Configuration

• Why do I get an asymmetric transfer rate on a leased line terminated with Tahoe modems?

  If the configured throughput is slightly too high for the given leased line, the modems may synchronise, but the data will be transmitted with errors. Packets containing errors will be discarded and will have to be retransmitted. Numerous retransmissions result in a reduced effective throughput. To avoid this, the throughput of the modems should be reduced slightly so that the noise margin at both ends of the link is >= 5 dB.
  
  It often happens that interference is greater at one end of the link than at the other. For example, at the Internet provider's exchange numerous DSL and ISDN lines generate strong interference that crosses over into neighbouring cables, so the noise margin at that end may be much lower than the same parameter at the customer's premises at the other end of the link (where relative 'electromagnetic silence' prevails). If the noise margin at one end drops below 5 dB, the transfer in one direction will be slower because of the transmission errors that occur. At the same time, transmission in the opposite direction may proceed without any problems. To avoid this problem, the throughput of the modems should likewise be reduced so that the noise margin at both ends of the link is >= 5 dB.

• How long does it take for the modems to synchronise?

  The synchronisation time depends primarily on the configured throughput. For HDSL modems, synchronisation at 2.9 Mb/s may take 10 seconds, and at 128 kb/s - 90 seconds. On G.shdsl modems synchronisation takes from 30 seconds to two minutes.
  
  If the line quality is poor (and the throughput is at the limit of what the line can support), synchronisation may fail at the first attempt and may be repeated several times. In that case it may take even several minutes.

• How do I remove an interface alias (e.g. eth0:1)?

  Enter 'ifconfig eth0:1 down'. Although the interface settings will be remembered (e.g. entering 'ifconfig eth0:1 up' will bring it up with the same IP address it had before it was disabled), once the settings have been saved and the device restarted the interface will no longer exist.

• How do I erase the configuration and restore the factory settings?

  On devices with a built-in router you can connect via telnet or the serial console and enter "con". The settings stored in the EEPROM memory will then be erased - after a restart the router will return to the factory settings. Until it is restarted the router will continue to work with the old settings, so entering "write" will write the current configuration back to the EEPROM. Therefore, to erase the settings, enter "con" and restart the router without entering write.
  
  On devices with a built-in keypad and LCD display the factory settings can also be restored by switching off the power, pressing the Esc and Enter keys, switching the power on and releasing the keys only when the main screen showing the device type and throughput appears on the display. After such a start-up the device operates with the factory settings. They are not, however, permanently stored in the EEPROM, so after a restart the device will return to the old settings. To make the factory settings permanent, select 'Save settings to EEPROM' from the menu, or alternatively save them by connecting via telnet or the serial console.