Modern Networking Concepts: An Introduction to Wireless LANs An Introduction to Wireless LANs Outline Wireless LAN architecture Medium access principle

Modern Networking Concepts: An Introduction to Wireless LANs

An Introduction to Wireless LANs

Outline Wireless LAN architecture Medium access principle 802.11 MAC control 802.11 MAC management

Lecturer: Lei Guo, Yahoo!


Wireless LAN Architecture


Comparison: infrastructure vs. ad-hoc networks

infrastructure network

ad-hoc network

APAP

AP

wired network

AP: Access Point


IEEE 802.11 - Architecture of an infrastructure network

Basic Service Set (BSS) group of stations using the same

radio channel

Station (STA) terminal with access mechanisms

to the wireless medium and radio contact to the access point

Access Point station integrated into the wireless

LAN and the distribution system

Portal bridge to other (wired) networks

Distribution System interconnection network to form

one logical network (EES: Extended Service Set) based on several BSS

Distribution System

Portal

802.x LAN

Access Point

802.11 LAN

BSS2

802.11 LAN

BSS1

Access Point

STA1

STA2 STA3

ESS


Your neighbor

Wireless router in your home

NAT Router

www.yahoo.com

Internet

Cable modem

Wireless router

AP

ESS1/BSS1

portal

ESS2/BSS2

channel 1

channel 6


802.11 - Architecture of an ad-hoc network

Direct communication within a limited range

Station (STA):terminal with access mechanisms to the wireless medium

Independent Basic Service Set (IBSS):group of stations using the same radio frequency

ad-hoc routing:

possible but not specified in IEEE 802.11

802.11 LAN

IBSS2

802.11 LAN

IBSS1

STA1

STA4

STA5

STA2

STA3


IEEE standard 802.11: protocol stack

mobile terminal

access point

fixedterminal

application

TCP

802.11 PHY

802.11 MAC

IP

802.3 MAC

802.3 PHY

application

TCP

802.3 PHY

802.3 MAC

IP

802.11 MAC

802.11 PHY

LLC

infrastructurenetwork

LLC LLC

LLC is same for all 802.x


802.11 - Layers and functions

PLCP Physical Layer Convergence Protocol

clear channel assessment signal (carrier sense)

PMD Physical Medium Dependent

modulation, coding

PHY Management channel selection, MIB

Station Management coordination of all management

functions

PMD

PLCP

MAC

LLC

MAC Management

PHY Management

MAC control access mechanisms, fragmentation,

encryption

MAC Management synchronization, roaming, MIB,

power management

PH

YD

LC

Sta

tion

Man

agem

ent

See video (in reference 4) for physical layer technologies


Medium Access Principle


Motivation

Can we apply media access methods from fixed networks?

Ethernet: 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 (original method in IEEE 802.3) stop when collision is detected

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”


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), neither can C A is “hidden” for C,

and vice versa

Exposed terminals B sends to A, C wants to send to another terminal D 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, and vice versa

Hidden and exposed terminals

BA C D


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 after RTS/CTS exchange, medium is reserved

Signaling packets contain sender address receiver address packet size

RTS/CTS collision probability is small, since RTS/CTS packets are very short


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

MACA avoids the problem of exposed terminals B wants to send to A, C

to another terminal D B sends RTS, and A

replies CTS now C does not have to wait,

for it cannot receive CTS from A

MACA examples

A B C

RTS

CTSCTS

A B C

RTS

CTS

RTS

D


Polling mechanisms

If one terminal can be heard by all others, this “central” terminal (e.g., AP) can poll all other terminals according to a certain scheme, e.g., round-robin polling

Polling is contention free, no collision

802.11 Infrastructure WLAN PCF (Point Coordination Function)

polling to avoid collision DCF (Distributed Coordination Function)

CSMA/CA with RTS/CTS to resolve hidden terminal problem no exposed terminal problem in infrastructure WLAN (why?)


