qpc of CIP in SBP2

ScotSmyers at aol.com ScotSmyers at aol.com
Fri Jun 27 05:50:17 PDT 1997


* From the SCSI Reflector (scsi at symbios.com), posted by:
* ScotSmyers at aol.com
*

In a message dated 6/26/97 10:17:55 PM, rlash at QNTM.COM (Rob Lash) wrote:
>My interpretation here is that complete data blocks are transmitted
>during an 
>isochronous cycle and not partial data blocks and therefore all data
>blocks are 
>indeed *full*.

This is correct.

The size of a source packet depends on the application, but all source
packets must fixed in size.  In the case of MPEG2, for example, a source
packet is 188 bytes.

A source packet may have a source packet header prepended (SPH=1) and it may
have padding added at the end (QPC != 0).  The resulting packet (SPH plus
source packet plus padding quadlets) may then be divided by 1, 2, 4 or 8
(FN=0, 1, 2 or 3 respectively) to form data blocks.  Only whole data blocks
are transmitted in an isochronous packet, beginning immediately after the CIP
header.  The DBC field in the CIP header (Data Block Counter field) counts
data blocks as they are transmitted.  The value in the DBC field in each CIP
header contains the count of the next data block to be transmitted.  If there
is more than one data block in an isoch packet, subsequent data blocks are
numbered DBC+1, DBC+2 and so on.

As another example, if FN =3, then all data blocks whose number is modulo 8
(i.e., low order 3 bits of the data block number =000) contain the start of a
SPH plus source packet plus padding concatenated block.  This makes an easy
way to locate source packet headers.

Scott Smyers
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