MAC protocol (1)

Today, I learned about the MAC protocol.

 

Where is the link layer implemented?

• At each interface of hosts, routers, switches, access points
• Link layer implemented mainly in "adoptor" (aka network interface card NIC) or on a chip.
  • e.g. Ethernet card (LAN), 802.11 card (WLAN)
  • NIC implements link layer and physical layer
• Link layer is implemented in both
  • NIC (hardware)
  • Operating System (software)

 

Frame transmission from src to dst

• Sending Side:
  • Rreceives datagram from Network layer
  • Encapsulates datagram in a link frame (adds link header)
  • Passes to Physical layer
• Receiving side:
  • Receives frame from Physical layer
  • Extracts datagram from frame (detaches Net header)
  • Passes to network layer

 

Multiple access links, protocols

Two types of "links":

1. Point-to-point: single sender, single receiver
   1. PPP link between a host and router using a switch
   2. PPP link between two routers
2. Broadcast (shared wire or medium): multiple sender, receiver
   1. Old-fashioned Ethernet (Ethernet using hub device)
   2. HFC (Cable Internet)
   3. 802.11 wireless LAN
   4. Satellite

 

Shared vs Dedicated channel

 

Multiple access protocols

• Challenge: single shared broadcast channel for multiple nodes
  • Two or more simultaneous transmissions by nodes: interference
  • Collision if node receives two or more signals at the same time thus, all the frames invovled in the collision are corrupted, and the channel BW is wated during the collision interval.
• Multiple access protocol is a distributed algorithm that determines how nodes share channel.
• Not only data packets, but also communication about channel sharing must use channel itself!
  • Challenge: no out-of-band channel for coordination