Local Area Networks NETW 901

Size: px

Start display at page:

Download "Local Area Networks NETW 901"

Edward Alexander Knight
5 years ago
Views:

1 Local Area Networks NETW 901 Lecture 2 Medium Access Control (MAC) Schemes Course Instructor: Dr. Ing. Maggie Mashaly maggie.ezzat@guc.edu.eg C

2 Contents Why Multiple Access Random Access Aloha Slotted Aloha CSMA Performance of Random Access MAC schemes 2

3 Why Multiple Access? A number of senders and receivers are connected to the same shared medium All stations can listen to and transmit on the same medium If two or more stations transmit at the same time collision will occur and the transmitted signal will not be received correctly There is a need to control how stations can gain access to the medium to transmit their data 3

4 Types of Multiple Access Time Division Multiple Access (TDMA) Frequency Division Multiple Access (FDMA) Code Division Multiple Access (CDMA) More common in Mobile Networks Random Access More common in Local Area Networks 4

5 Random Access How does it work? When a node has a packet to send: It transmits at full channel data rate No a priori coordination among nodes What if two nodes transmit at the same time? Collision Random Access MAC Protocols are required for specifying: 1. How to detect collisions 2. How to recover from collisions 5

6 Random Access Protocol: Aloha Aloha means love, peace,. (used as a greeting) University of Hawaii developed a low cost radio data network and named it Aloha The system is based on random access Proved useful communication can be achieved without a central manager Paved the way to Ethernet and then WiFi 6

7 Aloha Operation Each station transmits anytime it has data ready to be transmitted If two stations transmit at the same time: collision will occur In case of collision each station waits a random time then retransmits again (Back-off Algorithm) 7

8 Aloha Performance But a more accurate calculation method is required 8

9 Aloha Performance Due to the random access transmissions numerous packets are lost due to collisions For evaluating protocol s performance, number of lost packets need to be estimated, i.e.: Probability of Collision Rethinking Probability of Collision: Collision occurs when a packet or more are generated while another is being transmitted Required: What is the probability that one or more packets will be generated within another packet s transmission time? Or better: What is the probability that one or more packets will be generated within another packet s Vulnerable Period? 9

10 Aloha Performance Calculation using Transmission Time vs. Vulnerable Period Transmission Time=T t Vulnerable Period T v = 2T x t But a frame could have been already transmitted Now we need to figure how to calculate the probability of having a defined number of frames during vulnerability period 10

11 Aloha Performance Assumption: Packet Inter-Arrival Times for any user i are Exponentially distributed Exponential CDF: Mean: 11

12 Aloha Performance But we typically have more than one user i trying to access the shared medium Packet Inter-Arrival Distribution 12

13 Aloha Performance Exponential Packet Inter-Arrivals implies Poisson Arrivals Average number of arrivals during time T for Poisson distribution E = λt 13

14 Aloha Performance Probability of Collision Is the probability of having one or more frames transmitted during the vulnerability period of the transmitted frame Probability of a Successful Transmission Is the probability of having zero frames transmitted during the vulnerability period of the transmitted frame P Successful Transmission = P 0, T v Throughput Is the average number of successfully transmitted bits (during T transmission ) S = λt t P(0, T v ) = λt t P 0,2T = λt t e 2λT t Average number of bits during time T transmission for Poisson distribution Probability of a successful transmission 14

15 Slotted Aloha An enhancement for Aloha protocol Allow frames to be transmitted only at the beginning of a periodic time slot Results in a shorter Vulnerable Period T v = T t Slotted Aloha Throughput: S = λt t P(0, T v ) = λt t P 0, T t = λt t e λt t 15

16 Maximum Throughput What is the Maximum Throughput that can be achieved by both protocols: (maximize throughput w.r.t number of arrivals) 1. Aloha λt t = Maximum Throughput S = λt t e 2λT t = e

17 Maximum Throughput What is the Maximum Throughput that can be achieved by both protocols: (maximize throughput w.r.t number of arrivals) 2. Slotted Aloha ds dλt t = d dλt t λt t e λt t = e λt t λt t e 2λT t = 0 λt t = 1 Maximum Throughput S = λt t e λt t = e 1 17

18 Average Number of Frame Transmissions Think of it as follows: - Average of any variable is calculated as follows: E x = x. p(x) all x - What are the possible values of our variable (Average number of transmissions)? 1,2,3, - To calculate the average, we need to find the probability of each possible value.. 18

