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Knowledge Base

The Protocol Layer of Interface cannot Go Up Because the Resistance of E1 Interface of AR28-31 does not Match

Publication Date:  2012-07-27  |   Views:  91  |   Downloads:  0  |   Author:  Liang Zheng  |   Document ID:  EKB0000253987

Contents

Issue Description

AR28-31VRP is at Version 3.40 Release 0109 It is connected to DDF cabling rack via E1 interface. 
Failure Symptoms: the physical layer of E1 is up, and the link layer is down, with the following alarm information: 
%Sep 20 09:16:18:790 2006 dongtangzhongxin_AR2831 PHY/2/PHY:    Serial2/3:0: Exceeding packet error!This should be caused by poor signal or improper channel-config.
%Sep 20 09:16:18:991 2006 dongtangzhongxin_AR2831 PHY/2/PHY:    Serial2/2:0: Exceeding packet error!This should be caused by poor signal or improper channel-config.

Alarm Information

 %Sep 20 09:16:18:790 2006 dongtangzhongxin_AR2831 PHY/2/PHY:    Serial2/3:0: Exceeding packet error!This should be caused by poor signal or improper channel-config.
%Sep 20 09:16:18:991 2006 dongtangzhongxin_AR2831 PHY/2/PHY:    Serial2/2:0: Exceeding packet error!This should be caused by poor signal or improper channel-config.

Handling Process

Power off the router, and pull out the E1 module; turn the DIP switch of the resistance on module to 120ohm (there are four values in total; adjust the resistance according to using of interfaces). The problem disappears, and the link layer is up. 

Root Cause

E1 interface is connected to DDF cabling rack through twisted cables, and the resistance of twisted cables is 120 ohm; however, the E1 interface of a router is defaulted to 75 ohm of resistance, so the resistance values do not match. 

Suggestions

It is required to pay attention to the match of resistance of cables when we use E1 module. For the identification of resistance of cables, it is often 75ohm if there are two pieces of cables, and 120 ohm if four pieces.