USB 3.0 has been highly anticipated — and now, it’s finally here. Sort of. Find out about its availability, specifications, benefits, and limitations.
USB 3.0 is here! After long delays and much touted promotion of the new specification, USB 3.0 is now finally available or soon will be on some new ASUS and Gigabyte motherboards. ASUS has also announced an add-in PCIe x4 card with USB 3.0 support, though it is compatible only with its P55 series of motherboards after a BIOS upgrade. Dane-Elec has announced a family of external SuperSpeed SSD drives and add-in card, but you will have to pay a hefty premium for the extra performance.
There is some bad news, though: Intel has announced that it will not include USB 3.0 in its chipsets until 2011. AMD may not support USB 3.0 until 2011 either. That means that USB 3.0 is not likely to go mainstream until then.
Current versions of Windows do not support USB 3.0, but support is expected for Windows Vista and Windows 7 at a later date via an update or service pack. The Linux kernel supports USB 3.0 as of version 2.6.31.
Now that USB has finally arrived, albeit barely, this is a good time to compare the previous USB specifications with USB 3.0.
Note: I have taken great care to verify the accuracy of this information, but USB is complex. If you find an error in the documentation or have additional information, please post it in the forum. The examples in this document all use Windows 7.
This article is also available as a PDF download.
1: USB release dates
USB (universal serial bus) was developed as an alternative to serial and parallel data transfer protocols. USB 1.0 was introduced in January 1996. As you can see in Table A, it has been a long time since the USB 2.0 specification was released.
2: Changes in USB 3.0
USB 3.0 is one of the most anticipated changes to the PC in years. Here is a summary of the major changes:
- SuperSpeed — New higher signaling rate of 5Gbps (625MB/sec)
- Dual-bus architecture — Low-Speed, Full-Speed, and High-Speed bus plus SuperSpeed bus
- Asynchronous instead of polled traffic flow
- Dual-simplex simultaneous bi-directional data flow for SuperSpeed instead of half-duplex unidirectional data flow
- Support for streaming
- Fast Sync –N-Go technology
- Support for higher power
- Better power management
3: The Low-Speed, Full-Speed, High-Speed and SuperSpeed confusion
There are four distinct data rates - not to be confused with the four USB specifications. Each new major USB specification introduced a new data rate. Table B shows USB data rate types supported by the four USB specifications. Each new USB specification has been backward compatible.
Table C shows maximum data rates for the four data rate types.
USB 2.0 does not always mean High-Speed. This is usually, but not always, the case. A device labeled USB 2.0 can operate at Full-Speed instead of High-Speed.
Will this confusing labeling exist for USB 3.0? The USB 3.0 specification supports the three legacy speeds in addition to SuperSpeed. This is accomplished by referencing, not replacing, the USB 2.0 specification. Low-Speed, Full-Speed, and High-Speed devices are USB 2.0 compliant but not USB 3.0 compliant, so a USB High-Speed device should not be labeled as a USB 3.0 device. The USB Implementers Forum (USB-IF) has developed logos for each of the four data rates. Look for these logos when buying a USB device.
You can determine whether your USB 2.0 device is a High-Speed device in the Windows Device Manager (Figure A), although it is not a straightforward exercise. You will probably have to try more than one USB Root Hub before you find the device you are looking for.
Open the Device Manager and expand the Universal Serial Bus controllers item. Open the Properties window for a USB Root Hub. Tip: Start at the bottom USB Root Hub.
Next, click the Power tab (Figure B). If the device is attached to this hub it will appear in the Attached Devices section. In this example, I have attached a flash drive and it is displayed as a USB Mass Storage Device. Note that this Root Hub has six ports available — one of them used by the USB Mass Storage Device.
Finally, click the Advanced tab to see the USB speed (Figure C). On my system, the top six USB Root Hubs operate at Low-Speed and Full-Speed and the bottom two each operate at High-Speed.
4: Actual data throughput
Actual data throughput is usually much less than the maximum advertised USB specification and is a function of many variables, including overhead. Actual throughput in practice is typically up to 35 - 40MB/sec for USB 2.0 and may exceed 400MB/sec for USB 3.0. NEC recently demonstrated its new USB 3.0 controller transferring 500MB in 4.4 seconds or “only” 113.6MB/sec. Symwave and MCCI claim to have demonstrated over 270MB/sec data throughput at the Intel Developer Forum in September 2009.
Bottom line: Don’t expect actual SuperSpeed data rates approaching 400MB/sec anytime soon.
