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Local Area Networks. Chapter 10 – Wireless LANs. Wireless Communication. The proliferation of laptop computers and other mobile devices (PDAs and cell phones) created an obvious application level demand for wireless local area networking.

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local area networks

Local Area Networks

Chapter 10 – Wireless LANs

wireless communication
Wireless Communication
  • The proliferation of laptop computers and other mobile devices (PDAs and cell phones) created an obvious application level demand for wireless local area networking.
  • Companies jumped in, quickly developing incompatible wireless products in the 1990’s.
  • Industry decided to entrust standardization to the IEEE committee that dealt with wired LANS – namely, the IEEE 802 committee!!
  • Wireless communications compelling
    • Easy, low-cost deployment
    • Mobility & roaming: Access information anywhere
    • Supports personal devices
      • PDAs, laptops, data-cell-phones
    • Supports communicating devices
      • Cameras, location devices, wireless identification
    • Signal strength varies in space & time
    • Signal can be captured by snoopers
    • Spectrum is limited & usually regulated
wireless links
Wireless Links
  • Many end systems use wireless links:
    • Portable PCs within a wireless LAN
    • PDAs that connect to the Internet through wireless telephony infrastructure
    • Cameras, automobiles, etc.
  • Two standards for wireless networking:
    • IEEE 802.11b standard for wireless LANs (aka Wi-Fi)
    • Bluetooth standard that allows devices to communicate without being in line of sight
  • Wireless devices classified wrt power, range, and data rate
    • IEEE 802.11  high power, medium range, and high rate “access” technology
    • Bluetooth  low power, short range, low rate, “cable replacement” technology
ieee 802 11 wireless lan
IEEE 802.11 Wireless LAN
  • Wireless LANs: mobile networking
  • IEEE 802.11 standard:
    • MAC protocol
    • Unlicensed frequency spectrum: 2.4Ghz (802.11b) or 5-6 Ghz (802.11a)
    • Provides wireless Ethernet access at 11 Mbps or54 Mbps (802.11a)
  • Basic Service Set (BSS) (a.k.a. “cell”) contains:
    • wireless hosts
    • access point (AP): base station
  • BSS’s combined to form distribution system (DS)
wireless standards
Wireless Standards

Frequency, Hopping Spread Spectrum (FHSS)

Direct Sequence Spread Spectrum (FHSS) HR: High Rate

Orthogonal Frequency Division Multiplexing (OFDM, VOFDM, COFDM)

wireless physical layer
Wireless Physical Layer
  • Physical layer conforms to OSI (five options)
    • 1997: 802.11 infrared, FHSS, DHSS
    • 1999: 802.11a OFDM and 802.11b HR-DSSS
    • 2001: 802.11g OFDM
  • 802.11 Infrared
    • Two capacities 1 Mbps or 2 Mbps.
    • Range is 10 to 20 meters and cannot penetrate walls.
    • Does not work outdoors.
  • 802.11 FHSS (Frequency Hopping Spread Spectrum)
    • The main issue is multipath fading.
    • 79 non-overlapping channels, each 1 Mhz wide at low end of 2.4 GHz ISM band.
    • Same pseudo-random number generator used by all stations.
    • Dwell time: min. time on channel before hopping (400msec).
wireless physical layer1
Wireless Physical Layer

Frequency Hopping Spread Spectrum

wireless physical layer2
Wireless Physical Layer
  • 802.11 DSSS (Direct Sequence Spread Spectrum)
    • Spreads signal over entire spectrum using pseudo-random sequence (similar to CDMA see Tanenbaum sec. 2.6.2).
    • Each bit transmitted using an 11 chips Barker sequence, PSK at 1Mbaud.
    • 1 or 2 Mbps.
  • 802.11a OFDM (Orthogonal Frequency Divisional Multiplexing)
    • CompatiblewithEuropean HiperLan2.
    • 54Mbps in wider 5.5 GHz band  transmission range is limited.
    • Uses 52 FDM channels (48 for data; 4 for synchronization).
    • Encoding is complex ( PSM (Power saving mode) up to 18 Mbps and QAM above this capacity).
    • E.g., at 54Mbps 216 data bits encoded into into 288-bit symbols.
    • More difficulty penetrating walls.
wireless physical layer3
Wireless Physical Layer

