Internet speeds - what controls them?

I finally got a new cable line put on my house from the pole, and tested my speed. Using both speedtest and Xfinity’s test, I get a max of 15mbps download (not surprising, Xfinity’s test shows higher).

Looking at my plan, my speed is supposed to be 250mbps - that’s a huge difference.

My modem is a Netgear CMD31T but I am also running my laptop through an Airport Express (refurbished unit bought Feb 2016). The modem is plugged into the line coming into the basement and the tech said I’m at the high end for signal strength. I am currently sitting above the wireless and signal strength is good.

I’m hoping the cable at least resolves my dropout issues, but it certainly hasn’t helped speed any. But I’m wondering how much I am losing by going over wifi. Unfortunately I didn’t test prior to them disconnecting me.

Computer is 2015 MBP Retina running Sierra, 2.7 GHz Intel Core i5 16gb RAM



Most providers plans offer an ‘up to’ speed. Up to 250 includes 15… but the differential is extreme in your case. Our provider offers rebates when speeds drop below a certain percentage of the plans marketed speed. Your WiFi shouldn’t be that different from a wired connection.

Our plan offers up to 100 but it’s regularly around 50. They did warn me when the account was set up that our line wouldn’t sustain the full speed.


[Aside to everybody else: Apologies for the lengthy reply. Am I missing a way to reply directly to Diane?]

But I’m wondering how much I am losing by going over wifi.

Potentially, a lot. It depends on the hardware on both ends and conditions between them. However, there are other limitations in your setup.

The answer to the question in your subject is the slowest link/node in the chain limits the end-to-end speed.

Your modem may be dragging down your cable speed. In the product description on their website, NETGEAR says the CMD31T is rated for “Up to 150 Mbps†” (“† Maximum rate is derived from DOCSIS® 3.0 specifications and actual throughput will vary depending on the MSO provisioning and speed tier subscription.”)

The connection to your AirPort Express may be dragging your speed down further. A refurb AirPort Express bought in 2016 is likely to be the 2012 (last) model, so it can do real 802.11n on the wifi side, but it is limited to 10/100BASE-T Ethernet on the hardwire side (both WAN and LAN ports), dropping you further to 100 Mbps theoretical maximum of your 250.

Your 2015 MBP Retina should be capable of 802.11ac and compatible with 802.11n, so that hardware shouldn’t be a limiting factor.

The floor between the AirPort Express and your MBP will also have an effect on the speed achievable, even with good signal strength, depending on materials and thickness.

You can see what protocol and speed your AirPort Express and MBP are negotiating by holding down the option key while pulling down the wifi icon in your menu bar. (You may need to enable that icon in System Preferences > Network > Wi-Fi.) Check “Tx Rate” and “PHY Mode”; you would like to see something near 100Mbps and 802.11n respectively.

While you have it pulled down, you might check “CHANNEL” to see if you are using the 2.4 or 5 GHz band. 5 GHz can offer higher speeds, but it degrades more with distance and materials in the signal path.

Best of luck resolving your speed issues.


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You should try the speed test with your MBP connected directly to the modem with an ethernet cable. That will tell you what speed Xfinity is providing you at the modem. Any speed loss after that has to do with your wifi connection.

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I believe I’ve configured things here so you can reply directly to people by clicking the ••• button under a message, then clicking the flag button, and then using the private reply option. The system should send a private message and if the recipients have their notifications set right (probably a default), they should get it in email too.

I found it very informative. Thank you for not going private.

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Hi all and thanks so much to Mark for the incredible details! I have not gotten Tidbits email in a few days so I had to come online read this.

I really love the hidden commands - I really need to keep a list handy at all times (is there a list?!)

I figured out the modem speed awhile after I posted this. I did not plug directly in simply because the techs came back before that, but I did move around the house and test different locations. I would have had to use my 2008 unibody anyway as my MBP doesn’t have a port.

What I discovered after they left the second time was that my Airport was only set up for 2.4 and it’s a dual band! I had no idea (and feel like an idiot).

My MPB and even my 2008 unibody are now all set to 5 GHz. Neither of our Windows laptops sees any of the 5 GHz networks in the neighborhood.

