design aspects for wireless networks general structure of ... filechapter 2.8: wlan page 1 design...

Lehrstuhl für Informatik 4

Kommunikation und verteilte Systeme

Chapter 2.8: WLAN

Design Aspects for Wireless Networks (WLAN)

• World-wide standardization

• Low power consumption to enable battery operation for mobile devices• Usage is possible without special permission and/or licenses

• Robust transmission technology• Simplification of (spontaneous) cooperation at meetings

• Simple use and administration• Retain of former investments in the fixed network area

• Security regarding the tapping of confidential data and also regarding emissions

• Transparency regarding applications and protocols of higher layers



Page 2Chapter 2.8: WLAN

General Structure of Radio Networks

1. Networks with fixed infrastructure• Infrastructure means: stationary network e.g.

Ethernet or satellite routes

• Central Access Point (AP), wireless devices communicate only with the AP

• Control functionalities (media access, mobility management, authentication,…) are realized in the infrastructure

• Complexity lies in the infrastructure components, wireless devices only need to realize a minimum of functionality

2. Ad-hoc networks• No infrastructure – the wireless devices

are communicating directly

• Higher complexity of the devices, since every device has to implement all access and control mechanisms

InfrastructureL a p to pAP

APAPL a p to pL a p to p L a p to p L a p to p

LaptopLaptop

Laptop




Wireless Ethernet

• Wireless equivalent to Ethernet: “Wireless LAN” (WLAN)• Exclusively data-oriented, wide-band Internet access solution

• Standardized by the IEEE as IEEE 802.11� 1997: IEEE 802.11 (capacity of maximally 2 MBit/s)

� IEEE 802.11a with 54 MBit/s, use of a (more susceptible for disturbances) frequency band

� 1999: IEEE 802.11b (data rate of 11 MBit/s with a utilizable data rate of of up to 6-7 MBit/s)

� IEEE 802.11g: enhancement of 802.11b with up to 54 MBit/s� …

802.11• 1 or 2 MBit/s• 2.4 GHz• FHSS, DSSS

802.11a

• 54 MBit/s• 5 GHz• FHSS, DSSS

802.11b

• 11 MBit/s• 2.4 GHz• only DSSS

802.11g• 54 MBit/s• 2.4 GHz• only DSSS




WLAN: IEEE 802.11b

• Data rates– 1, 2, 5.5, 11 Mbit/s, depending on SNR (signal-to-noise ratio)– Utilizable data rate max. approx. 7 Mbit/s

• Range– 300m outside, 30m in buildings– Maximum data rate only can be used up to ~10m (in buildings)

• Frequency range– License-free 2,4 GHz ISM band (2,4 - 2,4835 GHz)

• Transmitting power– Maximally 100 mWatt

• Advantages:– Many installed systems, world-wide availability, free ISM-band, many

companies, integrated into laptops, simple system• Disadvantages:

– Strong disturbances on the ISM band (Bluetooth, microwave ovens,microwave lighting, analogue television, monitoring systems, license-free urban networks), no service quality, relatively low data rates




Structure of a WLAN

Integration into an existing fixed network:

Fixed networkLaptopAP

APAP

LaptopLaptop Laptop Laptop

• Access Points (APs) are attached to an existing fixed network

• Each AP manages all communication in its reception range

• APs using the same channel must have enough distance to avoid disturbances

LaptopLaptop

Laptop

Forming an Ad-hoc network:• If no AP is available, stations also can

build up an own LAN• The transmission now takes place directly

between the stations




APs in Aachen - MoPS




Architecture: Infrastructure Network

• Station (STA)

Computer with access mechanism to the wireless medium and by this radio connection to the AP

• Access Point (AP)

Station which is integrated both in the radio and the wired network (distribution system)

• Basic Service Set (BSS)

Group of stations incl. the AP within an AP transmission range

• Portal

Gateway to another fixed network• Distribution system

Connection of different AP areas to one logical network (EES: Extended service set). Simplest principle: switch

Distribution system

Portal

802.x LAN

AccessPoint

802.11 LAN

BSS2

802.11 LAN

BSS1

AccessPoint

STA1

STA2STA3

ESS




Architecture: Ad-hoc Network

• Direct communication within limited range

– Station (STA)Computer with access mechanism tothe wireless medium

– Independent Basic Service Set (IBSS)Group of stations which use the same carrier frequency within a transmission range

• Different IBSS are possible by spatial separation or by using different carrier frequencies

