Wireless Networks and Protocols MAP-Tele Manuel P. Ricardo - PowerPoint PPT Presentation

WNP-MPR-Fundaments 1 Wireless Networks and Protocols MAP-Tele Manuel P. Ricardo Faculdade de Engenharia da Universidade do Porto

WNP-MPR-Fundaments 2 Topics Scheduled for Today  Introduction to Wireless Networks and Protocols  Fundamentals of wireless communications Application » Transmission Transport » Wireless data links and medium access control Network » Networking Quality of Service » Mobility concepts and management Mobility Security Data link » Research issues Physical

WNP-MPR-Fundaments 3 Wireless Data Link, Wireless Medium Access Control

WNP-MPR-Fundaments 4  How to model an adaptive wireless data link layer?  How to implement duplex communications in a wireless link?  How to enable multiple access?  What is a random access method?  What is an hidden node? What is an exposed node?  Why is collision avoidance important?  How to avoid the hidden node?  How does the CSMA/CA work?  What is the minimum distance between nodes in CSMA/CA?  What are the services possibly provided by RLC?

WNP-MPR-Fundaments 5 Radio Link S     Radio link affected by propagation environment SNIR  N I  Modulation, coding, power Rx Tx used to overcome avoiding radio adversities  Service offered by the (wireless) Physical layer » characterized by data rate (bit/s) and bit error ratio » modern technologies  depends on the radio link operation modes  Operation mode » pair (modulation, code), typically » High-Speed Downlink Packet Access (HSDPA/UMTS)  12 modes » IEEE 802.11a  7 modes

WNP-MPR-Fundaments 6 Radio Link Model – Continuous Time Markov Chain  Radio link modeled as a Markov Chain Physical layer Adaptive Transmitter  Markov chain state r 0 e 0 r 1 e 1 r 2 e 2 r M-1 e M-1 m M-1 m 1 m 2 m 3 » Operation mode (modulation, code) … 0 1 M-1 2 » S i  l 2 l M-2 l 0 l 1 » Characterized by transmit bit rate r i and bit error ratio e i  Markov chain transition rates n  n - k+1 k » Process moves only to neighbor states  k+1   k » Estimating the transition rates: m k k l k

WNP-MPR-Fundaments 7 Frame Error Ratio, p E  - - L 1 ( 1 ) FER BER  Adaptive transmission tends to maintain BER constant p e by controlling modulation, coding, tx power, …  Frame Error Ratio – p e (  ) - bit error ratio of the uncoded system – G c (  ) - coding gain – L p – packet length in bits  If different codes are used for header and information fields

WNP-MPR-Fundaments 8 Information Rate (Goodput) - 1 M  Physical layer   Mean Information rate  R r Adaptive Transmitter c i i  r 0 e 0 r 1 e 1 r 2 e 2 r M-1 e M-1 0 i m M-1 m 1 m 2 m 3 … 0 1 2 M-1 l 2 l M-2 l 0 l 1   number of bits/symbol redundant bits introduced by codes Symbol duration

WNP-MPR-Fundaments 9 How to transmit signals in both directions simultaneously?

WNP-MPR-Fundaments 10 Duplex Transmission  Duplex – transference of data in both directions Uplink and Downlink channels required  Two methods for implementing duplexing » Frequency-Division Duplexing (FDD) – wireless link split into frequency bands – bands assigned to uplink or downlink directions – peers communicate in both directions using different bands » Time-Division Duplexing (TDD) – timeslots assigned to the transmitter of each direction – peers use the same frequency band but at different times

WNP-MPR-Fundaments 11 Duplex Transmission

WNP-MPR-Fundaments 12 How to enable one base station to communicate simultaneously with multiple mobile nodes?

WNP-MPR-Fundaments 13 Multi-Access Schemes  Multi-access schemes » Identify radio resources » Assign radio resources to users/terminals using some criteria  Types of multi-access schemes » Frequency-Division Multiple Access (FDMA) resources divided in portions of spectrum (channels) » Time-Division Multiple Access (TDMA) resources divided in time slots » Code-Division Multiple Access (CDMA) resources divided in orthogonal codes » Space-Division Multiple Access (SDMA) resources divided in areas

WNP-MPR-Fundaments 14 FDMA » Signal space divided along the frequency axis into non-overlapping channels » Each user assigned a different frequency channel » The channels often have guard bands » Transmission is continuous over time code channel k channel 2 channel 1 time

WNP-MPR-Fundaments 15 TDMA » Signal space divided along the time axis into non-overlapping channels » Each user assigned a different cyclically-repeating timeslot » Transmission not continuous for any user code … … » Major problem time synchronization among the users in the uplink channels users transmit over channels having different delays uplink transmitters must synchronize

