Straight-Through and Crossover Cables - N10-008 CompTIA Network+ : 2.3

Summary

The video explains the differences between straight-through and crossover Ethernet cables, commonly used in network connections. It covers how straight-through cables are typically used to connect workstations to network switches, and crossover cables are used when connecting similar devices like two switches or two computers. The script details how data flows in 10/100-Megabit and Gigabit Ethernet, highlighting the role of pins and Auto-MDI-X technology, which automatically determines if a crossover is needed. It also debunks misconceptions about cable wiring standards for crossover cables in gigabit networks.

Takeaways

• 🔌 Ethernet straight-through cables connect identical pins on both ends (e.g., pin 1 to pin 1, pin 2 to pin 2).
• 🖥️ Straight-through cables are commonly used to connect devices like workstations to network switches.
• 📡 For 10BASE-T and 100BASE-T Ethernet, only two pairs of wires are used (pins 1, 2, 3, and 6).
• 💡 The media-dependent interface (MDI) is responsible for transmitting data, while the MDI-X on the switch handles receiving.
• 🔄 In gigabit (1000BASE-T) Ethernet, all four pairs of wires are used, and each wire can both transmit and receive data.
• 🔀 A crossover cable is needed when connecting two MDI devices or two MDI-X devices (e.g., workstation to workstation or switch to switch).
• 🤖 Auto-MDI-X is a feature in modern Ethernet devices that automatically detects whether a crossover cable is needed and adjusts accordingly.
• 📊 In gigabit Ethernet, the concept of a crossover cable is different, with all eight wires crossing over, not just two pairs.
• 🛠️ Crossover cables for gigabit Ethernet are not simply a combination of TIA 568A on one side and 568B on the other.
• 🧠 Use a straight-through cable for different devices (e.g., workstation to switch), but use a crossover cable for like devices (e.g., switch to switch).

Q & A

What is a straight-through cable, and where is it commonly used?

A straight-through cable connects the same pin numbers on both ends (pin 1 to pin 1, pin 2 to pin 2, etc.). It's commonly used to connect workstations to network devices like switches.

What is a patch cable, and how does it relate to straight-through cables?

A patch cable is a type of straight-through cable used inside wiring closets to connect a patch panel to an Ethernet switch.

Which pairs of wires are used in 10BASE-T and 100BASE-T Ethernet straight-through cables?

In 10BASE-T and 100BASE-T Ethernet straight-through cables, only two pairs of wires (pins 1, 2, 3, and 6) are used for transmitting and receiving data.

What is an MDI, and what is its role in Ethernet networking?

MDI stands for Media-Dependent Interface. It refers to the network interface card (NIC) in a computer, responsible for transmitting and receiving data over Ethernet cables.

How do the pin configurations of MDI and MDI-X differ?

MDI uses pin 1 for transmit and pin 3 for receive, while MDI-X (typically in switches) uses pin 1 for receive and pin 3 for transmit, ensuring proper communication between devices.

Why do gigabit Ethernet (1000BASE-T) cables differ from 10/100 Mbps Ethernet cables?

In gigabit Ethernet, all four wire pairs are used for both transmitting and receiving data simultaneously, unlike 10/100 Mbps Ethernet, where only two pairs are used.

When should a crossover cable be used instead of a straight-through cable?

A crossover cable is used when connecting two devices of the same type, like two workstations or two switches. For different types (e.g., workstation to switch), a straight-through cable is used.

What is Auto-MDI-X, and how does it simplify network connections?

Auto-MDI-X is a feature in modern Ethernet devices that automatically detects the need for a crossover cable and adjusts the connection internally, eliminating the need for a physical crossover cable.

Why is the assumption that crossover cables use 568A on one side and 568B on the other incorrect?

This assumption is a misnomer, especially in gigabit Ethernet (1000BASE-T). The crossover configuration is based on the IEEE 802.3 Ethernet standard, which does not follow the 568A and 568B color coding for crossover cables.

What type of cable should be used when connecting a router to a switch?

A straight-through cable should be used when connecting a router to a switch because they are different device types (MDI to MDI-X).

Outlines

00:00

🔗 Understanding Straight-Through Ethernet Cables

This section introduces the concept of straight-through Ethernet cables, also referred to as patch cables. These cables connect the same pins (1 to 1, 2 to 2, etc.) on both ends. They are commonly used to connect workstations to network devices such as switches and are essential for 10BASE-T and 100BASE-T connections, which use only two pairs of wires. The paragraph also introduces the key components of Ethernet communication: the network interface card (MDI) and network switches (MDI-X), explaining how data flows through transmit and receive pins, and highlighting the difference between lower and higher bandwidths like gigabit Ethernet, which utilizes all four wire pairs.

05:01

🔄 Crossover Cables and Their Usage in Gigabit Ethernet

This section discusses crossover cables, used to connect devices of the same type (e.g., two workstations or two switches). It details how crossover cables differ from straight-through cables, specifically in gigabit Ethernet connections, where all eight wires are crossed. Modern Ethernet devices often support Auto-MDI-X, which eliminates the need for physical crossover cables by automatically adjusting the communication flow. The paragraph also dispels a common misconception that crossover cables must follow the TIA 568A and 568B color-coding scheme, emphasizing that the IEEE 802.3 standard dictates crossover wiring, especially for gigabit Ethernet, where this color scheme is not applicable.

