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Lecture 4 Wireless Medium Access Control Prof. Shamik Sengupta Office 4210 N [email protected] http://jjcweb.jjay.cuny.edu/ssengupta/ Fall 2010 Medium Access Control (MAC) Base Station Forward link Reverse link Mobile Station Mobile Station Mobile Station Mobile Station Earlier MAC Protocols: A quick overview Channel Partitioning: TDMA, FDMA – divide channel into “pieces” (time slots, frequency) – allocate piece to node for exclusive use A B f0 CB ACB ACB ACB A C Time Channel Partitioning: adv., disadv. – Share channel efficiently at high load – inefficient at low load: delay in channel access, 1/N bandwidth allocated even if only 1 active node! C C B B A f2 A f1 f0 Time Earlier MAC Protocols: A quick overview Packet Radio (PR) Access Technique: – Users attempt to access a single channel in an uncoordinated or random manner Random Access: Aloha, Slotted Aloha – allow collisions – “recover” from collisions Random access MAC protocols – efficient at low load: single node can fully utilize channel – high load: collision overhead Pure (unslotted) ALOHA Devised by Norman Abramson and his colleagues – University of Hawaii Simple, no synchronization when frame first arrives – transmit immediately collision probability increases: – frame sent at t0 collides with other frames sent in [t0-1,t0+1] Pure Aloha efficiency What is the efficiency? Slotted ALOHA Assumptions: all frames same size time divided into equal size slots (time to transmit 1 frame) nodes start to transmit only slot beginning nodes are synchronized if 2 or more nodes transmit in slot, all nodes detect collision Operation: when node obtains fresh frame, transmits in next slot – if no collision: node can send new frame in next slot – if collision: node retransmits frame in each subsequent slot with prob. p until success Slotted ALOHA Pros single active node can continuously transmit at full rate of channel highly decentralized: only slots in nodes need to be in sync simple Cons collisions, wasting slots idle slots nodes may be able to detect collision in less than time to transmit packet clock synchronization Slotted Aloha efficiency Efficiency : 37% At best: channel used for useful transmissions 37% of time! ! 5: DataLink Layer 5-9 Why Aloha protocols were disadvantageous? Aloha protocols do not listen to the channel before transmission – Do not exploit info about other users Listening to the channel if any user is transmitting is key to the efficient wireless access – This was the basic of CSMA protocols – Carrier Sense Multiple Access Protocol Carrier Sense Multiple Access (CSMA) Protocol Two imp parameters in CSMA – Detection delay – Propagation delay Detection delay – A function of the receiver hardware – Time reqd for a terminal to sense whether or not the channel is idle Propagation delay – Relative measure of how fast a packet travels from one station to another station (BS or AP) – Systems must be built taking this parameter significantly in account – High propagation delay impact efficiency – E.g., two extreme transmitting users may get into collision again and again due to high propagation delay Variations of CSMA 1-persistent CSMA – Listens to the channel, if idle transmit p-persistent CSMA – Listens to the channel, if idle, transmit with prob p in the first slot or (1-p) in the next slot CSMA/CD – Further improvement over earlier CSMA – Not only listens to channel before transmissions but also during transmissions – If collision is detected, transmissions are aborted immediately – Saves valuable resources from wastage – Combines “listen before talk” and “listen while talk” – Happens in Ethernet (because of full-duplex radios) CSMA in wireless The concept of CSMA/CD is interesting – How about applying it in wireless medium access control? 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 – a sender cannot “hear” the collision at the same time of transmission, because transmission power suppresses receiving power – i.e., CD does not work – furthermore, CS might not work if, e.g., a terminal is “hidden” Wireless MAC use variants of CSMA – CSMA/CA (collision avoidance protocol) – Does not make collision zero, just tries to reduce it – Very popular in IEEE 802.11 (WLAN) IEEE802.11 infrastructure network AP AP ad-hoc network wired network AP: Access Point AP 802.11 infrastructure mode Station (STA) 802.11 LAN 802.x LAN – terminal with access mechanisms to the wireless medium and radio contact to the access point Basic Service Set (BSS) STA1 BSS1 Portal Access Point Distribution System – station integrated into the wireless LAN and the distribution system – bridge to other (wired) networks Distribution System BSS2 STA2 Access Point Portal Access Point ESS – group of stations using the same radio frequency 802.11 LAN STA3 – interconnection network to form one logical network (ESS: Extended Service Set) based on several BSS 802.11: ad-hoc mode 802.11 LAN STA1 STA3 BSS1 – Station (STA): terminal with access mechanisms to the wireless medium – Basic Service Set (BSS): group of stations in range and using the same radio frequency STA2 BSS2 STA5 STA4 Direct communication within a limited range 802.11 LAN IEEE standard 802.11 fixed terminal mobile terminal server infrastructure network access point application application TCP TCP IP IP LLC LLC LLC 802.11 MAC 802.11 MAC 802.3 MAC 802.3 MAC 802.11 PHY 802.11 PHY 802.3 PHY 802.3 PHY How does the medium access work in WLAN? Contention Based Distributed Coordination Function (DCF) Contention Free Point Coordination Function (PCF) Access methods – DCF CSMA/CA (mandatory) – collision avoidance via exponential backoff – Minimum distance (IFS) between consecutive packets – ACK packet for acknowledgements (not for broadcasts) – DCF with RTS/CTS (optional) – Distributed Foundation Wireless MAC – avoids hidden terminal problem – PCF (optional) – access point polls terminals according to a list 802.11 – MAC Priorities – defined through different inter frame spaces – SIFS (Short Inter Frame Spacing) – highest priority, for ACK, CTS, polling