Direct Sequence Spread Spectrum

wireless physical layer4
Wireless Physical Layer
  • 802.11bHR-DSSS (High Rate Direct Sequence Spread Spectrum)
    • 11a and 11bshows a split in the standards committee.
    • 11b approved and hit the market before 11a.
    • Up to 11 Mbps in 2.4 GHz band using 11 million chips/sec.
    • Note in this bandwidth all these protocols have to deal with interference from microwave ovens, cordless phones and garage door openers.
    • Range is 7 times greater than 11a.
    • 11b and 11a are incompatible!!
  • 802.11g OFDM(Orthogonal Frequency Division Multiplexing)
    • An attempt to combine the best of both 802.11a and 802.11b.
    • Supports bandwidths up to 54 Mbps.
    • Uses 2.4 GHz frequency for greater range.
    • Is backward compatible with 802.11b.
infrastructure network
B1

A1

Gateway to

the Internet

Portal

Server

Distribution System

Portal

AP1

A2

B2

AP2

BSS A

BSS B

Infrastructure Network
  • Permanent Access Points provide access to Internet
802 11 definitions
802.11 Definitions
  • Basic Service Set (BSS)
    • Group of stations that coordinate their access using a given instance of MAC
    • Located in a Basic Service Area (BSA)
    • Stations in BSS can communicate with each other
    • Distinct collocated BSS’s can coexist
  • Extended Service Set (ESS)
    • Multiple BSSs interconnected by Distribution System (DS)
    • Each BSS is like a cell and stations in BSS communicate with an Access Point (AP)
    • Portals attached to DS provide access to Internet
ad hoc networks
Ad Hoc Networks
  • Ad hoc network: IEEE 802.11 stations can dynamically form network without AP
  • Formed “on the fly” when mobile devices are in proximity
  • Applications:
    • “Laptop” meeting in conference room, car
    • Interconnection of “personal” devices
    • Battlefield
  • IETF MANET (Mobile Ad hoc Networks) working group
hidden terminal problem
C

A

B

(b)

Data Frame

B

C

Data Frame

A

C transmits data frame & collides with A at B

Hidden Terminal Problem

(a)

Data Frame

A transmits data frame

C senses medium,

station A is hidden from C

  • New MAC: CSMA with Collision Avoidance
ieee 802 11 mac protocol csma ca collision avoidance
802.11 CSMA: sender

if sense channel idle for Distributed Inter Frame Space (DIFS) sec.

then transmit entire frame (no collision detection)

if sense channel busy then binary backoff

802.11 CSMA receiver:

if received OK

return ACK after Short Inter Frame Spacing (SIFS)

(DIFS = SIFS + 2 × slot time)

Time slot= 20 micro s, SIFS=10 micro s, DIFS=50 micro s.

IEEE 802.11 MAC Protocol: CSMA/CA (collision avoidance)
ieee 802 11 mac protocol
IEEE 802.11 MAC Protocol

802.11 CSMA Protocol: others

  • Other stations wait for a random backoff period after DIFS after current transmission
    • Avoids collisions
    • Collisions  uses exponentially increasing backoff period
  • Collisions detection is difficult:
    • Hidden terminal problem
    • Fading
  • NAV: Network Allocation Vector:
    • 802.11 frame has transmission duration field
    • Others (hearing stations) defer access (to save power) for NAV time units
hidden terminal effect
Hidden Terminal effect
  • Hidden terminals: A, C cannot hear each other
    • Obstacles, signal attenuation
    • Collisions at B
  • Goal: avoid collisions at B
  • CSMA/CA: CSMA with Collision Avoidance

Fading can also result in collisions

collision avoidance rts cts exchange
Collision Avoidance: RTS-CTS exchange
  • CSMA/CA: explicit channel reservation
    • sender: send short RTS: request to send
    • receiver: reply with short CTS: clear to send
  • CTS reserves channel for sender, notifying (possibly hidden) stations
    • Benefit: RTC-CTS avoids hidden station collisions
collision avoidance rts cts exchange1
Collision Avoidance: RTS-CTS exchange
  • CA with RTS-CTS:
    • Collisions less likely, of shorter duration
    • End result similar to collision detection
  • IEEE 802.11 allows:
    • CSMA
    • CSMA/CA: reservations
    • polling from AP
csma with collision avoidance
(a)

B

RTS

C

A requests to send

(b)

CTS

B

CTS

A

C

B announces A ok to send

(c)

B

Data Frame

A sends

C remains quiet

CSMA with Collision Avoidance
ieee 802 11 wireless lan4
IEEE 802.11 Wireless LAN
  • Stimulated by availability of unlicensed spectrum
    • U.S. Industrial, Scientific, Medical (ISM) bands
    • 902-928 MHz, 2.400-2.4835 GHz, 5.725-5.850 GHz
  • Targeted wireless LANs @ 20 Mbps
  • MAC for high speed wireless LAN
  • Ad Hoc & Infrastructure networks
  • Variety of physical layers
infrastructure network1
B1

