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Echo when Nortel PBX connected to Cisco 3825 T1 (CEMoIP)

This is the setup causing echo. ATM cloud is a 155M 30 mile diameter OC-3 ring.

............PhC

.............|

PhA--PSTN—NortelM1—3825A—ATM cloud—3825B—Nortel M1—AA

.............|..............................................................................|

.............PhB.......................................................................PhD

Echo could be consistently reproduced with topology above. When PhoneA from PSTN during business hours makes a call to PhoneB but no one answers, the call is sent to Auto Attendant(AA). The PhoneA makes selections with Auto Attendant. If the selection is sent the call back to PhoneC or PhoneB and someone picks up the call. The Phone C or B will hear echo.

Direct calls to Phone B from PSTN could not reproduce echo.

Direct calls to and from PhoneD may occasionally reproduce echo.

If 3825A is removed as below, then all echoes are gone.

............PhC

.............|

PhA--PSTN—NortelM1—ATM cloud—3825B—Nortel M1—AA

.............|........................................................................|

.............PhB...............................................................PhD

Do you have similar situation Nortel helped resolve the issue?

On 3825A and B CEM ports, we reduced de-jitter buffer from 100ms to 5ms, and packet size from 528Bytes to 256Bytes. The effect of echo was significantly reduced however not eliminated.

Any suggestion beyond is welcome.

Thanks in advance!!

1 REPLY
Silver

Re: Echo when Nortel PBX connected to Cisco 3825 T1 (CEMoIP)

Hybrid echo is caused by an impedance mismatch in the hybrid circuit, such as a two-wire to four-wire interface. This mismatch causes the Tx signal to appear on the Rx signal.

http://www.cisco.com/en/US/tech/tk652/tk698/technologies_tech_note09186a0080149a1f.shtml

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