The Data Link layer Flashcards
Ethernet and MAC addresses(1)
1 - traditional cable networks are still the most common option you find in the workplace and definitely in the data center.
2 - The protocol most widely used to send data across individual links is known as Ethernet. Ethernet and the data link layer provide a means for software at higher levels of the stack to send and receive data. One of the primary purposes of this layer is to essentially abstract away the need for any other layers to care about the physical layer and what hardware is in use.
3 - Ethernet, as a protocol, solved this problem by using a technique known as carrier sense multiple access with collision detection(CSMA/CD). This is used to determine when the communications channels are clear and when the device is free to transmit data.
4 - A way to identify which node the transmission was actually meant for, is called a media access control address or MAC address comes into play. A MAC address is a globally unique identifier attached to an individual network interface. It’s a 48-bit number normally represented by six groupings of two hexadecimal numbers(16 bit).
Ethernet and MAC addresses(2)
5 - Since we don’t have numerals to represent any individual digit larger than nine, hexadecimal numbers employed the letters A, B, C, D, E, and F to represent the numbers 10, 11, 12, 13, 14, and 15.
6 - Another way to reference each group of numbers in a MAC address is an octet. An octet, in computer networking, is any number that can be represented by 8 bits. In this case, two hexadecimal digits can represent the same numbers that 8 bits can.
7 -The total number of a possible MAC addresses that could exist is 2 to the power 48.
8 - The first three octets of a MAC address are known as the organizationally unique identifier or OUI. These are assigned to individual hardware manufacturers by the IEEE or the Institute of Electrical and Electronics Engineers. This is a useful bit of information to keeping your back pocket because it means that you can always identify the manufacturer of a network interface purely by its MAC address.
Ethernet and MAC addresses(3)
9 - The last three octets of MAC address can be assigned in any way that the manufacturer would like with the condition that they only assign each possible address once to keep all MAC addresses globally unique.
10 - Ethernet uses MAC addresses to ensure that the data it sends has both an address for the machine that sent the transmission, as well as the one that the transmission was intended for.
Unicast, Multicast, and Broadcast.
1 - A unicast transmission is always meant for just one receiving address.
2 - this is done by looking at a special bit in the destination MAC address. If the least significant bit in the first octet of a destination address is set to zero, it means that Ethernet frame is intended for only the destination address.
3 - This means it would be sent to all devices on the collision domain, but only actually received and processed by the intended destination. If the least significant bit in the first octet of a destination address is set to one, it means you’re dealing with a multicast frame.
Ethernet Frame(1)
1 - A data packet is an all-encompassing term that represents any single set of binary data being sent across a network link.
2 - 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.
3 - 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.
4 - 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.
Ethernet Frame(2)
5 - 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.
6 - VLAN header indicates that the frame itself is what’s called a VLAN frame.
7 - 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.
8 - 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.
Ethernet Frame(3)
9 - 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.
10 - 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.
11 - 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.
12 - 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.
13 - 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.
