Multiple Access-Data Communication and Computer Networks-Lecture Slides, Slides of Data Communication Systems and Computer Networks

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Multiple Access

 The data link layer is considered as two sub layers. The upper sub layer is

responsible for data control and lower sub layer is responsible for resolving

access to the shared media.

 We categories the protocols into three groups

o Random access protocol

o Controlled access protocol

o Channelization

Random Access

 In random access no station is superior to another station and none is

assigned the control over another.

 Each station is can transmit when it desires on the condition that it fallows

the predefined procedures.

 No scheduled time for the station to transmit, transmission is random

among the stations; that is why methods are called random access.

 Stations compete with one another to access the medium, that is why the

methods are called contention methods.

A collision involves two or more stations. If all these stations resend the frame after time-out period will happen again.

Pure ALOHA dictates that when the time-out period passes, each station waits for a random amount of time then resend the frame, randomness will help avoid more collisions; will call it back off time TB.

Vulnerable time for pure ALOHA

vulnerable time is the time in which there is a

possibility of collision.

Throughput for slotted ALOHA S = G × e−G^.

The maximum throughput Smax = 0.368 when G = 1.

 If a frame is generated during one frame transmission time, then 36.

percent of these frames reach their destination.

Vulnerable time for Slotted ALOHA

Example

A slotted ALOHA network transmits 200-bit frames on a shared channel of 200 kbps. What is the throughput if the system (all stations together) produces a. 1000 frames per second b. 500 frames per second c. 250 frames per second. Solution

The frame Transmission time is 200/200 kbps or 1 ms.

a) If the system creates 1000 frames per second, this is 1 frame per millisecond. The load is 1. In this case S = G× e−G^ or S = 0.368 (36.8percent). This mean that the throughput is 1000 × 0.0368 = 368 frames .Only 386 frames out of 1000 will probably survive.

b) If the system creates 500 frames per second, this is (1/2) frame per millisecond. The load is (1/2). In this case S = G × e−G^ or S = 0.303 (30.3 percent). This means that the throughput is 500 × 0.0303 = 151. Only 151 frames out of 500 will probably survive.

 At time t1, station B senses the medium and finds it idle, so it sends the frame. At time t2(t2>t1), station C senses the medium and finds it idle and sends its frame, as at that time, the first bits from station B have not reached station C. The frame collide and both frames destroyed.

Vulnerable Time

 Worst case the left most station A sends a frame at time t1, which reaches the rightmost station D at time t1+Tp.

Persistence Methods

Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

In this method , a station monitors the medium after it sends a frame to see if the transmission is successful.

• If so, the station is finished. If , however, there is a collision, the frame is sent again.

Flow diagram for CSMA/CD

Contention Window is an amount of time divide into slots.

 A station is ready to transmit chooses a random number of slots as its wait time.

 The number of slots in the window changes according to the binary exponential back-off strategy. It is set to one slot the first time and then doubles each time the station cannot detect an idle channel after the IFS time.

 However the station needs to sense the channel after each time slot.

 If station is busy , it does not restart the timer of the contention window; it stops the timer and restarts it when the channel becomes idle.

Acknowledgment

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.

Reservations

 In this method, 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.

 If there are N stations in the system, there are exactly N reservations

minislots in the reservation frame.

 Each minislot belongs to a station.

 When a station needs to send a data frame, it makes a reservation in

its own mini slot.

Polling in this method one device is designated as a primary and the other

devices are secondary devices.

 The primary device controls the while secondary device follow the instructions.

 All data exchanged must be made through the primary device even when the ultimate destination is secondary device.

SELECT function is used whenever the primary device has something to send.

 If primary device has something to send, the primary device always controls the link.

 So primary device must alert the secondary device about the upcoming transmission and waits for an acknowledgment of the secondary ready status.

 Before sending, the primary creates and transmits a select (SEL) frame, one field of which includes the address of the intended secondary.

POLL is used by the primary device to solicit transmission from the secondary devices.

 When the primary device is ready to receive data, it must ask (poll) each device in turn if it has anything to send.

 When the first secondary is approached, it responds either with NAK frame if it has nothing to send or with data (in form of data frame).

 If response is negative primary polls the next secondary device.

 When the response is positive , the primary reads the data frame and sends an acknowledgment (ACK fame), verifying its receipt.