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Agenda
What is isochronous transfer? Use of isochronous transfer Format of isochronous transactions Comparison of transfer in USB 2.0 & USB 3.0
a a orma Transfer Direction Service Period Bandwidth Allocation
Transfer Packet Size Power Management
Summary
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Agenda
What is interrupt transfer? Principal Use Handshake Phase
Format of Interru t Transactions Characteristics of interrupt transfer Comparison of transfer in USB 2.0 & USB 3.0
Data Format Transfer Direction Maximum Latency Bandwidth Allocation Packet Size Transfer Completion
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Isochronous Transfers
Synchronous connection with other devices Timeliness is more important than accurate delivery of
data Occur at fixed intervals
Spec a pr or ty ur ng execut on o eac rame Guaranteed access to USB bandwidth Guaranteed constant data rate No Handshake Phase
No retrying attempts in case of delivery failure
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Principal Use
Typically contain time sensitive information,such as an audio or video stream
e.g. microphone, speaker, CD audio etc.
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Format of Isochronous IN & OUTTransaction
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Data Format
USB imposes no data content structure oncommunication flows for isochronous pipes
Streaming pipe
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Transfer Direction
Isochronous pipes are unidirectional pipes
Endpoint descriptor defines the direction of data
flow
Two endpoints are required for data transfer inboth directions
This feature is same for USB 2.0 and USB 3.0
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Service Period
Isochronous transfers is scheduled to beperformed regularly during the service period
In USB 2.0, service interval isms or u pee en po n s
125s for High Speed endpoints
Isochronous transfers does not support LowSpeed endpoints
In USB 3.0, service interval for isochronoustransfer is 125 s
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Bandwidth Allocation
Isochronous transfers along with interrupttransfers can be allocated up to 80% of totalbandwidth on SuperSpeed
USB 2.0 requires maximum 90% of totalavailable bandwidth for Full Speed endpointsand maximum 80% of total bandwidth forHigh Speed endpoints
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Packet Size
In USB 2.0, maximum data payload is 1023bytes during each frame for Full Speedendpoints and 1024 bytes for High Speed
Maximum data payload size is 1024 bytesduring each frame for SuperSpeed
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Power Management
USB 2.0 does not support low power USB busstate b/w service intervals for isochronoustransfers
SuperSpeed allows isochronous devices toautonomously enter low power link states b/wservice intervals. It defines a PING &PING_RESPONSE mechanism to activate thelow power link
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Summary
USB 3.0 Protocol preserves key characteristics of USB2.0 Isochronous Protocol i.e., no Retries , Simpleprotocol
isochronous devices
Software Backward Compatibility
Maximum data transferred in 1 service interval isincreased to 48K bytes
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Interrupt Transfers
Interrupt transfers are small data transfersused to communicate information from theUSB device to the client software.
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Principal Use
HID such as mouse, keyboard, joysticks etc.
All data transfer requests are initiated by hostinstead of interrupt requests from input
device Host polls the input device periodically
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Handshake Phase
ACK (acknowledgment)Sent by the host in IN Transactions and by thedevice in OUT Transactions on successful
NAK(negative acknowledgment)
Sent by the device only when it is busy or hasno data to send
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Handshake Phase
Only device sends the STALL
STALL can be due to Unsupported Control Transfer Request Control request Failed Endpoint Failed
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Format of an Interrupt IN & OUTtransaction
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Characteristics of Interrupt Transfers
High reliability to communicate is required Guaranteed maximum service interval Guaranteed retry of transfer attempts in next service
Guaranteed bandwidth in each service interval Host waits for ready notification to service the endpoint Handshake Phase exists
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Data Format
In both USB 2.0 and USB 3.0, no data contentstructure is imposed on communication flowfor interrupt pipes
Streaming pipes
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Transfer Direction
Interrupt pipes are unidirectional pipes
Endpoint descriptor defines the direction of dataflow
Two endpoints are required for data transfer inboth directions
This feature is same for USB 2.0 and USB 3.0
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Maximum Latency
Endpoint descriptor specifies the latency or pollinginterval.
For Low Speed devices, the maximum latency variesfrom 10 ms to 255 ms
For Full Speed interrupt transfers, the maximum
latency varies from 1 ms to 255 ms For High Speed devices, the maximum latency varies
from 125 s to 4 sec
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Bandwidth Allocation
In USB 2.0, interrupt transfers & Isochronoustransfers can use up to 90% of the totalavailable bandwidth
Isochronous & Interrupt transfers can use upto 80% of the total available bandwidth onSuperSpeed
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Packet Size
Endpoint specifies the maximum data payload size
In USB 2.0, data payload size is
8 bytes or less for Low Speed Devices 64 bytes or less for Full Speed Devices
1024 bytes for High Speed Devices
In USB 3.0, maximum data payload size is 1024 bytes
No data padding in case of less data
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Packet Size (contd)
Host ensures that data payload size of anypacket is not greater than endpoints
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Transfer Completion
Interrupt transfer is completed when Has transferred the exact amount of data
expected
Transfers a packet with a payload lessthan the maximum packet size
Responds with a STALL handshake
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