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3/7/2012 1 Introduction Wireless Networking Wireless LAN Modes of Operation Module-07 Jerry Bernardini Community College
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3/7/2012
Introduction Wireless Networking Wireless LAN Modes of Operation Module-07 Jerry Bernardini Community College of Rhode Island 3/7/2012
Wireless Networking
J. Bernardini
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HT PPDU Formats • Non-HT Legacy – Mandatory for 802.11n – Only 20 MHz channels – Same format as 802.11ag
• HT Mixed – Two part preamble – First part can be decoded by 802.11ag – Second part can not be decoded by 802.11ag
• HT Greenfield 3/7/2012
– Preamble can not be decoded by 802.11ag Networking J. Bernardini – Can use bothWireless 20 MHz and 40 MHz channels
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Presentation Reference Material •
CWTS Certified Wireless Technology Specialist Official Study Guide , Chapter-7 • CWNA Certified Wireless Network Administration Official Study Guide (PWO-104), David Coleman, David Westcott, 2009, Chapter-7 •
The California Regional Consortium for Engineering Advances in Technological Education (CREATE) project
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IEEE 802.11 Topologies or Operating Modes • • • •
The purpose of 802.11 is to interconnect radio cards Every wireless device has a radio card All wireless devices are referred to as Stations (STA) Three topologies defined by 802.11 – Service Sets – Basic Service Set (BSS) – Extended Service Set (ESS) – Independent Basic Service Set (IBSS)
• Nonstandard Topologies – Bridging, Repeating, Workgroup bridging – Mesh networking (growing in importance) 3/7/2012
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Service Set Identifiers - SSID and BSSID •
SSID -Service Set Identifier is a 1-32 byte alphanumeric sequence that uniquely names an ESS (the network name). • Any SSID or Null SSID is a blank SSID used to associate with anyone. • BSSID- Basic Service Set Identifier is a 48-bits that uniquely identifies a BSS Wired LAN BSA Basic service Area –Physical Coverage Area
AP
ESS Id = SSID BSS Id = BSSID
Basic Independent Basic Service Set (IBSS) •
•
•
Independent Basic Service Set (IBSS): Wireless network that does not use an AP – Wireless devices communicate between themselves – Peer-to-peer or ad hoc mode BSS more flexible than IBSS in being able to connect to other wired or wireless networks IBSS useful for quickly and easily setting up wireless network – When no connection to Internet or external network needed
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Basic Service Set (BSS) BSS - The Basic Service Set is a term used to describe the collection of Stations which may communicate together within an 802.11 WLAN. •
•
•
•
Basic Service Set (BSS): Group of wireless devices served by single AP – infrastructure mode BSS must be assigned unique identifier – Service Set Identifier (SSID) • Serves as “network name” for BSS Basic Service Area (BSA): Geographical area of a BSS – Max BSA for a WLAN depends on many factors Dynamic rate shifting: As mobile devices move away from AP, transmission speed decreases
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Extended Service Set (ESS) • • •
ESS - is comprised of a number BSS’s ESS stations must have the same SSID The BSSID is the “name” of the BSS (not same as SSID)
• APs can be positioned so that cells overlap to facilitate roaming – Wireless devices choose AP based on signal strength – Stations going from one BSS to another will deal with Handoff
Wired LAN
ESS SSID BSS2 (BSSID2)
BSS1 (BSSID1)
BSS3 (BSSID3)
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Wireless Mesh Access Points • • • • • • • • •
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Mesh APs associate with multiple APs Association between APs is limited by vendor (3-5) Currently vendor dependent Clients can reach destinations thru multiple APs APs route packets to ovoid failures and optimal paths Mesh Networks are more resilient Not every AP has to be connected to a wired network Self-Healing, Self-Configuring using Layer-2 Protocol New standard IEEE 802.11s will allow interoperability between vendors
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Wireless Mesh Network Implementation
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Ethernet and 802.11 Frames • Ethernet Frame 7
1
Preamble
1518
6
6
Source
Destination
2
46 - 1500
4
Data
FCS
Type or Length Field
Start Of Frame
• Wireless Frame 10 or 18
2
4 or 6
Sync
PLCP Header Start Of Frame
2
2
6
6
6
Source
Destination
Rec. Adr
Duration ID Frame Cntrl
2
6
0 - 2304
4
Trans. Adr
Data
FCS
Sequence Cntrl MAC Packet DATA Unit, (MPDU)
Frame Categories / Types •
Management Frames o Beacon Frame o Probe Frames o Association Frames… more
•
Control Frames o RTS and CTS Frames o ACK – Acknowledgement Frames… more
•
Data Frames o Data Payload Frames
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Twelve Management Frame Types
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Eight Control Frames • Used to assist with the delivery of data frames
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Fifteen Data Frames • The frames that actually carry application data
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IEEE 802.11 Frame Formats Management
Duration (2)
Frame Control (2)
Control
Data
Frame Control (2)
Frame Control (2)
Duration (2)
Des. Address (6)
Duration (2)
Address 1 (6)
Source Address (6)
Receiver Address (6)
Address 2 (6)
Address 3 (6)
BSSID (6)
Seq. Control (2)
Transmit Address ( 6)
Frame Check Seq. (6)
Seq. Control (2)
Address 4 (6)
Frame Body ( 1 to 2311)
Data ( 1 to 2311)
Frame Check Seq. (6)
Frame Check Seq. (6)
(Bytes per field) CWNA Guide to Wireless LANs, Second Edition
