ATM vs IP

[mcaranci]

Can any of you who work for VZ tell me if there are ATM switches in the core network?

Many data networking textbooks seem to diss ATM (asynchronous transport mode), mainly because it's not a creation of the IETF (internet engineering task force) and is only used deep within the public telephone network core.

From my reading of what ATM is and what it was intended to do, it should've gotten more "air play", but seems to be overtaken by IP-based networks.

Any thoughts?

[JoeSmith]

ATM and IP work on seperate layers...

[jumpin68ny]

ATM is transport.  IP traffic can traverse an ATM network.  They are two very different things.coo

To compare, ATM is transport like Ethernet.  IP runs over ATM or Ethernet.

[mcaranci]

What you say is only partially true, for ATM AAL5 (ATM Adaptation Layer 5) was designed by the internet community specifically for variable bit rate (VBR) handling data.

However, there is also AAL1 for constant bit rate (CBR) signals, e.g., voice or real-time video, and not bursty data.

So while ATM can carry data, it need not.

And while data can travel over an ATM network, it need not.

My understanding was that ATM is both a transport protocol, as well as a network protocol in its own right.

[lasagna]

The key here really is that the "Internet" as we know is an IP-based protocol network.   Regardless of the underlying transport, at the end of the day the end-to-end communication is over IP.   So while ATM has the ability to do some pretty interesting things, particularly in the voice and video world (particularly because of the small cell size), it's actually not the greatest thing when it comes to large size packet protocols like IP. 

A more interesting discussion might be around MPLS and the use of traffic prioritization and multiple VRF's  for delivering multiple network services to a home market.

[jumpin68ny]

Keeping it simple for the average person is the way to go.  We can always take technical discussions off line.  MPLS is some pretty cool stuff.  The company I work for is in the market for carrier class Ethernet Networking and no, I do not work for Cisco.

[mcaranci]

The internet community tends to disdain anything that smacks of connection-oriented networks (X.25, ATM, frame relay), due to the latter's roots in the telephone industry. 

However, Andrew Tanenbaum in Computer Networks, 4th Ed. states the following:

 To make a long story short, ATM ... is now widely used deep within the telephone system, often for moving IP packets. Because it is now mostly used by carriers for internal transport, users are often unaware of its existence, but it is definitely alive and well. (p. 45f)

With regard to MPLS:

Of course, labeling flows this way comes perilously close to virtual circuits. X.25, ATM, frame relay, and all other networks with a virtual-circuit subnet also put a label (i.e., virtual-circuit identifier) in each packet, look it up in a table, and route based on the table entry. Despite the fact that many people in the Internet community have an intense dislike for connection-oriented networking, the idea seems to keep coming back, this time to provide fast routing and quality of service. However, there are essential differences between the way the Internet handles route construction and the way connection-oriented networks do it, so the technique is certainly not traditional circuit switching. (p. 319)

And Radia Perlman, whose Interconnections, 2nd Ed. was written before there were any formal MPLS specs, opined:

… the MPLS working group … seems to be converging on basically reinventing ATM. (p.275)

I love that.