Medium Access Control

1
Medium Access Control

• Zhibin Wu

2
Lecture Overview

• Introduction
• Random Access
• Aloha
• Slotted Aloha
• CSMA
• CSMA/CD
• CSMA/CA
• Scheduled Access
• TDMA
• Dynamic TDMA
• Spread-Spectrum/CDMA

3
Medium Access Sublayer
network
Link layer control
LLC
Data link
MAC
Medium access control
physical

• Medium access (MAC) sublayer is not relevant on
  point-to-point links
• The MAC sublayer is only used in broadcast or
  shared channel networks
• All communication entities share a common
  channel
• Examples
• Wired networks Ethernet LAN
• Wireless Mobile Networks Satellite, Cellular,
  Wireless LAN,
• Packet radio network?

4
Share a Channel Ideally

• Broadcast channel of rate R bps
• 1. When one node wants to transmit, it can send
  at rate R.
• 2. When M nodes want to transmit, each can send
  at average rate R/M

5
Random Access Protocols

• Single channel shared by a large number of hosts
• No coordination between hosts
• Control is completely distributed
• Examples ALOHA, CSMA, CSMA/CD

6
Scenarios of ALOHA

• A group of nodes trying to sending frames to a
  central node
• Star-topology.
• Not a complete solution for bi-directional
  communication
• For half-duplex device, what if a data packet
  arrives while it is receiving?

7
Pure Aloha

• In Pure Aloha, frames are transmitted at
  completely arbitrary times.

8
Aloha Algorithm

• Transmit whenever you have data to send
• Listen to the broadcast (probably a separate
  channel)
• Because broadcast is fed back, the sending host
  can always find out if its packet was destroyed
  just by listening to the downward broadcast one
  round-trip time after sending the packet
• If the packet was destroyed, wait a random amount
  of time and send it again
• The waiting time must be random to prevent the
  same packets from colliding over and over again

9
Vulnerable Period

• Vulnerable period for the shaded frame is 2t
• Note that if the first bit of a new packet
  overlaps with the last bit of a packet almost
  finished, both packets are totally destroyed. (No
  capture effect)

10
Analysis of Aloha

• Packet Arrival is Poisson Process
• P k arrivals in t seconds
• Let G be the total number of frames attempted in
  t seconds
• P k attempts in t seconds
• Conditional successful probability for one
  attempt is
• P0 P 0 other attempts in 2t seconds e-2Gt
• Set t as unit frame time
• Let S be the mean number of successful attempts
• SGP0Ge-2G
• S is optimum at G1/2
• S1/2e 0.184

11
Slotted Aloha

• Transmission of frames are synchronized slot by
  slot.
• Channel feedback about whether packet is received
  or not

12
Slotted Aloha (Continued)

• Slotted ALOHA cuts the vulnerable period for
  packets from 2t to t.
• Time is slotted. Packets must be transmitted
  within a slot.
• Procedure
• If a host has a packet to transmit, it waits
  until the beginning of the next slot before
  sending
• Listen to the broadcast and check if the packet
  was destroyed
• If there was a collision, wait a random number of
  slots and try to send again

13
Analysis of Slotted ALOHA

• Packet Arrival is Poisson Process
• P k arrivals in t seconds
• Let G be the total number of frames attempted in
  t seconds
• P k attempts in t seconds
• Successful probability for each slot is
• P 1 attempts in a t seconds slot Ge-Gt
• Set Slot time t as unit time, then SGe-G
• S is optimum at G1
• S1/e 0.368

14
Performance of ALOHA

• Throughput versus offered traffic for ALOHA
  systems
• The main reason for poor channel utilization of
  ALOHA (pure or slotted) is that all stations can
  transmit at will, without paying attention to
  what the other stations are doing.

15
CSMA

• Protocols in which stations listen for a carrier
  (i.e., a transmission) and act accordingly are
  called carrier sense protocols.
• There are several types of CSMA protocols
• Non-Persistent CSMA
• 1-Persistent CSMA
• P-Persistent CSMA

16
Assumptions with CSMA Networks

• Constant length packets
• No errors, except those caused by collisions
• No capture effect
• Each host can sense the transmissions of all
  other hosts
• The propagation delay is small compared to the
  transmission time

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Propagation Delay
A
C
D
B

• D only sense As transmission after a propagation
  delay t
• If t is larger than packet transmission time,
  there are too much time wastage.
• CSMA in satellite communication? No.

