Example Wireless Networks: WaveLAN, Bluetooth Y. Richard Yang 01/26/2004

2 Outline  Admin. and recap  Bluetooth networks

3 Admin: Homework 1 r A mini-paper on MAC protocols for directional antennas m due 11:59pm on Friday the 30th m send result to pdf, ps, or word should be fine m no longer than 6 pages (double/single space, or single/double column, your decision)

4 Recap: Cellular Networks r GSM and GPRS m GSM GSM combines TDMA and FDMA GSM data rate is low, e.g., 9.6kbps m GPRS: higher data rates m 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 r IMT-2000 m UMTS uses W-CDMA as radio interface

5 GPRS User Data Rates in kbps Coding scheme 1 slot 2 slots 3 slots 4 slots 5 slots 6 slots 7 slots 8 slots CS CS CS CS GPRS channel structure: - time is divided into multiframe (240ms) - each multiframe contains 48 data frames - 4 slots form a block

6 GPRS Coding Coding scheme Pre- cod. USF Infobits without USF Parity bits BC Tail bits Output conv encoder Punc tured bits Code rate Data rate kbit/s CS /29.05 CS ~2/313.4 CS ~3/415.6 CS ­456­121.4 g(1)(D) = 1 + D3 + D4 g(2)(D) = 1 + D + D3 + D4, convolution code:

7 Computation: Examples r CS-1 and 1 slot: r CS-4 and 8 slot:

8 Recap: Architecture Ad hoc mode Infrastructure mode

9 Recap: Wireless LAN (802.11) PHY

10 DSSS PHY 3 non-overlapping channels

11 Recap: MAC Layer r Traffic services m Asynchronous Data Service (mandatory) exchange of data packets based on “best-effort” support of broadcast and multicast m Time-Bounded Service (optional) exchange of bounded delay service m Use Inter framing spacing (IFS) to combine the two modes m Use RTS/CTS/DATA/ACK r Power saving mode m Use beacon interval to allow sleep

Frame Format r frame has more fields than other media type frames r 30 bytes frame header appears too long! r Duration ID: NAV r CRC: check sum

Frame Control Field

14 Background: Cyclic Redundancy Check r For a given data D, consider it as a polynomial D(x) m consider the string of 0 and 1 as the coefficients of a polynomial e.g. consider string as x 4 +x+1 m addition and subtraction are modular 2, thus the same as xor r Choose generator polynomial G(x) with r+1 bits, where r is called the degree of G(x) m For example the degree of the G(x) for is 32

15 Cyclic Redundancy Check: Objective r Given data G(x) and D(x), choose R(x) with r bits, such that m D(x)x r +R(x) is exactly divisible by G(x) r The bits correspond to T(x)=D(x)x r +R(x) are sent to the receiver r Since G(x) is global, when the receiver receives the transmission T’(x), it divides T’(x) by G(x) m If non-zero remainder: error detected! m If zero remainder, assumes no error +x

16 CRC: Steps and an Example Suppose the degree of G(x) is r Append r zero to D(x), i.e. consider D(x)x r Divide D(x)x r by G(x). Let R(x) denote the reminder Send to the receiver

17 The Power of CRC r Let T(x) denote D(x)x r +R(x), and E(x) the polynomial of the error bits, i.e m the received signal T’(x) = T(x)+E(x) r Since T(x) is divisible by G(x), we only need to consider E(x) divided by G(x) r A single bit of error: E(x) = x i m If G(x) contains two or more terms, E(x) is not divisible by G(x) r An odd number of errors: E(x) has an odd number of terms: m 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 m If G(x) contains x+1 as a factor, E(x) will not be divided by G(x) r Many more errors can be detected by designing the right G(x)

18 Outline  Admin. and recap  Bluetooth networks

19 Bluetooth Design Objective r Design objective: a cable replacement technology m 1 Mb/s m range 10+ meters m single chip radio + baseband (means digital part) low power low price point (target price $5)

20 Bluetooth: Use Scenarios r Synchronization r Data access points r Headset r Conference table r Business card exchange r Instant postcard r … File synchronization Cordless headset

21 Bluetooth Architecture

22 Bluetooth Radio Link r Bluetooth shares the same freq. range as r Radio link is the most expensive part of a communication chip (discussion: compare with ) m Bluetooth uses frequency hopping spread spectrum GHz + k MHz, k=0, …, 78 1,600 hops per second m GFSK (Gaussian FSK) modulation 1 Mb/s symbol rate m transmit power: 1mW

23 Bluetooth Physical Layer r Nodes form piconet: one master and upto 7 slaves m Each radio can function as a master or a slave r The slaves follow the pseudorandom jumping sequence of the master A piconet

24 Physical Channel Setup (Piconet formation) r An inquiry/scan/page protocol r Master: sends Inquiry messages, with Inquiry Access Code (IAC), hoping at a universal frequency hopping sequence (32 frequencies) m announce the master r Joining slave: m jump at a much lower speed m after receiving an Inquiry message, wait for a random time, then send a request to the master r The master sends a paging message to the slave to join it

25 Inquiry Hopping

26 The Bluetooth Link Establishment Protocol FS: Frequency Synchronization DAC: Device Access Code IAC: Inquiry Access Code

27 Bluetooth Links

28 Bluetooth Packet Format Header

29 Multiple-Slot Packet

30 Background: Forward Error Correction Code/Erasure Code r Widely used in wireless communications x y

31 FEC: Example r Suppose data signal is x, and the encoded signal y = Gx, where G is the generator matrix r Example: Vandermonde Matrix: g ij = a i j-1, where a i are different numbers

32 FEC: An Example r Suppose k=3, and n=5 r Suppose y 2 and y 3 are dropped, then we have y 1, y 4, and y 5. Given the relationship (we know they are y 1, y 4, y 5 ) r Since the matrix is not singular, we can recover x 1, x 2, and x 3

33 Further Enhancements of Bluetooth r Power management modes m e.g., PARK r Scatternets: multiple piconets