Mac

MAC layer Taekyoung Kwon

Media access in wireless - start with IEEE 802.11 • In wired link, – Carrier Sense Multiple Access with Collision Detection – send as soon as the medium is free, listen into the medium if a collision occurs (original method in IEEE 802.3)

• In wireless – Signal strength decreases in proportional to at least square of the distance – Collision detection only at receiver – Half-duplex mode – Furthermore, CS is not possible after propagation range

Hidden terminal problem • 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

A

B

C

Exposed terminal problem • Exposed terminals – B sends to A, C wants to send to D – C has to wait, CS signals a medium in use – but A is outside the radio range of C, thus waiting is not necessary – C is “exposed” to B A

B

C

D

Multiple access methods • FDMA • TDMA • CDMA • SDMA

ALOHA, Slotted-ALOHA • Mechanism – random, distributed (no central arbiter), timemultiplex – Slotted Aloha additionally uses time-slots, sending must always start at slotcollision boundaries

• Aloha sender A sender B sender C t

• Slotted Aloha

collision

sender A sender B sender C t

MACA (multiple access 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

• aka, virtual carrier sense

MACA operation • MACA avoids the problem of hidden terminals – A and C want to send to B – A sends RTS first – C waits after receiving CTS from B

RTS

RTS CTS

CTS B

A

C

• MACA avoids the problem of exposed terminals – B wants to send to A, C to another terminal – now C does not have to wait for it cannot receive CTS from A

RTS CTS

RTS

CTS A

B

C

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 reservation was empty in the 1 last 2 3 frame 4 5 6 7 8 time-slot ACDABA-F

ACDABA-F AC-ABAFA---BAFD ACEEBAFD

frame1 A C D A B A frame2 A C

F

A B A

frame3 A

B A F

frame4 A

B A F D

frame5 A C E E B A F D

collision at reservation attempts t

IEEE 802.11 wireless LAN infrastructure network AP AP

wired network

ad-hoc network

AP: Access Point

802.11 infrastructure mode •Station (STA) 802.11 LAN

STA1

802.x LAN

– terminal with access mechanisms to the wireless medium and radio contact to the access point

•Basic Service Set (BSS) BSS1 Portal

Access Point

•Access Point

Distribution System Access Point

ESS

– station integrated into the wireless LAN and the distribution system

•Portal – bridge to other (wired) networks

BSS2

STA2

– group of stations using the same radio frequency

•Distribution System

802.11 LAN

STA3

– interconnection network to form one logical network (ESS: Extended Service Set) based on several BSS

802.11 MAC requirements

802.11 MAC • Traffic services – Asynchronous Data Service (mandatory) • exchange of data packets based on “best-effort” • support of broadcast and multicast

– Time-Bounded Service (optional) • implemented using PCF (Point Coordination Function)

• Access methods – DFWMAC-DCF CSMA/CA (mandatory) • collision avoidance via randomized back-off mechanism • minimum distance between consecutive packets • ACK packet for acknowledgements (not for broadcasts)

– DFWMAC-DCF w/ RTS/CTS (optional) • Distributed Foundation Wireless MAC • avoids hidden terminal problem

– DFWMAC- PCF (optional) • access point polls terminals according to a list

802.11 layers • PLCP

Physical Layer Convergence

Protocol

• MAC – access mechanisms, fragmentation, encryption

• MAC Management – synchronization, roaming, MIB, power management

– clear channel assessment signal (carrier sense)

• PMD

Physical Medium Dependent

– modulation, coding

• PHY Management – channel selection, MIB

• Station Management – coordination of all management functions

Infrastructure mode fixed terminal mobile terminal

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

802.11 MAC • Priorities – defined through different inter frame spaces – no guaranteed, hard priorities – 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 DIFS

DIFS PIFS medium busy direct access if medium is free ≥ DIFS

SIFS

contention

next frame t

802.11 CSMA/CA DIFS

DIFS

contention window (randomized back-off mechanism)

medium busy direct access if medium is free ≥ DIFS

next frame t slot time

– station ready to send starts sensing the medium (Carrier Sense based on CCA, Clear Channel Assessment) – 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 backoff time of the station, the back-off timer stops (fairness)

802.11 CSMA/CA: contention resolution DIFS

DIFS

station1 station2

DIFS boe

bor

boe

busy

DIFS boe bor

boe

boe busy

boe bor

boe

boe

busy

busy

station3 station4

boe bor

station5

busy

bor t

busy

medium not idle (frame, ack etc.)

boe elapsed backoff time

packet arrival at MAC

bor residual backoff time

802.11 CSMA/CA: detailed • 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

802.11: 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) – acknowledgement via CTS after SIFS by receiver (if ready to receive) – sender can now send data at once, acknowledgement via ACK – other stations store medium reservations distributed via RTS andDIFS CTS sender

RTS

data

SIFS receiver

other stations

CTS SIFS

SIFS

NAV (RTS) NAV (CTS) defer access

ACK

DIFS

data t

contention

802.11 beaconing (infrastructure mode) beacon interval

access point medium

B

B busy

busy

B busy

B busy t

value of the timestamp

B

beacon frame

802.11 beaconing (ad hoc mode) beacon interval

station1

B1

B1 B2

station2 medium

busy

busy

B2 busy

busy t

value of the timestamp

B

beacon frame

random delay