Part 7 - High Speed Transactions

High Speed USB Transactions

We have looked at the mechanisms for communicating at 480 Mb/s. We now examine the packet formats in more depth, and then see how these are used to provide the various transactions: Control, Interrupt, Bulk and Isochronous.x

The New Packets

The packet types which have been added for high speed are:

• DATA2
• MDATA
• NYET
• ERR
• SPLIT
• PING

The table on the right gives brief details of the purposes of the six new packet types.

For the sake of completeness we will now look at the packet formats for all the available packets type, including the ones already covered for low and full speed.

There are five different packet formats based on which PID the packet starts with.

Token Packet

Used for SETUP, OUT, IN and PING packets. They are always the first packet in a transaction, identifying the targeted endpoint, and the purpose of the transaction.

(The SOF packet is also defined as a Token packet, but has a slightly different format and purpose, which is described below.)

The token packet contains two addressing elements:

Address (7 bits)

This device address can address up to 127 devices. Address 0 is reserved for a device which has not yet had its address set.

Endpoint number (4 bits)

There can be up to 16 possible endpoints in a device in each direction. The direction is implicit in the PID. OUT, SETUP and PING PIDs will refer to the OUT endpoint, and an IN PID will refer to the IN endpoint.

Data Packet

Used for DATA0, DATA1, DATA2 and MDATA packets. If a transaction has a data stage this is the packet format used.

Handshake Packet

Used for ACK, NAK, STALL and NYET packets. This is the packet format used in the status stage of a transaction, when required.

SOF Packet

The Start of Frame packet is sent every 1 ms on full speed links. The frame is used as a time frame in which to schedule the data transfers which are required. For example, an isochronous endpoint will be assigned one transfer per frame.

Split Packet

The SPLIT packet is the first packet in either a Start Split transaction or a Complete Split transaction, sent to a high speed hub when it is handling a low or full speed device.

Control, Interrupt or Bulk Endpoints

Isochronous Endpoints

High Speed Bulk Transactions

These work much like full speed bulk transactions. The differences are:

• The maximum packet size can only be specifed as 512 bytes.
• A more efficient method of doing OUT transfers has been introduced, involving the use of PING transactions, and the new NYET handshake packet.

Ping Protocol

Before sending an OUT transaction to a bulk endpoint, the host controller may send a PING packet. The response to the PING will be ACK or NAK. ACK means that the endpoint is ready to accept an OUT transaction of maximum packet size for the endpoint. NAK means it is not. The host may continue to send PINGs after a NAK, or it may choose to wait for a number of microframes before re-trying.

On high speed Bulk OUT endpoints, the endpoint descriptor value bInterval is required to specify the NAK rate of the endpoint. The specification is misleading when it defines this value. The value represents the number of microframes which the host would have to wait, after receiving a NAK in response to a PING, before a further PING is guaranteed to elicit an ACK response. 0 means the endpoint never NAKs.

A further new feature is that, after a successful OUT transaction, the endpoint may respond with ACK to indicate that it is already prepared to accept a further packet, or NYET to indicate that it received the data correctly, but is not yet ready to accept further data.

BULK Transfer Error Control Flow

High Speed Isochronous Transfers

High speed isochronous transfers have an extended control system to allow up to 3 isochronous transactions per microframe, allowing a data rate of up to 192 Mb/s. (The specification refers to an isochronous endpoint with more than 1 packet per microframe as a 'High Bandwidth' endpoint.)

As isochronous transactions do not include a acknowledgement (handshake) packet, a system was specified which allows the recipient to be aware, if a data packet is lost, which one it was*.

Isochronous IN

When requesting IN transactions, the device packages them in DATA2, DATA1 or DATA0 packets, depending on how many packets per microframe are specified, and which one it is. See diagram on the right.

Isochronous IN Protocol

Isochronous OUT

When sending OUT transactions, the host packages them either in MDATA or DATA0 DATA1 or DATA2 packets, as shown in the diagram on the right

*In fact this does not apply to Isochronous OUT transfers with three transactions per microframe because the first two packets use MDATA as a PID.

Isochronous OUT Protocol

High Speed Interrupt Transactions

These work much as for full and low speed, except that the endpoint descriptor may specify up to 3 interrupt transactions per microframe, allowing a data rate of up to 192 Mb/s. (The specification refers to an interrupt endpoint with more than 1 packet per microframe as a 'High Bandwidth' endpoint.)

The protocol is the same as for full and low speed, in that DATA0 and DATA1 packets are alternated.

If more than one transaction per frame is specified and a transaction is NAKed, then the host should attempt no more transactions on that endpoint within the same microframe.

Interrupt Transfer Error Control Flow

High Speed Control Transfers

High speed control transfers are the same as full and low speed transfers, with the following exceptions:

• For high speed control endpoints, the maximum packet size for the data stage is fixed at 64 bytes.
• During a data phase, using an OUT PID, the transaction follows the rules above for a Bulk OUT transaction, including the PING protocol.

