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Synchronization unable due to wrong pin assignments when using 3rd party balun

Publication Date:  2019-07-09 Views:  199 Downloads:  0

Issue Description

After swap to Huawei RTN, NSN NodeB is unable to recover the synchronization from the RTN910. The external clock port on the RTN910 is used to synchronize the NodeB. The clock port on the NSN NodeB is 75ohm, while it is 120ohm on the RTN910, so a balun is used in between.

Alarm Information

Sync alarm on NSN NodeB. DCN communication to NodeB is OK.

Handling Process

  1. Checked the synchronization of the PTN NE and the two RTN NEs. PTN is using 1588v2 for synchronization, and the status was OK. RTN950 is using Synchronous Ethernet with the PTN950 as clock source. PTN910 is using physical layer synchronization on the microwave link.
  2. Verified the clock output mode of the external clock port as 2Mbit/s.
  3. Checked the clock cables, and balun. We discovered that the balun provided by the subcontractor has different pin settings than the balun Huawei provides.

On Huawei RTN, the pin assignments for the external clock ports are as follows:

Pin External Clock
1 CLK receiving (-)
2 CLK receiving (+)
3 Not defined
4 CLK transmitting (-)
5 CLK transmitting (+)
6 Not defined
7 Not defined
8 Not defined

The Huawei balun and the balun provided by the subcontractor use different pin assignments:

Pin Huawei balun 3rd party balun
1 CLK receiving (-) CLK transmitting (-)
2 CLK receiving (+) CLK transmitting (+)
4 CLK transmitting (-) CLK receiving (-)
5 CLK transmitting (+) CLK receiving (+)

A picture of the 3rd party balun is shown below.

Therefore, the custom 120ohm cable made by subcontractor was incorrect, and after making a new cable with the correct pin assignment, the NSN NodeB was able to detect the synchronization output from the RTN910.

Root Cause

  1. Check the synchronization status of the Huawei NEs, starting with the upstream NE.
  2. Verified the settings of the external clock output.
  3. Check the clock cables.


Make sure subcontractor pay special attention to the pin assignments, especially when using 3rd party equipment.