# LAN Data-Link Layer Protocols

The most commonly used LAN [Data-Link Layer](https://app.gitbook.com/@mudassirs46/s/network-fundamentals/~/drafts/-MRZ8l67L5MHnaQIEh9W/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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