Communication Networks - Technische Universität Ilmenau

Communication Networks Winter 2017/18

Prof. Jochen Seitz 1

Communication Networks

Chapter 10 – Wireless Local Area Networks According to IEEE 802.11

Communication Networks: 10. IEEE 802.11 651

10. WLANs According to IEEE 802.11

Overview

• Organization of a WLAN according to IEEE 802.11

• Current IEEE 802.11 standards

• IEEE 802.11 layers

• IEEE 802.11 MAC

• MAC synchronization, power saving and roaming

• WLAN and ad hoc networks




10.1 Working Modes

802.11 – WLAN in Infrastructure Mode

• Station (STA)

Device with access to the wireless medium and connectivity to the access point

• Basic Service Set (BSS)

Group of devices working on the same radio frequency

• Access Point

Device that allows communication between stations and integrates them into the distribution system

• Portal

Gateway to some other network

• Distribution System

Connection of different WLAN cells to build an Extended Service Set EES


Distribution System

Portal

802.x LAN

AccessPoint

802.11 LAN

BSS2

802.11 LAN

BSS1

AccessPoint

STA1

STA2STA3

ESS

10.1 Working Modes

802.11 – WLAN in Ad-hoc Mode

• Direct communication with limited range

Station (STA):Device with access to the wireless medium

Basic Service Set (BSS):Group of devices working on the same radio frequency(Independent Basic Service Set IBSS)


802.11 LAN

BSS2

802.11 LAN

BSS1

STA1

STA4

STA5

STA2

STA3



10.1 Working Modes

Hidden Node Problem

• A would like to communicate with B

• C is already transmitting information to B using the same channel

• There are collisions in B

• Both A and C cannot detect these collisions


A B C

10.1 Working Modes

Exposed Node Problem

• B is already transmitting information to A

• C would like to communicate with D

• C finds the channel occupied and refrains from sending

• Free capacities are unused


A B C D



IEEE-Standard 802.11


10.2 The Standard IEEE 802.11

Mobile Terminal

AccessPoint

Server

Fixed Terminal

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

Infrastructure Network

IEEE 802.11 Important Substandards

• 802.11

Original standard from 1997

Data rate 1 or 2 Mbit/s

Frequency range 2.400 to 2.485 GHz (ISM band)

• 802.11a

Extended physical layer, published 1999

Data rate 54 Mbit/s

Frequency range around 5 GHz

• 802.11b


Data Rate 11 Mbit/s

Same frequency range as original 802.11

• 802.11g


Data rate 54 Mbit/s

Frequency range 2.400 to 2.4835 GHz

• 802.11n

Multiple-input multiple-output antennas (MIMO)

Publication by the IEEE in October 2009

Data rate up to 600 MBit/s

Frequency range 2.4 GHz or 5 GHz

• 802.11ac

Published 2013

Theoretical max. data rate 3.466 Gbit/s with 8x8 MIMO

Frequency: 5 GHz

• 802.11p

Wireless access in vehicular environments (WAVE)

Data exchange between high-speed vehicles and between the vehicles and the roadside infrastructure in the licensed ITS band of 5.9 GHz (5.85-5.925 GHz)

Published in November 2010





802.11 Layers



LLCLogical Link Control

MACMedium Access Control

PLCPPhysical Layer

Convergence Protocol

PMDPhysical Medium

Dependent

MAC Management

PHY Management

Stat

ion

Man

agem

ent

802.11 Functions

• MAC

Medium Access

Segmentation/Reassembly

Ciphering

• MAC Management

Synchronization

Roaming

MIB

Power Control

• PLCP

Clear Channel Assessment Signal (Carrier Sense)

• PMD

Modulation

Coding

• PHY Management

Channel Selection

MIB

• Station Management

Coordination of Management Functions





802.11 MAC Frame Format


10.3 802.11 MAC

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w.r

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10.3 802.11 MAC

Fields in the IEEE 802.11 MAC Frame

• Synch – Preamble: for FH PHY 80 bits

for DSSS PHY 128 bits

alternating '0's and '1's

• SFD – Start Frame Delimiter: 16 bits “0000 1100 1011 1101”

• PLW – PLCP-PDU Length Word: 12 bits indicating the number of bytes in the packet

first portion of the PLCP header

PLCP header is transmitted only at 1 Mbps!