• No designated stations for the forwarding of data, routing,…

802.11 LAN

IBSS2

802.11 LAN

IBSS1

STA1

STA4

STA5

STA2

STA3




Protocol Architecture

Medium Access Control• Access mechanism, fragmenting, encryption

• MAC management: synchronization, roaming between APs, power management

Physical layer• Channel selection, modulation, coding

Applications should not be aware of theexistence of the wireless network (except capacity, longer access times)

802.11 MAC

802.11 PHY

IP

TCP

Application

802.3 MAC

802.3 PHY

IP

TCP

Application

802.11 MAC

802.11 PHY

802.3 MAC

802.3 PHY

Fixed Terminal

AccessPoint

Mobile Terminal

Infrastructure Network

Server




802.11 - Physical Layer

Variants for transmission: 2 radio (in 2,4 the GHz band), 1 infrared

• FHSS (Frequency Hopping Spread Spectrum)

– 2 frequencies at 1 Mbit/s, 4 frequencies at 2 Mbits/s– Frequency band divided into 79 different channels of 1 MHz bandwidth

– min. 2.5 frequency changes per second– GFSK modulation

– Max. transmission power: 1 W (USA)/100 mW (EU), minimum: 1 mW

• DSSS (Direct Sequence Spread Spectrum)

– DBPSK modulation for 1 Mbit/s (Differential Binary Phase Shift Keying), DQPSK for 2 Mbit/s (Differential Quadrature PSK)

– Chip sequence: (+1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1), a barker code

– Maximum transmitting power: 1 W (USA)/100 mW (EU), minimum: 1 mW

• Infrared

– 850-950nm, diffuse light, typically 10m range




FHSS PHY Frame Format

Synchronization SFD PLW PSF HEC Utilizable data

Preamble Header

80 16 12 4 16 variable Bits

• Synchronization

– Synchronization of the receiver by sequence 010101…• SFD (Start Frame Delimiter)

– 0000110010111101 as starting pattern• PLW (PLCP_PDU Length Word)

– Length of the utilizable data in bytes inclusive 32-bit CRC (at the end of the utilizable data). Permitted values are between 0 and 4095

• PSF (PLCP Signaling Field)

– Data rate of the utilizable data (1 or 2 Mbit/s)• HEC (Header Error Check)

– CRC with x16+x12+x5+1

Transmission always with 1 Mbit/s

Transmission alternatively with 1 or 2 Mbit/s




DSSS PHY Frame Format

Synchronization SFD Signal Service HEC Pay load

Preamble Header


Length

16

• Synchronization– Snychronization, power management, signal detection, frequency adjustment

• SFD (Start Frame Delimiter)– 1111001110100000 as starting pattern

• Signal– Data rate of the payload (0A: 1 Mbit/s DBPSK; 14: 2 Mbit/s DQPSK)

• Service

– Reserved for later use, standard: 00 for 802.11 frames

• Length (length of the utilizable data) and HEC (CRC) as for FHSS

Transmission always with 1 Mbit/s

Transmission alternatively with 1 or 2 Mbit/s




802.11b - Physical Layer

Achieved bits/symbol

Used Symbol Rate

ModulationCode lengthData Rate

81,375 MS/sQPSK8 (CCK)11 Mbit/s

41,375 MS/sQPSK8 (CCK)5,5 Mbit/s

21 MS/sQPSK11 (barker code)2 Mbit/s

11 MS/sPSK11 (barker code)1 Mbit/s

Dynamic Rate Shifting: Adjustment of the data rate to the transmission quality:

• Only DSSS

• CCK: Complementary Code KeyingUse of an 8-Chips spreading sequence: select 64 (11 Mbit/s) resp. 4 (5,5 Mbit/s) of the 48 possible states, which have as good cross correlation characteristics as possible. I.e.: use spreading for the transmission of several bits at the same time

Thus the transmission becomes clearly more susceptible for disturbances than for 1 resp. 2 Mbit/s




Range of IEEE 802.11

10 30 60 100 m0

2

4

6

8

10

Data rate

Mbit/s

Distance

802.11

802.11b

Due to missing spreading, the higher data transmission rates are more susceptible for disturbances. Thus, a smaller range results:




Range of 802.11b




IEEE 802.11b – PHY Frame Formats

synchronization SFD signal service HEC Utilizable data

Preamble Header


length

16

192 µs with 1 Mbit/s DBPSK 1, 2, 5.5 or 11 Mbit/s

short synch. SFD signal service HEC Utilizable data

Preamble(1 Mbit/s, DBPSK)

Header(2 Mbit/s, DQPSK)


length

16

96 µs 2, 5.5 or 11 Mbit/s

Long frame format:

Short frame format, optionally:




Communication Channels in IEEE 802.11

The whole 2.4GHz ISM band is divided into several channels. To avoid interference, distances must be left between the channels. To avoid collisions, when configuring an access point a channel is assigned to it.

