets. At the terminating side, the digital information is extracted from the packet, modulated, and played out.
Q2) A
Q3) C
Q4) A, G
Q5) A
Q6) D