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Frame Types and Sizes Protocol Data Unit (PDU)
Frame Type and Vendor Support
MTU (Bytes)
TCP
Transport maximum segment size
1460
Layer-3 default size
1500
IEEE 802.3 Ethernet default
1500
MPDU
IEEE 802.11 default
1534
MPDU
IEEE 802.11 maximum
2304
MAC
Jumbo Frame
>1500
MAC
Cisco Baby giant
1552
1600
MAC
Cisco Catalyst 4000
9198
9216
MAC
Cisco Catalyst 6000
9216
9234
IP MAC
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Wireless Networking
MTU + Overhead (Bytes)
1518
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Beacons
• Beacon – information frame sent by an AP. Beacon frames are approximately 50-bytes with the following information: – – – –
Timestamp Beacon Interval Capability Info Service Set Identifier
Header
Payload
Support Rates Parameter Sets Traffic Indication Map
Trailer
Beacon Frame
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Beacon Management Frame • A special management frame that is used by a client stations seeking a wireless network to join. • Instead of beacon frames a station could use probe request and probe response frames • In an ad hoc (IBSS ) wireless network all stations take turns broadcasting the beacon frame
Beacon
Beacon
S2
S1 AP Control Point 3/7/2012
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Passive Scanning (Beacons) 1. Client stations listens for a beacon from an access point (AP) 2. If multiple beacons are received the strongest one is selected 3. The listening station then requests authentication and association
Beacons
Beacons
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AP Control Point Wireless Networking
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Active Scanning (Probes) • A station could use probe request and probe response frames Instead of beacon frames 1. Station is configured with SSID and switched to a channel 2. Probe request sent by requesting station 3. All stations that have the same SSID and have normal configurations respond with a Probe Response frame • The process also involves waiting for ProbeDelay and MinChannel Timers
Probe Response
Probe Request
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AP Control Point Wireless Networking
S2
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Authentication and Association • • 1. 2. 3. •
Using the IEEE 802.11 State Machine Stations are in one of three states Unauthenticated / Unassociated Authenticated / Unassociated Authenticated / Associated You cannot transmit data frames for processing until you are associated • You cannot transmit associated frames for processing until you are Authenticated
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Wireless Connection Process • 2-Step Connection Process – Authentication Phase – Association Phase Authentication Phase
Association Phase
Wireless Connection Process • Authentication is the process used by a station to verify that another station is approved for communications. This is a station authentication and not a user authentication. • Authentication Step – Two IEEE 802.11 standards: • Open System Authentication • Shared Key Authentication
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Authentication • Authentication - A means to establish or prove identity; verifying eligibility of users, devices, or applications. – Only authorized clients are allowed to gain access to the network.
• For this level of authentication it applies to the stations and not the user.
Association • Association – The binding of a wireless network client to an Access Point for the purpose of data transfer. Again the stations not the user.
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Connection State Machine Unauthenticated and Unassociated Successful Authentication
De-authentication Notification
Authenticated and Unassociated Disassociation Notification
Successful Association
Authenticated and Associated De-authentication Notification
Unauthenticated & Unassociated • In the first state: – Wireless Client/Node is not connected to the network. Wired LAN Access Points
Wireless Client
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Authenticated & Unassociated • In the Second State: – Wireless Client/Node has passed the authentication process but is not associated with the AP. Wired LAN Association Response
Access Points
Association Request
Wireless Client
Authenticated & Associated • In the Third State: – Wireless Client/Node is now connected and associated with the AP. Data can now be transferred between the devices. Wired LAN Access Points Data Link Wireless Client
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Open System Authentication • Open System / Null Authentication – – – –
IEEE 802.11 Default Authentications based on empty string SSID Client sends empty string SSID Receiving station, (AP) sends acknowledgment
Open System Authentication Wired LAN
Authentication Management Frame - SSID ACK
Wireless Client
Access Points
Authentication Management Frame ACK
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Closed System Authentication • Closed System – – – –
IEEE 802.11 Authentications based only on SSID Client sends SSID Receiving station, (AP) sends acknowledgment
Shared Key Authentication • Shared Key – IEEE 802.11 Wired Equivalent Privacy, (WEP). – Authentications based on Text and WEP Keys. – Challenge – Response Scheme
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Shared Key Authentication Wired LAN Authentication Management Frame - SSID Challenge Phrase Encrypted Phrase Wireless Client
WEP Key
Access Points
Authentication Secure Channel
WEP Key
Roaming • Roaming - The ability for a user to function when the serving network is different from their home/associated network. The process of a client moving seamlessly from one area or cell to another while maintaining a data link.
