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Can two packets, say from host A, propagate on the same communication link while the first one is still on its way to the next point, say host B?

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On the same physical link, no. The source needs to finish sending a packet before it can start a new one. Packets are atomic and cannot overlap.

Physically, with the limited propagation speed of a signal, the destination may still be receiving the previous packet while the source has already started transmitting the next one. With a 10 Gbit/s link and a propagation speed of 200,000 km/s, a single bit is just 2 cm "long". For a minimum sized Ethernet frame that's just 13.5 m, so a longer cable could actually hold one or more entire frames. [figures corrected]

On a logical link, consisting of multiple physical links, it is normal for the source to send many packets while the destination is still receiving previous packets. In the TCP transport-layer protocol, the window size defines the amount of data that can be "in flight".

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  • Even a single bit of a packet has to get the destination before the source can send the next one on the same physical link?
    – jeysmith
    Mar 10, 2019 at 10:54
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    @jeysmith No. Data is sent serialized in packets or frames. When a frame is finished a new one can be started. Maybe my edit to the answer makes this clearer.
    – Zac67 ♦
    Mar 10, 2019 at 10:59
  • What I want to figure out is what exactly happens at a physical layer (on the cable). As a bit is an electrical signal, I presume that a packet is a "bunch" of electrical signals, so, as far as I understood now, a few electrical signals can travel on the same cable simultaneously before the first one gets its destination. To make it simple, say computer A linked to computer B via one cable. Did I get it right?.
    – jeysmith
    Mar 10, 2019 at 11:13
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    Data is also transmitted on optical fiber, and the "bits" can also be multi-bit symbols, but yes, a packet or a frame is a bunch of those line symbols. It depends what you mean with "simulateously" - you can only feed one packet/frame at a time into a cable but generally, multiple, sequential frames can be travelling on a single cable in a given moment; see the italicized paragraph.
    – Zac67 ♦
    Mar 10, 2019 at 13:10
  • I think I've understood now. Thanks
    – jeysmith
    Mar 10, 2019 at 15:11

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