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Suppose we have nodes A and B sending data over a link and the collision happens exactly in the middle (that is half the propagation time). Then both A and B detect the collision at the same time and send a jamming signal at the same time. Can the jamming signal collide?

Assuming the collision happens near B, then why do we need to send a jamming signal to A from B? Why can't A sense the change in signal levels over the link while transmitting?

  • 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 Feb 19 '18 at 20:37
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With a shared electric medium - coax - the jam signals collide. The jam signal is designed to be reliably detected even when several sources are sending it. Note that on all other media - twisted pair, fiber - the transmit and receive channel are separate and a collision can be detected more easily and slightly faster.

The jam signal is sent so that every connected node registers the collision. Due to the nature of propagating jam signals originating from various points in the segment, the transmissions needs to have a minimum length.

edit: The exact collision detection levels for 10BASE5 (you didn't ask specifically) is detailed in IEEE 802.3 Clause 8.3.1.5:

For receive mode collision detection, the MAU’s collision detection threshold shall be within the range –1448 mV to –1590 mV. The actual dc voltage on the cable during a noncollision transmission has a maximum value of –1293 mV. The lower threshold limit of –1448 mV allows 55 mV for sending end overshoot during preamble and filter impulse response during the remainder of the packet. These limits take account of up to 12% collision detect filter impulse response.

Collision detection for 10BASE2 is detailed in Clause 10.4.1.5:

For receive mode collision detection the MAU shall have its collision detection threshold set in the range –1404 mV and –1581 mV. These limits take account of up to 8% collision detect filter impulse response. If a specific filter implementation has a higher value of impulse response, the lower threshold limit of –1404 mV is required to be replaced by –1300 mV × [1 + impulse response].

PS: All shared electric media are extremely obsolete and only of historical interest. Slightly more modern variants use separate transmit and receive channels and all seriously modern variants additionally use switching with full-duplex transmission without any collision domain.

  • Can you answer my second question about voltage levels or signal levels ? – Zephyr Dec 13 '17 at 4:23

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