Is it the modem or the line? How to tell
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The internet is bad; the technician says "the modem is fine", support says "the line looks clean", and the ball keeps landing in your court. Yet separating modem from line at home is usually possible — run these tests before booking a technician, and at least you will know who to argue with.
From symptom to suspect: the quick table
| Symptom | Primary suspect |
|---|---|
| Slow everywhere, but no disconnects | Line / congestion |
| Connection fully drops and returns several times a day | Modem or line — isolate further |
| Slow only on some devices | The device or Wi-Fi |
| Modem too hot to touch, freezes occasionally | Modem |
| Drops in the evening, recovers at night | Regional congestion (line side) |
| Gets worse in rainy weather | Line (copper wiring) |
Test 1 — Rule out your devices
First make sure the problem really lives in the network: run the speed test on at least two different devices (phone + computer). Slow on one device only → that device is the problem; modem and line are acquitted.
Test 2 — Go wired
Disable Wi-Fi entirely and measure over Ethernet. If speed is fine over cable, the modem's modem part and the line are healthy; the problem is wireless (placement, channel, interference). Calling your ISP about the line would be wasted breath — reposition the router first.
Test 3 — Ask the modem itself
Open the modem's interface (usually 192.168.1.1) and find two values:
- Sync speed: the rate negotiated between your modem and the exchange. If your plan is 100 Mbps but sync shows 35 Mbps, the problem is not inside your home — the line can only carry that much. A screenshot of this screen is powerful evidence in an ISP conversation.
- Uptime: if it keeps resetting even though you never noticed an outage, the line is dropping and reconnecting → line side.
On DSL, SNR/attenuation values help too: very high attenuation = long distance to the exchange = a structural speed ceiling.
Test 4 — Interrogate the modem: heat, age, restart
- Unplug for 30 seconds; if things improve but degrade again within days, the modem is tired.
- If it is uncomfortably hot and lives in a closed cabinet, ventilate it — an overheating modem both slows down and drops connections.
- A 5+ year old modem can be the bottleneck, especially after a fibre/VDSL upgrade; providers usually ship a new device on request.
- If possible, try another modem (a friend's): if the problem vanishes with a different modem on the same line, you have your answer.
Test 5 — Spread it over time
One day of data misleads. Test at different hours across several days; the history feature on this site archives the results for you. A regular evening dip points at the line/congestion; random drops at random hours point more at the modem.
The verdict: who do you call?
- Modem guilty: request a device swap from your ISP (your right if it is rented) or buy a compatible one.
- Line guilty: open a fault ticket armed with your sync screenshot and test history. For the process, your rights and the regulator stage: Not getting your advertised speed.
- Still unclear: book the technician, but keep your findings from this article handy — it is hard to dismiss a subscriber who says "slow over cable too, sync stuck at 35, same on two modems."