The physical layer turns bits into signals on a medium. On twisted-pair Ethernet that is a differential voltage, about plus or minus 2.5 V on a 10 Mbps link, coded so the receiver can recover the clock from the signal itself. Manchester coding at 10 Mbps puts the signal energy between 5 and 10 MHz, and 100 Mbps Ethernet sends 125 million symbols per second, which is why each step up in speed needed a better cable category.
Part of Networking
How bits become electrical or optical signals on cable, and what limits how fast and how far those signals travel.
Bits Travel As A Difference Between Two Wires
Ethernet over twisted pair does not send a voltage on one wire measured against ground. It sends the signal as the difference between the two wires of a pair, with one wire carrying the inverse of the other, and the receiver measures only that difference. Twisted Pair Wiring explains why this matters: interference that lands on both wires equally is common-mode, and it cancels in the difference. Twisting keeps the two wires exposed to the same interference, so the cancellation works.
Hub Construction gives the signalling voltage for 10 Mbps links as plus or minus 2.5 V differential. That is a small signal, which is why noise, poor connections and length are what degrade it.
The Clock Travels Inside The Signal
There is no separate clock wire on Ethernet, so the receiver has to extract timing from the data. 10 Mbps Ethernet uses Manchester coding, in which every bit is a transition in the middle of its bit period, so every bit contains at least one transition for the receiver to lock onto (Hub Construction makes the same point). The cost is bandwidth: the signal changes at least once per bit.
100 Mbps Ethernet (100BASE-TX) uses two schemes together, as Hub Construction describes. 4B5B coding turns every group of 4 data bits into a 5-bit symbol, which guarantees enough transitions for the clock. MLT-3 then sends those symbols on three voltage levels, which lowers the highest frequency in the signal.
Faster Links Need Better Cable Because Higher Frequencies Fade Faster
Hub Construction states that higher frequencies are attenuated more than lower ones in twisted pair, rounding off sharp edges. So the rule that follows is that a faster link, which uses higher frequencies, needs cable that loses less at those frequencies and leaks less between pairs. That is the whole content of the category system: Cat3 for 10 Mbps, Cat5 and above for 100 Mbps, and Cat5e, Cat6 and Cat6a for gigabit and beyond, as Hub Construction and Physical Connections list.
Three impairments set the working range:
- Attenuation — the signal weakens with length, and weakens faster at high frequency; the 100-metre channel limit exists so the weakened signal is still readable
- Crosstalk — a pair induces a signal in its neighbours; near-end crosstalk is one of the parameters certification testers measure, and leaving too much of a pair untwisted at a connector makes it worse
- Reflection — any change in impedance, such as a poorly made connector or a missing terminator on a bus, sends part of the signal back toward the sender; Connectors describes the connector case and Bus Topology the termination case (120 ohms for RS-485 twisted pair, 50 ohms for 10BASE2 coaxial cable)
Worked Example: What The Line Rate And The Delay Mean In Practice
Manchester at 10 Mbps. One bit lasts 1 / 10,000,000 s = 100 ns. Every bit has a mid-bit transition. For a run of identical bits, there is also a transition at each bit boundary, so the signal completes one full cycle per bit: 1 / 100 ns = 10 MHz. For alternating bits only the mid-bit transitions occur, one cycle per two bits: 5 MHz. The signal energy therefore lies between 5 and 10 MHz, which matches the 10 MHz crystal in the minimal hub design.
4B5B at 100 Mbps. Five line bits carry four data bits, so the symbol rate is 100 Mbps x 5 / 4 = 125 million symbols per second. This is a larger number than the data rate, which is the price of the clock guarantee.
Propagation along a full-length run. Network Fundamentals gives a copper propagation speed of about 200,000 km/s, which is 200,000,000 m/s. A 100 m run delays the signal by 100 / 200,000,000 = 0.0000005 s = 500 ns. At 100 Mbps one bit lasts 10 ns, so the cable holds 500 / 10 = 50 bits in flight. The data-link layer has to be designed around delay of this size, which is the subject of CD.
Common Mistakes
- Untwisting more than the connector needs — the pair stops rejecting interference exactly where the signal is weakest; Twisted Pair Wiring gives 13 mm as the Cat5e limit
- Mixing a good cable with a poor connector — the connector is often the reflection point, not the cable
- Assuming the link is fine because it lights up — a link can negotiate at a lower speed on a marginal cable and still show a light; check the negotiated speed
Safety
Long copper runs between buildings carry surges
A copper cable that leaves one building for another is a conductor that can pick up a lightning-induced surge and can join two points at different ground voltages. Network Interface notes that the transformer isolation in a port protects against voltage differences along a cable and against lightning-induced surges, but it is not a lightning arrester. Between separate buildings prefer fibre, which has no metal path (Fiber Optics). If copper must be used, follow the protection and earthing practice in Lightning Protection and Ground Rod, and keep people off the cable and the equipment during a storm.
What’s Next
- Learn next: Data Link Layer — what the layer above does with the bit stream
- Combine with: Cable Testing — measuring whether a real cable meets these limits