Chapter 13 Multiple Access

Figure 13.1 Multiple-access protocols

13.1 Random Access/ Contention Multiple Access CSMA Carrier Sense Multiple Access CSMA/CD Carrier Sense Multiple Access with Collision Detection CSMA/CA Carrier Sense Multiple Access with Collision Avoidance

CSMA/ Collision Detection CSMA/ Collision Avoidance Evolution of random-access methods or Contention method CSMA/ Collision Detection Media Access (ALOHA) Carrier Sense Multiple Access CSMA/ Collision Avoidance No station is superior to another station None is assigned the control over another No station permits, or does not permit, another station to send station that has data to send uses a procedure defined by protocol to make a decision on whether or not to send The decision depends on the state of medium (idle or busy)

MA (Media Access) ALOHA network When can the station access the medium ? What can the station do if the medium is busy? How can the station determine the success or failure of the transmission? What can the station do if there is an access conflict?

Media Access/ ALOHA protocol Station sends frame whenever it has a frame to send. there is only one channel to share, there is possibility of collision between frames from different stations Pure Aloha

Procedure for ALOHA protocol Kmax : Maximum No. of retransmission attemps Tb : Back-off time K: backoff = # attempt K=0 Tb = Random x Tp_max Random = [0, 1] Twaiting = 2 x Tp_max K>Kmax Tp_max = max. propagation time K=K++

Carrier Sense Multiple Access CSMA The chance of collision can be reduced if station sense medium before trying to use it. CSMA requires that each station first listen to the medium before sending ( “sense before transmit” or “listen before talk” )

Persistence Methods What should station do if channel if busy? What should station do if channel if idle? Three methods have been devised to answer these questions Nonpersistent method 1-Persistent method p-persistent method

Persistence strategies

Flow diagram for three persistence methods Highest chance of collision Reduce efficiency of N/W Channel has time slot : >= Max. propagation time Combine advantages of two methods Reduce chance of collision improve efficiency Behavior of three persistence methods

Collision Delay Delay

Collision in CSMA Possibility of collision still exists because of Propagation delay Collision in CSMA CSMA can reduce the possibility of collision, but it cannot eliminate

CSMA/CD procedure Collision Detection K=0 Nonpersistence p-persistence CSMA does not specify procedure following collision CSMA/CD augments algorithm to handle collision station monitors medium after sending frame transmission was successful, station is finished there is collision, frame is send again K=0 Tb = Random x Tp_max Random = [0, 2K - 1] Nonpersistence p-persistence K>Kmax K=K++ Kmax is normally 15

Timing in CSMA/CA In wireless network, much of sent energy is lost in transmission : not useful for effective collision detection We need to avoid collisions on wireless network because they cannot be detected CDMA/CA has three strategies Interframe space (IFS) Contention window Acknowledgement

CSMA/CA procedure

13.2 Control Access Reservation Polling Token Passing

Reservation access method

Controlled Access Protocol Polling

Polling

Selecting

Controlled Access Protocol TOKEN Passing

Token-passing network predecessor successor Token

Token-passing procedure

13.3 Channelization FDMA: Frequency Division Multiple Access TDMA: Time Division Multiple Access CDMA: Code Division Multiple Access

Channelization Use Multiplexing Techniques FDMA: Frequency Division Multiple Access the bandwidth is divided into channels. TDMA: Time Division Multiple Access the bandwidth is just one channel that is timeshared. CDMA one channel carries all transmissions simultaneously

Frequency-division multiple access (FDMA)

Time-division multiple access (TDMA)

Channelization CDMA

Simple idea of communication with code

Chip sequences

Encoding rules

CDMA multiplexer

Digital signal created by four stations in CDMA

CDMA demultiplexer

Decoding of the composite signal for one in CDMA

PN Sequence Generation (W1 and W2N) Walsh Table Walsh Code or Hadamard Matrix

Sequence generation Element W2 W2 W2 W2 N=1 N=2

Example 1 Check to see if the second property about orthogonal codes holds for our CDMA example. Solution The inner product of each code by itself is N. This is shown for code C; you can prove for yourself that it holds true for the other codes. C . C = [+1, +1, -1, -1] . [+1, +1, -1, -1] = 1 + 1 + 1 + 1 = 4 If two sequences are different, the inner product is 0. B . C = [+1, -1, +1, -1] . [+1, +1, -1, -1] = 1 - 1 - 1 + 1 = 0

Example 2 Check to see if the third property about orthogonal codes holds for our CDMA example. Solution The inner product of each code by its complement is -N. This is shown for code C; you can prove for yourself that it holds true for the other codes. C . (-C ) = [+1, +1, -1, -1] . [-1, -1, +1, +1] = - 1 - 1 - 1 - 1 = -4 The inner product of a code with the complement of another code is 0. B . (-C ) = [+1, -1, +1, -1] . [-1, -1, +1, +1] = -1 + 1 + 1 - 1 = 0