PACKET RADIO PROTOCOL

PRESENTED BY: PRIYA KAUSHAL ROLL NO. 152615

ME(ECE) REGULAR

PRESENTED TO:Dr. S B L SACHAN

IntroductionMultiple access control channels:•Each node is attached to at transmitter and receiver which communicates via a channel shared by other nodes.

Introduction•Transmission from any node is received by other nodes.

Introduction•Multiple access issues•If more than one node transmit at a time on the control channel to BS, a collision occurs•How to determine which node can transmit to BS?

•Multiple access protocols•Solving multiple access issues

Channel Sharing Techniques

Classification of Multiple Access Protocols

Contention protocols•Contention protocols resolve a collision after it occurs. These protocols execute a collision resolution protocol after each collision

Collision-free protocols•Collision-free protocols (e.g., a bit-map protocol and binary countdown) ensure that a collision can never occur.

RANDOM ACCESS•A station that has data to send uses a procedure defined by a protocol to make a decision on whether or not to send.

•This decision depends on the state of the medium(idle or busy).

RANDOM ACCESS•There is no scheduled time for a station to transmit. Transmission is random among the stations. That is why these methods are called random access.

• If more than one station tries to send at a time , there is an access conflict - collision

RANDOM ACCESS

•No rules specify which station should send next . Stations compete with one another to access the medium. That is why these methods are also called contention methods.

Packet Radio •In packet radio access techniques, many subscribers attempts to access a single channel in uncoordinated manner.•Transmission is done by using bursts of data.

Packet Radio •Collision from the simultaneous transmissions of multiple transmitters are detected at each base station receiver.• In which case an ACK or NACK signal is broadcast by the base station to alert the desired user ( and all other users) of received transmission .

Packet Radio •The ACK signal indicate an acknowledgment of received burst from particular user by the base station.

•NACK (negative acknowledgment) indicate that the previous burst was not received correctly by the base station.

Packet Radio • By using ACK and NACK signals, PR system

employs perfect feedback, even though traffic delay due to collisions may be high.

Packet Radio Multiple access •Easy to implement•But low spectral efficiency include delays•The subscriber use contention technique to transmit on common channel.•ALOHA protocol , developed for early satellite system, are the best example of contention technique

ALOHA Protocol•ALOHA allow each subscriber to transmit whenever they have data to send.

ALOHA Protocol•The transmitting subscriber listen to the acknowledgment feedback to determine if transmission has been successful or not.

ALOHA Protocol•If the collision occur, the subscriber wait a random amount of time, and then re-transmits the packet.

Advantage•The advantage of packet contention technique is the ability to serve a large number of subscriber with virtually no overhead.

Performance •The performance of contention technique can be evaluated by the

• Throughput(T): Which is defined as the average number of messages successfully transmitted per unit time.•Average delay(D): Experienced by a typical message burst.

Performance•Vulnerable Period (Vp): which is defined as the time interval during which the packets are susceptible to collisions with transmission from other users.

Vulnerable Period• The packet A will suffer a

collision if the other terminals transmit packets during the periods t ₁ to t₁+2τ.

• Even if only a small portion of packet A sustains a collision, the interference may render the message useless.

Packet Radio Protocol• It is assumed that all the packets sent by all users have a constant packet length and fixed channel data rate and all other users may generate new packets at random time intervals.•Furthermore, it is assumed that packet transmissions having a mean arrival rate of λ packet per second.

Packet Radio Protocol• If τ is the packet duration in seconds then the traffic occupancy or throughput R of a packet radio network is given by

R=λτ•R is the normalized channel traffic due to arriving and buffered packets and is a relative measures of the channel utilization.

Packet Radio Protocol• If R>1 , then the packets generated by the users exceed the maximum transmission rate of the channel. Thus, to obtain a reasonable throughput, the rate at which new packets are generated must lie within 0<R<1.•Under the condition of normal loading, throughput T is the same as the total offered load, L.

Packet Radio Protocol•The normalized throughput is given as the total offered load times the probability of successful transmission:

T=R.Pr[no collision]=λτ.Pr[no collision] (1)Where Pr[no collision] is the probability of the user making successful packet transmission.

