More on Wireless Ethernet, Token Ring, FDDI

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


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


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


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


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


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


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


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


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)


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


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


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!


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


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


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”


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”


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


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


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


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


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”


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


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


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

More on Wireless Ethernet, Token Ring, FDDI

Documents

Transcript of More on Wireless Ethernet, Token Ring, FDDI