Question 1

What is Power over Ethernet?

Accepted Answer

**Power over Ethernet (PoE)** delivers electrical power along the same cable that carries data, so a device needs only one connection rather than a network cable and a mains adapter.

**Why it is useful.** The devices that most need networking are frequently in awkward places — a ceiling-mounted access point, an outdoor camera, a doorbell, a wall-mounted display. Running mains power to those locations requires an electrician, a socket and, outdoors, weatherproofing. **A single data cable is far easier**, and can be installed by anyone.

**How it works.** Ethernet cable contains four twisted pairs. Depending on the standard, power is delivered either on the pairs not used for data, or superimposed on the data pairs using **phantom power**, which works because the data signal is differential and a common-mode voltage does not disturb it.

**Negotiation matters and is the safety feature.** A PoE source does not simply energise the cable. It **detects** whether a compatible device is attached and **classifies** how much power it requires before supplying anything. Plugging an ordinary non-PoE device into a PoE port is therefore harmless — nothing is delivered because nothing was negotiated.

**The standards, in outline:**

**802.3af (PoE)** — up to about 15W at the source, roughly 12.95W at the device after cable losses.

**802.3at (PoE+)** — up to about 30W, enough for pan-tilt-zoom cameras and more demanding access points.

**802.3bt (PoE++ / 4PPoE)** — substantially higher, up to around 60W or 90W depending on type, powering displays, lighting and small computers.

**Passive PoE** is a non-standard alternative that simply applies a fixed voltage with **no negotiation**, used by some equipment. It can damage devices that were not expecting it, so matching the manufacturer's specification matters.

**How power is supplied:** a **PoE switch**, powering many devices centrally — which also allows the whole lot to be backed up by one UPS; or a **PoE injector**, adding power to a single run from an ordinary switch.

**Practical limits:** the 100-metre Ethernet distance limit still applies, and voltage drop over long runs with thin cable reduces available power — so cable quality matters more with PoE than with data alone.

Question 2

What is the difference between Cat5e, Cat6 and Cat6a cable?

Accepted Answer

Successive specifications for twisted-pair Ethernet cable, differing in how well they resist interference — which is what determines the speed and distance they can support. What all of them share.

Question 3

How does fibre actually carry data?

Accepted Answer

As pulses of light down a strand of extremely pure glass, guided by a physical effect that keeps the light from escaping even as the cable bends. The physical mechanism. A fibre has a core of glass

Question 4

What is a VLAN, and why would a home network need one?

Accepted Answer

A virtual LAN splits one physical network into several logically separate ones, so devices on different VLANs cannot see each other even though they share the same switch and cabling. How it works.

Question 5

How does cable broadband work, and what is DOCSIS?

Accepted Answer

Cable broadband delivers internet over the coaxial television network, using a standard called DOCSIS — Data Over Cable Service Interface Specification — that defines how data shares that cable with

Question 6

What is a MAC address, and how is it different from an IP address?

Accepted Answer

A MAC address identifies a piece of hardware on a local network; an IP address identifies a location on the internet. One is fixed to the device, the other describes where the device currently is —

Question 7

What is the difference between FTTC and FTTP?

Accepted Answer

How far the fibre actually goes — and that single fact determines almost everything about the speed and reliability you get. FTTC — fibre to the cabinet. Fibre runs from the exchange to the green

Related Questions