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  • HDLC (Cisco Proprietary)
  • Flag (Eq to Preamble+SFD of Ethernet):
  • Address (Eq to DMAC of Ethernet):
  • Control (N/A):
  • Type (Eq to Type of Ethernet):
  • FCS (Eq to FCS of Ethernet):
  • Reference:

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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Last updated 4 years ago