Is it normal for the modem to feel hot and hot to the touch? Summer light decay causes high network speeds, three cooling tips tested in real tests

mid-July during that high heat, my home network had a strange problem: every afternoon, my internet speed dropped from 940Mbps to around 760Mbps, and every evening it would come back on its own. The optical modem indicator light is all green, and restarting the router doesn't help. The night I squatted in front of the weak current box to check the case, I stuck the optical modem shell on the back of my hand—hot to the touch, hotter than a router. Checked the backend of 192.168.2.1 and saw the light degradation—it had increased by almost 3 dB compared to the spring checkup.

the modem overheats and the router overheating

I wrote a dedicated article about

router overheating. The symptoms are the CPU can't handle the high load, WiFi disconnects, and background apps spinning in circles. The solutions are three key strategies: relocating, padding, and installing a fan. The optical modem is quite different. The most delicate part of the optical modem isn't the main chip, but the laser that keeps glowing—your data is sourced from this beam to the hallway splitter, then to the data room end. If you want to know about router setups, check out this article: Is it normal for routers to get hot and hot to the touch Below is a focus on optical modems.

lasers are especially sensitive to temperature; this is a physical property. When the temperature rises, the luminous power drops. When the light is weak, the signal received by the data center becomes weaker, and the "received optical power" value you see in the backend increases—that is, the light attenuation increases. So if the router overheats, it speaks to lag; if the modem overheats, it speaks to light degradation. Light degradation is the modem's thermometer.

another hard reference: the modem's rated operating temperature generally states 0 to 45 degrees. I checked Huawei's HG8145V manual, which clearly states that if the operating temperature exceeds 55 degrees, the cooling environment should be checked. But in a sealed low-voltage box in summer, it's not hard to hit 45 degrees—the optical modem, router, and switch all heat up together, and the cabinet is exposed to the sun, turning it into a mini sauna.

my week's timeline, light decay follows the box temperature

My weak current box is in the balcony wall. I cleaned it up last year, and the optical modem was moved to a lower position inside the box. But the wall of the box could be sun-dried in the afternoon. During the week of continuous high temperatures in July, I hung the kitchen probe thermometer inside the box and recorded the temperature every day at 2 p.m., then casually checked the light decay and velocity measurements in the backend, keeping records for six days.

pattern is rigid: the higher the cabin temperature, the greater the light decay, and the worse the speed in the afternoon. On July 18th, the cabin temperature was 49°C, which was the most typical. In the afternoon, light decay climbed to -24.6°C, with a speed test of 760 km/h; At 10 PM, the temperature dropped back to 41 degrees, nothing changed, light decay returned to -23.2, speed returned to 890. On this day, the leather wire and splitter weren't touched; what changed was the temperature. This "poor afternoon, good evening" trend is the signature of temperature-type light decay.

why does light decay increase when the temperature rises

explain the principles in plain language. The laser in the modem normally transmits light power between 0.5 and 5dBm, powered 24 hours a day. Laser sensitivity to temperature is innate: with each step higher in ambient temperature, the luminous power drops slightly, and the wavelength drifts a bit. When the light is weak, the data read by the receiver in the data center is poor, which shows in your backend as the light decay and the number increases.

light decay worsens, bit error rates also increase. If there are too many bits and packets need to be retransmitted, the speed drops—that's why my speed and case temperature were reversed that week. Once the light decay difference drops below -28, it's no longer a drop in speed—it's just a lot of bit errors causing disconnections and a flashing LOS red light. At that point, self-care is basically useless, so just honestly report for repair.

there's another thing to add fuel to the fire: ITMS is pushing its configuration in the middle of the night. At two or three in the morning, ITMS released the configuration for the Time Cat CPU, which was already hotter and coincided with the worst cooling period on summer nights. That week, I occasionally disconnected once in the early morning, which was closely related to this. I recorded the network disconnection patterns for a month, and I wrote this article about four patterns in total: Why does the Telecom optical modem always disconnect

try these three cooling techniques one by one, and you'll know how much light deteriorates when it returns

the root cause of illness is temperature, just follow the cooling method. The three moves are ranked from small to large at cost, and after each move, copy the light fading backend.

first trick: open the lid of the weak current box, and get zero yuan. night, I opened all the door doors, and by the afternoon the next day, the temperature dropped from 49 to 43, and the light decay dropped from -24.6 to -23.1. The drawbacks are obvious: the case looks ugly when exposed to dust, so it can only be used as a diagnostic tool—after opening the lid, the light degradation clearly returns, confirming it's a temperature issue.

second trick: stand the optical cat upright, don't stack it again. my router was directly pressed onto the optical modem, and the two burned each other. Move the router aside and place the optical modem upright (some models have hook ears that can be hung directly on the cabinet wall), leaving a fist's worth of space on both the top and bottom. Two days later, light decay was -22.7. This trick is free long-term profit, and it's strongly recommended to do it first.

third trick: use a USB mini fan to blow directly. USB fan for just over ten yuan, powered by the charging head, fixed at the box opening to blow on the optical modem. Two hours later, the light decay dries to -21.9°C—even better than before the heat. This is an experimental move, proving that the optical module itself is fine—it's purely that the ambient temperature trapped it.

my current final plan: replace the lid with a cooling vent (about 30 yuan online, measure the size before buying), put away the fan, light decay stabilizes around -22.5, and the 940 still runs full during evening peaks. If you find changing the lid too troublesome, simply moving the optical modem out of the weak current box is also an option. I've written about several ways to move the modem and its costs: Can modem be relocated

two situations, don't blame the temperature

First: Low temperature, large light decay. With the lid open and the fan blowing directly at the limit, the light decay is still below -27 km/h, so it's not about the temperature—most likely, the optical module is getting older. Lasers are lossy components, and prolonged high temperatures accelerate their aging. The hotter they get, the faster they deteriorate, creating a vicious cycle. At this point, don't fuss with cooling. Dial 10,000 and get a 'low light' alert. The technician brings a light power meter to measure in segments, so switch cats when needed. The optical modem is the carrier's asset and can be replaced for free if not caused by human factors. Before reporting for repair, take a photo of the SN code on the sticker on the device body. The technician can ask, and what is each string of codes on the sticker for? I once wrote a dictionary: Telecom optical modem sticker has . My used TEWA-750G was replaced once through the repair process, and I didn't charge for the whole process.

second type: light degradation is sometimes good or bad and doesn't match temperature. If the reading is poor when it's cool in the morning, don't doubt the cat for now—squat down and check the wires—if the box door is pressed down, bent in a bent way, or the zip ties are too tight, all will cause light decay and jump. Cable management is zero-cost; first smooth out the lines before counting. Three tips for dividing light degradation and self-checking before, which I wrote in detail before: -8 to -27 for health, -27 to -28 for marginal use, and beyond -28, don't mess around with it. Check out this article: What to do if the telecom modem has severe degradation.

take it personally, let's talk about two habits

two habits to wrap things up. First, don't try to stop and restart the optical modem after the router is done. Frequent power cuts damage the laser, not maintenance. I've done the math: should the telecom modem be restarted regularly Second, keep the light degradation base recorded on the day of installation. Every summer, during the hottest period, if the light deteriorates by one or two dB per year, it's normal aging; if it rises by four or five dB in a summer, it's a warning alert. If the PON light starts flashing and registration fails, that's a different issue. Check out this article: ONT and PON light keeps flashing and registration fails . The overall organization of the weak current box and the placement of the optical modem are particular. Huawei has a counterintuitive approach and has written: the weak current box is too messy, how should you organize it