This document discusses medium access control methods for wireless communication networks. It describes challenges with applying carrier sensing and collision detection from wired networks to wireless. It also covers hidden and exposed terminals and introduces basic multiple access methods like FDMA, TDMA, CDMA. Dynamic access methods like Aloha, slotted Aloha, polling and demand assigned multiple access are summarized.
This document discusses medium access control methods for wireless communication networks. It describes challenges with applying carrier sensing and collision detection from wired networks to wireless. It also covers hidden and exposed terminals and introduces basic multiple access methods like FDMA, TDMA, CDMA. Dynamic access methods like Aloha, slotted Aloha, polling and demand assigned multiple access are summarized.
This document discusses medium access control methods for wireless communication networks. It describes challenges with applying carrier sensing and collision detection from wired networks to wireless. It also covers hidden and exposed terminals and introduces basic multiple access methods like FDMA, TDMA, CDMA. Dynamic access methods like Aloha, slotted Aloha, polling and demand assigned multiple access are summarized.
This document discusses medium access control methods for wireless communication networks. It describes challenges with applying carrier sensing and collision detection from wired networks to wireless. It also covers hidden and exposed terminals and introduces basic multiple access methods like FDMA, TDMA, CDMA. Dynamic access methods like Aloha, slotted Aloha, polling and demand assigned multiple access are summarized.
1 Motivation • Can we apply media access methods from fixed networks?
• Example CSMA/CD – Carrier Sense Multiple Access with Collision Detection – send as soon as the medium is free, listen into the medium if a collision occurs (legacy method in IEEE 802.3) • Problems in wireless networks – signal strength decreases proportional to the square of the distance – the sender would apply CS and CD, but the collisions happen at the receiver – it might be the case that a sender cannot “hear” the collision, i.e., CD does not work – furthermore, CS might not work if, e.g., a terminal is “hidden”
9/10/2023 2 Hidden and exposed terminals • Hidden terminals – A sends to B, C cannot receive A – C wants to send to B, C senses a “free” medium (CS fails) – collision at B, A cannot receive the collision (CD fails) – A is “hidden” for C
• Exposed terminals A B C – B sends to A, C wants to send to another terminal (not A or B) – C has to wait, CS signals a medium in use – but A is outside the radio range of C, therefore waiting is not necessary – C is “exposed” to B 9/10/2023 3 Near and far terminals • Terminals A and B send, C receives – signal strength decreases proportional to the square of the distance – the signal of terminal B therefore drowns out A’s signal – 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!
9/10/2023 5 Fixed Access methods • SDMA (Space Division Multiple Access) – segment space into sectors, use directed antennas – cell structure • FDMA (Frequency Division Multiple Access) – assign a certain frequency to a transmission channel between a sender and a receiver – permanent (e.g., radio broadcast), slow hopping (e.g., GSM), fast hopping (FHSS, Frequency Hopping Spread Spectrum) • TDMA (Time Division Multiple Access) – assign the fixed sending frequency to a transmission channel between a sender and a receiver for a certain amount of time
9/10/2023 6 CDMA • CDMA (Code Division Multiple Access) – all terminals send on the same frequency probably at the same time and can use the whole bandwidth of the transmission channel • Disadvantages: – higher complexity of a receiver (receiver cannot just listen into the medium and start receiving if there is a signal) – all signals should have the same strength at a receiver • Advantages: – all terminals can use the same frequency, no planning needed – huge code space (e.g. 232) compared to frequency space – interferences (e.g. white noise) is not coded – forward error correction and encryption can be easily integrated
9/10/2023 7 Comparison 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 km² 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
9/10/2023 8 Dynamic access Advantages • Bandwidth utilization when demand is low • No coordination required
Disadvantages • Overhead at high demand • More difficult to ensure fairness
Q: What are the basic strategies for doing this?
