1. MEDIUM ACCESS CONTROL PROTOCOL
CHAPTER 6
MEDIUM ACCESS CONTROL PROTOCOL

2. Figure 6.1 Taxonomy of multiple-access protocols discussed in this chapter

3. Topics discussed in this section:
6-1 RANDOM ACCESS
In random access or contention methods, no station is superior to another station and none is assigned the control over another. No station permits, or does not permit, another station to send. At each instance, a station that has data to send uses a procedure defined by the protocol to make a decision on whether or not to send.
Topics discussed in this section:
ALOHA Carrier Sense Multiple Access
Carrier Sense Multiple Access with Collision Detection
Carrier Sense Multiple Access with Collision Avoidance

4. ALOHA
Wireless link to provide data transfer between main campus & remote campuses of University of Hawaii
Simplest solution: just do it
A station transmits whenever it has data to transmit
If more than one frames are transmitted, they interfere with each other (collide) and are lost
If ACK not received within timeout, then a station picks random backoff time (to avoid repeated collision)
Station retransmits frame after backoff time
First transmission
Retransmission
Backoff period B t
t0-X t0
t0+X
t0+X+2tprop + B
t0+X+2tprop
Vulnerable
period
Time-out

5. Throughput of ALOHA Collisions are means for coordinating access
Max throughput is rmax= 1/2e (18.4%)
Bimodal behavior:
Small G, S≈G
Large G, S↓0
Collisions can snowball and drop throughput to zero
e-2 = 0.184

6. Figure 6.3 Frames in a pure ALOHA network

7. Figure 6.4 Procedure for pure ALOHA protocol

8. Example 6.1
The stations on a wireless ALOHA network are a maximum of 600 km apart. If we assume that signals propagate at 3 × 108 m/s, we find
Tp = (600 × 105 ) / (3 × 108 ) = 2 ms.
Now we can find the value of TB for different values of K .
a. For K = 1, the range is {0, 1}. The station needs to| generate a random number with a value of 0 or 1. This means that TB is either 0 ms (0 × 2) or 2 ms (1 × 2), based on the outcome of the random variable.

9. Example 6.1 (continued)
b. For K = 2, the range is {0, 1, 2, 3}. This means that TB can be 0, 2, 4, or 6 ms, based on the outcome of the random variable.
c. For K = 3, the range is {0, 1, 2, 3, 4, 5, 6, 7}. This means that TB can be 0, 2, 4, , 14 ms, based on the outcome of the random variable.
d. We need to mention that if K > 10, it is normally set to

10. Figure 6.5 Vulnerable time for pure ALOHA protocol

11. Example 6.2
A pure ALOHA network transmits 200-bit frames on a shared channel of 200 kbps. What is the requirement to make this frame collision-free?
Solution
Average frame transmission time Tfr is 200 bits/200 kbps or 1 ms. The vulnerable time is 2 × 1 ms = 2 ms. This means no station should send later than 1 ms before this station starts transmission and no station should start sending during the one 1-ms period that this station is sending.

12. The throughput for pure ALOHA is S = G × e −2G .
Note
The throughput for pure ALOHA is S = G × e −2G .
The maximum throughput
Smax = when G= (1/2).

13. Example 6.3
A pure 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 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 frame per millisecond. The load is 1. In this case S = G× e−2 G or S = (13.5 percent). This means that the throughput is 1000 × = 135 frames. Only frames out of 1000 will probably survive.

14. Example 6.3 (continued)
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 −2G or S = (18.4 percent). This means that the throughput is 500 × = 92 and that only 92 frames out of 500 will probably survive. Note that this is the maximum throughput case, percentagewise.
c. If the system creates 250 frames per second, this is (1/4) frame per millisecond. The load is (1/4). In this case S = G × e −2G or S = (15.2 percent). This means that the throughput is 250 × = 38. Only frames out of 250 will probably survive.

15. Slotted ALOHA Time is slotted in X seconds slots
Stations synchronized to frame times
Stations transmit frames in first slot after frame arrival
Backoff intervals in multiples of slots
Backoff period B t kX
(k+1)X
t0 +X+2tprop
t0 +X+2tprop+ B
Time-out
Vulnerableperiod
Only frames that arrive during prior X seconds collide

16. Figure 6.6 Frames in a slotted ALOHA network

17. The throughput for slotted ALOHA is S = G × e−G .
Note
The throughput for slotted ALOHA is S = G × e−G .
The maximum throughput Smax = when G = 1.

18. Figure 6.7 Vulnerable time for slotted ALOHA protocol

19. Example 6.4
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 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 frame per millisecond. The load is 1. In this case S = G× e−G or S = (36.8 percent). This means that the throughput is 1000 × = 368 frames. Only 386 frames out of 1000 will probably survive.