Keywords

💡Straight-through cable

A straight-through cable is an Ethernet cable where the wires on one side connect directly to the corresponding wires on the other side (e.g., pin 1 to pin 1, pin 2 to pin 2, etc.). It's commonly used to connect devices like workstations to network switches. The script emphasizes this cable's role in connecting devices with different communication types, such as MDI to MDI-X.

💡Patch cable

A patch cable is another term for a straight-through cable, typically used in wiring closets to connect devices like patch panels to switches. The video mentions it as a common Ethernet cable used for connecting devices in a network, such as workstations to switches.

💡10BASE-T and 100BASE-T

These terms refer to standards for Ethernet networks operating at 10 and 100 megabits per second, respectively. In the script, it is explained that these types of networks only use two pairs of wires in Ethernet cables, and they are limited to specific pins for transmitting and receiving data.

💡Network Interface Card (NIC)

A Network Interface Card (NIC) is the hardware inside a computer that allows it to connect to a network. The script refers to it as an MDI (Media-Dependent Interface) and explains its role in transmitting and receiving data across an Ethernet cable, particularly in 10/100BASE-T networks.

💡MDI (Media-Dependent Interface)

MDI stands for Media-Dependent Interface, which is a term used to describe devices like network interface cards in computers. MDI devices transmit data through specific pins (pin 1 for transmit and pin 3 for receive). The script uses this term to explain the role of NICs in Ethernet networks.

💡MDI-X (Media-Dependent Interface Crossover)

MDI-X refers to a device, such as a network switch, that reverses the transmit and receive pins compared to an MDI device. The script explains that switches use MDI-X to ensure proper communication with MDI devices, as data is transmitted and received across different pins.

💡Gigabit Ethernet (1000BASE-T)

Gigabit Ethernet, or 1000BASE-T, is a standard for Ethernet networks that transmit data at 1 gigabit per second. Unlike 10BASE-T and 100BASE-T, it uses all four pairs of wires in an Ethernet cable. The script highlights that in Gigabit Ethernet, both transmit and receive operations occur on every wire, eliminating the need for separate transmit and receive pins.

💡Crossover cable

A crossover cable is used to connect two devices of the same type (e.g., two workstations or two switches). The script explains that instead of a straight-through connection, crossover cables 'cross over' the wiring (e.g., pin 1 on one side connects to pin 3 on the other), allowing devices to communicate without a switch.

💡Auto-MDI-X

Auto-MDI-X is a feature in modern Ethernet devices that automatically detects whether a crossover cable is needed and configures the connection without requiring a physical crossover cable. The script highlights how this feature simplifies network connections, as straight-through cables can be used for most connections.

💡TIA 568A and 568B

TIA 568A and 568B are standards for color-coding the wires in Ethernet cables. The script explains a common misconception that crossover cables are simply wired 568A on one side and 568B on the other, which is not accurate in gigabit communication (1000BASE-T).

Highlights

A straight-through cable connects Ethernet devices, mapping pins 1-to-1, 2-to-2, and so on, commonly referred to as a patch cable.

Straight-through cables are used to connect workstations to network devices like switches.

10BASE-T and 100BASE-T Ethernet cables use only two pairs of wires for networking, specifically pins 1, 2, 3, and 6.

The network interface card (NIC) is known as a media-dependent interface (MDI), with pin 1 for transmit plus and pin 2 for transmit minus.

A network switch is referred to as a media-dependent interface crossover (MDI-X), with transmit and receive functions on different pins.

In 10/100-megabit Ethernet, the NIC transmits to the switch's receive pins and receives from the switch's transmit pins.

Gigabit Ethernet (1000BASE-T) uses all four pairs of wires for simultaneous transmit and receive on each pair, eliminating the need for separate transmit and receive pins.

For gigabit connections, it is crucial that all eight wires in the Ethernet cable are properly connected.

Connecting two MDI or two MDI-X devices (e.g., two switches) requires a crossover cable.

The 1000BASE-T crossover cable wiring pattern involves crossing all eight wires between the two connected devices.

Modern Ethernet devices can use Auto-MDI-X to automatically detect when a crossover is needed, removing the need for a physical crossover cable.

There is a common misconception that a crossover cable follows TIA 568A on one side and TIA 568B on the other, which is not true for gigabit Ethernet.

In gigabit networking, crossover cables do not follow the TIA 568A/568B color pattern, as this only applies to earlier Ethernet standards.

To connect a workstation to a switch or router to a switch, use a straight-through cable, as these are different types of devices (MDI to MDI-X).

Connecting like devices (e.g., switch-to-switch, router-to-router, or workstation-to-workstation) requires a crossover cable.

Transcripts

When we're connecting ethernet devices,

we're commonly using a cable that

is a straight-through cable.

