Link Layer of the Internet Protocol Suite

The Link Layer is the lowest level of the network stack that makes up the Internet Protocol Suite. It is concerned with the physical and logical connections between hosts on the same local network.

The physical and logical connections are known as Links.

Link Layer protocols goven communication between devices. Examples include:

WiFi
- For wireless communication between devices using radio waves
Ethernet
- Wired communication

All devices connected to the internet participate in the Link Layer since it is the base level and the gateway out of the local network to other networks. The same is not true for other layers. For example a device like a laptop participates in all four layers up to the Application Layer but a network router does not:

Link Layer diagram

MAC addresses

Each device on a link has a network address that uniquely identifies it. For most Link Layer Protocols this is a Media Access Control (MAC) address.

A device receives data addressed to its MAC address.

MAC addresses are limited to local networks. Remote networks cannot send data to a specific MAC address. This is off-limits from the outside.

Frames

Link Layer data is divided into small units called “frames”. The anatomy of a frame is as follows:

Link Layer frame

The header contains the source and destination MAC address plus a descriptor of the type of data it contains. The data is the payload. The footer is used to detect errors.

Transport Layer of the Internet Protocol
- Transport Layer protocols provide a communications channel for data to be sent between hosts. The Transport Layer is dependent upon the Internet Layer for addressing and routing the data and the Link_Layer_of_Internet_Protocol for transferring the data to a specific host once it has reached the local network.
Internet fundamentals
- The Link Layer concerns communication between devices on the same local network. The transfer of information once it has been received from the internet or its transfer between devices before it sent out into the internet. Ethernet and WiFi are two core protocols utilised at this layer.
Internet Layer of the Internet Protocol Suite
- Data is sent accross the internet between hosts as packets. These packets, when sent and received within the Link Layer on the local network, are enclosed within a Link Layer frame (in its payload section):
Example scenario of data transfer accross the internet
- At the Link Layer of the client device, the IP packet is encapsulated in a frame with the MAC_address of the local router in its header. The frame is wirelessly transmitted by the client device’s WiFi hardware.
82f9c228_dissolution_of_ARPANET
- Later, these became commercial and subscription-based. A plethora of …NETS grew up (NSFNET, USENET, CSNET, TELENET etc.), all based on z:tag/packet-switching, the ARPANET nodes as the backbone and TCP/IP host communication.
653c0b1d_host_protocols_of_ARPANET
- The responsibility for enabling communication between nodes fell to the hosts, not the IMPs. At the lowest level (equivalent to the link layer in the subsequent internet?), this consisted in moving raw bits between hosts regardless of what specific type of data they encoded.