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Medium Access Control 192620010 Mobile & Wireless Networking Lecture 3: Medium Access Control [Schiller, Chapter 3] [Wikipedia: "Hybrid Automatic Repeat Request"] Geert Heijenk Mobile and Wireless Networking 2013/ 2014 Outline of Lecture 3 q Medium Access Control q Motivation q Channel partitioning l CDMA l Duplexing q Taking turns q Random access l Aloha / Slotted Aloha l Reservation Aloha l Packet reservation multiple access l Reservation TDMA l CSMA – CSMA/CA – RTS/CTS l Random access and CDMA (UMTS) q Hybrid ARQ 2 Mobile and Wireless Networking 2013/ 2014 Motivation Can we apply media access methods from fixed networks? Example CSMA/CD q Carrier Sense Multiple Access with Collision Detection q send as soon as the medium is free, listen into the medium if a collision occurs (original method in IEEE 802.3) Problems in wireless networks q signal strength decreases proportional to the square of the distance q the sender would apply CS and CD, but the collisions happen at the receiver q it might be the case that a sender cannot “hear” the collision, i.e., CD does not work q furthermore, CS might not work if, e.g., a terminal is “hidden” 3 Mobile and Wireless Networking 2013/ 2014 Motivation - hidden and exposed terminals Hidden terminals q A sends to B, C cannot receive A q C wants to send to B, C senses a “free” medium (CS fails) q collision at B, A cannot receive the collision (CD fails) q A is “hidden” for C Exposed terminals A B C q B sends to A, C wants to send to another terminal (not A or B) q C has to wait, CS signals a medium in use q but A is outside the radio range of C, therefore waiting is not necessary q C is “exposed” to B 4 Mobile and Wireless Networking 2013/ 2014 Motivation - near and far terminals Terminals A and B send, C receives q signal strength decreases proportional to the square of the distance q the signal of terminal B therefore drowns out A’s signal q C cannot receive A A B C If C for example was an arbiter for sending rights, terminal B would drown out terminal A already on the physical layer Also severe problem for CDMA-networks - precise power control needed! 5 Mobile and Wireless Networking 2013/ 2014 Outline of Lecture 3 q Medium Access Control q Motivation q Channel partitioning l CDMA l Duplexing q Taking turns q Random access l Aloha / Slotted Aloha l Reservation Aloha l Packet reservation multiple access l Reservation TDMA l CSMA – CSMA/CA – RTS/CTS l Random access and CDMA (UMTS) q Hybrid ARQ 6 Mobile and Wireless Networking 2013/ 2014 Channel partitioning protocols q SDMA (Space Division Multiple Access) q segment space into sectors, use directed antennas q cell structure q FDMA (Frequency Division Multiple Access) q assign a certain frequency to a transmission channel between a sender and a receiver q permanent (e.g., radio broadcast), slow hopping (e.g., GSM), fast hopping (FHSS, Frequency Hopping Spread Spectrum) q special case: Orthogonal FDMA (OFDMA) q TDMA (Time Division Multiple Access) q assign the fixed sending frequency to a transmission channel between a sender and a receiver for a certain amount of time q CDMA (Code Division Multiple Access) q assign a code to a transmission channel between a sender and a receiver for a certain amount of time 7 Mobile and Wireless Networking 2013/ 2014 Code Division Multiple Access (CDMA) q all terminals send on the same frequency probably at the same time and can use the whole bandwidth of the transmission channel q each sender has a unique random number, a code, the sender XORs the data with this code q the receiver can “tune” into this signal if it knows the pseudo random number, tuning is done via a correlation function q different codes should be orthogonal q inner product should be 0 q ideally, code should have good autocorrelation q inner product with itself should be large, inner product with shifted version should be low q good for synchronization 8 Mobile and Wireless Networking 2013/ 2014 CDMA example In the following example let us suppose that a 0 is coded as a positive signal (+1), and a 1 as a negative signal (-1). Now we can represent the XOR operation as simple multiplication. 