The Data Link Layer Read Chapters 3 and 4

CSC 3352 Computer Communications Chapter 3&42 Where in the OSI Reference Model ? Session Transport Network Data Link Layer Physical Presentation Application Logical Link Layer Medium Access Control

CSC 3352 Computer Communications Chapter 3&43 The Data Link Layer

The Data Link Layer: The Medium Access Control Reading: Chap. 4 in Tanenbaum Chap. 7 + Section 14.1 in Stallings

CSC 3352 Computer Communications Chapter 3&45 Why Do We Need a MAC Layer ? Let us consider different topologies Point-to-point channel  full duplex  half duplex Broadcast channel

CSC 3352 Computer Communications Chapter 3&46 Management of Broadcast Channels Allocating statically shares of the channel to all stations (by TDM, FDM, or other) Let stations compete for the ALL channel

CSC 3352 Computer Communications Chapter 3&47 A Fundamental Result In Queueing Theory In the context of LANs, one powerful bandwith for all is better than one part of it for each one ! Why ? Better utilization, as a dedicated channel may stay IDLE.

CSC 3352 Computer Communications Chapter 3&48 Multiple Access Protocols Competing stations (possible collisions) Aloha Slotted Aloha CSMA CSMA/CD Collision-free Bit-map protocol Binary countdown Limited contention Adaptive tree walk protocol

CSC 3352 Computer Communications Chapter 3&49 The Pure Aloha Model

CSC 3352 Computer Communications Chapter 3&410 The Slotted Aloha Model

CSC 3352 Computer Communications Chapter 3&411 CSMA-Persistent

CSC 3352 Computer Communications Chapter 3&412 CSMA Non-Persistent

CSC 3352 Computer Communications Chapter 3&413 CSMA with Collision Detection

CSC 3352 Computer Communications Chapter 3&414 The Contention period

CSC 3352 Computer Communications Chapter 3&415 Collision-free: the bit-map protocol All previous methods can lead to collisions… Divide the contention period into N intervals slots (N is the machine number in the LAN, 1bit per slot)… The sender broadcast one bit to reserve and then, once all the bit- map is passed by, the channel can be seized by stations with respect to their reservation order… See figure 4.6, page 254 Slots’ number correpsond to the order of machines on the LAN… We have an overhead of 1bit per machine…

CSC 3352 Computer Communications Chapter 3&416 Collision-free: the Binary countdown A station willing to use the channel sends its Binary address starting from the high level bits… Priority rules are clearly set to select the next broadcaster based on its Address… Bit 1 beats bit 0… In case of equality, we go to the next bit… See figure 4.7, page 256

CSC 3352 Computer Communications Chapter 3&417 Limited contention: Adaptive tree walk protocol D 7 ABC 1 E H GF The LAN machines