Introduction to the link layer

The link layer moves a network-layer datagram between adjacent nodes (hosts, switches, routers, or Wi-Fi access points).

Types of network links

point-to-point links:
- connect one sender to one receiver over a dedicated link
- examples:
  - two routers connected by a long-distance link
  - a computer connected to an Ethernet switch
- common protocols:
  - HDLC (High-Level Data Link Control)
  - PPP (point-to-point protocol)
broadcast links:
- shared by multiple senders and receivers
- examples:
  - wireless LANs
  - satellite networks
  - hybrid fiber-coaxial (HFC) networks
- common protocols:
  - Cable Internet: DOCSIS over hybrid fiber-coaxial (HFC) networks
  - Wi-Fi

node: any device that runs a link-layer (layer 2) protocol
- hosts
- routers
- switches
- Wi-Fi access points
link: a communication channel connecting two adjacent nodes
- a Wi-Fi link between a host and a Wi-Fi access point
- an Ethernet link between an access point and a link-layer switch
link-layer frame: encapsulates a network-layer packet (e.g., IP datagram) as payload

Aspect	Description
Trip planning	A travel agent plans a tourist's trip from Princeton (New Jersey) to Lausanne (Switzerland) by booking: a limousine from Princeton to JFK Airport a flight from JFK Airport to Geneva Airport a train from Geneva Airport to Lausanne
Responsibility	Each transportation company is responsible only for its own segment: Limousine company: Princeton → JFK Airport Airline: JFK Airport → Geneva Airport Swiss rail service: Geneva Airport → Lausanne
Each segment	connects two adjacent locations is managed by a different transportation company uses a different mode of transportation (limousine, plane, or train)
Key idea	Although services vary, each segment just moves the passenger from one adjacent stop to the next.
In this analogy	the tourist is a datagram each transportation segment is a link the transportation mode is a link-layer (layer 2) protocol the travel agent is a network-layer (layer 3) routing protocol

Basic service:

Other possible services:

Service	Description
Framing	encapsulate each network-layer datagram within a link-layer frame before transmission over the link the frame format depends on the link-layer protocol
Link access	controls how devices share and use the communication link e.g., Medium Access Control (MAC) protocol
Reliable delivery	ensures a datagram reaches the next node without errors uses acknowledgments (ACKs) and retransmissions if needed: mainly useful on unreliable links, such as wireless networks often omitted on reliable wired links (fiber, coaxial cable, twisted-pair copper)
Error detection and correction	detects or corrects bit errors during transmission introduced by signal attenuation or electromagnetic noise many link-layer protocols detect bit errors so corrupted frames are not forwarded some protocols can also correct certain errors

The link layer is mostly implemented in a chip called a network adapter also called a network interface controller (NIC).

This chip handles tasks like:

The link-layer uses both hardware and software:

Published on 03/08/2026