I have a USB flash drive that can read at 26MB/sec and write at 6.6MB/sec and is typical of flash drives available as of late 2009. These data rates are within the actual High-Speed data rate. But Faster USB 3.0 flash drives are on the way that can take advantage of the SuperSpeed data rate.
Most hard disk drives can transfer data faster than 40MB/sec. USB 3.0 will be welcomed by those who like to back up data to an external hard drive or SSD drive or who have any USB device that transfers large amounts of data.
5: Cabling and maximum cable length
During my days at Hughes Aircraft Company, I was always looking for ways to save money. I suggested that my supervisor, who sat in the next cube, share a laser printer with me. But printing over the long parallel cable caused characters to be intermittently printed as the gibberish that is so familiar when data loss or corruption occurs. USB cables have a similar constraint. But unlike my parallel cable problem, there is a solution.
Table D shows the maximum cable and total lengths.
*The USB 3.0 spec does not detail a maximum cable length, but 3.0 meters or 9.8 feet has been recommended.
A total of six cables can be strung together using five hubs to achieve the maximum total length. In practice, the cable to the USB device counts as one of the six cables, reducing the maximum total length.
If the USB 2.0 five-meter limit is not long enough for your needs, you can purchase one or more USB hubs or special cables. There are two types of hubs: powered and unpowered. Higher power draw devices may require a powered hub.
Longer total lengths can be realized using repeater extension cables and CAT5 extenders for USB 1.0, 1.1, and 2.0. There is also a special class of USB 3.0 cables that contain circuitry to achieve a length of six meters (19.7 ft). The USB-IF Web site recommends a USB bridge to achieve lengths greater than 30 meters.
The USB 2.0 specification for a Full-Speed/High-Speed cable calls for four wires, two for data and two for power, and a braided outer shield.
The USB 3.0 specification calls for a total of 10 wires plus a braided outer shield. Two wires are used for power. A single unshielded twisted pair (UTP) is used for High-Speed and lower data transfer and allows for backward compatibility.
Two shielded differential pairs (SDPs) have been added. Each SDP contains three wires, two for signal transmission and one drain wire. The two SDPs are used for transferring SuperSpeed data allowing for simultaneous bi-directional data flow.
See the Author’s Notes section at the end of the article for a reference to a USB 3.0 cable cross-section diagram.
One of the most significant innovations in USB over serial and parallel protocols is the addition of power to the specification. Plug in a USB device and it can be powered from the host computer.
To find the power requirements for USB devices open the Device Manager, expand the Universal Serial Bus controllers item, Right-click on Generic USB Hub as in this example or USB Root Hub (Figure B), select Properties and click the Power tab, as shown in Figure D.
More power has been added in the USB 3.0 specification for power hungry devices. Table E shows the maximum amperage per port in milliamps.
There are four basic power states to accommodate a variety of devices and device states. For information about USB hubs and power, read Greg Shultz’s article Understand and exploit USB topology in Windows XP.
Note: The USB 3.0 specification details more power states, including idle and sleep.
We’ve already discussed some of the USB limitations:
- Maximum data rates
- Actual data throughput
- Cable length and total length
There are several other limitations that you should know about.
Though you will likely never find it an issue, there is a 127 device limitation per controller.
Each USB 2.0 Enhanced Host Controller Interface (EHCI) host controller has a 60MB/sec total bandwidth limitation, and the bandwidth is shared by all attached High-Speed USB devices. If, for example, two High-Speed devices like a digital video camera and an external hard drive are in use at the same time, the last High-Speed device attached may operate at a lower data rate or a USB Controller Bandwidth Exceeded error may occur. If you have two EHCI host controllers on your system, you may be able to resolve the bandwidth error by moving one of the High-Speed devices to another USB port. Wikipedia has a list of I/O Controller Hubs with two or more EHCI host controllers.
Want to know how much bandwidth has been allocated for each USB device in Windows? According to this MSDN article, you can check Device Manager, if you use Vista or later:
“Starting with Windows Vista, users can see how much bandwidth a USB controller has allocated by checking the controller’s properties in the Device Manager. Select the controller’s properties then look under the Advanced tab. This reading does not indicate how much bandwidth USB hubs have allocated for transaction translation.
“The Device Manager feature that reports the bandwidth usage of a USB controller does not work properly in Windows XP.”