9/10/2023 9 Classes of strategies • Dynamic channel access
• Dynamic reservation requests
– polling – demand assignment protocols
9/10/2023 10 Aloha/slotted aloha • Mechanism – random, distributed (no central arbiter), time-multiplex – Slotted Aloha additionally uses time-slots, sending must always start at slot boundaries • Aloha collision
sender A sender B sender C t
• Slotted Aloha collision
sender A sender B sender C t 9/10/2023 11 Polling mechanisms • If one terminal can be heard by all others, this “central” terminal (a.k.a. base station) can poll all other terminals according to a certain scheme – now all schemes known from fixed networks can be used (typical mainframe - terminal scenario) • Example: Randomly Addressed Polling – base station signals readiness to all mobile terminals – terminals ready to send can now transmit a random number without collision with the help of CDMA or FDMA (the random number can be seen as dynamic address) – the base station now chooses one address for polling from the list of all random numbers (collision if two terminals choose the same address) – the base station acknowledges correct packets and continues polling the next terminal – this cycle starts again after polling all terminals of the list
9/10/2023 12 DAMA - Demand Assigned Multiple Access • Channel efficiency only 18% for Aloha, 36% for Slotted Aloha (assuming Poisson distribution for packet arrival and packet length)
• Reservation can increase efficiency to 80%
– a sender reserves a future time-slot – sending within this reserved time-slot is possible without collision – reservation also causes higher delays – typical scheme for satellite links
• Examples for reservation algorithms:
– Explicit Reservation according to Roberts (Reservation-ALOHA) – Implicit Reservation (PRMA) – Reservation-TDMA 9/10/2023 13 DAMA: Explicit Reservation • Explicit Reservation (Reservation Aloha): – two modes: • ALOHA mode for reservation: competition for small reservation slots, collisions possible • reserved mode for data transmission within successful reserved slots (no collisions possible)
– All stations must keep the consistent reservation list at any
point in time; therefore, all stations have to synchronize from time to time collision
t Aloha reserved Aloha reserved Aloha reserved Aloha
9/10/2023 14 Access method DAMA: PRMA • Implicit reservation (PRMA - Packet Reservation MA): – a certain number of slots form a frame, frames are repeated – stations compete for empty slots according to the slotted aloha principle – once a station reserves a slot successfully, this slot is automatically assigned to this station in all following frames as long as the station has data to send – competition for this slots starts again as soon as the slot was empty in the last frame
reservation 1 2 3 4 5 6 7 8 time-slot ACDABA-F frame1 A C D A B A F ACDABA-F frame2 A C A B A AC-ABAF- frame3 A B A F collision at reservation A---BAFD frame4 A B A F D attempts ACEEBAFD frame5 A C E E B A F D t 9/10/2023 15 DAMA: Reservation-TDMA • Reservation Time Division Multiple Access – every frame consists of N mini-slots and x data-slots – every station has its own mini-slot and can reserve up to k data- slots using this mini-slot (i.e. x = N * k). – other stations can send data in unused data-slots according to a round-robin sending scheme (best-effort traffic)
e.g. N=6, k=2
N mini-slots N * k data-slots
reservations other stations can use free data-slots
for data-slots based on a round-robin scheme
9/10/2023 16 MACA - collision avoidance • MACA (Multiple Access with Collision Avoidance) uses short signaling packets for collision avoidance – RTS (request to send): a sender request the right to send from a receiver with a short RTS packet before it sends a data packet – CTS (clear to send): the receiver grants the right to send as soon as it is ready to receive
• Signaling packets contain
– sender address – receiver address – packet size
• Variants of this method can be found in IEEE802.11
as DFWMAC (Distributed Foundation Wireless MAC) 9/10/2023 17 MACA examples • MACA avoids the problem of hidden terminals – A and C want to send to B – A sends RTS first – C waits after receiving RTS CTS from B CTS CTS A B C
• MACA avoids the problem of exposed terminals
– B wants to send to A, C to another terminal RTS RTS – now C does not have to wait for it cannot CTS receive CTS from A A B C
9/10/2023 18 MACA variant: DFWMAC in IEEE802.11 sender receiver
idle idle packet ready to send; RTS data; ACK RxBusy time-out; wait for the RTS RTS; time-out CTS ACK right to send data; time-out NAK NAK; RTS CTS; data wait for wait for ACK data
9/10/2023 19 ISMA (Inhibit Sense Multiple Access) • Current state of the medium is signaled via a “busy tone” – the base station signals on the downlink (base station to terminals) if the medium is free or not – terminals must not send if the medium is busy – terminals can access the medium as soon as the busy tone stops – the base station signals collisions and successful transmissions via the busy tone and acknowledgements, respectively (media access is not coordinated within this approach) – mechanism used, e.g., for CDPD (USA, integrated into AMPS)
