Table of Contents
- IEEE 802.3 standards (copper)
- Straight-through and crossover cables
- Common device types and their Tx/Rx pin pairs
- Auto MDI-X
What is Ethernet ?
Network standards are essential rules that enable devices from different brands to seamlessly connect and communicate with each other, whether via wired or wireless connections. These standards ensure that data is exchanged efficiently and without errors.
Key Standards:
Ethernet (IEEE 802.3):
- Governs wired connections.
- Defines how devices connect via cables to exchange data.
- Widely used in local area networks (LANs) for fast and reliable connections.
Wi-Fi (IEEE 802.11):
- Governs wireless connections.
- Allows devices to connect without cables, ensuring flexibility and convenience.
Why Network Standards Matter:
Thanks to these standards, devices can easily communicate regardless of their connection type—whether they use cables or wireless signals.
In Summary:
- Ethernet is a cornerstone of wired networks.
- It’s part of a broader set of standards that enable seamless communication between all types of devices.
Network standards make the modern digital world possible, ensuring that our devices can always stay connected and share information effortlessly.
Binary: Understanding Bits and Bytes
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Bit: The smallest unit of information, a bit can be either 0 or 1. It’s the fundamental language that computers use to process and store data.
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Byte: A byte is made up of 8 bits. It’s commonly used to measure data size, such as representing one character of text (e.g., “A”).
Computers exchange information by transmitting sequences of bits (0s and 1s) through cables or wireless signals. When data is sent over cables, such as in Ethernet connections, these bits are converted into electrical signals or light pulses that travel along the wire.
How Ethernet Transmits Data
In an Ethernet network, devices like computers and switches communicate by sending electrical signals through copper cables. Each signal represents either a 0 or a 1, depending on the voltage level.
- High voltage might represent a 1.
- Low voltage could represent a 0.
These electrical signals are then decoded by the receiving device to reconstruct the data, allowing devices on the network to exchange information rapidly and efficiently.
Example
A router sends 1 byte of data to a switch. Changes in the voltage of the electric signal indicate values of 0 or 1.
Network Speeds
Network speeds are measured in bits per second (bps). Because modern networks transfer data at incredibly high speeds, we use larger units:
- Kilobits (kb) = 1,000 bits
- Megabits (Mb) = 1,000,000 bits
- Gigabits (Gb) = 1,000,000,000 bits
- Terabits (Tb) = 1,000,000,000,000 bits
Example speeds:
- 56 kbps (56 kilobits per second)
- 100 Mbps (100 megabits per second)
- 10 Gbps (10 gigabits per second)
1,000 or 1,024 bits?
There can be confusion about whether 1 kilobit is 1,000 bits or 1,024 bits. Here’s a breakdown to help clear things up:
Decimal (Base-10) System:
In most networking contexts, we use the decimal system, where:
- 1 kilobit (kb) = 1,000 bits
- 1 megabit (Mb) = 1,000 kilobits
- 1 gigabit (Gb) = 1,000 megabits
- 1 terabit (Tb) = 1,000 gigabits
These values are used to measure things like network speeds.
Binary (Base-2) System:
In computing, binary prefixes are used, based on powers of 2 (e.g., 2^10 = 1,024):
- 1 kibibit (Kib) = 1,024 bits
- 1 mebibit (Mib) = 1,024 kibibits
- 1 gibibit (Gib) = 1,024 mebibits
- 1 tebibit (Tib) = 1,024 gibibits
The Key Difference:
- Decimal system (1,000) is used for network speeds and large data transfer rates.
- Binary system (1,024) is used for computer memory and storage.
Copper UTP connections
Copper Ethernet cables (commonly referred to as UTP - Unshielded Twisted Pair) are used for networking connections. These cables are popular because they are cost-effective, reliable, and suitable for short to medium distances.
Key Features of UTP Cables:
- Unshielded: No metallic shielding around the wires, which makes UTP cables more affordable but more susceptible to electromagnetic interference (EMI).
- Twisted Pair: The 8 wires inside the cable are twisted in pairs to reduce interference between them.
Connectors:
- 8P8C connectors: Often referred to as RJ45, these connectors have 8 pins, one for each wire inside the cable.
Example
Two 8P8C ports on a Cisco switch (left) and an 8P8C connector on a copper UTP network cable (right)
IEEE 802.3 standards (copper)
The IEEE (Institute of Electrical and Electronics Engineers) defines a range of standards for Ethernet cables that cover different speeds, cable types, and distances.
Types of Names for Each Standard:
- Speed-derived name: Indicates the speed, such as 10BASE-T (10 Mbps), 100BASE-T (100 Mbps).
- IEEE Task Group Name: Identifies the IEEE group responsible for creating the standard, e.g., IEEE 802.3i.
- Informal Name: A commonly used name, typically based on the speed and cable type, like Gigabit Ethernet for 1000BASE-T.
