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.