Medium Access Control for Wireless Links CS 515 Mobile and Wireless Networking Ibrahim Korpeoglu Computer Engineering Department Bilkent University

Outline Random MAC Schemes What we have see so far? PHY layer functions and parameters General Wireless System Architecture Media Access Control Classes of MAC protocols Simplex and Duplex Channels Coordinated MAC Schemes FDMA TDMA Capacity of TDMA systems and which factors affect the capacity. Spread Spectrum Access Methods FHMA Case study: Bluetooth CDMA Hybrid Spread Spectrum Schemes. Random MAC Schemes CSMA MACA and MACAW Case Study: IEEE 802.11 MAC CS 515 © Ibrahim Korpeoglu

What we have seen so far? Physical layer functions Get stream of bits and transport them to the other end. Modulation/Demodulation We have seen that this is not an easy task Large-scale path loss and Small-scale fading and multipath effects causes the received power at the receiver to Fluctuate (hard to decode the symbols (bits)) To decrease (Affects of interfering sources increases) Received signal power level affect the quality of the signals (information) that is transported. Received signal power level defines the Signal-to-Noise (SNR) ratio CS 515 © Ibrahim Korpeoglu

What we have seen so far? We have seen That SNR and bandwidth of a channel affects Datarate – bps) of the wireless channel by Shannon limit The bit error rate (BER) on the channel. That multipath fading results in a wireless channel error model that changes states between good (low-error rate) and bad (high error-rate) Large-scale path loss defines the range of stations for different environments (LOS, urban,…) The above factors are important channel characteristics that affect the design of wireless systems architectures and design of the protocols and applications for wireless links/networks In short, we have seen so far some of Fundamental Concepts of Wireless Communication. CS 515 © Ibrahim Korpeoglu

What we will do now? We will look now to the protocols, algorithms, schemes that are developed over this wireless channels. How can we share a wireless channel: Results in Wireless Media Access Control Protocols How we can change base stations: Results in Handoff algorithms and protocols How can we seamlessly support mobile applications over wireless links: Results in mobility protocols like Mobile IP, Cellular IP, etc. How can we design efficient transport protocols over wireless links: Results in solutions like SNOOP, I-TCP, M-TCP, etc. How different wireless networks/systems are designed? Bluetooth, IEEE 802.11, GSM, etc. CS 515 © Ibrahim Korpeoglu

Wireless System Architecture and Functions Applications TCP/IP Neighbor Discovery and Registration, Multicasting, Power Saving Modes, Address Translation (IP-MAC), Routing, Quality of Services, Subnet Security Wireless Subnet Controller Wireless Link Layer (Layers 1 and 2 in ISO/OSI Network Reference Model) Link Controller Medium Access Control, MAC level Scheduling, Link Layer Queueing, Link Layer Reliability – ACKs, NACKs, …. Transceiver Frame Controller Framing and frame synchronization, error control, CRC, bit scrambling, widening, …. Carrier frequency, channel bandwidth, carrier detect, Captude detect, channel data rate, modulation, Received signal strength (RSSI), transmit power, Power control, … Physical Radio CS 515 © Ibrahim Korpeoglu

Medium Access Control Wireless spectrum (frequency band) is a very precious and limited resource. We need to use this resource very efficiently We also want our wireless system to have high user capacity A lot of (multiple) users should be able to use the system at the same time. For these reasons most of the time, multiple users (or stations, computers, devices) need to share the wireless channel that is allocated and used by a system. The algorithms and protocols that enables this sharing by multiple users and controls/coordinates the access to the wireless channel (medium) from different users are called MEDIUM ACCESS, or MEDIA ACCESS or MULTIPLE ACCESS protocols, techniques, schemes, etc…) CS 515 © Ibrahim Korpeoglu

Wireless Media Access Control Random Schemes (Less-Coordinated) Examples: MACA, MACAW, Aloha, 802.11 MAC,… More suited for wireless networks that are designed to carry data: IEEE 802.11 Wireless LANs Coordinated Schemes Examples: TDMA, FDMA, CDMA More suited for wireless networks that are designed to carry voice: GSM, AMPS, IS-95,… Polling based Schemes Examples: Bluetooth, BlueSky,… Access is coordinated by a central node Suitable for Systems that wants low-power, aims to carry voice and data at the same time. CS 515 © Ibrahim Korpeoglu

Duplexing It is sharing the media between two parties. If the communication between two parties is one way, the it is called simplex communication. If the communication between two parties is two- way, then it is called duplex communication. Simplex communication is achieved by default by using a single wireless channel (frequency band) to transmit from sender to receiver. Duplex communication achieved by: Time Division (TDD) Frequency Division (FDD) Some other method like a random access method CS 515 © Ibrahim Korpeoglu

Duplexing Usually the two parties that want to communication in a duplex manner (both send and receive) are: A mobile station A base station Two famous methods for duplexing in cellular systems are: TDD: Time Division Duplex FDD: Frequency Division Duplex CS 515 © Ibrahim Korpeoglu

