ShoreTel, Fax Machines and Modems

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Fax Machines and Modems

The ShoreTel system supports fax machines and modems in the United States and Canada (and not elsewhere).

Fax and modem calls are more sensitive to network problems than voice conversations.

The human ear does not notice a lost packet during a voice conversation, but when a packet is lost during a fax transmission the line may be dropped. During a modem call, a lost packet can cause a  retransmission. In the worst case, fax machines and modems will not establish a connection or may drop the call altogether.

In general, fax and modem calls work across a local area network, but work on wide area networks only when there is virtually no packet loss and little jitter.

The ShoreTel system automatically detects both fax and modem tones, and boosts the voice encoding to a higher value to increase throughput. (G.711 at 64 Kbps is recommended.) It also stops the nonlinear processing of the echo canceller and fixes the size of the jitter buffer to a preset level. In addition, for modems, the echo canceller is frozen or stopped, since the modems use their own network echo cancellers.


The ShoreTel system supports “moderate-use” modem applications on the system.

This is generally considered to be modem calls up to 28.8 Kbps that do not last longer than 15 minutes.

If your application demands greater performance, you should bypass the ShoreTel system or move your modem application to a pure IP-based solution.

Modem Performance

The expected modem performance in different configurations is as follows:

  • Analog connection speeds will not exceed 33.6 Kbps and could be lower. External factors, including poor-quality trunk lines, ISP limitations, and multiple analog- digital conversions in the network, can have a significant impact on connection speeds.
  • Modem calls demand a high-quality network with virtually no packet loss. Packet loss should not exceed 0.1%, which can be achieved on a local area network or in a wide area network using leased T1 facilities.
  • Analog trunk ports should not be used if a digital trunk (T1) is available, since performance will be limited to 28.8 Kbps maximum. Digital trunks should be used instead.
  • Connection speeds are significantly affected by multiple packet-to-circuit conversions (including modem calls from one ShoreTel system to another). If a T1 line is used, modems should be able to connect at K56Flex/V.90 or approximately 48 Kbps.

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