Chapter 2 More on Wireless Ethernet, Token Ring, FDDI

1
Chapter 2More on Wireless Ethernet, Token Ring,
FDDI

• Professor Rick Han
• University of Colorado at Boulder
• rhan_at_cs.colorado.edu

2
Announcements

• Programming assignment 1 is now available on Web
  site, due Feb. 6
• Homework 2 should be online by Friday night
• This weeks lectures should be online by Friday
  night
• Next, Chapter 2, more on Wireless Ethernet, Token
  Ring, FDDI

3
Program 1 TCP/UDP Ports

• Ports are used at transport layer 4 to
  differentiate/demultiplex between incoming
  traffic from different applications

Source ports on Host A
Destination ports on Host B
App 1
App 2
App 2
App 1
5200
5307
7447
6010

• A port is like a mailbox
• Need ports in both hosts so ACKs know where to go
• Can also send data in both directions

UDP
UDP
IP
IP
Link/MAC
Link/MAC
Phys
Phys
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Program 1 TCP/UDP Ports (2)

• Application source dest ports gt 5000
• Web/HTTP uses well-known reserved port 80

Source port on Host A For Stop-and-Wait Client
Destination port on Host B For Stop-and-Wait
Server
5200
8110

• Normally, link layer protocol doesnt use ports
• Were emulating layer 2 from above layer 4, so we
  have to specify ports

UDP
UDP
IP
IP
Link/MAC
Link/MAC
Phys
Phys
5
Announcements

• Programming assignment 1 is now available on Web
  site, due Feb. 6
• Homework 2 should be online by Friday night
• This weeks lectures should be online by Friday
  night
• Next, Chapter 2, more on Wireless Ethernet, Token
  Ring, FDDI

6
Recap of Previous Lecture

• Multiple Access Protocols
• Designed for shared-media links
• Channel reservation protocols TDMA, FDMA, CDMA
• Random access protocols CSMA/CD, CSMA/CA
• Random Access Protocols
• ALOHA, slotted ALOHA packet collisions
• CSMA listen before you talk
• CSMA/CD listen while you talk Ethernet
• CSMA/CA 802.11 wireless Ethernet

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802.11 MAC Layer

• Uses CSMA/CA CSMA Collision Avoidance
• Hidden terminal RTS/CTS is required feature but
  may be disabled
• exponential backoff also helps avoids collisions
• 802.11s CSMA/CA is called the Distributed
  Coordination Function (DCF)
• Useful to send non-delay-sensitive data such as
  Web, ftp, email lt- asynchronous traffic
• 802.11bs MAC is 70 efficient
• slotted ALOHA 37
• Ethernets efficiency 1/(15Tprop/Ttrans),
• 70 for common values of prop. delay and max
  pkt size,
• -gt100 for small prop. delays small pkts

8
802.11 MAC Layer (2)

• Contention in CSMA causes delay
• Point Coordination Function (PCF) Mode gives
  delay-sensitive traffic priority over
  asynchronous traffic
• Useful for interactive audio/video
• Define a superframe. Delay-sensitive traffic
  gets access to first part of superframe via
  shorter random wait times.
• Inside the first part of superframe, a central
  PCF master polls each user with delay-sensitive
  data
• In second part of superframe, asynchronous data
  is carried
• Built on top of DCF

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Physical Layers of 802.11 Variants

• What does 802.11 use for its physical layer?

Original 802.11 Standard
802.11b
802.11a
Also, 802.11g at 2.4 GHz, OFDM or PBCC, up to 54
Mbps. 802.11a _at_ 5 GHz ok in U.S., but conflicts
abroad
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802.11b Direct Sequence Spread Spectrum

• Multiply data bit stream d(t) by a faster
  chipping sequence c(t) BPSK example 1/-1

1
time
Data d(t)
-1
110011101001110010
1
Chipping Sequence c(t)
time
-1

• Chipping sequence c(t) also called Pseudo-Noise
  (PN) spreading sequence depending on usage

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Direct Sequence Sender
1
time
Data d(t)
-1
110011101001110010
1
Chipping Sequence c(t)
time
-1
1
d(t)c(t)
time
-1
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Direct Sequence Receiver
1
Receive d(t)c(t)
time
-1
110011101001110010
1
Receiver also has c(t)
time
-1
1
d(t)c(t)c(t) Data d(t), since c(t)c(t) 1!
time
-1
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Direct Sequence Spreads the Spectrum

• Benefit of modulating data d(t) by chipping
  sequence spreading the spectrum to improve
  immunity to noise and fading

Spectrum of data d(t)
frequency
Spectrum of chipping sequence c(t)
frequency
Spectrum of d(t)c(t)
frequency
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CDMA via Direct Sequence

• Each DSSS chipping sequence c(t) can be used as a
  code
• In CDMA, assign different DSSS codes to different
  hosts
• Assign code c1(t) between a base station and user
  1, assign code c2(t) between base station and
  user 2,
• Base station transmits summed signal
• d1(t)c1(t) d2(t)c2(t)
• Ideally, choose c1(t) to be orthogonal to c2(t)
• ? c1(t)c2(t) 0 (reality only orthogonal)
• In general, ? cj(t)ck(t) 0 for j?k

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CDMA via Direct Sequence (2)

• At receiver 1, received signal is multiplied by
  c1(t) and integrated
• ? c1(t)d1(t)c1(t) d2(t)c2(t) ?
  d1(t)
• Can extract data bit sequence d1(t) from ? d1(t)
  using a threshold detector, and then youre done!