Duplexing - FDD F A duplex channel consists of two simplex channel with different carrier frequencies Forward band: carries traffic from base to mobile Reverse band: carries traffic from mobile to base M B R Base Station Mobile Station Reverse Channel Forward Channel fc,R fc,,F frequency Frequency separation Frequency separation should be carefully decided Frequency separation is constant CS 515 © Ibrahim Korpeoglu

Duplexing - TDD A single radio channel (carrier frequency) is shared in time in a deterministic manner. The time is slotted with fixed slot length (sec) Some slots are used for forward channel (traffic from base to mobile) Some slots are used for reverse channel (traffic from mobile to base) B M Base Station Mobile Station Slot number 0 1 2 3 4 5 6 7 … channel F R F R F R F R …. Reverse Channel Forward Channel time Ti Ti+1 Time separation CS 515 © Ibrahim Korpeoglu

Duplexing – TDD versus FDD FDD is used in radio systems that can allocate individual radio frequencies for each user. For example analog systems: AMPS In FDD channels are allocated by a base station. A channel for a mobile is allocated dynamically All channels that a base station will use are allocated usually statically. More suitable for wide-area cellular networks: GSM, AMPS all use FDD TDD Can only be used in digital wireless systems (digital modulation). Requires rigid timing and synchronization Mostly used in short-range and fixed wireless systems so that propagation delay between base and mobile do not change much with respect to location of the mobile. Such as cordless phones… CS 515 © Ibrahim Korpeoglu

Multiple Access - Coordinated We will look now sharing the media by more than two users. Three major multiple access schemes Time Division Multiple Access (TDMA) Could be used in narrowband or wideband systems Frequency Division Multiple Access (FDMA) Usually used narrowband systems Code Division Multiple Access Used in wideband systems. CS 515 © Ibrahim Korpeoglu

Narrow- and Wideband Systems Narrowband System The channel bandwidth (frequency band allocated for the channel is small) More precisely, the channel bandwidth is large compared to the coherence bandwidth of the channel (remember that coherence bandwidth is related with reciprocal of the delay spread of multipath channel) AMPS is a narrowband system (channel bandwidth is 30kHz in one-way) Wideband Systems The channel bandwidth is large More precisely, the channel bandwidth is much larger that the coherence bandwidth of the multipath channel. A large number of users can access the same channel (frequency band) at the same time. CS 515 © Ibrahim Korpeoglu

Narrowband Systems Wideband systems Could be employing one of the following multiple access and duplexing schems FDMA/FDD TDMA/FDD TDMA/TDD Wideband systems Could be employing of the following multiple access and duplexing schemes CDMA/FDD CDMA/TDD CS 515 © Ibrahim Korpeoglu

Cellular Systems and MAC Multiple Access Technique AMPS FDMA/FDD GSM TDMA/FDD USDC (IS-54 and IS-136) PDC CT2 Cordless Phone FDMA/TDD DECT Cordless Phone US IS-95 CDMA/FDD W-CDMA CDMA/TDD cdma2000 CS 515 © Ibrahim Korpeoglu

Frequency Division Multiple Access B Individual radio channels are assigned to individual users Each user is allocated a frequency band (channel) During this time, no other user can share the channel Base station allocates channels to the users f1,F f2,F fN,F f1,R f2,R fN,R … M M M CS 515 © Ibrahim Korpeoglu

Features of FDMA An FDMA channel carries one phone circuit at a time If channel allocated to a user is idle, then it is not used by someone else: waste of resource. Mobile and base can transmit and receive simultaneously Bandwidth of FDMA channels are relatively low. Symbol time is usually larger (low data rate) than the delay spread of the multipath channel (implies that inter-symbol interference is low) Lower complexity systems that TDMA systems. CS 515 © Ibrahim Korpeoglu

Capacity of FDMA Systems Frequency spectrum allocated for FDMA system … Guard Band channel Guard Band Bt : Total spectrum allocation Bguard: Guard band allocated at the edge of the spectrum band Bc : Bandwidth of a channel AMPS has 12.MHz simplex spectrum band, 10Khz guard band, 30kHz channel bandwidth (simplex): Number of channels is 416. CS 515 © Ibrahim Korpeoglu

Time Division Multiple Access The allocated radio spectrum for the system is divided into time slots In each slot a user can transmit or receive A user occupiess a cyclically repeating slots. A channel is logically defined as a particular time slot that repeats with some period. TDMA systems buffer the data, until its turn (time slot) comes to transmit. This is called buffer-and-burst method. Requires digital modulation CS 515 © Ibrahim Korpeoglu

