Wireless networks - Lecture 7: CSMA and spread spectrum. The main topics covered in this chapter include: carrier sense multiple access (CSMA); employs different node behaviour when channel found busy; problem of radio transmission; direct sequence spread spectrum (DSSS);...
Wireless Networks Lecture CSMA and Spread Spectrum Dr Ghalib A Shah Review of previous lecture #6 CSMA ► Versions of CSMA ► CSMA/CA ► Example Spread Spectrum ► Frequency Hoping ► Direct Sequence Summary of today’s lecture Last Lecture Review FDMA TDMA CDMA Random Access ► ALOHA ► Slotted ALOHA ► Reservation-based ALOHA Carrie r S e ns e Multiple Ac c e s s (CS MA) Dis advantag e s o f ALOHA ► us e rs do no t lis te n to the c hanne l be fo re (and while) trans mitting ► suitable for networks with long propagation delays Carrie r S e ns e Multiple Ac c e s s ► po lite ve rs io n o f ALOHA ► Listen to the channel before transmitting • if sensed channel busy, back-off (defer transmission), and sense channel again after a random amount of time • if channel idle, transmit entire frame Versions of CSMA Emplo ys diffe re nt no de be havio ur whe n c hanne l fo und bus y ► no npe rs is te nt CS MA • after sensing busy channel, node waits entire back-off period before sensing again ► pe rs is te nt CS MA • after sensing busy channel, node continues sensing until the channel becomes free; then … ► 1pe rs is te nt CS MA • node transmits immediately with probability ► ppe rs is te nt CS MA • node transmits with probability p; or, it defers transmission with probability (1-p) CS MA / Co llis io n Avo idanc e Us e d whe re CS MA/CD c anno t be us e d ► 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 inte rvals –contention window –intervals used for contention and transmission of packet frames Bac ko ff c o unte r–used only if two or more stations compete for transmission CSMA/CA Algorithm Frame to transmit Medium Idle? No Yes Wait IFS Wait IFS Still Idle? Wait until Trans ends No Yes Transmit frame Still Idle? No Yes Exp b/o while Medium idle If medium becomes busy during the backoff time, the backoff timer is halted and resumes when the medium becomes idle Transmit frame Example Spread Spectrum Pro ble m o f Radio Trans mis s io n ► fre que nc y de pe nde nt fading c an wipe o ut narro wband s ig nals fo r duratio n o f inte rfe re nc e solution: ► spread narrow band signal into a broad band signal using a special code ► initially developed for military in order to combat jamming and inte rc e ptio n ► power of spread signal is the same as of narrow band signal, resulting in a lower power spectral density due to larger bandwidth 10 Spread Spectrum 11 Types of spreading: ► dire c t s e que nc e s pre ad s pe c trum (DS S S ) ► fre que nc y ho pping s pre ad s pe c trum (FHS S ) 12 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 13 Frequency Hoping Spread Spectrum Receiver, hopping between frequencies in synchronization with transmitter, picks up message Advantages ► Eavesdroppers hear only unintelligible blips ► Attempts to jam signal on one frequency succeed only at knocking out a few bits 14 Frequency Hoping Spread Spectrum 15 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 slowfrequencyhop spread spectrum Tc