On Jul 31, 2006, at 5:58 AM, Matthew J Whelan wrote:
Henric,
The maximum theoretical data rate of the CC2420 is 250kbps.
However to achieve anywhere near this rate, you have to carefully
schedule transmissions to avoid packet collisions and loss. I
can't quote you any rates achievable using any of the standard
radio libraries or base applications, as I have had some problems
with transparency of the code and would rather write my own than
try to figure out someone elses. To be honest, I have had a
problem using the base applications provided with TinyOS when
sending at a high rate from multiple motes and I would very much
recommend that you write your own base application if you intend to
push a lot of data across the radio.
Using my own base application, I have recieved data from 10 motes,
sending 99byte packets at a rate of 7 packets per second (approx.
700bytes/mote/sec or 70kbps total) with virtually no packet loss at
all (nearly all motes have 100% success after 5-10 minutes). I am
convinced that the radio can handle MUCH HIGHER rates than that,
even the 250kbps, though it takes careful software development and
knowledge of the CC2420 chip. I need to invest in more motes
before I push my network to the limits in terms of data rate, but I
thought that I would let you know that the numbers you have seen
have grossly underestimated the radio data rate.
A lot of it comes down to MAC backoff times.
Out of the box, a TinyOS 1.x or 2.x node can send about 240 pps,
where each packet has a 28 or 29 byte payload. If you do not obey
CSMA backoff, 1.x can get about 300 pps and 2.x can get something
like 600 pps. Boomerang, IIRC, can get about 500 pps when it batches
transmissions (disables CSMA).
On the software side, the limiting factor is usually reception, not
transmission. That is, the pps rate isn't limited by how fast the
transmitter can transmit them, but by how fast a receiver can receive
them. E.g., 1.x can send ~600 pps, but can only receive around 300
before the receive path runs into issues (the FIFO flush in response
to multiple packets being buffered).
These numbers are all CC2420 specific.
Phil
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