Characteristics of Wireless Environment

1
Characteristics of Wireless Environment
2
Radio Propagation Mechanism
3
Characteristics of Wireless Channel

• Path loss
• Pr/Pt O(d-?), where d distance
• ? 2 (free space), 5 (strong attenuation)
• Fading fluctuation of signal strength
• Fast fading due to multipath propagation
• Slow fading occurs when objects absorb the
  transmission
• May reduced by diversity or adaptive modulation
• Interference
• Adjacent channel interference ?guard band
• Co-channel interference ? cellular, directional
  antenna, dynamic channel allocation
• Inter-symbol interference ? adaptive equalization
• Doppler shift

4
Multiple Access Techniques

• FDMA
• OFDM
• TDMA
• Hard to compute good schedules in a distributed
  fashion.
• Schedule needs to be traffic dependent.
• Need synchronized clocks in hardware to implement
  slots
• CDMA
• FHSS
• DSSS
• SDMA
• Duplexing
• FDD
• TDD

5
CDMA DSSS

• used in several wireless broadcast channels
  (cellular, satellite, etc) standards
• unique code assigned to each user i.e., code
  set partitioning
• all users share same frequency, but each user has
  own chipping sequence (i.e., code) to encode
  data
• encoded signal (original data) X (chipping
  sequence)
• decoding inner-product of encoded signal and
  chipping sequence
• allows multiple users to coexist and transmit
  simultaneously with minimal interference (if
  codes are orthogonal)

6
CDMA Encode/Decode
channel output Zi,m
Zi,m di.cm
data bits
sender
slot 0 channel output
slot 1 channel output
code
slot 1
slot 0
received input
slot 0 channel output
slot 1 channel output
code
receiver
slot 1
slot 0
7
CDMA two-sender interference
8
Wireless MAC

• MAC (Medium Access Control)
• Sharing a Single Broadcast Medium among Multiple
  Users
• Contention Most Widely Used, Suffer from
  Collision
• Non-Contention Reservation/Round-Robin,
  Collision Free
• Wireless MAC vs. Ad Hoc MAC ?
• Ad Hoc Network Multi-Hop Wireless Network

9
ALOHA and CSMA

• ALOHA - University of Hawaii (1970)
• Transmit whenever it has data to send.
• Listen to the acknowledgement feedback from the
  receiver.
• If a collision occurs (no ACK), retransmits
  after a random delay.
• Utilization pure Aloha 18.5, slotted Aloha
  37
• CSMA - Kleinrock (1975)
• Listen (Carrier Sense) before transmission
• 1. If channel is idle, transmit
• 2. Otherwise, do one of the followings
• Wait until channel become idle and transmit ? 1
  Persistent-CSMA
• Wait until idle and transmit with probability p ?
  p Persistent-CSMA
• Defer transmission and try again after a random
  delay ? NP-CSMA
• Carrier sense not foolproof
• Propagation delay (also a problem in wireline).
• Can sense only at transmitter but collision
  happens at receiver (a wireless problem).

10
CSMA/CA (Collision Avoidance)

• RTS/CTS Dialog before Data Transmission
• RTS (Request To Send Sender) / CTS (Clear To
  Send Receiver) / DATA
• Contention Window
• How about CSMA/CD (Collision Detection) ?
• Need the ability to Listen while transmitting to
  detect collision
• The strength of its own transmission would mask
  all other signals on the air

11
Hidden and Exposed Terminal Problem
A
C
B
A
C
B
D
C wants to transmit to D. It hears Bs
transmission, and unnecessarily defers, although
it could transmit in parallel as A cant hear Cs
transmission
B transmits to A
C wants to transmit to B. It does not hear As
transmission, accesses the channel and collides
A transmits to B
Time
Packet Transmission From B to A
Packet Transmission From B to A
Time
Packet Transmission From A to B
Packet Transmission From C to B
Packet can be transmitted from C to D, But dont
it.
Waste Resource
Collision
C and A are Hidden Terminals relative to each
other one cant sense the others transmission
C is an Exposed Terminal relative to B. Bs
transmission inhibits C, although there would be
no collision at the receiver (D). If C were to
transmit.
12
Sense Carrier at Receiver