TDMA Concept Downstream Traffic: Forward Channels: (from base to mobiles) 1 2 3 … N 1 2 3 …. N … Logical forward channel to a mobile Base station broadcasts to mobiles on each slot Upstream Traffic: Reverse Channels: (from mobile to base) 1 2 3 … N 1 2 3 …. N … Logical reverse channel from a mobile A mobile transmits to the base station in its allocated slot Upstream and downstream traffic uses of the two different carrier frequencies. CS 515 © Ibrahim Korpeoglu

TDMA Frames Multiple, fixed number of slots are put together into a frame. A frame repeats. In TDMA/TDD: half of the slots in the frame is used for forward channels, the other is used for reverse channels. In TDMA/FDD: a different carrier frequency is used for a reverse or forward Different frames travel in each carrier frequency in different directions (from mobile to base and vice versa). Each frame contains the time slots either for reverse channels or forward channel depending on the direction of the frame. CS 515 © Ibrahim Korpeoglu

General Frame and Time Slot Structure in TDMA Systems One TDMA Frame Preamble Information Trail Bits Slot 1 Slot 2 Slot 3 … Slot N Guard Bits Sync Bits Control Bits Information CRC One TDMA Slot A Frame repeats in time CS 515 © Ibrahim Korpeoglu

A TDMA Frame Preamble contains address and synchronization info to identify base station and mobiles to each other Guard times are used to allow synchronization of the receivers between different slots and frames Different mobiles may have different propagation delays to a base station because of different distances. CS 515 © Ibrahim Korpeoglu

Efficiency of a Frame/TDMA-System Each frame contains overhead bits and data bits. Efficiency of frame is defined as the percentage of data (information) bits to the total frame size in bits. bT: total number of bits in a frame Tf: frame duration (seconds) bOH: number of overhead bits Number of channels in a TDMA cell: m: maximum number of TDMA users supported in a radio channel CS 515 © Ibrahim Korpeoglu

TDMA TDMA Efficiency TDMA is usually combined with FDMA GSM: 30% overhead DECT: 30% overhead IS-54: 20% overhead. TDMA is usually combined with FDMA Neighboring cells be allocated and using different carrier frequencies (FDMA). Inside a cell TDMA can be used. Cells may be re-using the same frequency if they are far from each-other. There may be more than one carrier frequency (radio channel) allocated and used inside each cell. Each carrier frequency (radio channel) may be using TDMA to further multiplex more user (i.e. having TDMA logical channels inside radio channels) For example: GSM uses multiple radio channels per cell site. Each radio channel has 200KHz bandwidth and has 8 time slots (8 logical channels). Hence GSM is using FHMA combined with TDMA. CS 515 © Ibrahim Korpeoglu

Contemporary TDMA Systems GSM (Europa) IS-54 (USA) PDC (Japan) DECT (European Cordless) Bit Rate 270.8 Kbps 48.6 Kbps 42 Kbps 1.152 Mbps Bandwidth 200 KHz 30 KHz 25 KHz 1.728 MHz Time Slot 0.577 ms 6.7 ms 0.417 ms Upstream slots per frame 8 3 12 Duplexing FDD TDD Efficiency of Time Slots 73 % 80 % 67 % Modulation GMSK p/4 DQPSK Adaptive Equalized Mandatory Optional None CS 515 © Ibrahim Korpeoglu

Features of TDMA Enables the sharing of a single radio channel among N users Requires high data-rate per radio channel to support N users simultaneously. High data-rate on a radio channel with fixed bandwidth requires adaptive equalizers to be used in multipath environments (remember the RSM delay spread s parameter) Transmission occurs in bursts (not continues) Enables power saving by going to sleep modes in unrelated slots Discontinues transmission also enables mobile assisted handoff Requires synchronization of the receivers. Need guard bits, sync bits. large overhead per slot. Allocation of slots to mobile users should not be uniform. It may depend on the traffic requirement of mobiles. This brings extra flexibility and efficiency compared to FDMA systems. CS 515 © Ibrahim Korpeoglu

Capacity of TDMA Systems Capacity can be expressed as System Capacity (the capacity of the overall system covering a region) Depends on: Range of cells Whether the system can support macro-cells, micro-cells or pico-cells. Cell Capacity Depends on the radio link performance between a base-sation and mobiles: The lowest C/I (carrier-to-interence) ratio the system can operate for example quality of transmission. This in turn depends on the speech coding technique, desired speech quality, etc. Data-rate over the channel which depends modulation efficiency (bits_per_second/Hz) and channel bandwidth. The frequency re-use factor CS 515 © Ibrahim Korpeoglu

System Capacity: Cluster: 7 cells constitute a cluster. C B Cluster size = 7 A y B D A x G G E A z F Frequency reuse factor is 1/7: same frequency is used every 7 cell. A is one set of frequencies, B is an other, etc. A mobile in cell x receives carrier signal from base x and interferences from base stations at cells y and z. The carrier signal strength of all combined signal strength from interfering base stations is called C/I or S/I ratio. CS 515 © Ibrahim Korpeoglu

