Hi Ryan, The MAC on the CC2420 is a CSMA MAC. This means that there is an initial and congestion backoff period that delays packets and results in lower data rate. It is common knowledge (and was proven in many Ethernet papers) that the maximum reliable achievable data rate on a CSMA channel with congestion is 85% utilization. For higher data rates, you will need to acquire the channel through a protocol of your choice, eliminate the backoff periods, and send your data continuously.
Futhermore, I would recommend reading the CC2420 datasheet. Before each packet, the radio must switch from RX mode to TX mode which takes 450 microseconds. Similarly, the radio automatically transfers back from TX mode to RX mode after transmission which requires an additional 450 microseconds. Thus, if your packet is 1.2ms long, then the total time required to send that packet is 2.1ms; however the channel is only used for roughly half that time. I have published a paper in last year's Sensys conference that shows the data throughput on the CC2420 under differing loads. The paper is called "A Unifying Link Abstraction for Wireless Sensor Networks" and is available at www.polastre.com/pubs.html -Joe Polastre Moteiv Corporation [EMAIL PROTECTED] On 8/17/06, Ryan Aures <[EMAIL PROTECTED]> wrote:
Hello all, We have been testing the Tmote Sky motes to see what is the maximum achievable data rate, and have gotten very discouraging results. Of the 250kbps that is supposed to be possible, we are only getting reliable transmission up to about 50kbps. Beyond this we start dropping packets, and can not get anywhere near 250kbps. We think the problem is on the transmission end, but it is difficult to tell where packets are being dropped, and what is happening at the radio. Is this a radio problem, a TinyOS problem, something else...? It seems like several people have encountered this problem, does anyone know what is really happening here? We have also tested the beta release of TinyOS 2.0 but there seems to be no improvement. Any advice would be greatly appreciated. Thanks Ryan Aures University of Rochester Department of Electrical and Computer Engineering SENSORS group _______________________________________________ Tinyos-help mailing list [email protected] https://mail.millennium.berkeley.edu/cgi-bin/mailman/listinfo/tinyos-help
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