• Busy Tone Multiple Access (BTMA)
• Receiver sounds a tone when busy receiving.
• Carrier sense on busy tone before transmission.
• Perfect solution. But need a busy tone channel
  and extra interface. Channel gains on data and
  busy tone channels may be different.
• In band solution
• Use virtual carrier sensing. Used in 802.11

13
IEEE 802.11 WLAN
14
Wireless LAN Standards
IEEE802.11
ETSI BRAN
802.11f Inter Access Point Protocol
UMTS Integration
IEEE 802.11
802.11e QoS Enhancements
MAC
802.11i Security Enhancements
HiperLAN
802.11h DFS TPC
DFS TPC 5GHz 54Mbps
802.11a 5GHz 54Mbps
802.11g 2.4GHz 20Mbps
802.11b 2.4GHz 11Mbps
802.11 2.4GHz 2Mbps
PHY
15
WLAN Standards
802.11
802.11b
802.11g
802.11a
Hperlan2
Frequency
2.42.4835 GHz (83.5MHz)
2.42.4835 GHz (83.5MHz)
2.42.4835 GHz (83.5MHz)
5.1505.350 GHz 5.7255.825 GHz (455MHz)
5.1505.350 GHz 5.4705.725 GHz (300MHz)
Modulation
DBPSK, DQPSK
DBPSK/CCK, DQPSK/CCK
CCK, OFDM
OFDM BPSK, QPSK 16QAM, 64QAM
OFDM BPSK, QPSK 16QAM, 64QAM
Max. PHY rate
1,2 Mbps
1,2,5.5,11Mbps
54 Mbps (1,2,5.5,6,9, 11,12,18,24, 36,48,54Mbps)
54 Mbps (6,9,12,18,24, 36,48,54Mbps)
54 Mbps (6,9,12,18,24, 36,48,54Mbps)
Max. Data Rate(layer 3)
1.2 Mbps
5 Mbps
2232 Mbps
32 Mbps
32 Mbps
MAC
CSMA/CA
CSMA/CA
CSMA/CA
CSMA/CA
TDMA/TDD
Connectivity
Connection-less
Connection-less
Connection-less
Connection-less
Connection -oriented
Fixed Network support
Ethernet
Ethernet
Ethernet
Ethernet
Ethernet, IP, ATM, UMTS, FireWire, PPP
16
IEEE 802.11 Protocol Stack
For centralized contention-free channel access
For distributed contention-based channel access
17
Possible Network Topologies
BSS mode
ESS mode
18
802.11 Channels, association

• 802.11b 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

19
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
  )

20
IEEE 802.11 MAC Protocol DCF

• 802.11 sender
• 1 if sense carrier idle for DIFS then
• transmit entire frame (no CD)
• 2 if sense (physical or virtual) carrier busy
  then
• Choose random backoff interval in 0, cw
• counts down while medium is idle
• Count-down is supended if medium becomes busy
• transmit when backoff interval 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)

• DCF is a CSMA/CA protocol
• 802.11 DCF is suitable for multi-hop ad hoc
  networking

21
Distributed Coordination Function (DCF)
22
Binary Exponential Backoff

• Backoff Counter is randomly selected from
  0,CW,where CW is contention window
• For each unsuccessful frame transmission, CW
  doubles (from CWmin to CWmax)
• CW ? 2 (CW1)-1
• If successful transmission,
• CW ? CWmin
• Reduces the collision probability

23
Avoiding collisions (more) RTS/CTS

• 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 AP using CSMA
• RTSs may still collide with each other (but
  theyre short)
• AP broadcasts clear-to-send CTS in response to
  RTS
• RTS heard by all nodes
• sender transmits data frame
• other stations defer transmissions

24
RTS/CTS Mechanism (Optional)

• RTC/CTS solves HTP
• But, non-negligible overhead
• If frame size gt RTSthreshhold,
• RTS-CTS-DATA-ACK
• Otherwise,
• DATA-ACK

802.11b 802.11b
tslot 20usec
SIFS 10usec
PIFS SIFS tslot
DIFS SIFS 2tslot
EIFS gt DIFS
25
Priorities in 802.11

