Cabling and Addresses

Introduction

Welcome to Part 2 of the Network Fundamentals study notes! If you haven’t already, we recommend watching the video first.

In this part, we’ll look at how devices connect to a network — both with cables and wirelessly. We’ll explore the Ethernet protocol, the different types of copper and fiber cables, and finish with an overview of how devices are identified on a network using MAC and IP addresses.

Connecting Devices

When it comes to connecting devices to a network, there are two options: wired and wireless.

Wired connections have been around since the beginning of networking — back in the late 1960s. A wired network uses cables to physically connect devices together.

Wireless technology has existed for much longer than networking itself — think of radios and mobile phones. The wireless technology we associate most with networking is called Wi-Fi, which has been in use since the early 1990s.

Ethernet

A wired LAN uses a protocol called Ethernet. Remember that a protocol is a set of rules that devices on a network agree to follow.

Ethernet is made up of many different parts. Some parts describe cable types and link speeds. Other parts describe how data should be formatted and sent — this is called Media Access Control (MAC). Because Ethernet is structured this way, devices with different cables or speeds can still communicate with each other.

For example, imagine a workstation with a 1 Gigabit connection wants to send data to a server with a 10 Gigabit connection. The workstation formats the message according to the MAC rules, then sends it according to its physical connection rules. When the server receives it, it decodes the physical layer — but the underlying message is the same. The layered design of Ethernet makes this possible.

Ethernet was created by a group called the IEEE (sometimes said as “I Triple E”). The IEEE creates standards for many different technologies, each with a code number. Standards starting with 802 are used for LANs. Ethernet specifically is IEEE 802.3.

Ethernet Naming

Because codes like “802.3an” are hard to remember, each Ethernet standard also has a friendly name. For example, 802.3an is better known as 10GBASE-T. This name gives us useful information:

• 10G – the speed of the connection (10 Gigabits per second)
• BASE – short for baseband, meaning it uses a digital signal (the alternative, broadband, uses an analogue signal)
• T – the cable type; in this case, UTP (explained below)

You might also see letters like LX, which refers to a type of fiber optic cable. If you’d like to see the full list of Ethernet standards, the Wikipedia page on IEEE 802.3 is a good reference.

Copper Cables – UTP

Copper cables carry data using electrical signals — a pattern of ones and zeros that the receiver decodes. This pattern is called an encoding scheme. To send electrical signals, a complete circuit is needed, which is why copper cables contain multiple wires inside.

The most common copper cable is called Unshielded Twisted Pair, or UTP. Modern UTP cables contain four pairs of wires, where each pair forms a circuit.

Crosstalk

Electricity flowing through a copper wire creates a small magnetic field. When pairs of wires run parallel to each other, this field from one pair can interfere with the signal on another pair. This interference is called crosstalk.

UTP solves this by twisting the pairs around each other. Twisted pairs no longer run in parallel, so they don’t generate the same interfering field. If you cut open an old network cable, you’ll find the four twisted pairs inside — each pair colour-coded with a solid colour and a striped colour (for example, brown and striped brown).

Cable Categories

Not all UTP cables are equal. You’ll hear terms like Cat 5, Cat 5e, Cat 6, and Cat 6a. The category defines things like the number of pairs, the wire thickness, and how tightly they’re twisted.

Older Ethernet standards like 10BASE-T and 100BASE-T only needed two pairs. Gigabit and 10 Gigabit speeds require all four pairs. As a general guide: Cat 5e is needed for Gigabit, Cat 6 supports 10 Gigabit up to around 55 metres, and Cat 6a extends that to 100 metres. Each newer category is backward compatible — a Cat 7 cable works fine on a 100 Mbps network.

You may also see an ‘e’ or ‘a’ suffix, such as Cat 5e or Cat 6a. These indicate an improved version of the base standard, supporting better speeds or longer distances.

Cable Connectors and Pinouts

Each end of a UTP cable terminates in an RJ45 connector — the rectangular plastic plug you push into a network card or switch port. The connector has eight pins that line up with the eight wires inside the cable. A standard colour scheme used to arrange the wires is called 568B.

Straight-Through vs Crossover

A standard cable — called a straight-through cable — has the same pin layout at both ends. Pin 1 connects to Pin 1, Pin 2 to Pin 2, and so on.

This works when connecting a device like a PC to a switch, because the switch uses the opposite pairs to the PC. So if the PC transmits on Pair 1, the switch receives on Pair 1 — the signals line up correctly.

But when connecting two similar devices — PC to PC, or switch to switch — both ends would try to transmit on the same pair. The solution is a crossover cable, which swaps the transmit and receive pairs at one end so the signals line up.

Auto MDI-X

Having to choose between two cable types is inconvenient. Modern devices support a feature called Auto MDI-X, which detects if the wrong cable type is in use and automatically swaps the pin functions to compensate.

From 100BASE-T onwards, Auto MDI-X is supported. In practice, crossover cables have become quite rare. That said, if you’re studying for a networking exam, you should still know the difference between straight-through and crossover cables.

