ITT300 - CHAPTER 7 ACCESS CONTROL.pdf

ITT300 INTRODUCTION TO DATA COMMUNICATION AND NETWORKING CHAPTER 7 ACCESS METHOD ADAPTED FROM: Behrouz A. Forouzan 7-1 INTRODUCTION Commonly, nodes connected to a common link called a multipoint (bus topology). The problem is how to...

ITT300 INTRODUCTION TO DATA COMMUNICATION AND NETWORKING CHAPTER 7 ACCESS METHOD ADAPTED FROM: Behrouz A. Forouzan 7-1 INTRODUCTION Commonly, nodes connected to a common link called a multipoint (bus topology). The problem is how to control the access of nodes to the multipoint (single link). That is to control which node that should have the right access to the link to make sure there is no packet rammed happen. Figure 7.1 Data link layer divided into two functionality-oriented sublayers 7.2 MULTIPLE ACCESS METHODS There is a method called multiple-access method or protocols to coordinate access to the link. Figure 7.2 Taxonomy of multiple-access protocols  7-3 RANDOM ACCESS In random access methods, no station is superior to another station, and none is assigned the control over another. Mean, all nodes are in the same level of right access. One of the popular random-access method is called Carrier Sense Multiple Access (CSMA). A station that has data to send uses a procedure defined by the protocol to make a decision on whether or not to send. CARRIER SENSE MULTIPLE ACCESS (CSMA) This method was developed to minimize the chance of collision and, therefore, increase the performance. Each station first listen to the medium before sending. In other words, “sense before transmit” or “listen before talk”. CSMA can reduce the possibility of collision, but it cannot eliminate it. CSMA protocol consist of 3 strategy that is 1- Persistent strategy, Nonpersistent strategy and p- Persistent strategy.  Persistence Strategy The persistence strategy defines the procedures for a station that sense a busy medium. 1-Persistent In the 1-persistent, if the station finds the line idle, the station sends its frame immediately (with a probability of 1). This method is facing the chance of collision because two or more stations may send their frames after finding the line idle. 1) Sense, and if the line is clear 2) Then it will transmit If the line is busy, then it will not transmit the packet Figure 7.3 Behavior of 1-Persistent methods Persistence Strategy Nonpersistent In nonpersistent strategy, a station that has a frame to send senses the line If the line is idle, the station sends immediately, otherwise, the station waits a random period of time and then senses the line again, reduces the chance of collision However this method reduces the efficiency. FREE Figure 7.4 Behavior of Nonpersistent methods Persistence Strategy p-Persistent The p-persistent approach combines the advantages of the other two strategies: reduces the chance of collision and improve the efficiency. 2)1) Time Continuously sense is the characteristics for one persistent slot, is the characteristics for one persistent Figure 7.5 Behavior of p-Persistent methods CSMA/CD Energy Level The level of energy in a channel can have three values: Zero – The channel is idle Normal – A station has successfully captured the channel and is sending its frame Abnormal – There is a collision, and the level of energy is twice the normal Figure 7.6 Energy level during transmission, idleness, or collision 7-4 CONTROLLED ACCESS In 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. There are three popular controlled- access methods: reservation, polling, and token passing. POLLING Polling works with topologies in which one device is designated as a primary station and other devices are secondary stations. The primary device controls the link and the secondary devices follow its instructions. All data exchanges must be made through the primary device. Polling Modes Select Mode Select function used whenever the primary device has something to send. Poll Mode Poll function is used by the primary device to receive transmission from the secondary device. Figure 7.8 Select and poll functions in polling access method  TOKEN PASSING In token passing method, the stations in a network are organized in a logical ring. Each station has a predecessor and a successor Predecessor is the station which is logically before the station in the ring (sender); successor is the station which is after the station in the ring (receiver). A special packet called a token circulates through the ring If a station needs to send data, it waits for the token. The station captures the token and sends one or more frames and finally releases the token to be used by the successor station. 1.Random Access Method 1. CSMA (Carrier Sense Multiple Access): 1.All nodes have equal access rights. 2.Stations decide whether to send data based on protocol-defined procedures. 3.CSMA minimizes collision chances but cannot eliminate them. 4.Three strategies: 1.1-Persistent: Transmits immediately if the line is idle, risking collision. 2.Nonpersistent: Waits a random time before re-sensing if the line is busy, reducing efficiency but lowering collision risk. 3.p-Persistent: Balances collision risk and efficiency, waiting for a probability before transmitting. 5.CSMA/CD (Collision Detection): 1.Detects collision by monitoring energy levels in the channel. 2.Levels: Zero (idle), Normal (successful transmission), Abnormal (collision). 2.Controlled Access Method 1. Stations coordinate to determine which has the right to send. 2. Methods: 1.Polling: 1.Involves primary (controls link) and secondary (follows instructions) stations. 2.Primary station uses "Select" to send data and "Poll" to receive data from secondaries. 2.Token Passing: 1.Stations form a logical ring. 2.A token circulates; stations send data when they hold the token, then pass it to the next station. These methods ensure orderly access to a shared communication medium, reducing collisions and improving data transfer efficiency.