Emulating ADSL bandwidth shaping

Frank Bulk - iName.com frnkblk at iname.com
Tue May 4 20:26:29 UTC 2010

We're an ISP that has four access technologies.  Both cable and DSL modem
link times are affected by configured rate and sync rate, respectively.  

My home CM is at 15/1 Mbps and one-way latency is 4 to 5 msec.  My home DSL
modem is at 15/1 Mbps (with interleaving) and has a one-way latency of 15 to
16 msec.  And FTTH at 15/1 Mbps is about 2 msec.

In regards to burst mode, the cable modem file specifies how many bytes are
given that top speed, not time.  If the port is heavily utilized, "top"
speed may not be attained during that burst session.


-----Original Message-----
From: Patrick Giagnocavo [mailto:patrick at zill.net] 
Sent: Monday, May 03, 2010 10:19 PM
To: Srikanth Sundaresan; NANOG
Subject: Re: Emulating ADSL bandwidth shaping

Srikanth Sundaresan wrote:
> I'm trying to model ADSL access link bandwidth shaping. With a link of
> 18Mbps, I'm using a token bucket filter (tc + netem) to model 10Mbps,
> 8Mbps and 2Mbps access plans. I have a couple of questions:
> - do ISPs typically use token bucket filters with large bursts to shape
> - what kind of burst sizes and latencies/limits are typically used for
> the filter?

You will definitely have to account for latency.

For emulating cable traffic, latencies (in the USA) will be about
60-80ms to typical sites.  Burst mode in my experience occurs only for
about the first 15 seconds, then is throttled back (though not always;
seems to depend on time of day).

For DSL, I seem to recall latency being about 90-110ms (note, I haven't
used DSL in many years).  Burst mode was generally not noticeable or
available, that is, you got the same speed regardless of downloading a
1MB jpeg or a 640MB .iso  file.



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