A1

Gateway to

the Internet

Portal

Distribution System

Server

Portal

AP1

A2

B2

AP2

BSS A

BSS B

Infrastructure Network
distribution services
Distribution Services
  • Stations within BSS can communicate directly with each other
  • DS provides distribution services:
    • Transfer MAC SDUs between APs in ESS
    • Transfer MSDUs between portals & BSSs in ESS
    • Transfer MSDUs between stations in same BSS
      • Multicast, broadcast, or stations’s preference
  • ESS looks like single BSS to LLC layer
infrastructure services
Infrastructure Services
  • Select AP and establish association with AP
    • Then can send/receive frames via AP & DS
  • Reassociation service to move from one AP to another AP
  • Dissociation service to terminate association
  • Authentication service to establish identity of other stations
  • Privacy service to keep contents secret
ieee 802 11 mac
IEEE 802.11 MAC
  • MAC sublayer responsibilities
    • Channel access
    • PDU addressing, formatting, error checking
    • Fragmentation & reassembly of MAC SDUs
  • MAC security service options
    • Authentication & privacy
  • MAC management services
    • Roaming within ESS
    • Power management
mac services
MSDUs

MSDUs

Contention-free service

Contention service

Point coordination

function

MAC

Distribution coordination function (DCF)

(CSMA-CA)

Physical

MAC Services
  • Contention Service: Best effort
  • Contention-Free Service: time-bounded transfer
  • MAC can alternate between Contention Periods (CPs) & Contention-Free Periods (CFPs). MAC Service Data Unit (MSDU)
distributed coordination function dcf
Contention

window

DIFS

PIFS

DIFS

SIFS

Busy medium

Next frame

Time

Wait for

reattempt time

Defer access

Distributed Coordination Function (DCF)
  • DCF provides basic access service
    • Asynchronous best-effort data transfer
    • All stations contend for access to medium
  • CSMA-CA
    • Ready stations wait for completion of transmission
    • All stations must wait Interframe Space (IFS)
priorities through interframe spacing
Contention

window

DIFS

PIFS

DIFS

SIFS

Busy medium

Next frame

Time

Wait for

reattempt time

Defer access

Priorities through Interframe Spacing
  • High-Priority frames wait Short IFS (SIFS)
    • Typically to complete exchange in progress
    • ACKs, CTS, data frames of segmented MSDU, etc.
  • PCF IFS (PIFS) to initiate Contention-Free Periods
  • DCF IFS (DIFS) to transmit data & MPDUs
contention backoff behavior
Contention & Backoff Behavior
  • If channel is still idle after DIFS period, ready station can transmit an initial MPDU
  • If channel becomes busy before DIFS, then station must schedule backoff time for reattempt
    • Backoff period is integer # of idle contention time slots
    • Waiting station monitors medium & decrements backoff timer each time an idle contention slot transpires
    • Station can contend when backoff timer expires
  • A station that completes a frame transmission is not allowed to transmit immediately
    • Must first perform a backoff procedure
slide37
(a)

B

RTS

C

A requests to send

(b)

CTS

B

CTS

A

C

B announces A ok to send

(c)

B

Data Frame

A sends

C remains quiet

(d)

B

ACK

ACK

B sends ACK

carrier sensing in 802 11
Carrier Sensing in 802.11
  • Physical Carrier Sensing
    • Analyze all detected frames
    • Monitor relative signal strength from other sources
  • Virtual Carrier Sensing at MAC sublayer
    • Source stations informs other stations of transmission time (in msec) for an MPDU
    • Carried in Duration field of RTS & CTS
    • Stations adjust Network Allocation Vector to indicate when channel will become idle
  • Channel busy if either sensing is busy
transmission of mpdu without rts cts
DIFS

Data

Source

SIFS

ACK

Destination

DIFS

NAV

Other

Wait for Reattempt Time

Defer Access

Transmission of MPDU without RTS/CTS
transmission of mpdu with rts cts
DIFS

RTS

Data

Source

SIFS

SIFS

SIFS

CTS

Ack

Destination

DIFS

NAV (RTS)

NAV (CTS)

Other

NAV (Data)

