Dissecting an Ethernet Frame

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From the course by Google

The Bits and Bytes of Computer Networking

3744 ratings

Google

The Bits and Bytes of Computer Networking

3744 ratings

Course 2 of 5 in the Specialization Google IT Support Professional Certificate

This course is designed to provide a full overview of computer networking. In this course, we’ll cover everything from the fundamentals of modern networking technologies and protocols to practical applications and network troubleshooting. By the end of this course, you’ll be able to: - describe computer networks in terms of a five-layer model. - understand all of the standard protocols involved with TCP/IP communications. - grasp powerful network troubleshooting tools and techniques. - learn network services like DNS and DHCP that help make computer networks run. Except as otherwise noted, the contents on this site are licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

From the lesson

Introduction to Networking

Welcome to the Networking course of the IT Support Professional Certificate! In the first week of this course, we will cover the basics of computer networking. We will learn about the TCP/IP and OSI networking models and how the network layers work together. We'll also cover the basics of networking devices such as cables, hubs and switches, routers, servers and clients. We'll also explore the physical layer and data link layer of our networking model in more detail. By the end of this module, you will know how all the different layers of the network model fit together to create a network.

Ethernet and MAC Addresses6:43

Unicast, Multicast, and Broadcast1:58

Dissecting an Ethernet Frame5:35

Victor Escobedo: Practical Experience in IT1:20

Meet the Instructors

Google

To wrap up, we'll round out your understanding of the basics of networking,

by dissecting and Ethernet frame.

Understanding the networking basics is the first step in building a really strong

foundation of networking knowledge that you'll need in IT support.

A data packet is an all-encompassing term

that represents any single set of binary data being sent across a network link.

The term data packet isn't tied to any specific layer or technology.

It just represents a concept.

One set of data being sent from point A to Point B.

Data packets at the Ethernet level are known as Ethernet frames.

An Ethernet frame is a highly structured collection of information

presented in a specific order.

This way network interfaces at the physical layer can convert a string of

bits, travelling across a link into meaningful data or vice versa.

Almost all sections of an Ethernet frame are mandatory and

most of them have a fixed size.

The first part of an Ethernet frame is known as the preamble.

A preamble is 8 bytes or 64 bits long and can itself be split into two sections.

The first seven bytes are a series of alternating ones and zeros.

These act partially as a buffer between frames and

can also be used by the network interfaces to synchronize internal clocks they use,

to regulate the speed at which they send data.

This last byte in the preamble is known as the SFD or start frame delimiter.

This signals to a receiving device that the preamble is over and

that the actual frame contents will now follow.

Immediately following the start frame delimiter,

comes the destination MAC address.

This is the hardware address of the intended recipient.

Which is then followed by the source MAC address, or

where the frame originated from.

Don't forget that each MAC address is 48 bits or 6 bytes long.

The next part of an Ethernet frame is called the EtherType field.

It's 16 bits long and used to describe the protocol of the contents of the frame.

We'll be doing a deep dive on what these protocols are a little later.

It's worth calling out that instead of the EtherType field,

you could also find what's known as a VLAN header.

It indicates that the frame itself is what's called a VLAN frame.

If a VLAN header is present, the EtherType field follows it.

VLAN stands for virtual LAN.

It's a technique that lets you have multiple logical LANs

operating on the same physical equipment.

Any frame with a VLAN tag will only be delivered

out of a switch interface configured to relay that specific tag.

This way you can have a single physical network that operates

like it's multiple LANs.

VLANs are usually used to segregate different forms of traffic.

So you might see a company's IP phones operating on one VLAN,

while all desktops operate on another.

After this, you'll find a data payload of an Ethernet frame.

A payload in networking terms is the actual data being transported,

which is everything that isn't a header.

The data payload of a traditional Ethernet frame

can be anywhere from 46 to 1500 bytes long.

This contains all of the data from higher layers such as the IP, transport and

application layers that's actually being transmitted.

Following that data we have what's known as a frame check sequence.

This is a 4-byte or 32-bit number that represents a checksum value for

the entire frame.

This checksum value is calculated by performing

what's known as a cyclical redundancy check against the frame.

A cyclical redundancy check or CRC, is an important concept for

data integrity and is used all over computing, not just network transmissions.

A CRC is basically a mathematical transformation that uses polynomial

division to create a number that represents a larger set of data.

Anytime you perform a CRC against a set of data,

you should end up with the same checksum number.

The reason it's included in the Ethernet frame is so

that the receiving network interface can infer if it received uncorrupted data.

When a device gets ready to send an Internet frame,

it collects all the information we just covered, like the destination and

originating MAC addresses, the data payload and so on.

Then it performs a CRC against that data and attaches the resulting checksum number

as the frame check sequence at the end of the frame.

This data is then sent across a link and received at the other end.

Here, all the various fields of the Ethernet frame are collected and

now the receiving side performs a CRC against that data.

If the checksum computed by the receiving end doesn't match the checksum

in the frame check sequence field, the data is thrown out.

This is because some amount of data must have been lost or

corrupted during transmission.

It's then up to a protocol at a higher layer to decide if that data

should be retransmitted.

Ethernet itself only reports on data integrity.

It doesn't perform data recovery.

You've gotten the basics of networking now, nice work.

Next up is a quiz.

You got this, but even if you don't,

just review the material until you get more comfortable with this stuff.

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