The Universal Serial Bus (USB) protocol has undergone radical enhancement in the field of communication, given the different dimensions of utilization for reliable and efficient transfer of data/information. The current direction is pointed towards Enhanced SuperSpeed USB Protocol that defines a new SuperSpeedPlus of 10Gbps alongside backward compatibility to older versions. While the previous attributes have been retained, additional enhancements have paved the way for advantages in terms of reliability and efficiency. This white paper gives a straight forward and broader idea of features, goals and advantages encompassing the USB Enhanced SuperSpeed Protocol and its scope in today’s market catering to the needs of customers.
Universal Serial Bus (USB) is one of the most familiar and routinely used digital communication protocols used in current world scenarios to connect PCs to peripherals right from customary mobile devices to “out-of-ordinary” industrial automations. A paradigm shift in improvisation of the protocol since decades is necessarily achieved, considering the requirements of vast areas of applications of globalization. USB’s most recent version is Enhanced SuperSpeed Gen X USB that includes the newer signaling rate of USB 3.1(SuperSpeedPlus / Gen 2 – 10Gbps) with earlier versions of USB 3.0 (SuperSpeed / Gen 1 – 5 Gbps) and USB 2.0 supporting low (1.5Mbps), full (12Mbps) and high speed (480Mbps) modes; adhering to the standards of USB-IF (Implementers’ Forum). Following sections describe in detail about the features, comparison and merits of Enhanced SuperSpeed in general and USB 3.1 in specific over previous versions –
The following points not only depict the features of Enhanced SuperSpeed but also provide information on why the features are advantageous with respect to earlier versions of USB.
- Host Directed and Scheduled protocol – This refers to the data transfer being initiated always by the host with respect to the peripheral devices and is further controlled by the host based on the response from devices. This is advantageous over previous versions in switching between multiple endpoints of devices asynchronously, thus reducing latency in waiting for an endpoint temporally incapable of data transfer. Support for dynamic attach, configure, detach and use is also available when host and peripheral devices are in operational modes.
- Dual Bus Architecture – This refers to parallel buses of USB 3.1/3.0 which are concurrently active. This contributes towards increasing efficiency of bus utilization with both buses operating independently.
- Star Tiered Topology – This refers to a hierarchical topology where the host is at Tier 1 or upstream and more than one device at lower tiers or downstream. This feature allows concurrent communication between host and multiple devices, thus achieving parallelism.
- Transfer Modes – Four modes of data transfer such as Bulk, Isochronous, Control, and Interrupt are supported. While Bulk supports for mass storage, Isochronous for continuous data such as audio/video, interrupt for intermittent short interval communication such as mouse clicks, keyboard strokes; control transfers support for control mechanisms. Bulk Streaming is an added feature that supports many logical connections to a single endpoint. Every device may have individual modes or a combination of any of the modes with respect to different endpoints, thus supporting robustness.
- Dual Simplex Data Interface – This feature is implemented to support bi-directional data flow. This is primarily advantageous as transmit and receive paths are isolated and consequently higher throughput is achievable. This supports Split Transactions wherein more than one IN/OUT transaction can be active on the bus simultaneously.
- Asynchronous Transfer – This feature allows multiple endpoints or devices to asynchronously request service from the host. Consequently, the host may initiate data transfer based on host scheduling to specific endpoints of devices that may be ready. This enables host to switch between endpoints or devices thus increasing efficiency.
- Efficient Power Management – Power Management is achieved through defining low power states U1 to U3, to which transitions are made based on links that do not involve data communication. Power Management is also applicable to different functions of composite devices. Links that are not active on data communication are placed in low power states and can be driven out with indications of transition events. Additionally, Support Smart Isochronous is a new feature for USB 3.1 to support Smart Isochronous Scheduling functionality by host controller. This information allows device to drive its link to low power states in between the times when host is polling the isochronous endpoint in its service interval.
- Precision Time Measurement – This feature characterizes link delays and propagation delays through hubs through Link Delay and Hub Delay Measurement mechanisms. This is used to improve device bus boundary interval accuracy. Both the host and hub shall implement PTM.
- Signal Integrity through differential drivers and shielding where isolation of transmission & reception paths helps avoid cross-talk and interference between signals.
- Error Detection and Handling – With BER being 1 in 1012 bits, CRC protection provides guard against multiple bit errors. Error recovery is invoked by hardware to support data integrity. Line encoding variation from 8b/10b of USB 3.0 to 128b/132b reduces overheads to just 3% which is quite substantial.
- Reliable Data Delivery – Packets are retried end-to-end thus avoiding any loss of data contributing to reliable data transfer across host and devices. Furthermore, data and control pipes ascribe to isolation of interactions between data and control information.