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Roaming Wired LAN
Access Points
1
1
2
1
Wireless Clients
Usually a 20%-30% cell overlap
Unauthenticated & Unassociated • In the first state: – Wireless Client/Node is not connected to the network. Wired LAN Access Points
Wireless Client
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Authenticated & Unassociated • In the Second State: – Wireless Client/Node has passed the authentication process but is not associated with the AP. Wired LAN Association Response
Access Points
Association Request
Wireless Client
Authenticated & Associated • In the Third State: – Wireless Client/Node is now connected and associated with the AP. Data can now be transferred between the devices. Wired LAN
Access Points Data Link Wireless Client
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IEEE 802.11 MAC Functions • • • • • • • • • •
Scanning- discover AP or BSS Synchronization- all stations have the same clock Frame Transmission- rules for frame transfer Authentication-allow device in network Association-after authentication associate with AP Reassociation-roaming and association with new AP Data Protection-data encryption protects data Power Management-save power by sleeping transceiver Fragmentation-breakup frame for efficiency and interfer. RTS/CTS- solution to hidden node problem
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Beacon Management Frame • A special management frame that is used by a client stations seeking a wireless network to join. • Instead of beacon frames a station could use probe request and probe response frames • In an ad hoc (IBSS ) wireless network all stations take turns broadcasting the beacon frame
Beacon
Beacon
S2
S1 AP Control Point 3/7/2012
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Passive Scanning (Beacons) 1. Client stations listens for a beacon from an access point (AP) 2. If multiple beacons are received the strongest one is selected 3. The listening station then requests authentication and association
Beacons
Beacons
S1 3/7/2012
AP Control Point Wireless Networking
S2 43
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Active Scanning (Probes) • A station could use probe request and probe response frames Instead of beacon frames 1. Station is configured with SSID and switched to a channel 2. Probe request sent by requesting station 3. All stations that have the same SSID and have normal configurations respond with a Probe Response frame • The process also involves waiting for ProbeDelay and MinChannel Timers
Probe Response
Probe Request
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AP Control Point Wireless Networking
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Communications Options • MAC Layer – Access Methods • DCF – RTS/CTS (optional) Distributed function Wireless MAC Avoids hidden node problem
• DCF – PCF (optional) AP polls stations Superframes to allow station to eventually get access Superframe = Beacon + CFP + CP CFP = Contention-Free Period CP = Contention Period
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 and CTS
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Fragmentation • •
Every network has an MTU (Maximum Transmission Unit) size. Packets larger than the allowable MTU size must be broken down into multiple smaller packets, or fragments, to enable them to traverse the network with lower bit error rates, (BER). Fragment size can typically be set by the user using a threshold setting between 256 and 2,048 bytes.
Header
Data
CRC
Threshold Header
Data -1
CRC
Header
Data -2
CRC Drawing not to scale.
Distributed Coordination Function • The Distributed Coordination Function (DCF) is the fundamental access mechanism in IEEE 802.11 Medium Access Control (MAC). • DCF can be used in all wireless topologies: IBSS, BSS, and ESS.
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DIFS • DIFS – Is used by default on all 802.11 stations. DIFS is the lowest priority and is used for data and management frames. For DSSS, DIFS is 50 s. DIFS
Station Backoff Timers
PIFS DIFS
sender
medium busy
SIFS
contention
frame
A look at all three: 10 s, 30 s, 50 s, for DSSS.
