"I'm only getting 30 megabits — I'm supposed to be on 80." Half my broadband callouts start with that sentence. The customer's done a speed test on their phone, the number is lower than the tier their ISP signed them up to, they've decided the ISP is misleading them.
Sometimes they're right. Often they're not. The speed test isn't telling the whole story. Here's what's actually happening, and what we measure when we want to know what the line can really do.
What speedtest.net actually measures
When you run a speed test on speedtest.net (or fast.com, or any of the others), what you're measuring is this:
The speed at which your specific device — phone, laptop, whatever — managed to transfer data, over your WiFi or ethernet, through your router, over your broadband line, through your ISP's network, to a specific test server somewhere in the country, at that precise moment.
That's a lot of links in the chain. Each one can introduce a bottleneck.
The most common cause of a low speed test isn't a slow line. It's WiFi. The WiFi between your phone and the router is almost always the slowest link in the whole chain, especially if you're more than a few metres from the router or you're on the 2.4GHz band. A 65Mbps line will show 22Mbps on a speed test because the phone-to-router hop is doing 22Mbps.
The fix for that one is easy: plug a laptop into the router with an ethernet cable and run the same test wired. If wired is significantly faster than WiFi, your line is fine and your WiFi is the bottleneck. That's a different conversation entirely.
What engineers actually measure
When we want to know what a line is really doing, we plug a tester into the test socket on the master socket (behind the front faceplate of the NTE5 / NTE5c) and read off four numbers. Three of them tell you everything you need to know.
1. Sync speed
The rate at which your modem (the bit inside the router) is talking to the DSLAM card in the cabinet at the end of your street. This is the actual maximum throughput your line is capable of right now, in the current physical conditions. It's not the same as your "headline speed" — your ISP sells you a tier, and sync speed is what your line actually achieves on that tier.
On a healthy 80Mbps FTTC line in central London, sync speeds tend to land at 75–82Mbps down. On a faulty or noisy line, the modem retrains lower to maintain stable connection — you might be paying for 80 and synced at 24.
2. Line attenuation
How much signal strength has been lost between the cabinet and your master socket. Measured in decibels (dB). Higher attenuation = more signal lost = slower sync.
Distance from the cabinet is the main driver. A flat 200m from a Mayfair cabinet might show 8dB attenuation. A house 1.5km from a North London cabinet might show 35dB. There's nothing anyone can do about your distance from the cabinet — that's a function of where you live — but unexpectedly high attenuation can indicate water in the cable, corroded joints, or aluminium cable (yes, some bits of London are still on aluminium phone cable from the 1960s).
3. SNR margin (signal-to-noise ratio)
This is the one most engineers care about most. SNR margin is the difference between your line's signal strength and the background noise. Measured in dB, higher is better, and it's the leading indicator of internal-wiring problems.
A healthy line shows 12–18dB SNR margin and stays steady. A line affected by REIN (the LED dimmer / phone charger problem — see the broadband-drops-evening guide) will show wildly fluctuating SNR — jumping from 14dB down to 3dB and back up again at exact times of day. A line with corroded internal terminals will show steady but low SNR — say 5dB constantly — which means it's always teetering on the edge of dropping sync.
If your SNR margin is jumping around at the same time every evening, you've got REIN. If it's low and flat, you've got a wiring problem. If it's high and steady, your line is fine and any speed issues are elsewhere.
4. CRC and HEC errors
Less important for diagnosis but worth knowing. CRC (Cyclic Redundancy Check) errors and HEC (Header Error Correction) errors are the modem's count of how often it had to ask for data to be re-sent because what arrived was corrupt. Even small numbers — a few hundred per minute — indicate noise issues that the SNR margin alone won't always catch.
Where the engineer's view differs from yours
If your router has a "broadband statistics" or "DSL status" page (most do — check 192.168.1.1 in a browser), you can see most of these numbers yourself. The problem is that ISP-supplied routers usually simplify the display — they'll show "connected, 50Mbps" and not much else.
An engineer's test set (Fluke OneTouch, EXFO Maxtester, or the older Megger and JDSU kit) plugs into the test socket directly. It bypasses everything inside your house, sees the line exactly as the cabinet sees it, and gives us numbers your router can't show: real-time SNR fluctuation, frequency-by-frequency attenuation, noise spectrum.
That's how we can plug in for 10 minutes and tell you the difference between "your line is fine, your WiFi is the problem", "your internal wiring is taking 15Mbps off you", and "there's a fault in the cable two streets away — ring your ISP and have them raise an Openreach ticket."
Aisha in Camden was getting 18Mbps on a line that should've been 50+ on FTTC. She'd done a hundred speed tests, all returning the same 18. Sky said the line "tested fine." Aisha's interpretation was that Sky was lying. They weren't — exchange-end tests don't see the wiring inside her building.
