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Re: [Discuss-gnuradio] One question regarding "-B FUSB_BLOCK_SIZE" and

From: Thomas Tsou
Subject: Re: [Discuss-gnuradio] One question regarding "-B FUSB_BLOCK_SIZE" and "-N FUSB_NBLOCKS"
Date: Mon, 28 Sep 2009 15:21:04 -0400

On Mon, Sep 28, 2009 at 1:50 PM, Bishal Thapa <address@hidden> wrote:
> Dear Tim,
>  First of all, thanks for your time answering my question. Secondly, one
> point you left out is: I am sending 104*128 bytes in reality, because I am
> spreading a packet of size 128 bytes with a 104 bits long spreading sequence
> right inside the pkt.py file in the
> gnuradio-core/src/python/gnuradio/blk2imp/pkt.py location. Does not that
> mean I am not running into the USB block-size issue?

I do not believe you are running into USB block size issues. By
default, the Linux usrp implementation will submit to the kernel all
data passed to it in the write call.

> Lastly, question to the list and for Tim also, what does -N and -B options
> in ./benchmark_tx.py script represent? It says it is the block size for fast
> usb option and number of fast usb blocks in the description, what does this
> mean? Please help me understand. Thank you.

Loosely speaking, FUSB_BLOCK_SIZE and FUSB_NBLOCKS refer to the
individual size and number of outstanding asynchronous transfers that
can be submitted to the underlying USB interface. The defaults
(specified with 0) use operating system dependent values. For example,
for Linux the default block size is 16-kb, which is then broken up by
the kernel into 512-bytes payloads that are sent over the hi-speed USB

Other combinations are possible. These include values less than or
non-multiples of 512-bytes, but this results in poor use of limited
USB bandwidth and ineffective use of existing code based around
512-byte payloads. For most general use cases, the default values
should suffice.

Hope that helps.


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