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USB 2.0 data rates
The theoretical maximum data rate in USB 2.0 is 480 Mbit/s (60 MB/s) per controller and is shared amongst all attached devices. Some chipset manufacturers overcome this bottleneck by providing multiple USB 2.0 controllers within the southbridge.

Transfer speeds in practice

Typical hi-speed USB hard drives can be written to at rates around 25-30 MB/s, and read from at rates of 30-42 MB/s, according to routine testing done by CNet.^[62] This is 70% of the total bandwidth available.
According to a USB-IF chairman, "at least 10 to 15 percent of the stated peak 60 MB/s (480 Mbit/s) of Hi-Speed USB goes to overhead — the communication protocol between the card and the peripheral. Overhead is a component of all connectivity standards."^[63] Tables illustrating the transfer limits are shown in Chapter 5 of the USB spec.
For isochronous devices like audio streams, the bandwidth is constant,  and reserved exclusively for a given device. The bus bandwidth therefore only has an effect on the number of channels that can be sent at a time, not the "speed" or latency of the transmission.

Data packets
USB communication takes the form of packets. Initially, all packets are sent from the host, via the root hub and possibly more hubs, to devices. Some of those packets direct a device to send some packets in reply.
After the sync field, all packets are made of 8-bit bytes, transmitted least-significant bit first. The first byte is a packet identifier (PID) byte. The PID is actually 4 bits; the byte consists of the 4-bit PID followed by its bitwise complement. This redundancy helps detect errors. (Note also that a PID byte contains at most four consecutive 1 bits, and thus will never need bit-stuffing, even when combined with the final 1 bit in the sync byte. However, trailing 1 bits in the PID may require bit-stuffing within the first few bits of the payload.)

USB PID bytes

Type	PID value (msb-first)	Transmitted byte (lsb-first)	Name	Description
Reserved	0000	0000 1111
Token	1000	0001 1110	SPLIT	High-bandwidth (USB 2.0) split transaction
	0100	0010 1101	PING	Check if endpoint can accept data (USB 2.0)
Special	1100	0011 1100	PRE	Low-bandwidth USB preamble
Handshake			ERR	Split transaction error (USB 2.0)
	0010	0100 1011	ACK	Data packet accepted
	1010	0101 1010	NAK	Data packet not accepted; please retransmit
	0110	0110 1001	NYET	Data not ready yet (USB 2.0)
	1110	0111 1000	STALL	Transfer impossible; do error recovery
Token	0001	1000 0111	OUT	Address for host-to-device transfer
	1001	1001 0110	IN	Address for device-to-host transfer
	0101	1010 0101	SOF	Start of frame marker (sent each ms)
	1101	1011 0100	SETUP	Address for host-to-device control transfer
Data	0011	1100 0011	DATA0	Even-numbered data packet
	1011	1101 0010	DATA1	Odd-numbered data packet
	0111	1110 0001	DATA2	Data packet for high-bandwidth isochronous transfer (USB 2.0)
	1111	1111 0000	MDATA	Data packet for high-bandwidth isochronous transfer (USB 2.0)

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