Cables and the physical layer: copper, fibre and Ethernet
Lesson 3 of 4
3Cables and the physical layer: copper, fibre and Ethernet
The big pictureBefore any address or protocol, your message must turn into something that crosses space: pulses of electricity in copper, flashes of light in a hair of glass, or waves in the air. If that road breaks, the smartest configuration in the world will not help you.
What the picture shows: Layer 1 understands nothing it carries: it turns ones and zeros into a signal, moves it, and the other end turns it back into ones and zeros.
Map of this part
The concepts in order, click any one to jump to it:
Two ways into the same idea
Think of the network as a city. Addresses and protocols are the postal system, but cables are the roads. Copper cable is a road inside the neighbourhood: cheap and easy, but no longer than about 100 metres. Fibre is a motorway between cities: it carries light over very long distances and electrical noise does not bother it. Wi-Fi is a road through the air: comfortable but shared and crowded. When the network goes down, look first: is the road intact? Is the port light on?
Layer 1 defines the medium, the connector, the signal and the timing. Twisted pair (UTP) is four pairs on an RJ45 connector wired T568A or T568B (they differ by swapping the orange and green pairs). TIA-568 caps the permanent link at 90 m and the whole channel at 100 m including patch cords. Cat5e carries up to 1 Gbps, Cat6 up to 10 Gbps over only about 55 m, and Cat6A 10 Gbps over the full 100 m (500 MHz). Multimode fibre (MMF) covers hundreds of metres inside a building and single-mode (SMF) kilometres between buildings and cities, through SFP modules in the switch. On top sits the Ethernet frame: destination and source MAC, the payload type (EtherType), up to 1500 bytes of payload (the MTU) and an FCS to catch errors.
Before You Start
🧅The layers
You know layer 1 is the bottom of the OSI model.
🕸️What is a network?
You know the switch, the router and bandwidth.
The Core Idea
1What exactly does layer 1 do?
Its job is moving the bit: a zero or a one. It knows no addresses and no frames. It defines three things: the medium (copper, glass or air), the connector (RJ45, LC or an antenna) and the signal (voltage, light, a radio wave). Its simple devices are the cable, the connector, the media converter, and the old hub that repeats everything it gets to every port.
| Medium | Signal | Typical reach | Where you see it |
|---|---|---|---|
| Twisted-pair copper (UTP) | Electricity | Up to 100 m | Offices and homes |
| Multimode fibre | Light (LED or short laser) | Hundreds of metres | Between floors and inside data centres |
| Single-mode fibre | Laser | Kilometres and beyond | Between buildings, cities and providers |
| Air (Wi-Fi) | Radio waves | Tens of metres | Mobile devices |
2Copper: the categories and the 100 metre limit
Twisted pair holds four pairs of wires, each twisted around itself to cancel noise. The higher the category, the higher the frequency it carries and so the speed. Whatever the category, the electrical signal fades with distance, so TIA-568 caps the whole channel at 100 metres: 90 fixed in the wall and the rest patch cords. Beyond that you need a switch in between or fibre.
| Category | Common top speed | Frequency | Distance |
|---|---|---|---|
| Cat5e | 1 Gbps | 100 MHz | 100 m |
| Cat6 | 10 Gbps | 250 MHz | About 55 m at 10 Gbps, 100 m at 1 Gbps |
| Cat6A | 10 Gbps | 500 MHz | 100 m at 10 Gbps |
Change the category and lengths and try. This is the question a contractor asks you before pulling cable.
# Will this copper run work? The channel limit is 100 m (TIA-568).
LIMITS = {"Cat5e": {1: 100}, "Cat6": {1: 100, 10: 55}, "Cat6A": {1: 100, 10: 100}}
def check(cable, gbps, wall_m, patch_m):
total = wall_m + patch_m
reach = LIMITS[cable].get(gbps)
if reach is None:
return f"{cable} is not rated for {gbps} Gbps"
if wall_m > 90:
return f"{total} m: the fixed part is {wall_m} m, over the 90 m permanent link"
return f"{total} m of {cable} at {gbps} Gbps: " + ("OK" if total <= reach else f"too long, max {reach} m")
print(check("Cat6", 10, 70, 6))
print(check("Cat6A", 10, 85, 10))
print(check("Cat5e", 10, 20, 2))
print(check("Cat6A", 1, 95, 3))3Fibre: when and why?
