Friday, July 3, 2009

Cisco IOS hints and tricks: What went wrong: end-to-end ATM

I enjoy reading Ivan Pepelnjak's Cisco IOS hints and tricks blog. Having been a partner in a state wide ATM wide area network that implemented end to end RSVP, his thoughts on What went wrong: end-to-end ATM are interesting.

I can' figure out how to leave a comment on his blog though, so I'll comment here:
I'd add a couple more reasons for ATM's failure.

(1) Cost. Host adapters, switches and router interfaces were more expensive. ATM adapters used more CPU, so larger routers were needed for a given bandwidth.

(2) Complexity, especially on the LAN side. (On a WAN, ATM isn't necessarily more complex than MPLS for a given functionality. It might even be simpler).

(3) 'Good enough' QOS on ethernet and IP routing. Inferior to ATM? Yes. Good enough? Considering the cost and complexity of ATM, yes.

Ironically, core IP routers maintain a form of session state anyway (CEF).
On an ATM wide are a network, H.323 video endpoints would connect to a gatekeeper and request a bandwidth allocation for a video call to another endpoint (384kbps for example). The ATM network would provision a virtual circuit and guarantee the bandwidth and latency end to end. There was no 'best effort'. If bandwidth wasn't available, rather than allowing new calls to overrun the circuit and degrade existing calls, the new call attempt would fail. If a link failed, the circuit would get re-routed at layer 2, not layer 3. Rather than  band-aid-add-on QoS like DSCP and priority queuing, ATM provided reservations and guarantees.

It was a different way of thinking about the network.

1 comment:

  1. Thanks additional thoughts. BTW, CEF is not session state, Netflow is.

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