Data Pathways – Why The Whole Chain Matters

There are typically a number of steps involved in getting data from one place to the other.  Whether burning a CD, downloading a memory card, or copying files from one hard drive to another, it’s never quite as simple as “my drive is fast, so the copy will be fast.”

This could become an extraordinarily long and technical discussion.  I’m going to do my best to communicate the concept simply through metaphor and examples.  The super-geek stuff will come below the jump, so you’re welcome to ignore it if you like.

Why a “Chain”?

A chain is a simple way to illustrate the concept.  Imagine if you will, a short length of chain rated to hold 20 tons.  Each link in the chain must individually be rated to 20 tons.  If we change just a single link to one with a 5 ton rating, then the entire chain is downgraded to 5 tons.  Likewise if we upgrade just one link in the 20 ton chain to 100 tons, overall the chain is still only capable of 20.  Upgrading a single link does not upgrade the entire chain.  Downgrading a single link downgrades the entire chain.  The same is true for the data pathway from point A to point B.

The Data Chain

Consider this (simplified) list of steps an image file takes when going from your CF card to your hard drive.  Every step represents a link in the chain, and  impacts the time it takes the file to copy.

The CF card itself → USB cable → USB hub → USB cable → USB port on the computer → (ABSTRACT*) → HDD (hard disk drive) interface  → HDD cable →HDD.

Lots of steps, yes?  Just like in the example of the 20 ton chain with a single 5 ton link, the slowest single step in the chain determines how long the file takes to copy.

Some Examples

Vivian has a computer that’s a few years old.  It has USB2 ports, an internal HDD that was top-of-the-line at the time of purchase, and she recently bought some new high-performance 600x CF cards for her cameras.  Vivian’s good friend tells her about some fancy new USB3** card readers, and how amazingly fast they are.  Viv loves the idea of getting her cards downloaded faster, so she buys herself a couple.  Has she improved her situation?  Unfortunately no.  Vivian’s fancy new card readers will work in that they will dutifully transfer data***, but the download time will remain exactly the same as it was on her USB2 card readers.  She needs USB3 along every USB step.  Laptops typically have limited upgrade-ability.  Viv may not have many options in this case short of buying a new laptop with USB3, though some laptops support minor upgrades through express ports.  If you’re unsure, contact your neighborhood nerd for help.

Sebastian has a desktop computer that he bought last year.  It’s pretty fast and he’s happy with it, but it didn’t come with USB3 support.  He’s got fast memory cards, a USB3 card reader, and his internal hard drives are all lightning quick.  Sebastian wants to be able to download multiple CF cards at once, and have the downloads run as fast as the card is capable.  All he needs to do is buy a USB3 interface card, pop the lid off his case, and install it.  This isn’t nearly so hard as it sounds, but if you’re unsure consult your neighborhood nerd.

Josephine just unboxed her new pride-and-joy: a 32 core super-computer with 128GB of RAM, four SSDs in striped RAID, and three 30″ displays for an obscene amount of desktop space.  Naturally she has USB3 ports, cables, and card readers to match.  Josephine made this purchase because she upgraded to a new 64 megapixel camera, and she expects the files to be large.  Josephine shoots digital, but she’s very thoughtful and only presses the shutter button when she knows it’s going to be an amazing photo.  Because of this, Josephine has never felt the need to upgrade her aging CF cards, which frankly, are pretty slow.  Josephine’s computer will process those files crazy-fast once they’re inside the computer, but because the CF cards are old and slow it’s unlikely her download times are going to improve over her previous system.


If I’ve written this well, then by now you get the point.  The top speed is only as fast as the slowest step along the way.  Don’t make the mistake of running out and buying USB3 card readers if you only have USB2 in your computer and cannot upgrade.

This article is likely to live on the internet for quite a long time.  Today, in 2012, USB3 and Thunderbolt are both considered very fast.  If you’re reading this in 2016 they’re likely average.  If you’re reading this in 2020, they’re probably pretty slow.  Certainly you’re on USB4, or Thunderbolt2, or something altogether newer.  There was a time when USB2 was amazing.  While the specific names and specs will change, the concepts remain sound.

*There are many steps inside the core of the computer that most users have no control over, and can only be upgraded through the purchase of a new machine.  I’ll gloss over them in the interest of simplicity.

