Hi Elad,
In
the scenario which you were talking about it is obvious that the PDU SN
extended sub-header attached to all other PDU's other than the first PDU
is redundant.
Let
us think of in this way, at the MAC level during the process of formation
of MAC PDU(MAC header, Payload & 16-bit MAC CRC) it doesn't know whether
this particular PDU will be the first PDU of the HARQ sub-burst or not.
Hence it must invariably attach the PDU SN sub-header. Only at the HARQ
level(Next to the formation of MAC PDU) if it identifies that this particular
PDU is not the first PDU of the HARQ sub-burst then it must remove the
PDU SN sub header because of which it'll end up in changing the MAC header
and 16-bit MAC CRC also.
Might
be to discriminate these two process the standard might have taken this
method.
Regards,
P.Muthukumar
Sr.Software Engineer
Communication and Embedded Systems,
Larsen &Toubro Infotech,
Bangalore,INDIA
Assume an HARQ sub-burst holding four MAC PDUs. They invariably have successive
PDU SN numbers (x, x+1, x+2, x+3). The sub-burst is protected by CRC-16.
To transfer the PDU SN information, the sub-burst uses 4 or 8 bytes of
information (for PDU SN short and PDU SN long subheaders, respectively.
But the same information could have been transmitted in just 1 or 2 bytes,
by attaching the PDU SN subheader only to the first PDU.
1. As far as my perception is concerned your
assumption towards the first question is right. Two cosecutive PDU's must
have two consecutive sequence numbers that's ofcourse its significance
is.
If two consecutive PDUs are numbered PDU SN x and PDU SN y, then
y= ((x+1)modulo 0xFF) incase of PDU SN short and,
y= ((x+1)modulo 0xFFFF) incase of PDU SN long.
2. My perception towards your second question,
In sec 11.13.35 of std 16e, it is mentioned that a single connection can
have more than one HARQ channels (sub-burst), hence a single connection
can have simultaneously operated multiple Stop&Wait protocols running
in (Not one) by which the sequencing of the MAC PDU packets is not ensured.
Hence extended SN is required in MAC PDU and can't be wasteful.
Rgds,
P.Muthukumar
Sr.Software Engineer
Communication and Embedded Systems,
Larsen &Toubro Infotech,
Bangalore,INDIA
I'm in need of some clarifications regarding PDU SN.
1. Section 6.3.2.2.7.8 says that PDU SN "Specify the PDU sequence
number in a monotonic increasing manner." If two consecutive PDUs
are numbered PDU SN x and PDU SN y, is it OK to assume y=x+1, or only that
y>x ?
2. If an HARQ sub-burst holds several MAC PDUs and the connection has PDU
SN enabled, the PDU SN extended subheader is added to every PDU in the
sub-burst. Isn't this wasteful?
Thank you,
Elad Eyal
Software Architect
Comsys
Tel +972 (9) 9717895
Fax +972 (9) 9717887
elad.eyal@comsysmobile.com
www.comsysmobile.com
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