This document introduces the features and benefits of USB 2.0 and
describes its impact to users, PC manufacturers and PC peripheral
manufacturers. Following a recap of USB 1.1, this paper overviews
the technical aspects of USB 2.0 whose details are in the
specification draft released in October.
USB 2.0 Executive Summary
A core team from Compaq, Hewlett Packard, Intel, Lucent, Microsoft, NEC and
Philips is leading the development of the USB Specification, version 2.0, that will
increase data throughput by a factor of 40. This backwards-compatible
extension of the USB 1.1 specification uses the same cables, connectors and
software interfaces so the user will see no change in the usage model. They will,
however, benefit from an additional range of higher performance peripherals,
such as video-conferencing cameras, next-generation scanners and printers, and
fast storage devices, with the same ease-of-use features as today’s USB
peripherals.
Impact to User
From a user’s perspective, USB 2.0 is just like USB, but with much higher
bandwidth. It will look the same and behave the same, but with a larger choice of
more interesting, higher performance devices available. Also, all of the USB
peripherals the user has already purchased will work in a USB 2.0-capable
system.
Impact to PC Manufacturer
USB 2.0 will provide system manufacturers the ability to connect to high
performance peripherals in the least expensive way. The additional performance
capabilities of USB 2.0 can be added with little impact to overall system cost.
Indeed, high-bandwidth interfaces such as SCSI adapters may no longer be
required in some systems, leading to a net saving of system cost. Simpler
construction will result since only USB connectors will be needed on many future
PCs. Today’s ubiquitous USB connectors will become USB 2.0, superceding
USB 1.1.
Impact to Peripheral Manufacturer
Today’s USB devices will operate with full compatibility in a USB 2.0 system.
The added capabilities of USB 2.0 will expand the market segment for USB
peripherals, while enabling retail products to transition with the installed base.
Support of USB 2.0 is recommended for hubs and higher bandwidth peripherals.
Designing a USB 2.0 peripheral will be a similar engineering effort to that of
designing a USB 1.1 peripheral. Some low-speed peripherals, such as HID
devices, may never be redesigned to support the USB 2.0 high-speed capability
in order to maintain the absolute lowest cost.
Historical Perspective – Universal Serial Bus
The Universal Serial Bus was originally developed in 1995 by many of the same
industry leading companies currently working on USB 2.0. The major goal of
USB was to define an external expansion bus which makes adding peripherals to
a PC as easy as hooking up a telephone to a wall-jack. The program’s driving
goals were ease-of-use and low cost. These were enabled with an external
expansion architecture, as shown in Figure 1, which highlights:
· PC host controller hardware and software,
· robust connectors and cable assemblies,
· peripheral friendly master-slave protocols,
· expandable through multi-port hubs.
Figure 1. Example USB 1.1 System Configuration
(available in full version)
Today, USB is enjoying tremendous success in the marketplace, with most
peripheral vendors around the globe developing products to this specification.
Virtually all new PCs come with one or more USB ports on the box. In fact, USB
has become a key enabler of the Easy PC Initiative, an industry initiative led by
Intel and Microsoft to make PCs easier to use. This effort sprung from the
recognition that users need simpler, easier to use PCs that don’t sacrifice
connectivity or expandability. USB is one of the key technologies used to provide
this.
Recap of USB 1.1 Operation
An understanding of the roles of each of the major elements within a USB 1.1
system will better show the evolutionary step that USB 2.0 provides.
Role of Host PC hardware and software.
The role of the system software is to provide a uniform view of IO system for all
applications software. It hides hardware implementation details so that
application software is more portable. For the USB IO subsystem in particular, it
manages the dynamic attach and detach of peripherals. This phase, called
enumeration, involves communicating with the peripheral to discover the identity
of a device driver that it should load, if not already loaded. A unique address is
assigned to each peripheral during enumeration to be used for run-time data
transfers. During run-time the host PC initiates transactions to specific
peripherals, and each peripheral accepts it’s transactions and responds
accordingly. Additionally the host PC software incorporates the peripheral into
the system power management scheme and can manage overall system power
without user interaction.
Role of the hub.
Besides the obvious role of providing additional connectivity for USB peripherals,
a hub provides managed power to attached peripherals. It recognizes dynamic
attachment of a peripheral and provides at least 0.5W of power per peripheral
during initialization. Under control of the host PC software, the hub may provide
more device power, up to a maximum of 2.5W, for peripheral operation. A newly
attached hub will be assigned its unique address, and hubs may be cascaded up
to five levels deep. During run-time a hub operates as a bi-directional repeater
and will repeat USB signals as required on upstream (towards the host) and
downstream (towards the device) cables. The hub also monitors these signals
and handles transactions addressed to itself. All other transactions are repeated
to attached devices. A hub supports both 12Mb/s (full-speed) and 1.5Mbs (lowspeed)
peripherals.
Role of the peripheral.
All USB peripherals are slaves that obey a defined protocol. They must react to
request transactions sent from the host PC. The peripheral responds to control
transactions that, for example, request detailed information about the device and
it’s configuration. The peripheral sends and receives data to/from the host using
a standard USB data format. This standardized data movement to/from the PC
host and interpretation by the peripheral gives USB it’s enormous flexibility with
little PC host software changes. USB 1.1 peripherals can operate at 12Mb/s or
1.5Mb/s.
What does USB 2.0 add?
