Anand, either you clocks are off a lot more than you think or you have not set-up the test instrument correctly. Ethernet has a minimum interpacket delay that both prevents 100% utilization and the problem you describe. Steve. Anand Srinivasan wrote: > Thanks Steve, > As you said that ethernet is designed to use independent clocks. The > problem that I am seeing is due to the fact that I am transmitting at > 100% line rate all the time for testing purpose and am seeing the fifo's > overrunning. (though in real systems ethernet will be a bursty traffic). > I have a strange observation that in none of previous switches did I > have to reduce the ppm in IXIA to trasmit data at 100% line rate, but I > am required to do so in this switch. I believe that in any set-up there > be a clock variation between the sending and receiving parties. At 100% > line rates shouldn't this accumulation lead to overruns or underruns on > the buffers or fifo's all the time? > > Thanks, > Anand > > > -----Original Message----- > From: steve weir [mailto:weirsi@xxxxxxxxxx] > Sent: Wednesday, April 16, 2008 10:15 AM > To: Anand Srinivasan > Cc: Si-list@xxxxxxxxxxxxx > Subject: Re: [SI-LIST] ppm related question > > Anand, > > 1. Yes, a PLL generates a slave clock that tracks the reference within > the bandwidth of the loop, subject to disturbances affecting the VCO. > 2. No, variation means any deviation from the reference up to and > including the specified limit. Usually this is unsigned as for example: > > +/-50ppm > 3. Yes, there are many SI and PI phenomena that impair clock timing. > 4. Yes, PLL, DLL etc. > > Ethernet is designed to use independent rather than hierarchical clocks. > > You should not need to lock clocks between stations. > > Steve. > > Anand Srinivasan wrote: > >> Hi All, >> I have an IXIA packet generator/analyzer providing ethernet data >> > streams > >> to my switch. I am finding that the switch is receiving data at a rate >> that is slightly higher than the rate at which switch is transmitting >> data. I attribute this to some clock ppm variations in the switch. I >> have some questions related to the ppm variation in clocks: >> >> 1. If i have a clock of X MHz and if i feed this clock to a pll to >> > get > >> a clock of freq Y Mhz, will the ppm variation of the clocks X and Y >> remain the same? >> 2. When we state that there is a +100 ppm variation from X MHz in the >> clock, does this mean that the clock now is a constant frequency of (X >> MHz + 100ppm)? or does it dynamically vary between X MHz and (X MHz + >> 100ppm)? >> 3. Would signal integrities issues on the board affect ppm variations >> > in > >> clocks? >> 4. Is there any on-board correction methodologies that can be applied >> > to > >> adjust the ppm variations in clocks? >> >> Thanks, >> Anand >> ------------------------------------------------------------------ >> To unsubscribe from si-list: >> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field >> >> or to administer your membership from a web page, go to: >> //www.freelists.org/webpage/si-list >> >> For help: >> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field >> >> >> List technical documents are available at: >> http://www.si-list.net >> >> List archives are viewable at: >> //www.freelists.org/archives/si-list >> or at our remote archives: >> http://groups.yahoo.com/group/si-list/messages >> Old (prior to June 6, 2001) list archives are viewable at: >> http://www.qsl.net/wb6tpu >> >> >> >> >> > > > -- Steve Weir Teraspeed Consulting Group LLC 121 North River Drive Narragansett, RI 02882 California office (866) 675-4630 Business (707) 780-1951 Fax Main office (401) 284-1827 Business (401) 284-1840 Fax Oregon office (503) 430-1065 Business (503) 430-1285 Fax http://www.teraspeed.com This e-mail contains proprietary and confidential intellectual property of Teraspeed Consulting Group LLC ------------------------------------------------------------------------------------------------------ Teraspeed(R) is the registered service mark of Teraspeed Consulting Group LLC ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.net List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu