Hi Joe, I calcualted all overhead according to IEEE 802.15.4 2003 version as follows:
1. Turn around time, aTurnaroundTime is 16 symbols specified in the standard. Since the symbol rate is 62.5 Ksymbols/second, aTurnaroundTime = 16 * (1/62.5) = 192 us. (where did you get 450 us?) 2. IFS (Interframe Spacing) Since I used packet size > aMaxSIFSFrameSize (=18 byre), IFS = aMinLIFSPeriod = 40 symbols = 40*(1/62.5) = 640 us. 3. Average Backoff Time = 1.8 ms, as calculated in the last email. 4. ACK I am not sure of ACK is enabled or not. just suppose it is enabled. Tack = 11 bytes / 250 K = 352 us 5. packet transmission time = 3.84 ms Therefore, the total overhead for one packet is: AverageBackoff + aTurnaroundTime + Tack + IFS + aTurnaroundTime = 1.8 + 0.192 + 0.352 + 0.64 + 0.192 = 3.176 ms (according to Fig. 16 on page 142 in the standard) The achievable throughput should be: 3.84/(3.84+3.176)*250K = 136.8 Kbps >> 35 Kbps. Am I missing any delay/overhead? Thanks. Chonggang ----- Original Message ----- From: Joe Polastre <[EMAIL PROTECTED]> Date: Wednesday, July 11, 2007 2:33 pm Subject: Re: [Tinyos-help] throughput of tmote sky To: Chonggang Wang <[EMAIL PROTECTED]> Cc: Philip Levis <[EMAIL PROTECTED]>, [email protected] > Remember that the radio takes 450us to switch from RX>TX and another > 450us to switch back from TX>RX. There are other associated delays. > > -Joe > > On 7/11/07, Chonggang Wang <[EMAIL PROTECTED]> wrote: > > Thanks for the posting, Phil. Yes, There is a backoff in CSMA-CA. > However the backoff is not that much to make 250 Kbps down to 35 > Kbps. The reason is below. > > > > According the IEEE 802.15.4 standard 2003 version, the average > backoff time will be (2^BE * aUnitBackOffPeriod * aUnitSymbolTime) > /2 . Also according to the standard, aUnitBackOffPeriod = 20 and > aUnitSymbolTime = 1/SymoblRate = 1/ 62.5K = 16 us (for 2.45 GHz > PHY). In this experiement, there is no collision, therefore BE = > macMinME. macMinME is between 0 and 3. If it is 0, there is no > backoff. Let us choose the maximum 3 and BE=3. > > > > Therefore, for each packet, the average backoff time is about > (8*20*16)/2 = 1.28 ms. In my experiment, the UDP packet length is > 110 Bytes and consider 10 bytes or so overhead, it is 120 bytes in > total, and the consumed time is 120*8/250K = 3.84 ms. > > > > Therefore the achieved throughput should be around > (3.84/(3.84+1.28))*250K = 187.5 Kbps. Am my understanding right? > > > > 187.5 Kbps is still far larger than 35 Kbps. I still do not know > the reason. > > > > Thanks, > > Chonggang > > > > > > ----- Original Message ----- > > From: Philip Levis <[EMAIL PROTECTED]> > > Date: Wednesday, July 11, 2007 1:57 pm > > Subject: Re: [Tinyos-help] throughput of tmote sky > > To: Chonggang Wang <[EMAIL PROTECTED]> > > Cc: [email protected] > > > > > On Jul 11, 2007, at 10:48 AM, Chonggang Wang wrote: > > > > > > > Hello, > > > > > > > > I am trying to test the pure throughput of 802.15.4 and conduct > > > an > > > > experiment as follows: > > > > > > > > \/ \/ > > > > | | > > > > ----------- ------- -------- > > > > ----------- > > > > | PC1 | |Mote1| |Mote2| | > PC2> > | > > > > | |-----| | | > > > > |---------| | > > > > ---------- ------- -------- > > > > ---------- > > > > > > > > > > > > Both motes are placed close (guarantee good channel condition) > > > and > > > > configured as Base Station by being installed TOSBase > > > application. > > > > There is a sender program at PC1 and a receiver program at PC2. > > > > Both PC1 and PC2 start SerailForwarder (SF) (I changed the > > > original > > > > SF in order to support UDP and avoid the effect of TCP > Congestion> > > > > > Control and ACK). The baud rate for both motes is 115200. > > > > > > > > Then the sender program at PC1 continuously send a file (large > > > > enough) to SF at PC1 through UDP socket. The receiver program at > > > > PC2 listens to the SF's UDP port at PC2 and calculate the > > > received > > > > data rate. I repeated this experiment for lots of times and > found> > > > > > the received data rate at PC2 is only about 35 Kbps. Since tmote > > > > sky uses chipcon 2420 with 250 Kbps data rate, 35 Kbps is > > > > surprisingly pretty slow since in this case there should not be > > > any > > > > collision. I am not sure if those two motes automatically go to > > > > sleep state. Also i did not find any bad packet due to CRC > > > failure > > > > in SerialForwarder. > > > > > > > > Could any one give me a clue? > > > > > > MAC backoff. > > > > > > Phil > > > > > _______________________________________________ > > Tinyos-help mailing list > > [email protected] > > https://mail.millennium.berkeley.edu/cgi- > bin/mailman/listinfo/tinyos-help > > > _______________________________________________ Tinyos-help mailing list [email protected] https://mail.millennium.berkeley.edu/cgi-bin/mailman/listinfo/tinyos-help
