North American Network Operators Group
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Re: Do ATM-based Exchange Points make sense anymore?

  • From: William B. Norton
  • Date: Fri Aug 30 19:21:23 2002 
At 01:13 PM 8/9/2002 -0700, Bill Woodcock wrote:
> Personally, I don't believe that ATM is 'bad' for
> shared-fabric exchange point. I mean, it works, and solves several
> problems quite easy: a) it's easily distributed via SONET services to
> folks who are not next to the ATM switch, b) it makes interconnection
> between networks safer (ie, not dealing with broadcast issues on a
> ethernet nap), c) virtual PI connections are easily accomplished, d) there
> are varying degrees of interconnection speed (agreeably, less important),

All of the above are true of frame relay as well, which has the additional
benefit of not being funamentally incompatible with data networking. :-)
You guys might find this interesting.... I'd like to share the more common "Religious debate points" regarding ATM-based vs. Ethernet-based IXes that I heard during the walk throughs (about 50 so far) of this paper (v1.6):

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ATM Advocate: First off, make sure you mention that ATM solves the key problem with "Broadcast Domain Internet Exchanges". Broadcast Domain Issues refers to problems when all ISP attachments are on the same Ethernet segment. Broadcast storms and other anomalies caused by one attached customer can adversely affects all others. Ethernet-based IX operators try and solve this problem administratively with MOUs Memorandum of Understanding (see the Appendix of the http://www.linx.net/joining/mou.thtml for an example of this) but it is solved in ATM by the private nature of PVCs, yielding a more stable peering infrastructure.

Ethernet Advocate: Ethernet-based IXes can address these "Broadcast Domain" issues technically via private VLANs and direct cross connects between members. The "nature" of ATM as you describe it has a high overhead associated with it, specifically, by statically allocating bandwidth to a peering session with a historically spiky cyclical traffic characteristic. This static allocation of bandwidth prevents the multiplexing benefits of aggregation of lots of peering traffic sources.

In addition to Ethernet-IXes being generally less expensive than ATM-based IXes, Ethernet interfaces are generally less expensive. This reduces the total peering costs, breakeven points, and increases the attractiveness of the Ethernet-based IX and therefore its likelihood of succeeding.

Finally, Ethernet is what ISPs know, it is what they love. This ubiquity of and familiarity with Ethernet reduces the costs of operation, since operations folks only need to know how the Ethernet stuff works.

ATM Advocate: Most Ethernet-based IXes primarily use the default LAN and are therefore subject to the "Broadcast Domain" issues.

Ethernet may be less expensive but it is not shaping the traffic as ATM does. You pay for that functionality and stability.

As for the operations argument, naturally one technology is easier to support than multiple technologies. This isn't a specific fault of ATM or Ethernet but rather and indication that choosing one and sticking with one is advantageous.

Ethernet Advocate: In the Ethernet-based IX model, private cross connects allow one to scale beyond OC-12, the maximum reasonable capacity of ATM. The OC-48 ATM cards cost $250K, making it unreasonably expensive for the exchange of Internet Peering traffic.

At the same time, the Ethernet-based model allows collocated folks to interconnect at Gigabit Ethernet, 10-GigE, whatever the peers choose. These direct connects are typically not allowed (for policy reasons) to occur at collocated ATM IXes.

And PVCs are not the same things as a PNI as there are active electronics in the middle, some thing that can break, obscure troubleshooting, and limit flexibility with respect to interconnect types.

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Interesting points, and although orthogonal to the analysis in "Do ATM-based Internet Exchange Points Make Sense Anymore?", I am including these in the appendix to show these alternate views of the world. Am I missing any of the major (fact-based) views?

Bill