Elements of a wireless network

1
Elements of a wireless network
2
Elements of a wireless network
3
Elements of a wireless network

• wireless link
• typically used to connect mobile(s) to base
  station
• also used as backbone link (WiMAX)
• multiple access protocol coordinates link access
• various data rates, transmission distance

4
Elements of a wireless network
5
Elements of a wireless network

• Ad hoc mode
• no base stations
• nodes can only transmit to other nodes within
  link coverage
• nodes organize themselves into a network route
  among themselves

6
Wireless Link Characteristics

• Differences from wired link .
• decreased signal strength radio signal
  attenuates as it propagates through media (path
  loss)
• interference from other sources standardized
  wireless network frequencies (e.g., 2.4 GHz)
  shared by other devices (e.g., phone) devices
  (motors) interfere as well
• multipath propagation radio signal reflects off
  objects, ground, arriving at destination at
  slightly different times
• . make communication across (even a point to
  point) wireless link much more difficult

7
Wireless network characteristics

• Multiple wireless senders and receivers create
  additional problems (beyond multiple access)

• Hidden terminal problem
• B, A hear each other
• B, C hear each other
• A, C can not hear each other
• means A, C unaware of their interference at B

• Signal fading
• B, A hear each other
• B, C hear each other
• A, C can not hear each other interferring at B

8
Special challenge for Wireless MAC

• Collision detection is difficult
• Transmission power is much higher than receiving
  power
• Hidden station problem
• A station is not able to detect a potential
  competitor for the medium because the competitor
  is too far away
• Example A is transmitting to B. C cannot hear
  the transmission, thus falsely concludes that it
  can transmit.
• Exposed station problem
• A station hears the on-going transmission, and
  falsely assume it cannot transmit
• Example B tis transmitting to A. C hears the
  transmission, thus falsely concludes that it
  cannot transmit o D.

9
IEEE 802.11 Wireless LAN

• 802.11b
• 2.4-2.485 GHz unlicensed radio spectrum
• up to 11 Mbps
• direct sequence spread spectrum (DSSS) in
  physical layer
• all hosts use same chipping code (insensitive to
  multipath fading)
• widely deployed, using base stations

• 802.11a
• 5.1-5.8 GHz range
• up to 54 Mbps
• OFDM
• 802.11g
• 2.4-2.485 GHz range
• up to 54 Mbps
• DSSS
• All use CSMA/CA for multiple access
• All have base-station and ad-hoc network versions

10
Spread Spectrum

• Idea
• spread signal over wider frequency band than
  required
• originally deigned to thwart jamming
• Frequency Hopping
• transmit over random sequence of frequencies
• sender and receiver share
• pseudorandom number generator
• seed
• 802.11 uses 79 x 1MHz-wide frequency bands

11
Spread Spectrum (cont)

• Direct Sequence
• for each bit, send XOR of that bit and n random
  bits
• random sequence known to both sender and receiver
  
• called n-bit chipping code
• 802.11 defines an 11-bit chipping code

12
OFDM

• A large number of closely-spaced
  orthogonalsub-carriers are used to carry data.
  The data are divided into several parallel data
  streams or channels, one for each sub-carrier.
  Each sub-carrier is modulated with a conventional
  modulation scheme (such as quadrature amplitude
  modulation or phase shift keying) at a low symbol
  rate, maintaining total data rates similar to
  conventional single-carrier modulation schemes in
  the same bandwidth.

13
Base Station Approach

• Wireless host communicates with a base station
• base station access point (AP)
• Basic Service Set (BSS) (a.k.a. cell) contains
• wireless hosts
• access point (AP) base station
• BSSs combined via a distribution system (DS)
• The DS runs at layer 2 of the ISO architecture!

