Data Link Multiple Access

1
Data Link Multiple Access

• CS4323

2
Figure 12.2 Taxonomy of multiple-access
protocols discussed in this chapter
3
Random Access Protocols

• Aloha
• CSMA
• CSMA/CD
• CSMA/CA

4
Random Access Protocols

• All stations on these networks have equal access
  to data transfer resources.
• No station controls access to the data transfer
  resources.
• Resources are used based on availabilty
• Availability is determined by the protocol

5
Aloha

• A station sends a frame when the station needs to
  send a frame.
• If frames overlap, the frames are damaged beyond
  repair, and must be resent.
• An acknowledgement frame is sent by the receiver
  station after a frame arrives.
• If an acknowledgement is not received, the sender
  resends the frame.

6
Aloha

• If a collision occurs, the energy of the damaged
  frame is greater than a normal frame.
• This property leads to limits on line lengths due
  to attenuation.
• All damaged frames are dropped and no
  acknowledgement frame is sent.

7
Figure 12.3 Frames in a pure ALOHA network
8
Aloha

• Frames are re-sent using an algorithm that
  determines when to resend.
• A common algorithm is the binary exponential
  back-off time.

9
Figure 12.4 Procedure for pure ALOHA protocol
10
Vulnerability Time

• Let T(fr) be the average transmission time for a
  frame.
• Then the vulnerability time for a frame is 2T(fr).

11
Figure 12.5 Vulnerable time for pure ALOHA
protocol
12
Vulnerability

• When station A sends a message, it is subject to
  being corrupted by a previous message or a future
  message.
• The vulnerability time window is 2T for station
  A.

13
Aloha BEBOT

• Binary exponential back-off time
• Tb R Tp
• R is computed based on choosing a random value
  from 0 to 2k -1, and k 1, 2, ..., 10
• k is the number of retries.

14
Figure 12.4 Procedure for pure ALOHA protocol
15
Aloha Efficiency

• Pure Aloha has a maximum efficiency of about
  18.4.
• S G e(-2G)
• Where G is the load ratio of usage to capacity.
• See example 12.3 page 368

16
Slotted Aloha

• The stations are synchronized to only send a
  frame at the start of a time slot.
• The time slot is set to be equal to the average
  transmission time for a frame.

17
Figure 12.6 Frames in a slotted ALOHA network
18
Slotted Aloha Efficiency

• Slotted Aloha is about 36.8 efficient
• S G e(-G).
• See example 12.4 page 370

19
Slotted Efficiency

• 200 bits/frame with a capacity of 200kbps
• Throughput (efficiency) for 1000 frames/sec.
• G 1
• S G e ( -G )

20
CSMA

• Carrier Sense Multiple Access
• A station senses the media before attempting to
  send.

21
Figure 12.8 Space/time model of the collision in
CSMA
22
CSMA Vulnerable Time

• Tp is the propagation time from one end of the
  transmission medium to the other.
• Vulnerable time Tp

23
Figure 12.9 Vulnerable time in CSMA
24
CSMA Persistence Methods

• Persistence methods are used to determine what to
  do when the channel is either
• Idle
• busy

25
CSMA Persistence Methods

• 1-Persistent
• Non-Persistent
• p-Persistent

26
Figure 12.10 Behavior of three persistence
methods
27
1-Persistent

• Persistently sense the medium until it is idle.
• Send a message as soon as the medium is idle
• This protocol is most likely to produce
  collisions, since many stations could be
  simultaneously waiting for the medium to become
  idle.
• No more than 50 efficient

28
Non-Persistent

• If the medium is busy, wait a random amount of
  time before sensing again.
• Less likely to produce collisions, since the
  likelihood of the stations sending at the same
  time is reduced.
• Does not fully use the available bandwidth

29
P-Persistent

• The medium is continuously sensed until idle.
• Messages are only sent when the medium is idle at
  the start of a synchronized time slot.
• Each time slot is equal to the medium's
  propagation time (vulnerability time).
• An algorithm determines randomly which time slot
  is used for sending data.

30
CSMA/CD

• Carrier sense multiple access collision detection
• This adds the protocol for determining what to do
  when a collision occurs.

31
Figure 12.15 Energy level during transmission,
idleness, or collision
32
Figure 12.12 Collision of the first bit in
CSMA/CD
33
CSMA/CD

• The station does not save any message sent or
  wait for an acknowledgement of successful
  delivery.
• A station will only detect a collision while
  transmitting.
• A sending station listens to a feedback loop of
  its own transmission to detect a collision.

34
CSMA/CD

• If a collision is detected during transmission,
  the current message is resent after waiting a
  random amount of time.

35
Figure 12.14 Flow diagram for the CSMA/CD
36
CSMA/CA

• CSMA/Collision Avoidance
• Used for wireless transmission
• The energy of a collision cannot be detected
  using CSMA/CD. Therefore, collisions must be
  avoided.

37
CSMA/CA 3-Simultaneous Strategies

• IFS, Inter-frame space
• Contention window
• Acknowledgement

38
CSMA/CA IFS

• Minimum IFS time is Tp (aka vulnerability time)
• This time can be adjusted on each station to
  establish a priority for each station.

39
CSMA/CA Contention Window

• The CW is divided into time slots.
• The station waits a random number of time slots
  before sending.
• The window of time slots is computed using the
  binary exponential back-off algorithm until the
  medium is idle.

40
CSMA/CA Acknowledgement

• The sender waits for an acknowledgement frame to
  confirm delivery.

41
Figure 12.16 Timing in CSMA/CA
42
Figure 12.17 Flow diagram for CSMA/CA
43
In CSMA/CA, the IFS can also be used to define
the priority of a station or a frame.
44
Controlled Access

• Reservation
• Polling
• Token Passing

45
Controlled Access Reservation

• A reservation frame contains time slots for each
  station sharing the network.
• The number of time slots in the reservation frame
  equals the number of stations.
• The reservation frame has a bit mask to determine
  if a station is transmitting data.

46
Figure 12.18 Reservation access method
47
Controlled Access Polling

• Stations are categorized as either primary or
  secondary stations.
• All data exchanges must be made through the
  primary device.
• The primary device controls which secondary
  device will transmit.

48
Figure 12.19 Select and poll functions in
polling access method
49
Controlled Access Polling

• SEL this packet is sent by the primary station
  to alert a secondary station when the primary is
  about to send data.
• Poll this function is used by the primary
  station to solicit transmissions from the
  secondary stations. Each station is solicited in
  order.

50
Controlled Access Polling

• When a secondary station is polled, it will
  either
• Send data
• Send a NAK, which is interpreted as no data to
  send at this time (not-acknowledgement)
• When any data is received, the receiving stations
  sends back an acknowledgement frame.

51
Controlled Access Token Passing

• AKA the token ring.
• The ring can be configured logically.
• A station has a predecessor and a successor.
• Predecessor before the current node
• Successor after the current node

52
Controlled Access Token Passing

• A special packet known as a token moves from
  station to station.
• A station can only send when the token is in its
  possession.
• When transmission is complete, the station sends
  the token to the successor.

53
Controlled Access Token Management

• The token can be regenerated by the MAU if the
  token is lost.
• The stations can be configured to hold the token
  for different amounts of time.

54
Figure 12.20 Logical ring and physical topology
in token-passing access method
55
Token Ring

• FDDI fiber distributed data interface is a common
  implementation of a token ring.
• FDDI uses a dual ring, with one ring acting as a
  backup.