Bottom line is my modem may be having intermittent problems (or not - Comcast has been at my pole and around the neighborhood continually since they left here Saturday afternoon, so something is still going on out there). So I’ll be buying a faster one anyone. DOCSIS 3.1 is much more expensive than the 2.1 I have now, I must’ve bought further into the product cycle last time.

One of the techs got a 130mbs reading directly off the modem and I am certainly getting much higher readings when I am right next to it, even wireless.

Am I correct in thinking that moving the Airport back upstairs (ethernet plugged directly into modem in the basement and airport in my office) will not degrade speed at all? (unless I am surfing in the basement).

I will look into a DOCSIS 3.1 with wireless as well but think they will be way more than I want to spend. It feels like I bought this one only 3 or so years ago but maybe it was longer.

Thanks again for the help! It’s been an interesting weekend - but hey I finally got my 1980s cable replaced!


You shouldn’t be able to notice the delay caused by a few yards/meters of properly rated ethernet cable.

Remember that there are various kinds of ethernet cable. Use cable rated at 100Mbps or higher with your existing hardware, so it doesn’t form a new slowest link in the chain. If you think you might swap out the AirPort Express for something faster, go for gigabit cable.


Yes and yes. I have messages saved from a speed discussion awhile back. I could run the cable modem back up here as well, in either case it’s great to know everything is new.

My current switch is gigabit. If I bring the wireless back up and plug the switch into that, will it NOT pass gigabit through to the devices plugged into it? (basically a new laser and my 2003 FW800)

If I am correct on that, the modem definitely has to come back upstairs.


Also, remember that the Airport Express, being 10/100 Ethernet only, will be a bandwidth limiter on your WiFi connection if your direct modem speed is faster than 100, even if the AEX is hooked up to a gigabit switch. The fastest WiFi speeds you’d be able to get with that setup would be maybe 80, assuming that your direct cable modem speed is faster than 100 and depending on a bunch of other variables.

Hi - specs say the Express will have a raw throughput of 300 mbps in 5GHz and 150 mbps in 2.4

Are you saying in reality it will only be 80, even with a 300mpbs modem?

A Macworld review (below) claims the wifi data will not be affected by the 10/100 ethernet port.

Please let me know if I read this wrong - thanks!

I’m still running with everything in the basement and speed is much improved but of course not near what I should be getting simply due to distance and the modem. I’ve also found my 2008 unibody and the iPhone SE are more sensitive to the 5G if they are far away.


If you are only concerned about the speed between wifi devices, the ethernet speed will not matter. However traffic to and from the internet will be limited, as it will have to pass through the 10/100 ethernet port to/from the router (whether or not there is a switch between them).

Keep asking yourself, “What’s the slowest thing in the path?” That’s going to throttle down the flow that passes through it. Slower devices can pull down faster devices, but not the other way around.


Yes. If the AEX is hooked up to your modem via an Ethernet cable plugged into the AEX 10/100 Ethernet jack, then the fastest theoretical WiFi Internet speed you’ll get is 100, assuming that the modem is getting above 100 speeds. Real world speeds for 10/100 Ethernet over the cable between the modem and the AEX would probably top out around 80/90, and the WiFi’s Internet speeds are limited to the speed of this cabled Ethernet connection.

Perfect - thanks! I’m looking at something newer anyway but it’s what I have at the moment.

Mark, I have a G4 FW800 on the network in addition to a new laser printer.


You mentioned that. The G4 FW800 specs include “10/100/1000BASE-T Ethernet”. That interface won’t be the slow link. (The G4 and the backplane might be the limiting factor inside that computer.)

Maybe you should replace the flag by a mail icon or something. I would have not known that there is a mail function hidden underneath it.

It’s a flag because the primary purpose of the dialog is to report spam or other “bad” forum messages. The other option, “I want to talk to this person directly and personally about their post,” may still not send an email to the individual, depending on how they have their forum notifications set.

@cwilcox is right—the flag icon is mostly used for other things. You can also always click your avatar, click the envelope icon to go to your messages, and click the New Message button to create a new message from scratch. Discourse is good about autocompleting usernames, so you can usually guess at it easily.