Networks eventually have to move bits through something physical. This lesson covers copper and fibre Ethernet, common connectors, interface speeds, duplex and the practical differences between the media you'll see on real networks.
At the bottom of the OSI model, the Physical layer is responsible for transmitting bits. Those bits might be represented by electrical signals over copper, pulses of light through fibre or radio waves through the air.
Common for Ethernet connections to PCs, phones, printers and access points. Uses twisted copper pairs to reduce interference and typically terminates in an 8P8C modular connector commonly called RJ45.
Uses light instead of electrical signalling. Fibre supports long distances and high speeds and is resistant to electromagnetic interference.
Uses radio rather than a physical cable. Wireless still connects into the same Ethernet/IP network, usually through an access point.
Cisco expects you to recognise common Ethernet speeds and media. Exact capability depends on the standard and cable category.
| Standard | Speed | Typical medium | Common max distance |
|---|---|---|---|
| 100BASE-TX | 100 Mbps | Twisted-pair copper | 100 m |
| 1000BASE-T | 1 Gbps | Twisted-pair copper | 100 m |
| 2.5GBASE-T | 2.5 Gbps | Twisted-pair copper | Up to 100 m |
| 5GBASE-T | 5 Gbps | Twisted-pair copper | Up to 100 m |
| 10GBASE-T | 10 Gbps | Twisted-pair copper | Up to 100 m depending on cabling |
Designed primarily for shorter-distance links such as within a building or campus.
It has a larger core than single-mode fibre and allows multiple light paths or modes.
Common for switch uplinks and data-centre connections.
Uses a much smaller core and is designed for long-distance transmission.
It supports much greater distances and is commonly used between buildings, across campuses and by service providers.
Best suited to long-distance high-bandwidth links.
The familiar modular connector used on twisted-pair Ethernet cabling.
A small fibre connector commonly used with modern network transceivers.
A larger push-pull fibre connector found in many fibre installations.
Removable transceiver modules allowing network devices to support different fibre or copper interfaces.
SFP/SFP+ port
Multimode or single-mode
Matching transceiver
Ethernet interfaces have a speed and duplex setting. Modern Ethernet commonly uses auto-negotiation to agree suitable settings.
A device can transmit and receive at the same time.
✓ Normal operation for modern switched Ethernet.
A device cannot transmit and receive simultaneously.
Older Ethernet behaviour and generally not what you expect on modern switched networks.
When troubleshooting an interface, start by checking whether it is operational and what speed and duplex it negotiated.
The important things to look for include:
Interface state: is the interface actually up?
Speed: has it negotiated the expected speed?
Duplex: is it operating full-duplex?
Errors: increasing errors can indicate a physical problem, damaged cabling or negotiation issue.
Next we'll look at the structure of an IPv4 address, subnet masks, private address ranges, default gateways and how a host decides whether a destination is local or needs to be sent to a router.