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USB and Firewire

Moving large amounts of data from one source to another is becoming one of the most important attributes of computers as users demand that they take their data and information with them, everywhere, especially with the increasing need for more mobility. This whitepaper describes and highlights the features of Firewire (also known as IEEE 1394 or i.LINK®), USB 1.0, USB 1.1 and the new USB 2.0 standard which will be implemented in most of Toshiba's new notebook series.
 

A little bit of history
 

As computers gained popularity, most communication between them was done via floppy disks and archaic networks, or possibly using very cumbersome tape drive units.For most users, parallel and serial ports were the only way to connect devices to their units, whether it was for communication with a modem or with a printer. Some people connected their computers with null modem cables and communication software to move data back and forth, but this method was too slow so that most users still used floppy disks because of their benefit of speed. Eventually, programs and data files increased in size which meant that available ports on the computer were no longer adequate. The demand for easier functionality, increased speed, and "plug-and-play" ability, gave rise to the Universal Serial Bus (USB) 1.0.

USB 1.0 and 1.1
 


USB 1.0 surfaced on the market in 1995. USB 1.1 appeared soon after and brought with it minor improvements in hardware recognition and data transfer from its predecessor. Both have the same small square physical connector that we are familiar with today. The four contact plug is easy to remove and insert and has a cable length between three to five metres.

USB 1.0 and 1.1 have two speed modes: low-speed at 1.5 Mbps (megabits per second) and full-speed at 12 Mbps. The low-speed rate is used with peripherals such as keyboards and mice, while the full-speed rate is more practical for peripherals such as modems and printers.

The nomenclature of USB can be defined as follows: All USB devices are clients, meaning they require a computer to talk to one another. This means that the computer is always the host. The bus has a speed of 12 Mbps maximum and each device can request up to 6 megabits per second. Theoretically, up to 127 devices can be connected to a host via a hub or directly. Realistically, however, up to ten devices can be connected. Cable length can range from one to 5 metres or up to 30 metres if hubs are connected in between.

A USB cable contains four wires; two of these wires carry 5 volt power, with a current of 500ma (milliamps) and the other two carry data. Small USB devices, such as mice and memory sticks, draw their power straight from the USB port, while bigger devices such as scanners have their own power supply.

USB hubs come in two types, powered and non-powered; the notable difference is that the powered ones can provide power to the USB devices attached to it, while the non-powered are limited by the 500ma current limit. All USB devices are hot-swappable, meaning that you can plug them and unplug them without having to restart the machine, also referred to as plug & play. Although some operating systems may complain when you remove an item, it usually will not cause a unit to stop.

So what exactly happens when a USB device is plugged in? When a computer is powered on with USB devices attached, it polls or queries all the devices that are connected via USB and assigns them an address. The same occurs when a device is plugged in while the computer is already on. This is called bus enumeration. At this time, the host also determines what type of communication each device wants to perform.

There are three types of data transfer between the computer and respective peripherals. Interrupt mode is used for peripherals that transmit very little data, like mice. Bulk mode is used for peripherals that use one big block of data, such as a printer. Isochronous mode is used with those peripherals that deliver data in streams such as a modem. Governing the travel of information is done by the host using control packets and by dividing the available bandwidth into frames of 1500 bites every millisecond. This ensures that isochronous and interrupt mode transfers get the bandwidth they require while bulk transfers use up the remaining available space.

USB 2.0

USB 2.0 has all the flexibility that USB 1.1 allows with all its benefits - ease-of-use, expandability - but with one immense improvement. USB 2.0 transfer rates at 480 Mbps are 40 times faster than USB 1.1. USB 2.0 uses the same protocol as USB 1.1 and is backward compatible, meaning you can still use all your old cables. All your USB 1.1 peripherals will work, even hubs, but at a slower rate of 12 Mbps. To get the maximum throughput, a USB 2.0 hub is needed.

The transfer speed increases by adding another transfer mode, in addition to the original low-speed and full-speed modes. The high-speed mode allows for a transfer speed approaching hard disk drive UDMA-66 levels, namely 480 Mbps (see diagram). This operation speed is perfect for hard disk drives, CD-R/RW, and any other peripherals that require high bandwidth.