• FHSS: The band is divided into 79 sub-bands, the channel number determines the hop sequence

• DSSS: The band is divided into 11 resp. 13 sub-bands, each of these forms an own channel. Signal spreading is performed in the sub-bands:

→One sub-band has a bandwidth of 22 MHz. The sent data are spread to those bandwidth to avoid environmental disturbances:

Channel n

22 MHz

Purpose: even if the frequency range is disturbed partly, enough of the signal power reaches the receiver. If only on one frequency transmission would take place, the whole data would be lost.

Signal is spread to 11 frequencies (when using a barker code)




Channel Selection with IEEE 802.11b

2400[MHz]

2412 2483.52442 2472

Channel 1 Channel 7 Channel 13

22 MHz

• Channels are 22 MHz wide• Channels are overlapping

• 13 channels in Germany (2412, 2417, 2422,… 2467, 2472 MHz), 11 in the USA/Canada

• Non overlapping channel selection:

• Ideally: assign only e.g. channels 1, 6 and 11: 116

1

611

1




Medium Access Control

Procedure for Ethernet: CSMA/CD• Send as soon as the medium is free, listen whether a collision took place

Problems in wireless networks• Signal strength decreases squarely with the distance• CS/CD are used by the sender, but collisions happen at the receiver

• CS can deliver wrong results, e.g. if a terminal is “hidden” (Hidden station problem)• Therefore, a collision possibly is not detectable by the sender, i.e. CD fails

BA C

• A sends to B, C does not receive signal from A

• C wants to send to B, medium is unused for C (CS failed)

• Collision at B, A does not detect it (CD failed)




Further Problems with CSMA/CD

Exposed station

• B sends to A, C wants to send to D• C must wait, because CS signals an

“occupied” medium

• Since A however is outside the range of C, C is blocked unnecessarily

BA C D

Terminals A and B send, C has to receive

• the signal strength weakens squarely with the distance, therefore the signal of B “drowns” the signal of A

• C cannot hear A

• Accurate efficiency control is necessary!A B C

Solution for the problems, especially hidden station:

CSMA/CA – CSMA with Collision Avoidance




Collision avoidance

Collision detection does not work. Therefore: Collision Avoidance

Multiple Access with Collision Avoidance (MACA)• Three-way handshake minimizes number of hidden terminals

• Signaling frames contain sender and receiver address as well as frame size• Sender sends a short Request to Send (RTS) frame• Receiver answers with Clear to Send (CTS) frame

• Sender sends data

Multiple Access with Collision Avoidance by Invitation (MACA-BI)• Sender needs “invitation” before transmitting data to the receiver• RTS is omitted, Ready to Receive (RTR) instead of CTS

• Less complex than MACA, since fewer signaling frames are sent• But: the receiver must be able to estimate the traffic volume of the sender




Avoidance of the hidden station problem:• A and C want to send to B

• A first sends RTS• C waits since it hears the CTS of B

RTS/CTS Handshake

A B C

RTS

CTSCTS

A B C

RTS

CTS

RTS

Avoidance of the exposed station problem:• B wants to send to A,

C wants to send to D

• C does not wait unnecessarily, because it does not receivethe CTS of A

D




Further Procedures for Collision Avoidance

Collision avoidance through out-of-band signaling• Use of an additional channel for signaling

Busy Tone Multiple Access (BTMA)• Everyone who hears a continuous transmission on the data channel, sends “Busy

Tone” on another transmission channel (control channel)

• All devices in the range of 2 hops of an active station wait• No hidden stations, but many exposed stations

Receiver Initiated Busy Tone Multiple Access (RI-BTMA)• Only the receiver sends “Busy Tone”

• Hardly exposed stations, but the Busy Tone can be sent only if the receiver decoded the transmission wish

Wireless Collision Detect (WCD) Protocol• Combines BTMA and RI-BTMA: Two kinds of “Busy Tones”• First like BTMA: Stations send Busy Tone “collision detect”

