15-441 Communications and Networking

1
15-441Communications and Networking

• Lecture 6
• Gregory Kesden

2
Medium Access Control (MAC) Layer
3
Medium Access Control

• Allow multiple stations to share a common
  communication channel
• Approaches
• Take turns token-passing, coordinator-polling
• Spread spectrum
• Collision-safe

4
Master-Slave Polling
Master
Slave
Slave
Slave

• Master relays for slaves slaves cant talk
  directly
• Okay when peer-to-peer is less common that
  client-server
• Master is single point of failure
• Can pass more quickly by slaves with no data,
  while still limiting time for those with backlog

5
Token Passing

• Provide equal time slices to each station
• No starvation, predictable, possibly wasted slots
• Can be enforced by hardware
• Allows flexible time slices
• Possibility of starvation, more dynamic
• Broken link requires election

6
Code Division Multiple Access (CDMA)

• Each unit is given a unique id known as a chip
  sequence.
• All chip sequences are the same length
• All chip sequences have the same number of 0s as
  1s
• A sender sends a 1-bit by transmitting its chip
  sequence
• A sender sends a 0-bit by transmitting the 1s
  complement (flip each bit) of its chip sequence
• Collision can occur, but will not destroy data

7
CDMA, cont.

• Since each chip sequence must be of the same size
  and have as many 0s as 1s, the dot-product of two
  chip sequences is 0, unless they are the same, in
  which case it is 1
• .
• When multiple stations transmit concurrently,
  their signals add linearly
• To ensure that their power is the same, they
  transmit with the inverse power of the base
  stationss signal strength
• Sender and receiver must be synchronized for the
  linear combination to work. This is accomplished
  by training the receiver to the sender using a
  known bit sequence.

8
CDMA, cont
If we represent 0s and 1s as 1s and 1s, the dot
product of a chip sequence and itself is 1,
whereas the dot product of a chip sequence And
any other chip sequence is 0.
9
CDMA, cont.

• Recall that collision will result in the linear
  combination of chip sequences.
• The dot product of this linear combination of
  chip sequences and the senders chip sequence
  will result in a 1 if the sender sent a 1 or a
  1, otherwise.
• The other sequences balance out and become 0.
  The interesting chip sequence is multiplied by
  itself, or the compliment of itself, yielding 1
  or 1.
• Consider 0101 vs 1010 vs 1100 vs 0011 vi 0110 vs
  1001.

10
Pure Aloha

• Circa 1970
• Originally developed at Univ. of Hawaii for use
  across multiple campus on multiple islands
• Avoided costly phone lines
• Illustrates the principles of a contention-based
  MAC protocol on any broadcast media.

11
Pure Aloha, Illustration
Coordinator
Station
Station
Station
Station

• Uplink is broadcast Any station can transmit at
  any time
• (Downlink was time-sliced)
• If ACK isnt received within RTT, assume lost
• Transmission problem/interference
• Collision detected by waiting for ACK, any
  overlap destroys transmission
• Retransmit after random time
• Wastes network time
• Can be lost, themselvesrepeat

12
Pure Aloha, Collision Example
A
B
C
D
E
t
window of vulnerability (2t)
13
Slotted Aloha

• Coordinator signals slot boundaries
• Tolerance needs to be built in for timing
  signals transmission latency
• Send only at beginning of slot.
• This reduces the window of vulnerability to a
  single frames transmission time (1 slot)
• Of course, if contention is low and collision is
  not likely, it will increase average transmission
  latency.

14
Slotted Aloha, Collision
tic
tic
tic
tic
tic
tic
tic
tic
tic
tic
tic
tic
tic
A
B
C
D
E
t
window of vulnerability (t)
Potentially unnecessary delay
t
15
Real Islands, Real problems
Coordinator King of the mountain
! mountains

• Stations cant hear each other. They cant listen
  for garbled transmissions

16
Carrier Sense Multiple Access (CSMA)

• But, what all stations can listen?
• if, instead of using slots, stations just
  listen first, and only transmit if no one else
  is?
• This means that collisions would only occur in
  the event of a tie two stations listened and
  then transmitted?
• This leads to much better performance the
  window of vulnerability is very small just the
  transmission latency.

17
Persistent CSMA

• Under persistent CSMA stations continually sense
  the channel and transmit as soon as it is free.
• The problem is that a queue could have been
  building up during the prior transmission.
• This ensures almost certain collision to follow.

18
Nonpersistent CSMA

• Nonpersistent CSMA will transmit immediately if
  the channel is free. The recent past is a good
  indicator of the near future. A collision isnt
  likely.
• If however, a station listens and hears a
  transmission, it stops listening and waits a
  random amount of time before trying again.
• This helps to mitigate the queuing of requests
  during a transmission from leading to a collision

19
p-Persistent CSMA

• p is a parameter. It specifies the probability
  with which a station should transmit upon
  detecting an idle channel.
• In other words, if the channel is free, a station
  begins to transmit immediately with a probability
  of p.
• The other (1-p) time, the station waits a random
  amount of time. After this random delay, it rolls
  the dice again and transmits or waits,
  accordingly.
• This system is designed to further reduce the
  likelihood of collision by spreading out bursts.