USB 2.0 is an evolution of the USB 1.1 specification, providing a higher
performance interface. Today’s USB 1.1 connectors and full-speed cables will
support the higher speeds of USB 2.0 without any changes. Characterization
that has already been done on these cables confirms this compatibility. Analysis
that has been done by the electrical team suggests that a target of 480Mbs is
achievable on USB 2.0. USB 2.0 will specify a microframe, which will be 1/8th of
a 1msec frame. This will allow USB 2.0 devices to have small buffers even at
high data rates.
Support of higher speed USB 2.0 peripherals connected to a hub assumes USB
2.0 hubs as shown in Figure 2. The higher transmission speed is negotiated on a
device-by-device basis and if the higher speed is not supported by a peripheral,
then the link operates at a lower speed of 12Mb/s or 1.5Mb/s as determined by
the peripheral.
Figure 2. Example Future USB 2.0 System Configuration
(available in full version)
As shown in Figure 2, high-speed connections were negotiated between the root
hub and the external USB 2.0 hub and between the external USB 2.0 hub and
the video-conferencing camera (a USB 2.0 peripheral). All other connections are
at USB 1.1 data rates, i.e. 12Mb/s automatically downshifting to 1.5Mb/s for lowspeed
peripherals. Note that the external USB 2.0 hub has different signaling
rates on its ports. Using a 40x multiplier for USB 2.0, the USB 2.0 hub example
in Figure 2 has an input rate of 480Mb/s and output rates of 480Mb/s for attached
high speed USB 2.0 peripherals, and 12Mb/s or 1.5Mb/s for attached USB 1.1
peripherals. Any downstream port of a USB 2.0 hub can support attachment of
any speed USB device. The USB 2.0 hub must match the data rates sent out of
its downstream ports to the data rate appropriate to the attached device. This
increases the hub’s role in a USB 2.0 system as outlined below.
Overview of USB 2.0 Operation
The external view of a USB 2.0 system looks no different from a USB 1.1 system
as evidenced by comparing Figures 1 and 2. A casual observer will not be able
to discriminate between the two system versions – which is exactly the view the
user should have. However, the user will have to be able to distinguish between
USB 2.0 hubs and USB 1.1 hubs in order to optimize the placement of USB 2.0
high-speed devices. The roles of the components of the 2.0 system have minor
changes from the roles in a USB 1.1 system.
Role of Host PC software.
Current applications software on the PC continues to operate with USB 1.1
peripherals and is unchanged. The system software will comprehend the
increased capabilities of USB 2.0 peripherals so that it can optimize
performance. The system software will also detect sub-optimal configurations,
i.e. a USB 2.0 peripheral attached to a USB 1.1 hub, and will alert the user and
recommend a better configuration for attaching the peripherals. New
applications will be written to take advantage of the higher speed capabilities and
ease-of-use of USB 2.0 peripherals and drivers.
Role of the hub.
A USB 2.0 hub accepts high-speed transactions at the faster frame rate andmust deliver them to high-speed USB 2.0 peripherals and USB 1.1 peripherals.
This data rate matching responsibility will require some increased hub complexity
and temporary buffering of the incoming high-speed data. In the simplest case of
communicating with an attached USB 2.0 peripheral, the hub repeats the highspeed
signals on appropriate USB 2.0 upstream and downstream cables just as
a USB 1.1 hub repeats full and low-speed signals today on USB 1.1 devices.
This allows USB 2.0 peripherals to utilize the majority of USB 2.0 bandwidth.
To communicate with USB 1.1 peripherals, a USB 2.0 hub contains a mechanism
that supports the concept of matching the data rate with the capabilities of the
downstream device. In other words, the hub manages the transition of the data
rate from the high speed of the host controller to the lower speed of a USB 1.1
device. This feature of USB 2.0 hubs means that USB 1.1 devices can operate
along with USB 2.0 devices and not consume disproportionate amounts of USB
2.0 bandwidth. This new hub architecture is intended to be as simple and cost
effective as possible, and yet deliver the full capabilities of 1.1 connections.
The new USB 2.0 hub will be completely defined in the USB 2.0 specification
providing clear implementation guidelines for hub vendors and allowing a single
software driver to service USB 2.0 hub products from multiple vendors.
Role of the peripheral.
Current peripheral products do not require any changes to operate in a USB 2.0
system. Many Human Interface Devices, such as mice, keyboards and game
pads will not require the additional performance that USB 2.0 offers and will
remain as full or low speed peripherals as defined by USB 1.1.
The higher data rate of USB 2.0 will, however, open up the possibilities of
exciting new peripherals. Video-conferencing cameras will perform better with
access to higher bandwidth. Next-generation, higher-speed and higher
resolution printer and scanner devices will be enabled at the high-end. High
density storage devices such as R/W DVD, and high capacity CDROM jukeboxes
will also be enabled by USB 2.0. These devices require minor changes to the
peripheral interface, as defined in the USB 2.0 specification. Overall, the
additional cost to support USB 2.0 is expected to be minimal to the peripheral.
Both USB1.1 and USB 2.0 devices will inter-operate in a USB 2.0 system.
Summary
The USB Specification is currently at Version 1.1 and supports a wide range ofproducts. Many vendors are moving towards USB, drawn by its inclusion on
virtually all PC platforms and its ease-of-use. More and more types of innovative
new peripherals are taking advantage of USB, which further enhance the
available USB product portfolio.The version 2.0 specification that is under development is an evolutionary step
that increases performance capabilities at low cost for USB peripherals in a
backwards compatible fashion. It is expected to broaden the market for new and
higher performance PC peripherals, and supercede USB 1.1 on future PCs.
