Universal Serial Bus

Universal serial bus was first developed in 1990. It has now become one of the most commonly used peripherals in personal computers and laptops. In USB protocol, the computer acts as the host system and up to 127 devices can be connected to it at the same time. When the host powers up, the enumeration system assigns each one, an address. Data transfer is then done using this address.

Data Transfer

Data transfer can be initiated by software, by sending an I/O request packet to the USB. Software is acknowledged for both, successful transition as well as if an error occurs. Data is transferred as packets. It contains source address, destination address, length of the data, and also error detection information. Packet is made up of a set of components called fields. Different fields in a packet are shown in the given figure.

Different fields in a packet

USB packet starts with an eight bit "SYNC”, which is used for the synchronization and timing correction. The 8 bit Sync is followed by 8 bit Packet Identifier (PID). First four bits in PID are used to determine the type (token, data, handshake, and special) and the remaining 4 bits are complement of the first bits. Third field is for the address of the function. Four bit endpoint field follows address, giving the details of the endpoint which sends or receives the packet. Finally the data field, which consists of 0-1023 bytes of data. Except the PID, errors are detected in each field using Cyclic Redundancy Check (CRC).

Type of Data Transfer

(a). Control Transfer

In the USB protocol, control transfer includes configuration, command and status checking. Bi-directional data transfer is initiated by the host. If the transfer is successful, USB replies with an ACK handshake else it ignores the data totally. Control transfer is pictorially represented in the following figure. At first, a control token is send from the host to the USB device. For the successful control transfer, USB device sends acknowledge signal back to the host.

Control Transfer

If the control transfer fails then no response is send back from USB device to the Host system.

If the control transfer fails

(b). Interrupt Transfer

Interrupt data transfer includes sending and reception of a small amount of data, for example mouse and key board. Here also data transfer is initiated from the Host system. Host will query the USB device by a token packet. The Device returns data if it has any interrupt related information available to transmit.

Interrupt Transfer

NAK (Negative Acknowledge) signal is send back instead, if interrupt data is not available.

if interrupt data is not available.

(c). Bulk Transfer

Purpose of bulk data transfer is to transfer one big packet of data. Like in printers, where large amount of data is to be transferred, bulk transfer is used. Bulk transfer includes verification methods to make sure the data transferred is correct. Transfer is initiated by token packet, which specifies whether data transfer is input or output. For input data transfer, token is send and the system waits for data.

Bulk Transfer

And for the output, the host will send both transmission request and corresponding data and will wait for an appropriate handshake from the recipient.

(d). Isochronous Transfer

Isochronous data transfer is real-time data transfer between host and the device. Expecting enough tolerance, no error correction is used in this method. Data transfer begins with an appropriate token packet followed by continuous data transfer.

Isochronous Transfer

During operation Isochronous and interrupt devices can consume up to maximum of 90 percent of the total bandwidth. Bulk transfers can use only the bandwidth left after the use of isochronous and interrupt devices. 

USB Versions

So far three versions of USBs are available in market.

USB 1.0

It is the first commercial version of USB with a data rate of 12 Mbps. In almost every field it is getting replaced by USB 2.0.

USB 2.0

USB 2.0 is the updated version of USB 1.0. At a time, bus can either send or receive data at a maximum rate of 480 megabits per second. At 5v, USB 2.0 consumes a power of 500 milliamps.

USB 3.0

USB 3.0 is the latest version. Data rate is 10 times larger than that of USB 2.0. It can transmit data in both directions simultaneously at a rate of 4.8 gigabits. Power consumption is more than USB 2.0 that is 900 milliamps power at 5v.


  Song / Pic SD-Movie HD-Movie
Size 4 MB 6 GB 25 GB
USB 1.0 5.3 sec 2.2 hr 9.3 hr
USB 2.0 0.1 sec 3.3 min 13.9 min
USB 3.0 0.01 sec 20 sec 70 sec


USB Connectors

Universal Serial Bus (USB) was developed in the 1990s to simplify the connections between computers and peripheral devices. Some most common USB connectors used for different applications are listed here.

(a). USB A-Type

USB A types is the most commonly used USB connector. It is rectangular, flat interface found in USB hubs and computers.  Connection is established by friction and thus making it easy for end user to make connection and disconnection. The flat contacts in USB withstand multiple removal and attachment of the connector.




 (b). USB B-Type

The B-type USB interface has a square in shape with top ends slightly beveled for the connectors. Connection is established by usage of friction with the contact area. The usage of B-type connectors is limited to peripheral devices.

(c). Micro-USB A

Micro-USB A is physically smaller than A-type and B-type USB. Micro USB A connectors are found in mobiles, PDAs etc. with an exchange rate of 480Mbps. It comes with a receptacle, that is, white colored and is a concise design of 5 pins.





(d). Micro-USB B

Micro-USB B also has 5 pins and is smaller than USB A and B types. Figure of a Micro-USB B is shown below. It has characteristics similar to Micro-USB A.





(e). USB Mini-b

The size of the USB B -style restricts its use in mobiles, PDAs etc. and has given way for the development of USB Mini-b. It has 5 pins and has the On-The Go, by which peripheral devices co-exist with host controller can communicate with each other.




(g). USB 3.0 A-Type


USB 3.0 A-type is the type of USB connector similar to USB 2.0 with additional pins. But shape and size of both USB 3.0 A and USB 2.0 A is same. This makes USB 3.0 A-type physically in sync with USB 2.0. USB 3.0 A is commonly in blue colour to distinguish them from earlier versions.



(h). USB 3.0 B-Type

The USB 3.0 B-Type is developed for carrying power and data in applications that comes under SuperSpeed applications. This type of connector cables is not backward compatible with other connectors such as USB 1.1, USB 2.0. But the devices with these kinds of connections can readily accept USB 1.1 and 2.0.

(i). USB 3.0 Micro B

This is similar to USB 3.0 B-Type, which is, used to transfer of power and data in SuperSpeed operations.

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