0 XOR 0 = 0 à 1 1 = 1 0 XOR 1 = 1 à 1 -1 = -1 1 XOR 0 = 1 à -1 1 = -1 1 XOR 1 = 0 à -1 -1 = 1 9 Mobile and Wireless Networking 2013/ 2014 CDMA encode/decode channel output Z . i,m Zi,m= di cm data d0 = 1 1 1 1 1 1 1 1 1 d1 = -1 bits - - sender -1 -1 1 -1 -1 -1 -1 1 slot 1 slot 0 code 1 1 1 1 1 1 1 1 -1 -1 -1 -1 -1 -1 -1 -1 channel channel output output slot 1 slot 0 M D = Z .c i Σm=1 i,m m M received 1 1 1 1 1 1 1 1 d0 = 1 input -1 -1 -1 -1 -1 -1 -1 -1 d1 = -1 slot 1 slot 0 code 1 1 1 1 1 1 1 1 -1 -1 -1 -1 -1 -1 -1 -1 channel channel output output receiver slot 1 slot 0 10 Mobile and Wireless Networking 2013/ 2014 CDMA: decoding with an interfering transmitter channel sums together transmissions by sender 1 and 2 Sender 1 Sender 2 using same code as sender 1, receiver recovers sender 1’s original data from summed channel data! 11 Mobile and Wireless Networking 2013/ 2014 CDMA Disadvantages: q higher complexity of a receiver (receiver cannot just listen into the medium and start receiving if there is a signal) q all signals should have the same strength at a receiver Advantages: q all terminals can use the same frequency, no planning needed q huge code space (e.g. 232) compared to frequency space q interferences (e.g. white noise) is not coded q forward error correction and encryption can be easily integrated 12 Mobile and Wireless Networking 2013/ 2014 Duplexing q Simultaneous transmission and reception of up and down-link channels q Time and frequency domain techniques: q FDD: Frequency division duplex q TDD: Time division duplex (Code division duplex would give an extreme near-far problem) 13 Mobile and Wireless Networking 2013/ 2014 FDD/FDMA - general scheme, example GSM f 960 MHz 124 935.2 MHz 1 200 kHz 20 MHz 915 MHz 124 890.2 MHz 1 t 14 Mobile and Wireless Networking 2013/ 2014 TDD/TDMA - general scheme, example DECT 417 µs 1 2 3 11 12 1 2 3 11 12 t downlink uplink 15 Mobile and Wireless Networking 2013/ 2014 Comparison SDMA/TDMA/FDMA/CDMA Approach SDMA TDMA FDMA CDMA Idea segment space into segment sending segment the spread the spectrum cells/sectors time into disjoint frequency band into using orthogonal codes time-slots, demand disjoint sub-bands driven or fixed patterns Terminals only one terminal can all terminals are every terminal has its all terminals can be active be active in one active for short own frequency, at the same place at the cell/one sector periods of time on uninterrupted same moment, the same frequency uninterrupted Signal cell structure, directed synchronization in filtering in the code plus special separation antennas the time domain frequency domain receivers Advantages very simple, increases established, fully simple, established, flexible, less frequency capacity per km? digital, flexible robust planning needed, soft handover Dis- inflexible, antennas guard space inflexible, complex receivers, needs advantages typically fixed needed (multipath frequencies are a more complicated power propagation), scarce resource control for senders synchronization difficult Comment only in combination standard in fixed typically combined still faces some problems, with TDMA, FDMA or networks, together with TDMA higher complexity, CDMA useful with FDMA/SDMA (frequency hopping lowered expectations; will used in many patterns) and SDMA be integrated with mobile networks (frequency reuse) TDMA/FDMA 16 Mobile and Wireless Networking 2013/ 2014 Outline of Lecture 3 q Medium Access Control q Motivation q Channel partitioning l CDMA l Duplexing q Taking turns q Random access l Aloha / Slotted Aloha l Reservation Aloha l Packet reservation multiple access l Reservation TDMA l CSMA – CSMA/CA – RTS/CTS l Random access and CDMA (UMTS) q Hybrid ARQ 17 Mobile and Wireless Networking 2013/ 2014 Taking turns protocols q If one terminal can be heard by all others, this “central” terminal (master or base station) can poll all other terminals according to a certain scheme q master-slave scheme q now all schemes known from fixed networks can be used (typical mainframe - terminal scenario) q round robin, random, reservation based q Used in Bluetooth, IEEE802.11 (option), LTE q Downlink from the master / base station (centralized) scheduling can be used. 18 Mobile and Wireless Networking 2013/ 2014 Outline of Lecture 3 q Medium Access Control q Motivation q Channel partitioning l CDMA l Duplexing q Taking turns q Random access l Aloha / Slotted Aloha l Reservation Aloha l Packet reservation multiple access l Reservation TDMA l CSMA – CSMA/CA – RTS/CTS l Random access and CDMA (UMTS) q Hybrid ARQ 19 Mobile and Wireless Networking 2013/ 2014 Aloha / Slotted Aloha Mechanism q random, distributed (no central arbiter), time-multiplex q no carrier sense, retransmission (after collision) with probability p q Slotted Aloha additionally uses time-slots, sending must always start at slot boundaries collision Aloha sender A sender B sender C t Slotted Aloha collision sender A sender B sender C t 20 Mobile and Wireless Networking 2013/ 2014 Reservations in dynamic TDM Channel efficiency only 18% for Aloha, 36% for Slotted Aloha (assuming Poisson distribution for packet arrivals) Reservation can increase efficiency to 80% q a sender reserves a future time-slot q sending within this reserved time-slot is possible without collision q reservation also causes higher delays q typical scheme for satellite links Examples for reservation algorithms: q Explicit Reservation (Reservation-ALOHA)
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