Defer access

Transmission of MPDU with RTS/CTS
collisions losses errors
Collisions, Losses & Errors
  • Collision Avoidance
    • When station senses channel busy, it waits until channel becomes idle for DIFS period & then begins random backoff time (in units of idle slots)
    • Station transmits frame when backoff timer expires
    • If collision occurs, recompute backoff over interval that is twice as long
  • Receiving stations of error-free frames send ACK
    • Sending station interprets non-arrival of ACK as loss
    • Executes backoff and then retransmits
    • Receiving stations use sequence numbers to identify duplicate frames
point coordination function
Point Coordination Function
  • PCF provides connection-oriented, contention-free service through polling
  • Point coordinator (PC) in AP performs PCF
  • Polling table up to implementor
  • CFP repetition interval
    • Determines frequency with which CFP occurs
    • Initiated by beacon frame transmitted by PC in AP
    • Contains CFP and CP
    • During CFP stations may only transmit to respond to a poll from PC or to send ACK
pcf frame transfer
TBTT

Contention-free repetition interval

SIFS

SIFS

SIFS

SIFS

SIFS

Contention period

CF End

B

D2+Ack+Poll

D1 + Poll

U 2 + ACK

U 1 + ACK

PIFS

Reset NAV

NAV

CF_Max_duration

D1, D2 = frame sent by point coordinator

U1, U2 = frame sent by polled station

TBTT = target beacon transmission time

B = beacon frame

PCF Frame Transfer
distributed coordination function dcf1
Distributed Coordination Function (DCF)
  • DCF is the access method used to support asynchronous data transfer on a best effort basis
  • All stations must support the DCF (DCF operates solely in the ad hoc network)
  • Operates solely or coexists with the PCF in an infrastructure network
  • DCF sits directly on top of the physical layer and supports contention services:
    • Each station with an MSDU queued for transmission must contend for access to the channel
    • Once the MSDU is transmitted, must recontend for access to the channel for all subsequent frames
    • Contention services promote fair access to the channel for all stations.
  • The DCF is carrier sense multiple access with collision avoidance (CSMA/CA).
    • CSMA/CD is not used because a station is unable to listen to the channel for collisions while transmitting
    • In IEEE 802.11, carrier sensing is performed at both the air interface, referred to as physical carrier sensing, and at the MAC sublayer, referred to as virtual carrier sensing
    • Physical carrier sensing detects the presence of other IEEE 802.11 WLAN users by analyzing all detected packets, and also detects activity in the channel via relative signal strength from other sources
  • Virtual carrier sensing
    • Stations include MPDU duration in the header of request to send (RTS), clear to send (CTS), and data frames
    • An MPDU is a complete data unit that is passed from the MAC sublayer
    • to the physical layer
    • The MPDU contains header information, information, payload, and a 32-bit CRC
    • The duration field indicates the time (in microseconds) after the end of the present frame the channel will be utilized tocomplete the successful transmission of the data or management frame.
    • Stations in the BSS use the duration field to adjust their network allocation vector (NAV)
    • NAV indicates the amount of time that must elapse until the current transmission session is complete
distributed coordination function dcf2
Distributed Coordination Function (DCF)
  • DCF operates under the Contention Period (CP)
  • Three types of frames: management, control, and data
    • Management F: station association dis-association with AP
    • Control F: handshaking in CP, ACK data in CP, and end CFP
  • Basic DCF Access Method (no RTS-CTS):
    • When ST finds chaneel idle, it waits for DIFS and checks it again
    • If it is still idle, it transmits MPDU with medium busy time (including SIFS and ACK times)
    • Receiving st computes Checksum, if correct sends an ACK to source
    • All other STs in BSS hearing above messages adjust their NAV timers
distributed coordination function dcf3
Distributed Coordination Function (DCF)
  • RTS-CTS Data Mode
  • Priority Accsess: SIFS, PIFS (SIFS+1), and DIFS (SIFS+2)
  • In BSS, STs hearing RTS, CTS, F0, and ACK adjust their NAV
  • Sts: Basic mode, RTS/CTS mode if MPDU exceeds L, or always use RTS/CTS mode
  • Fairness: BEB starts with (1,8) and end at some maximum
distributed coordination function dcf4
Distributed Coordination Function (DCF)
  • MPDU (2300 bytes): collision lead to bandwidth loss
  • RTS is 20 bytes and CTS is 14 bytes
  • Fragmentation increases transmission reliability
  • Fragment MPDU, transmit Frag, receive ACK to completion
  • If no ACK, re-contend for medium and stat al last Frag.
  • In RTS-CTS mode, RTS-CTS used only in first frag.
point coordination function pcf on top of dcf
Point Coordination Function (PCF on top of DCF)
  • PCF (optional) operates under the Contention-Free Period (CFP)
  • Medium access contr. by Point Coordinator PC (AP/BSS, polling)
  • Polled Sts can transmit (No CSMA)
  • CFP Repetition Interval (Manag duration): (1) PCF, and (2) DCF
point coordination function pcf on top of dcf1
Point Coordination Function (PCF on top of DCF)
  • Light traffic: shorter CFP if previous DCF traffic is not complete
  • PC: PIFS, Beacon, (CF-poll/data/Data+CF-poll), CF-end.
  • CF-aware st:
    • Gets CF-poll,
    • Responds: CF-ACK, Data+CF-ACK,
    • Then PC responds by Data+CF-ACK+CF-poll
point coordination function pcf on top of dcf2
Point Coordination Function (PCF on top of DCF)
  • When ST receives a poll from IP:
    • Transmit a F to another ST in the BSS
    • When Dest receives F, a DCF-ACK is returned to source
    • PC waits for PIFS after ACK before continuation
frame types
Frame Types
  • Management frames
    • Station association & disassociation with AP
    • Timing & synchronization
    • Authentication & deauthentication
  • Control frames
    • Handshaking
    • ACKs during data transfer
  • Data frames
    • Data transfer
frame structure
Frame Structure

