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?

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    – Ron Maupin
    Commented Dec 23, 2021 at 16:29

2 Answers 2


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
    Commented Feb 13, 2021 at 12:13
  • 1
    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
    Commented Feb 14, 2021 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
    Commented Feb 14, 2021 at 8:22
  • xxxxx @Ricky ? xxxxx
    – Noob_Guy
    Commented Feb 16, 2021 at 3:51

ATM provides switchable L2 and PPP provides point-to-point L2. If your requirement is a link connectivity between two points then you can use PPP with HDLC framing. If you plan to build an L2 network then ATM is your option. ATM VCI can be used to switch its payload within your transport (SONET/SDH) network.

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