I plugged a test set into her master socket. Sync was 22Mbps (slightly higher than what was making it through to her router, because her router was at the end of a daisy-chain of extension wiring). Line attenuation was OK. SNR margin was 4dB — bumping along the absolute minimum for stable sync. The line was capable of much more but couldn't hold it because of the noise.
I opened the communal riser cupboard and found six flats' worth of phone wiring tee'd off with corroded scotchlok connectors from the 1990s. Re-terminated her pair on a Krone strip, ran a fresh cable from the riser to her flat. SNR jumped to 16dB. Sync speed climbed to 48Mbps over the next hour as the modem retrained. Her speed tests now read 48Mbps. The "line fault" Sky couldn't see was real, fixed, and not on Openreach's side at all.
The five-minute home test
Before you ring anyone, do this. It tells you whether your problem is broadband or WiFi, which is the single most useful piece of information for any engineer you might call.
- Take a laptop with an ethernet port (or borrow one — most modern laptops don't have ethernet built in, but a basic USB-C to ethernet adapter is easy to come by).
- Plug an ethernet cable from a LAN port on your router to the laptop.
- Turn off the laptop's WiFi.
- Run a speed test on speedtest.net.
- Compare the wired result to your usual WiFi result.
If wired is significantly faster than WiFi (more than 30% difference) — your WiFi is the bottleneck. The line is probably fine. Look at the WiFi coverage guide.
If wired is similar to WiFi and both are well below your headline speed — the line itself is slow. Look at your router's broadband stats for sync speed and SNR margin. If sync is at or near your tier (e.g. 75Mbps+ on an 80Mbps line) but speed tests are still low, the problem is somewhere beyond the modem (ISP backhaul, route congestion). If sync is well below tier (e.g. 30Mbps on an 80Mbps line), the problem is in the line — and that's where an engineer comes in.
Sarah in Stoke Newington is one of my favourite case histories because the diagnosis happened in two minutes. She had a brilliant connection until about 7pm every evening, then it tanked. Sky told her the line was fine. She'd done dozens of speed tests — most showed 60+Mbps, but the evening ones showed 8.
I turned up at 8pm deliberately. Plugged the tester in. Watched the SNR margin. It was sitting at 14dB, then dropping to 3dB, then back to 14dB, in a regular pattern every 7 seconds. Classic REIN signature — something electrical was switching on and off downstairs.
It turned out to be her neighbour's LED dimmer switch in their hallway. A cheap online unit with no proper EMC filtering, dumping noise straight into the building's shared phone wiring. A small ferrite choke on the dimmer's mains lead, fitted by me with the neighbour's permission, took the noise away. Sarah's evening speed went back to 60Mbps. Total time: about an hour, most of it diplomatic.
What to look at in your router
Open a browser, type 192.168.1.1 (or 192.168.0.1 — depends on the router) into the address bar. Log in (the password is usually on a sticker on the router itself). Find a page called something like "Broadband", "DSL Status", "WAN" or "Diagnostics".
You're looking for four things:
- Sync speed (down/up) — should be close to your headline tier on FTTC. If far below, the line has a problem.
- SNR margin — should be 8dB or above and stable. Below 6 is bad. Fluctuating significantly is bad.
- Line attenuation — set by your distance from the cabinet. Mostly a sanity check.
- Error counters (CRC, HEC, FEC) — small numbers fine. Counts in the thousands per minute = wiring problem.
Take a screenshot of that page. When you ring an engineer, that screenshot saves about ten minutes of diagnostics on our side.
Common questions
Is speedtest.net useless?
Not useless, but limited. It measures the speed of your specific device, over your WiFi or ethernet, through your router, to a specific server, at that moment. Lots of variables. It's a good quick check, not a definitive measurement of your line.
What's the difference between sync speed and download speed?
Sync is the maximum your line can transfer to/from the cabinet — set by physics (distance, noise). Download speed is what you actually achieved through your router and WiFi at the moment of testing. Sync is the ceiling; download is what you got.
What does SNR margin mean?
Signal-to-noise margin — the cushion between your line's signal and background noise. Higher is better. 12dB+ is healthy. Below 6dB = sync drops are likely. Fluctuating margin = electrical interference (REIN).
How do I see my line's real stats?
Log into the router's admin page (usually 192.168.1.1). Find "broadband statistics" or "DSL status". Sync speed, attenuation, SNR margin are all there. Take a screenshot before ringing an engineer — it saves time.
Speed test result doesn't match your tier?
Take a screenshot of your router's broadband stats page, ring me, and I'll tell you in 90 seconds whether it's the line, the wiring, the WiFi or the ISP.