20
Quiet _at_ it!

• Why do stations keep transmitting during a
  collision?
• Consider Aloha used radios
• Most cant concurrently transmit and receive (to
  protect receiver), so they cant hear the
  garbling
• Even if they could listen while transmitting,
  theyd likely only hear self
• Signal weakens with inverse-square, so they
  out-power other sender
• These assumptions change if medium isnt radio,
  but instead copper wire.

21
CSMA/CD

• Another level of sophistication is the addition
  of collision detection.
• Ethernet
• Stations listen while transmitting
• If a station hears something different than what
  it is sending, it immediate stops
• What it hears is different from what it sends, if
  another transmission garbles the original.
• IEEE 802.3 (including Ethernet) protocols are
  1-persistent CSMA/CD.

22
CSMA/CD Jamming Signal

• What if two colliding signals, propagating in
  opposite directions, cancel each other out?
• If any station detects a collision, it sends a
  jamming signal to make sure that the colliding
  signals dont cancel each other out, preventing
  detection after attenuation.
• This reduces the time wasted in the event of a
  collision sending already garbled packets.

23
CSMA/CD Min. Frame Size

• Because of latency, collisions cannot be detected
  immediately. This signal from one transmitter
  much reach the other transmitter.
• In practice, this imposes a minimum size on the
  length of a frame. The frame must be long enough
  to permit a collision to be detected.
• If the frame is too short, it will be gone before
  the signal from one transmitter reaches another.
• The frames will be corrupted as the cross in the
  mail
• But neither sender will be able to detect it,
  until the whole frame has been sent, which
  defeats the purpose.

24
Minimum Frame Size for CSMA/CD

• The frame needs to be sufficiently long to
  require more than a round-trip time for
  transmission.
• We must consider the bit-rate of the channel as
  well as its length to determine the round trip
  time.
• The frame must require more than this round-trip
  time for transmission or the sender wont
  necessarily still be transmitting when the
  conflicting signal is first heard.

BS

• B bits/second
• S seconds
• BS bit time

Sender1
Sender2
Sender 1 must wait for its frame to (almost) make
it to Sender2, when the collision occurs, and
then for the energy from Sender2s frame to make
it back, before detecting the collision.
Sender2 detects the Collision almost immediately
25
CSMA/CD Observations

• Frame size is a function of
• Wire size
• Transmission speed
• So
• Faster speeds or longer wires imply larger frames
• Smaller frames require slower speeds or shorter
  wires
• Consider the bandwidth-delay product
• Minimum frame size
• (2length)/propagation_speedbit_rate
• Think of it this way
• How much time does it take a single to make a
  round trip?
• (2length) X propagation speed (distance/s)
• How fast are bits being placed on the wire?
• bit-rate (b/s)
• How many bits are placed on the wire during this
  round trip?
• (2length)/propagation_speedbit_rate

26
Can we use Carrier Detection Over a Satellite?

• Nope. Why not?
• Stations transmit up to a satellite, which then
  echo everything back down.
• Stations cant sense each others transmissions,
  until they are echoed back down the
  transmitters are too far apart and focused on the
  satellite.
• The propagation delay between the transmission to
  the satellite and the echo back down is about
  270ms
• Throughput might be on the order of 36Mbps
• Consider the size of the frame needed to detect
  collision.

27
What Mountains Back to Wireless

• Consider shorter-range wireless No Mountains
• But, still challenges Reachability is not
  transitive
• A can reach B
• B can Reach C
• A cannot reach C

A
B
C
D
28
What Mountains Back to Wireless

• A and C cant hear each other
• Cant detect that they are colliding at B
• A and C are hidden from each other
• B can hear and C, but C and A cant hear each
  other
• C hears B transmit to A, so B waits before
  transmitting to D
• This is unnecessary, because D cant hear B
• B and D are both exposed to C

A
B
C
D
29
MACA Multiple Access with Collision Avoidance

• Dont assume we know what the receiver can hear.
  Instead, ask the receiver
• Request to send (RTS) w/data size (number of
  frames)
• If RTS not ACKed, assume collision
• Data should also be ACKed (to let other senders
  know they can RTS)

30
MACA Example
A
B
C
D

• White RTS from B to C (heard also by A)
• White-Dashed RTS-ACK from C to B (heard also by
  D)
• Black Data from B to C (heard also by A)
• Black-Dashed Data-ACK from C to B (heard also by
  D)

31
MACA Example Another View
A
B
C
D
Not intended recipient
Not intended recipient