Wireless Communications

1
Wireless Communications

• Objectives
• Understand 802.11
• Bluetooth

2
Wireless (802.11)
Hedy Lamarr, seen in 1946, had a string of hit
films in the 1930s and 1940s.
3
Hedy Lamarr

• Deseret News Jan 30 2000 page E10
• Hedy Lamarr, the Austrian movie siren whose
  assets included buckets of beauty and a thimble
  of acting talent, was found dead at her home in
  Orlando, Fla., last week. She was 86 and only
  recently had begun to enjoy recognition for her
  real-life role as the godmother of cell phone
  technology.     

4
On the acting side

• She was celebrated more for quotability than
  ability. Her declamation that "any girl can be
  glamorous all you have to do is stand still and
  be stupid," remains the most accurate description
  of her presence in movies such as "Algiers"
  (1938), "Ziegfeld Girl" (1941) and "Tortilla
  Flat" (1942).

5
Just a pretty face?

• While her colleagues in Hollywood plotted their
  next radio appearance, she immersed herself in
  the intricacies of spread spectrum radio
  transmission, the forerunner of cellular
  technology. It is possible that without Lamarr,
  modern military communications and cordless
  phones would not exist. About no other screen
  legend can it be said that her invention has
  provided more pleasure than did contemplation of
  her gorgeous face.

6
Background

• Hedwig Eva Maria Kiesler was born in Vienna on
  Nov. 9, 1913, to Jewish parents, a banker father
  and a pianist mother.
• One of the leading arms manufacturers in Europe,
  Mandl operated a factory that helped prepare
  Mussolini for Abyssinia and would later supply
  Hitler in his European campaigns.
• In the luscious Hedwig Kiesler, Mandl found the
  ideal trophy wife. He forbade her to act and
  encouraged her to direct her talents to
  entertaining his "business associates."
• Sensing that Jews had no future in Austria, she
  left homeland and her husband in 1937.

7
Torpedo Communications

• The woman who had learned about the latest in
  German and Austrian technology at her husband's
  plants met composer George Antheil at a dinner
  party in 1940 and shared what she knew about the
  design of remote-controlled torpedoes. Mandl had
  never gone into production with these torpedoes
  because their radio signals were vulnerable to
  detection and jamming.
• Lamarr believed the solution was to broadcast the
  weapons' signals on rapidly shifting frequencies.
  She and Antheil developed a frequency-hopping
  system by which both the transmitting and
  receiving stations of a remote-control torpedo
  changed at intervals. They received U.S. Patent
  2,292,387 in August, 1942, and their research was
  put to limited use by the U.S. Navy during World
  War II.

8
What else?

• While she raised her children, the military and
  private sector took a growing interest in spread
  spectrum technology. However Miss Lamarr didn't
  receive a nickel for her work until 1997, when a
  Canadian wireless data communications company
  acquired the original patent rights from the
  actress in exchange for an undisclosed number of
  shares in the firm.
• "Films have a certain place in a certain time
  period," said Miss Lamarr last year in what could
  be her epitaph. "Technology is forever."

9
Frequency Hopping (Bluetooth, some 802.11)

• To avoid Jamming, transmit over a random sequence
  of frequencies with both ends knowing the random
  sequence.
• The FCC requires a channel to use 75 or more
  frequencies with a maximum dwell time of 400ms.
• If an error occurs on one frequency, retransmit
  on the next one.
• If two stations are transmitting, they wont
  interfere if they have different hop sequences
• Limited to 2Mbps

10
Direct Sequence (Most others including 802.11
ethernet)

• Direct Sequence exor the signal with a random
  sequence and transmits over a wider frequency
  band
• Transmitters are higher cost and can achieve
  higher bandwidth, however each transmitter must
  have its own frequency.
• They also draw a lot more power

11
Chipping sequence
1
Data stream 1010
0
1
Random sequence 0100101101011001
0
1
XOR of the two 1011101110101001
0
12
Recovering the Signal
Direct Sequence 
13
Collisions (hidden node) (C and A want to send
to B)
A
B
C
D
14
Collisions (Exposed node) (B-gtA and C-gtD)
A
B
C
D
15
MACA

• Multiple Access with Collision Avoidance (MACA)
• RTS and CTS signals before transmission starts.
  This allows all other nodes to know of where the
  transmission is occurring
• Any node who sees the CTS knows that it cant
  transmit or it will interfere
• Any node who sees the RTS, but not the CTS is not
  close to the receiver, so it can transmit without
  interfering.
• An ACK is sent when the frame is received

16
MACA (hidden node) (C and A want to send to B)
RTS
RTS
A
B
C
D
17
MACA (Exposed Node)
RTS
RTS
A
B
C
D
18
Distribution System

• Access Points (AP) are connected by a
  distribution system and are not mobile.
• Roaming Nodes can communicate directly, or
  through Access Points.
• Scanning
• Node sends a Probe frame
• All APs within range respond
• Node selects AP and sends it a Association
  Request frame
• AP responds with an Association Response frame

