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 yry@cs.yale.edu

• 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