The size (length) of the network must be limited!
18
Non-persistent CSMA

• To send data, a station first listens to the
  channel to see if anyone else is transmitting.
• If so, the station waits a random period of time
  (instead of keeping sensing until the end of the
  transmission) and repeats the algorithm.
  Otherwise, it transmits a frame.
• If a collision occurs, the station waits a random
  amount of time and starts all over again.
• Assumption
• propagation delay is a constant common to all
  nodes
• a is the ratio of propagation delay to packet
  transmission time

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Analysis of Non-persistent CSMA
Unsuccessful transmission period
Successful transmission period
Normalized Time
a
a
Y
1
1
Busy period
Idle period
a
Busy period

• S U/(BI)
• B Y 1 a , I 1/G
• U e-aG
• FY(y)Pno packet occur in an duration of a-y
  e-G(a-y)

20
Discussion of Collisions

• What's the effect of signal propagation delay a?
  
• The longer the delay, the more the collisions,
  and the worse the performance of the protocol.
• How about zero propagation delay ?
• There still exist chances of collisions. S
  G/(1G)
• Is this protocol any better than ALOHA (both pure
  and slotted) ?
• Yes, because both stations have the decency to
  desist from interfering with the third station's
  frame.

21
1-persistent CSMA

• 1-persistent CSMA (Carrier Sense Multiple
  Access)
• To send data, a station first listens to the
  channel to see if anyone else is transmitting.
• If so, the station waits (keeps sensing it) until
  the channel becomes idle. Otherwise, it transmits
  a frame.
• If a collision occurs, the station waits a random
  amount of time and starts all over again.
• It is called 1-persistent because the station
  transmits with a probability of 1 whenever it
  starts sensing the channel and finds the channel
  idle. (Greedy)
• This protocol has worse channel utilization than
  non-persistent CSMA.

22
Tradeoff between Non-persistent and 1-persistent

• If B and C become ready in the middle of As
  transmission,
• 1-Persistent B and C collide
• Non-Persistent B and C probably do not collide
• If only B becomes ready in the middle of As
  transmission,
• 1-Persistent B succeeds as soon as A ends
• Non-Persistent B may have to wait

23
P-persistent CSMA

• Assume channels are slotted
• One slot contention period (i.e., one round
  trip propagation delay)
• Algorithm
• Sense the channel
• If channel is idle, transmit a packet with
  probability p
• if a packet was transmitted, go to step 2
• if a packet was not transmitted, wait one slot
  and go to step 1
• If channel is busy, wait one slot and go to step
  1.
• In other words, wait until idle and then transmit
  with probability p
• Detect collisions
• If a collision occurs, wait a random amount of
  time and go to step 1

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Persistent and Non-persistent CSMA

• Comparison of the channel utilization versus load
  for various random access protocols.

25
CSMA with Collision Detection

• CSMA/CD (Carrier Sense Multiple Access with
  Collision Detection) protocol further improves
  ALOHA by aborting transmissions as soon as a
  collision is detected.
• The conceptual model
• To send data, a station first listens to the
  channel to see if anyone else is transmitting.
• If so, the station waits until the end of the
  transmission (1-persistent) or wait a random
  period of time and repeats the algorithm
  (non-persistent). Otherwise, it transmits a
  frame.
• If a collision occurs, the station will detect
  the collision, abort its transmission, waits a
  random amount of time, and starts all over again.
  

26
How to Detect Collision
Tx
Rx

• Prerequisite A node can listening while talking
• Ethernet cables

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CSMA/CD Continued

• CSMA/CD can be in one of three states
  contention, transmission, or idle
• The minimum time to detect the collision is the
  time it takes the signal to propagate from one
  station to the other.
• How long could the transmitting station be sure
  it has seized the network ? (? or 2? ? where ?
  is time equal to the full propagation)
• Model the contention interval as slotted aloha
  with slot width 2?

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CSMA/CA

• Wireless LAN
• How can detect collision if you cannot listening
  while talking?
• Collision Avoidance
• Random Backoff (instead of 1-persistent)
• RTS/CTS
• CS no longer works well
• Rules
• carrier gt do not transmit
• no carrier gt OK to transmit
• But the above rules do not always apply to
  wireless.

