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

Computer Networks2 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 ? MAC Sublayer

Computer Networks3 the need for determine who gets to use the channel -When there are two or more 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. MAC Sublayer

Computer Networks4 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. MAC Sublayer

Computer Networks5 The Channel Allocation Problem There are two schemes to allocate a single channel among competing users: 1)Static Channel Allocation. 2) Dynamic Channel Allocation MAC Sublayer

Computer Networks6 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. MAC Sublayer

Computer Networks7 Dynamic Channel Allocation: Before the discussion of algorithms used for dynamic allocation we need to consider the following assumptions. 1)Station Model: N independent stations generate frames for transmission. (Generate >Block >Transmission) 2) Single channel Assumption: Single channel is available for all communication. 3)Collision Assumption 4)Continuous Time, or Slotted Time 5)Carrier Sense, or No Carrier sense MAC Sublayer

Computer Networks8 Multiple Access Protocols: - ALOHA is a system proposed for solving the channel allocation problem. -there are two versions of ALOHA: 1)Pure ALOHA; 2) Slotted ALOHA The basic difference with respect to timing is: Pure ALOHA does not require global time synchronization; Slotted ALOHA does MAC Sublayer

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

Computer Networks10 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. MAC Sublayer

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

Computer Networks12 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. MAC Sublayer

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

Computer Networks14 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. MAC Sublayer

Computer Networks15 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. MAC Sublayer

Computer Networks16 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) MAC Sublayer

Computer Networks17 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. MAC Sublayer

Computer Networks18 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. MAC Sublayer

Computer Networks19 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. MAC Sublayer

Computer Networks20 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. MAC Sublayer

Computer Networks21 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. MAC Sublayer

Computer Networks22 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. MAC Sublayer

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

Computer Networks24 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. MAC Sublayer

Computer Networks25 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. MAC Sublayer