I ordered a new M1 MacBook Pro to upgrade my existing model from 2016. I’m still waiting on it to arrive by managed to catch a sensationalist headline in the process:
“New MacBook Wi-Fi Slower than Intel Model!”
The article referenced this spec sheet from Apple referencing the various cards and capabilities of the MacBook Pro line. I looked it over and found that, according to the tables, the wireless card in the M1 MacBook Pro is capable of a maximum data rate of 1200 Mbps. The wireless card in the older model Intel MacBook Pro all the way back to 2017 is capable of 1300 Mbps. Case closed! The older one is indeed faster. Except that’s not the case anywhere but on paper.
PHYs, Damned Lies, and Statistics
You’d be forgiven for jumping right to the numbers in the table and using your first grade inequality math to figure out that 1300 is bigger than 1200. I’m sure it’s what the authors of the article did. Me? I decided to dig in a little deeper to find some answers.
It only took me about 10 seconds to find the first answer as to one of the differences in the numbers. The older MacBook Pro used a Wi-Fi card that was capable of three spacial streams (3SS). Non-wireless nerds reading this post may wonder what a spatial stream is. The short answer is that it is a separate unique stream of data along a different path. Multiple spacial streams can be leveraged through Multiple In, Multiple Out (MIMO) to increase the amount of data being sent to a wireless client.
The older MacBook Pro has support for 3SS. The new M1 MacBook Pro has a card that supports up to 2SS. Problem solved, right? Well, not exactly. You’re also talking about a client radio that supports different wireless protocols as well. The older model supported 802.11n (Wi-Fi 4) and 802.11ac (Wi-Fi 5) only. The newer model supports 802.11ax (Wi-Fi 6) as well. The quoted data rates on the Apple support page state that the maximum data rates for the cards are quoted in 11ac for the Intel MBP and 11ax for the M1 MBP.
Okay, so there are different Wi-Fi standards at play here. Can’t be too hard to figure out, right? Except that the move from Wi-Fi 5 to Wi-Fi 6 is more than just incrementing the number. There are a huge number of advances that have been included to increase efficiency of transmission and ensure that devices can get on and off the air quickly to help maximize throughput. It’s not unlike the difference between the M1 chip in the MacBook and its older Intel counterpart. They may both do processing but the way they do it is radically different.
You also have to understand something called Modulation Coding Set (MCS). MCS defines the data rates possible for a given definition of signal-to-noise ratio (SNR), RSSI, and Quadrature Amplitude Modulation (QAM). Trying to define QAM could take all day, so I’ll just leave it to GT Hill to do it for me:
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The MCS table for a given protocol will tell you what the maximum data rate for the client radio is. Let’s look at the older MacBook Pro first. Here’s a resource from NetBeez that has the 802.11ac MCS rates. If you look up the details from the Apple support doc for a 3SS radio using VHT 9 and an 80MHz channel bandwidth you’ll find the rate is exactly 1300 Mbps.
Here’s the MCS table for 802.11ax courtesy of Francois Verges.. WAY bigger, right? You’re likely going to want to click on the link to the Google Sheet in his post to be able to read it without a microscope. If you look at the table and find the row that equates to an 11ax client using 2SS, MCS HE 11, and 80MHz channel bandwidth you’ll see that the number is 1201. I’ll forgive Apple for rounding it down to keep the comparison consistent.
Again, this all checks out. The Wi-Fi equivalent of actuarial tables says that the older one is faster. And it is under absolutely perfect conditions. Because the quoted numbers for the Apple document are the maximums for those MCSes. When’s the last time you got the maximum amount of throughput on a wired link? Now remember that in this case you’re going to need to have perfect airtime conditions to get there. Which usually means you’ve got to be right up against the AP or within a very short distance of it. And that 80MHz channel bandwidth? As my friend Sam Clements says, that’s like drag racing a school bus.
The World Isn’t Made Out Of Paper
If you are just taking the numbers off of a table and reproducing them and claiming one is better than the other then you’re probably the kind of person that makes buying decisions for your car based on what the highest number on the speedometer says. Why take into account other factors like cargo capacity, passenger size, or even convertible capability? The numbers on this one go higher!
In fact, when you unpack the numbers here as I did, you’ll see that the apparent 100 Mbps difference between the two radios isn’t likely to come into play at all in the real world. As soon as you move more than 15 feet away from the AP or put a wall between the client device and your AP you will see a reduction in the data rate. The top end of these two protocols are running in the 5GHz spectrum, which isn’t as forgiving with walls as 2.4GHz is. Moreover, if there are other interfering sources in your environment you’re not going to get nearly the amount of throughput you’d like.
What about that difference in spatial streams? I wondered about that for the longest time. Why would you purposely put fewer spatial streams in a client device when you know that you could max it out? The answer is that even with that many spatial streams reality is a very different beast. Devin Akin wrote a post about why throughput numbers aren’t always the same as the tables. In that post he mentioned that a typical client mix in a network is 2018 is about 66% devices with 1SS, 33% devices with 2SS, and less than 1% of devices have 3SS. While those numbers have probably changed in 2021 thanks to the iPhone and iPad now having 2SS radios, I don’t think the 3SS numbers have moved much. The only devices that have 3SS are laptops and other bigger units. It’s harder for a unit to keep the data rates from a 3SS radio so most devices only include support for two of them.
The other thing to notice here is that the value of what a spatial stream brings you is different between the two protocols. In 802.11ac, the max data rate for a single spatial stream is about 433 Mbps. For 802.11ax it’s 600 Mbps. So a 2SS 11ac radio maxes out at 866 Mbps while a 3SS 11ax radio setup would get you around 1800 Mbps. It’s far more likely that you’ll be using the 2SS 11ax radio more efficiently more often than you’ll see the maximum throughput of a 3SS 11ac radio.
This whole tale is a cautionary example of why you need to do your own research, even if you aren’t a Wi-Fi pro. The headline was both technically correct and wildly inaccurate. Yes, the numbers were different. Yes, the numbers favored the older model. No one is going to see the maximum throughput under most normal conditions. Yes, having support for Wi-Fi 6 in the new MacBook Pro is a better choice overall. You’re not going to miss that 100 Mbps of throughput in your daily life. Instead you’re going to enjoy a better protocol with more responsiveness in the bands you use on a regular basis. You’re still faster than the gigabit Ethernet adapters so enjoy the future of Wi-Fi. And don’t believe the numbers on paper.