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MAC Sublayer. COMPUTER NETWORKS Data-link Layer (The Medium Access Control Sublayer). MAC Sublayer. Questions to be answered ? In broadcast networks, How the channel is divided between competing users? What is Medium Access Control (MAC)?

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Presentation Transcript
slide1

MAC Sublayer

COMPUTER NETWORKS

Data-link Layer

(The Medium Access Control Sublayer)

slide2

MAC Sublayer

  • Questions to be answered ?
  • In broadcast networks, How the channel is divided between competing users?
  • What is Medium Access Control (MAC)?
  • What protocols are used for allocating a multiple access channel ?

Computer Networks

slide3

MAC Sublayer

  • the need for determine who gets to use the channel
  • When there are two ormore users trying to use a shared single channel there should be an algorithm to control this access.
  • This problem occurs in broadcast networks which are known as multiaccess channels.

Computer Networks

slide4

MAC Sublayer

  • What is MAC?
  • Medium Access Control (MAC) is a sublayer of the Data-link layer.
  • The protocols used to determine who goes next on a multiaccess channel belongs to a MAC sublayer.
  • MAC is important in LAN which use a multiaccess channel as the basis for communication.

Computer Networks

slide5

MAC Sublayer

  • The Channel Allocation Problem
  • There are two schemes to allocate a single channel among competing users:
    • Static Channel Allocation.
    • 2) Dynamic Channel Allocation

Computer Networks

slide6

MAC Sublayer

  • Static Channel Allocation:
  • In this scheme a Frequency Division Multiplexing (FDM) is used for allocating a single channel among competing users.
  • Example
  • if we have N users, the bandwidth will be divided into N equal-size portions.
  • ++ FDM is a simple and efficient allocation mechanism.
  • - -Waste of resources when the traffic is bursty, or the channel is lightly loaded.

Computer Networks

slide7

MAC Sublayer

  • Dynamic Channel Allocation:
  • Before the discussion of algorithms used for dynamic allocation we need to consider the following assumptions.
  • Station Model: N independent stations generate frames for transmission. (Generate >Block >Transmission)
  • Single channel Assumption: Single channel is available for all communication.
  • Collision Assumption
  • Continuous Time, or Slotted Time
  • Carrier Sense, or No Carrier sense

Computer Networks

slide8

MAC Sublayer

  • Multiple Access Protocols:
  • - ALOHA is a system proposed for solving the channel allocation problem.
  • there are two versions of ALOHA:
  • Pure ALOHA; 2) Slotted ALOHA
  • The basic difference with respect to timing is:
  • Pure ALOHAdoes not requireglobal time synchronization;
  • Slotted ALOHA does

Computer Networks

pure aloha
Pure ALOHA

In pure ALOHA, frames are transmitted at completely arbitrary times.

slide10

MAC Sublayer

  • Pure ALOHA
  • The system is working as follows:
  • 1- let users transmit whenever they have data to be sent.
  • 2- expected collisions will occur.
  • 3- the collided frames will be destroyed.
  • 4- using a feedback mechanism to know about the status of frame.
  • 5- retransmit the destroyed frame.

Computer Networks

pure aloha 2
Pure ALOHA (2)

Vulnerable period for the shaded frame.

slide12

MAC Sublayer

  • Pure ALOHA
  • The main disadvantage of Pure ALOHA is a low channel utilization.
  • This is expected due to the feature that all users transmit whenever they want.

Computer Networks

pure aloha 3
Pure ALOHA (3)

Throughput versus offered traffic for ALOHA systems.

slide14

MAC Sublayer

  • Slotted ALOHA
  • In this method the proposal was to divide the time into discrete intervals each interval corresponding to one frame.
  • In Slotted ALOHA, a computer can not send anytime, instead it is required to wait for the beginning of the time slot.
  • The big advantage of Slotted ALOHA is the increase in channel utilization.

Computer Networks

slide15

MAC Sublayer

  • Slotted ALOHA
  • There is a limit for the best channel utilization using Slotted ALOHA.
  • To reduce the chance of collisions the station should be able to detect what other stations are doing.
  • In LAN networks this is possible, therefore they can achieve better utilization than Slotted ALOHA.
  • Carrier Sense Protocols are protocols in which stations listen for a carrier.

Computer Networks

slide16

MAC Sublayer

  • Carrier Sense Multiple Access (CSMA) Protocols
  • There are several versions of carrier sense protocols:
  • 1-persistent CSMA
  • Non-persistent CSMA
  • P-persistent CSMA
  • CSMA with Collision Detection (CSMA/CD)

Computer Networks

slide17

MAC Sublayer

  • Carrier Sense Multiple Access (CSMA) Protocols
  • - In 1-persistent CSMA, a station prior to send data it listen to the channel to see if anyone else is transmitting at that moment.
  • if the channel is busy, the station waits until it becomes idle.
  • If the channel is idle, the station transmits a frame.
  • If a collision occurs, the station waits a random amount of time and starts all over again.

Computer Networks

slide18

MAC Sublayer

  • Carrier Sense Multiple Access (CSMA) Protocols
  • Although this protocol has disadvantages, it is better than ALOHA and Slotted ALOHA
  • -- 1) It’s performance depends on the propagation delay.
  • -- 2) There is a chance when two stations start transmission at the same time.

Computer Networks

slide19

MAC Sublayer

  • Carrier Sense Multiple Access (CSMA) Protocols
  • In Non-persistent CSMA a station makes conscious attempt to sense the channel.
  • After the first attempt, if the channel is idle, it sends, however, if the channel is already in use, it waits a random period of time and repeats the algorithm.
  • (+ -)This algorithm has better utilization but longer delays than 1-persistent CSMA.

Computer Networks

slide20

MAC Sublayer

  • Carrier Sense Multiple Access (CSMA) Protocols
  • In p-persistent CSMA a station transmits if the channel is idle with a probability p and with probability q=1-p it waits until the next slot.

Computer Networks

slide21

MAC Sublayer

  • Carrier Sense Multiple Access (CSMA) Protocols
  • The main advantage of persistent and non-persistent over ALOHA is that they ensure no station begins to transmit when it senses the channel busy.

Computer Networks

slide22

MAC Sublayer

  • CSMA with Collision Detection
  • It is important that stations should terminate transmission as soon as they detect a collision.
  • This protocol is called CSMA/CD.
  • It is widely used on LANs in the MAC sublayer.
  • It is the basis of the popular Ethernet LAN.

Computer Networks

csma with collision detection
CSMA with Collision Detection

CSMA/CD can be in one of three states: contention, transmission, or idle.

slide24

MAC Sublayer

  • CSMA with Collision Detection
  • A collision can be detected by looking at the power or pulse width of the received signal and comparing it to the transmitted signal.
  • After a station detects a collision it terminates its transmission, waits a random period of time, and then tries again, assuming that no other station has started transmitting in the meantime.

Computer Networks

slide25

MAC Sublayer

  • CSMA with Collision Detection
  • Collision detection is an analog process. Therefore, special encoding is commonly used.
  • A sending station must continually monitor the channel, listening for noise bursts that might indicate a collision. For this reason, CSMA/CD with a single channel is inherently a half-duplex system.

Computer Networks