14
Ad Hoc Network Approach

• No access point (i.e., base station)
• peer-to-peer mode
• wireless hosts communicate with each other
• to get packet from wireless host A to B may need
  to route through wireless hosts X,Y,Z
• Applications
• laptop meeting in conference room, car
• interconnection of personal devices
• battlefield
• IETF MANET (Mobile Ad hoc Networks) working
  group

15
802.11 Channels, association

• 802.11b/g 2.4GHz-2.485GHz spectrum divided into
  11 channels at different frequencies
• AP admin chooses frequency for AP
• interference possible channel can be same as
  that chosen by neighboring AP!
• host must associate with an AP
• scans channels, listening for beacon frames
  containing APs name (SSID) and MAC address
• selects AP to associate with
• may perform authentication
• will typically run DHCP to get IP address in APs
  subnet

16
Definitions

• Definitions
• MAC-level Acknowledgement
• Indicate the received frame is correct
• The source should wait ACKTimeout amount of time
  for ACK
• Inter-frame Space (IFS)
• SIFS short IFS
• PIFS PCF IFS
• DIFS DCF IFS
• DIFSgtPIFSgtSIFS

17
IEEE 802.11 multiple access

• avoid collisions 2 nodes transmitting at same
  time
• 802.11 CSMA - sense before transmitting
• dont collide with ongoing transmission by other
  node
• 802.11 no collision detection!
• difficult to receive (sense collisions) when
  transmitting due to weak received signals
  (fading)
• cant sense all collisions in any case hidden
  terminal, fading
• goal avoid collisions CSMA/C(ollision)A(voidance
  )

18
IEEE 802.11 MAC Protocol CSMA/CA

• 802.11 sender
• 1 if sense channel idle for DIFS then
• transmit entire frame (no CD)
• 2 if sense channel busy then
• start random backoff timer
• timer counts down while channel idle
• transmits when timer expires
• if no ACK, increase random backoff interval,
  repeat 2
• 802.11 receiver
• - if frame received OK
• return ACK after SIFS (ACK needed due to
  hidden terminal problem)

sender
receiver
19
Avoiding collisions Option I

• idea allow sender to reserve channel rather
  than random access of data frames avoid
  collisions of long data frames
• sender first transmits small request-to-send
  (RTS) packets to BS using CSMA
• RTSs may still collide with each other (but
  theyre short)
• BS broadcasts clear-to-send CTS in response to
  RTS
• RTS heard by all nodes
• sender transmits data frame
• other stations defer transmissions

Avoid data frame collisions completely using
small reservation packets!
20
Collision Avoidance RTS-CTS exchange
A
B
AP
defer
time
21
IEEE 802.11 MAC Layer Option II

• Protocol Architecture
• Distributed Coordination Function (DCF)
• Point Coordination Function (PCF)

22
802.11 MAC Layer Carrier Sensing

• Carrier sense at two levels
• Physical carrier sense done by physical layer
• Virtual carrier sense at MAC layer using Network
  Allocation Vector (NAV) set while
  RTS/CTS/Data/Ack are overheard partially solves
  problem of Hidden and Exposed terminal
• The Duration field reserves the media!
• Reduces collision by deferring transmission if
  any of the carrier sense mechanisms sense the
  channel busy

23
DCF Basic Access

• Basic Access
• When a STA has data to send, it senses medium
• The STA may transmit a MAC Protocol Data Unit
  (MPDA) when medium idle time is greater or equal
  to DIFS
• If medium is busy, wait for a random backoff time
  

24
DCF

• Backoff Procedure
• Backoff procedure is invoked for a STA to
  transfer a frame but the medium is busy
• Set Backoff Timer to be random backoff time
• Backoff Timer start decreasing after an idle time
  of DIFS following the medium busyness
• Backoff Timer is suspended when medium is busy,
  and wont resume until the medium is idle for
  DIFS
• A frame may be transmitted immediately when
  Backoff Timer is 0

25
DCF

• Recovery procedures
• Collision may happen during contention
• When collision happens, retransmission
• STAs maintain a station short retry counter
  (SSRC) and long retry counter (SLRC) for each
  MSDU and MMPDU
• SSRC increases by one for each failed RTS or MPDU
  whose length is lt dot11RTSThreshold
• SLRC increases by one for each failed MPDU whose
  length is gt dot11RTSThreshold
• Both counter is reset upon a successful MPDU
• Retry is aborted when SSRCgtdot11ShortRetryLimt
  7 or SLRCgtaLongRetryLimit