**USB3, USB3.0, Super Speed USB – these are all the same spec / version of USB.

***The beauty of the USB spec is that new devices are always backward compatible with older devices.  A USB3 card reader will transfer data when plugged in to a USB2 port, but it will slow down to USB2.  A USB2 card reader will work on a USB1 port, but it will slow down to USB1 speeds.

Getting Nerdy

If you’re hungering for more examples and more complicated setups, roll up your sleeves and hit the “continue reading” button below the sharing icons.

Everything has limits.  USB3 is wicked fast (props to my East Coast visitors) and Thunderbolt is wicked faster, but even they max out.  I avoided numbers and tried to keep things conceptual above, but it’s time to bring in the maths.

Most current implementations of USB3 are rated to 5Gb/s.  Currently Thunderbolt is rated to 10Gb/s.  Most people with some computer literacy are accustomed to thinking of things in terms of MB/s or GB/s, so let’s convert.

Most current implementations of USB3 are rated to 625MB/s.  Currently Thunderbolt is rated to 1,250MB/s.  Is Thunderbolt better?  Maybe.  Bring in the COST BENEFIT ANALYSIS (say it like you’re announcing The Muppets’ “Pigs In Space” and it will be more entertaining.)

Thunderbolt has two drawbacks.  It’s available in fewer computers, and it’s more expensive to implement which leads to more expensive accessories.  This will change with time, but as of today these are the facts.  Even if a Thunderbolt CF card reader were available, it would likely be quite pricey.  And to what end?

What End Indeed…

Consider.  At a max speed of 625MB/s, you have enough bandwidth to download six 90MB/s CF (or ten 60MB/s) cards at once and still have a little headroom.  If you’re doing seven or more cards simultaneously Thunderbolt may interest you, but you still have a speed problem: your target HDD.  The highest performing mechanical HDDs right now are just barely hitting top speeds of 120 – 150 MB/s.  With a limitation like that, even two 90MB/s cards are already maxing out your chain.  Two 60MB/s cards are just about the right fit.

“I have an SSD,” you exclaim.  Awesome, I love SSDs.  If you have a really good one sitting on a SATA6 interface, you’re probably getting about 520MB/s write speeds, which is roughly equal to the 90MB/s x 6 = 540MB/s that our theoretical six-card-transfer needs, but it’s not more.  Thunderbolt still doesn’t solve any problems.  There may be lots of reasons to love Thunderbolt, but it does not dodge this particular issue.

What About USB2?

USB 2.0 is rated to 480 Mb/s (or 60 MB/s.)  If you’re calculating how many old CF cards you can download at once and still realize a speed benefit, use 35MB/s instead of the stated 60MB/s max.  The reasons have to do with the USB communication protocol, and the impact of this signaling on the real-world transfer rate.  This is why even under ideal circumstances, you’ll never see an external USB2 HDD perform better than 40MB/s, and typically closer to 30MB/s.  Protocol overhead impacts every interface [WikiPedia] not just USB2.  I’m intentionally going to avoid getting any further in to this can of worms.

Many older CF cards and some modern ones use the X (“ex,” or “by”) rating to indicate speed.  What does the x speed mean? [article]  Multiply the x number by 150KB/s to get a usable performance number.

FireWire, FireWire800, eSATA, in brief

FireWire is rated to 400Mb/s (50MB/s.)  In spite of its apparent shortfall when compared to USB2, in practice it is actually faster.  This is due to better efficiency in the protocol.  External FireWire HDDs regularly see performance in excess of 40MB/s.  FireWire800 (aka FW800) doubles the speed to 800Mb/s (100MB/s.)  eSATA is simply an externally exposed SATA port – the performance metrics remain the same regardless of internal or external.  There are three versions of SATA in existence.  The first version, simply “SATA” was rated to 1.5Gb/s (187MB/s.)  This version is still present in many usable computers, but is no longer being sold.  SATA2 doubled the speed to 3Gb/s (375 MB/s) and the most recent SATA6 doubled it to 6Gb/s (750MB/s.)  Notice what they did with the name there?  The 2 in SATA2 means “version 2” but the 6 in “SATA6” is for 6Gb/s.  Some companies do call it SATA3 for “version 3”- they are the same.


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