Carrier Sense Multiple Access (CSMA) and Spread Spectrum in Computer Networks, Slides of Wireless Networking

Download Carrier Sense Multiple Access (CSMA) and Spread Spectrum in Computer Networks and more Slides Wireless Networking in PDF only on Docsity! 2  Review of previous lecture #6  CSMA ► Versions of CSMA ► CSMA/CA ► Example  Spread Spectrum ► Frequency Hoping ► Direct Sequence  Summary of today’s lecture docsity.com 3 Last Lecture Review  FDMA  TDMA  CDMA  Random Access ► ALOHA ► Slotted ALOHA ► Reservation-based ALOHA docsity.com 6 CSMA / Collision Avoidance  Used where CSMA/CD cannot be used ► e.g. in wireless medium collision cannot be easily detected as power of transmitting overwhelms receiving antenna ► CSMA/CA is designed to reduce collision probability at points where collisions would most likely occur • when medium has become idle after a busy state, as several users could have been waiting for medium to become available ► key elements of CSMA/CA: • IFS –interframe spacing –priority mechanism–the shorter the IFS the higher the priority for transmission • CW intervals –contention window –intervals used for contention and transmission of packet frames • Backoff counter–used only if two or more stations compete for transmission docsity.com 7 CSMA/CA Algorithm Frame to transmit Medium Idle? Wait IFS Still Idle? Transmit frame Exp b/o while Medium idle Wait until Trans ends Wait IFS Transmit frame Still Idle? No Yes No No Yes Yes If medium becomes busy during the backoff time, the backoff timer is halted and resumes when the medium becomes idle. docsity.com Example IFS cw IFS CW IFS CW IFS cw Sener — — — Sender B a a = ; Al ! maiFone] | Sender C Sender D Sender E docsity.com Spread Spectrum Output data Channel Input date Channel Modulator encoder Spreading Spreading code code Pseudonoise Pseudonoise generator generator Figure 7.1 General Model of Spread Spectrum Digital Communication System & docsity.com 12  Types of spreading: ► direct sequence spread spectrum (DSSS) ► frequency hopping spread spectrum (FHSS) docsity.com 13 Frequency Hoping Spread Spectrum (FHSS)  Signal is broadcast over seemingly random series of radio frequencies ► A number of channels allocated for the FH signal ► Width of each channel corresponds to bandwidth of input signal  Signal hops from frequency to frequency at fixed intervals ► Transmitter operates in one channel at a time ► Bits are transmitted using some encoding scheme ► At each successive interval, a new carrier frequency is selected  Channel sequence dictated by spreading code docsity.com 16 FHSS Using MFSK  MFSK signal is translated to a new frequency every Tc seconds by modulating the MFSK signal with the FHSS carrier signal  For data rate of R: ► duration of a bit: T = 1/R seconds ► duration of signal element: Ts = LT seconds  Tc  Ts - slow-frequency-hop spread spectrum  Tc < Ts - fast-frequency-hop spread spectrum docsity.com 17 Slow-frequency Hop Spread Spectrum using MFSK M = 4, k=2 T Ts Ws Fr eq ue nc y Wd Wd Wd Wd Tc 0 1 1 1 0 0 1 1 1 1 0 1 1 0 0 0 0 0 1 1MFSK 00 11 01 10 00PN Sequence docsity.com 18 Fast-frequency Hop Spread Spectrum using MFSK M = 4, k=2 T Tc Ws Fr eq ue nc y Wd Wd Wd Wd Ts 0 1 1 1 0 0 1 1 1 1MFSK 00 11 01 10 00 10 00 11 10 00PN Sequence docsity.com Transmitter Receiver Transmitted signal Data input A 1LOOLTOTLOTOLTILOIOILOODLIOLOOIOOI Locally generated PN bit stream OLLOOTIOOIILO oO1110 OOOTTIOLTLILOILO C=A@B LJ Received signal C Locally generated PN bit stream identical to B above Data output A=C@B Figure 7.6 Example of Direct Sequence Spread Spectrum docsity.com 22 Summary  CSMA ► Versions of CSMA ► CSMA/CA ► Example  Spread Spectrum ► Frequency Hoping ► Direct Sequence  Next Lecture ► Evolution of wireless networks docsity.com