Wireless Networks

• 802.11 (WiFi)
• 802.15 (Bluetooth)
• 3G (Cellular wireless networks)
• 802.16 (WiMAX)

802.11

IEEE standard 802.11 defines physical and data link layers.

Two modes of operation:

• infrastructure mode (with base station)
• ad hoc mode (without base station (peer-to-peer))

We will consider infrastructure mode.

BSS - Basic Service Set.  A base station (known as an access point) and additional hosts.  The access point connects to the Internet (or some other distribution system).

Physical layer

• Transmission is by infrared or radio waves (most prevalent)
• 2.4 GHz band
• ISM (Industrial, Scientific, Medical)
• no licensing necessary
• susceptible to interference from microwave ovens, cordless phones, garage door openers, etc.
• Six different physical layer protocols are defined (1 infrared, 5 microwave).  (Three were in the original 1997 protocol, 802.11a and 802.11b were added in 1999, 802.11g in 2001)
• All use some form of spread spectrum.  That is, they use more bandwidth than is strictly necessary.

FHSS - Frequency Hopping Spread Spectrum

• Invented by actress Hedy Lamarr (patented 1942)
• 79 channels defined using different frequencies in ISM band
• sender and receiver "hop" from frequency to frequency, at least 2.5 hops per second
• sequence determined by random number generator
• avoids interference
  
• avoids multipath fading
• provides some security
• data rate:  1 or 2 Mbps

DSSS - Direct Sequence Spread Spectrum

• 1 wide channel is defined, also in ISM band
• each bit is sent as an 11-bit chip sequence, similar to CDMA
• chip sequence is determined by random number generator
• sender sends code = message xor (random sequence)
• receiver computes code xor (random sequence) = message
• data rate:  1 or 2 Mbps

802.11a (1999)

• uses OFDM - Orthogonal Frequency Division Multiplexing
• up to 48 data channels are defined within 5 GHz range using FDM
• bit stream is sent in parallel over multiple channels (similar to ADSL)
• data rate:  up to 54 Mbps

802.11b (1999)

• uses HR-DSSS - High Rate DSSS
• approved before 802.11a
• data rate up to 11 Mbps
• slower but longer range than 802.11a
• uses same chipping rate as DSSS, but a more complex modulation scheme to allow more bits per baud

802.11g (2001)

• uses OFDM in 2.4 GHz range
• up to 54 Mbps

MAC layer

• CSMA/CA - Carrier Sense Multiple Access with Collision Avoidance
• based on MACA we studied earlier
• uses a variety of collision avoidance techniques
• issues:
• not all stations can hear each others' transmissions, due to range limitations, obstacles
• a station cannot detect the channel while it is transmitting, because its own signal is so much stronger than incoming ones (half duplex)
• two versions of multiple access:  DCF (Distributed Coordination Function) and PCF (Poinr Coordination Function)
  

DCF

• RTS (Request to Send) and CTS (Clear to Send) messages are used to set up a transmission
• Sender senses channel before sending, waits until it is clear.  Two forms of sensing:
• Physical - listen to the channel
• Virtual - listen to RTS and CTS messages.  If one is heard, estimate the length of time until the data frame is sent.  Wait until the expected transmision time is completed.
• Once the sender begins transmitting, it sends the entire frame.  Why?
• It does not detect the channel while transmitting, and therefore does not detect a collision if it occurs
• Hidden station problem
• Exposed station problem
• ACK messages are sent from receiver to sender
• Frames may be broken into fragments (because of the higher error rate)
• Fragments may be sent in bursts, using only one RTS-CTS pair.  Each fragment is separately ACKed.

PCF (Point Coordination Function)

• A centralized multiple access protocol, with the base station acting as coordinator. 
• The base station transmits a beacon frame to poll other stations to see if they have data to transmit.

DCF and PCF actually coexist on the same channel, using a prioritization based on the wait time after an ACK.  The highest priority activity waits the least amount of time.  If nothing happens, the next priority activity can take place after a slightly longer wait interval, etc.  The classes of activity, from high to low priority, are:

1.  The same sender or receiver in a single dialog.  This allows fragment bursts to proceed without addtional RTS and CTS messages.

2.  Base station, for beacon or polling frame.  (PCF)

3.  Any sender wishing to claim the channel with an RTS message.

4.  A receiver reporting a bad or unknown frame.

Frame structure

The same frame structure is used by all the 802.11 variations.

bytes	contents
2	frame control RTS, CTS, ACK flags To/From DS (Distribution system) flags etc.
2	duration how long will transmission of this frame last?
24	4 6-byte MAC addresses source destination to/from DS station 4th is used for ad hoc networking
2	sequence number for fragments
0-2312	data
4	checksum

Services

The 802.11 specifications outline various services to be provided:

• Association/Disassociation
• establishing a connection to a base station
• AP sends out beacon frames periodically, containing name and MAC address
• host can then request association using 802.11 association protocol
• if all is OK, host becomes part of the AP's BSS
• Distribution  -  Internetworking with a wired part of the network
• Authentication
• Privacy / Encryption
• Data Delivery

example  Suppose that a wireless host H1 is connected to an access point AP1, which is connected on an Ethernet to an Internet router R1 and another access point AP2.  An IP datagram is sent to H1 from a host through R1.  What happens?

1. The datagram contains the IP address of H1.  The router uses ARP to get H1's MAC address.
2. The router puts the datagram in an Ethernet frame and sends it on the Ethernet, with H1's address as the destination address.
3. AP1 receives the frame, removes the Ethernet framing and puts the contents in a 802.11 frame with
• address 1 = H1 (wireless destination)
• address 2 = AP1 (wireless source)
• address 3 = R1 (DS source)
• from DS flag set
4. When H1 sends a reply, it constructs an 802.11 frame with
• address 1 = AP1
• address 2 = H1
• address 3 = R1
• to DS flag set
5. When AP1 receives the frame, it converts it to an Ethernet frame with source = H1 and destination = R1

Bluetooth

Bluetooth is another wireless technology.

• Designed for wireless connections between computers and peripherals
• "wireless personal area network" WPAN
• shorter range, lower power than 802.11
  
• IEEE 802.15 standard
• Architecture
• piconet - A master node and up to 7 slave nodes, within a distance of up to 10 meters
• uses centralized TDM.  Master allocates time slots to slaves
• no slave-to-slave communication.  Everything goes through master
• multiple piconets can be connected by "bridge slaves"
• Physical layer
• uses low power radio waves in ISM band (2.4 GHz)
• FSM modulation
• frequency hopping spread spectrum, on 79 channels
• operates in the same band as 802.11

Next:  WiMAX and 3G wireless networks