Overview of Some Copper Ethernet Standards
The following table summarizes some key copper Ethernet standards, their speeds, and the cables used.
| Speed | Speed-Derived Name | IEEE Task Group | Informal Name | Max Cable Length | Cable Name |
|---|---|---|---|---|---|
| 10 Mbps | 10BASE-T | IEEE 802.3i | Ethernet | 100 m | Cat 3 |
| 100 Mbps | 100BASE-T | IEEE 802.3u | Fast Ethernet | 100 m | Cat 5 |
| 1 Gbps | 1000BASE-T | IEEE 802.3ab | Gigabit Ethernet | 100 m | Cat 5e |
| 10 Gbps | 10GBASE-T | IEEE 802.3an | 10 Gig Ethernet | 100 m | Cat 6a |
NOTE
- IEEE 802.3: This standard defines how Ethernet networks work. It specifies the rules for data transmission, the communication protocol, and the speed at which data can travel over the network.
- Physical Cables: While IEEE 802.3 defines the network protocol, it does not define the physical cables used to connect devices in an Ethernet network.
- The cables (like Cat 5 and Cat 6) are actually defined by two other organizations:
- EIA (Electronic Industries Alliance)
- TIA (Telecommunications Industry Association)
- Ethernet Cable: The term “Ethernet cable” is commonly used, but it’s a bit misleading. While it refers to the cables used for Ethernet networks, the physical specifications of these cables are set by EIA and TIA, not by IEEE.
Straight-through and crossover cables
Straight-Through cables are used to connect different devices, such as a PC to a switch.
Wiring Structure:
- Pair 1: Connected to pins 1 and 2 (used for transmitting data - Tx).
- Pair 2: Connected to pins 3 and 6 (used for receiving data - Rx).
How it Works:
- The PC uses pins 1 and 2 to transmit (Tx), while the switch uses these same pins to receive (Rx).
- The switch uses pins 3 and 6 to transmit (Tx), while the PC uses these same pins to receive (Rx).
Example Use Case:
- PC to Switch Connection: The straight-through cable works perfectly here because the devices have opposite roles in communication (one transmits, the other receives).
Example
A PC and a switch connected via a straight-through cable
In this configuration, the PC uses pins 1 and 2 to transmit data (Tx) and the switch uses pins 1 and 2 to receive data (Rx). Similarly, the switch uses pins 3 and 6 to transmit data (Tx), and the PC uses pins 3 and 6 to receive data (Rx).
This wiring works perfectly when connecting devices that communicate in opposite directions, such as a PC to a switch.
Now, consider connecting two similar devices, such as two routers, or two PCs together using a straight-through cable. This can cause issues:
- Problem: Both devices are trying to transmit data on the same pins.
- The Tx pins (1-2) from one device will be connected to the Tx pins (1-2) of the other device, which causes communication failure.
Example
Two routers connected via a straight-through cable. Because both routers transmit data using the same pin pair, communication fails.
Crossover Cable is used to connect similar devices (like router to router or PC to PC), a crossover cable is used. The crossover cable swaps the transmission and reception pins, allowing proper communication between the two devices. PC).
- Router to Router: The crossover cable swaps the Tx and Rx pins for each device, allowing them to communicate correctly.
Example
Two routers connected via a crossover cable. The Tx pin pair of one router connects to the Rx pin pair of the other router.
Common device types and their Tx/Rx pin pairs
| Device Type | Transmit (Tx) Pins | Receive (Rx) Pins |
|---|---|---|
| Router | 1 and 2 | 3 and 6 |
| Firewall | 1 and 2 | 3 and 6 |
| PC/Server | 1 and 2 | 3 and 6 |
| Switch | 3 and 6 | 1 and 2 |
Auto MDI-X
Overview
With modern networking equipment, you no longer have to worry about choosing between straight-through and crossover cables. This is thanks to a feature called Auto Medium-Dependent Interface Crossover (Auto MDI-X).
How Auto MDI-X Works
Auto MDI-X automatically detects the type of device it’s connected to and adjusts the pins it uses to transmit and receive data. This means that a device can reverse its transmit and receive pins based on whether it is connected to another similar device (like router-to-router or PC-to-PC) or a different device (like PC-to-switch or router-to-switch).
Why This Matters
- Straight-through and crossover cables are still important to understand, especially for exams, but in practice, Auto MDI-X removes the need to worry about which type of cable to use.
- Example: Two routers connected via a straight-through cable. Normally, the routers transmit on pins 1-2 and receive on pins 3-6, but with Auto MDI-X, the router on the right can automatically reverse the pins and transmit on pins 3-6 while receiving on pins 1-2.
Example
Two routers connected via a straight-through cable. The router on the right uses Auto MDI-X to adjust which pins it uses to transmit and receive data.
