Thursday, August 19, 2004
Isochronous Transfers
Isochronous transfers occur continuously and periodically. They typically contain time sensitive information, such as an audio or video stream. If there were a delay or retry of data in an audio stream, then you would expect some erratic audio containing glitches. The beat may no longer be in sync. However if a packet or frame was dropped every now and again, it is less likely to be noticed by the listener.
Isochronous Transfers provide
->Guaranteed access to USB bandwidth.
->Bounded latency.
->Stream Pipe - Unidirectional
->Error detection via CRC, but no retry or guarantee of delivery.
->Full & high speed modes only.
->No data toggling.
The maximum size data payload is specified in the endpoint descriptor of an Isochronous Endpoint. This can be up to a maximum of 1023 bytes for a full speed device and 1024 bytes for a high speed device. As the maximum data payload size is going to effect the bandwidth requirements of the bus, it is wise to specify a conservative payload size. If you are using a large payload, it may also be to your advantage to specify a series of alternative interfaces with varying isochronous payload sizes. If during enumeration, the host cannot enable your preferred isochronous endpoint due to bandwidth restrictions, it has something to fall back on rather than just failing completely. Data being sent on an isochronous endpoint can be less than the pre-negotiated size and may vary in length from transaction to transaction.
The above diagram shows the format of an Isochronous IN and OUT transaction. Isochronous transactions do not have a handshaking stage and cannot report errors or STALL/HALT conditions.
Isochronous Transfers provide
->Guaranteed access to USB bandwidth.
->Bounded latency.
->Stream Pipe - Unidirectional
->Error detection via CRC, but no retry or guarantee of delivery.
->Full & high speed modes only.
->No data toggling.
The maximum size data payload is specified in the endpoint descriptor of an Isochronous Endpoint. This can be up to a maximum of 1023 bytes for a full speed device and 1024 bytes for a high speed device. As the maximum data payload size is going to effect the bandwidth requirements of the bus, it is wise to specify a conservative payload size. If you are using a large payload, it may also be to your advantage to specify a series of alternative interfaces with varying isochronous payload sizes. If during enumeration, the host cannot enable your preferred isochronous endpoint due to bandwidth restrictions, it has something to fall back on rather than just failing completely. Data being sent on an isochronous endpoint can be less than the pre-negotiated size and may vary in length from transaction to transaction.
The above diagram shows the format of an Isochronous IN and OUT transaction. Isochronous transactions do not have a handshaking stage and cannot report errors or STALL/HALT conditions.