• PSF – PLCP Signaling Field: 4 bits to show the rate of the MAC payload transmission

Bit 0 is reserved and is always '0'

Bits 1 to 3 indicate the data rates




10.3 802.11 MAC

802.11 – MAC

• Distributed Foundation Wireless MAC (DFWMAC)

• Different traffic types

Asynchronous data transmission (standard)

Exchange of MAC frames without Quality of Service („best-effort”)

Broadcast and multicast

Time-limited transmission (optional)

Point Coordination Function (PCF) only in infrastructure mode


10.3 802.11 MAC

802.11 MAC Procedures

• Distributed Coordination Function: Carrier Sense Multiple Access with Collision Avoidance (DFWMAC-DCF CSMA/CA) (standard)

Collision avoidance based on arbitrary backoff algorithm

Minimum time span between two MAC frames (so called inter frame spacing)

Correct transmission signaled with ACK-frame (except Broadcast or Multicast)

• Distributed Coordination Function with “Request to Send” / “Clear To Send” Frames (DFWMAC-DCF with RTS/CTS) (optional)

Avoidance of hidden node problem

• Point Coordination Function(DFWMAC-PCF) (optional)

List-based polling done in the Access Point




10.3 802.11 MAC

802.11 – MAC: Inter Frame Spacing

• Implementation of Priorities

No guarantees

Shorter inter frame spacing allows earlier sending time for the frame:

SIFS (Short Inter Frame Spacing)

Highest priority, for ACK, CTS, response to polling

PIFS (PCF IFS)

Medium priority, for time limited services in PCF

DIFS (DCF, Distributed Coordination Function IFS)

Lowest priority, for asynchronous data transmission


t

medium busy SIFS

PIFS

DIFSDIFS

next framecompetition

direct access, if medium is free DIFS

10.3 802.11 MAC

802.11 – CSMA/CA I

• Carrier sense based on clear channel assessment signal

• Station may send, if medium is free for the appropriate IFS

• If medium is busy, station sets backoff time to an arbitrary number of time slots

• After the medium is free again, station waits the appropriate IFS and the backoff time

• If medium gets busy during backoff time, backoff time is frozen


t

medium busy SIFS

PIFS

DIFSDIFS

next frame

Competition Window(arbitrary backoff time)

time slotwaiting time



802.11 – CSMA/CA II


10.3 802.11 MAC

t

busy

boe

Station1

Station2

Station3

Station4

Station5

data arrival at MAC-SAP

DIFS

boe

boe

boe

busy

elapsed backoff time

bor residual backoff time

busy medium busy (frame, ack etc.)

bor

bor

DIFS

boe

boe

boe bor

DIFS

busy

busy

DIFS

boe busy

boe

boe

bor

bor

10.3 802.11 MAC

802.11 – CSMA/CA III

• Sending unicast frames

Frames can be sent after DIFS (plus backoff time as described before)

Receiver responds immediately (after SIFS), if frame has been correctly received (CRC)

If an error occurs, the frame is automatically repeated


t

SIFS

DIFS

Data

Ack

waiting time

Further Stations

Receiver

SenderData

DIFS

competition



10.3 802.11 MAC

802.11 – RTS / CTS

• Sending unicast frames using RTS/CTS

Before transmitting a data frame, an RTS frame has to be sent including the duration of the data frame (after DIFS)

Receiver acknowledges RTS frame with a CTS frame (after SIFS)

Sender may then send the data frame after SIFS, which is acknowledged as usual

Other stations store the time the medium is busy (as contained in the RTS and CTS frames)


twaiting time

FurtherStations

Receiver

Sender

competition

SIFS

DIFS

data

ACK

data

DIFS

RTS

CTSSIFS SIFS

NAV (RTS)NAV (CTS)

NAV = Network Allocation Vector

802.11 – RTS / CTS: Fragmentation


10.3 802.11 MAC

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



Medium busy

DFWMAC-PCF I


10.3 802.11 MAC

PIFS

NAV of thestations

Stations

Point Coordinator

D1

U1

SIFS

NAV

SIFSD2

U2

SIFS

SIFS

Super Framet0 t1

Point Coordination Function

DFWMAC-PCF II


10.3 802.11 MAC

t

D3

NAV

PIFSD4

U4

SIFS

SIFSCFend

competition

t2 t3 t4

NAV of thestations

Stations

Point Coordinator

period without competition



10.3 802.11 MAC

MAC Address Format

Frame Type to DS from DS Address 1 Address 2 Address 3 Address 4

Ad-hocNetwork

0 0 DA SA BSSID -

Infrastructure Network, from AP

0 1 DA BSSID SA -

Infrastructure Network, to AP

1 0 BSSID SA DA -

Infrastructure Network, in DS

1 1 RA TA DA SA


DS : Distribution SystemAP : Access PointDA : Destination AddressSA : Source AddressBSSID : Basic Service Set IdentifierRA : Receiver AddressTA : Transmitter Address

10.4 802.11 MAC Management

802.11 – MAC Management

• Synchronization

Finding and staying in a WLAN

Timer etc.

