WAN Data-Link Layer Protocols
The most commonly used WAN Data-Link Layer protocol is Ethernet & WAN Data-Link Layer protocols are High-Level Data Link Control (HDLC) and Point-to-Point Protocol (PPP).
The two most commonly used WAN Data-Link Layer protocols are High-Level Data Link Control (HDLC) and Point-to-Point Protocol (PPP).
On WAN Links Ethernet Frames are De-encapsulated & Re-encapsulated with HDLC Header&Trailer or they use EoMPLS to avoid Serial Technology using HDLC WAN Protocol.
HDLC (Cisco Proprietary)
Flag | Address | Control | Type | Data | FCS |
1 Byte | 1 Byte | 1 Byte | 2 Bytes | (Variable) | 2 Bytes |
Header | Data | Trailer |
Flag (Eq to Preamble+SFD of Ethernet):
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)
Address (Eq to DMAC of Ethernet):
Identifies the destination device.
Control (N/A):
no longer in use today
Type (Eq to Type of Ethernet):
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.
FCS (Eq to FCS of Ethernet):
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 Discarded as a Corrupt Frame.
Reference:
CCNA 200-301 OCG, Volume 1, Pg. 63 - Wendell Odom.
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