Causes a Physical layer to place the line in loopback mode

    startloop [node node-id] modem connect line line-name -
              mode mode-type

    The Mode argument is required.  It specifies the type of loopback
    used on the line.  The possible mode-types are:

    connector        Data is looped through a loopback connector
                     attached to the communications device.

    device           Data is looped in the communications device.

    driver           Data is looped in the driver of the
                     communications device.

    external         Data is looped through a null modem or a modem in
                     loopback mode.

    local            The communications device switches its local
                     modem into loopback mode by asserting the local
                     loopback signal. Data is then looped through that
                     modem.

    remote           The communications device switches the remote
                     modem into loopback mode by asserting the remote
                     loopback signal. Data is then looped through that
                     modem.