Probability•The probability that n packets are generated by the user population during a given packet duration interval is assumed to be the poisson distributed and is given by:

Pr(n)= Rⁿe⁻ᴿ/nᴉ

Probability•A packet is assumed successfully transmitted if there are no other packets transmitted during the given time interval. The probability that zero packets are generated (i.e., no collision) during this interval is given by

Pr(0)=e⁻ᴿ

Multiple access protocols

RANDOM ACCESS•ALOHA (Pure ALOHA and Slotted ALOHA)•CSMA•CSMA/CD•CSMA/CA

ALOHA Protocol•ALOHA is developed in the 1970s at the University of Hawaii. •The basic idea is simple:•Let users transmit whenever they have data to be sent.

• If two or more users send their packets at the same time, a collision occurs and the packets are destroyed.

ALOHA Protocol• The original ALOHA protocol is called pure

ALOHA.• Each station sends a frame whenever it has a frame

to send.• However , there is only one channel for them to

share.

ALOHA Protocol• If two or more stations transmit at same time, there is a collision • the sender waits a random amount of time and sends it again.

•The waiting time must be random. Otherwise, the same packets will collide again

Types of ALOHA Protocol

Pure ALOHA•The Pure ALOHA protocol is a random access protocol used for data transfer.•A user accesses a channel as soon as a message is ready to be transmitted.•After a transmission, the user wait for an acknowledgement on either the same channel or a separate feedback channel.

Pure ALOHA• In case of collisions (i.e., when NACK is received), the

terminal waits for a random period of times and retransmits the message.

• As the number of users increase, a greater delay occurs because the probability of collision increases.

Pure ALOHA• In fig there are four stations that contended with one another for access to shared channel. All these stations are transmitting frames.

Pure ALOHA•Some of these frames collide because multiple frames are in contention for the shared channel. Only two frames, frame 1.1 and frame 2.2 survive. All other frames are destroyed.

Pure ALOHA•Whenever two frames try to occupy the channel at the same time, there will be a collision and both will be damaged.

Pure ALOHA• If first bit of a new frame overlaps with just the last bit of a frame almost finished, both frames will be totally destroyed and both will have to be retransmitted.

Pure ALOHA•For the ALOHA protocol, the vulnerable is double the packet duration.•Thus the probability of no collision during the interval of 2τ is found by evaluating Pr(n) given as

Pr(n)=(2R)ⁿe⁻²ᴿ/nᴉ at n=0 (2)

Pure ALOHA•One may evaluate the mean of eqn(2) to determine the average number of packets sent during 2τ.•The probability of no collision is Pr(0)=e⁻²ᴿ.•The throughput of the ALOHA protocol is found by equation(1)

T=Re⁻²ᴿ

Slotted ALOHA•In slotted ALOHA, time is divided into equal time slots of length greater than the packet duration τ.

Slotted ALOHA•The subscriber each have synchronized clocks and transmit a message only at the beginning of a new time slot, thus resulting in a discrete distribution of packets .

Slotted ALOHA•This prevents partial collisions, where one packet collides with a portion of another.

Slotted ALOHA•As the number of users increase, a greater a delay will occur due to complete collision and resulting repeated transmissions of those packets originally lost.

Slotted ALOHA•The number of slots which a transmitter waits prior to retransmitting also determines the delay characteristics of the traffic.•The vulnerable period for slotted ALOHA is only one packet duration, since partial collisions are prevented through synchronization.

Slotted ALOHA•The probability that no other packets will be generated during the vulnerable period is e⁻ᴿ.•The throughput for the case of slotted ALOHA is thus given by

T=R e⁻ᴿ

ALOHA and slotted ALOHA

Numerical(1)Problem: 56 kbps unslotted Aloha channel shared

by N stations. Stations: 1 packet(1000 bits) every 100 seconds.

Solution:a) Required data rate

Numerical(1)b). Available data rate:

•The pure Aloha has a maximum efficiency of 18.4 % therefore the maximum available data rate is:•Available Data Rate = 0.184*56000bps = 10.304kbps

Numerical(1) c). Maximum Number of Stations:

The maximum sustainable number of stations is:N=10304/10 =1030

Numerical(2)•Problem: Slotted aloha channel with 10% of the slots are idle.•Solution:a). What is the channel load R? The fraction of idle slots is: P(idle)=0.1 Since P (idle)= e⁻ᴿ →R=−ln(P(idle) = −ln(0.1) = 2.3 R=2.3

Numerical(2)b). What is the system throughput in packets/slot?

The throughput is: throughput = P(success)=Re⁻ᴿ=23%

Numerical(2)c). Is the channel overloaded or under-loaded?

• The maximum throughput (36.8%) is achieved for R=1. •For R>1 the channel efficiency drops because it is overloaded.

Thank You