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IP Over ATM

With the need for fast broadband outstripping available network technology, ISP’s were forced to deploy the only equipment available, ATM switching. This infrastructure equipment provided the required bandwidth, deterministic forwarding performance, forwarding capacity, and traffic engineering to support the immediate requirements, the solution becoming known as IP over ATM.

IP Over ATM

The IP over ATM overlay model consists of a logical IP routed topology running independently over an underlying ATM [Layer 2 switched] topology.
The functionality requires every system in the core network to have:

ATM software controls [signaling and routing]
Hardware forwarding [label swapping]

The model recognised the software based routers as the key source of poor network performance, limited the role of IP routing to the edges of the network. Using Layer 3 functionality at the edges of the network, it maximized network throughput by relying on high speed, label swapping ATM switches and PVCs [Permanent Virtual Circuits] in the core.

Scalability Issues

Transporting IP over an ATM network creates scalability and network administration problems. Each router has to be joined to each other router via an ATM VC.

Scalability issues arise due to the lack of scalability of a fully meshed network topology created by the exponential number [N[N-1]/2] virtual circuits [where N equals the number of nodes] required as the network grows. This is known as the ‘n-squared’ PVC problem.

Network Administration Issues

IP and ATM are two different technologies, with totally different characteristics, and developed for entirely different tasks. These differences include connectionless vs. connection-oriented, different routing protocols, different addressing schemes etc.

Hence the IP over ATM network requires double administration and configuration. This complexity is difficult for service providers to justify.

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