USB Explained

What is USB?
USB or Universal Serial Bus is a standard which enables several devices to be linked together to exchange data. These devices include PCs, laptops, digital cameras, printers, scanners, mouse, keyboard, MP3 players, mobile phones, mobile broadband modems and more. Today, a total of 127 devices support USB connectivity.

USB is basically designed for devices to communicate with a PC or a laptop. Hence the laptop or PC is referred to as the host or the master. The other device to which the PC connects via USB is referred to as the slave or a peripheral. The host is called the master because, it has the power to initiate data transfers. The slave that connects to the host cannot on its own start to transfer data. It can only obey the master’s command.

Using the host as a medium, data can be exchanged between various devices too. For example, a digital camera can send a picture to a PC/laptop via USB. The computer/laptop can then send the picture to a printer for printing via USB again.

Serial ports and Parallel ports…
Before the USB ports came into being, serial and parallel ports were used for inter-device communication. However, with the advent of the USB connectivity, many of the disadvantages of the serial and parallel ports were overcome. Serial and parallel ports were fewer in number and were slow. USB ports proved to be faster than them.

USB Hubs…

Most laptops/PC, come with 2 or more USB ports. Some of them offer even 4 to 5. Plus, with the help of USB hubs, you can connect several devices together at one go. Even chaining of hubs is possible (connecting hub after hub).

USB Cables…
Devices can be connected to the hosts via USB cables. Some devices, come built with a USB male connector, like USB flash drive, USB mobile broadband modem etc. While others have female ports and can be hooked to the PC or laptop via a USB cable. Even the PC/laptop has a female USB port. Inside a USB cable, there is a set of power wires and a twisted pair of wires. The set of power wires consists of +5V and ground. The twisted pair carries the data.  USB cables can be as long as 5m. These cables can be used to connect hub after hub to allow devices separated by about 30m to communicate as well.

Brief History of USB…
The first USB (version 1.0), was available in 1996. This supported a speed of 12Mbps. The next version that was created, USB 1.1, came into being in 1998. It allowed a speed of 12Mbps for fast data transfers and 1.5Mbps for slower transfers. The next version, USB 2.0, which is the most which is the most widespread today, succeeded the 1.1, in 2000. It offers data transfer speeds of 480Mbps. Plus it also allows backward and forward compatibility.

USB for charging devices…
When a device is connected via USB for the first time, the drivers for the USB connectivity are installed. You might be prompted for it. Next time onwards, the USB drivers are automatically activated when the device is connected. The power required by the USB is automatically drawn from the PC or laptop. That is why, many devices, like mobile phones and MP3 players use USB connectivity for their charging. However, certain devices like printers and scanners have their own power supply and do not require power from the laptops or PCs. Hence such devices draw little power from the hosts(laptops/PCs).

Caution – During the process of data transfer between the host and the peripheral, the cable or the devices must not be unplugged. Else, there are chances of the devices getting hung.

How does USB work?
Before getting to know how the data transfer happens it is essential to know the classifications of USB ports. There are 3 types:-
Interrupt, Bulk and Isochronous. The Interrupts as the name suggests does not require continuous data to be transferred. It is interrupted, like when using a mouse or a keyboard. The second type, bulk, requires data to be transferred in bulk, like a printer. The third one, isochronous, requires data to be streamed, like a speaker. When these devices are linked to the host via USB, the host divides the available bandwidth into slots. The interrupt and isochronous types are given up to 90% of the available bandwidth i.e. 480Mb. The rest of the bandwidth is given to the bulk type. This is because. the interrupt and isochronous types transfer data that needs immediate attention. Whereas the bulk transfers can afford to wait. Interrupt and isochronous are always definite to get slots. The remaining (atleast 10%) is assigned to bulk transfers.

Future…
The USB 3.0 will soon become popular. A few devices have already implemented this USB connectivity. This is supposed to offer speeds of about 4.8 Gbps. USB 3.0 is supposed to allow full duplex communication. Two-way data transfer can happen at the same time. In previous versions (2.0, 1.1, 1.0), this was not possible.

Types of USB…

Standard A – The Standard A USB connector has a rectangular shape. The USB ports on the laptops and computers are Standard-A sockets (female). The connector on the USB cable that fits into these sockets is the Standard-A plug (male). The USB Standard-A plug draws power from the host (laptop or PC).

Standard B – The Standard B USB connector is smaller in width than the Standard-A. It is more squarish. Here again, there are the female sockets and the male plugs. The Standard-B socket (female) is usually present in devices like the printer. The USB Standard-B plug delivers power to the device it is connected to.

The USB cable used to connect devices to laptops/PC, have a Standard A plug on one end and a Standard B plug on the other end. Since the Standard A and B dimensions are different, users will never get confused about which end of the cable goes where.

Mini and Micro USB…
These are the USB ports present in small devices like smartphones, PDAs, MP3 players etc. The mini-USB connectors are rare these days. The micro-USB ports, which are smaller than mini-USB ports, are becoming more commonly used.

There are 2 kinds of micro-USB sockets (female):– micro-B and micro-AB.
There are also 2 kinds of micro-USB plugs(male):– micro-A and micro-B.

Micro-B USB sockets are found in devices like the digital camera, MP3 players, smartphones etc. They are usually peripherals or slaves. A micro-B plug can fit into the micro-B socket, and deliver power to the device.

USB On-the-go devices (USB OTG)…
Devices with a micro-AB socket are generally referred to as On-the-go devices. These devices have the capability of behaving as hosts as well as peripherals. Thus they can either draw power or supply power. They are compatible with both micro-A and micro-B plugs. When a micro-A plug is plugged in, the OTG device behaves as host and supplies power. When a micro-B plug is plugged in, it behaves as a peripheral and draws in power. Such devices are also referred to as dual-role devices. An example of an OTG device is the Nokia N810 Internet Tablet.

Host Negotiation Protocol (HNP)…
Two OTG devices can be connected to each other as well. They use a USB OTG cable for connecting to each other. This cable has a micro-A plug at one end and a micro-B plug at the other end. The OTG device that gets the micro-A plug end of the cable fitted in it becomes the host and the other OTG device which gets the micro-B plug, becomes the peripheral. A very interesting aspect comes into the picture now – role reversal. In role reversal, the OTG devices exchange their roles as host and peripheral. The original host becomes the peripheral and vice-versa. This is possible because of the introduction of a new protocol called Host Negotiation Protocol (HNP).

With HNP protocol, there is no need to reverse the cable at all. The micro-A plug and the micro-B plug, each have an additional pin, called ID pin, apart from the power wires and the twisted pair. The micro-AB socket too has a corresponding pin for the ID. The ID pin helps determine the electrical identities of the cable ends. In the micro-A plug, the ID pin is grounded. In the micro-B plug, the ID pin is left floating.

Role Reversal…
Consider an example of 2 OTG devices being connected together – say an OTG digital camera and an OTG printer. Let’s assume that the digital camera has the printer driver to initiate the printing. Therefore the digital camera has to become the host and the printer should become the peripheral. However, if the user puts the micro-A end of the cable into the printer and the micro-B end in the digital camera, then the printer becomes the host and the digital camera becomes the peripheral. The Host Negotiation Protocol now comes into picture and without the intervention of the user, the roles are reversed. The digital camera becomes the host and the printer becomes the peripheral.

Micro-AB port on the Nokia N810 image via Pocketables.