I came across a decent Cisco article that discusses how to calculate traffic shaping parameters for links that are on one end ATM and the other Frame relay. http://www.cisco.com/warp/public/121/frf8_shaping.html The second to the last paragraph in that article suggests that ATM SCR's should be shaped at 15-20% higher than Frame CIR's. But there is no corresponding conclusion about how to equate burst space. There's been a few responses from people interested in helping get a tool together to make these calculations simpler and more accurate. I'm still seeking comments on this though so if you've got something to add, please do! Once I feel like we've choked out enough comments to get to work on this, I'll contact those interested parties offline about next steps. Thanks, BM -----Original Message----- From: Peter E. Fry [mailto:pfry@swbell.net] Sent: Thursday, January 09, 2003 10:46 PM To: nanog@merit.edu Subject: RE: frame relay to atm conversion tool? On 9 Jan 2003 at 17:45, Swaminathan, Sekar wrote:

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: [...]

Specifying an "average packet size" is rough: I've observed that on an "average Internet connection" around 35% of packets are 64 bytes, 35% 1500 bytes, and the remainder scattered about. The average is guaranteed to be a poor match for at least 70% of your traffic. Not that it matters. Most carriers configure FRATM VCCs as "1536kbps frame relay = 4500cps", treating fractions accordingly. The last thing you want to do is exceed this cell rate (around 1710kbps at a 1500-byte packet size), as on a policed link you'll lose nonconforming cells. (Actually, a 1536kb frame relay seems to be closer to a 1705kb ATM VCC. Eh, the IWF has to buffer some anyway.) If you increase your cell rates you potentially oversubscribe your frame link (and the IWF may disallow it in any case). So if you're tempted to exceed this ratio be very certain of your link characteristics. Add to that: when a sustained rate is defined (VBR service category) the ATM peak rate probably doesn't mean what you would expect, so I recommend sustained = peak. (ABR is the best match for data, but isn't often used.) As to the original question, I wouldn't recommend translating a burstable frame to ATM unless you can order an ABR VCC or get UPC and/or frame discard disabled on your link (don't bet on it). It's not likely you can match parameters, so without flow control you're essentially looking at reduced performance or packet loss under load. Whatever you do get, I recommend you ask your carrier for a spec. You have plenty of variables and relatively robust protocols, meaning you could take a shot, miss, and only find out for certain that you had when you'd really rather not. Get it reasonably right, though, and it'll do right by you. Huh. Now that I look at it, I need to rethink my own figures yet again (I haven't even implemented the last rethink) as I appear to have tended toward the conservative in some areas. Grrr. Take it easy. Peter E. Fry