This article describes the clocking-mode setting recommended for a serial interface on the SRX when it is functioning as data terminal equipment (DTE).

The topology below depicts the following:

• SRX650 with a serial interface installed.
• SRX650 works as a DTE.
• Clocking mode is loop-timed, by default.
• Serial interface is up, but protocol is down.
• Framing errors increasing quickly.

Topology

SRX650(DTE) --- Modem(DCE) ------------- Modem(DCE) --- SRX650(DTE)

root@SRX650> show interfaces terse se-8/0/0 
Interface               Admin Link Proto    Local                 Remote
se-8/0/0                up    up  
se-8/0/0.0              up    down inet     1.1.1.1/30    

  Input errors:
    Errors: 101987, Drops: 0, Framing errors: 101986, Runts: 0, Giants: 0, Policed discards: 0, Resource errors: 0
--snip--
  LCP state: Down
  NCP state: inet: Down, inet6: Not-configured, iso: Not-configured, mpls: Not-configured
--snip--
  Serial media information:
    Line protocol: v.35
    Resync history:
      Sync loss count: 0
    Data signal:
      Rx Clock: OK
    Control signals:
      Local mode: DTE
      To DCE: DTR: up, RTS: up
      From DCE: CTS: up, DCD: up, DSR: up
    Clocking mode: loop-timed
    Loopback: none
    Tx clock: non-invert
    Line encoding: nrz

Frame check sequence (FCS) errors can occur for one of the following reasons:

• The serial line is noisy.
• The serial cable is too long, or the cable from the channel service unit/data service unit (CSU/DSU) to the router is not shielded.
• Improper clock setting.

The PIM card, receiver, and fiber are all good, so it's time to focus on the clocking mode.

After configuring serial-options clocking-mode with data circuit-terminating equipment (DCE) instead of loop, this issue is resolved.

"loop" is the default clocking-mode on Junos. When the device is functioning as DTE, you must use the DCE clocking mode for all serial interfaces except X.21 serial interfaces (X.21 does not have a TX clock).

Difference between loop and DCE

• When set to loop, DCE use the RX clock.
• When set to DCE, DCE use the TX clock.
• The figure below shows the clock sources of loop, DCE, and internal clocking modes.

Set the serial options clocking mode to DCE instead of loop-timed.

For more information, see Configuring the Serial Clocking Mode in this Tech Doc .

• Understanding Frame Check Sequences