How to 'tailend' a leased line with the FRM220-DATA

The FRM220-DATA is a fibre optic modem for serial data. Its flexibility allows it to be used in a multitude of different scenarios. The following gives some insight into its many features :-

  • Support for X.21, RS530, RS449, V.35 and RS232
  • Supports both Sync and Async operation
  • Data rates from 50bps to 8.192Mbps
  • It can be configured for DTE & DCE operation without the need for serial cross over cables
  • Multiple clock modes - Internal, External, Recovered, Transparent and Asymmetric
  • Optics can be single or multimode with options for single fibre and C/DWDM support
  • Can be configured via onboard DIP switches or via companion SNMP/HTTP NMC card within FRM220 chassis

One of the most common ways for the FRM220-DATA to be deployed is when it is used to extend an existing serial leased line. The existing service, likely from a network operator, will be already be providing a clocking signal out of their serial interface. The FRM220-DATA has to be configured to 'take' this clock and present it to the far end equipment. This type of extension is known as 'tailending'.

The following diagram shows how the FRM220-DATA is configured when 'tailending' a 2.048Mbps serial leased line. This shows how the onboard DIP switches need to be set. If the FRM220-DATA is being used in conjunction with the FRM220-NMC management card, the DIP switches settings are not required and all configuration is achieved via the NMC.

Tailending X.21 Circuit

Switches SW#1 and 2 configure the speed whereas SW#3 configures the interface type, the DTE/DCE selection and the configuration of flow control circuits. SW#4 can be used to invoke various test loops that could be useful to aid troubleshooting if required.