response – PIFS (PCF IFS) – medium priority, for time-bounded service using PCF – DIFS (DCF, Distributed Coordination Function IFS) – lowest priority, for asynchronous data service, competing stations DIFS DIFS medium busy PIFS SIFS direct access if medium is free DIFS contention next frame t WLAN CSMA/CA access method DIFS DIFS medium busy direct access if medium is free DIFS t slot time starts sensing the medium (Carrier Sense) If the medium is free for the duration of an Inter-Frame Space (IFS), the station can start sending (IFS depends on service type) If the medium is busy, the station has to wait for a free IFS, then the station must additionally wait a random back-off time – next frame Station ready to send – contention window (randomized back-off mechanism) collision avoidance, multiple of slot-time If another station occupies the medium during the back-off time of the station, the back-off timer freezes WLAN access scheme details Sending unicast packets – station has to wait for DIFS before sending data – receivers acknowledge at once (after waiting for SIFS) if the packet was received correctly (CRC) – automatic retransmission of data packets in case of transmission errors DIFS sender data SIFS receiver ACK DIFS other stations waiting time data t contention Contention for channel When the other stations find the channel idle, they would like to transmit their own packets – Contention for channel If all the waiting stations attempt at once, this will surely result in collision – Some CA scheme is necessary – Backoff intervals can be used to reduce collision probability Backoff Interval When transmitting a packet, choose a backoff interval in the range [0,cw] – cw is contention window Count down the backoff interval when medium is idle – Count-down is suspended if medium becomes busy When backoff interval reaches 0, transmit packet B1 = 25 B1 = 5 wait data data B2 = 20 Assume cw = 31 wait B2 = 15 B2 = 10 B1 and B2 are backoff intervals at nodes 1 and 2 Backoff Interval The time spent counting down backoff intervals is a part of MAC overhead – Choosing a large cw leads to large backoff intervals and can result in larger overhead – Choosing a small cw leads to a larger number of collisions (when two nodes count down to 0 simultaneously) Since the number of nodes attempting to transmit simultaneously may change with time, some mechanism to manage contention is needed – IEEE 802.11 DCF: contention window cw is chosen dynamically depending on collision occurrence – Follows Binary exponential backoff algorithm Binary Exponential Backoff (BEB) in DCF Even before the first collision, nodes follow BEB Initial backoff interval (before 1st collision) – [0,7] If still packets collide, double the collision interval – [0,15], [0,31] and so on… Express this binary exponential backoff interval as a function of collision number Numerical example #1 Two nodes, A and C both waiting for a busy channel to be idle so that they can proceed with their first transmission. After the channel becomes idle, what is the probability of A and C colliding in their first transmissions? Numerical example #2 Two nodes, X and Y intend to transmit frames of 10 and 5 timeslots. Initially after waiting for DIFS, X and Y both generate random backoff number, rX and rY as 2. In the next stage, X generates rX =1 and Y generates rY =3. What will be the time (slots) taken to complete both transmissions and receive acks? – Assume, SIFS=1 timeslot, DIFS=2 timeslots Avoiding collisions (more) idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets to BS using CSMA – RTSs may still collide with each other (but they’re short) BS broadcasts clear-to-send CTS in response to RTS CTS heard by all nodes – sender transmits data frame – other stations defer transmissions avoid data frame collisions completely using small reservation packets! Collision Avoidance: RTS-CTS exchange B A AP reservation collision DATA (A) defer time 802.11 access scheme details – RTS/CTS Sending unicast packets – station can send RTS with reservation parameter after waiting for DIFS (reservation determines amount of time the data packet needs the medium) – ack via CTS after SIFS by receiver (if ready to receive) – sender can now send data at once, acknowledgement via ACK – other stations store reservations distributed via RTS and CTS DIFS sender RTS data SIFS receiver other stations CTS SIFS SIFS NAV (RTS) NAV (CTS) defer access ACK DIFS data t contention 802.11 Steps – RTS/CTS All backlogged nodes choose a random number, R Each node counts down R – Continue carrier sensing while counting down – Once carrier busy, freeze countdown Whoever reaches ZERO transmits RTS – Neighbors freeze countdown, decode RTS – RTS contains (CTS + DATA + ACK) duration = T_comm – Neighbors set NAV = T_comm – Remains silent for NAV time 31 802.11 Steps – RTS/CTS Receiver replies with CTS – Also contains (DATA + ACK) duration. – Neighbors update NAV again Tx sends DATA, Rx acknowledges with ACK – After ACK, everyone initiates remaining countdown – Tx chooses new R = rand (0, CW) If RTS or DATA collides (i.e., no CTS/ACK returns) – Indicates collision – RTS chooses new random no. following BEB 32 Numerical example #3 Two nodes, X and Y intend to transmit frames of 10 and 5 timeslots. Initially after waiting for DIFS, X and Y both generate random backoff number, rX and rY as 2. In the next stage, X generates rX =1 and Y generates rY =3. What will be the time (slots) taken to complete both transmissions and receive acks? – Assume, SIFS=1 timeslot, DIFS=2 timeslots – RTS threshold = 8. Another special access – with Fragmentation DIFS sender RTS frag1 SIFS receiver CTS SIFS frag2 SIFS ACK1 SIFS SIFS ACK2 NAV (RTS) NAV (CTS) other stations NAV (frag1) NAV (ACK1) DIFS contention data t Point Coordination Function t0 t1 medium busy PIFS point coordinator wireless stations stations‘ NAV SuperFrame SIFS D1 SIFS SIFS D2 SIFS U1 U2 NAV Point Coordination Function t2 point coordinator wireless stations stations‘ NAV D3 PIFS SIFS D4 t3 t4 CFend SIFS U4 NAV contention free period contention period t