3 Shannon’s limitFor a channel without shadowing, fading, or ISI, the maximum possible data rate on a given channel of bandwidth B isR=Blog2(1+SNR) bps,where SNR is the received signal to noise ratio
4 Bits mapped to signal (analog signal waveform) Adds redundancy to protect the digital information from noise and interferenceBits mapped to signal (analog signal waveform)e.g., GFSKe.g., TDMA,CDMAThe information is represented as a sequence of binary bits, the binary bits are then mapped (modulated) to analog signal waveforms and transmited over a communication channel. The communication channel introduces noise and interference to corrupt the transmitted signal. At the receiver, the channel corrupted transmitted signal is mapped back to binary bits.The received binary information is estimate of the transmitted binary information.Channel coding is often used in digital communication systems to protect the digital information from noise and interference and reduce the number of bit errors. Channel coding is mostly accomplished by selectively introducing redundant bits into the transmitted information stream. These additional bits will allow detection and correction of bit errors in the received data stream and provide more reliable information transmission.
6 Mobile radio channelA single direct path between the base station and the mobile is seldom the only physical means for propagationHence, the free space propagation model is inaccurate when used aloneTwo-ray ground reflection model considers both the direct path and a ground reflected propagation path between transmitter and receiverReasonably accurate for predicting the large-scale signal strength over distances of several km for mobile radio systems that use tall tower (heights which exceed 40m) or for line-of-sight micro-cell channels in urban environment
7 Hidden node problem Node 3 Node 1 Node 2 From the perspective of node 1, node 3 is hiddenIf node 1 and node 3 communicate simultaneously, node 2 will be unable to make sense of anythingNode 1 and node 3 would not have any indication of the error because the collision was local to node2
8 Fading problem Node 3 Node 1 Node 2 Node 1 and 3 are placed such that their signal is not strong enough for them to detect each other’s transmissions, and yet their transmissions are strong enough to have interfered with each other at node 2
9 Carrier-Sensing Functions Physical carrier-sensingExpensive to build hardware for RF-based mediaTransceivers can transmit and receive simultaneously only if they incorporate expensive electronicsHidden nodes problemFading problemVirtual carrier-sensingCollision avoidance: stations delay transmission until the medium becomes idleReduce the probability of collisionsUndetectable collisions
10 Carrier-Sensing Functions Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA)Virtual sensingMAC layercontrol messages (RTS, CTS, ACK)network allocation vector (NAV) to ensure that atomic operations are not interruptedDifferent types of delay depending on the priority of the frame (e.g., SIFS, DIFS, backoff)
11 Question: [answers @ log] Transceivers that transmit and receive simultaneously
13 IEEE family802.11b:Direct Sequence Spread Spectrum (DSSS) or Frequency Hopping (FH), operates at 2.4GHz, 11Mbps bitrate802.11a: between 5GHz and 6GHz uses orthogonal frequency-division multiplexing, up to 54Mbps bitrate802.11g: operates at 2.4GHz up to 54Mbps bitrateAll have the same architecture & use the same MAC protocol
14 Code Division Multiple Access (CDMA) CDMA assigns a different code to each nodeCodes orthogonal to each other (i.e inner-product = 0)Each node uses its unique code to encode the data bits it sendsNodes can transmit simultaneouslyMultiple nodes per channelTheir respective receivers correctly receive a sender’s encoded data bits assuming the receiver knows the sender’s code in spite of interfering transmissions by other nodes.Has been used extensively in military for some time due to its antijamming properties and is now beginning to find widespread civilian use, particularly for use in wireless multiple access channels.
