1 / 25

COMPUTER NETWORKS Data-link Layer (The Medium Access Control Sublayer)

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)?

delu
Download Presentation

COMPUTER NETWORKS Data-link Layer (The Medium Access Control Sublayer)

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. MAC Sublayer COMPUTER NETWORKS Data-link Layer (The Medium Access Control Sublayer)

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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

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

  10. 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

  11. Pure ALOHA (2) Vulnerable period for the shaded frame.

  12. 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

  13. Pure ALOHA (3) Throughput versus offered traffic for ALOHA systems.

  14. 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

  15. 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

  16. 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

  17. 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

  18. 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

  19. 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

  20. 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

  21. 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

  22. 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

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

  24. 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

  25. 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

More Related