LAN Data-Link Layer Protocols
The most commonly used LAN Data-Link Layer protocol is Ethernet & WAN Data-Link Layer protocols are High-Level Data Link Control (HDLC) and Point-to-Point Protocol (PPP).
Ethernet Frame is Ethernet Header+Trailer encapsulating the Data.
Ethernet
Preamble
SFD
DMAC
SMAC
Type
Data & PAD
FCS
7 Bytes
1 Byte
6 Bytes
6 Bytes
2 Bytes
46-1500 Bytes
4 Bytes
Header
Data
Trailer
Preamble:
For Synchronization
To provide 5MHz clock timing which allows the receiving Computer/device to lock the incoming bitstream.
It looks like this (10101010 10101010 10101010 10101010 10101010 10101010 10101010)
SFD (Start Frame Delimiter):
The last ON bit of this field Signifies that the next byte begins the DMAC or DMAC is incoming.
It looks like this (10101011)
DMAC
The Receiver of the Frame
SMAC
The Sender of the Frame
Type
Type of Layer 3 Network Layer packet (IPv4/IPv6).
The Frame is however in a Hexadecimal format so instead of putting 'IPv4'/'IPv6', they put Ether type as 0800 for IPv4 & 86DD for IPv6. These DIFFERENT ETHER TYPES are assigned by IEEE for different Protocols.
By looking at this type of field a Networking device that received the frame understands which Protocol stack it belongs to (IPv4/IPv6)
Data & PAD
L3PDU (Data+IP Header) + PAD to meet the minimum length requirement of the Data Field (i.e. 46 Bytes)
FCS(Frame Check Sequencing)/ (CRC Errors):
To check transmission error.
Errors occur when Frame's bits changes due to Electrical interference or due to Corrupt NIC.
Sender's Ethernet Trailer applies a complex Math formula to the frame & store the results in the FCS field & then send it.
The Receiver on the other side applies the same Math Formula after receiving the Frame & then compares the Sender's results with its own results.
If the results are the same then the frame is Accepted or else it will show the errors as CRC Errors & Discard them as a Corrupt Frame.
Reference:
CCNA 200-301 OCG, Volume 1, Pg. 49-50,52-53 - Wendell Odom.
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