Hello and welcome back to this lecture on Ethernet. In the last lecture, we spoke about the TC/IP model and we also compared the TC/IP model with the OSI model. In this video, we wanna talk about a Layer 2 technology known as Ethernet.
First step, what is the Ethernet? Ethernet is a suite of technologies or protocols that define communication at Layer 2, also known as Local Area Network. Ethernet is not the only technology at Layer 2.
There are other technologies as well, such as Token Ring, FDDI, which stands for Fiber Distributed Data Interphase, et cetera. Ethernet was first developed by the company called as Xerox Corporation, and over time it became the most popular and widely adopted technology for Layer 2 communication. It was later on standardized as IEEE 802.3. Now originally, Ethernet was designed to use coaxial cables for communication, as a shared medium.
This means multiple devices would be connected to the same coaxial medium, for communicating over the network. The original standard for Ethernet was known as 10BASE5, and it supported speeds up to 10 megabits per second. Now we have FastEthernet which supports speeds up to 100 megabits per second, and we also have GigabitEthernet which is 1000 megabits per second, or 1 gigabit per second. In addition to providing standards for speed, it also provides standards or specifications for the cables, such as coaxial cables, twisted pair cables like Cat 5, Cat 5e, and Cat 6.
It also provides specifications for the connectors, like RJ45. Just in case you’ve never seen coaxial cables or twisted pair cables, this is how it looks like. The one of the left hand side is coaxial cables. That’s what Ethernet was originally designed for. It’s no longer used for communication over land, but even today it is used for sending signals for televisions. The one on the right hand side is the twisted pair cable, which is what we use these days.
Ethernet frame format
Now let’s talk about the Ethernet frame format. So in the video for OSI model, we discussed that data at Layer 1, or at the physical layer, is in the form of zeroes and ones. When this data moves one level up, or when it moves to Layer 2. It gets transformed into what is known as Frames. So when data moves from Layer 1 to Layer 2, some headers get added onto the data and that’s how the data gets transformed into what is known as Frames. The Ethernet Frame format looks like what you see on the screen right now.
It first starts with seven bytes of preamble. Now, preamble is basically seven bytes of alternating ones and zeroes. Now I’m sure you must be knowing that one byte is eight bits of data. But this one is seven bytes which means it’s 56 bits of alternating ones and zeroes used by the sending and receiving devices to synchronize their internal clocks. After the preamble, we have one byte of SFD, also known as Start Frame Delimiter.
It looks like 10, 10, 10, 11. The Start Frame Delimiter signals the end of the preamble and the start of data. It is then followed by six bytes of Destination MAC address. Now I know we’ve not discussed about MAC addresses so far; it is coming up in the next lecture. But just remember that MAC address is the real address of the computer on the real address of a network device. It is the address that is burnt onto the chip. So the Ethernet Frame Format has six bytes of destination MAC address; it’s then followed by six bytes of source MAC address, and then it has the two bytes of length.
The length field indicates the length of the payload that is about to follow. You then have data, which is the actual payload. The payload must be 46 bytes to 1500 bytes in length. Followed by payload, we have a field known as padding. So if your data is less than the minimum, which is 46 bytes, you add some padding onto it to make sure it has at least 46 bytes of length. And finally, you have FCS; it is also known as Frame Check Sequence, and it is four bytes in length.
It is used for error detection. Alright, now let’s talk about CSMA/CD. So we just discussed that Ethernet was originally designed for communicating over a shared medium, which means multiple devices would be connected to the same wire, or the same cable, and they would try to send data over the same medium. Now, CSMA/CD stands for Carrier Sense Multiple Access with Collision Detection. When multiple devices try to send data at the same time, now since all these devices are connected to the same shared medium, if multiple devices send data at the same time, it would result in collision.
CSMA/CD is a technology that defines the standards for sending data over a shared medium without resulting in collisions. So this is how it works: every device that wants to send data must first listen and check for traffic on the shared medium. A device is allowed to transmit only when the shared medium is determined to be free, which means nobody else is transmitting data. Now, if two or more devices transmit data at the same time, it results in what is known as a collision, or loss of data.
If a collision is detected, sending devices back-off and then wait a random amount of time before the attempt to re-transmit the data. So that’s it guys for this lecture, let me know if you have any questions. If not, I’d like to thank you for watching, and I’ll catch you in the next lecture.
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