To USB or Not to USB


Latency, WiFi, and USB 3.0

In a WiFi group I visit, I received a response that said something like “I’ve reviewed WiFi-AC products with USB 2.0 and it seems that WiFi-AC can’t really take adantage of USB 3.0 speeds”

First, Iappreciated the response.
Second, I thought, “If I am an early adopter, I already use WiFi-N 2×2 and I get maybe 0.300 Gbps.  This is pretty close to the maximum effective throughput of USB 2.0”

“Why would I possibly upgrade to WiFi-AC if it is only USB 2.0 enabled?  I think I would prefer a PCIexpress Add-In card or ExpressCard for my laptop (if my laptop actually has an ExpressCard slot).  That could support up to 2Gbps.  Of course, if I own an ultrathin or ultrabook PC I won’t have an ExpressCard slot, so I still need a USB 3.0 port.”

Then I started think about: “How would I measure throughput in a system using WiFi-AC and USB 2.0 or USB 3.0?”

“What are the sources of latency?”

So you have a PC connected by USB 2.0 or 3.0 to the USB Adapter with a WiFi-AC radio

You have a WiFi-AC router with a USB 3.0 (or 2.0 Host port) connected to a USB 3.0 or 2.0 Hard Drive.

You have the following latencies.

  1. The PC Operating System
  2. The PCIe bus connecting the PC to the USB 3.0 Host (Small in a system with a PC chipset)
  3. The USB  Host itself
  4. The USB Device in the Adapter
  5. The WiFi-AC Baseband/System
  6. Over the air transmission time
  7. The WiFi_AC Router receiving and processing the signal
  8. The bridge/interface (maybe PCIe or AXI)  to the USB Host
  9. THe USB Host in the router
  10. The USB Device in the USB Hard Drive/Flash Drive/SSD
  11. The SATA interface into the USB Hard Drive/Flash Drive/SSD (USB Storage)
  12. The speed of the USB Storage itself (Use a cheap flash drive and you will never get even USB 2.0 speeds)

So all these pieces could drag down the throughput regardless of the USB 2.0 or USB 3.0 drive attached.

If you used a Fastest USB 3.0 Flash Drive in the Universe that we demonstrated here, the Lexar Triton, which uses fast flash memory, then it will definitely operate at USB 3.0 speeds and will NOT slow down the system at all.

Use a cheap give-away USB 2.0 Flash Drive and you will be limited the read speeds of 0.017 Gbps and will never get anything out of the system.

My point is, I interpret the commenter on in that WiFi forum as follows:

 If all the pieces can’t operate at the necessary speeds, then there is little advantage to adopting WiFi-AC at this point.  Therefore, be sure that you adopt USB 3.0 if you are going to a WiFi-AC project to make sure you maximize throughput.


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USB Christmas Tree

My collegue, Norma,  kindly sent me this picture of a USB Christmas Tree for this blog entry.

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