Figure E shows that three USB devices have been allocated 4% of the bandwidth available for this Universal Host Controller. The Fujifilm FinePix S700 digital camera is a USB Full-Speed device and is therefore listed under one of the Universal Host Controllers and not one of the Enhanced Host Controllers. The USB specification defines four data transfer types: Control, Interrupt, Isochronous, and Bulk. The 10% System reserved value shown here is used for Control and Bulk data transfers and cannot be changed.
During system boot-up and when a USB device is plugged in, a process called enumeration occurs. The device is recognized, its speed is identified, and a unique address is assigned. For devices using the Interrupt or Isochronous data transfer types, a specific amount of the remaining available bandwidth is requested, thus guaranteeing that the bandwidth will be available. If the bandwidth is available, it’s allocated, and the device description and reserved bandwidth will be listed on the Advanced tab.
Note: Don’t bother looking for the bandwidth used by a Mass Storage device like a flash drive. This class of USB device typically uses the Bulk data transfer type and is not listed on the Advanced tab.
In addition to any of the System Reserved bandwidth that may be available, devices using the Bulk data transfer mode may use the remaining non-reserved bandwidth. The Bandwidth Used column heading is misleading. The bandwidth is allocated/reserved but may not actually be used.
As you can see in Figure F, ICH9R Southbridge supports six Universal Host Controller Interface (UHCI) host controllers and two Enhanced Host Controller Interface (EHCI) host controllers. The number of UHCI and EHCI host controllers may be different on your system. The ICH9R supports a total of 12 USB ports. The six Universal Host Controllers operate at Low-Speed and Full-Speed and each shares its bandwidth with two USB ports. The two USB2 Enhanced Host Controllers operate at High-Speed and each shares its bandwidth with six USB ports. The Advanced tab shows that 20% of the bandwidth is reserved by each Enhanced Host Controller for Control and Bulk data transfers.
Note: There is another host controller type, not shown, called USB Open Host Controller Interface (OHCI) that supports Low-Speed and Full-Speed devices. The name of the new Intel SuperSpeed host controller specification is Extensible Host Controller Interface (xHCI).
8: Connector and receptacle types
There are a number of USB 3.0 connector and receptacle types:
- Standard-A connector and receptacle
- Standard-B connector and receptacle
- Powered-B connector and receptacle (new in USB 3.0)
- Micro-AB receptacle
- Micro-A connector
- Micro-B connector and receptacle
The matrix in Table F shows the types of USB 2.0 and USB 3.0 connectors that will work with USB 2.0 and USB 3.0 receptacles. Note that according to the USB 3.0 specification Table 5.1, the only USB 3.0 connector that will work in a USB 2.0 receptacle is the Standard-A connector.
A new multi-tiered system has been developed for the extra pins needed for USB 3.0. The Standard-A connector is slightly longer and the receptacle slightly deeper to accommodate the new design. Five pins have been added to the Standard-A connector and receptacle specifically for SuperSpeed transmit and receive data and ground.
The USB 3.0 specification recommends using a blue color scheme for USB 3.0 Standard-A connectors and receptacles to distinguish them from USB 2.0 Standard-A connectors and receptacles.
The USB 3.0 specification includes a new type of connector and receptacle called a USB 3.0 Powered-B Connector and USB 3.0 Powered-B Receptacle. They are identical to the USB 3.0 Standard-B Connector and receptacle, except that two pins have been added for power and ground. It is designed to provide power to a USB device without the need for any other power source. The USB 3.0 Powered-B Receptacle can accept both Standard-B and Powered-B connectors.
The Micro family of connectors and receptacles are defined for handheld devices. Unlike the Standard-A connectors with their elegant design, the Micro connectors and receptacles have a more complex design with two plugs and receptacles sitting side by side — one for USB 2.0 and the other for USB 3.0.
See the Author’s Notes section for references to diagrams for the USB 3.0 Standard-A Connector, the USB 3.0 Standard-B Connector, and the USB 3.0 Micro Connector Family.
9: Hot-swappable devices and data corruption
I can’t write an article about USB without bringing up the issue of data corruption. Removing any USB device capable of writing data can cause data corruption if done improperly. There are three ways to minimize the risk of data corruption:
- Verify write-back caching is off
- Pay attention to device LEDs
- Safely remove/eject device
First, verify that write-back caching is turned off for the USB device. To verify write caching status, open Device Manager and right-click on the USB device. Select Properties from the drop-down list (Figure G). In this example, I am checking a SanDisk Cruzer flash drive.