• CTS and ACK have priority over RTS
• After channel becomes idle
• If a node wants to send CTS/ACK, it transmits
  SIFS duration after channel goes idle
• If a node wants to send RTS, it waits for DIFS gt
  SIFS

26
Ranges and Zones

• Transmission range
• Frame can be successfully received
• Carrier-sensing zone (C-Zone)
• Signal can be detected, but not decoded.
• Interfering range
• Receiving node can be interfered by another
  transmission ? collision

27
Collisions are not completely avoidedin IEEE
802.11 !!

• H does not sense any signal during Ds DATA tx
• H may transmit
• Collision in Es reception

28
Energy Conservation Powercontrol

• Power control has two potential benefit
• Reduced interference increased spatial reuse
• Energy saving
• If C reduces transmit power, it can still
  communicate with D
• Reduces energy consumption at node C
• Allows B to receive As transmission (spatial
  reuse)

29
Point Coordination Function (PCF)

• To provide real-time service
• Poll-and-response MAC for nearly Isochronous
  service
• In infrastructure BSS only Point Coordinator
  (PC) resides in AP
• Alternating Contention-Free Period (CFP) and
  Contention Period (CP)

30
Contention Free Operation

• Two consecutive frames are separated by SIFS
• CFP lengths depend on traffic amount
• Maximum length announced by AP used for NAV set

31
802.11 frame addressing
Address 4 used only in ad hoc mode
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
32
802.11 frame addressing
H1
R1
33
802.11 frame more
frame seq (for reliable ARQ)
duration of reserved transmission time (RTS/CTS)
frame type (RTS, CTS, ACK, data)
34
802.11 mobility within same subnet

• H1 remains in same IP subnet IP address can
  remain same
• switch which AP is associated with H1?
• self-learning (Ch. 5) switch will see frame from
  H1 and remember which switch port can be used
  to reach H1

hub or switch
BBS 1
AP 1
AP 2
H1
BBS 2
35
WEB Wired Equivalent Privacy

• RSA RC4 algorithm with 40-bit secret key
• Data encryption and integrity

36
Other MAC Layer Functionalities

• Synchronization
• Quasi periodic beacon frame are transmitted by AP
  (may be deferred if medium is busy)
• Beacon contains time stamp
• Power management
• Sleep and awake states
• Sleeping stations wake up periodically
• Sender has to buffer the data if receiver is on
  sleep state
• Roaming
• Active scanning send a probe on each channel and
  waiting for response
• Passive scanning listen into medium to find
  other network

37
The Other IEEE 802.11 Efforts

• 802.11e
• Provides QoS support by differentiating traffic
  streams
• Applicable to 802.11 PHY a, b, and g
• 802.11h
• Supplementary to MAC layer so as to comply with
  European regulations for 5 GHz WLAN
• 802.11i
• Security enhancement
• 802.11n
• Enhancement for higher throughput (gt 100 Mbps )
• Decrease overhead within 802.11 protocol
• Packet preamble, CW, ACK, IFS parameters
• 802.11r
• Speed up handoff between APs (Fast
  BSS-Transition)
• Important for VoWLAN
• 802.11s
• Support mesh networks

38
HIPERLAN
39
HIPERLAN Standards

• ETRI BRAN Project
• HIPERLAN/1
• RLAN without a wired infrastructure
• Suited to both ad hoc and infra-based net
• 5.15 GHz, 17.1 GHz 23.5 Mbps
• HIPERLAN/2
• Short range (200m) wireless access to IP, ATM,
  other infra-based net
• To integrate WLANS into cellular systems
• 5 GHz 6 54 Mbps
• HIPERACCESS (HIPERLAN/3)
• HIPERLINK (HIPERLAN/4)

40
HIPERLAN/1 EY-NPMA

• Elimination Yield Non-Preemptive MA
• Efficiency 8 83 for packet size 50B 2KB

41
HIPERLAN/2

• IEEE 802.a (54Mbps) QoS handoff data
  integrity
• To integrate WLANs into cellular system (3G)
• ATM-compatible WLAN
• CO
• Fixed size packets
• support QoS
• MAC based on TDMA/TDD
• 2msec MAC frame consists of
• BCH broadcast control
• FCH frame control
• ACH access feedback control
• DL downlink data
• UL uplink data
• DiL direct link (for Ad Hoc)