MAC header (bytes)

2

2

6

6

6

2

6

0-2312

4

  • MAC Header: 30 bytes
  • Frame Body: 0-2312 bytes
  • CRC: CCITT-32 4 bytes CRC over MAC header & frame body

Frame

Control

Duration/

ID

Address

1

Address

2

Address

3

Sequence

control

Address

4

Frame

body

CRC

frame control 1
MAC header (bytes)

2

2

6

6

6

2

6

0-2312

4

Frame

Control

Duration/

ID

Address

1

Address

2

Address

3

Sequence

control

Address

4

Frame

body

CRC

2

4

1

1

1

1

1

1

1

1

2

Protocol

version

Type

Subtype

To

DS

From

DS

More

frag

Retry

Pwr

mgt

More

data

WEP

Rsvd

Frame Control (1)
  • Protocol version = 0
  • Type: Management (00), Control (01), Data (10)
  • Subtype within frame type
  • Type=00, subtype=association; Type=01, subtype=ACK
  • MoreFrag=1 if another fragment of MSDU to follow
frame control 2
2

2

6

6

6

2

6

0-2312

4

Frame

Control

Duration/

ID

Address

1

Address

2

Address

3

Sequence

control

Address

4

Frame

body

CRC

2

4

1

1

1

1

1

1

1

1

2

Protocol

version

Type

Subtype

To

DS

From

DS

More

frag

Retry

Pwr

mgt

More

data

WEP

Rsvd

To

DS

1

0

0

1

1

0

From

DS

1

0

Destination

address

Destination

address

BSSID

Address

1

Receiver

address

Source

address

BSSID

Transmitter

address

Address

2

Source

address

Address

3

Source

address

Destination

address

Destination

address

BSSID

N/A

N/A

Source

address

N/A

Address

4

Meaning

Data frame from station to station within a BSS

Data frame exiting the DS

Data frame destined for the DS

WDS frame being distributed from AP to AP

Frame Control (2)

To DS = 1 if frame goes to DS; From DS = 1 if frame exiting DS

frame control 3
MAC header (bytes)

2

2

6

6

6

2

6

0-2312

4

Frame

Control

Duration/

ID

Address

1

Address

2

Address

3

Sequence

control

Address

4

Frame

body

CRC

2

4

1

1

1

1

1

1

1

1

2

Protocol

version

Type

Subtype

To

DS

From

DS

More

frag

Retry

Pwr

mgt

More

data

WEP

Rsvd

Frame Control (3)
  • Retry=1 if mgmt/control frame is a retransmission
  • Power Management used to put station in/out of sleep mode
  • More Data =1 to tell station in power-save mode more data buffered for it at AP
  • WEP=1 if frame body encrypted
physical layers
LLC PDU

LLC

MAC

layer

MAC

header

MAC SDU

CRC

Physical layer

convergence

procedure

Physical

layer

Physical medium

dependent

PLCP

preamble

PLCP

header

PLCP PDU

Physical Layers
  • 802.11 designed to
    • Support LLC
    • Operate over many physical layers
802 11 mac management
802.11 - MAC management
  • Synchronization
    • try to find a LAN, try to stay within a LAN
    • timer etc.
  • Power management
    • sleep-mode without missing a message
    • periodic sleep, frame buffering, traffic measurements
  • Association/Reassociation
    • integration into a LAN
    • roaming, i.e. change networks by changing access points
    • scanning, i.e. active search for a network
  • MIB - Management Information Base
    • managing, read, write
synchronization using a beacon infrastructure
Synchronization using a Beacon (infrastructure)

beacon interval

B

B

B

B

access

point

busy

busy

busy

busy

medium

t

B

value of the timestamp

beacon frame

synchronization using a beacon ad hoc
Synchronization using a Beacon (ad-hoc)

beacon interval

B1

B1

station1

B2

B2

station2

busy

busy

busy

busy

medium

t

B

value of the timestamp

beacon frame

random delay

power management
Power management
  • Idea: switch the transceiver off if not needed
  • States of a station: sleep and 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?)
power saving with wake up patterns infrastructure
T

