seminar on multiple access control protocol

1. shared wire or medium
Ms. Sheetal Agrawal Munesh meena

2. shared wire or medium
Ms. Sheetal Agrawal Munesh meena

3. Multiple Access Links and Protocols
Two types of “links”:
• point-to-point
– PPP for dial-up access
– point-to-point link between Ethernet switch and host
• broadcast (shared wire or medium)
– old-fashioned Ethernet
– 802.11 wireless LAN
humans at a
cocktail party
shared wire (e.g., shared RF
shared RF (shared air, acoustical)
cabled Ethernet) (e.g., 802.11 WiFi)
(satellite)
MUNESH MEENA
MCA 3rd Sem (MLSU)

4. Point-to-point networks
Point-to-point networks are those in which,when a message is sent from
one computer to another, it usually has to be sent via other computers in
the network.
MUNESH MEENA
MCA 3rd Sem (MLSU)

5. Broadcast networks
Broadcast networks have a single communication channel that is shared by all the
machines on the network.
Shared channel
MUNESH MEENA
MCA 3rd Sem (MLSU)

6. Multiple Access protocols
• single shared broadcast channel
• two or more simultaneous transmissions by nodes:
interference
– collision if node receives two or more signals at the same time
multiple access protocol
• distributed algorithm that determines how nodes share
channel, i.e., determine when node can transmit
• communication about channel sharing must use channel itself!
– no out-of-band channel for coordination
MUNESH MEENA
MCA 3rd Sem (MLSU)

7. Need of protocols in Broadcast channel
Issues in multi-access channel :
• WHO is going to use the channel ?
• WHEN the channel is going to be used ?
• For HOW much time the channel is used ?
MUNESH MEENA
MCA 3rd Sem (MLSU)

8. Types of Multiple –access protocol
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MCA 3rd Sem (MLSU)

9. RANDOM ACCESS
• Single channel shared by a large number of hosts
• No coordination between hosts
• Control is completely distributed
• Examples: ALOHA, CSMA, CSMA/CD
MUNESH MEENA
MCA 3rd Sem (MLSU)

10. ALOHA
There are two versions of Aloha system which differ with respect to
whether or not time is divided up into discrete slots into which all
frames must fit. :
PURE ALOHA
SLOTTED ALOHA
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MCA 3rd Sem (MLSU)

11. Pure Aloha
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MCA 3rd Sem (MLSU)

12. If the first bit of a new frame overlaps with just the last bit of a frame
almost finished, both the frames will be totally destroyed. It does not
distinguish between a total loss or a near miss.
MUNESH MEENA
MCA 3rd Sem (MLSU)

13. Slotted Aloha
• Divide time up into discrete intervals, each corresponding to one packet.
• The stations can only transmit data in one of the time slots only.
• The vulnerable period is now reduced in half. If the frames collide they will
overlap completely instead of partially.
MUNESH MEENA
MCA 3rd Sem (MLSU)

14. Vulnerable time for slotted ALOHA protocol
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MCA 3rd Sem (MLSU)

15. MUNESH MEENA
MCA 3rd Sem (MLSU)

16. CSMA (Carrier Sense Multiple Access)
CSMA: listen before transmit:
• If channel sensed idle: transmit entire frame
• If channel sensed busy, defer transmission
• human analogy: don’t interrupt others!
MUNESH MEENA
MCA 3rd Sem (MLSU)

17. CSMA collisions
spatial layout of nodes
collisions can still occur:
propagation delay means
two nodes may not hear
each other’s transmission
collision:
entire packet transmission
time wasted
note:
role of distance & propagation delay in
determining collision probability
MUNESH MEENA
MCA 3rd Sem (MLSU)

18. Carrier sense multiple access with collision detection
• Carrier Sense:- listen for traffic on medium and determines gap between frames.
• Multiple Access:- Station can transmit any time when find network quite.
• Collision Detection:- Occurs when two stations transmits at same time. It gets detected
and trans. Holds for random time.
MUNESH MEENA
MCA 3rd Sem (MLSU)

19. CSMA/CD (Collision Detection)
CSMA/CD: carrier sensing, deferral as in
CSMA
– collisions detected within short time
– colliding transmissions aborted,
reducing channel wastage
• collision detection:
– easy in wired LANs: measure signal
strengths, compare transmitted,
received signals
– difficult in wireless LANs: received
signal strength overwhelmed by local
transmission strength
• human analogy: the polite
conversationalist
MUNESH MEENA
MCA 3rd Sem (MLSU)

20. Station is
Station is ready to send
ready to send
New attempt
Wait according to
backoff strategy
Station is ready to send
(6)
Sense
channel
(1) Channel busy (3)
Channel free
(2)
Collision detected Transmit
Transmit data and sense jam signal
Station is ready to send
Station channelsend
is ready to (5)
(4)
MUNESH MEENA
MCA 3rd Sem (MLSU)

