Data packets
A data packet consists of the PID followed by 0–1,023 bytes of data payload (up to 1,024 in high bandwidth, at most 8 at low bandwidth), and a 16-bit CRC.
There are two basic data packets, DATA0 and DATA1. They must always be preceded by an address token, and are usually followed by a handshake token from the receiver back to the transmitter. The two packet types provide the 1-bit sequence number required by Stop-and-wait ARQ. If a USB host does not receive a response (such as an ACK) for data it has transmitted, it does not know if the data was received or not; the data might have been lost in transit, or it might have been received but the handshake response was lost.
To solve this problem, the device keeps track of the type of DATAx packet it last accepted. If it receives another DATAx packet of the same type, it is acknowledged but ignored as a duplicate. Only a DATAx packet of the opposite type is actually received.
When a device is reset with a SETUP packet, it expects an 8-byte DATA0 packet next.
USB 2.0 added DATA2 and MDATA packet types as well. They are used only by high-bandwidth devices doing high-bandwidth isochronous transfers which need to transfer more than 1024 bytes per 125 µs microframe (8,192 kB/s).

Isochronous Transfer Packet Size Constraints
An endpoint in a given configuration for an isochronous pipe specifies the maximum size data payload that
it can transmit or receive. The USB System Software uses this information during configuration to ensure
that there is sufficient bus time to accommodate this maximum data payload in each (micro)frame. If there
is sufficient bus time for the maximum data payload, the configuration is established; if not, the
configuration is not established.
The USB limits the maximum data payload size to 1,023 bytes for each full-speed isochronous endpoint.
High-speed endpoints are allowed up to 1024-byte data payloads. A high speed, high bandwidth endpoint
specifies whether it requires two or three transactions per microframe. Table 5-4 lists information about
different-sized full-speed isochronous transactions and the maximum number of transactions possible in a
frame. The table is shaded to indicate that a full-speed isochronous endpoint (with a non-zero wMaxpacket
size) must not be part of a default interface setting. The table does not include the overhead associated with
bit stuffing.