- On 7/24/06, [EMAIL PROTECTED] <[EMAIL PROTECTED]> wrote: > > > ---------- Forwarded message ---------- > From: Heo Heo <[EMAIL PROTECTED]> > To: ns-users@ISI.EDU > Date: Sat, 22 Jul 2006 17:18:26 -0700 (PDT) > Subject: [ns] Wireless packet size limit + wireless bandwidth question > Hi all, > > I was doing a simple wireless simulation in which 2 nodes send are > placed next to each other, with one being the sender and the other being > the receiver (based on the Simple.tcl on ns website). The sender sends > 500 packets to the receiver. > > 1) I tried to change the packet size from 512 to 1024 bytes. I parsed > the trace file I find out that: > - If the packet size is 512 bytes, 500 packets were sent. > - If the packet size is 1024 bytes, 1000 packets were sent. > > It seems to me that some layer has automatically split up the 1024-byte > packets > into smaller parts. > > So, my question is: what is the default wireless packet size, and how > to change it ? and Which layer actually split up the big packets ?
The network layer. The splitting-process depends upon the MTU (Maximum Transmission Unit) used by the network layer of the network from which the packet passes. For a very draft explanation, this is the default maximum packet size that a sender can put onto the channel for transmission, which has been agreed according to a maximum. For example, the Internet de facto standard for the MTU is 576 bytes, while for Ethernet networks the maximum is 1500 bytes (to account for typical TCP packets). However, this can change per connection, according to the initial 3-way handshake parameters exchange. The NS-2 simulator uses an MTU of approximately 576 bytes, so this is probably the reason of the packet-doubling. For more information concerning the MTU, check the following site: http://searchnetworking.techtarget.com/sDefinition/0,,sid7_gci213605,00.html and http://searchnetworking.techtarget.com/sDefinition/0,,sid7_gci830944,00.html 2) My next question is about the wireless bandwidth. > > By default, ns uses a wireless link bandwidth of 2 Mbps. By > changing the packet interval, I have at most 670 Kbps. Because there are > only > 2 nodes, the 2 nodes must be using full bandwidth for this. Given that > there are some overhead (RTS/CTS, packet headers, etc.), I would think > I should get about 1.5Mbps. So, why is the actual speed so much lower > than that ? Is it natural ? Did you account for re-transmission errors and the error rate in general? It is a very common thing the error rate factor to be somehow high in wireless networks. Thanks a lot for your help. Hope that I've helped you. -Fk Heo > > > > > - > -- Filippos N Kolovos Software Systems Analyst & Engineer M.Sc. (Eng.) in Data Communications Automation & Networking Department University of Macedonia Library Egnatia 156, P.O.Box 1591 540 06 Thessaloniki, Greece E-Mail: [EMAIL PROTECTED], [EMAIL PROTECTED] ----------------------------------------------