One of the most positive developments is that Intel® will include support for USB 2.0 on its upcoming chipsets, making the cost of USB 2.0 negligible and thus making market penetration almost guaranteed, as most functionalities incorporated by Intel® have become de-facto standards.

Firewire

Firewire, also know as IEEE 1394 or i.LINK®, has essentially the same goals as USB, but with different uses. While it is also a serial interface, the difference emerges in the way data is transmitted along with some of the basic fundamental design characteristics.

Firewire is usually associated with high-end video cameras and hard drives. Firewire communicates in two modes: asynchronously and isochronously. Asynchronous channels are used to control the bus and to transfer bulk data. Isochronous channels are used for transferring video and audio streams. The reason for these two channels is to ensure data stability and to prevent timing issues that can arise from moving synchronization dependent data.

Firewire, unlike USB 2.0, is not host based. This means that Firewire devices can communicate with one another without a computer in the middle, in a peer-to-peer fashion; for example, one can transfer video from their camcorder straight to their VCR, if Firewire equipped. This leads us to the next point, that all of the communication requirements are contained within Firewire.

Power is also supplied by Firewire from 8-40 volts and up to 1.5 amperes, which is enough to charge some batteries and other small devices. A Firewire cable consists of six wires, two for power and four for data in two twisted pairs. Two different types of connectors are available - one consisting all six connections, thus providing power and one with only four connections, only for data. The maximum cable length is 4.5 metres and can be expanded to 76.5 metres through the use of hubs.

Some analysis and the future

Connectivity technology will always improve and bandwidth will increase as demand grows. USB 2.0 is a technology that will be as prominent as USB 1.1, due to common cabling, backward compatibility, and the low cost provided by the support in chipsets from Intel®. Devices, such as external hard disk drive and CD-ROM enclosures are already available using USB 2.0, ready to benefit from the newly afforded speed. Firewire will coexist with USB 2.0, but in a more specialized manner geared toward video and audio streaming where price is less important. The consumer will state which will prevail with the assistance of creative manufacturers; just imagine a one gigabyte USB 2.0 memory stick. Now that is fast backup.

USB 1.0, 1.1, 2.0 and Firewire Comparison

 

USB 1.0 and 1.1

USB 2.0

Firewire

Data transfer mode

Control Transfer: Control signals Interrupt Transfer: Polling mode (i.e. mouse, keyboard)

Bulk Mode: Timely not secured transmission (i.e. scanner, printer)

 

Isochronous Mode: Transmission with fixed bandwidth (i.e. ISDN)

Control Transfer: Control signals

Interrupt Transfer: Polling mode (i.e. mouse, keyboard)

Bulk Mode: Timely not secured transmission (i.e. Scanner, printer)

Isochronous Mode: Transmission with fixed bandwidth (i.e. ISDN)

Asynchonus Mode: Transmission on request (i.e HDD)

Isochronous Mode:Transmission with fixed bandwidth (i.e. video digital camera)

Max. data rate

12Mbs (Megabit)

Practical: up to 1MB (Megabyte) possible

480Mbs

Practical: up to 50MBs should be possible

400Mbs

Practical: up to 40MBs possible

Max. number of clients

127 Clients

Practical: problems occur with more than 7-8 clients

127 Clients

Practical: not known

63 Clients

As 1,023 buses are possible this means, 64,449 peripherals could be used.

Practical: problems occur with more than 10 peripherals

Powersupply thru host

0.5A, Hubs up to 1.2A

0.5A, Hubs up to 1.2A

< 1.5A

Cables

0.8 - 3.0m Low Speed 0.8 - 5m High Speed (drilled + shielded)Single cables are not extendableTotal transmission distance only extendable via HUBs.(Max. 6 HUBs = 30m)

0.8 - 3.0m Low Speed 0.8 - 5m High Speed (drilled + shielded)Single cables are not extendableTotal transmission distance only extendable via HUBs.(Max. 6 HUBs = 30m)

 

Up to 4.5 mTotal transmission distance only extendable via HUBs.(Max. 16 HUBs = 76.5m)

Notes: Mbps refer to megabits per second. MBps refer to megabytes per second. All trademarks are property of their respective owners.


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