Lecture 4 Wireless Medium Access Control Prof Shamik

Lecture 4 Wireless Medium Access Control Prof. Shamik Sengupta Office 4210 N ssengupta@jjay. cuny. edu http: //jjcweb. jjay. cuny. edu/ssengupta/ Fall 2010

Medium Access Control (MAC) Base Station Forward link Reverse link 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 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! cy Time C C f 2 B B A qu en · f 0 CBACBA Fre C A Fre B qu en cy · A f 1 f 0 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 t 0 collides with other frames sent in [t 0 -1, t 0+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: Data. Link 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)

IEEE 802. 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) STA 1 BSS 1 Portal Access Point Distribution System – station integrated into the wireless LAN and the distribution system – bridge to other (wired) networks ·Distribution System BSS 2 STA 2 ·Access Point ·Portal Access Point ESS – group of stations using the same radio frequency 802. 11 LAN STA 3 – interconnection network to form one logical network (ESS: Extended Service Set) based on several BSS

802. 11: ad-hoc mode · 802. 11 LAN STA 1 STA 3 BSS 1 – 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 STA 2 BSS 2 STA 5 STA 4 Direct communication within a limited range 802. 11 LAN

IEEE standard 802. 11 fixed terminal mobile terminal server infrastructure network access point application TCP IP IP LLC LLC 802. 11 MAC 802. 3 MAC 802. 11 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 medium busy DIFS PIFS SIFS direct access if medium is free DIFS contention next frame t

WLAN CSMA/CA access method DIFS medium busy direct access if medium is free DIFS · contention window (randomized back-off mechanism) next frame t slot time Station ready to send – 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 – 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 B 1 = 25 B 1 = 5 wait data B 2 = 20 Assume cw = 31 wait B 2 = 15 B 2 = 10 B 1 and B 2 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 1 st 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, r. X and r. Y as 2. In the next stage, X generates r. X =1 and Y generates r. Y =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 RTS(B) RTS(A) reservation collision RTS(A) CTS(A) DATA (A) defer time ACK(A)

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 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, r. X and r. Y as 2. In the next stage, X generates r. X =1 and Y generates r. Y =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 frag 1 SIFS receiver CTS SIFS frag 2 SIFS ACK 1 SIFS ACK 2 NAV (RTS) NAV (CTS) other stations NAV (frag 1) NAV (ACK 1) DIFS contention data t

Point Coordination Function t 0 t 1 medium busy PIFS D 1 point SIFS coordinator wireless stations‘ NAV Super. Frame SIFS D 2 SIFS U 1 U 2 NAV

Point Coordination Function t 2 point coordinator wireless stations‘ NAV D 3 PIFS SIFS D 4 t 3 t 4 CFend SIFS U 4 NAV contention free period contention period t