Globalstar Product Management – Technical Support

How to Fix Analog Modem Slowdown

How to Configure Legacy Analog Modem Software for Optimal Data Transfer Speed

Legacy analog modem communications software systems were historically designed for systems where the server and the remote data system are connected directly to analog modems, and the analog link is the only possible source of jitter and delay.  These legacy systems often monitor the end-to-end communications and slow down the data transfer rate of the link if they detect any variation in the delay of end-to-end communications.   Analog modem communications make it possible for centralized data servers to collect data from distant data collection sites, which might not have any data network available, and only have an analog phone line available.

Globalstar provides analog asynch data communications, which makes it possible for legacy analog modem systems to reach across almost unlimited distances.  The addition of a satellite link changes the data transfer characteristics (delay and jitter), and legacy software sometimes mis-interprets these variations and slows down the data transfer speed.

To correct this problem and achieve the maximum speed data link possible over Globalstar, one or both of the following steps are recommended.

  1. Reconfigure GSP-1720:
    1. By default, Globalstar sets up a V.42bis analog link between the Globalstar gateway and the customer’s landline modem.
      1. This type of link runs at 14400bps and uses error correction.
      2. Both of those features contribute to variability in the pace of the data being received by the legacy software platform.
    2. There is no need to use compression or error correction on the landline.
    3. Eliminate these sources of data flow variability by issuing a custom AT+MS command to the GSP-1720.
  2. Reconfigure Legacy Software:
    1. Reconfigure the legacy analog communications software so that it is insensitive to delays as long as 90 seconds and to ignore jitter (variation between delay times).
    2. If the legacy software cannot be configured to completely ignore jitter, then configure the trigger value for jitter to be a high number, e.g. 120 seconds
    3. Explanation:
      1. Landlines:  On landline links, it has become common practice to interpret variations in delay, a.k.a. jitter, as an indication of a poor quality data link that needs to be slowed down.
      2. Globalstar:
        1. Globalstar does NOT use an analog link over the satellite to carry analog modem links.
        2. Globalstar carries the data over the air via an error-corrected digital serial link which provides nearly perfect data over the satellite link, with only one error in 10^9 parts.
        3. Globalstar converts the serial data to analog tones at the gateway interface to the public landline phone system (PSTN)
        4. It is safe to say that Delay and Jitter on a Globalstar data link are not at all related to the quality of the data link.
          1. Delay is primarily the result of analog-digital conversions at the Globalstar gateway and the user’s landline modem.
          2. Jitter simply indicates that there was some error correction being carried out on the satellite link to perfect the data before the data was handed off to the analog modem in the Globalstar ground station gateway.
        5. Unlike landlines, jitter and delay on a Globalstar data link do not in any way indicate a poor quality data link that needs to be slowed down.  Although there is some contribution to delay and jitter from the analog lines between the Globalstar gateway and the landline analog modem, it almost never indicates a bad link.
          1. Delay on the PSTN link is inconsequential and would not cause legacy landline software to slow down the analog link.
          2. The jitter is almost completely the result of the error correction that happens within the satellite radio link layer before the data is handed off to the analog modem in the Globalstar gateway.
          3. In other words, the jitter, which is causing the slowdown of the legacy analog modem software, is simply an indication of the corrections that are being carried out in order to carry data over the air link with perfect quality.
      3. Explanation:  Think about a download from a website to a laptop PC.
        1. If you start a download and then unplug your RJ45 cable for a couple of seconds and put it back into the computer, the laptop doesn’t restart the whole data transfer.
        2. It continues and there’s no problem at all; you don’t normally even see any hiccup.
      4. Summary:  On the legacy software, expand the delay/jitter tolerance windows wide open and let the data flow
      • The AT+MS command can be used to force the following types of analog links:
        1. v.21 300bps, full duplex
        2. v22 at 1200 bps
        3. v22 at 2400 bps
        4. v.27ter 4800/2400 bps half duplex
        5. v.32 9600 bps or 4800 bps full duplex
        6. v.32 bis 14000 bps full duplex
        7. v.42 error correction
        8. v.42bis compression/ error correction.
        9. . . . plus others . . .
      • For example, if the AT+MS command for v.32 9600 is used, it will configure the Globalstar landline analog interface so that when it answers an incoming call to the GSP-1720, the Globalstar Gateway will ONLY negotiate a landline link which operates at a fixed data speed of 9600 bps, and does NOT use compression and error correction (on the landline link between the Globalstar gateway and the customer’s analog modem)

Fully Tested AT+MS Modem Configuration Commands:


Additional keywords:  GSP-1700, GSP-1620, GSP-2900, SDM, SDVM, slowdown

Written by Joseph Crowley

September 4, 2017 at 9:33 pm

Posted in Uncategorized

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