• Power Management

Sleep modus without loosing a frame

Periodically sleeping, buffering of frames, traffic map

• Association / Reassociation

Associating with a distribution system

Roaming, i.e. changing networks when changing access points

Scanning, i.e. actively looking for a WLAN

• MIB - Management Information Base

Administering, reading, writing




MAC Synchronization in Infrastructure Mode



beacon interval

(20ms – 1s)

tmedium

access point

busy

B

busy busy busy

B B B

value of the timestamp B beacon frame

MAC Synchronization in Ad hoc Mode



tmedium

station1

busy

B1

beacon interval

busy busy busy

B1

value of the timestamp B beacon frame

station2

B2 B2

random delay



Power Saving

• Idea

switch the transceiver off if not needed

• States of a station sleep

awake

• Timing Synchronization Function (TSF)

stations wake up at the same time

• Infrastructure Traffic Indication Map (TIM)

list of unicast receivers transmitted by AP

Delivery Traffic Indication Map (DTIM)

list of broadcast/multicast receivers transmitted by AP

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

announcement of receivers by stations buffering frames

more complicated - no central AP

collision of ATIMs possible (scalability?)

• APSD (Automatic Power Save Delivery) new method in 802.11e replacing above scheme


• 10.4 802.11 MAC Management

Power Saving in Infrastructure Mode



TIM interval

t

medium

access

pointbusy

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

ddata transmission

to/from the station



Power Saving in Ad hoc Mode



awake

A transmit ATIM D transmit data

t

station1

B1 B1

B beacon frame

station2

B2 B2

random delay

A

a

D

d

ATIM

window beacon interval

a acknowledge ATIM d acknowledge data

Roaming

• No or bad connection? Then perform:

• Scanning

scan the environment, i.e., listen into the medium for beacon signals or send probes into the medium and wait for an answer

• Reassociation Request

station sends a request to one or several AP(s)

• Reassociation Response

success: AP has answered, station can now participate

failure: continue scanning

• AP accepts Reassociation Request

signal the new station to the distribution system

the distribution system updates its data base (i.e., location information)

typically, the distribution system now informs the old AP so it can release resources

• Fast roaming – 802.11r

e.g. for vehicle-to-roadside networks


• 10.4 MAC Management



10.5 802.11 in Ad-hoc Mode

IEEE 802.11s

• IEEE 802.11 amendment for mesh networking, since 2012 incorporated in 802.11 standard

• Broadcast/multicast and unicast delivery using “radio-aware metrics over self-configuring multi-hop topologies”

• Default mandatory routing protocol Hybrid Wireless Mesh Protocol (HWMP), inspired by a combination of AODV (RFC 3561) and tree-based routing, based on MAC addresses

• Peer authentication methods defined for security

• “One Laptop per Child” project (laptop.org) uses the 802.11sdraft standard for its OLPC XO laptop and OLPC XS schoolserver networking



Ad-hoc Networks and QoS (I)

• Guarantee of QoS even when topology keeps changing all the time!

• Normal procedure:

Find route with enough resources

Reserve the required resources

Keep on controlling the achieved QoS

• For ad-hoc networks:

Limited range and energy

Restricted availability of channels / bit rate (shared medium)

Unforeseeable radio problems

Vertical and horizontal handover




Ad hoc Networks and QoS (II)



Node cannot supplythe required QoS


Ad hoc Networks and QoS (III)

• Trying to guarantee QoS through redundancy

• If QoS cannot be supplied: best effort transmission or communication breakdown


Simultaneous transmission

Backup routes established and

reservedBackup routes selected

Parallel Routes



LTE Cell


Interoperability with Other Networks

• One of the nodes in the ad-hoc network allows access to some other network

• Problem: different network characteristics

Addresses

Capacity / QoS

Routing and Signaling

• Example: WLAN-based ad hoc network


Ad-hocNetwork Internet


Multimedia Transmission in an Ad hoc Network

• Idea: Transmitting a video stream in an ad hoc network based on AODV

• Problem:





Summary on Ad hoc Networks

• Currently many research projects in this area

• Still not very well accepted

Security?

Benefits?

Standards?

• Well suited to enhance an infrastructure network

• Perfect for communication in underdeveloped areas


References

References

• Gast, Matthew S. (2017): 802.11 Wireless Networks. The Definitive Guide. Sebastopol, CA: O'Reilly Media.

• Olenewa, Jorge L. (2017): Guide to Wireless Communications. Fourth edition. Australia: Cengage Learning.

• Perahia, Eldad; Stacey, Robert (2013): Next Generation Wireless LANs. 802.11n, 802.11ac, and Wi-Fi direct. 2nd edition. Cambridge: Cambridge University Press.

• Schiller, Jochen H. (2009): Mobile Communications. United Kingdom: Pearson Education Limited.

• Slingerland, Janet (2018): Wi-Fi. How It Works. Lake Elmo, MN: Focus Readers.


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Communication Networks - Technische Universität Ilmenau