Gigabit Ethernet

At Gigabit speeds, all four wire pairs are used. There are two approaches to how they work:

• 1000BASE-TX – uses two pairs for sending and two pairs for receiving. Requires Cat 6 or higher.
• 1000BASE-T – all four pairs send and receive simultaneously using a more complex technique. Only requires Cat 5e, and is the more common standard.

Both 1000BASE-T and 1000BASE-TX require Auto MDI-X support.

Fiber Cables

The alternative to copper cabling is fiber optic cable. Fiber cables use strands of glass (sometimes called a core or pipe). Rather than electrical signals, fiber uses light pulses to carry data. This means fiber is not affected by electromagnetic interference, and works over much longer distances than copper. Fiber is commonly used between networking devices like switches and routers, and in servers.

Duplex

Duplex refers to how a link handles sending and receiving. A full duplex device can send and receive at the same time. A half duplex device can only do one at a time — it sends for a while, stops, then receives.

Fiber illustrates this well. A single core fiber cable can only carry light in one direction at a time, making it half duplex. A dual core configuration uses one core for sending and one for receiving, giving you full duplex. Enterprise networks almost always use dual core fiber — between switches, routers, and servers. If you connect fiber and it doesn’t work, try swapping the cores at one end; it’s easy to get them mixed up.

Single Mode vs Multimode

There are two types of fiber, and while they may look similar, they use different types of light:

• Multimode fiber (MMF) – uses an LED light source. Cheaper, and suitable for shorter distances — typically up to around 500 metres. Good for connections within a building.
• Single mode fiber (SMF) – uses a laser light source. More expensive, but capable of much longer distances — 2 km or more depending on the hardware. Used between buildings, or when a service provider runs fiber into your premises.

Bend Radius

Even though fiber is made of glass, it’s flexible — but only to a point. Every fiber cable has a maximum bend radius. Bend the cable too tightly and attenuation occurs — the signal is degraded or lost. The cable may still work, but not reliably. Check the manufacturer’s specifications for the acceptable bend radius.

Fiber Connectors

The two main connector types used in data networking are LC and SC. LC connectors are smaller and commonly found on switches and routers, usually in a dual core configuration. SC connectors are older and larger — less common today, but still seen in wiring closets.

Transceivers (SFP Modules)

Some switch ports look empty — these accept transceiver modules, commonly called SFPs. Transceivers let you mix and match cable types on the same switch. Different transceivers support different fiber types (single mode or multimode), different speeds (1G, 10G), and different cable lengths — a transceiver for a 40 km run costs considerably more than one for a 1 km run.

You can even use an RJ45 transceiver to run a copper UTP cable from an SFP port. This flexibility is one of the reasons many switches are built with SFP ports.

Wireless – Wi-Fi

The other way to connect devices is wirelessly, using Wi-Fi. Wireless networks use access points — think of an access point as a wireless switch. Multiple devices like phones and laptops connect to it without cables. The access point can also connect to the wired network, so wired and wireless devices can all be part of the same network.

Wireless is well suited to end-user devices like laptops, phones, and tablets. You wouldn’t typically connect a server or router to Wi-Fi.

Wi-Fi uses the IEEE 802.11 standard — not the 802.3 Ethernet standard used by wired networks. 802.11 describes how radio waves are used to format and transmit data at different speeds. While the two standards are different, they share many similarities in how data is structured — something we’ll explore in a future video.

Network Addresses

Now that we know how devices connect, the next question is: how does one device find another on the network?

Imagine your computer wants to send a print job to a printer. Your network has several devices — so how does the computer know exactly where to send the data? Sending a message to every device every time would be inefficient, insecure, and messy. If all devices were printing the same job, or if private data were visible to everyone, that would be a problem.

The answer is addresses. Just like your home has a unique postal address, each device on a network has addresses that identify it. In a LAN, each device has two types of address: a MAC address and an IP address.

MAC Addresses

A MAC address (Media Access Control address) is permanently assigned to a network card at the time of manufacture. It’s sometimes called the burned-in address. Because every network card gets its own unique MAC, no two devices should share one. If a device has more than one network card, it will have more than one MAC address.

MAC addresses are used when a device needs to communicate with another device within the same LAN segment.

IP Addresses

An IP address is different — it’s assigned by us, the network administrators. Unlike a MAC address, we can choose IP addresses that are logical and easy to remember.

IP addresses can be used for communication within a LAN segment, but their real power is in reaching devices on different LAN segments. Imagine a company that has grown and now has two separate networks, joined by a router. To reach a device on the other side, you use its IP address.

How MAC and IP Work Together

Here’s a quick example. Your computer wants to print to a printer on a different network segment:

1. The computer prepares a message with the printer’s IP address as the destination.
2. Since the printer is on another segment, the computer needs the router’s help. It adds the router’s MAC address to the message and sends it.
3. The router receives the message, strips off its own MAC address, replaces it with the printer’s MAC address, and forwards it on.

So IP addresses carry the message across networks, while MAC addresses handle the local delivery at each hop. We’ll look at how this works in much more detail in later videos.

Resources

Try out your understanding with the Connecting Devices and Addressing quiz!