C/I affect on capacity C/I ratio affects the cluster size, hence the frequency reuse factor. Frequency_reuse_factor = 1 / cluster_size Cluster size can be 3, 7, 12, 13, … Cluster size affects the cell capacity (it affects the maximum number of frequencies that can be used in a cell) A low C/I requirement for appropriate quality enables smaller cluster sizes, hence larger frequency reuse factor, meaning that larger cell capacities CS 515 © Ibrahim Korpeoglu

Comparing Systems GSM Parameters AMPS Parameters Channel banwidth: 200 KHz Required bandwidth per user = 200/8 = 25 Khz. Required minimum C/I: 9dB Frequency re-use factor: 1/3 (cluster size=3) AMPS Parameters Channel bandwidth = 30Khz Required C/I: 18 dB Frequency re-use factor: 1/7 (cluster size=7) Required bandwidth per user = 30kHz. CS 515 © Ibrahim Korpeoglu

Spread Spectrum Access SSMA uses signals that have transmission bandwidth that is several orders of magnitued larger than minimum required RF bandwidth. Provides Immunity to multipath interference Robust multiple access. Two techniques Frequency Hopped Multiple Access (FHMA) Direct Sequence Multiple Access (DSMA) Also called Code Division Multiple Access – CDMA CS 515 © Ibrahim Korpeoglu

Frequency Hopping (FHMA) Digital muliple access technique A wideband radio channel is used. Same wideband spectrum is used The carrier frequency of users are varied in a pseudo-random fashion. Each user is using a narrowband channel (spectrum) at a specific instance of time. The random change in frequency make the change of using the same narrowband channel very low. The sender receiver change frequency (calling hopping) using the same pseudo-random sequence, hence they are synchronized. Rate of hopping versus Symbol rate If hopping rate is greather: Called Fast Frequency Hopping One bit transmitted in multiple hops. If symbol rate is greater: Called Slow Frequency Hopping Multiple bits are transmitted in a hopping period GSM and Bluetooth are example systems CS 515 © Ibrahim Korpeoglu

Case Study - Bluetooth Uses Frequency Hopping in cell (piconet) over a 79 MHz wideband radio channel. Uses 79 narrowband channels (carrier frequencies) to hop through. Freq (f) = 2402+k MHz, k = 0,...,78 Channel spacing is 1 MHz (narrowband channel bandwidth) Wideband spectrum width = 79 MHz. Hopping Rate = 1600 Hops/Second Hopping sequence is determined by Bluetooth Hardware address and Clocks that are syncrozied between sender and receiver 79 MHZ 1 2 3 ..... 77 78 79-Hop System 1 MHZ A hop sequence could be: 7,1,78,67,0, 56,39,....... CS 515 © Ibrahim Korpeoglu

Case Study: Bluetooth – Piconet and FHSS Each node is classified as master or slave. Master defines a piconet (a cell). Maximum 7 slaves can be connected to a master. Master coordinates access to the the media. All traffic has to go over master. Slaves can not talk to each-other directly. Picocell S Range = 10m Raw Data-rate: 1 Mbps/piconet Radio channel used by devices in a piconet is 79MHz channel, which Is frequency hopped: hopping though 789 channels. Hoprate = 1600 hops/sec FHSS M S S All slaves and the master hops according to the same hopping sequence. The hopping sequence is determined by the clock and BT_address of the master. CS 515 © Ibrahim Korpeoglu

Case Study: Bluetooth – Scatternet and FHSS Piconet S S Piconet can be combined into scatternets. Red slave acts as a bridge between two piconets. M2 Piconet S FHSS S FHSS M1 Each piconet uses FHSS with different hopping sequences (masters are different). This prevents interference between piconets. S S CS 515 © Ibrahim Korpeoglu

Case Study: Bluetooth - Media access in a piconet Inside a piconet, access to the frequency hopped radio channel is coordinated using time division multiple access: TDMA/TDD. Slot duration = 1/1600 sec = 625ms Piconet S1 FHSS M In an even slot, master transmits to a slave. In an odd slot, the slave that is addressed in the previous master-to-slave slot transmits. S3 S2 0 1 2 3 4 5 6 7 ….. M-S1 S1-M M-S2 S2-M M-S3 S3-M M-S1 S1-M …… slot time=625ms CS 515 © Ibrahim Korpeoglu

Code Division Multiple Access (CDMA) In CDMA, the narrowband message signal is multiplied by a very large bandwidth signal called spreading signal (code) before modulation and transmission over the air. This is called spreading. CDMA is also called DSSS (Direct Sequence Spread Spectrum). DSSS is a more general term. Message consists of symbols Has symbol period and hence, symbol rate Spreading signal (code) consists of chips Has Chip period and and hence, chip rate Spreading signal use a pseudo-noise (PN) sequence (a pseudo-random sequence) PN sequence is called a codeword Each user has its own cordword Codewords are orthogonal. (low autocorrelation) Chip rate is oder of magnitude larger than the symbol rate. The receiver correlator distinguishes the senders signal by examining the wideband signal with the same time-synchronized spreading code The sent signal is recovered by despreading process at the receiver. CS 515 © Ibrahim Korpeoglu

