I understand this is a simple question considering the quality of this forum, but i need you help to clarify on some of the concepts of ATM.
I understand ATM is a Layer2 Technology which has many benefits over other technologies like LL, FR etc. What i want to know is how is ATM terminate on a router. Does the router need a separate ATM module to support it? Is the ATM/ IMA interface on a router a physical or a logical interface. Any good links that give some good description of this technology would be greatly appreciated. Thanks in advance.
Agree with Maria, i LOL my self :), but its very nice to meet people looking into such details, i mean having an engineering point of view.
1. SAR should be handled at the device responsible to send non-ATM traffic out of its ATM interface as ATM traffic, which can be a CPE or a provider Core router with an ATM interface, it is an essential process for any ATM device and usually done by a dedicated chip (SAR engine).
2. To add another comment to Maria's, ATM suffers design complexity in the hardware design, for example faster SAR engines are more complex and expensive, which causes the ATM technology to be speed limited and expensive as a WAN technology for SP backbones, plus the complexity of managing devices of multiple various technologies.
3. The most common layer 1 technology used with ATM is SONET/SDH (fiber interfaces) as illustrated by Maria.
4. The main factor that has driven ADSL to adopt the use of ATM was the ability to provision multiple PVCs on the same physical link (which can't be done via PPP alone), more over the invention of PPPoA and PPPoE solved the issue of requiring PPP (with all its addons) over such implementations.
I laughed out loud when I read this last post of yours, because although you claim to be confused, it seems to me that you got straight into the point of this whole ATM story and even more. Tough questions you put into the table! :-)
1. In most cases the answer is yes. Inside the provider network SAR is done at their own routers as well for their own point-to-point backbone VCs (that have nothing to do with direct customer physical connectivity).
2. The networking history is full of stories of the pretty-cool-new-technology that takes over the fading-away-legacy-technology. ATM has been in both of these categories. Besides the economical interests of networking device companies to keep pushing new things for us to buy and the intellectual-economical interests of researchers to justify their existance, sometimes those new technologies do have something to offer. Not that much that the marketing claims, but a few new things are there. From a technological standpoint in many cases it seems like going back-and-forth or in circles. MPLS was supposed to be fast, but nobody puts it in their network for this reason. The providers for example, see MPLS as an opportunity to sell new services. The driving factors for new things are supposed to be cost, speed, scalability to name a few. Yet, nothing new comes without a cost. High speed networking and high performance anything is full of comprimises.
3. ATM is L2 technology. There are various options for its L1. One common option is optical interface on the ATM module card where 2 optical cables are attached (trasmit/receive):
Another cabling option is coaxial with BNC connectors : http://www.cisco.com/en/US/docs/interfaces_modules/port_adapters/install_upgrade/atm/pa-a3_ATM_install_config/5117ovr.html#wp1038876
4. ADSL is L1 technology. To speed up deployments, ATM was specified as the L2 used over ADSL. (For Ethernet both L1 and L2 specifications exist.). The PPPoE, PPPoA and friends are there for the logical termination of the customer connection. While you physical ADSL connection terminates on a DSL port of a card in a DSLAM, your logical connection goes further in the network to get IP address and other information and terminates logically somewhere else. By the way, your home telephone cable is another type of cable that can be attached in an ATM interface (since ATM is L2 for ADSL, but others could be as well).
I hope you did not really mean that this was just a starter. :-)