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802.11b via Direct Sequence

• Original 802.11 at 1 Mbps
• used 11 chips/bit (Barker sequence), and BPSK
  (1/-1 signalling) for 11 Mcps, or 11 MHz
• 802.11b is more sophisticated
• 8 chips per symbol, and 8 bits/symbol, chipping
  rate is 11 MHz 1.375 Msps 11 Mbps
• 2.4 GHz ISM band has 14 channels (11 in U.S.)
• Each channel occupies 22 Mhz. Within each
  channel, uses Direct Sequence CDMA

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802.11 Channel Allocation

• 2.4 GHz ISM band has 14 channels (11 in U.S.)
• Interference from adjacent Access Points (AP) or
  base stations Only 3 channels (1,6,11) are
  non-overlapping
• reuse frequencies in beehive pattern to avoid
  degraded throughput

• Interference from Bluetooth, microwaves, garage
  door openers unlicensed spectrum!

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802.11 Modes of Operation

• Infrastructure mode
• Access point acts as gateway to wired Internet
• One-hop wireless access

Internet
Access Point/ Base station
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802.11 Modes of Operation

• Ad hoc mode
• Group of laptops form isolated wireless LAN no
  AP
• Ad hoc meeting in conference room
• One-hop wireless communication
• Not multi-hop

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802.11a OFDM

• OFDM Orthogonal Frequency Division Multiplexing
• Special case of Multi-Carrier Modulation (MCM),
  or Discrete Multi-Tone (DMT)
• Divide data bit stream d(t) over different
  frequencies. For example
• Transmit(t) d1(t)cos(2p3000t) d2(t)cos
  (2p6000t)
• 48 subcarriers in 802.11a over a 20 MHz channel
• Delivers better performance than DSSS, especially
  indoors
• High spectral efficiency, resistance to
  multipath,
• Various flavors of DSL also employ this technique

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Token Ring

• Not very popular, even being phased out at IBM
  primarily of historical interest
• Why did Ethernet win? Cheaper and good enough
• Conceptual Topology of Token Ring

Token Ring
Ethernet
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Token Ring (2)

• Links are unidirectional
• Each node has a downstream neighbor and an
  upstream neighbor

• Topology resembles N point-to-point links forming
  a ring rather than continuous wire loop
• but access to ring is shared via tokens
• A token is a special flag that circulates
  around the ring

Token Ring
010010
Token
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Token Ring (3)

• Each node receives token, then transmits it to
  its downstream neighbor
• Round-robin ensures fairness, as every node
  eventually can transmit when it receives token

• Suppose token was passed from source to
  destination rather than around the ring as in
  Token Ring
• some hosts could be passed over indefinitely
  unfair!

Token Ring
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Token Ring (4)

• When a node has a frame to send, it takes token,
  and transmits frame downstream

• Each node receives a frame and forwards it
  downstream
• Destination host saves copy of frame, but keeps
  forwarding frame.
• Inefficient
• Forwarding stops when frame reaches original
  source

Token Ring
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Token Ring Example
Destination
Source
(1)
Token Ring
(7) Stop Data Frame
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Token Rings Robustness To Failure

• A given node can fail at any time
• Without the token
• With the token

• If a node fails without the token
• An electromechnical relay closes at failing node,
  keeping the ring intact
• Data frame continues to be forwarded as before

Token Ring
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Token Rings Robustness To Failure (2)

• In Token Ring, when frame reaches a destination
  node, it is marked as read
• When marked-as-read frame reaches sender, it acts
  as ACK to sender

Destination

• If a destination node fails without the token
• Sender receives unmarked frame, and can
  retransmit it later

Token Ring
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Token Rings Robustness To Failure (3)

• If a node fails with the token, then the ring
  must somehow introduce a new token
• After a timeout, in which no token is detected, a
  designated monitor introduces a new token

• If designated monitor fails
• Its periodic keep-alive not detected
• A node sends claim token around ring
• If claim token returns to sender, then sender
  becomes designated monitor

Token Ring
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Token Ring Other Points

• Token Holding Time (THT) by default is 20 ms
• Token Ring data rates are 4 and 16 Mbps
• If a token is held until data frame returns, then
  called delay-release
• Inefficient, original version of 802.5
• Solution release token as soon as send has
  transmitted data frame
• More efficient, called early release, now
  supported in later version of 802.5
• Token Rotation Time
  lt ( Nodes)THT Ring Latency

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FDDI

• Fiber Distributed Data Interface
• Dual ring topology originally using optical
  fibers instead of copper wire
• 100 Mbps

• Second ring helps with robustness/ fault recovery
• Some nodes may be part of only one ring single
  attachment station (SAS)

FDDI
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FDDI (2)

• Recall the inefficiency of Token Ring frames are
  forwarded even after theyve reached destination

• Solution in FDDI, destination node removes frame
  from ring

Destination
FDDI