19 Average Number of Frame Transmissions Think of it as follows: - What is the probability of 1 transmission? When the frame is successfully transmitted from the first time p x = 1 = P No Collision = P 0, T v = e λt v - What is the probability of 2 transmissions? When the frame has collided once then retransmitted successfully p x = 2 = P No collision P one collision = e λt v(1 e λt v) - What is the probability of 3 transmissions? When the frame has collided twice then retransmitted successfully p x = 3 = P No collision P two collisions = e λt v(1 e λt v) 2 Can you see the pattern? 19

20 Average Number of Frame Transmissions - What is the probability of n transmissions? When the frame is collided n-1 times then retransmitted successfully p x = n = P No collision P n 1 collisions = e λt v(1 e λt v) n 1 Average number of frame transmissions E n = n. p(n) n=1 = ne λt v(1 e λt v) n 1 n=1 = e λt v 20

21 Average Delay Frame delay is the duration took by the frame to be successfully delivered to its destination, including any retransmission delays E d T + d prop + (e λt v 1)(T + d prop + B) 21

22 Carrier Sense Multiple Access (CSMA) A different approach than Aloha/Slotted Aloha! A station wishing to transmit first listens to the medium to determine if another transmission is in progress (Carrier Sensing) - If the medium is busy: The station must wait - If the medium is idle: The station may transmit So do collisions still occur? Yes! If Two stations sense the medium at the same time to find it idle and start transmission, their frames will collide After collision stations wait a random time (Back-off period) before sensing the medium again 22

23 Carrier Sense Multiple Access (CSMA) CSMA Operation Start No Station has Tx data Yes Listen to the channel Channel Idle No Wait Random Time Yes Transmit No Collision Yes 23

24 Time Carrier Sense Multiple Access (CSMA) Sensing Time & Collisions A B C D 24

25 Carrier Sense Multiple Access (CSMA) CSMA Variations With CSMA, an algorithm is needed to specify what a station should do if the medium is found busy. 3 approaches exist: 1. Non-Persistent CSMA Persistent CSMA 3. P-Persistent CSMA 25

26 Carrier Sense Multiple Access (CSMA) 1. Non- Persistent CSMA A station wishing to transmit listens to the medium and obeys the following rules: i. If the medium is idle: transmit ii. If the medium is busy: wait an amount of time drawn from a probability distribution and re-sense medium again Random delays reduce the probability of collisions - If two stations back-off for he same duration they will collide again However; capacity is wasted because medium remains idle during back-off although stations has data ready to be sent 26

27 Carrier Sense Multiple Access (CSMA) 2. 1-Persistent CSMA A station wishing to transmit listens to the medium and obeys the following rules: i. If the medium is idle: transmit ii. If the medium is busy: continue to listen until the channel is sensed idle then transmit immediately Avoids medium idle time compared to non-persistent CSMA 1-persistent is an aggressive algorithm, stations with data will definitely collide, things get sorted out only after the collision 27

28 Carrier Sense Multiple Access (CSMA) 2. 1-Persistent CSMA Start No Station has Tx data Yes Listen to the channel Channel Idle No Wait Random Time Yes Transmit with Probability 1 No Collision Yes 28

29 Carrier Sense Multiple Access (CSMA) 3. P-Persistent CSMA A station wishing to transmit listens to the medium and obeys the following rules: i. If the medium is idle: transmit with probability p, and delay one time unit with probability (1-p) (Time unit is typically equal to the maximum propagation delay) ii. iii. If the medium is busy: continue to listen until the channel is sensed idle and repeat step i If transmission is delayed one time unit, repeat step i 29

30 Carrier Sense Multiple Access (CSMA) 3. P-Persistent CSMA Start No Station has Tx data Yes 1-P Listen to the channel Channel Idle Probability No Wait Random Time No P Collision Yes 30

31 Carrier Sense Multiple Access (CSMA) 3. P-Persistent CSMA Problem: How to choose P value? High P: allows more stations transmitting data and thus more probable collisions Low P: stations must wait longer to attempt transmission So what is a good balance? According to system load: At High loads: np must be smaller than one to avoid excessive collisions (where np is the number of stations that will transmit their data) At Low loads: P must not be of a very small value to avoid excessive delays 31

32 Carrier Sense Multiple Access (CSMA) Back-off Duration Different back-off distributions can be used to introduce priority - Stations with low average waiting time will have a high probability of accessing the medium - Stations with high average waiting time will have a low probability of accessing the medium Back-off duration typically follows an exponential distribution where the back-off time increases with the increased number of collisions Imagine the case of fixed back-off time? 32

6.1 Multiple Access Communications

6.1 Multiple Access Communications Chap 6 Medium Access Control Protocols and Local Area Networks Broadcast Networks: a single transmission medium is shared by many users. ( Multiple access networks) User transmissions interfering or colliding