Next, click the Policies tab (Figure H). The Quick Removal (Default) option should be selected. If not, select it to reduce the risk of data corruption.
Second, pay attention to device LEDs. Some USB devices will tell you when data is being transferred to or from the device with a flashing LED. Simply put, don’t remove the USB device when the LED is trying to tell you not to.
Third, safely remove/eject device. No doubt you already know how to safely remove a USB device but I am including it to be thorough. To safely remove a USB device in Windows 7, click the Taskbar Notification area Up-arrow and click on the USB icon (Figure I).
Click the USB device you want to eject — Cruzer Micro, in this example (Figure J).
The Safe To Remove Hardware notification balloon will appear when it is safe to remove your flash drive (Figure K).
There is an alternate method for ejecting a USB device that you might not be familiar with. To safely remove a flash drive using Explorer, right-click on the logical drive assigned to the flash drive and select Eject from the drop-down list (Figure L). You can eject attached drives in Explorer, but be aware that more than one drive may need to be ejected.
10: USB downsides
USB can cause problems that can be difficult to debug. For example, on one occasion I was unable to install XP until I disconnected the USB to parallel cable attached to my printer.
USB is so convenient and easy to use, it poses problems in the workplace. Flash drives are the biggest concern to IT managers. Flash drives are so small that they are easy to bring into the workplace in a pocket or purse. The flash drive is a conduit for sensitive or confidential data leaving the office or malware sneaking in.
In addition, people who are conscious of the risks of transferring viruses via a floppy, CD, or DVD don’t think twice about plugging in a flash drive and transferring files to/from home. Perhaps the best solution to this problem is education. Flash drives are banned in some government agencies and companies, though the effectiveness of that policy is questionable. Interestingly, the DOD is partially lifting its flash drive ban.
The final word
USB has been such a huge success that a more than 10 times improvement in speed and an 80% increase in power is almost certainly guaranteed to be just as successful, right? Well, maybe not. Intel’s original conceptual designs for the USB 3.0 cable specified optical fiber cabling to carry the SuperSpeed data. Copper replaced fiber optics in the final USB 3.0 spec, but Intel continues to work on a variation of this design known as Light Peak. It may be available as early as 2010 in Apple products before Intel plans to support USB 3.0 in its chipsets.
Light Peak promises double the data rate of USB 3.0 now, with speeds possibly reaching 20 times the USB 3.0 speeds as the new technology matures. Perhaps more important in the short term, Light Peak cables may reach 100 meters (328 feet) in length and may be smaller in diameter and lighter.
Could Light Peak make its way to the Wintel platform? It certainly could, and its data transfer capability would leapfrog it past USB 3.0. So don’t bet just yet that USB 3.0 will be as successful as its predecessors. Regardless of what happens with Light Peak, USB SuperSpeed should satisfy USB device data rate requirements for many years to come.
Resources used in writing this article
- USB Implementers Forum
- Universal Serial Bus Revision 2.0 Specification (Zip, 10.2MB)
- Universal Serial Bus Revision 3.0 Specification (Zip, 3.8MB)
- Wikipedia - Universal Serial Bus
- Everything USB
- Windows USB FAQ: Introductory Level
- Beyond Logic - USB in a Nutshell - Making sense of the USB standard (PDF, 175KB)
I would like to thank Microsoft for its invaluable help.
In item 5, Cabling and maximum cable length, you will also need a USB switch and the appropriate USB cables if you are sharing a USB printer with two computers.
I wanted to include some of the USB-IF diagrams that showcased its new cable and connector designs. I also wanted to include its USB logos to show you what to look for when buying a High-Speed or SuperSpeed USB device. I was denied the use of all USB-IF diagrams. Here is a list of the images so you can look them up yourself if you so choose. They do a good job of detailing the changes you will see in USB 3.0 cables, connectors, and receptacles.
Items 5 and 8 — Cabling and maximum cable length and Connector and receptacle types:
From SuperSpeed_USB_DevCon_Physical_Heck.pdf (PDF, 2.33MB):
Page 11 — USB 3.0 Cable Assembly (Cable cross-section diagram)
Page 11 — USB 3.0 Cable Assembly (Type-A connector pin diagram)
Item 8 — Connector and receptacle types:
From SuperSpeed_USB_DevCon_Architecture_Overview_Dunstan.pdf (PDF, 3.15MB):
Page 9 — USB 3.0 Standard-A Connector
Page 10 — USB 3.0 Standard-B Connector
Page 11 — USB 3.0 Micro Connector Family