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Faxes Fail Due to Network Delay

Publication Date:  2015-07-24 Views:  939 Downloads:  0

Issue Description

 U1960: V100R001C01SPC300
 IAD1224: V300R001C07SPC700

 The U1960 connects to outer-office devices through the PRI trunk.
 The IAD registers with the U1960 using SIP.
 PSTN phones and fax machines are connected to the IAD.

The customer switches the network in January. Before the switching, frontline delivery engineers perform fax tests for three to four times. The fax speed is low, but no fax failure occurs.
During two months after the switching, some fax users report that faxes sent to China often fail, and faxes sent from China to the local end often fail or cannot be printed completely. The failure probability for intra-city fax sending and receiving is low.

When the fault is reported, Huawei engineers go to the customer site to collect fault information.
  • View the U1960 alarm console. No E1 frame slip occurs.
  • When faxes are being sent, the complete fax tone can be heard in most cases.
  • Fax tests can be completed correctly in most cases, but fax machines print the fax failure report.
Various methods are tried to locate the fault, such as replacing fax machines with another model, changing fax machine numbers, and connecting fax machines directly to the distribution frame. However, the fault persists. After massive tests, the fax failure probability exceeds 80%.

Handling Process

Step 1 Perform mirror packet capture on the switch, and use the Wireshark to capture packets on the service port of the U1960 when using different fax numbers to send faxes. Record the packet capture result for each successful or failed fax separately.

Step 2 Analyze the packets captured by the Wireshark to find the differences between packets for successful and failed faxes.

It is found that faxes fail in the following situations:
 When fax training starts, the training confirmation tone returned by the peer fax machine overlaps the next training tone sent by the local fax machine. As a result, the local fax machine cannot recognize the confirmation tone, and the fax fails.

The following figure shows the correct training tone.

 After the local fax machine sends the EOP message when fax training ends, the peer fax machine returns the MCF message, which overlaps the DCN message sent by the local fax machine. As a result, the local fax machine cannot recognize the MCF message, and the fax fails.

The following figure shows the correct fax ending tone.

Based on the preceding packet analysis, it is preliminarily confirmed that the U1960 and IAD run properly.

Step 3 Provide the preceding analysis result to the customer. The customer decides to contact the E1 line provider to check the network delay cause.


Root Cause

The T.30 protocol defines the procedure for exchanging files on the PSTN between a pair of G3 fax terminals, including manual and automatic communication. The entire transmission process consists of signaling and data transmission. Signaling transmission uses High-Level Data Link Control (HDLC) format and V.21 second channel 300 bps modulation signals. Data transmission uses the modulation protocol negotiated by fax terminals. A fax protocol is an upper-level transmission protocol. The protocol itself generally does not ensure reliable data transmission. The data correctness depends on the line quality.

The following table describes fax phases.

The following table describes common T.30 messages.

The following figure shows the process for sending a single-page fax.