Ethernet & IEEE 802.3 Ethernet Frame Format


Computer Network / Friday, July 17th, 2020

Ethernet is a commonly used protocol for LAN. IEEE 802.3 Ethernet uses 1 persistent CSMA/CD access method.

What is Ethernet?

Ethernet is the most widely installed local area network (LAN) technology. Specified in a standard, IEEE 802.3, Ethernet was originally developed by Xerox from an earlier specification called Alohanet (for the Palo Alto Research Center Aloha network) and then developed further by Xerox, DEC, and Intel. An Ethernet LAN typically uses coaxial cable or special grades of twisted pair wires. Ethernet is also used in wireless LANs. The most commonly installed Ethernet systems are called 10BASE-T and provide transmission speeds up to 10 Mbps. Devices are connected to the cable and compete for access using Carrier Sense Multiple Access with Collision Detection (CSMA/CD) protocol.

Fast Ethernet or 100BASE-T provides transmission speeds up to 100 Megabytes per second and is typically used for LAN backbone systems, supporting workstations with 10BASE-T cards. Gigabit Ethernet provides an even higher level of backbone support at 1000 Megabits per second (1 gigabit or 1 billion bits per second). 10-Gigabit Ethernet provides up to 10 billion bits per second.

Ethernet was named by Robert Metcalfe, one of its developer, for the passive substance called, “luminiferous (light-transmitting) ether” that was once thought to pervade the universe, carrying light throughout. Ethernet was so named to describe the way that cabling, also a passive medium could similarly carry data everywhere throughout the network.

IEEE 802.3 Ethernet with various types of cabling

Characteristics IEEE 802.3 Ethernet cable
Characteristics Ethernet cable

IEEE 802.3 Ethernet Frame Format

IEEE 802.3 Ethernet accesses the channel using 1-persistent CSMA/CD method in LAN. Now we will discuss MAC frame structure for IEEE 802.3 Ethernet with the help of Figure.

IEEE 802.3 Ethernet Frame Format
IEEE 802.3 Ethernet Frame Format

Each frame has seven fields explained as follows:

Preamble: The first field of 802.3 frame is 7 byte (56 bits) long with a sequence of alternate 1 and 0 i.e., 10101010. This pattern helps the receiver to synchronize and get the beginning of the frame.

Starting Delimiter (SD): The second field start delimiter is 1 byte (8 bit) long. It has pattern 10101011. Again, it is to indicate the beginning of the frame and ensure that the next field will be a destination address. Address, here, can be a single address or a group address.

Destination Address (DA): This field is 6 byte (48 bit) long. It contains the physical address of the receiver.

Source Address (SA): This filed is also 6 byte (48 bit) long. It contains the physical address of the sender.

Length of Data Field: It is 2 byte (16 bit) long. It indicates the number of bytes in the information field. The longest allowable value can be 1518 bytes.

Data: This field size will be a minimum of 46 bytes long and a maximum of 1500 bytes as will be explained later.

Pad: This field size can be 0 to 46 bytes long. This is required if, the data size is less than 46 bytes as a 802.3 frame must be at least 64 bytes long.

Frame Checksum (FCS): This field is 4 bytes (32 bit) long. It contains information about error detection. Here it is CRC-32.

Minimum and Maximum Length of Frame

Minimum frame length = 64 bytes = 512 bits, Maximum frame length= 1518 bytes = 12144 bits

Minimum length or lower limit for frame length is defined for normal operation of CSMA/CD. This is required so that, the entire frame is not transmitted completely before its first bit has been received by the receiver. If, this happens then the probability of the occurrence of collision will be high (the same has been explained earlier in the previous post CSMA/CD).

Hence, Ethernet frame must be of 64 bytes long. Some of the bytes are header and trailer parts of the frame. If, we consider 6 bytes destination address, 6 bytes source address, 2 bytes length and 4 bytes FCS (6+6+2+4=18) then, the minimum length of data will be 64-18= 46 bytes. If, frame is less than 46 bytes then, padding bits fill up this difference.

As per 802.3 standard, the frames maximum length or upper limit of frame is = 1518 bytes (excluding preamble and SD). If we subtract 18 bytes of header and trailer then, the maximum length will be 1500 bytes.

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