Do ATM-based Exchange Points make sense anymore?

William B. Norton wbn at equinix.com
Fri Aug 30 23:15:56 UTC 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):

------------------------------------------------------------------------------------------------------------------

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.

------------------------------------------------------------------------
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




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