Subject Name: Computer Communication Networks Subject Code: 10EC71 Prepared By: Kala H S & Megha Gupta Department: ECE Date: 9/17/2018

Unit 3 MULTIPLE ACCESSES 9/17/2018

TOPICS TO BE COVERED Multiple Access Mechanism Random Access Controlled Access Channelization 9/17/2018

2 Functionality-Oriented Sublayers of Data Link Layer: Upper sublayer is responsible for data link control. Lower sublayer is responsible for resolving access to the shared media. If the channel is dedicated, we do not need the lower sublayer. 9/17/2018

Multiple Access Mechanism When nodes or stations are connected and use a common link, we need a multiple-access protocol to coordinate access to the link. These protocols regulate the contention between many transmitters which tend to use the media anywhere, anytime and independently. 9/17/2018

RANDOM ACCESS Also called as contention method. No station is superior to another station and none is assigned the control over another. Two features give this method its name. Random Access: there is no scheduled time for a station to transmit. Transmission is random among the stations. (b) Contention Method: Stations compete with one another to access the medium. 9/17/2018

ALOHA It is the earliest random access method. It was developed at the University of Hawaii in early 1970. It was designed for a radio (wireless) LAN, but it can be used on any shared medium. The medium is shared between the stations. When a station sends data, another station may attempt to do so at the same time. The data from the two stations collide and become garbled. 9/17/2018

PURE ALOHA It is the original Aloha Protocol. Each station sends a frame whenever it has a frame to send. Since there is only one channel to share, there is the possibility of collision between frames from different stations. 9/17/2018

PROCEDURE FOR PURE ALOHA PROTOCOL 9/17/2018

VULNERABLE TIME FOR PURE ALOHA 9/17/2018

SLOTTED ALOHA It was invented to improve the efficiency of pure ALOHA. In slotted ALOHA we divide the time into slots and force the station to send only at the beginning of the time slot. 9/17/2018

VULNERABLE TIME FOR SLOTTED ALOHA 9/17/2018

CSMA CSMA would send a frame only if it senses no carrier. There are two strategies for that: Non-persistent and persistent strategy. 9/17/2018

CSMA/CD CSMA method does not specify the procedure following a collision. Carrier sense multiple access with collision detection (CSMA/CD) augments the algorithm to handle the collision. A station monitors the medium after it sends a frame to see if the transmission was successful. If so, the station is finished. If, however, there is a collision, the frame is sent again. 9/17/2018

CSMA/CD FLOW DIAGRAM 9/17/2018

CSMA/CD Carrier Sense Multiple Access with Collision Avoidance. Used in a network where collision cannot be detected. Eg: Wireless LAN 9/17/2018

contention window size is 2K-1 CSMA/CA FLOW DIAGRAM contention window size is 2K-1 After each slot: - If idle, continue counting - If busy, stop counting 9/17/2018

CONTROLLED ACCESS The stations consult one another to find which station has the right to send. A station cannot send unless it has been authorized by other stations. Three common methods: Reservation Polling Token passing 9/17/2018

RESERVATION A station needs to make a reservation before sending data. Time is divided into intervals. In each interval, a reservation frame precedes the data frames sent in that interval. 9/17/2018

POLLING One device is designated as a primary station and the other devices are secondary stations. All data exchanges are made through the primary device even when the ultimate destination is a secondary device. The primary device controls the link; the secondary devices follow its instructions. Primary device determines which device is allowed to use the channel at a given time. Primary device is always the initiator of a session. 9/17/2018

POLLING and ACCESS METHOD 9/17/2018

TOKEN PASSING The stations in a network are organized in a logical ring. For each station, there is a predecessor and a successor. Stations do not have to be physically connected in a ring, the ring can be a logical one. 9/17/2018

CHANNELISATION It is a multiple-access method in which the available bandwidth of a link is shared in time, frequency, or through code, between different stations. Three channelization protocols: FDMA, TDMA, CDMA. 9/17/2018

FDMA The bandwidth is divided into channels. Each channel is used by a single station. FDMA is data link layer protocol (MAC sub layer) which uses FDM at the physical layer. 9/17/2018

TDMA The entire bandwidth is just one channel. The stations share the bandwidth through time slots. TDMA is data link layer protocol (MAC sub layer) which uses TDM at the physical layer. 9/17/2018

CDMA All stations share the same bandwidth and the same time (no frequency division, no time division, all data transmitted simultaneously). One channel carries all transmissions at the same time. Each channel is separated by code. 9/17/2018

CDMA: CHIP SEQUENCES Each station is assigned a unique chip sequence Chip sequences are orthogonal vectors Inner product of any pair must be zero With N stations, sequences must have the following properties: They are of length N Their self inner product is always N 9/17/2018

CDMA: BIT REPRESENTATION 9/17/2018

CDMA: TRANSMISSION 9/17/2018

CDMA ENCODING 9/17/2018

CDMA DECODING 9/17/2018

Acknowledgement : MY SINCERE THANKS TO By Kala H S & Megha Gupta THE AUTHOR PROF.BEHROUZ A FOROUZAN. BECAUSE THE ABOVE PRESENTATION MATERIALS ARE HEAVILY BORROWED FROM HIS TEXTBOOK “DATA COMMUNICATION & NETWORKING” 4TH EDITION, PUBLISHER TATA MCGRAW HILL SURESHA V. PROFESSOR, DEPT. OF E&C, KVG COLLEGE OF ENGINEERING. SULLIA, D.K - 574 327 By Kala H S & Megha Gupta Assistant Professor 9/17/2018