CDMA Advantages Low power spectral density. Signal is spread over a larger frequency band Other systems suffer less from the transmitter Interference limited operation All frequency spectrum is used Privacy The codeword is known only between the sender and receiver. Hence other users can not decode the messages that are in transit Reduction of multipath affects by using a larger spectrum Random access possible Users can start their transmission at any time Cell capacity is not concerete fixed like in TDMA or FDMA systems. Has soft capacity Higher capacity than TDMA and FDMA No frequency management No equalizers needed No guard time needed Enables soft handoff CS 515 © Ibrahim Korpeoglu

CDMA Principle Represent bit 1 with +1 Represent bit 0 with -1 1 1 One bit period (symbol period) 1 1 Data PN-Code (codeword) 1 1 1 0 1 0 1 1 1 1 1 0 1 0 1 1 Coded Signal Input to the modulator (phase modulation) Chip period CS 515 © Ibrahim Korpeoglu

Processing Gain Main parameter of CDMA is the processing gain that is defined as: Gp: processing gain Bspread: PN code rate Bchip: Chip rate R: Data rate IS-95 System (Narrowband CDMA) has a gain of 64. Other systems have gain between 10 and 100. 1.228 Mhz chipping rate 1.25 MHz spread bandwidth CS 515 © Ibrahim Korpeoglu

Near Far Problem and Power Control At a receiver, the signals may come from various (multiple sources. The strongest signal usually captures the modulator. The other signals are considered as noise Each source may have different distances to the base station In CDMA, we want a base station to receive CDMA coded signals from various mobile users at the same time. Therefore the receiver power at the base station for all mobile users should be close to eacother. This requires power control at the mobiles. Power Control: Base station monitors the RSSI values from different mobiles and then sends power change commands to the mobiles over a forward channel. The mobiles then adjust their transmit power. B pr(M) M M M M CS 515 © Ibrahim Korpeoglu

DSSS Transmitter Message Baseband sss(t) + BPF m(t) Transmitted Signal p(t) PN Code Generator Oscillator fc Chip Clock CS 515 © Ibrahim Korpeoglu

DSSS Receiver Phase Shift Keying IF Wideband Demodulator Filter Received Data Received DSSS Signal at IF PN Code Generator Synchronization System CS 515 © Ibrahim Korpeoglu

Spectra of Received Signal Spectral Density Spectral Density Interference Signal Interference Signal Frequency Frequency Output of Correlator after dispreading, Input to Demodulator Output of Wideband filter CS 515 © Ibrahim Korpeoglu

CDMA Example (*) R Receiver (a base station) Data=1011… Data=0010… A B Transmitter (a mobile) Transmitter Codeword=101010 Codeword=010011 Data transmitted from A and B is multiplexed using CDMA and codewords. The Receiver de-multiplexes the data using dispreading. (*) This example is adapted from the CDMA example of Prof. Randy Katz at UC-Berkeley. CS 515 © Ibrahim Korpeoglu

CDMA Example – transmission from two sources 1 0 1 1 A Data 0 1 0 0 1 1 0 1 0 0 1 1 A Codeword 0 1 0 0 1 1 0 1 0 0 1 1 1 0 1 1 0 0 0 1 0 0 1 1 1 0 1 1 0 0 1 0 1 1 0 0 A Signal 0 0 1 0 B Data 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 B Codeword 1 0 1 0 1 0 1 0 1 0 1 0 0 1 0 1 0 1 1 0 1 0 1 0 B Signal Transmitted A+B Signal CS 515 © Ibrahim Korpeoglu

CDMA Example – recovering signal A at the receiver A+B Signal received A Codeword at receiver Integrator Output Comparator Output 0 1 0 1 Take the inverse of this to obtain A CS 515 © Ibrahim Korpeoglu

CDMA Example – recovering signal B at the receiver A+B Signal received B Codeword at receiver Integrator Output Comparator Output 1 1 0 1 Take the inverse of this to obtain B CS 515 © Ibrahim Korpeoglu

CDMA Example – using wrong codeword at the receiver A+B Signal received Wrong Codeword Used at receiver Integrator Output Comparator Output X 0 1 1 Noise Wrong codeword will not be able to decode the original data! CS 515 © Ibrahim Korpeoglu

Hybrid Spread Spectrum Techniques FDMA/CDMA Available wideband spectrum is frequency divided into number narrowband radio channels. CDMA is employed inside each channel. DS/FHMA The signals are spread using spreading codes (direct sequence signals are obtained), but these signal are not transmitted over a constant carrier frequency; they are transmitted over a frequency hopping carrier frequency. CS 515 © Ibrahim Korpeoglu

