If SONET/SDH can carry IP, and ATM rides on SONET/SDH, then why do we need to encapsulate IP inside ATM?

That makes "IP over ATM over SONET/SDH". What's its benifit over "IP over SONET/SDH"?

Why do we need this extra layer? For transport? But doesn't SONET/SDH already provides us the transport network?


SONET is a physical layer protocol (layer-1). You need a datalink layer protocol (layer-2) to handle framing. That was originally ATM, because that was one of the dominant L2's used by carrier networks. (vs. frame relay, ATM easily mixes voice and data.) IP (layer-3) "directly" over SONET still needs framing. Packet-over-SONET (POS) uses PPP (actually PPP+HDLC, but nothing ever exposes the HDLC-like parts.) So what you might be thinking of as "IP over SONET" is actually IP over PPP over SONET.

(PPP has obvious efficiencies over ATM. A frame size greater than 53 bytes being key.)

  • Oh ok. What about telephony. Why does voice telephony need ATM if it can ride directly on SONET? – Noob_Guy Feb 13 at 12:13
  • It doesn't "ride directly" over SONET. Voice needs framing, too. ATM brings muxing, qos, etc. along with it. Remember, ATM was primarily designed for voice traffic -- short chunks to avoid delay and jitter. (slicing up a very large data packet into dozens of cells that could be mixed with higher priority voice traffic without adding enormous - for voice - delay was key.) [ a max ethernet frame would take 32 cells. a max IP packet could take over a thousand (1366), not that anything sends 64k packets ] – Ricky Feb 14 at 5:46
  • Oh, so ATM was developed for telephony. I thought the technologies that were developed for telephony were T-Carrier and SONET. So does it mean telephony will not work without ATM? – Noob_Guy Feb 14 at 8:22
  • xxxxx @Ricky ? xxxxx – Noob_Guy Feb 16 at 3:51

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