Ethernet Frame Format

Ethernet is a widely used local area networking technology that enables devices to communicate with each other. Ethernet frames are basic units of data that are transmitted over Ethernet network. In this article, we will explore Ethernet frame format in detail, including various fields that make up frame and purpose of each field.

Introduction to Ethernet Frames

An Ethernet frame is a packet of data that is transmitted over an Ethernet network. frame is composed of a header, a payload, and a trailer. header and trailer contain information about frame, such as source and destination addresses, while payload contains actual data being transmitted.

Ethernet frames are sent between devices on a network using a method called Carrier Sense Multiple Access with Collision Detection (CSMA/CD). This means that each device listens for traffic on network before sending a frame to avoid collisions. If two devices try to send a frame at same time, a collision will occur, and devices will wait a random amount of time before attempting to resend their frames.

Ethernet Frame Structure

An Ethernet frame is made up of several fields that provide information about frame and data being transmitted. frame structure is as follows −

Preamble	Destination Address	Source Address	EtherType	Payload	FCS
7B	6B	6B	2B	Variable	4B

The following subheadings will provide an explanation of each field.

Preamble

The preamble is a seven-byte (56-bit) field that serves as a synchronization sequence. It consists of alternating 1s and 0s and is used to signal start of a frame to receiving device. preamble allows receiver to synchronize its clock with sender's clock so that it can correctly interpret data that follows.

Destination Address

The destination address field is a six-byte (48-bit) field that identifies recipient of frame. This field contains Media Access Control (MAC) address of device that is intended to receive frame. MAC address is a unique identifier assigned to each network interface controller (NIC) on network.

For example, if a device wants to send a frame to a printer with a MAC address of 00:11:22:33:44:55, it will populate destination address field with that MAC address.

Source Address

The source address field is a six-byte (48-bit) field that identifies sender of frame. This field contains MAC address of device that is sending frame.

For example, if a device wants to send a frame to a printer with a MAC address of 00:11:22:33:44:55, it will populate source address field with its own MAC address.

EtherType

The EtherType field is a two-byte (16-bit) field that specifies type of data that is contained in payload. It is used by receiving device to determine how to interpret payload data.

For example, if EtherType field is set to 0x0800, it indicates that payload contains an Internet Protocol (IP) packet. If EtherType field is set to 0x0806, it indicates that payload contains an Address Resolution Protocol (ARP) packet.

Payload

The payload field is a variable-length field that contains actual data being transmitted. length of payload can vary depending on type of data being transmitted.

For example, if a device is sending an email message, payload would contain text of message. If a device is sending a file, payload would contain contents of file.

Frame Check Sequence (FCS)

The Frame Check Sequence (FCS) field is a four-byte (32-bit) field that is used to detect errors in frame. FCS field is calculated by sender using a cyclic redundancy check (CRC) algorithm, and receiver calculates FCS field again to verify that frame was received correctly. If calculated FCS field does not match FCS field in frame, it indicates that frame was corrupted during transmission, and receiver discards frame.

For example, if a device sends a frame with an incorrect FCS field, receiving device will detect error and discard frame. sending device will then need to resend frame with a correct FCS field.

Ethernet Frame Examples

Let's take a look at some examples of Ethernet frames to better understand structure and purpose of each field.

Example 1: Sending an Email Message

Suppose a user wants to send an email message to a colleague on same Ethernet network. user's device has a MAC address of 00:11:22:33:44:55, and colleague's device has a MAC address of 66:77:88:99:aa:bb.

The Ethernet frame that would be sent to deliver email message might look something like this −

Preamble	Destination Address	Source Address	EtherType	Payload	FCS
10101010. ..1010	66:77:88: 99:aa:bb	00:11:22: 33:44:55	0x0800	Email message data	Calculated

In this example, preamble consists of alternating 1s and 0s, and destination address field contains MAC address of colleague's device. source address field contains MAC address of user's device. EtherType field is set to 0x0800 to indicate that payload contains an IP packet. payload field contains actual email message data, and FCS field is calculated by sender using a CRC algorithm.

Example 2: Sending a File

Suppose a user wants to send a file to another device on same Ethernet network. user's device has a MAC address of 00:11:22:33:44:55, and recipient's device has a MAC address of 77:88:99:aa:bb:cc.

The Ethernet frame that would be sent to deliver file might look something like this −

Preamble	Destination Address	Source Address	EtherType	Payload	FCS
10101010 ...1010	77:88:99 :aa:bb:cc	00:11:22 :33:44:55	0x0800	File contents data	Calculated

In this example, preamble consists of alternating 1s and 0s, and destination address field contains MAC address of recipient's device. source address field contains MAC address of user's device. EtherType field is set to 0x0800 to indicate that payload contains an IP packet. payload field contains contents of file, and FCS field is calculated by sender using a CRC algorithm.

Advantages and Disadvantages of Ethernet Frame Format

Ethernet frame format has been standard for data transmission in local area networks (LANs) for many years. Here are some of advantages and disadvantages of Ethernet frame format −

Advantages

• Efficient use of bandwidth − Ethernet frames allow for efficient use of bandwidth by enabling multiple devices to transmit and receive data at same time.

• Flexibility − Ethernet frames are flexible in terms of types of data that can be transmitted. They can carry a wide range of data, including email messages, files, video, and voice.

• Reliability − FCS field in Ethernet frames provides a reliable way to detect errors in data transmission. This helps ensure that data is transmitted accurately and reliably.

• Compatibility − Ethernet frames are compatible with a wide range of devices, making them a versatile and widely used data transmission standard.

Disadvantages

• Limited distance − Ethernet frames are limited in terms of distance that data can be transmitted. maximum distance for data transmission is around 100 meters, beyond which signal degradation occurs.

• Limited bandwidth − Ethernet frames have limited bandwidth, which can limit amount of data that can be transmitted at any given time.

• Vulnerability to security threats − Ethernet frames can be vulnerable to security threats such as hacking and data theft, which can compromise confidentiality and integrity of transmitted data.

• Limited scalability − Ethernet frames are limited in terms of scalability, which can limit their use in large networks.

Conclusion

Ethernet frames are basic units of data transmission in Ethernet networks. They consist of several fields, including preamble, destination address, source address, EtherType, payload, and FCS fields. Each field serves a specific purpose in transmission of data, including identifying destination and source devices, indicating type of data being transmitted, and detecting errors in frame. Understanding format and structure of Ethernet frames is essential for troubleshooting network issues and optimizing network performance.