Re: A tricky problem concerned with fragmentation and Logical link Layer service.

On Aug 7, 1:07 am, Rich Seifert <use...@xxxxxxxxxxxxxxxxxxxxxxx>
wrote:
In article <1186373802.476534.296...@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,

lander <lostlander....@xxxxxxxxx> wrote:

Thanks very much for replying!!!

I'm not sure which standard you refer to, I'm refering to DSRC
european stanadard. However, the layer2 should be the same with what
you refer to, i.e connectionless-acknowledged(LLC-3). I'm not sure
about whether Layer7 is the same.

It doesn't matter whether the two systems use the identical Application
protocol; whatever protocol is used, it must assume responsibility for
both fragmentation and reliable delivery in your stated architecture.

Now I've got several problems below:
1, Nevertheless, you said that upon receipt of RSU's command, OBU's
LLC should respond with an ACK immediately, but the standard doesn't
specify this.

Yes, it does. This is covered by the LLC standard (IEEE 802.2), not the
DSRC standard.

2, You said that Upon receipt of the OBU's LLC-3 ack, Layer7 sends the
next fragment, how should RSU's LLC informs RSU's Layer7? The standard
doens't seem to specify this kind of primitive.

Again, check the 802.2 LLC-3 specification; all of the layer primitives
are provided there.

3, You said the layer7 should be in charge of restrasmitting commands,
the standard doesn't speicify it too.

The standard does not state this because it does not have to. *IF* your
application requires reliable delivery (i.e., retransmission in the
event of a discarded frame), then the application must provide this
function, since it is clearly not provided by the lower layers. Note,
however, that this is an *application* requirement, not related to the
architecture.

If the application does *not* require reliable delivery (many such
applications exist--think of idempotent routing protocols), then the
application need not retransmit in the event of a discarded frame. That
is, an application that can tolerate occasional loss can ignore
retransmissions; an application that cannot tolerate such losses must
compensate by retransmitting. It's all up to the application.

4, And, I've looked log files of layer2&&layer7 of the current
implementations(i.e QFree), there is no independent ack from OBU's LLC
if the request is a "Data Exchange"(P=1). In the case of "Data
Transmitting"(P=0), it is okay!!!

I have no idea what you are talking about on this point. I am not
familiar with the implementation you describe.

--
Rich Seifert Networks and Communications Consulting
21885 Bear Creek Way
(408) 395-5700 Los Gatos, CA 95033
(408) 228-0803 FAX

Send replies to: usenet at richseifert dot com

Thanks very much for your reply.

Now, the problem can be concluded as such:

(For clarification: RSU can only send command(i.e request with respect
to Layer7), OBU can only send response))

1, From my understanding, when the OBU's LLC-3 receives a command from
RSU to request a response from OBU, it doesn't send back the ack
immediately, instead, it will pass the LLC's SDU up to OBU's Layer7
for further processing, then OBU's LLC-3 waits for OBU's layer7 to
pass down a response(L7 uses the DL-REPLY-UPDATE primitive) which will
then be transmitted by OBU's LLC immediately (and the ack is sent, i.e
piggybacked by the LLC SDU).

2, But from your description and the LLC-3 spec, the OBU's LLC-3 will
send back the ack immediately upon receipt of RSU's LLC-3 command. If
this is true, the OBU will send two frames for a independent request
from RSU, one is the 'ack', the latter is the actual 'response'. In
order to send the latter one, the OBU has to request a new
window(timeslot) since the previous 'ack' has used the window
allocated by the RSU's LLC-3 command. Or if the window allocated
hasn't expired before the latter actual 'response' is prepared, it
will be transmitted with the 'ack' in the same window.
****I'm sorry if the text decribed has something you're not familiar
with, especially the window(timeslot) which is stated in the MAC layer
of DSRC)***

What do you think of point1 vs 2?

Thanks in advance!!! ^_^

.

Relevant Pages