Mike's PBX Cookbook

Fax Issues with MGC's

Fax settings and performance over VoIP solutions vary depending on the network configuration. In order to achieve a successful faxing environment the VoIP solution has to be engineered properly. Following are configuration and network design aspects that need to be taken into consideration when implementing faxing in VoIP solutions:

CODECs:

T.38:

Modem Pass Through (G.711):

Modem Pass Through:

The Modem Pass Through feature will detect the phase reversal tone negotiation used for higher speeds and will tell the DSPs involved in the call to disable echo-cancellation and all other non linear components.

Note: The MPTA feature is only supported when a Mindspeed DSP is used (NTDW62, NTDW64, NTDW65, NTDW78, NTDW56, NTDW59, NTDW20). In order for MPTA/MPTD to work, a system bandwidth strategy of BQ (Best Quality) must be used.
Faxing at 33.6 requires all network elements to support it. When high speed connections cannot be made with a consistent success rate, it is recommended to set the fax units to a max. speed of 14.4.

Scenarios:

Typical supported scenarios for faxing in a CS1K solution:

  1. Two faxes connected to analog lines in the same MGC (TDM call, no DSPs)
  2. Two faxes connected to analog lines in different MGCs of the same CS1K (DSPs are used)
  3. One fax connected to analog line in an MGC to IP Trunk to a remote system with a fax (DSPs are used)
  4. One fax connected to analog line in an MGC to Analog trunk in same MGC to PSTN (Local or LD) fax (No DSPs are involved)
  5. One fax connected to analog line in an MGC to Digital trunk in same MGC to PSTN (Local or LD) fax (No DSPs are involved)
  6. One fax connected to analog line in an MGC to Digital trunk in different MGC of the same CS1K to PSTN fax (DSPs are used)

The following scenario is not fully supported for faxing:

  1. One fax connected to analog line in an MGC to Analog trunk in different MGC of the same CS1KE to PSTN (Local or LD) fax

Data Network:

Depending on the scenario, fax calls can traverse the IP network either internally (ex: MGC to MGC) or externally (ex: IP Trunks).
It is important to engineer the data network to support the following:

Media card configuration:

Performance degrades significantly with packet loss (which must be less than 0.5%), when the delay (round trip) is greater than 50 msec, and when mean jitter is greater than 5msec.

Avaya does not guarantee that all fax brands will operate properly over all G.711 (VoIP) networks.
Before you deploy faxes, test the fax within the network to verify reliable operation.

Call Server Settings:

Check the following settings for fax and/or modem stations connected to an analog line card in a media gateway:

1. Media Gateways (Element Manager):

  • Enable Modem/Fax Pass Through

  • Enable V.21 FAX tone detection

  • MGC's (and VGW trunks) should be in a Zone with Best Quality (BQ)

2. Station Class of Service (LD 10):

  • Both: CLS FAXA. Fax allowed. ISDN call is generated with 3.1 KHz Bearer Capability. Set is a modem or a FAX machine.

  • Faxes: CLS MPTD. This setting will allow lower speed faxes (up to 14.4) to use T.38, and higher speed faxes to use G711 clear channel (no echo cancelation, no nonlinear DSP features).

  • Modems: CLS MPTA. This setting will force all calls to use G711, supporting rates up to 33.6 kbit/s (eg: for data modem 'devices').

Additional Note:

  • Digital trunk routes (LD 16, RDB) should be set to DSEL 3VCE for voice and data/fax applications. The default is VOD, and sometimes problematic with data/fax.

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