The link layer handles node-to-node delivery over a local link. PSC questions often test MAC addressing, ARP, Ethernet frame logic, collision handling, switches versus hubs and wireless LAN differences.

Engineering Definitions

Link layer

Standard definition: The layer responsible for framing, error detection and local node-to-node delivery over a physical link.

Exam meaning: Same LAN/link भित्र frame deliver गर्ने layer।

ARP

Standard definition: Address Resolution Protocol maps an IPv4 address to a MAC address in a local network.

Exam meaning: IP थाहा छ तर Ethernet frame पठाउन MAC चाहिन्छ; ARP ले MAC खोज्छ।

CSMA/CD

Standard definition: Carrier Sense Multiple Access with Collision Detection is an Ethernet media access method for shared half-duplex links.

Exam meaning: Shared Ethernet मा channel सुनेर पठाउने र collision detect गर्ने method।

Concept Teaching

IP packet local network मा सिधै पठिँदैन; यसलाई frame मा राख्नुपर्छ। Frame ले destination MAC चाहिन्छ। If destination is inside same LAN, ARP asks “who has this IP?” If destination is outside, host ARPs for default gateway MAC.

Engineering Mechanism

  • Sender checks routing decision: same LAN or default gateway.
  • Sender needs next-hop MAC address.
  • ARP request is broadcast; ARP reply is unicast.
  • Ethernet frame carries destination MAC, source MAC, type, payload and FCS.
  • Switch learns source MAC on incoming port and forwards based on MAC table.

Diagrams / Models To Draw

  • Draw Ethernet frame fields.
  • Draw ARP request/reply between host and gateway.
  • Draw hub vs switch collision domains.
  • Draw WLAN station and access point association.

Formulas, Fields and Algorithms

  • Ethernet MAC address length: 48 bits.
  • CRC/FCS is used for error detection, not correction.
  • Collision domain: hub shares one; switch separates per port.
  • CSMA/CD applies to legacy shared Ethernet, not modern full-duplex switched Ethernet.
Item Role Key distinction
Hub Repeats bits to all ports One collision domain
Switch Forwards frames using MAC table Separate collision domains
Bridge Connects LAN segments Software/older switching concept
ARP IP to MAC mapping Broadcast request, unicast reply

Exam Point

  • ARP is a favorite MCQ topic: IP-to-MAC, not MAC-to-IP.
  • Explain switch learning: source MAC learning and destination MAC forwarding.
  • Mention FCS/CRC for error detection.

Worked Example

Host A wants to send to 192.168.1.20. It checks ARP cache. If no MAC exists, it broadcasts ARP request. Host B replies with its MAC. Host A then sends Ethernet frame to that MAC.

Subjective Answer Pattern

  • Define link layer services.
  • Explain framing and MAC addressing.
  • Explain ARP with request/reply flow.
  • Compare hub, bridge and switch.
  • Add Ethernet/WLAN/PPP note based on question.

Common Engineering Mistakes

  • Saying ARP works across the whole internet.
  • Confusing IP address with MAC address.
  • Writing CSMA/CD for modern full-duplex switched Ethernet without qualification.

MCQ Revision

  • ARP maps what to what?
  • MAC address length?
  • Switch forwards using which table?
  • FCS is for detection or correction?

Final Summary

  • Link layer is local-hop delivery.
  • ARP bridges IP and MAC addressing.
  • Switching reduces collision domains and improves LAN performance.