Hi Frank,
> Are you suggesting that I send a SOB tag along with the Flag
0x7E (1 byte) and the radio will continuously output 0x7E's until I
send an EOB ?
Whatever "flag 0x7e" is, no, that's not how it works. You specify a
transmit time at the start of your burst, and that's the time that
the USRP will start transmitting the burst. You need to supply the
samples that it will transmit. Setting another tx_time later on will
define a "stop and wait for that time to come before continuing to
transmit".
Note that I have no actual clue of HDLC; I was presuming it was a
very packet-oriented thing?
The project I am on is a cubesat project and the team working on
the cubesat have stipulated that to maintain lock on the signal,
when not transmitting data the ground station transmitter (my
work) is to send continuous HDLC Flags. So regardless of my view I
need to somehow meet this requirement.
That conflicts with my impression that HDLC is a packet format with
sufficient framing? Because then I don't really see a possibility of
"continuous HDLC flags"; just closely enough timed HDLC frames that
help to maintain channel state information on the ground, right?
In that case, I'd expect that these "keepalive" frames are just a
special kind of data frame, and you'd basically have a timer
somewhere that runs out occasionally, initiating a frame
transmission if it does, and that gets reset to 0 every time a frame
was sent; or something similar.
Sorry for taking up so much of your time.
Your assistance is appreciated.
Hey, sorry if I sometimes come across a little harsh :) I was just
really confused by the whole aspect of that obsolete thread.
So, let's give a quick overview of what I understand, so that we
have a good base for discussion:
- You're implementing the satellite-side end of a HDLC comms
line
- You'll have two types of packets
- actual payload packets
- packets only meant to keep sync with the ground station
So, the current situation is this:
Assuming you want some kind of GMSK modulation, this would "work"
for you:
so what happens here is
- The message strobe generates a message every 500 ms containing
a pair with an arbitrary value in the first element (the HDLC
framer demands this, but doesn't use it), and a byte vector
(ascending values) in the second element.
- The HDLC framer has a message port, which means it can
asynchronously get messages (asynchronous to the work() )
function; every time the work() is called, the HDLC framer
checks the new-style message queue (that's a bit different from
the old-style messgae queues you'll find in old GNU Radio).
Sadly, in GNU Radio's current version (3.7) this means that it
"spins"; but that should, as far as I can tell, change with 3.8.
(I'm testing this right now)
- if there's a message in the queue, get it, turns it to a
HDLC frame, and adds an output stream tag to the first
produced sample telling the downstream blocks "hey, here
starts a frame of N items"
- the GMSK modulator takes one item and turns that into 8
complex samples (hence the "tagged stream multiply length tag"
block – the length tag doesn't reflect the correct length
anymore)
- The USRP sink has been set to "wait for a frame (tagged stream
block) tag" mode, and will send a burst of specified sample
number upon receiving an appropriately named tag
Best regards,
Marcus
On 15.08.2016 13:34, Inspire wrote:
Hi Marcus
Thank you for the response. I will take your advice and go
over all the tutorial information again as well as [2] &
[3].
However I have completed the tutorial's and for the most
part understand the information. Admittedly I am still
struggling with the use of stream tag's, I have gone back over
the tutorial on stream tags multiple times and whilst I
understand how to create stream tags and their propagation, I
haven't really grasped how and when to use them. I have also
read [2] but don't really understand how this will help me.
Please excuse my lack of understanding. Are you
suggesting that I send a SOB tag along with the Flag 0x7E (1
byte) and the radio will continuously output 0x7E's until I
send an EOB ?
The project I am on is a cubesat project and the team
working on the cubesat have stipulated that to maintain lock
on the signal, when not transmitting data the ground station
transmitter (my work) is to send continuous HDLC Flags. So
regardless of my view I need to somehow meet this requirement.
Sorry for taking up so much of your time. Your assistance
is appreciated.
Kind Regards
Frank
View this message in context: Re:
Constant carrier digital transmission
Sent from the GnuRadio mailing list
archive at Nabble.com.
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