If there is IP fragmentation, is there also Ethernet fragmentation?

I tried to google it but there seems to be no such thing.

So what if, for example, we encapsulate Ethernet into ATM, what would the ATM payload look like?

The payload size of ATM is only 48 bytes, but the minimum frame size of Ethernet is 64 bytes. How can we fit an Ethernet frame into the payload of an ATM frame?

Or what if, for example, we encapsulate Ethernet into a T1 frame;

A T1 frame is only 193 bits, with 1 bit of header and 192 bits (24 bytes) of payload. How can an Ethernet frame fit inside the payload of a T1 frame?


If there is IP fragmentation, is there also Ethernet fragmentation?

No, there's no such thing.

Of course, you can break a frame into multiple pieces when transporting over another protocol like ATM. The frame splitting and reassembly would need to happen somewhere in between the original Ethernet transport and that other protocol. However, Ethernet has no such mechanism, so you can't do it natively.

  • "...but on Ethernet itself you cannot do that." What do you mean by that? You first said we can break a frame into multiple pieces, but then not Ethernet? – Noob_Guy Apr 18 at 2:48
  • @Noob_Guy I've tried to clarify that a bit. – Zac67 Apr 18 at 6:43
  • Oh so you mean, if we were to encapsulate Ethernet into ATM, then there must be a protocol in between Ethernet and ATM (in terms of OSI model or TCP/IP model) that will handle the fragmentation and reassembly of Ethernet frame? – Noob_Guy Apr 18 at 7:31
  • Not necessarily a protocol in between, but some kind of mechanism between Ethernet and the underlying transporter (depending on the perspective, you could also call that tunneling, as Ron has pointed out). – Zac67 Apr 18 at 10:05

Ethernet interfaces specify a Maximum Transmission Unit (MTU), which defines the maximum frame size it can transmit. If the interface receives a frame larger than the MTU, it will drop it.

Ethernet and ATM are both layer 2 protocols, so normally you would use one or the other. It is possible to encapsulate Ethernet in ATM (see RFC 1483), but in that case, you are essentially tunneling Ethernet over another L2 protocol.

T1 CAS is a circuit-switched connection. It uses a different layer 2 protocol (some version of HDLC) for IP packets.

I'll also remind you that ATM and T1 are nearly extinct. You can study them for historical interest, but you are unlikely to ever see it in the real world.

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