Reassociation Wired LAN
Access Points
Link Fading
1
2
1
Reassociation Request
Wireless Clients
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Dynamic Rate Switching • Dynamic Rate Switching =Dynamic Rate Selection =Automatic Rate Shifting----All mean the same thing • Process of reducing or increasing the data rate based upon RF signal levels • RF signals attenuate over distance or by absorption • AP will reduce data rate for weaker signals • AP will increase data rate for higher signals
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Dynamic Rate Selection (DRS) •
Dynamic Rate Selection or Adaptive Rate Selection/shifting. – 802.11a, 802.11g modes: 54, 48, 36, 24, 18, 12, 9, 6 Mbps – 802.11b mode: 11, 5.5, 2, 1 Mbps – Orinoco 2X mode: 108, 96, 72, 48, 36, 24, 18, 12 Mbps
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Data Rate Coverage Areas -85 dBm -76 dBm -72 dBm
24 Mbps 18 Mbps
9 Mbps
Power Management •
•
A WLAN laptop must remain “awake” in order to receive network transmissions – Original IEEE 802 standard assumes stations always ready to receive network messages Power management: Allows mobile devices to conserve battery life without missing transmissions – Transparent to all protocols – Differs based on WLAN configuration – AP records which stations awake and sleeping – Buffering: If sleeping, AP temporarily stores frames
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WLAN Power Management Features • Active Mode – No power saving but improved station and AP performance – For desktops and line powered laptops • Power Save Mode – Dozing and Wake modes – Switches to wake to check for frames • WMM Power Save – U-APSD Unscheduled Automatic Power-Save Delivery – This is an industry certification – IEEE 802.11e-2005 3/7/2012
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Power Management • At set times AP send out beacon to all stations – Contains traffic indication map (TIM) – At same time, all sleeping stations switch into active listening mode
• Power management in ad hoc mode: – Ad hoc traffic indication message (ATIM) window: Time at which all stations must be awake
• Wireless device sends beacon to all other devices – Devices that previously attempted to send a frame to a sleeping device will send ATIM frame indicating that receiving device has data to receive and must remain awake
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Continuous Aware Mode • Constantly Awake Mode provides the best performance allowing the client a strong connection between the wireless card and the AP; however, it also rapidly drains the client’s battery, resulting in shorter battery life.
Power Management with TIM/DTIM/ATIM • Traffic Indication Map (TIM) – A table stored on the AP of all STA’s in Power Save mode – TIM is used to determine which STA’s require frame buffering – Every Beacon contains a TIM • Delivery Traffic Indication Message (DTIM) – Used to manage STAs and to program wakeup – Sent on every few (third or some interval)Beacon • Ad Hoc Traffic Indication Message (ATIM) – Use to power manage IBSS
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Protection Mechanisms • Protection Mechanisms allow for backward compatibility • Older technology and newer technology compatibility • Two Broad Categories of protection mechanism – Extended rate physical (ERP) – High throughput (HT)
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Review 802.11g Protection • Before an 802.11g client can transmit to an 802.11g AP it must reserve the medium. • Must transmit so 802.11b will understand. • Two Protection Methods – CTS-to self at 802.11b modulation (slow Clear to Send) – RTS-CTS at 802.11b modulation • CTS-to-self is more efficient but may not be seen by hiddennode • RTS-CTS is more reliable but has more overhead • Both Methods dramatically reduce the 802.11g throughput CCRI
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802.11b/g Mixed Mode Operation Station80 2.11g
Station80 2.11g
AP 802.11g
Station80 2.11b
1-Slow CTS 2-Slow CTS
2-Slow CTS 3-Fast Data
3-Fast Data
3-Fast Data
CCRI
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802.11 Summary Characteristics Range (Radius Range (Radius Indoor) Outdoor) Loss Depends, # includes one and type of wall walls
Protocol
Release Date
Op. Frequency
Throughput (Typical)
Data Rate (Max)
Modulation Technique
802.11a
1999
5 GHz
23 Mbps
54 Mbps
OFDM
~35 Meters
~120 Meters
802.11b
1999
2.4 GHz
4.3 Mbps
11 Mbps
DSSS -CCK
~38 Meters
~140 Meters
802.11g
2003
2.4 GHz
19 Mbps
54 Mbps
OFDM & DSSS
~38 Meters
~140 Meters
June 2010
2.4 GHz 5 GHz
74 Mbps
248 Mbps
OFDM MIMO
~70 Meters
~250 Meters
802.11n
CCK-Complementary Code Keying OFDM-Orthogonal Frequency Division Multiplexing DSSS-Direct Sequence Spread Spectrum MIMO-Multi-Input Multi-Output CCRI
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HT Mode • Non-HT Legacy – Mandatory for 802.11n – Only 20 MHz channels – Same format as 802.11ag • HT Mixed – Two part preamble – First part can be decoded by 802.11ag – Second part can not be decoded by 802.11ag • HT Greenfield – Preamble can not be decoded by 802.11ag – Can use both 20 MHz and 40 MHz channels 3/7/2012
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