This means that the wire on pin 1 on one side

is connecting to the wire on pin 1 on the other side,

pin 2 to pin 2, pin 3 to pin 3, and so on.

You'll sometimes hear this referred to as a patch cable,

because this is the cable we use inside of a wiring

closet to patch from a patch panel into an ethernet switch.

This is a very common Ethernet cable.

If you were to look at the Ethernet cable being used

on your computer, it's probably a straight-through cable,

and it's most commonly used to connect workstations

to network devices, such as switches.

This is a wire map of 10BASE-T and 100BASE-T

straight-through cables.

You can see that there are only two pair

being used here, because only two pairs of wires

are used for 10-megabit and 100-megabit networking.

This block on the left is a network interface card.

This is the adapter that's inside of your computer,

and it's technically referred to as a media-dependent interface,

or an MDI.

You can see that pin 1 is a transmit plus,

pin 2 is a transmit minus, pin 3 is a receive plus,

and pin 6 is a receive minus.

You can see that, with 10 and 100-megabit Ethernet,

we don't use pins 4, 5, 7, and 8.

On the right side is a network switch.

This is a media-dependent interface crossover, or MDI-X.

Sometimes we'll refer to this as "em-dix."

This is a receice plus and receive minus on pins 1 and 2,

and transmit plus and transmit minus on pins 3 and 6.

This means that pin 1 transmit on one side

is connected to pin 1 receive on the other side.

If we were to look at the traffic flows

across this 10 or 100-megabit Ethernet cable,

the network interface card would be transmitting to the receive

pins and receiving traffic from the transmit pins

on the other side.

This traffic flow changes a bit when

we get into gigabit networking and higher.

With a gigabit straight-through or 1000BASE-T

straight-through connection between a network interface

card or MDI communicating to a network switch or MDI-X,

you can see that there is no longer a transmit

and receive side.

That's because, with gig communication,

there's both transmit and receive

on every single wire used.

And we're using all four pairs of wires in a single Ethernet

cable.

In this diagram I've labeled the pairs of wires as data A,

data B, data C, and data D. This is

why, whenever we're connecting a gigabit Ethernet cable,

we have to make sure that all eight wires are connecting

on both sides of the cable.

If we're connecting two devices to each other that

are both MDI devices--

for example, two separate workstations

to each other-- or we're connecting MDI-X devices

to each other-- for example, connecting two switches

to each other-- we would not use a straight-through cable.

Instead, we would use a crossover cable.

This is the crossover cable pin out for 1000BASE-T

based on the 802.3 standard.

You can see that pin 1 goes to pin 3, pin 2 to 6,

3 to 1, 4 to 7, 5 to 8, 7 to 4, and 8 to 5.

Because all eight of the wires are

used in a gigabit communication, we

would also need to cross over all eight of those wires

to create a crossover cable.

One nice feature on most modern Ethernet devices is that it can

automatically identify when a crossover cable is needed

and implement what's known as Auto-MDI-X, or Auto "em-dix."

this means that both devices will

identify that they're both MDI devices or both MDI-X devices,

and internally automatically crossover the communication

without needing a separate physical crossover cable.

This certainly simplifies things if you're

connecting to a network, because you

can plug in a single straight-through cable,

and the devices themselves will determine if a crossover is

needed or not.

One common misnomer with crossover cables

is that one side of the crossover cable

is wired to be TIA 568A colors and the other side

is wired to be TIA 568B colors.

Well, as you can see, with this crossover cable for 1000BASE-T,

that's actually not the case.

One side is configured in this case for 568B,

but the other side is neither 568A or B colors.

The determination of what colors are

used on each pin come from the TIA 568 standard,

but the determination of what makes up a crossover cable

comes directly from the IEEE 802.3 Ethernet standard.

This confusion with a crossover cable

being 568A on one side and 568B on the other

probably started when we only had two pairs of wires

that we needed to worry about.

And with 10 and 100-megabit Ethernet,

you could see that the wires on one side and the wires

and the other side do have a similarity to A and B wiring.

Obviously, we're not using pins 4, 5, 7, and 8.

This definition files apart, though,

when we get into gigabit communication,

because obviously, a 1000BASE-T crossover is not

568A on one side and 568B on the other.

I realize there are many books and many websites that

will tell you that a crossover cable is 568A on one side

and 568B on the other, but as you can see,

that isn't actually the case when you

get into gigabit networking.

So if you're connecting devices together,

do you use a straight-through cable

or do you use a crossover cable?

If you're connecting a workstation to a switch--

this would be an MDI device connecting to MDI-X--

we would use a straight-through cable.

If you're connecting a router to a switch,

it's a similar configuration with a straight-through cable.

If you're connecting a switch to a switch,

those are both like devices, which

means we would not use a straight-through cable.

We would use a crossover cable.

A router to router is also a scenario where

both devices are identical.

We would use a crossover cable with those as well.

And the same applies when connecting a workstation

to a workstation.

Because those are the same device,

we would use a crossover cable.

A workstation to a router looks like these

would be two separate devices, but both a workstation

and a router are both MDI devices,

so you would use a crossover cable with that configuration

as well.