WNP-MPR-Fundaments 16 CDMA  Each user assigned a code to spread his information signal » Multi-user spread spectrum (Direct Sequence, Frequency Hopping) » The resulting spread signal – occupy the same bandwidth – transmitted at the same time code  Different bitrates to users  control length of codes channel k … channel 2  Power control required in uplink channel 1 time » to compensate near-far effect » If not, interference from close user swamps signal from far user

WNP-MPR-Fundaments 17 SDMA  SDMA uses direction (angle) to assign channels to users  Implemented using sectorized antenna arrays » the 360º angular range divided in N sectors » TDMA or FDMA then required to channelize users MT-1 BS MT-2 MT-k

WNP-MPR-Fundaments 18 Combined Multi-Access Techniques  Current technologies  combinations of multi-access techniques » GSM: FDMA and then TDMA to assign slots to users  The cell concept  combined multi-access technique » SDMA + FDMA  Cellular planning f 3 f 3 f 3 f 2 f 3 f 7 f 2 f 2 f 5 f 2 f 2 f 2 f 2 f 1 f 1 f 1 f 1 f 1 f 1 f 4 f 6 f 5 f 5 f 5 f 3 f 3 f 3 f 4 f 4 f 3 f 3 f 1 f 4 f 8 f 8 f 8 f 6 f 6 f 7 f 7 f 7 f 2 f 2 f 2 f 3 f 7 f 1 f 9 f 9 f 9 f 1 f 1 f 2 f 3 f 3 f 3 f 3 f 6 f 5 f 2 a) Group of 3 cells b) Group of 7 cells c) Group of 3 cells, each having 3 sectors

WNP-MPR-Fundaments 19 Wireless Medium Access Control  Medium Access Control (MAC) assigns radio resources to terminals along the time  3 type of resource allocation methods » dedicated assignment resources assigned in a predetermined, fixed, mode (TDMA) » random access terminals contend for the medium (channel) » demand-based terminals ask for reservations using dedicated/random access channels

WNP-MPR-Fundaments 20 Hidden, Exposed and Capture Nodes  Signal strength decays with the transmitter-receiver distance  Carrier sensing depends on the position of the receiver  MAC protocols using carrier sensing  3 type of problematic nodes » hidden nodes – C is hidden to A » exposed nodes D – C is exposed to B B A C » capture nodes – D captures A

WNP-MPR-Fundaments 21 Hidden, Exposed and Capture Nodes  Hidden node  C is hidden to A » A transmits to B; C cannot hear A » If C hears the channel it thinks channel is idle; C starts transmitting  interferes with data reception at B » » In the range of receiver; out of the range of the sender D A B C  Exposed node  C is exposed to B » B transmits to A; C hears B; C does not transmit; » but C transmission would not interfere with A reception » In the range of the sender; out of the range of the receiver  Capture  D captures A » A and D transmit simultaneously to B; but signal strength from D much higher than that from A

WNP-MPR-Fundaments 22 MAC Protocols - Aloha, S-Aloha, CSMA  Aloha  Efficiency of 18 % if station has a packet to transmit  transmits the packet  waits confirmation from receiver (ACK)  if confirmation does not arrive in round trip time, the station computes random backofftime  retransmits packet  Slotted Aloha  Efficiency of 37 % stations transmit just at the beginning of each time slot  Carrier Sense Multiple Access (CSMA)  Efficiency of 54 % – station listens the carrier before it sends the packet – If medium busy  station defers its transmission  ACK required for Aloha, S-Aloha and CSMA

WNP-MPR-Fundaments 23 Aloha versus T ime D ivision M ultiplexing

WNP-MPR-Fundaments 24 CSMA/CD – Not Used in Wireless  CDMA/Collision Detection  Efficiency < 80% – station monitors de medium (carrier sense)  medium free  transmits the packet  medium busy  waits until medium is free  transmits packet  if, during a round trip time, detects a collision  station aborts transmission and stresses collision (no ACK packet)  Problem of CDMA/CD in wireless networks Collision detection near-end interference makes simultaneous transmission and reception difficult

WNP-MPR-Fundaments 25 How to minimize collision in a wireless shared medium?

WNP-MPR-Fundaments 26 CSMA with Collision Avoidance (CSMA/CA) DIFS DATA S1 DIFS S2-bo DATA S2 S3-bo DIFS DIFS S3-bo-r S3-bo-e S3-bo-r DATA S3 DATA DIFS S2-bo - Packet arrival - Transmission of DATA - Time interval DIFS - Backoff time, station 2 - Elapsed backoff time, station 3 - Remaining backoff time, station 3 S3-bo-e S3-bo-r