Ethernet, Wireless LAN

1
Ethernet, Wireless LAN
2
Ethernet Frame Format
(a) DIX Ethernet, (b) IEEE 802.3
3
Minimum Frame Size

• Why a minimum frame size is needed?
• How long does it take for a station to notice a
  collision?

4
Worst case
5
Minimum Frame Size

• So, if maximum delay is t, the minimum frame size
  is 2tbit rate.
• t is about 50us.
• So the minimum frame size of 10M Ethernet is 512
  bits.
• What if the speed goes up?

6
Ethernet Performance

• Suppose there are k stations. Let p be the
  probability that a station has a frame to send
  when the channel is idle. Assume it is
  independent across stations, and is independent
  for one station at different times. Find the
  average number of collisions before a frame is
  sent.
• First, the probability that one station got the
  chance to send is Akp(1-p)k-1.
• Second, maximized when p1/k. So A is bounded by
  (1-1/k)k-1.
• Third, each contention is independent, so average
  number of collision is 1/A, which is e when k is
  large.
• Each contention is 2t, so channel efficiency is
  P/P2et.

7
Ethernet

• Physical medium
• thin cable/thick cable/twisted pair/fiber
• 10Base5 500 meters thick (cable) Ethernet
  100 nodes/seg
• 10Base2 200 meters thin (cable) Ethernet
  30 nodes/seg
• 10BaseT 100 meters twist pair
  1024 nodes/seg
• 10BaseF 2000 meters fiber optics
  1024 nodes/seg
• 10Base5/10Base2, cable connected to each machine
  
• 10BaseT -- connecting to a hub
• 10BaseF -- between building Connecting

8
Ethernet

• Fast Ethernet
• Keep everything in Ethernet, make the clock
  faster 100Mbps.
• Cable
• 100Base-T4 100m category 3 UTP, 4 lines.
• 100Base-Tx 100m category 5 twisted pair
• 100Base-Fx 2000m Fiber optic

9
IEEE 802 LAN Protocol Stack
ISO/OSI Reference Model
10
Wireless LAN

• Basic structure
• Stations plus an access point
• Stations talk to the access point, then to
  outside
• Access point talks to stations
• Stations talk to stations
• Design goal
• A MAC protocol to determine who talks next

11
Wireless communications

• Signal decays according to a power law with the
  distance, at least to the power of -2 with
  distance
• Comparing to Ethernet, what is the difference (as
  far as MAC is concerned)?
• When a station is sending, not all stations can
  hear. No real 100 carrier sense.
• In Ethernet, everybody can hear everybody

12
Wireless communications

• When a station is sending, he cannot hear other
  stations cannot decide if there is a collision.
  No CD in wireless LAN.
• In Ethernet, the sender can determine if there is
  collision and abort immediatelly.

13
Wireless communications

• Being able to sense the carrier does not mean
  that you can decode the data
• If received signal having power P means that you
  can decode the data, it may be true that at power
  P/2 you can realize that there is something going
  on

14
Wireless communication

• The received signal can be read if the signal to
  noise ratio is larger than a certain threshold.
  Whether there is a collision depends on the
  signal to noise ratio at the receiver.
• You may allow two transmissions at the same time
  without collision.
• In Ethernet, two simultaneous transmission means
  collision

A
D
C
B
A
D
C
B
A-gtB, D-gtC
A-gtB, C-gtD
15
Wireless communications

• Hidden terminal

A
D
C
B

• Exposed terminal

A
D
C
B
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Medium Access Control (MAC) Layer

• Asynchronous Data Service
• DCF (Distributed Coordination Function)
• Contention-Based Medium Access Control
• CSMA/CA Carrier Sense Multiple Access/Collision
  Avoidance
• For elastic applications like email, file
  transfer
• Time-Bounded Service
• PCF (Point Coordination Function)
• Contention Free Medium Access Control
• Optional access method works like polling
• For time-sensitive voice/video applications

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Goals

• How to design an efficient contention-based MAC
  protocol for wireless LAN?
• Goals
• Collision avoidance to reduce wasted
  transmissions
• Reasonable fairness
• Cope with hidden terminals
• Allow exposed terminals to talk

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Problems

• What problems will occur if apply Ethernet MAC?
• No CD, does not know whether there is a collision
• No CD, channel waste could be large using
  1-persistent
• Cannot hear all other people means the sender
  cannot be sure that he can reserve the whole
  channel.

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Fixes

• No CD, use ACK. If there is no ACK, assume there
  is collision
• No CD, has to use non-persistent to reduce
  collision by AVOIDING COLLISION, CA
• Cannot hear other people, so devise some channel
  reservation technique

20
DCF

• When got a packet to send, sense the channel, if
  idle, send immediately (not completely
  non-persistent! Why?)
• When channel is busy, wait until idle, then
  backoff a random time. If still idle, send. (The
  non-persistent part. The CA feature). If busy
  before reaching zero, freeze it, and reactivate
  when idle again.
• After receives a packet, send ACK.
• If collision, use the exponential backoff.

21
DCF

• Do you want the ACK to have the same priority as
  data packets?
• How do you make sure that ACK has higher
  priority?
• Use time. You have to wait for a certain amount
  time before you can send.
• High priority packets wait shorter.

22
DCF

• The SIFS, DIFS. SIFS is for control packets. DIFS
  is for data packets.
• When a station wants to send, if it is a control
  packet, sense the channel for SIFS, then send. If
  it is a data packet, sense the channel for DIFS,
  then send.

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DCF
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Further improvement

• Further improvement by improving carrier sense
• The problem is other people cannot hear me
  sending, so they will send.
• So, how to make sure that they will know I am
  sending?

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RTS/CTS

• RTS/CTS in the place for carrier sense
• RTS reserves channel for a bit of time, if
  sender hasnt heard other CTSes
• CTS sender replies if it hasnt heard any other
  RTSes
• Both messages include time. Network Allocation
  Vector (NAV)
• If no CTS, exponential backoff
• RTS-CTS-DATA

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RTS/CTS

• 802.11 standardized both CSMA/CA and RTS/CTS
• In practice, most operators disable RTS/CTS
• Very high overhead!
• RTS/CTS packets sent at base rate (often 1Mbit)
• Avoid collisions regardless of transmission rate
• Most deployments are celluar (base stations), not
  ad hoc. Neighboring cells are often configured
  to use non-overlapping channels, so hidden
  terminals on downlink are rare
• Hidden terminal on uplink possible, but if
  clients mostly d/l, then uplink packets are
  small.
• THIS MAY CHANGE. And is likely not true in your
  neighborhood!
• When CS range gtgt reception range, hidden terminal
  less important

27
PCF

• The AP acts as the master and sends out beacon
  signals for polling stations and stations can
  sign up for certain amount of bandwidth use
• Co-exists with DCF.
• How to make sure that beacon signals have higher
  priority?
• PIFS