D

awake

TIM

DTIM

data transmission

to/from the station

B

p

d

broadcast/multicast

PS poll

Power saving with wake-up patterns (infrastructure)

TIM interval

DTIM interval

D

B

T

T

d

D

B

access

point

busy

busy

busy

busy

medium

p

d

station

t

power saving with wake up patterns ad hoc
A

transmit ATIM

Power saving with wake-up patterns (ad-hoc)

ATIM

window

beacon interval

B1

A

D

B1

station1

B2

B2

a

d

station2

t

B

D

beacon frame

random delay

transmit data

a

d

awake

acknowledge ATIM

acknowledge data

802 11 roaming
802.11 - 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
wlan ieee 802 11b
What’s new?

Define a new PHY layer. All the MAC schemes, management procedures are the same

User data rate max. approx. 6 Mbit/s

Frequency

On certain frequencies in the free 2.4 GHz ISM-band

Security

Limited, WEP insecure, SSID

Cost

100€ adapter, 250€ base station, dropping

Availability

Many products, many vendors

Special Advantages/Disadvantages

Advantage: many installed systems, lot of experience, available worldwide, free ISM-band, many vendors, integrated in laptops, simple system

Disadvantage: heavy interference on ISM-band, no service guarantees, slow relative speed only

WLAN: IEEE 802.11b
bluetooth
Bluetooth
  • Most compelling application addressed by Bluetooth:
    • A convenient, untethered means to interconnect electronic devices
    • Examples: portable phones, PDAs, laptops, desktops, digital cameras, fax machines, printers, keyboard, mouse, etc.
  • Line-of-sight infrared technology has been used for such communications
    • Using RF wireless communication, Bluetooth does not require LoS
    • It can support multipoint as well as point-to-point communication
  • Bluetooth architecture:
    • Mobile devices need short-range transceivers
    • Transceivers operate in 2.5 Ghz unlicensed frequency band
    • Provide data rates of up to 721 kbps + 3 voice channels (64 kbps)
    • Operating range is 10 to 100 meters
    • Each device is identified by a 12-bit address
bluetooth cont d
Bluetooth (Cont’d)
  • Frequency hopping
    • Transceiver minimizes the effect of interference from other signals
    • Hops to a new frequency after transmitting or receiving a packet
  • Error recovery:
    • Transceiver forward error correction (FEC)
    • Automatic Repeat reQuest (ARQ) for retransmission
  • Bluetooth protocol suite includes:
    • Baseband protocol
      • Enables physical RF wireless connection between devices
      • A connection of 2-7 Bluetooth devices forms a small networkpiconet
    • Link manager protocol
      • Handshaking between two devices to establish connection
    • L2CAP protocol
      • During a connection, adapts upper layer protocols for transmission over the baseband
bluetooth physical upwards
Bluetooth - Physical Upwards!
  • 79 channels, each 1MHz, using FSK, with 1 bit per
  • symbol = 1Mbps
  • Much of the 1Mbps is taken up with protocol overheads – caused
  • by frequency hopping (250-260 ms needed to stabilise radio after
  • the hop!)
  • Leaves about 366 bits for actual data – of which 126 bits are headers
  • – leaving 240 bits for data per slot!
point to point data link control
Point to Point Data Link Control
  • One sender, one receiver, one link: easier than broadcast link:
    • No Media Access Control
    • No need for explicit MAC addressing
    • Examples:
      • Dialup link phone line  56 Kbps modem connections
      • SONET/SDH link
      • X.25 connection
      • ISDN line
  • Popular point-to-point DLC protocols:
    • PPP (point-to-point protocol)
    • HDLC: High level data link control (Data link used to be considered “high layer” in protocol stack!)
ppp design requirements rfc 1547
PPP Design Requirements [RFC 1547]
  • Packet framing:
    • Encapsulation of network-layer datagram in data link frame
    • Carry network layer data of any network layer protocol (not just IP)
    • Ability to demultiplex upwards
  • Bit transparency:
    • Must carry any bit pattern in the data field with no constraints
  • Error detection (no correction)
    • PPP receiver must be able to detect bit errors
  • Connection liveness:
    • Detect, signal link failure to network layer
  • Network layer address negotiation:
    • Endpoint can learn/configure each other’s network address
ppp non requirements
PPP Non-Requirements
  • No error correction/recovery
  • No flow control
    • PPP receiver is expected to receive frames at full physical layer speed  higher layer could drop packets or throttle sender
  • Out of order delivery OK
  • No need to support multipoint links (e.g., polling)
    • Other link layer protocols can support multipoint links
    • E.g., HDLC