Error Control Flow

SETUP STAGE

Notice that it is not permitted for a device to respond to a SETUP with a NAK, STALL or NYET.

DATA STAGE

(same as for high speed bulk transfer)

STATUS STAGE

(same as for bulk transfer)

Split Transactions

Transaction Translator

Split transactions form a high speed only protocol between host controllers and high speed hubs, which are handling full or low speed traffic. This is the means by which the full or low speed traffic is prevented from degrading the high speed bus performance. Each high speed hub is required to have a 'Transaction Translator' which handles the full or low speed transactions to particular ports.

Packet Sequences

To overcome the difference in speeds, each full or low speed transaction is split into two parts on the high speed bus; a Start Split Transaction and a Complete Split Transaction. In between, the Transaction Translator independently handles the full or low speed transaction.

The illustration to the right shows how the host defines the required full or low speed transaction, allows the hub to deal with the transaction at that speed and then checks back later for the result.

The actual packet sequence in a split transaction varies, depending on the type of transaction involved, and will be described in more detail, in the following paragraphs.

This will be a brief summary of what to expect, however the full USB specification contains masses of detail which would be needed to actually implement a host controller or hub.

Transaction Translator

This is a much simplified diagram of the transaction translator within a high speed hub.

Example Split Transaction Sequence

This illustrates a typical split transaction sequence; in this case a split interrupt IN.

Split Bulk OUT Transaction

For a split bulk OUT transaction, the host sends a Start Split packet specifying the hub address and port number, and speed (full), with the 'endpoint type' set to 'bulk'.

It then sends an OUT token packet to identify the full speed transaction type, device and endpoint address, followed by the data packet for onward transmission to the target endpoint.

If the Transaction Translator receives these packets correctly and has an available buffer, it will respond with an ACK packet, otherwise it will respond with a NAK packet.

At this point the Transaction Translator proceeds to send this transaction to the specified full speed endpoint, and to get a response, which it stores, ready to pass back to the host during a Complete Split transaction.

When the host controller decides it is appropriate, it checks for the result by issuing a Complete Split transaction. The Complete Split packet and the OUT token packet are the same as for the Start Split transaction, except for the Start/Complete bit of the packet. This allows the hub to verify which transaction it is to present the results of.

If the full speed transaction is not yet completed, the hub will respond with a NYET packet and the host will attempt the Compete Split transaction sometime later. If it is complete, then a packet containing the actual result (ACK, NAK or STALL) is sent to the host.

Split Bulk IN Transaction

A split bulk IN transaction is similar to a split bulk OUT, except that the DATA0/1 packet occurs during the Complete Split stage because it is now part of the response. If data was returned then the data packet appears in lieu of a handshake packet.

The actual ACK of the full speed transaction was sent by the Transaction Translator to the device on receiving the DATA0 or DATA1 packet from the device, and the host will keep track of which data it has received. If it fails to receive a valid packet it will retry the Complete Split Transaction.

Split Bulk IN Transaction

Split Interrupt OUT Transaction

For an interrupt OUT transaction the maximum data size is 64 bytes, so these are always transferred within a single Start Split transaction. There is no need for a handshake packet from the hub, as the interrupt will not be retried in the same frame, if missed.

As with a Bulk transaction, the Complete Split will elicit a handshake packet from the device if it responded with one itself, or with ERR if there was an error on the full or low speed bus, or NYET if the Transaction Translator has not completed its task.

Split Interrupt OUT Transaction

Split Interrupt IN Transaction

For a split interrupt IN, we issue a Start Split (without expecting a handshake from the hub).

The Complete Split may result in the complete data packet (DATA0 / DATA1) being returned, but it is also possible that during the microframe, only part of the data was so far collected, in which case an MDATA PID is used to indicate that a further Complete Split will be necessary in the next microframe.

Handshake packets NAK or STALL will indicate that the device responded with this packet. ERR indicates that there was an error on the full or low speed bus. NYET indicates that the Transaction Translator has not completed its task.

Split Interrupt IN Transaction

Split Isochronous OUT Transaction

On the face of it, this is the simplest split transaction. There is no handshake from the hub, and no Complete Split, as the delivery of isochronous transactions is not checked.

It is complicated by the fact that single full speed isochronous transactions are divided up into separate Start Split transactions of at most 188 bytes each, sent one per microframe.

The Start Split packet has its Start and End bits set to identify which part of the payload is being transferred.

Split Isochronous OUT Transaction

Split Isochronous IN Transaction

For a split isochronous IN, we issue a Start Split (without expecting a handshake from the hub).

The Complete Split may result in the complete data packet (DATA0) being returned, but it is also possible that during the microframe, only part of the data was so far collected, in which case an MDATA PID is used to indicate that a further Complete Split will be necessary in the next microframe.

ERR indicates that there was an error on the full or low speed bus. NYET indicates that the Transaction Translator has not completed its task.

Split Isochronous IN Transaction

Summary

We have examined how each type of data transfer works at high speed.

Coming up...

...to be announced...