• Receiver then sends “feedback tone” as soon as he detects transmission wish




802.11 - MAC Layer: DFWMAC• Traffic types

– Asynchronous data service (standard)• Exchange of packets on “best effort” basis

• Support of broadcast and multicast– Time-limited service (optional)

• implemented via PCF (Point Coordination Function)

• Access methods– DFWMAC-DCF CSMA/CA (standard, DCF: Distributed Coordination Function)

• DFWMAC: Distributed Foundation Wireless MAC• Collision avoidance by random access with “backoff” mechanism

• Short time interval introduced between successive frames• Receipt acknowledgment by ACK (not with broadcast)

– DFWMAC-DCF with RTS/CTS (optional)• Avoidance of the hidden station problem

– DFWMAC-PCF (optional)• Collision-free, centralized polling with a list of stations in the AP




802.11 - MAC LayerPriorities for media accesses

• Access times are controlled by introducing three waiting periods

• No guaranteed priorities

• SIFS (Short Inter-Frame Spacing) – 10µs– highest priority, for ACK, CTS, answer to polls

• PIFS (PCF Inter-Frame Spacing) – 30µs– medium priority, for time-limited services using PCF

• DIFS (DCF Inter-Frame Spacing) – 50µs– lowest priority, for asynchronous data services

t

Medium occupied SIFSPIFS

DIFSDIFS

next frameCompetition

direct access, if the channel was unused for a time ≥ DIFS




t

Medium occupied SIFSPIFS

DIFSDIFS

next frame

Competition window(random backoffmechanism)

802.11 - CSMA/CA Procedure

• All implementations have to support the procedure• A station, which wants to send, listens to the medium• If the medium is free for at least the duration of a DIFS, the station may send

• If the medium is occupied, when becoming free the station waits for one DIFS and then randomly chooses a backoff time (collision avoidance, in multiples of a slot time). The station continues to listen to the medium

• If the medium is occupied by another station during the backoff time, the backofftimer stops. In the next try, no new backoff time is chosen randomly, but the old timer is gone on with.

• Also usable for broadcast

Time slot (20 µs)Waiting period




boebor

boebor

boebor

Stations in Competition

t

busy

boe

Station1

Station2

Station3

Station4

Station5

Sending request

DIFSboe

boe

boe

busy

applied backoff time

bor remaining backoff time

busy Medium occupied (Frame, ACK, etc.)

DIFS

boe

boe

boe

DIFS

busy

busy

DIFSboe busy

boe

boe

bor

bor

The size of the competition window (Contention Window, CW) affects the efficiency. Therefore (similar to Ethernet) it starts with CW = 7 and is doubled with each collision up to CWmax = 255




802.11 - CSMA/CA Procedures

Unicast transmission: the receipt is additionally confirmed, since collisions possibly are not detected by the transmitter

• Data can be sent after waiting for DIFS

• Receivers answer immediately (after SIFS, without additional backoff time), if the frame arrived correctly (CRC)

• In case of an error the frame is repeated automatically. No special treatment of a transmission repetition, same access mechanism as before

t

SIFS

DIFS

Data

ACK

Waiting period

FurtherStations

Receiver

SenderData

DIFS

Competition




802.11 – DFWMAC with RTS/CTS

Optional extension for the avoidance of the hidden station problem:

• RTS with holding time as parameter can be sent after waiting for DIFS (plus backoff time)

• Confirmation of the receiver by CTS after SIFS (also containing holding time)

• Immediate sending of the data is possible, confirmation by ACK• Other stations store the holding time, which were sent in the RTS and CTS, in their

NAV (Network Allocation Vector)

• Collisions are only possible with RTS/CTS messages, but substantial overhead through RTS/CTS messages

tWaiting period

FurtherStations

Receiver

Sender

Competition

SIFS

DIFS

Data

ACK

Data

DIFS

RTS

CTSSIFS SIFS

NAV (RTS)NAV (CTS)




802.11 – DFWMAC with RTS/CTS

t

SIFS

DIFS

Data

ACK1

frag1

DIFS

Competition

RTS

CTSSIFS SIFS

NAV (RTS)NAV (CTS)

NAV (frag1)NAV (ACK1)

SIFSACK2

frag2

SIFS

FurtherStations

Receiver

Sender

• Fragmenting data can decrease the damage caused by transfer errors • Special mechanism: adapt size of the fragments to current error rate of the

medium

• First: normal reservation with RTS/CTS• Fragments and ACKs (except the last for each case) contain reservation durations




