Section 9

• The frames are encoded and decoded into bits at Layer 2.
• Error detection and correction for the physical layer can be provided here.
• Ethernet is the Layer 2 medium used on Local Area Network.

When a packet is composed, the sender goes from the top of the OSI model down. During the 7-5 layers, it’s called data, in the Layer 4 (Transport), it’s called segment, in Layer 3 (Network), it’s called packet and in this layer it’s called frame.

It’s composed of:

• Preamble: 8 bytes.
• Destination address: 6 bytes.
• Source address: 6 bytes.
• Length/Ethertype: 2 bytes.
• Data: from 45-1500 bytes.
• Frame Check Sequence (FCS): 4 bytes.

• Ethernet uses a 48 bit hexadecimal MAC address.
• The first 24 bits is the Organizationally Unique Identifier or OUI, which uniquely identifies the manufacturer of the Ethernet port.
• The last 24 bits is vendor assigned.
• The burned in MAC address on every NIC port in the world is globally unique.