No, you can not assume there will be no signal on the line just because you're not deliberately sending something at the network packet layer. There are things called link pulses, and there can be other negotiation going on between the switch and the PHY and possibly the MAC.
The best thing to do would be to let the MAC and PHY do their jobs. You can use a microcontroller with a MAC and PHY to blip a pin whenever one of your packets is received. You'd write a very bare "driver" that just waits for a packet at the MAC layer, blips a pin, then clears the packet. With such dedicated firmware, the jitter it adds should be quite small.
If you are willing to slow down your packet rate, you can do this even more easily. Most switches and other ethernet devices have send and/or receive LEDs. These are often blipped for a few 10s of ms when a packet is sent or received, which is why sending one every 5 ms won't work. If you send a packet every 100 ms, the LED should be off again when the next packet arrives. You can also wire up your own PHY and set it up to blip a pin a short time for each packet. That's basically what the chip in the switch is doing when it drives the LED.
The most basic CMOS/TTL logic uses a voltage within a specified min/max range to represent either a "logic low" 0 or a "logic high" 1. These include the discrete logic gates like 7400 / 7402 / 7432 that were used in the 1970's (and are still used sometimes on solderless breadboards), as well as more modern higher-integration chips. The exact voltage range is listed in the device's datasheet Electrical Characteristics table:
- VOH = voltage output high; specified as a minimum limit
- VOL = voltage output low; specified as a maximum limit
The device that is driving the output, is guaranteed to drive a logic low 0 as some voltage between GND and VOLmax; and also to drive a logic high 1 as some voltage between VOHmin and VCC (power supply rail). The gap between VOLmax and VOHmin is a dead band where the output is undefined -- this is what provides the noise immunity of digital signalling as compared to analog signals.
- VIH = voltage input high; specified as a minimum limit
- VIL = voltage input low; specified as a maximum limit
The device that receives the input, will interpret a voltage between GND and VILmax as meaning a logic low 0, and interpret a voltage between VIHmin and VCC as meaning a logic high 1. Any input between VILmax and VIHmin is not valid. And any input below GND or above VCC may violate the Absolute Maximum Ratings (i.e. permanently damage or degrade the device).
For CMOS, the VOH/VOL and VIH/VIL thresholds are usually a percentage of the power supply, like 30%VCC / 70%VCC. For TTL, the thresholds are absolute with 2.4V the usual VOLmin voltage.
Timing is a separate concern. If the logic is just implementing some Boolean equation ("glue logic"), the output signal simply follows the input after some specified propagation delay time. If the logic implements a state machine or a CPU, there will be a clock signal that determines the system's timing.
You also mentioned the Ethernet and USB communications protocols; these are a lot more complicated. It's a lot harder to even frame the question in terms of sending a single binary bit, since there is a lot more information that is required (such as host IP address, frame number, etc.) These build on the basic idea I described above, but add a lot more layers that are specific to each standard.
Ethernet has several layers of communications protocols; the datalink layer is different even for different types of Ethernet -- 10Mbit and 100Mbit are not just different speeds but different signalling protocols. This is described in IEEE standard 802.3
The USB protocols are described in the USB Standard, as well as on Jan Axelson's USB Complete website.
If you've never read a standards specification document before, I'd recommend starting with USB -- it's comparatively a bit simpler than Ethernet.
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Because patch panels are simply cheap connectors that just connect wires together. They would have to terminate a link from both devices with Ethernet chips to know what speeds they work at. Which is what an Ethernet switch already does.