15 CDMA Example Sender Zi,m=di*cm Data bits d0=1 d1=-1 Spread code 1 1 1 Time slot 1Time slot 011111111Channel output-1-1-1-1-1-1-1-1
16 CDMA ExampleWhen no interfering senders, the receiver would receive the encoded bits and recover the original data bit, di, by computingdi= — S Zi,m*cmInterfering transmitted bit signals are additiveM1Mm=1CDMA’s assumption that interfering transmitted bit signals are additive
17 Frequency Hopping Timing the hops accurately is the challenge slot5User A4User B321Time slot
18 Modulation techniques DSSSAs CDMA except all mobile hosts and base stations use the same chipping codeSpreads the energy in a signal over a wider frequency rangeSpreading ratio (i.e., number of chips) should be as low as possible to meet design requirements and avoid wasting bandwidthFH divides the ISM band into a series of 1-MHz channelsDivides hopping sequences into non-overlapping setsAny two members of a set are orthogonal hopping sequencesOFDMDistributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the "orthogonality" in this technique which prevents the demodulators from seeing frequencies other than their own. The benefits of OFDM are high spectral efficiency, resiliency to RF interference, and lower multi-path distortion.
19 802.11 direct-sequence Uses the Barker sequence (11-bit sequence) It is applied to each bit in the stream by a modulo-2 adder:when 1 is encoded, all the bits in the spreading code change;when 0 is encoded, they stay the sameFCC imposes legal limits on the power transmission:One watt of transmitter output power and four watss of effective radiated poewr.Effective radiated power is transmitters power output * gain of the antenna-loss in the transmission line.
20 Media Access ProtocolCoordinates the access & use of the shared radio frequencyCarrier Sense Multiple Access protocol with collision avoidance (CSMA/CA)Physical layer monitors the energy level on the radio frequency to determine whether another station is transmitting and provides this carrier-sensing information to the MAC protocolIf channel is sensed idle for DIFS, a station can transmitWhen receiving station has correctly & completely received a frame for which it was the addressed recipient, it waits a short period of time SIFS and then sends an ACK
21 Carrier-Sensing Functions IEEE to avoid collisionsCarrier Sense Multiple Access/Collision Avoidance (CSMA/CA)MAC layerRTS, CTS, ACKnetwork allocation vector (NAV) to ensure that atomic operations are not interruptedDifferent types of delayShort Inter-frame space (SIFS):highest priority transmissions (RTS, CTS, ACK)DCF inter-frame space (DIFS):minimum idle time for contention-based servicesEIFS: minimum idle time in case of “erroneous” past transmission
22 RTS/CTS clearing Node 1 Node 2 Node3 Node 1 RTS Time CTS frame Node 2 (4) ACKCTSframeNode 2ACKRTS: reserving the radio link for transmissionRTS, CTS: Silence any station that hear them
23 Positive acknowledgement of data transmission Node 1Node 2TimeframeACKallows stations to lock out contention during atomic operation so that atomic sequences are not interrupted by otherHosts attempting to use the transmission medium
24 Media Access ProtocolIf channel is sensed busy will defer its access until the channel is later sensed to be idleOnce the channel is sensed to be idle for time DIFS, the station computes an additional random backoff time and counts down this time as the channel is sensed idle. When the random backoff timer reaches zero, the station transmits its frameBackoff process to avoid having multiple stations immediately begin transmission and thus collide
25 Using the NAV for virtual carrier sensing (eg 4-8KB)RTSFrameCTSACKSenderReceiverNAVDIFSSIFSNAV (RTS)NAV(CTS)(e.g.10ms)Carrier-sensing functionsContentionWindowAccess to medium deferredNAV is carried in the headers of CTS & RTS
26 Backoff with DCFContention window (or backoff window) follows the DIFSWindow is divided in time slotsSlot length is medium-dependentWindow length limited and medium-dependentHosts pick a random slot and wait for that slot before attempting to access the medium;All slots are equally likely selectionsHost that picks the first slot (earlier number) winsEach time the retry counter increases, the contention window moves to the next greatest power of two
27 Contention window size DIFSPreviousFrame31 slotsInitialAttempt63 slots127 slots1st retransmission2nd retransmission3rd retransmission255 slotsSlot time:20sThe contention window is reset to its minimum size when frames are transmitted successfully, or the associated retry counter is reached and the frame is discarded