example wireless networks: wavelan, bluetooth y. richard yang 01/26/2004
TRANSCRIPT
Example Wireless Networks:WaveLAN, Bluetooth
Y. Richard Yang
01/26/2004
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Outline
Admin. and recap Bluetooth networks
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Admin: Homework 1
A mini-paper on MAC protocols for directional antennasdue 11:59pm on Friday the 30th send result to [email protected]
• pdf, ps, or word should be fineno longer than 6 pages (double/single
space, or single/double column, your decision)
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Recap: Cellular Networks
GSM and GPRS GSM
• GSM combines TDMA and FDMA• GSM data rate is low, e.g., 9.6kbps
GPRS: higher data rates discussion: a comparison of GSM and GPRS
• which one is better:– Eight servers each can process X packets per second
– One server which can process 8 X packets per second
IMT-2000 UMTS uses W-CDMA as radio interface
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GPRS User Data Rates in kbpsCoding scheme
1 slot
2 slots
3 slots
4 slots
5 slots
6 slots
7 slots
8 slots
CS-1 9.05 18.2 27.15
36.2 45.25
54.3 63.35
72.4
CS-2 13.4 26.8 40.2 53.6 67 80.4 93.8 107.2
CS-3 15.6 31.2 46.8 62.4 78 93.6 109.2
124.8
CS-4 21.4 42.8 64.2 85.6 107 128.4
149.8
171.2GPRS channel structure:
- time is divided into multiframe (240ms) - each multiframe contains 48 data frames - 4 slots form a block
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GPRS Coding
Coding scheme
Pre-cod. USF
Infobits without USF
Parity bits BC
Tail bits
Output conv encoder
Punctured bits
Code rate
Data rate kbit/s
CS-1 3 181 40 4 456 0 1/2 9.05
CS-2 6 268 16 4 588 132 ~2/3 13.4
CS-3 6 312 16 4 676 220 ~3/4 15.6
CS-4 12 428 16 456 1 21.4
g(1)(D) = 1 + D3 + D4 g(2)(D) = 1 + D + D3 + D4 ,
convolution code:
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Computation: Examples
CS-1 and 1 slot:
kbps05.9456
181 ebytes/fram 114ameframes/mfr 48
ms/mframe 240
ms 1000
CS-4 and 8 slot:
kbps2.171456
428 ebytes/fram 114 8ameframes/mfr 48
ms/mframe 240
ms 1000
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Recap: 802.11 Architecture
Ad hoc mode Infrastructure mode
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Recap: Wireless LAN (802.11) PHY
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DSSS PHY
3 non-overlapping channels
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Recap: 802.11 MAC Layer
Traffic services Asynchronous Data Service (mandatory)
• exchange of data packets based on “best-effort”• support of broadcast and multicast
Time-Bounded Service (optional)• exchange of bounded delay service
Use Inter framing spacing (IFS) to combine the two modes
Use RTS/CTS/DATA/ACK Power saving mode
Use beacon interval to allow sleep
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802.11 - Frame Format
802.11 frame has more fields than other media type frames
30 bytes frame header appears too long!
Duration ID: NAV CRC: check sum
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802.11 Frame Control Field
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Background: Cyclic Redundancy Check For a given data D, consider it as a polynomial D(x)
consider the string of 0 and 1 as the coefficients of a polynomial
• e.g. consider string 10011 as x4+x+1 addition and subtraction are modular 2, thus the same as
xor Choose generator polynomial G(x) with r+1 bits,
where r is called the degree of G(x) For example the degree of the G(x) for 802.11 is 32
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Cyclic Redundancy Check: Objective Given data G(x) and D(x), choose R(x) with r
bits, such that D(x)xr+R(x) is exactly divisible by G(x)
The bits correspond to T(x)=D(x)xr+R(x) are sent to the receiver
Since G(x) is global, when the receiver receives the transmission T’(x), it divides T’(x) by G(x) If non-zero remainder: error detected! If zero remainder, assumes no error
+x
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CRC: Steps and an Example
Suppose the degree of G(x) is r
Append r zero to D(x), i.e. consider D(x)xr
Divide D(x)xr by G(x). Let R(x) denote the reminder
Send <D, R> to the receiver
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The Power of CRC Let T(x) denote D(x)xr+R(x), and E(x) the polynomial of the
error bits, i.e the received signal T’(x) = T(x)+E(x)
Since T(x) is divisible by G(x), we only need to consider E(x) divided by G(x)
A single bit of error: E(x) = xi
If G(x) contains two or more terms, E(x) is not divisible by G(x)
An odd number of errors: E(x) has an odd number of terms: Lemma: if E(x) has an odd number of terms, E(x) cannot be
divisible by (x+1)• suppose E(x) = (x+1)F(x), let x=1, the left hand will be 1, while the
right hand will be 0 If G(x) contains x+1 as a factor, E(x) will not be divided by G(x)
Many more errors can be detected by designing the right G(x)
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Outline
Admin. and recap Bluetooth networks
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Bluetooth Design Objective
Design objective: a cable replacement technology 1 Mb/s
range 10+ meters
single chip radio + baseband (means digital part)
• low power • low price point (target price $5)
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Bluetooth: Use Scenarios
Synchronization Data access points Headset Conference table Business card exchange Instant postcard …
File synchronization
Cordless headset
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Bluetooth Architecture
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Bluetooth Radio Link
Bluetooth shares the same freq. range as 802.11 Radio link is the most expensive part of a
communication chip (discussion: compare with 802.11) Bluetooth uses frequency hopping spread spectrum
• 2.402 GHz + k MHz, k=0, …, 78• 1,600 hops per second
GFSK (Gaussian FSK) modulation• 1 Mb/s symbol rate
transmit power: 1mW
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Bluetooth Physical Layer Nodes form piconet: one master and upto 7 slaves
Each radio can function as a master or a slave The slaves follow the pseudorandom jumping sequence of the
master
A piconet
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Physical Channel Setup (Piconet formation) An inquiry/scan/page protocol Master: sends Inquiry messages, with Inquiry
Access Code (IAC), hoping at a universal frequency hopping sequence (32 frequencies) announce the master
Joining slave: jump at a much lower speed after receiving an Inquiry message, wait for a
random time, then send a request to the master
The master sends a paging message to the slave to join it
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Inquiry Hopping
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The Bluetooth Link Establishment Protocol
FS: Frequency Synchronization
DAC: Device Access Code
IAC: Inquiry Access Code
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Bluetooth Links
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Bluetooth Packet Format
Header
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Multiple-Slot Packet
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Background: Forward Error Correction Code/Erasure Code
Widely used in wireless communications
xy
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FEC: Example
Suppose data signal is x, and the encoded signal y = Gx, where G is the generator matrix
Example: Vandermonde Matrix: gij= aij-1, where ai
are different numbers
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FEC: An Example Suppose k=3, and n=5
Suppose y2 and y3 are dropped, then we have y1, y4, and y5. Given the relationship (we know they are y1, y4, y5)
Since the matrix is not singular, we can recover x1, x2, and x3
3
2
1
222
211
5
4
3
2
1
1
1
100
010
001
x
x
x
aa
aa
y
y
y
y
y
3
2
1
222
211
5
4
1
1
1
001
x
x
x
aa
aa
y
y
y
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Further Enhancements of Bluetooth
Power management modes e.g., PARK
Scatternets: multiple piconets