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Sorry if this is a stupid question.

Given you could observe signals going through RJ45 pins, would you see single TCP packets or their fragments?

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  • The bits of the frames are encoded. For example, one ethernet version uses 4B5B where each four bits is encoded into five bits for transmission on the wire. Different ethernet standards have different encodings.
    – Ron Maupin
    Jan 23 '19 at 15:44
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer.
    – Ron Maupin
    Dec 14 '19 at 18:57
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Remember that TCP segments are encapsulated in IP packets, which in turn are encapsulated in Ethernet frames. You will be able to see the Ethernet signalling on the pins. From there, you will need to decode the Ethernet PDU and then the IP PDU in order to "see" the TCP segment.

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This depends on the layer-2 used:

In the case of Ethernet (note that there are different types) you would observe Ethernet frames.

Each Ethernet frame contains exactly one IP fragment so one IP packet can be split into multiple fragments and therefore multiple Ethernet frames.

In the case of ADSL you could observe ATM cells. I'm not going too deep into detail here, but multiple ATM cells represent one IP fragment (while one IP packet can be represented by multiple fragments).

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The signal on the RJ45 (or 8P8C) pins is defined by the physical layer variant, most commonly 1000BASE-T, 100BASE-TX or 10BASE-T. The physical layer encodes bits on the cable.

Those bits are used to form data link layer frames, most often Ethernet frames. These frames group bits together into a meaningful entity for local transport. Frames are transported by MAC addresses set by the device manufacturers.

Transported (encapsulated) by those frames are network layer packets - this is where you find IP addresses that are globally routed.

IP packets move (encapsulate) transport layer protocols like TCP or UDP. TCP segments provide a transparent, bilateral communication channel for an application layer protocol - e.g. HTTP for web access or IMAP for mailbox access.

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