Late reply to an old post, but something to remenber.
Regarding the CSU mentioned in the original post, there is a very good possibility that that CSU may have timimg options that require selection based on installation requirement. Inthis case SF to ESF frame format conversion towardsthe CPE..
FOr example. The TXPort-3000 CSU/DSU, now Verilink, performes SF to ESF frame format, as well as AMI/B8ZS lince code conversion. Additionally, there is a clock option within the TXPort that must be selected.
1. Internal DS1 clock reference
2. Recover clock from the Network T1 port
3. Recover clock from the CPE T1 port
4. External clock from a HS EIA 530 data interface.
In the situation described above, the CSU requires to have the timing option configured as NETWORK.
This will then pass network clock towards the PBX port. The PBX recovers the timing from the CSU to reference the PBX internal oscillator to network timing, thus matchng the clock (DS1 line rate) from the LEC switch.
NOTE:
Maximum DS1 line rate frequency deviation is +/- 50Hz nominal 1544000Hz. REF: ANSI T1.403