Fiber-optic connections
Copper UTP cables are common in LANs due to their low cost and broad compatibility, but they are limited to a 100-meter distance. While sufficient for devices on the same floor, this isn’t ideal for multi-floor or inter-building connections. For longer distances, fiber-optic cabling is preferred. Unlike copper, fiber-optic cables transmit data using light signals through glass fibers, offering superior range but requiring careful handling to avoid damage or signal loss.
The anatomy of a fiber-optic cable
A typical fiber-optic connection uses two cables: one for transmitting and one for receiving data. These cables connect to an SFP (Small Form-Factor Pluggable) transceiver, which is inserted into an SFP port on the device. SFP transceivers are modular and must be purchased separately, and they can be surprisingly expensive.
Example
A Cisco switch with an SFP transceiver inserted into one of its SFP ports. An additional SFP is placed on top of the switch.
A fiber-optic cable consists of several layers:
- Core: A thin glass fiber that carries the light signal (core thickness varies by cable type).
- Cladding: A reflective layer that helps guide the light along the core.
- Buffer: Provides protection to the core.
- Outer Jacket: The protective outer layer of the cable.
Example
The typical structure of a fiber-optic cable. An outer jacket (4) and buffer (3) serve to protect and contain the inner components. A layer of reflective cladding (2) helps carry the light signal along the glass core (1).
All types of fiber-optic cabling can carry a signal farther than copper cabling, but even within the category of fiber-optic cabling, the maximum supported length can vary greatly. There are two main types of fiber-optic cabling: multimode fiber (MMF) and single-mode fiber (SMF).
Example
Light travels down an MMF cable at multiple angles (modes), whereas light travels down SMF cables at a single angle.
MMF (Multimode Fiber) cables have a wider core and use LED transmitters that send light at multiple angles. They typically support distances of several hundred meters.
SMF (Single-Mode Fiber) cables have a narrower core and use laser transmitters that send light in a single direction, allowing for longer distances—up to tens of kilometers. SMF transmitters are more expensive than MMF’s LED transmitters.
Overview of Some Fiber Optic Standards
| Speed | Standard | Connection Speed | Mode Support | Max Transmission Distance |
|---|---|---|---|---|
| 1000BASE-LX | 802.3z | 1 Gbps | Multimode / Single | 550 meters (Multi) / 5 km (Single) |
| 10GBASE-SR | 802.3ae | 10 Gbps | Multimode | 400 meters |
| 10GBASE-LR | 802.3ae | 10 Gbps | Single | 10 kilometers |
| 10GBASE-ER | 802.3ae | 10 Gbps | Single | 30 kilometers |
UTP vs. Fiber
| Aspect | Copper UTP | Fiber-Optic |
|---|---|---|
| Distance Support | Limited to 100 meters | Supports much greater distances (up to tens of kilometers) |
| Cost | Lower cost (cheaper cables and no need for SFP transceivers) | Higher cost (expensive SFP transceivers) |
| Common Use | Connections from switches to end hosts on the same floor | Connections between network infrastructure (e.g., switches and routers on different floors or buildings) |
| Vulnerability to EMI | Can be affected by electromagnetic interference (EMI) | Immune to EMI |
| Signal Leakage | Can emit signals outside the cable, posing security risks | No signal leakage, more secure |
| Device Compatibility | Most devices (e.g., PCs) use UTP connections, no need for SFP ports | Devices must have SFP ports, limiting its use for client devices |
| Maximum Distance | Limited to short distances (100 meters) | Suitable for long-distance connections (hundreds of meters to kilometers) |
Summary
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Network Standards: These are rules that ensure devices can communicate over a network. Ethernet, defined by the IEEE 802.3 standard, is used for wired connections.
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Binary Communication: Computers communicate using 0s and 1s (bits). A group of 8 bits is called a byte.
-
Network Speeds: Measured in bits per second (bps), with units like kilobits (kb), megabits (Mb), gigabits (Gb), and terabits (Tb).
-
UTP Cables: Most Ethernet networks use unshielded twisted pair (UTP) cables. These cables have 8 wires twisted into 4 pairs to reduce electromagnetic interference (EMI). The connectors used are RJ45 (8P8C).
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Ethernet Standards:
- 10BASE-T and 100BASE-T: Use 2 wire pairs.
- 1000BASE-T and 10GBASE-T: Use all 4 pairs.
- All of these have a maximum cable length of 100 meters.
- Auto MDI-X lets devices automatically adjust how they send and receive data.
-
Fiber Optic Cables: They send light signals and support much longer distances than UTP cables.
- Single-mode fiber (SMF): Supports longer distances (kilometers), but is more expensive.
- Multi-mode fiber (MMF): Supports shorter distances (hundreds of meters) and is cheaper.
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Cost and Usage:
- UTP Cables: Less expensive and commonly used to connect devices to switches (e.g., a PC to a switch).
- Fiber Optic Cables: More expensive but are used for connecting network devices over long distances, such as between different floors or buildings.