USB terms

In the USB world, the most basic data transfer is done in packet, but to understand the on-the-wire protocol, there are other things to understand.

Packet
USB Packet types: (Source: http://www.usbmadesimple.co.uk/ums_3.htm)

PID Type	PID Name	PID<3:0>*
Token	OUT	0001b
	IN	1001b
	SOF	0101b
	SETUP	1101b
Data	DATA0	0011b
	DATA1	1011b
	DATA2	0111b
	MDATA	1111b
Handshake	ACK	0010b
	NAK	1010b
	STALL	1110b
	NYET	0110b
Special	PRE	1100b
	ERR	1100b
	SPLIT	1000b
	PING	0100b
	Reserved	0000b

Transaction: 
   
One transaction has 3 packets: Token + Data + Handshake for a successful transaction. (*). Nak and Stall error condition could happen after Token when no data is available.

There are 3 types of transactions: IN, OUT, SETUP

 *: For IN and OUT transactions used for isochronous transfers, there are only 2 packets; there is no handshake packet.

Transfer:
There are 4 types of transfer:

• Control 
• Interrupt
• Bulk
• Isochronous (such as audio data)

HID device only uses Control and Interrupt

Control transfer = 3 transactions: SETUP + IN/OUT DATA + STATUS(IN/OUT transaction)

Other transfers has just one transaction: IN/OUT data.

In general, you want to pay attention to Transfer or Transaction level, not packet level.