21. Carrier sense multiple access with collision avoidance(CSMA/CA)
• Motivation
– In a wireless network, the received signal has very little energy due
to energy loss in transmission  a collision may add only 5~10%
additional energy  not useful for effective collision detection
• Features
– Tries to avoids collision
– Used in Wireless LAN
 Difficult to detect collisions
 Hidden node problem!
(CTS/RTS used in 802.11)
MUNESH MEENA
MCA 3rd Sem (MLSU)

22. • Timing in CSMA/CA
• Interframe Space (IFS)
– When an idle channel is found, the station does not send immediately but
waits for a period of time call IFS
– The IFS time allows the front of the transmitted signal by the distant station
to reach this station
– The IFS variable can also be used to prioritize stations or frame types
• e.g., DIFS(Distributed IFS) , SIFS((Short)
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MCA 3rd Sem (MLSU)

23. 1-persistent
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MCA 3rd Sem (MLSU)

24. Controlled Access
• Approach
– The stations consult one another to find which station has the right
to send
• Controlled access methods
– Reservation
• Ex. DQDB, Cable modem, …
– Polling
• Select and Poll
• Ex. HDLC (normal response mode)
– Token passing
• Ex. Token ring (FDDI), token bus, …
MUNESH MEENA
MCA 3rd Sem (MLSU)

25. Reservation
• Features
– Time is divided into intervals
• In each interval, a reservation frame precedes the data frames sent in
that interval
– N stations  N reservation minislots in the reservation frame
• When a station needs to send a data frame  it makes a reservation
in its own minislot
MUNESH MEENA
MCA 3rd Sem (MLSU)

26. Polling
• Motivation and features
– Communication for primary-secondary stations topology
• Ex. NRM mode of HDLC
– The primary station is always the initiator of a session
• By polling (PS) and selecting (PS)
• Select and Poll
MUNESH MEENA
MCA 3rd Sem (MLSU)

27. Token Passing
• Features
– A station is authorized to send data when it receives a special frame called
a token
– Stations are arranged around a ring (physically or logically)
• A token circulates around 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 as long as
the allocated time has not expired
– It releases the token to be used by the successor station
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MCA 3rd Sem (MLSU)

28. • Procedure
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MCA 3rd Sem (MLSU)

29. Channelization
• Definition
– A Multiple-access method in which the available BW of a link is
shared in time, frequency, or through code, between different
stations
• Features
– FDMA, TDMA are based on FDM and TDM in the physical layer
– CDMA is a data link multiple-access protocol
MUNESH MEENA
MCA 3rd Sem (MLSU)

30. FDMA
• Approach and features
– A data link layer protocol that uses FDM at the physical layer
– The available BW is shared by all stations
– Each stations uses its allocated band to send its data
– Ex. AMPS (1G cellular networks)
MUNESH MEENA
MCA 3rd Sem (MLSU)

31. MUNESH MEENA
MCA 3rd Sem (MLSU)

32. TDMA
• Approach and features
– A data link layer protocol that uses TDM at the physical layer
– The stations share the capacity of the channel in time
– Each station is allocated a time slot during which it can send data
– Ex. GSM (2G cellular networks - Europe)
MUNESH MEENA
MCA 3rd Sem (MLSU)

33. MUNESH MEENA
MCA 3rd Sem (MLSU)

34. Code Division Multiple Access (CDMA)
• used in several wireless broadcast channels (cellular, satellite,
etc) standards
• unique “code” assigned to each user; i.e., code set partitioning
• all users share same frequency, but each user has own “chipping”
sequence (i.e., code) to encode data
• encoded signal = (original data) X (chipping sequence)
• decoding: inner-product of encoded signal and chipping
sequence
• allows multiple users to “coexist” and transmit simultaneously
with minimal interference (if codes are “orthogonal”)
MUNESH MEENA
MCA 3rd Sem (MLSU)

35. CDMA Encode/Decode
.
d0 = 1
Zi,m= di cm
data d1 = -1
1 1 1 1 1 1 1 1
sender bits -1 -1 -1 -1 -1 -1 -1 -1
slot 1 slot 0
1 1 1 1 1 1 1 1
channel channel
code -1 -1 -1 -1 -1 -1 -1 -1 output output
slot 1 slot 0
M
.
Di = S Zi,m cm
m=1
1 1 1 1 1 1
M
received 1 1
d0 = 1
-1 -1 -1 -1 -1 -1 -1 -1
receiver input d1 = -1
slot 1 slot 0
1 1 1 1 1 1 1 1
channel channel
code -1 -1 -1 -1 -1 -1 -1 -1 output output
slot 1 slot 0
MUNESH MEENA
MCA 3rd Sem (MLSU)

36. CDMA: two-sender interference
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MCA 3rd Sem (MLSU)

37. Thanks!
MUNESH MEENA
MCA 3rd Sem (MLSU)