Industrial communication operations personnel almost universally rely on ping to assess link status. If ping succeeds, they assume everything is fine; if ping fails, they begin troubleshooting. But this logic is wrong in many scenarios. A real-world case: a substation experienced frequent connect/disconnect cycles. Ping to the master station showed 0% packet loss and 20ms latency—all metrics appeared normal—yet the business connection kept dropping. The root cause likely lies in the fundamental differences between ICMP and TCP mechanisms.
I. What Exactly Does Ping Measure?
Ping operates on the ICMP protocol with a very simple mechanism: the local device sends an Echo Request, the remote device replies with an Echo Reply, and that’s it.
It establishes no connection, maintains no state, assigns no sequence numbers, and requires no acknowledgment. Every ping is an independent, one-time “shout.”
If ping succeeds, it proves exactly one thing: the IP-layer physical pathway is open.
But industrial communication protocols (such as IEC 104) run over TCP. TCP operates completely differently:
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First, a three-way handshake establishes the connection
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Every data packet carries a sequence number, and the receiver must send an acknowledgment
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Every NAT device along the path maintains a “connection registration table” recording the state of this TCP connection
Here’s the critical point: this registration table has a timeout. In 4G carrier environments, this timeout is typically around 5 minutes. Once timed out, the NAT deletes the entry.
After deletion, the following occurs:
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Data packets from the master station: the NAT finds no matching entry and drops them directly
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Data packets from the DTU: the NAT treats this as a new connection and assigns a new port, which the master station doesn’t recognize, so they are also dropped
The TCP connection is actually broken, but ICMP doesn’t check the registration table—ping continues to succeed.
Ping proves the road exists; TCP proves the road is open. The road existing does not equal the road being open.
II. Why Is Wireless Communication More Vulnerable?
Wired networks typically have only one layer of NAT, with longer timeout durations, making connections relatively stable.
4G wireless communication passes through at least three layers of NAT:
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First layer: The CPE, responsible for local routing translation
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Second layer: The base station gateway, a wireless access network NAT
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Third layer: The carrier CGNAT, responsible for public address translation
Each layer maintains its own connection registration table, each with independent timeout mechanisms. TCP long connections in this multi-layer NAT environment are inherently fragile. This is not poor device quality—it is architecturally determined.
This also explains why the same site worked fine after switching to fiber optic—wired networks have no base station handovers, no multi-layer NAT timeouts, and none of these intermediate variables.
III. When “Ping Succeeds but Frequent Disconnections Occur,” Follow This Diagnostic Sequence
After learning from detours, here is an efficient troubleshooting path that can compress three days of work into several hours:
Step 1: Check Physical Layer Signals
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Inspect CPE indicator light status
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Review critical 4G metrics: RSRP (signal strength), SINR (signal quality)
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Confirm whether frequent base station handovers are occurring
Judgment criteria: RSRP below -90dBm indicates weak signal, requiring amplifier installation or antenna repositioning; frequent base station handovers indicate connection instability, requiring base station locking.
This step resolves weak signal and base station handover issues. If signal strength is full but disconnections persist, proceed to the next step.
Step 2: Capture Packets
Pull up the master station’s recent connect/disconnect logs, focusing on whether regular patterns appear before each event:
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Message 50 (encryption authentication request): If this appears before every disconnect, the TCP connection has already broken, and the master station is attempting to rebuild it
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Monitoring frame interruption, U-frame interruption: These indicate application-layer communication has already ceased
Core operation: Capture packets simultaneously at both the master station and DTU ends, then compare them. Examining only one side easily leads to misjudgment. Whether the master station sent but the DTU didn’t receive, or the DTU sent but the master station didn’t receive—this reveals the true link state.
Step 3: Test TCP Communication Quality
Don’t perform just a single ping—conduct sustained testing:
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ping -tfor continuous ping, observing whether intermittent packet loss occurs -
tracertto check routing hop count, determining whether the path is circuitous -
telnet IP portto directly test TCP port connectivity -
Time-segmented testing (day vs. night, weekdays vs. weekends), determining whether network load accelerates NAT timeout
Judgment criteria: If ping is normal but TCP connection is unstable, the problem lies at the application layer, not the network layer; if instability occurs during specific time periods, NAT timeout acceleration is the likely trigger.
Step 4: Change Communication Method
Priority ranking: Fiber optic > Wired network > Change carrier > Optimize wireless solution
If conditions permit, deploy fiber optic directly. Wired networks have no multi-layer NAT timeouts or base station handover issues—their stability is fundamentally different. If the problem disappears after changing communication methods, you can basically confirm it was an architectural defect of the original method.
Practical experience: we ultimately resolved the issue only after switching to fiber optic. No matter how much wireless communication is optimized, its stability cannot match wired connections.
IV. Conclusions
Ping is the minimum viable test and cannot represent business-layer status. ICMP does not check connection registration tables; NAT forwarding requires no state maintenance for ICMP. Next time you encounter a communication fault, capture packets first—don’t ping first.
Packet analysis is the core method for problem localization. Indicator lights can mislead; ping can mislead. But packets record the actual communication behavior that occurred. The breakthrough in this troubleshooting effort came from finding the pattern in Message 50.
TCP long connections in wireless communication have structural weaknesses. Multi-layer NAT connection registration table timeouts are the primary cause of TCP disconnections in wireless communication scenarios. This is not a problem with any particular device—it is an inherent characteristic of 4G architecture. In scenarios where fiber optic is available, prioritize wired connections. This is not “wireless is bad,” but rather “the complexity of wireless has been underestimated.”
