# Data Link Layer
###### tags: `computer network`

## Functioanlity of Link Layer
The Link Layer is more focus on node-to-node transmisission via link. To put it another way, it is responsible for data transfer between two adjacent machine
## Service
### Reliable Delivery
| | unacknowledged connectionless | acknowledged connectionless | acknowledged connection |
| -------------------- | ----------------------------- | --------------------------- |:----------------------- |
| establish connection | no | no | yes |
| ack frame | no | yes | yes |

### Framing
#### Frame Count - Bad Approach
Add at the number of bytes at the front for a fram at the begining

#### Byte Stuffing
Add falg at start and the end
more info on [here](https://www.tutorialspoint.com/what-is-byte-stuffing-in-computer-networks)
#### Bit Stuffing
Each frame ends with start and end of a pattern, and if message have the pattern, use the following approach:

### Flow Control
Set rules for sender to send frame (avoid frame loss)
### Error Control
#### Terminology
A codeword is the message + the redundant check bit

A code is a collection of code

Hamming distance d of a code is the smallest hamming distance among all valid codeword

With a code of Hamming distance d:
• We can detect d-1 single-bit errors
• We can correct (d-1)/2 single-bit errors
#### Hamming Code : Error Detection and Correction
see slide
#### CRC: Error Detection
[reference](https://www.youtube.com/watch?v=A9g6rTMblz4&list=PLBlnK6fEyqRgMCUAG0XRw78UA8qnv6jEx&index=48&ab_channel=NesoAcademy)