802.11 Wireless LANs: MAC Control


802.11 - MAC layer I - DFWMAC

Traffic services Asynchronous Data Service (infrastructure/ad-hoc, mandatory)

station sends packets with “best-effort”, no upper bound for delay support of broadcast and multicast implemented using DCF (Distributed Coordination Function)

Time-Bounded Service (infrastructure only, optional) implemented using PCF (Point Coordination Function) with polling

DFWMAC (Distributed Foundation Wireless MAC) DFWMAC-DCF CSMA/CA (mandatory)

collision avoidance via randomized “back-off” mechanism reliable delivery with packet ACK (for unicast only)

DFWMAC-DCF w/ RTS/CTS (optional) based on CSMA/CA, avoids hidden terminal problem optional, but supported by most commodity products

DFWMAC-PCF (optional) AP polls terminals according to a list, contention free optional, not supported by most commodity products


802.11 - MAC layer II

Frame types data frame: normal data packets control frame: ACK, RTS, CTS management frame: beacon, probe, association, authentication, …

Priorities for different frames defined through different IFS (Inter Frame Spacing) duration SIFS (Short IFS): highest priority, for ACK, CTS, polling response, … PIFS (PCF IFS): medium priority, for time-bounded service (PCF) DIFS (DCF IFS): lowest priority, for asynchronous data service (DCF) no guaranteed, hard priorities

t

medium busySIFS

PIFS

DIFSDIFS

next framecontention

direct access if medium is free DIFS

random


t

medium busy

DIFSDIFS

next frame

contention window(randomized back-offmechanism)

802.11 - CSMA/CA: sending unicast data frame

when has data to send: starts sensing the medium (Carrier Sense) if the medium is free for the duration of DIFS, starts sending if the medium is busy, wait for a free DIFS, then do back-off:

wait a random back-off time (contention window) to avoid collision if the medium is busy again during the back-off time: stop back-off,

wait for a free DIFS, then continue back-off (the contention window is set to the residual back-off time)

if collision: stop back-off, wait for a free DIFS, then redo back-off by doubling contention window size (until maximum), re-send data

direct access if medium is free DIFS total waiting time


802.11 - competing stations - simple version

t

busy

boe

station1

station2

station3

station4

station5

packet arrival at MAC

DIFSboe

boe

boe

busy

elapsed backoff time

bor residual backoff time

busy medium not idle (frame, ack etc.)

bor

bor

DIFS

boe

boe

boe bor

DIFS

busy

busy

DIFSboe busy

boe

boe

bor

bor


802.11 - CSMA/CA: data frame acknowledgment

Sending a unicast packet: complete procedure station has to wait for DIFS before sending data receiver send ACK at once if the packet was received correctly

ACK cannot be delayed since it just wait a SIFS automatic retransmission of data packets in case of transmission errors Compared with CSMA/CD (Ethernet)

no collision detection (Ethernet has CD) use ACK to confirm delivery except multicast/broadcast (Ethernet no ACK)

t

SIFS

DIFS

data

ACK

waiting time

otherstations

receiver

senderdata

DIFS

contention

atomic operation


802.11 – DFWMAC with RTS/CTS

Sending unicast packets A sends RTS with net allocation vector (NAV, amount of time to use the

medium) after waiting for DIFS, to reserve medium B replies CTS after SIFS (if ready to receive) A sends data to B, B replies ACK other stations know medium reservation through NAV in RTS and CTS

t

SIFS

DIFS

data

ACK

defer access

receiver

senderdata

DIFS

contention

RTS

CTSSIFS SIFS

NAV (RTS)NAV (CTS)other

stations

A

B

C

atomic operation


802.11 Wireless LANs: MAC Management


802.11 - MAC management

Synchronization synchronize internal clocks functions to support finding a WLAN

Power management sleep-mode without missing a message periodic sleep, frame buffering, traffic measurements