- Hubs – This features a logic combination of USB 3.1/3.0 and USB 2.0 hubs. Hubs support attach/removal, remote wake–up, power distribution among devices as well as hardware suspend/resume signaling. The store-forward feature of hubs allows to receive packets before the hubs can transfer them i.e. allows for outstanding packets. This helps to route packets and preserve their ordering both along upstream and downstream.
- Peripheral Devices – This features SuperSpeed Only or Enhanced SuperSpeed. While the former supports Gen 1 PHY mode only, the latter provides support for Gen X.
- Host – This features attach/removal detection, control/data flow, providing power to devices etc.
WHY “GRAVITATE” TOWARDS ENHANCED SUPERSPEED?
The following points depict the major differences between Enhanced SuperSpeed and USB 2.0 and related reasons to shift towards Enhanced SuperSpeed protocol.
- Tokens – In contrast to USB 2.0, tokens are included in data packet headers of Enhanced SuperSpeed USB OUT transactions and IN transactions use handshake signals to provide information on readiness of endpoint for data transfer.
- Continuous bursting – While USB 2.0 implements data polling, Enhanced SuperSpeed supports data bursting. This improves efficiency as wait time for acknowledgment of each packet is eliminated. While an IN transaction burst size can be limited by a value, an OUT transaction burst size can easily be controlled by the host.
- Dual Simplex Uni-casting – Enhanced SuperSpeed uni-casts packets directed towards a specific path between host and targeted device with the help of routing information. This also improves reliability and integrity of data transfer. Uni-casting alongside asynchronous notifications cause inactive links to remain in low power states, thus enabling power management. This plays an important role in isochronous transfers where links go to low power states between/ within service intervals and brought back to activity through PING signals. Since dual simplex essentially provides separate buses for transmit & receive paths, simultaneous IN/OUT transactions are achievable.
- Latency Tolerance Messaging in low power states – This enables the host to have information of how much latency the devices can tolerate when host enters low power states.
- Relaxed Isochronous Timestamp Intervals – As opposed to transmission of fixed uSOF/SOF with very tight duration and jitter specifications of USB 2.0, Enhanced SuperSpeed protocol transmits ITPs with relaxed window from bus interval boundary.
- Power Management – Power Management in Enhanced SuperSpeed is much more effective than the previous versions with the utilization of inactivity timers. A deferred response from the device hub to the host shall enable the host to schedule transfers to other active links thus saving power.
Thus, the above points give ample appreciation to transit towards USB Enhanced SuperSpeed Protocol.
OTHER DIMENSIONS OF MERITS
Additionally, some fields of consideration in the viewpoint of efficiency to upgrade technologies to support Enhanced SuperSpeed protocol are data rate, latency and power distribution.
Efficiency in terms of data rate is supposedly higher in Gen 2 compared to Gen 1. While USB 3.0 gives a raw throughput of 500MBps reduced to 450MBps with overheads, USB 3.1 promises around 1.1 to 1.2 GBps with overheads. This poses one of the strong reasons to upgrade existing versions to SuperSpeedPlus. Significantly, latency is reduced with up-gradation of versions from milliseconds to nano-seconds. Power Distribution, being based on low power states provides aggressive power management.
The following tables give a broader idea of comparison between different versions of USB based on various factors. It is to be noted that newer versions are backward compatible and older versions are not forward compatible.
||500 – 450 MBps
||1.2 – 1.1 GBps
Table 1: Values for Various USB versions
Some of the challenges that Enhanced SuperSpeed Protocol may face are listed as follows –
- Host Processing Power – With increase in bandwidth for USB 3.1, host processing power needs to be scaled up for incoming data. Consequently, older systems may slow down when using devices such as USB 3.1 cameras
- Cable Length – Irrespective of specified cable length, around 5m or less of copper appears to be reliable. However, longer lengths may be under consideration to achieve reliability.
- Image / Communication Issues – Devices such as cameras that are fast enough to process data in comparison to USB 3.1 are still a challenge failing which throughput will be reduced.
USB Enhanced SuperSpeed Protocol has paved a way towards achieving much more data rate, efficiency, robustness, reliability compared to earlier versions. Owing to cater to the needs of present day technology that is continuously evolving in short span of time, applications based on the protocol are finding a place in the hot trends of Gen-Next industrial and business realms. The enhancement with respect to USB 2.0 or USB 3.0 only not only tackles issues based on requirements, but also poses new challenges to IP design and verification. However, Enhanced SuperSpeed protocol promises to bring a considerably substantial advancement in the field of digital communication, which in today’s market settles down as a trend-setter for both intuitive research domains and business-oriented environment.