wireless-lan (1)
TRANSCRIPT
Wireless LAN
Wireless LANs
• Evolution and Technology
• IEEE 802.11
• Bluetooth
• Zigbee and IEEE 802.15
Evolution
• Early experiences (1970-72): IBM, HP, Motorola– Abandoned due to limited performance and unavailability of
frequency bands• Early challenges:
– Complexity and cost– Bandwidth– Coverage– Interference– Frequency administration
• Emergence of unlicensed bands– Release of Industrial, Scientific and Medical (ISM) bands in 1985
• Applications: military, home and enterprise networks, mobile networks, teetherless access
Media Access
• Media in wireless networks is shared and is scarce – access must be controlled
• Observations:– Contention is at the receiver, not at the sender –
makes the carrier sense approach inappropriate– Unlike Ethernet, congestion is location-dependent– The media access protocol should propagate
congestion information explicitly rather than having each device learn about congestion independently
– Media access protocol should propagate synchronization information about contention periods, so that all devices can contend effectively
IEEE 802.11
• Standardization group formed in 1990, first standards completed in 1997
• IEEE 802.11 is the first WLAN standard; only one to secure a market
• 802.11b: PHY layer supports 11 Mbps using CKK (complementary code keying) technology
• 802.11a: PHY layer supports 54 Mbps using OFDM
• Uses CSMA/CA for contention data• Supports both infrastructure as well as ad hoc
modes
Requirements
• Single MAC to support multiple PHY layers
• Mechanism to support multiple overlapping network
• Provisions to handle interference
• Mechanism to handle hidden terminals
• Privacy and access control
Logical link control
Point coordination function (PCF)
Distributed coordination function (DCF)
2.4-Ghz frequency-
hopping spread
spectrum 1Mbps 2Mbps
2.4-Ghz direct
sequence spread
spectrum 1Mbps 2Mbps
Infrared 1Mbps 2Mbps
5-Ghz orthogonal FDM 6, 9. 12. 18, 24, 36, 48, 54
Mbps
2.4-Ghz direct
sequence spread
spectrum 5.5 Mbps 11 Mbps
Contention-free service Contention
service
MAC layer
IEEE 802.11 Protocol Architecture
IEEE 802.11 IEEE 802.11a IEEE 802.11b
Topology
Basic Service Set (BSS)
BSS
BSS
An Extended Service Set
(ESS)
Infrastructure Network
Ad hoc Network
Layered Protocol Architecture
• MAC sublayer is responsible for access mechanisms and fragmentation/reassembly
• MAC management is responsible for roaming in Extended Service Set (ESS), power management, association/dissociation/reassociation/ process for registration connection management
• PHY management: decides on channel tuning– Physical Layer convergence protocol (PLCP): carrier sensing
and forming packets– Physical Medium Dependent (PMD): modulation and coding
techniques for signaling
• Station management: coordination of interaction between MAC and PHY layers
Low Layer Protocol Stack
MAC Management
Data Link Layer
Sta
tion
Man
agem
ent
PHY Management
LLC
MAC
PLCP
PMD
Physical Layer
PLCP: Physical Layer Convergence ProtocolPMD: Physical Medium Dependent
PHY Layer
• When the MAC protocol data unit (MPDU) arrive at the PLCP layer, a header is attached that is designed specifically for the PMD
• The PLCP packet is then transmitted by the PMD according to specification of the signaling techniques
• IEEE 802.11 defines three PLCP packet formats:– FHSS (frequency hopping spread spectrum)– DSSS (direct sequence spread spectrum)– DFIR (diffused infrared)
FHSS
• PMD hops over 78 channels of 1 MHz each in the center of 2.44 GHz ISM bands
• Each BSS can select one of the three patterns of 26 hops:– (0, 3, 6, 9, …, 75)– (1, 4, 7, 10, …, 76)– (2, 5, 8, 11, …, 77)
• IEEE 802.11 specifies specific random hopping pattern for each of these frequency groups that facilitates multivendor interpretability
• Multiple Basic Service Set (BSS) can co-exist in the same area by up to three APs using different frequency groups
DSSS
• DSSS communicates using non-overlapping pulses at 11 Mcps
• The ISM band at 2.4 GHz is divided into 11 overlapping channels spaced at 5 MHz
• A PHY layer management sublayer of AP covering a BSS can select one of the choices
• Because of wider bandwidth, DSSS provides a better coverage and a more stable signal
Carrier Sense Multiple Access (CSMA appropriateness?)
