Instead of Frame Relay frames, you have to look at the payload which is usually IP packets. Here is the formula that I would use: BW = IP bandwidth in bits per second (bps) PS = average IP packet size; typically 256 bytes AO = AAL5 Overhead = 8 bytes OH = overhead factor Ceiling((PS+AO)/48) * 53 = ------------------------ PS Ceiling((256+8)/48) * 53 6 * 53 = ------------------------ = ------ = 1.24 (for our 256B example) 256 256 EBW = effective ATM bandwidth in cells per second BW * OH = ------- 53*8 Thus for 512 Kbps IP bandwidth, you need 1497 cps ATM bandwidth assuming 256 byte average packet size. SCR should be at least this. The PCR could be the line rate or equal to SCR. The MBS could be 32 cells (1 MTU packet size at least) for low jitter applications and as high as 256 cells for bursty applications. -Sekar -----Original Message----- From: Brennan_Murphy@NAI.com [mailto:Brennan_Murphy@NAI.com] Sent: Thursday, January 09, 2003 3:08 PM To: nanog@merit.edu Subject: frame relay to atm conversion tool? I'm trying to locate a tool (eg, spreadsheet) to convert throughput on a frame circuit to an ATM circuit. So, for example if you have a 512k CIR with burst to 1024k and 80% of the time you get your burst bandwidth and you want to convert that link to ATM, what corresponding SCR/PCR/MBS values would you need. I'm less concerned with Frame vs ATM protocol overhead than I am with calculating the difference in the way the burst bandwidth is meted out. Any recommendations? Supposing no one knows of such a tool. Would anyone be interesting in helping me put one together? Best Regards, -BM