Packet radio protocols allow multiple subscribers to access a shared channel for transmitting data packets. They use contention-based random access techniques like ALOHA. Pure ALOHA protocol has low efficiency due to partial packet collisions. Slotted ALOHA synchronizes transmissions to time slots to prevent partial collisions, improving efficiency. Performance is evaluated using metrics like throughput, which is highest at optimal channel load and drops off above and below this point.

1. PACKET RADIO
PROTOCOL
PRESENTED BY:
PRIYA KAUSHAL
ROLL NO. 152615
ME(ECE) REGULAR
PRESENTED TO:
Dr. S B L SACHAN

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

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

4. 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

5. Channel Sharing Techniques

6. Classification of Multiple Access
Protocols

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

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

9. 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).

10. 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

11. 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.

12. 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.

13. 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 .

14. 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.

15. Packet Radio
• By using ACK and NACK signals, PR system
employs perfect feedback, even though
traffic delay due to collisions may be high.

16. 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

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

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

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

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

21. 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.

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

23. 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.

24. 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.

25. 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.

26. 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.

27. 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.

28. 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

29. 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

30. Multiple access protocols

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

32. 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.

33. 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.

34. 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

35. Types of ALOHA Protocol

36. 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.

37. 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.

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

39. 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.

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

41. 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.

42. 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)

43. 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

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

45. 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 .

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

47. 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.

48. 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.

49. 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

50. ALOHA and slotted ALOHA

51. 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

52. 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

53. Numerical(1)
c). Maximum Number of Stations:
The maximum sustainable number of stations is:
N=10304/10 =1030

54. 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

55. Numerical(2)
b). What is the system throughput in
packets/slot?
The throughput is:
throughput = P(success)=Re⁻ᴿ=23%

56. 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.

57. Thank You