• Carrier sense provides information about potential collision at the sender, but not at the receiver
• Since the receiver and sender are not co-located, carrier sense does not provide adequate information for collision avoidance – interference at the sender does not imply interference at the receiver
Carrier Sensing
• Carrier sensing in IEEE 802.11 is performed physically or virtually
• PHY sensing is through the clear channel assignment (CCA) signal produced by PLCP
• CCA is generated by sensing detected bits or by checking the RSS
• Virtual carrier sensing is done based on a network allocation vector (NAV) – more later
MAC Layer
• MAC Sublayer:– Defines the access mechanisms and packet
formats
• MAC Management:– Defines roaming support in the ESS, power
management and security
MAC Sublayer
• Reliable data delivery
• Access mechanisms– Contention-based
• CSMA/CA
– Contention-free• RTS/CTS• Point Coordination Function (PCF)
Reliable Data Delivery
• High degree of unreliability and large timers for retransmissions used in higher layers motivates to deal with errors at the MAC layer
• Each transmission is followed by an ACK as an atomic unit. Retransmission is done if the ACK is not received
• RTS/CTS exchange
Hidden Terminal Problem
A
B
XNode X finds that the mediumis free, and transmits a packet
No carrier ≠OK to transmit
A is transmitting a packet to B
Exposed Terminal Problem
A is transmitting a packet to B
X can not transmit to Y, eventhough it will not interfere at B
A
B
XY
Presence of carrier ≠ holds off transmission
Busy Tone
A
B
XA
B
XY
X OK to transmit X not OK to transmit
1. Receiver transmits busy tone when receiving data2. All nodes hearing busy tone keep silent3. Requires a separate channel for busy tone
B is receiving a packet from A
RTS/CTS dialog
RTS = Request to Send
RTS
Any node that hears this RTS will defer medium access.
Defer
RTS/CTS Dialog
CTS = Clear to Send
CTS
Any node that hears this CTS will defer medium access.
Defer
Defer
RTS
RTS/CTS Dialog
ACK
Defer
Defer
Data
Access Control
• Distributed Coordination Function (DCF)
• Point Coordinated Function (PCF) Centralized
Distributed Coordination Function (DCF)
• DCF sublayer makes use of a simple CSMA algorithm
• Collision detection (CD) is not included because of its impracticability in wireless networks
• DCF includes a set of delays called interframe space (IFS) to provision priority
Wait for frame to transmit
Medium idle?
Wait IFS
Still idle?
Transmit frame
Wait until current transmission ends
Wait IFS
Still idle?
Exponential backoff while medium idle
Transmit frame
Yes
No
No
Yes
No
Yes
IEEE 802.11 Medium Access Control Logic
IEEE 802.11 DCF
• Uses RTS-CTS exchange to avoid hidden terminal problem– Any node overhearing a CTS cannot transmit for the
duration of the transfer– Any node receiving the RTS cannot transmit for the
duration of the transfer• To prevent collision with ACK when it arrives at the sender
• Uses ACK to achieve reliability
IEEE 802.11 DCF
• CSMA/CA– Contention-based random access– Collision detection not possible while a node is
transmitting
• Carrier sense in 802.11• Physical carrier sense• Virtual carrier sense using Network Allocation Vector (NAV)
– NAV is updated based on overheard RTS/CTS packets, each of which specified duration of a pending Data/Ack transmission
• Collision avoidance• Nodes stay silent when carrier sensed busy (physical/virtual)• Backoff intervals used to reduce collision probability
Backoff Interval
• When the channel is busy, choose a back-off interval in the range [0,cw]– cw is contention window
• Count down the back-off interval when medium is idle– Count-down is suspended if medium becomes
busy
• When back-off interval reaches 0, transmit RTS
Dynamic Contention Window
• Binary Exponential Back-off in 802.11 DCF – When a node fails to receive CTS in response
to its RTS, it increases the contention window• cw is doubled (up to an upper bound)
– When a node successfully completes a data transfer, it restores cw to cwmin
Priority-based Access Provisioning
• Using different values of inter frame space (IFS)• SIFS (short IFS): used for immediate response
actions• PIFS (Point coordination function IFS): used by
the centralized controller while issuing polls• DIFS (Distributed coordination function IFS):
minimum delay for asynchronous frames contending for access
DIFS > PIFS > SIFS
802.11 CSMA/CA
S2
S1
R
S2 S1 R X
X
Channel B
usy
DIFS
Channel Id
le
DIFS: DCF Inter-Frame Space
RTS
SIFS: Short Inter-Frame Space
CTS
SIFS
NAV
NAV
SIFS
DATASIFS
ACK
B2=9
B1=5
cw = 15
RTS
B2=4
B1=7
DIFSC
hannel Id
le
Point Coordination Function (PCF)
• PCF is implemented on top of DCF• The time sensitive traffic are controlled by the PCF and
the remaining traffic contend for access using CSMA/CA• The centralized polling master (point coordinator) issues
polls using PIFS • The poll responses use SIFS• The point coordinator could issue polls in a round robin
fashion• Seizing of the medium by the PCF is avoided by using
superframes where the point coordinator is allowed to poll for a fixed duration and then idle for the rest of the superframe period to allow the asynchronous traffic to contend for the medium.
