Ethernet Frame format

 Ethernet Frame format:

The basic frame consists of seven elements as follows:

Preamble (PRE) - This is seven bytes long and it consists of a pattern of alternating ones and zeros, and this informs the receiving stations that a frame is starting as well as enabling synchronisation. (10 Mbps Ethernet)

Start Of Frame delimiter (SOF) - This consists of one byte and contains an alternating pattern of ones and zeros but ending in two ones.

Destination Address (DA) - This field contains the address of station for which the data is intended. The left most bit indicates whether the destination is an individual address or a group address. An individual address is denoted by a zero, while a one indicates a group address. The next bit into the DA indicates whether the address is globally administered, or local. If the address is globally administered the bit is a zero, and a one of it is locally administered. There are then 46 remaining bits. These are used for the destination address itself.

Source Address (SA) - The source address consists of six bytes, and it is used to identify the sending station. As it is always an individual address the left most bit is always a zero.

Length / Type - This field is two bytes in length. It provides MAC information and indicates the number of client data types that are contained in the data field of the frame. It may also indicate the frame ID type if the frame is assembled using an optional format.(IEEE 802.3 only).

Data - This block contains the payload data and it may be up to 1500 bytes long. If the length of the field is less than 46 bytes, then padding data is added to bring its length up to the required minimum of 46 bytes.

Frame Check Sequence (FCS) - This field is four bytes long. It contains a 32 bit Cyclic Redundancy Check (CRC) which is generated over the DA, SA, Length / Type and Data fields.

 Ethernet Frame format:

Here's what each field in the frame format means:

• Preamble: This is a sequence of 7 bytes (56 bits) used to synchronize the receiver's clock with the sender's clock. The preamble consists of alternating 1s and 0s, followed by a final 1, and is used to identify the beginning of a frame.

• Start Frame Delimiter (SFD): This is a single byte (8 bits) used to indicate the start of the Ethernet frame. It has a fixed value of 10101011.

• Frame Payload: This is the actual data being transmitted, which can range from 46 to 1500 bytes in length. The payload can include anything from network layer packets to application layer data.

• Frame Check Sequence (FCS): This is a 4-byte (32-bit) cyclic redundancy check (CRC) used to ensure that the frame has been transmitted without errors.

Now, let's talk about the physical media that Ethernet can run over. Ethernet was originally designed to run over coaxial cables, but it can now run over a variety of physical media, including:

• Twisted pair copper cables: This is the most common type of Ethernet cabling today, with variations like Cat5e, Cat6, and Cat7. These cables are used for Ethernet connections in homes, businesses, and data centers.

• Fiber optic cables: Ethernet can also run over fiber optic cables, which offer higher bandwidth and longer distance transmissions than copper cables.

• Wireless media: Ethernet can be transmitted over wireless media, such as Wi-Fi or microwave links.

• Powerline communication: Ethernet can also be transmitted over power lines, although this is less common.

In summary, Ethernet frames consist of a preamble, SFD, frame payload, and FCS. Ethernet can run over various physical media, including twisted pair copper cables, fiber optic cables, wireless media, and power lines.