20. Carrier Sensing Multiple Access (CSMA)
A station senses the channel before it starts transmission
If busy, either wait or schedule backoff (different options)
If idle, start transmission
Vulnerable period is reduced to tprop (due to channel capture effect)
When collisions occur they involve entire frame transmission times
If tprop >X (or if a>1), no gain compared to ALOHA or slotted ALOHA A
Station A begins transmission at t = 0
Station A captures
channel at t = tprop

21. CSMA Options Transmitter behavior when busy channel is sensed
1-persistent CSMA (most greedy)
Start transmission as soon as the channel becomes idle
Low delay and low efficiency
Non-persistent CSMA (least greedy)
Wait a backoff period, then sense carrier again
High delay and high efficiency
p-persistent CSMA (adjustable greedy)
Wait till channel becomes idle, transmit with prob. p; or wait one mini-slot time & re-sense with probability 1-p
Delay and efficiency can be balanced
Sensing

22. Figure 6.8 Collision of the first bit in CSMA/CD

23. Figure 6.9 Collision and abortion in CSMA/CD

24. Example 6.5
A network using CSMA/CD has a bandwidth of 10 Mbps. If the maximum propagation time (including the delays in the devices and ignoring the time needed to send a jamming signal, as we see later) is 25.6 μs, what is the minimum size of the frame?
Solution
The frame transmission time is Tfr = 2 × Tp = 51.2 μs. This means, in the worst case, a station needs to transmit for a period of 51.2 μs to detect the collision. The minimum size of the frame is 10 Mbps × 51.2 μs = 512 bits or 64 bytes. This is actually the minimum size of the frame for Standard Ethernet.

25. Figure 6.10 Flow diagram for the CSMA/CD

26. Figure 6.11 Energy level during transmission, idleness, or collision

27. Figure 6.12 Timing in CSMA/CA

28. Figure 6.13 Flow diagram for CSMA/CA

29. Topics discussed in this section:
6-2 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. We discuss three popular controlled-access methods.
Topics discussed in this section:
Reservation Polling Token Passing

30. Reservations Systems
Centralized systems: A central controller accepts requests from stations and issues grants to transmit
Frequency Division Duplex (FDD): Separate frequency bands for uplink & downlink
Time-Division Duplex (TDD): Uplink & downlink time-share the same channel
Distributed systems: Stations implement a decentralized algorithm to determine transmission order
Central
Controller

31. Reservation Systems Transmissions organized into cycles
interval
Frame
transmissions r d d d r d d d
Time
Cycle n
Cycle (n + 1) r = 1 2 3 M
Transmissions organized into cycles
Cycle: reservation interval + frame transmissions
Reservation interval has a minislot for each station to request reservations for frame transmissions

32. Reservation System Options
Centralized or distributed system
Centralized systems: A central controller listens to reservation information, decides order of transmission, issues grants
Distributed systems: Each station determines its slot for transmission from the reservation information
Single or Multiple Frames
Single frame reservation: Only one frame transmission can be reserved within a reservation cycle
Multiple frame reservation: More than one frame transmission can be reserved within a frame
Channelized or Random Access Reservations
Channelized (typically TDMA) reservation: Reservation messages from different stations are multiplexed without any risk of collision
Random access reservation: Each station transmits its reservation message randomly until the message goes through

33. Example Initially stations 3 & 5 have reservations to transmit frames8
(a)
Station 8 becomes active and makes reservation
Cycle now also includes frame transmissions from station 8 t r 3 5 8
(b)

34. Efficiency of Reservation Systems
Assume minislot duration = vX
TDM single frame reservation scheme
If propagation delay is negligible, a single frame transmission requires (1+v)X seconds
Link is fully loaded when all stations transmit, maximum efficiency is:
TDM k frame reservation scheme
If k frame transmissions can be reserved with a reservation message and if there are M stations, as many as Mk frames can be transmitted in XM(k+v) seconds
Maximum efficiency is:

35. Random Access Reservation Systems
Large number of light traffic stations
Dedicating a minislot to each station is inefficient
Slotted ALOHA reservation scheme
Stations use slotted Aloha on reservation minislots
On average, each reservation takes at least e minislot attempts
Effective time required for the reservation is 2.71vX
X X(1+ev) v
ρmax = =

36. Polling Systems
Centralized polling systems: A central controller transmits polling messages to stations according to a certain order
Distributed polling systems: A permit for frame transmission is passed from station to station according to a certain order
A signaling procedure exists for setting up order
Central
Controller

37. Polling System Options
Service Limits: How much is a station allowed to transmit per poll?
Exhaustive: until station’s data buffer is empty (including new frame arrivals)
Gated: all data in buffer when poll arrives
Frame-Limited: one frame per poll
Time-Limited: up to some maximum time
Priority mechanisms
More bandwidth & lower delay for stations that appear multiple times in the polling list
Issue polls for stations with message of priority k or higher

38. Figure 6.14 Select and poll functions in polling access method

39. Comparison of MAC approaches
Aloha & Slotted Aloha
Simple & quick transfer at very low load
Accommodates large number of low-traffic bursty users
Highly variable delay at moderate loads
Efficiency does not depend on a
CSMA-CD
Quick transfer and high efficiency for low delay-bandwidth product
Can accommodate large number of bursty users
Variable and unpredictable delay

40. Comparison of MAC approaches
Reservation
On-demand transmission of bursty or steady streams
Accommodates large number of low-traffic users with slotted Aloha reservations
Can incorporate QoS
Handles large delay-bandwidth product via delayed grants
Polling
Generalization of time-division multiplexing
Provides fairness through regular access opportunities
Can provide bounds on access delay
Performance deteriorates with large delay-bandwidth product