26
DCF

• Random backoff timerandom()xaSlotTime
• aSlotTime the value of the correspondingly named
  PHY characteristic (20?s for DSSS)
• Random() a random integer uniformly distributed
  over 0, CW
• CW (contention window)
• Increases exponentially after each retry fails
  (so does average backoff time. Why to do this?)
• Keep constant after reaching the maximum
• Reset after a successful transmission

27
DCF RTS/CTS Scheme

• RTS/CTS Scheme
• Four way handshake RTS-CTS-DATA-ACK
• NAV (Network Allocation Vector)
• An indicator, maintained at each STA, for the
  period that transmission will not be initiated
• Setting and resetting NAV according to Duration
  in MAC header when receiving a valid frame

28
DCF -- Fragmentation

• Control of the channel
• Once the STA has contented for the channel, it
  shall continue to send fragments until
• All fragments of a MSDU or MMPDU have been sent
• An ACK is not received
• STA is restricted from sending additional
  fragments by PHY layer
• Duration field
• RTS/CTS time till the end of ACK0
• Fragments/ACK time till the end of the ACK for
  the next fragment
• Last fragment/ACK length of ACK/0

29
DCF

• Directed (Unicast) MPDU
• STA uses RTS/CTS for directed MPDU only when the
  length of a MPDU gt dot11RTSThreshold
• Always use RTS/CTS set dot11RTSThreshold0
• Dont use RTS/CTS set dot11RTSThresholdmaximum
  MPDU length2304 octets
• Broadcast and multicast
• Regardless of the length of frame, no RTS/CTS
• No ACK
• No MAC layer recovery

30
PCF

• Point Coordinator
• PCF provides contention free frame transfer
• PC resides in AP It is an option of AP to become
  PC
• Fundamental Access
• PC senses the medium
• When medium is idle for PIFS, PC transmit a
  Beacon frame
• After beacon, PC shall wait for SIFS, and then
  transmit
• Data frame
• CF-Poll (contention free poll) frame
• Data CF-Poll frame

31
PCF

• Polling list
• STA indicates its CF-Pollability via Association
  and Reassociation
• PC shall send a CF-Poll to at least one STA when
  there are entries in the polling list
• NAV
• Each STA set the NAV to CFPMaxDuration
• PC shall transmit a CF-End frame at the end of CFP

32
PCF

• Only available for infrastructured architecture,
  why?
• PCF is on top of DCF
• Super frame contains a contention-free period and
  a contention period
• Question how time-bounded service is provided?

33
802.11 frame addressing
Address 4 for Intra DS communication
Address 1 MAC address of wireless host or AP to
receive this frame
Address 3 MAC address of router interface to
which AP is attached
Address 2 MAC address of wireless host or AP
transmitting this frame
34
802.11 frame more
frame seq (for reliable ARQ)
duration of reserved transmission time (RTS/CTS)
Frame subtype (RTS, CTS, ACK, Beacon, etc.)
frame type (management, control, data)
35
Interpretation of the MAC Addrs
To AP From AP Addr 1 Addr 2 Addr 3 Addr 4
0 0 DA SA BSSID -
0 1 DA BSSID SA -
1 0 BSSID SA DA -
1 1 RA TA DA SA
36
802.11 frame addressing
H1
R1
37
Supporting Mobility

• Case 1 ad hoc net working
• Case 2 access points (AP)
• tethered
• each mobile node associates with an AP

38
Mobility (cont)

• Scanning (selecting an AP)
• node sends Probe frame
• all APs w/in reach reply with ProbeResponse
  frame
• node selects one AP sends it AssociateRequest
  frame
• AP replies with AssociationResponse frame
• new AP informs old AP via tethered network
• When
• active when join or move
• passive AP periodically sends Beacon frame