Hybrid Spread Spectrum Techniques Time Division CDMA (TCDMA) Each cell is using a different spreading code (CDMA employed between cells) that is conveyed to the mobiles in its range. Inside each cell (inside a CDMA channel), TDMA is employed to multiplex multiple users. Time Division Frequency Hopping At each time slot, the user is hopped to a new frequency according to a pseudo-random hopping sequence. Employed in severe co-interference and multi-path environments. Bluetooth and GSM are using this technique. CS 515 © Ibrahim Korpeoglu

Random Access Packet Radio Protocols CSMA Protocols Multihop radio network that carries packets Not circuit oriented like GSM, CDMA, etc. Example Protocols Pure Aloha Slotted Aloha CSMA Protocols 1-persistent CSMA non-persistent CSMA p-persistent CSMA CSMA/CD Reservation Protocols Reservation Aloha PRMA Others MACA, MACAW IEEE 802.11 MAC CS 515 © Ibrahim Korpeoglu

Pure Aloha Algorithm: A mobile station transmits immediately whenever is has data. It then waits for ACK or NACK. If ACK is not received, it waits a random amount of time and retransmits. Ignoring the propagation delay between mobiles and base station: B The time difference between the time a mobile send the first bit of packet and the time the base station receives the last bit of the packet is given by 2T. T = C/P T: packet time. C: channel data rate (bps) P: packet length (bits) Ack/Nack Data M1 M3 M2 During this 2T period of time, the packet may collide with someone elses packet. CS 515 © Ibrahim Korpeoglu

Throughput of Aloha ~0.185 0.5 Normalized Throughput Normalized Channel Occupancy CS 515 © Ibrahim Korpeoglu

CSMA: Carrier Sense Multiple Access Aloha does not listen to the carrier before transmission. CSMA listen to the carrier before transmission and transmits if channel is idle. Detection delay and propagation delay are two important parameters for CSMA Detection delay: time required to sense the carrier and decide if it is idle or busy Propagation delay: distance/speed_of_ligth. The time required for bit to travel from transmitter to the receiver. CS 515 © Ibrahim Korpeoglu

CSMA Variations 1-persistent CSMA: Non-persistent CSMA: A station waits until a channel is idle. When it detects that the channel is idle, it immediately starts transmission Non-persistent CSMA: When a station receives a negative acknowledgement, it waits a random amount of time before retransmission of the packet altough the carrier is idle. P-persistent CSMA P-persistent CSMA is applied to slotted channels. When a station detects that a channel is idle, it starts transmission with probability p in the first available timeslot. CSMA/CD Same with CSMA, however a station also listen to the carrier while transmitting to see if the transmission collides with someone else transmission. Can be used in listen-while-talk capable channels (full duplex) In single radio channels, the transmission need to be interrupted in order to sense the channel. CS 515 © Ibrahim Korpeoglu

MACA – Medium Access with Collision Avoidance CSMA protocols sense the carrier, but sensing the carrier does not always releases true information about the status of the wireless channel There are two problems that are unique to wireless channels (different than wireline channels), that makes CSMA useless in some cases. These problems are: Hidden terminal problem Exposed terminal problem. CS 515 © Ibrahim Korpeoglu

MACA – Hidden Terminal Problem A’s cell C’s cell A B C Hidden terminal A is transmitting to B. C is sensing the carrier and detects that it is idle (It can not hear A’s transmission). C also transmits and collision occurs at B. A is hidden from C. CS 515 © Ibrahim Korpeoglu

MACA – Exposed Terminal Problem B’s cell C’s cell A B C D Exposed terminal B is transmitting to A. C is hearing this transmission. C now wants to transmit to D. It senses the existence of carrier signal and defers transmission to D. However, C can actually start transmitting to D while B is transmitting to A, Since A is out of range of C and C’s signals can not be heard at A. C is exposed to B’s transmission. CS 515 © Ibrahim Korpeoglu

Ali, lets talk! I am available. MACA Solution Concept Ali, lets talk! I am available. Can Can, I want to talk to you! Can, I want to talk to you! Biltepe Mountain Ali Veli CS 515 © Ibrahim Korpeoglu

MACA Protocol When a station wants to transmit data It sends an RTS (Ready-to-Send) packet to the intended receiver The RTS packet contains the length of the data that needs to be transmitted Any station other than the intended recipient hearing RTS defers transmission for a time duration equal to the end of the corresponding CTS reception The receiver sends back CTS (Clear-to-Send) packet back to sender if it is available to receive. The CTS packet contains the length of the data that original sender wants to transmit Any station other than the original RTS sender, hearing CTS defers transmission until the data is sent. The original sender upon reception of the CTS, starts transmitting. CS 515 © Ibrahim Korpeoglu

MACA Solution for Hidden Terminal Problem A is transmitting to B. A’s cell C’s cell RTS(n) RTS(n) CTS(n) X A B C X defers transmission until expected CTS reception time by RTS sender. CTS(n) C defers transmission for duration of n bytes of data transmission. Node A is no longer hidden from C effectively. Data(n) Waiting time of node X is much smaller than waiting time of node C. CS 515 © Ibrahim Korpeoglu

