fcd-2 has the flexibility to meet the timing requirements of various system configurations. the timing modes for the main link and for the user channel are selected from the front panel or from the supervision terminal.

    main link timing

    fcd-2 main link receive path always operates on the receive clock. fcd-2 recovers the receive clock from the received main link data signal. the source of the fcd-2 main link transmit clock can be selected by the user. the following main link transmit timing modes are available:

    1 loopback timing (lbt): the fcd-2 main link transmit signal is locked to the recovered receive clock. this is usually the timing mode selected for network operation.

    2 internal (int): the fcd-2 main link transmit signal is derived from the internal clock oscillator. this timing mode is necessary in point-to-point applications over leased lines, where one fcd-2 must provide the timing reference for the link. in this case, one fcd-2 must use the internal oscillator, and the other must operate on the recovered clock signal.

    the internal timing mode is also useful during system troubleshooting, since it provides fcd-2 with an independent timing capability.

    3 external: the fcd-2 main link transmit clock is locked to the clock signal provided by the user dce connected to the fcd-2 user data channel. when a user data channel is used as source, the channel must use clock timing mode 3 (dte2). this mode can be used to maintain hierarchical timing distribution in large networks.

    main link fallback timing

    fcd-2 has provisions for maintaining timing integrity in case the source selected to supply the main link transmit timing fails. these are the fallback timing source options:

    1 fcd-2 generally switches to the internal clock oscillator in case the timing source fails, for example, if the receive signal is lost, and vice versa.

    2 in addition to the internal oscillator, fcd-2 can be configured to use the external transmit clock connected at the user data channel interface in the dte2 mode described below. in this case, fcd-2 will switch to the internal clock oscillator only when the two other sources fail.

    user data channel timing

    fcd-2 has three user clocking modes:

    1 clocking mode 1 (dce): the fcd-2 data channel operates as a dce and provides the receive and transmit clocks to the data terminal equipment (dte) connected to the user channel. the clocks are locked to the corresponding main link clock signals.

    2 clocking mode 2 (dte1): the fcd-2 data channel supplies the receive clock to the synchronous user dce, and accepts a transmit clock from the dce. the dce must transmit data at the rate of the clock signal supplied by fcd-2.

    3 clocking mode 3 (dte2): the fcd-2 data channel operates as a dte and accepts both receive and transmit signals from the user equipment (a dce). the dte2 mode must be selected on the data channel that is to serve as a clock source whenever the transmit path of the fcd-2 main link uses external timing, or external timing is used as a fallback source.

    the d te1 mode is not available on x. 21 channels.