19
Access Points
20
Changing APs
APs periodically send a beacon frame (passive
scanning, or nodes recognize reduced signal
strength (active scanning)
21
Frame Format

• Up to 2312 bytes of data
• 48 bit source, dest addresses
• 4 addresses can identify two endpoints and two
  intermediate Access Points

22
Bluetooth

• Spread spectrum frequency hopping radio
• 79/23 one MHz channels
• Hops every packet
• Packets are 1, 3 or 5 slots long
• Frame consists of two packets
• Transmit followed by receive
• Nominally hops at 1600 times a second (1 slot
  packets)

23
Bluetooth

• Radio Designation
• Connected radios can be master or slave
• Radios are symmetric (same radio can be master or
  slave)
• Piconet
• Master can connect to 7 simultaneous or 200
  active slaves per piconet
• Each piconet has maximum capacity (1 MSPS)
• Unique hopping pattern/ID

24
Piconet

• All devices in a piconet hop together
• In forming a piconet, master gives slaves its
  clock and device ID
• Hopping pattern determined by device ID (48-bit)
• Phase in hopping pattern determined by Clock
• Non-piconet devices are in standby
• Piconet Addressing
• Active Member Address (AMA, 3-bits)
• Parked Member Address (PMA, 8-bits)

25
Which Technology

• Why Bluetooth?
• Why 802.11?
• What about IRDA?

26
Network Adaptors
27
Overview

• Typically where data link functionality is
  implemented
• Framing
• Error Detection
• Media Access Control (MAC)

Network link
Bus
Link
Host I/O bus
interface
interface
Adaptor
28
Host Perspective

• Control Status Register (CSR)
• Available at some memory address
• CPU can read and write
• CPU instructs Adaptor (e.g., transmit)
• Adaptor informs CPU (e.g., receive error)
• Example
• LE_RINT 0x0400 Received packet Interrupt (RC)
• LE_TINT 0x0200 Transmitted packet Interrupt
  (RC)
• LE_IDON 0x0100 Initialization Done (RC)
• LE_IENA 0x0040 Interrupt Enable (RW)
• LE_INIT 0x0001 Initialize (RW1)

29
Moving Frames Between Host and Adaptor

• Direct Memory Access (DMA)
• Programmed I/O (PIO)

30
Device Driver

• Interrupt Handler
• interrupt_handler()
• disable_interrupts()
• / some error occurred /
• if (csr LE_ERR)
• print_and_clear_error()
• / transmit interrupt /
• if (csr LE_TINT)
• csr LE_TINT LE_INEA
• semSignal(xmit_queue)
• / receive interrupt /
• if (csr LE_RINT)

31

• Transmit Routine
• transmit(Msg msg)
• char src, dst
• Context c
• int len
• semWait(xmit_queue)
• semWait(mutex)
• disable_interrupts()
• dst next_xmit_buf()
• msgWalkInit(c, msg)
• while ((src msgWalk(c, len)) ! 0)
• copy_data_to_lance(src, dst, len)
• msgWalkDone(c)
• enable_interrupts()
• semSignal(mutex)
• return

32

• Receive Interrupt Routine
• receive_interrupt()
• Msg msg, new_msg
• char buf
• while (rdl next_rcv_desc())
• / create process to handle this message /
• msg rdl-gtmsg
• process_create(ethDemux, msg)
• / msg eventually freed in ethDemux /
• / now allocate a replacement /
• buf msgConstructAllocate(new_msg, MTU)
• rdl-gtmsg new_msg
• rdl-gtbuf buf
• install_rcv_desc(rdl)

33
Memory Bottleneck
With 114MBps max, if there are 5 data copies, the
best throughput will be 22MBps (114/5)
34
Divergence

• Underwater acoustical modems

35
What is a acoustical modem?
External modems connect directly to the serial
port, internal modems are devices on the I/O bus.
Acoustical modems provide a cradle for the
telephone and must produce sound for the
telephone handset
36
How do you do this underwater

• Submarines would like to send email
• Connect to an underwater hydrophone transmitter
• Frequency modulation
• Communicate between a submersible and aircraft or
  ships
• From http//guinness.cs.stevens-tech.edu/mtalreja
  /seniord/

37
Architecture
38
FM for alternating 0s and 1s
39
Details

• Bandwidth 100bps
• Speed of sound 1400-1500m/s (varies with
  salinity)
• Mark1600Hz, Space1000 Hz
• RS232 framing

40
Considerations

• High frequencies are absorbed more quickly than
  low frequencies in water

41
RS232
42
RS232 Details

• 1 (MARK, LOW) means -3 V to -15 V
• 0 (SPACE, HIGH) means 3 V to 15 V
• Start bitHigh, Stop bitsLow
• Data is transmitted LSB to MSB, (LSB, Bit 0)
  first, 0 HIGH,1LOW.