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USB has built upon the benefits
and shortcomings of numerous prior peripheral interfaces to
create a user friendly and universal connection protocol that
eliminates many of the problems associated with legacy interfaces
such as RS-232 or RS-422.
USB offers:
- Plug-and-play ease of use. No configuration or complex
setup required.
- A high bandwidth interface providing advanced functionality
and a quality user experience.
- Versatility. Multiple device types providing a variety
of user applications and functions can be connected to the
same port type.
- Low cost, enabling implementation into very cost sensitive
CE devices.
- Powered interface to enable simple device connectivity
without additional power cables, or batteries.
- Embedded support for the USB communications protocol
in the operating system to simplify system integration and
operation.
By providing a powered, high data rate, low cost serial interface
that enables PC connectivity to multiple device types, USB
handles virtually all of the connection responsibilities on
today’s PCs. Input devices, storage devices, music players,
web cameras and printers all connect via USB, enriching and
simplifying the experience every time one sits down in front
of a PC to make a VoIP call or upload photos from the day’s
activities.
As a tradeoff to the simplicity and efficiency of the USB
interface, USB imposes a very strict transmit/acknowledge
timing protocol that limits the wired reach of a USB connection
to 5m. As USB becomes more and more prevalent in today’s
electronics devices, the distance limitation restricts advanced
use cases and further expansion of the value that USB has
brought to the modern PC. There are a number of USB Extension
technologies that can overcome the 5m distance limitation
of USB and therefore offer the benefits of USB peripheral
connectivity for TV 2.0.
USB Extension Overview
USB extension can be achieved by the following methods:
| ExtremeUSB: |
Icron’s patented ExtremeUSB
technology eliminates the round-trip delay limitation
by creating separate timing domains at each end of the
link. Distances are essentially unlimited and transmission
delays in the millisecond range can be accommodated.
ExtremeUSB is a hardware solution that is totally cross-platform
applicable, fully supports USB 2.0, and requires no
software additions to the host PC. |
USB Over IP: |
This technology requires software to
intercept traffic occurring over the USB software stack
running in the PC. The traffic is diverted away from
the local host controller and rerouted over the PC’s
TCP/IP stack to a remote host controller. The system
has the disadvantage of requiring custom software that
alters the operation of the OS. It is by design platform
specific and is often limited in the types of devices
it can support (often printers and hard drives only).
Isochronous endpoints are rarely, if ever, provided. |
Hub Chain: |
This is the method provided by the USB
2.0 specification. USB allows 5 hubs to be connected
in series by cables that are each five meters in length.
When the device cable is added, a maximum distance of
30 meters can be obtained. The system is clearly clumsy
and expensive. It also requires that power be provided
to every second hub. |
Delay Budget Use: |
The USB specification allows approximately
1µs for round-trip transmission through the maximum
length chain of hubs. If this budget is reallocated
to a dedicated cable with no hubs, then approximately
50 meters distance can be achieved. The system only
supports USB 1.1. Icron’s Rover products exploit
this method. |
Custom Host /
Device Sharing |
This is a more invasive scheme that requires
custom host controller hardware to be provided in the
PC. The approach is designed for sharing printers and
USB hard drives and for server farms / blade server
implementations. |
Table 4. USB Extension Alternatives
Of the USB solutions listed, only ExtremeUSB and USB over
IP options support the TV 2.0 architecture. There are some
implementation and functionality differences between the two
solutions however, as listed in the following comparison chart.
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