Roaming scanning changing access points joining a network

MIB - Management Information Base managing, read, write


Synchronization using a Beacon (infrastructure)

beacon interval

tmedium

accesspoint

busy

B

busy busy busy

B B B

value of the timestamp B beacon frame

each station maintains an internal clock

synchronized clocks are needed for power management PCF coordination hopping sequence of radio frequency in physical layer (see video)

how to synchronize: AP broadcasts quasi periodic beacon frames (100 ms)

synchronization in ad-hoc WLAN: more complex


Power management: motivation

Mobile devices are usually driven by battery powersWireless card consumes a great amount of energy in mobile devices

over 50% total energy for PDA up to 10% total energy for laptop

Power modes of wireless card

Transmit mode data transmission (sending) power consumption: high

Receive mode data receiving and listening power consumption: medium

Sleep mode power consumption: low

power consumption of a wireless card


802.11 power management mechanism

Idea: switch the transceiver off if not needed power saving mode: a station sleep and awake sender knows when to wake up for transmission receiver needs to know when to wake up for receiving

unicast in infrastructure WLAN AP buffers data for a station working in power saving mode AP periodically broadcasts with beacon a Traffic Indication Map (TIM),

indicating which station has data to receive station periodically wakes up to receive TIM station polls AP to receive data

broadcast/multicast in infrastructure WLAN similar but no buffer and no poll (broadcast/multicast has no ACK) needs another Traffic Indication Map (called Delivery TIM, DTIM) period of DTIM is multiple of TIM

ad-hoc WLAN more complex


Power saving with wake-up patterns (infrastructure)

TIM interval

t

medium

accesspoint

busy

D

busy busy busy

T T D

T TIM D DTIM

DTIM interval

BB

B broadcast/multicast

station

awake

p PS poll

p

d

d

d data transmissionto/from the station

t0 t1 t2 t3

before sleep: station notifies AP to have data buffered


802.11 – Roaming

Scan passive scan: listen into the medium for beacon signals active scan: send probes into the medium and wait for an answer

Reassociation station sends a Reassociation Request to new AP(s) new AP replies Reassociation Response

success: join the new AP failure: continue scanning

Handover new AP: signals the new station to the distribution system (DS) DS: updates data base (i.e., location information) old AP: sends buffered data to new AP through DS, release resources

Advantage seamless to end users, IP layer not affected TCP/IP socket not broken (roaming in an ESS)


IEEE 802.11 family

802.11: legacy (1997) 2.4 GHz 1, 2 Mbps

802.11a: not widely used (2001) 5-6 GHz range 6, 9, 12, 18, 24, 36, 48, and 54 Mbps

802.11b: widely used before (1999) 2.4 GHz 1, 2, 5.5, 11 Mbps

802.11g: most popular now (2003) 2.4 GHz 6, 9, 12, 18, 24, 36, 48, and 54 Mbps backwards compatible with b

802.11n: MIMO antenna (2010) up to 300 Mbps, 600 Mbps in the future backwards compatible with a, b, g product for 802.11n draft already on market

And others: 802.11 d, e, f, h, i, …


Summary: Key points

Architecture 802.11 protocol stack

Access principle of radio medium hidden terminal problem RTS/CTS, polling

802.11 MAC control DCF CSMA/CA DCF with RTS/CTS

802.11 MAC management synchronization (infrastructure) power saving mechanism (infrastructure)

References Basic: J. F. Kurose, K. W. Ross, Computer Networking: A Top-Down Approach

(4 ed.), Chapter 6 Advanced: J. Schiller, Mobile Communications (2 ed.), Chapter 7 Lab book: M. Gast, 802.11® Wireless Networks: The Definitive Guide, O'Reilly Physical layer video: http://public.yahoo.com/~lguo/download/cisco-aironet.zip,

click radio frequency technology to watch (optional)

Documents

Modern Networking Concepts: An Introduction to Wireless LANs An Introduction to Wireless LANs Outline Wireless LAN architecture Medium access principle