DFWMAC-PCF

PIFSD1

U1

SIFS

NAV

SIFSD2

U2

SIFS

SIFS

Super-framet0 t1

• PCF for guarantees concerning bandwidth and access delay

• AP controls medium access and cyclic queries all stations (Polling)• Super-frames with competition-free period and competition period (like before)• If the medium gets free (t1) after the begin of the super-frame (t0), the coordinator

cyclic asks all stations x (Dx) for sending needs. If necessary, they answer with Ux(the data to be sent)

• If the phase is ended earlier than planned (t2 instead of t3), more time remains for the competition phase (end is announced by a control frame CFend)

t

D3

PIFSD4

U4

SIFS

SIFSCFend

Competitioncompetition-free period

t2 t3 t4




802.11 - Frame Format

Frame Control• Protocol version, frame type (administration, control, data), fragmenting,

encryption information, meaning of the following address fields

Duration ID• Sent along with RTC, CTS for setting the NAV

Addresses• In each case contains 48-Bit MAC addresses. MAC frames can be transferred

between two stations, between station and AP or between two APs by the distribution system. In the field Frame Control, two bits are determining the current meaning of the addresses. Addresses can be: Final destination, source address, BSS Identifier, intermediate sender address, intermediate receiver address

Sequence Control

• Recognition of duplicated frames

FrameControl

Duration/ID

Address1

Address2

Address3

SequenceControl

Address4

Data CRC

2 2 6 6 6 62 40-2312Byte




802.11 - MAC Management

• Synchronization– Find a LAN, try to remain in the LAN

– Synchronization of internal clocks (e.g. FHSS, PCF, power savingmechanisms)

– Timer etc.

• Power management– Sleep mode without missing a message

– Periodic sleeping, frame buffering, traffic monitoring

• Association/Re-association

– Integration into a LAN– Roaming, i.e. moving between networks from one Access Point to another– Scanning, i.e. active search for a network




Interval of the periodic radio

signal (beacon): 20ms - 1s

tMedium

AccessPoint

busy

B

busy busy busy

B B B

Value of the time stamp B Beacon frame

Synchronization by “Beacons”

• Beacon frame contains time stamps and administrative information for power saving mechanisms and roaming

• Varying times between beacon frames, since the medium can be occupied

• In infrastructure networks: AP takes over the sending of the beacons




Synchronization by “Beacons” (ad-hoc)

tMedium

Station1

busy

B1

Beacon interval

busy busy busy

B1

Value of the time stamp B beacon frame

Station2B2 B2

random delay

• All stations try to send a Beacon frame in fixed intervals

• Standard access procedure with backoff

• One station wins and sends a beacon frame at first. All other stations synchronize to this frame.




Power Management

• Idea: Switch off the sending/receiving device when not needed• Timing Synchronization Function

– Regular activation of all stations. Transmissions for sleeping stations are buffered; when waking up, the stations receive the transmission

• Infrastructure:– AP can store all pending frameworks for sleeping stations– With each beacon frame, a Traffic Indication Map (TIM) is sent along which

indicates, for which stations frames are buffered.– Additionally: List for broadcast/multicast receivers (Delivery Traffic Indication

Map, DTIM)

• Ad-hoc– Ad-hoc Traffic Indication Map (ATIM)

• The storing stations announce the receivers of stored packages

• More complex, since no central AP: all stations have to temporarily store frames

• Collisions of ATIMs possible (scalability?)




Energy Saving with Activity 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

active

p poll confirmation

p

D

D

D data transmissionfrom/to the station




Energy Saving with Activity Patterns (Ad-hoc)

awake

A ATIM transmission D Data transmission

t

Station1B1 B1

B Beacon frame

Station2B2 B2

random delay

A

a

D

d

ATIMwindow Beacon interval

a confirmation of ATIM d Confirmation of the data




802.11 - Roaming

No or only bad connection?

• Scanning

– Scanning of the environment (listen to medium for beacons of APs or send a probe into the medium and wait for an answer)

• Re-association Request– Station sends inquiry at AP(s)

• Re-association Response– If success, i.e. an AP answered, the station now enters the network

– Further scanning, if no response is received

• AP accepts Re-association Request

– Indicate the distribution system about the new station– Distribution system updates location database (i.e. who is where)– Normally, the AP which was responsible for the station before, is

informed by the distribution system

design aspects for wireless networks general structure of ... filechapter 2.8: wlan page 1 design...

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