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  • DMAC
  • Reference:

LAN Data-Link Layer Protocols

PreviousEthernet Standards (Fiber Optic)NextEthernet Addressing (MAC)

Last updated 4 years ago

The most commonly used LAN 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.

Data-Link Layer