MACA Solution for Exposed Terminal Problem B is transmitting to A B’s cell C’s cell RTS(n) RTS(n) A B C D RTS(m) CTS(n) CTS(m) Data(n) Data(m) C defers transmission upon hearing B’s RTS until B could get CTS from A. After that C can start transmission to D. For that it first sends an RTS. C is not longer exposed to the data transmission of B. CS 515 © Ibrahim Korpeoglu

Case Study: IEEE 802.11b MAC IEEE 802.11b: High Data-rate Wireless LAN standard. Operates in 2.4-2483 MHz ISM RF Band. 83 MHz spectrum width Max data-rate: 11Mbps simplex. Spectrum Usage: FHSS or DHSS Modulation Technique: CCK with QPSK For 11Mbps: Symbol rate = 1,375 MSps Number of symbol states = 8 One symbol can encode 3 bits of information. Range: around 100m. CS 515 © Ibrahim Korpeoglu

802.11b Infrastructure Mode Works in Two Operational Modes Ad-Hoc Mode Infrastructure Mode Access Point Access Point Wireless Link Wireless Link Wireless Link Mobile Station Extended Service Set (ESS) Basic Service Set (BSS) All traffic has to go through access points Access point provides connectivity to the wired backbone CS 515 © Ibrahim Korpeoglu

802.11b Ad-Hoc Mode Independent Basic Service Set (IBSS) Mobile Stations can talk directly with each-other. All stations in an IBSS need to be in the range of each-other. CS 515 © Ibrahim Korpeoglu

802.11b MAC Sublayer Support two different MAC modes depending on the operational mode of the Wireless LAN 1) DCF: Distributed Coordination Function Based on CSMA/CA Carrier Sensing: Physical and Virtual. 2) PCF: Point Coordination Function Connection oriented Contention free service Polling based CS 515 © Ibrahim Korpeoglu

802.11b PHY Layer 802.11b Data Rate Specifications Can support data rates at: 1,2,5.5,11 Mbps 802.11b Data Rate Specifications Data rate Code Length Modulation Symbol Rate Bits/Symbol 1 Mbps 11 (Barker Sequence) BPSK 1 MSps 1 2 Mbps QPSK 2 5.5 Mbps 4 (CCK) 1.375 MSps 4 11 Mbps 8 (CCK) 8 CS 515 © Ibrahim Korpeoglu

FHSS 2.4 GHz band is divided into 75 one-MHz subchannels. The sender and receiver hops through this 75 channels in a synchronized manner using a hopping pattern. Can not support more than 2 Mbps data-rate. CS 515 © Ibrahim Korpeoglu

DSSS Divides the 2.4 GHz band into 14 twenty-two MHz channels Adjacent channels can overlap partially. 3 of 14 channels are completely non-overlapping Data is sent over one 22 MHz channel without hoppling using DSSS technique (chipping and code words are used like CDMA) Each access point uses a different 22 MHz channel if possible. All mobiles in the coverage of the access point uses the channel that is used by the access point. 802.11b MAC is used to coordinate the access to the shared 22 MHz channel. Original 802.11 systems use 11 bit chipping (code words of length 11). Later 802.11b systems use 8 bit chipping (code words of length 8 bits). Defines 64 different codewords from a space of 256. CS 515 © Ibrahim Korpeoglu

Spectrum Allocated for 802.11b DSSS Channels 25 MHz 25 MHz 2.412 GHz 2.437 GHz 2.462 GHz Channel 1 22 MHz Channel 6 22 MHz Channel 11 22 MHz 2.400 GHz 2.484 GHz Spectrum Allocated for 802.11b Channel 1, 6, and 11 are non-overlapping. CS 515 © Ibrahim Korpeoglu

Channel Assignment and Registration. In multi-access environment, the operator should try to allocate non-overlapping channels to the physically adjacent channels. If adjacent access points use overlapping channels, then interference can be high. A mobile station periodically tunes to all channels and evaluates the signal strength received over each channel Depending on the signal strength received over the channels, a mobile selects an access point and registers with that provided that the access points accepts the mobile. This is also called association. Re-association with a new access point occurs when the mobile moves away from the current access point. When the signal conditions changes between the mobile and current access point. When there are a lot of users associated with the current access point. CS 515 © Ibrahim Korpeoglu

Re-association at the PHY layer. Access Point (AP) A Access Point (AP) B Signal from A Signal from B Associated with Access Point B Associated with Access Point A Mobile tunes to the channel of AP B when it moves into its range. CS 515 © Ibrahim Korpeoglu

An example 3-cell Reuse scheme for WLAN deployment 1 11 11 6 6 1 1 1 11 11 6 6 An access point is located in the center of each hexagon. CS 515 © Ibrahim Korpeoglu