Fibre carries light in a hair of glass, so electrical noise and lightning do not affect it, and it covers distances copper cannot. Multimode has a wider core, is cheaper with shorter reach, and is used inside buildings and data centres. Single-mode has a very thin core and uses lasers over kilometres. In the switch you plug an SFP module that fits the fibre type and speed, and the golden rule: the modules at both ends must match in type and wavelength.
4The Ethernet frame and the port
On top of the cable comes Ethernet, the layer 1 and 2 standard in almost every office. It wraps data in a frame: the destination MAC, then the source, then the payload type (0x0800 means IPv4 and 0x86DD IPv6), then up to 1500 bytes of data, then the FCS, a check number that catches a damaged frame so it gets dropped. The port negotiates speed and duplex with the other end (full duplex sends and receives at once). Some ports also carry power through PoE, running cameras, Wi-Fi access points and phones on one cable.
notconnect means no signal: an unplugged or damaged cable, or a device that is off. A rising CRC counter means a bad cable or connector. The camera here negotiated only 100 Mbps.
SW1# show interfaces status
Port Name Status Vlan Duplex Speed Type
Gi1/0/1 PC-01 connected 10 a-full a-1000 10/100/1000BaseTX
Gi1/0/2 PC-02 notconnect 10 auto auto 10/100/1000BaseTX
Gi1/0/3 CAM-01 connected 30 a-full a-100 10/100/1000BaseTX
Gi1/0/24 UPLINK connected trunk full 10G SFP-10GBase-SR
SW1# show interfaces gi1/0/3 | include errors
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignoredWorked example
The task: a company on two floors. The server room is on the first floor, and the farthest office on the second is 120 metres along the cable path. The corridor cameras need power. What do you propose?
The answer: 120 metres is past the copper limit, so do not run one cable. Put a switch on the second floor and join it to the server room with multimode fibre and two matching SFP modules, then spread to the offices with Cat6A (every run under 90 metres). Make that switch PoE so it powers the cameras with no power sockets. After installing, check each port: status connected, the expected speed, and a zero CRC counter.
Myth vs Reality
| Myth | Reality |
|---|---|
| A pricier cable means faster internet. | Your internet speed is set by your plan and provider. A good cable only stops a bottleneck inside your network. |
| Fibre is only for providers. | It is the norm between floors and buildings and inside data centres. |
| If the light is on, the cable is fine. | It may run at a lower speed or with CRC errors. Check the speed and the counters. |
Where you meet it at work
🧰IT support
Starts every "the network is down" ticket with the cable, the port and the light.
🏢Network engineer
Picks the categories, fibre and SFP modules in the design of any new building.
📹Surveillance
Relies on PoE to run cameras on one cable.
Test your understanding
What is the longest copper channel under TIA-568, patch cords included?
90 metres fixed, and patch cords make up the rest to 100.
You want 10 Gbps over 95 metres of copper. Which category?
Cat6 carries 10 Gbps for only about 55 metres; Cat6A covers the full distance.
Between two buildings 3 km apart, what do you run?
Single-mode with lasers for kilometres; multimode for hundreds of metres.
What does the FCS field in an Ethernet frame do?
A check number the sender computes and the receiver recomputes.
A port's CRC counter keeps climbing. What do you suspect first?
CRC errors are usually a layer 1 signal problem.
What lets a camera run on one network cable with no power socket?
Power over Ethernet sends power over the cable's own pairs.
Quick review cards
Check Your Understanding
Why are the cable's wires twisted in pairs?
Because twisting cancels electrical noise from outside and from the neighbouring pairs, so the signal stays clean for longer.
A port says connected but at 100 Mbps instead of 1000. What could it be?
A damaged pair (1 Gbps needs all four pairs while 100 Mbps needs two), an old cable, or a device that cannot go faster. Try another cable first.
When do you pick fibre even though copper is cheaper?
For runs over 100 metres, between buildings (electrical isolation from lightning), in noisy environments, and for very high speeds.
Resources for this lesson
Grouped by type. Start with the official docs for precision, or an article or video for the explanation.
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