MAC Frame Format
• Frame Control (FC): Indicated type of frame, provides control information
• Duration/connection ID (D/I): If used as a duration field -indicates time (in s) for which the channel will be allocated for transmission of a MAC frame. In some control frames, it contains an association, or connection identifier
• Addresses: Context dependent. Types include source, destination, transmitting station, receiving station
• Sequence Control: Used for fragmentation/reassembly. • Frame Body: Contains an MPDU or its fragment• Cyclic Redundancy Check (CRC): 32-bit frame check sequence
FC D/I Address Address Address SC Address Frame Body CRC2 2 6 6 6 2 6 0-2312 4
Frame Control Field
• Protocol Version (PV): 802.11 version, currently version 0• Type: Identifies the frame as control, management, or data• Subtype: Identifies the function of frame• To DS: The MAC coordination sets this bit to 1 in a frame destined to the
distribution system• From DS: The MAC coordination sets this bit to 1 in a frame leaving the
distribution system• More Fragments (MF): Set to 1 if more fragments follow• Retry (RT): Set to 1 if retransmission• Power Management (PM): Set to 1 if transmitting station is in sleep mode• More Data (MD): Indicates that a station has additional data to send• Wired Equivalent Privacy (WEP): WEP implemented• Order (O): The frames must be processed in order if set to 1.
PV Type SubType TO
DSFROM
DSMF RT PM MD W O
422 1 1 1 1 1 1 1 1
IEEE 802.11 Management Sublayer
• Registration
• Handoff
• Power Management
• Security
Registration
• A management frame called beacon is transmitted periodically by the AP to establish the timing synchronization function (TSF)
• TSF contains: BSS id, timestamp, traffic indication map (TIM), power management, and roaming information
• Received Signal Strength (RSS) measurements are done on the beacon message
• Association: process by which an MS registers with an AP
Handoff
• Mobility Types:– No transition – MS is static or moving within a
BSA– BSS transition – MS moves from one BSS to
another within the same ESS– ESS transition – MS moves from one BSS to
another BSS which belong to a different ESS
• Reassociation service is used when an MS moves from one BSS to another within the same ESS
Handoff procedure in IEEE 802.11
Beacon Periodically AP1
AP2
AP3
1. Strong Signal
2. Weak Signal; start scanning for handoff
4. Pro
be
Resp
onse
5. Choose AP with strongest response
8. IAPP indicates
reassociation to old AP
7. Reassociation Response
6. Reassociation
Request3. Probe Request
Power Management
• How to power-off during idle periods?• IEEE 802.11 buffers data at the AP, and sends
the data when the MS is awakened• Using TSF, all MSs are synchronized – they
wake up at the same time to listen to beacon• With every beacon a TIM is sent that has a list of
stations having buffered data• An MS learns that it has buffered data by
checking beacon and TIM
Security
• There are provisions for authentication and privacy in IEEE 802.11
• Open system authentication (default)– Request frame sends the authentication algorithm id – the response frame sends the result
• Shared key authentication– Request frame sends the authentication frame id for
the shared key that is shared between itself and the AP
– The second station sends a challenge text– The first station sends the encrypted challenge as the
response– The second station sends the authentication result