MPDU (Variable Length) 802.11b PHY Layer MAC PLCP: Physical Layer Convergence Protocol PMD: Physical Medium Dependent Sublayer PLCP PHY Layer PMD Sublayer PLCP Frame Format SYNC (128) SFD (16) Signal (8) Service (8) Length (16) CRC (16) MPDU (Variable Length) SYNC: Synchronization field SFD: Start frame deliminer Signal: Indicated how fast the data will be transmitted Service: Reserved Length: MAC Protocol Data Unit (MPDU) length CRC: used for error detecting on the frame CS 515 © Ibrahim Korpeoglu

802.11b MAC Sublayer Supports both infrastructure and ad-hoc modes of operation. CRC is added to each MAC frame Packet fragmentation is supported to chop large higher layer (IP) packets into small pieces. Has advantages: Probability a packet gets corrupted increases with the packet size. In case of corruption, only a small fragment needs to be re-transmitted. CS 515 © Ibrahim Korpeoglu

Inter-frame Space (IFS 4 types of Inter-frame spaces: Short IFS (SIFS): period between completion of packet transmission and start of ACK frame Point Coordination IFS (PIFS): SIFS plus a slot time. Distributed IFS (DIFS): PIFS plus a slot time. Extended IFS (EIFS): longer IFS used by a station that has received a packet that it could no longer understand. Needed to prevent collisions. CS 515 © Ibrahim Korpeoglu

MAC Protocol 802.11b uses CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) MAC protocol. CSMA/CA is the protocol to implement the distributed coordination function (DCF) of the MAC sub-layer. RTS/CTS is used to avoid collisions. Use of RTS/CTS can be enabled or disabled depending on the traffic load (probability of collisions). CS 515 © Ibrahim Korpeoglu

CSMA – Transmission of MPDU (Data) without use of RTS/CTS DIFS Source Data SIFS Destination ACK Contention Window (Slot Times) DIFS Others Data Defer Access Backoff after Defer A station backoffs a random number of slot times. CS 515 © Ibrahim Korpeoglu

CSMA/CA – Transmission of MPDU (Data) using RTS/CTS DIFS Source RTS DATA SIFS SIFS SIFS Destination CTS ACK Others DIFS Defer Access for NAV(RTS) Defer Access for NAV(CTS) Backoff after Defer Defer Access for NAV(Data) CS 515 © Ibrahim Korpeoglu

CSMA/CA Collision Avoidance RTS/CTS is used to reserve channel for the duration of the packet transmission. This prevents hidden and exposed terminal problems ACK is required to understand if the packet is correctly received (without any collisions ) at the receiver. Ethernet does not require ACK to be sent, since the transmitter can detect the collision on the channel (cable) without requiring an explicit feedback from the receiver. A wireless transmitter can not detect collision, because: 1) Transmit power is much larger than the received power: received signal is regarded as noise (not collision). 2) There could be a hidden terminal Access Point Mobile RTS CTS DATA ACK CS 515 © Ibrahim Korpeoglu

802.11b Frame Format IEEE 802.11b MAC Frame Format FC (2 bytes) ID (2) Add1 (6) Add2 (6) Add3 (6) SC (2) Add4 (6) Data (0-2312 bytes) CRC (4) Frame Control Format (2 bytes) Protocol (2 bits) Type (2) Subtype (4) To DS (1) From DS (1) More Frag (1) Retry (1) Pw Mgt (1) More Data (1) WEP (1) Order (1) Protocol Version: version of 802.11 standard Type: Management. Control, Data frame Subtype: RTS, CTS, ACK frame To DS: 1 if frame is sent to Distribution System (DS) From DS: 1 if frame is received from Distribution System More fragment: 1 if there are more fragments belonging to the same frame following the current frame. Retry: indicates that is fragment is retransmission of previously transmitted fragment. Power Management: the type of power management mode that the station will be after the transmission of the frame. More Data: indicates that there are more frames buffered at the sender for this station. WEP: indicates that frame body is encrypted according to WEP. Order: indicates that the frame is sent using the strictly-ordered service class. Frame Control (FC): protocol version and frame type Duration/ID (ID): power-save poll message frame type and for NAV calculation Address Fields: contains up-to 4 MAC addresses Sequence Control: fragmentation and sequence number. Data: higher layer data that is maximum 2312 bytes. CRC: 32 bit cyclic redundancy check for detecting error on the frame. CS 515 © Ibrahim Korpeoglu

Mobility What happens when a station moves between access points Re-association function of the PHY layer associates a mobile with a new access point. Some vendor specific, layer-2 (datalink layer) solutions solves the mobility at layer. Solutions like Mobile IP needed to provide seamless mobility to higher layers (transport and application layers). DHCP is also a method but not as convenient as Mobile IP. We will see in the forthcoming classes how Mobile IP works. CS 515 © Ibrahim Korpeoglu