Signaling - Universal Serial Bus:
USB supports following signaling rates:
o A low speed rate of 1.5 Mbit/s is defined by USB 1.0. This is so much similar to "full speed" operation except each bit takes 8 times as long to transmit. It is intended primarily to save price in low-bandwidth human interface components (HID) such as mice, keyboards and joysticks.
o A hi-speed (USB 2.0) rate of 480 M bit/s was introduced in the year2001. All hi-speed components are capable of falling back to full-speed operation if needed; they are backward compatible. Connectors are identical.
o The full speed rate of 12 Mbit/s is the fundamental USB data rate which is defined by USB 1.1. All USB hubs hold up full speed.
o A Super Speed (USB 3.0) rate of 5.0 Gbit/s. The USB 3.0 requirement was released by Intel and partners in the year 2008, according to early reports from CNET news. The first USB 3 controller chips were sampled by NEC May in the year 2009  and generate by using the 3.0 specification are expected to arrive beginning in Q3 2009 and 2010. USB 3.0 connectors are usually backwards compatible, but include new wiring and full duplex operation. There is little incompatibility with older connectors.
USB signals are transmitted on a braided pair data cable having 90Ω ±15% Characteristic impedance, labeled D- and D+ Prior to USB 3.0, These all collectively use in half-duplex differential signaling to reduce the effects of electromagnetic noise on longer lines. Transmitted signal levels are in the range 0.0-0.3 volts for low and in the range 2.8-3.6 volts for high in full speed (FS) and low speed (LS) modes, and -10-10 mV for low and 360-440 mV for high in hi- speed (HS) mode. In Full Speed mode the cable wires are not terminated, but in the HS mode has termination of 45 Ω to ground, or 90 Ω differentials to match the data cable impedance, by reducing interference of specific kinds. USB3.0 introduces 2 extra pairs of shielded twisted wire and new, mostly interoperable contacts in USB3.0 cables, for them. They allow the higher data rate, and also full duplex operation.
A USB connection is always among a host or hub at the "A" connector end, and a component or hub's "upstream" port at the other end. Initially, it was a "B' connector, saving from erroneous loop connections, but extra added upstream connectors were precise, and some cable vendors designed and sold cables which allowed erroneous connections (and potential damage to the circuitry). USB interconnections are not as perfect or as effortless as originally intended.
The host includes 15 kΩ pull-down resistors on each data line. While no device is connected, this pulls both data lines low into the so-called "single-ended zero" state (SE0 in the USB documentation), and mention a disconnected or reset connection.
A USB device pulls 1 of the data lines high having a 1.5 kΩ resistor. it overpowers 1 of the pull-down resistors in the host and leaves the data lines in an idle state which is called "J". For USB 1.x, the option of data line indicates a device's speed support; full-speed components pull D+ high, while low-speed devices pull D- high.
USB data is transmitted by toggling the data lines between the opposite K state and the J state. USB encodes data by using the NRZI convention; a 0 bit is transmitted by toggling the data lines from J to K or vice-versa, when a 1 bit is transmitted by leaving the data lines as-is. To ensure a minimum density of signal transitions USB uses bit stuffing technique; an extra 0 bit is inserted into the data stream after any appearance of 6 consecutive 1 bits. 7 consecutive 1 bits is all the time an error. USB 3.00 has included extra added data transmission encodings.
A USB packet starts with an 8-bit synchronization sequence 00000001. That is data lines toggle KJKJKJKK, after the initial idle state J. The final 1 bit (repeated K state) marks the end of the sync pattern and the starting of the USB frame.
A USB packet's end which is called EOP (end-of-packet), is specified by the transmitter driving 2 bit times of SE0 (D+ and D- both below max) and 1 bit time of J state. After this, the transmitter ceases to drive the D-/D+ lines and the aforementioned pull up resistors hold it in the J (idle) state. Sometimes skew due to reason of hubs may add as much as one bit time before the SE0 of the end of packet. This extra bit can be result in a "bit stuff violation" if the 6 bits before it in the CRC are '1's. This bit should be avoided by receiver.
A USB bus can reset by using a prolonged (10 to 20 milliseconds) SE0 signal.