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Problems with carrier sensing
Hidden terminal problem
Y
Z
W
W finds that medium is free and it transmits a
packet to Z
no carrier gt OK to transmit
/
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Problems with carrier sensing
Exposed terminal problem
Z
W
Z is transmitting to W
Y
X
Y will not transmit to X even though it cannot
interfere
/
Presence of carrier gt hold off transmission
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Solving Hidden Node problem with RTS/CTS
Y
Z
X
W
Note RTS/CTS does not solve exposed terminal
problem. In the example above, X can send RTS,
but CTS from the responder will collide with Ys
data.
32
RTS/CTS exchange example
SIFS
DIFS
Frame
RTS
Src
ACK
CTS
Dest
8192 ?s
352 µs
304 µs
304 µs
10 µs
10 µs
10 µs
Dest
NAV (RTS)
NAV (CTS)

• RTS CTS Frame ACK exchange invoked when
  frame size is large
• NAV (Network Allocation Vector)
• NAV maintains prediction of future traffic on the
  medium based on duration information that is
  announced in RTS/CTS frames prior to actual
  exchange of data

33
Pros Cons of Random Access

• Advantages
• Short delay for bursty traffic
• Simple (due to distributed control)
• Flexible to fluctuations in the number of hosts
• Fairness
• Disadvantages
• Low channel efficiency with a large number of
  hosts
• Not good for continuous traffic (e.g., voice)
• Cannot support priority traffic
• High variance in transmission delays

34
Scheduled Access

• TDMA
• Dynamic TDMA
• Widely used
• cellular,
• Wi-Fi (HyperLAN),
• IEEE 802.16
• Wireless ATM

35
TDMA

• Time Division Multiple Access

36
TDMA Continued

• access to channel in "rounds"
• each station gets fixed length slot (length
  packet transmission time) in each round
• unused slots go idle
• example 6-station LAN, 1,3,4 have packets, slots
  2,5,6 idle

37
Dynamic TDMA

• In dynamic time division multiple access, a
  scheduling algorithm dynamically reserves a
  variable number of timeslots in each frame to
  variable user data streams, based on the traffic
  demand of each user data stream.
• Negotiations (beforehand) to determine how to
  allocate slots dynamically.

38
Summary of Scheduled Access Protocols

• Avoid of contention/collision better channel
  efficiency with a large number of hosts
• predetermined channel allocation
• Need centralized control
• Require global synchronization
• Guard time period to protect slots
• Delay?

39
Spread Spectrum and CDMA

• What if not divide up the channel by time (as in
  TDMA), or frequency (as in FDMA)? Is collision
  inevitable?
• Not if collision is no longer damaging!
• Is there any way to decode bits garbled by other
  overlapping frames?
• CDMA based on Spread Spectrum
• A new perspective to solve multiple access
  problems
• Spread Spectrum is a PHY innovation, not a MAC
  technique.
• CDMA encodes data with a special code associated
  with each user and uses the constructive
  interference properties of the special codes to
  perform the multiplexing.

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Spread Spectrum

• Idea
• spread signal over wider frequency band than
  required
• originally deigned to thwart jamming
• Frequency Hopping
• transmit over random sequence of frequencies
• sender and receiver share
• pseudorandom number generator
• seed

41
Spread Spectrum (cont)

• Direct Sequence
• for each bit, send XOR of that bit and n random
  bits
• random sequence known to both sender and receiver
  
• called n-bit chipping code
• 802.11 defines an 11-bit chipping code

42
Code Division Multiple Access (CDMA)

• Multiplexing Technique used with spread spectrum
• Start with data signal rate D
• Called bit data rate
• Break each bit into k chips according to fixed
  pattern specific to each user
• Users code
• New channel has chip data rate kD chips per
  second
• E.g. k6, three users (A,B,C) communicating with
  base receiver R
• Code for A lt1,-1,-1,1,-1,1gt
• Code for B lt1,1,-1,-1,1,1gt
• Code for C lt1,1,-1,1,1,-1gt

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CDMA Example
44
CDMA Explanation

• Consider A communicating with base
• Base knows As code
• Assume communication already synchronized
• A wants to send a 1
• Send chip pattern lt1,-1,-1,1,-1,1gt
• As code
• A wants to send 0
• Send chip pattern lt-1,1,1,-1,1,-1gt
• Complement of As code
• Decoder ignores other sources when using As code
  to decode
• Orthogonal codes

45
Topics Not Covered in This Lecture

• Dynamic behavior of Aloha
• Strict mathematical analysis
• Stabilize Aloha systems with channel feedback
• Taking Turns MAC protocols
• Token Ring
• FDMA