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Chapter 2 More on Wireless Ethernet, Token Ring, FDDI. Professor Rick Han University of Colorado at Boulder [email protected]. Announcements. Previous lecture now online Homework #1 is on the Web site, due Feb. 5 - PowerPoint PPT Presentation
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Chapter 2More on Wireless Ethernet,
Token Ring, FDDIProfessor Rick Han
University of Colorado at [email protected]
Prof. Rick Han, University of Colorado at Boulder
Announcements• Previous lecture now online• Homework #1 is on the Web site, due
Feb. 5• Programming assignment #1 is now
available on Web site, due Feb. 19 (3 weeks)
• Next, Chapter 2, more on Wireless Ethernet, Token Ring, FDDI
Prof. Rick Han, University of Colorado at Boulder
Recap of Previous Lecture• Multiple Access Protocols
• Designed for shared-media links• Channel reservation protocols: TDMA,
FDMA, CDMA• Random access protocols: CSMA/CD
(Ethernet), CSMA/CA (802.11 wireless Ethernet)
• Random Access Protocols• ALOHA, slotted ALOHA – packet collisions• CSMA – “listen before you talk”• CSMA/CD – “listen while you talk”• CSMA/CA – see next slide
Prof. Rick Han, University of Colorado at Boulder
802.11 MAC Layer• Uses CSMA/CA = CSMA + Collision
Avoidance• Collision Avoidance equated with exponential
backoff• Hidden terminal RTS/CTS is required feature
but may be disabled• 802.11’s CSMA/CA is called the Distributed
Coordination Function (DCF)• Useful to send non-delay-sensitive data such
as Web, ftp, email <- asynchronous traffic• 802.11b’s MAC is ~70% efficient
• slotted ALOHA ~37%• Ethernet’s efficiency: ~ 1/(1+5Tprop/Ttrans),
• ~ 70% for common values of prop. delay and max pkt size,• ->100% for small prop. delays & small pkts
Prof. Rick Han, University of Colorado at Boulder
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
Prof. Rick Han, University of Colorado at Boulder
Physical Layers of 802.11 Variants
• What does 802.11 use for its physical layer?
2.4 GHzFreq. Hop1,2 Mbps
2.4 GHzDir. Seq.1,2 Mbps
Infrared1,2 Mbps
2.4 GHzDir. Seq.
5.5,11 Mbps
5 GHzOFDM
6-54 Mbps
Original 802.11 Standard802.11b 802.11a
Also, 802.11g at 2.4 GHz, OFDM or PBCC, up to 54 Mbps.802.11a @ 5 GHz ok in U.S., but conflicts abroad
Prof. Rick Han, University of Colorado at Boulder
802.11b: Direct Sequence Spread Spectrum
• Multiply data bit stream d(t) by a faster chipping sequence c(t) : BPSK example +1/-1
ChippingSequencec(t)
time
+1
-1
1 0 0 1 1 0
110011101001110010
Data d(t)
• Chipping sequence c(t) also called Pseudo-Noise (PN) spreading sequence depending on usage
+1
-1
time
Prof. Rick Han, University of Colorado at Boulder
Direct Sequence Sender
ChippingSequencec(t)
time
+1
-1
1 0 0 1 1 0
110011101001110010
Data d(t)
+1
-1
d(t)*c(t)
+1
-1time
time
Prof. Rick Han, University of Colorado at Boulder
Direct Sequence Receiver
Receiver also has c(t)
time
+1
-11 0 0 1 1 0
110011101001110010
d(t)*c(t)*c(t)= Data d(t), sincec(t)*c(t) = 1!
+1
-1
time
Received(t)*c(t)
+1
-1time
Prof. Rick Han, University of Colorado at Boulder
Direct Sequence Spreads the Spectrum
• Benefit of modulating data d(t) by chipping sequence: spreading the spectrum to improve immunity to noise and fading
frequency
Spectrum of data d(t)
frequency
Spectrum of chipping sequence c(t)
frequency
Spectrum of d(t)*c(t)
Prof. Rick Han, University of Colorado at Boulder
CDMA Employs Direct Sequence
• Each c(t) can be looked upon as a code that only the sender and receiver pair both know• Assign code c1(t) between a base station and
user 1, c2(t) between base station and user 2, …• Base station sends d1(t)*c1(t) + d2(t)*c2(t)
• Ideally, choose c1(t) to be orthogonal to c2(t), i.e. c1(t)*c2(t) =0 (reality: only ~orthogonal)• At receiver 1, received signal is multiplied by
c1(t): c1(t)*[d1(t)*c1(t) + d2(t)*c2(t)] = d1(t)!• CDMA: multiple data streams
simultaneously access the same medium using ~orthogonal DSSS codes
Prof. Rick Han, University of Colorado at Boulder
CDMA Employs Direct Sequence (2)
• 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
Prof. Rick Han, University of Colorado at Boulder
802.11 Specifics (2)• 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!
Prof. Rick Han, University of Colorado at Boulder
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(2t) + d2(t)*cos
(2t)• 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
Prof. Rick Han, University of Colorado at Boulder
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:
TokenRing
Ethernet
Prof. Rick Han, University of Colorado at Boulder
Token Ring (2)• Links are unidirectional• Each node has a downstream neighbor
and an upstream neighbor
TokenRing
• 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
010010
“Token”
Prof. Rick Han, University of Colorado at Boulder
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
TokenRing
• 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!
010010
“Token”
Prof. Rick Han, University of Colorado at Boulder
Token Ring (4)• When a node has a frame to send, it takes
token, and transmits frame downstream
TokenRing
• 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
010010
“Token”
1110011010
Data Frame
Prof. Rick Han, University of Colorado at Boulder
Token Ring Example
TokenRing 010010
“Token”
Destination Source
1110011010
Data Frame
1110011010
Data Frame
(2)
1110011010
Data Frame(5)
1110011010
Data Frame
(4)
1110011010
Data Frame
(3)
1110011010
Data Frame(6)
(1)
(7) Stop DataFrame
Prof. Rick Han, University of Colorado at Boulder
Token Ring’s Robustness To Failure
• A given node can fail at any time:• Without the token• With the token
TokenRing
• 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
010010
“Token”
1110011010
Data Frame
Prof. Rick Han, University of Colorado at Boulder
Token Ring’s 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
TokenRing
• If a destination node fails without the token:• Sender receives
unmarked frame, and can retransmit it later
010010
“Token”
1110011010
Data Frame
Destination
Prof. Rick Han, University of Colorado at Boulder
Token Ring’s 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
TokenRing
• 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”
010010
“Token”
Prof. Rick Han, University of Colorado at Boulder
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
<= (# Nodes)*THT + Ring Latency
Prof. Rick Han, University of Colorado at Boulder
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
Prof. Rick Han, University of Colorado at Boulder
FDDI
FDDI (2)• Recall the inefficiency of Token Ring:
frames are forwarded even after they’ve reached destination• Solution: in
FDDI, destination node removes frame from ring Destination
1110011010
Data Frame