Error recovery, flow control, data re-ordering

all relegated to higher layers!|

ppp data frame
PPP Data Frame
  • Flag: delimiter (framing)
  • Address: does nothing (only one option)
  • Control: does nothing; in the future possible multiple control fields
    • PPP sender can allow sender to skip address and control bytes
  • Protocol: upper layer protocol to which frame delivered
    • Examples: PPP-LCP, IP, IPCP, etc
    • RFC 1700 and RFC 3232 define 16-bit protocol codes for PPP
ppp data frame cont d
PPP Data Frame (Cont’d)
  • Info:
    • Variable length upper layer data being carried
    • Default maximum is 1500 bytes
    • Can be changed when the link is initially configured
  • Check:
    • Uses cyclic redundancy check (CRC) for error detection
    • Two or 4 bytes CRC
byte stuffing
Byte Stuffing
  • “Data transparency” requirement: data field must be allowed to include flag pattern <01111110>
    • Q: is received <01111110> data or flag?
  • Sender:
    • Adds (“stuffs”) an escape byte < 01111101> before each <01111110> data byte
  • Receiver:
    • Discards the escape byte and continues data reception
    • Single 01111110 flag byte
    • If two <01111101> bytes in a row  discard the first escape byte and continue data reception
byte stuffing1
Byte Stuffing

flag byte

pattern

in data

to send

flag byte pattern plus

stuffed byte in transmitted data

ppp link and network control protocols
PPP Link and Network Control Protocols

Before exchanging network-layer data, data link peers must:

  • Configure PPP link (max. frame length, authentication)
  • Learn/configure network

layer information

    • For IP: carry IP Control Protocol (IPCP) msgs (protocol field: 8021) to configure/learn IP address

PPP link always begins andends in the dead state

ppp link control protocol lcp
PPP Link Control Protocol (LCP)
  • Link establishment state:
    • Entered on an event that indicates presence of a physical layer, which is ready to be used: carrier detection, user intervention
    • One end of the link uses configure-request frame to indicate its configuration options
      • PPP frame with protocol filed set equal to LCP
      • Information field contains the specific configuration request
    • Options:
      • Maximum frame size for the link
      • Specification of authentication protocol to be used (if any)
      • Option to skip the address and control fields in PPP frames
    • The other side responds with configure-ack, configure-nak, or configure-reject frame
  • Network layer configuration begins after link is established:
    • Options negotiation done and authentication performed (if any)
    • Network layer specific control packets are exchanged with each other
ppp network control protocol ipcp
PPP Network Control Protocol (IPCP)
  • If IP is running over PPP, IP control protocol (IPCP) is used
    • IPCP is carried within a PPP frame
      • Protocol field will have IPCP  indicated by 0x8021
    • IPCP allows two IP modules to exchange or configure IP addresses
    • IPCP also allows two IP modules to negotiate whether or not IP datagrams will be sent in compressed form
    • Similar network control protocols for other network protocols:
      • Examples: DECnet, AppleTalk, etc.
  • Link goes in open state after network configuration
    • PPP can start exchanging network layer datagrams
    • To check the link status, use echo-request and echo-reply LCP frames
  • Terminating state
    • One side sends LCP terminate-request and other responds with LCP terminate-ack frame
    • Link goes to the dead state again
asynchronous transfer mode atm
Asynchronous Transfer Mode (ATM)
  • Two types of networks have existed side by side:
    • Telephone networks carry real-time voice
    • Data networks carry non real-time datagrams
  • ATM standards were developed in mid-1980’s
    • Goal: design a network technology that will be appropriate for both types of traffic
    • Standard developed by ATM Forum and ITU for broadband digital services networks
  • ATM technology:
    • A full suite of communication protocols form application to physical layer
    • Calls for packet switching within virtual circuits  virtual channels
    • Deployed in both telephone networks and Internet backbones
    • High performance ATM switches can deliver terabits per second!
    • Still could not replace TCP/IP based networks at desktop level
characteristics of atm
ATM service models:

Constant bit rate (CBR)

Variable bit rate (VBR)

Available bit rate (ABR)

Unspecified bit rate (UBR)

ATM uses fixed-length packets cells

Header: 5 bytes and payload: 48 bytes

Fixed length cell and simple header facilitate high speed switching

ATM VCs  virtual channels

Header includes virtual channel identifier (VCI) field

VCI is used by switches to forward the cells

Connection-oriented service

Cells always arrive in-order

ATM does not provide acks as other connection-oriented protocols do

Effectively, a VC is full duplex

Channel capacity and other properties may be different in two directions

Date rates:

155 Mbps, 622 Mbps, and higher

Characteristics of ATM
characteristics of atm cont d
Characteristics of ATM (Cont’d)
  • No link-by-link retransmissions
    • If an ATM switch detects error in a header, it tries to correct it
    • Simply drops the cell if error cannot be correctedno retransmission request
  • Congestion control
    • Only for ABR service class
    • Network provides feedback to sender to regulate its rate
  • ATM protocol stack consists of three layers:
    • ATM physical layer
    • ATM layer
    • ATM adaptation layer (AAL)
      • Analogous to transport layer in TCP/IP stack
      • Multiple types of AALs
cell header formats
Cell Header Formats
  • In both cases, cells consist of:
    • 5 byte header and 48 byte payloads
    • Headers are slightly different for two interfaces (GFC field is unused any way)
  • Header fields
    • VPI is a small integer that selects a particular virtual path
    • VCI selects a particular VC from within the chosen virtual path
cell header formats cont d
VPI and VCI

At UNI, 8 bit VPI means that host may have up to 256 virtual paths, each containing 65,536 VCs (16 bits)

Actually slightly less as some VCs are used for control functions

PTI field defines the type of payload

E.g., 000 means user data cell with no congestion and cell type 0 while 010 means user data cell that experienced congestion

A cell sent by the user as 000 may arrive as 010

Types are user supplied but congestion info is network supplied

CLP is set by a host to differentiate between high and low priority traffic

In case of congestion, switch will first drop cells with CLP 1 before dropping cells with CLP 0

HEC byte provides error control over the header

All single bit and 90% of multibit errors can be corrected

A 48 byte payload follows header

Not all 48 bytes available for payload as some of the AAL protocols put their headers and trailers inside the payload

Cell Header Formats (Cont’d)
connection setup
Connection Setup
  • ATM supports two types of VCs
    • Permanent VCs: present at all times like leased lines
    • Switched VCs: have to be setup for each session
      • Connection setup is not part of ATM layer
      • Described by ITU protocol Q.2931, which is part of control plane
  • Connection setup is a two-step process
    • First, a VC is acquired for signaling
      • To establish such a circuit, cells containing a request are sent to virtual path 0, VC 5
    • If first step is successful, a new VC is opened on which connection setup request and replies are transmitted
messages for connection setup in atm
Messages for Connection Setup in ATM
  • Four messages are used for establishment
    • Host sends a SETUP message on a special VC
    • Network responds with CALL PROCEEDING at each hop
    • When SETUP arrives at destination it responds with CONNECT that propagates back towards originator
    • Each switch returns a CONNECT ACK to originator
  • Two messages are used for release of a VC
    • Host wishing to release sends a request
    • Intermediate switches respond as request propagates
connection setup cont d
Connection Setup (Cont’d)
  • Multicast connection setup
    • A multicast channel has one sender and multiple receivers
    • Constructed by first setting up connection to one destination
    • ADD PARTY messages are sent to add more receivers to the VC previously returned
  • ATM addresses
    • Setup messages include destination address
    • ATM addresses come in three forms
      • Type 1: 20 bytes long OSI addresses
        • First byte indicates which of three formats
        • Bytes 2 and 3 specify country; byte 4 gives format for the rest of address that contains 3-byte authority, 2-byte domain, 2-byte area, and 6-byte add.
      • Type 2: bytes 2 and 3 designate an international organization and rest is same as in type 1
      • Type 3: 15 digit decimal ISDN telephone number
atm adaptation layer
ATM Adaptation Layer
  • ATM layer does not provide error or flow control to applications
    • Only 53 byte cells are output
    • Not directly useable for applications
    • ATM Adaptation Layer (AAL) was defined to bridge this gap
  • AAL protocols:
    • Four protocols to handle four classes of service
      • AAL1 – AAL4
      • Requirements for classes C and D were so similar that AAL3 and AAL4 are combined into AAL ¾
      • AAL1 for CBR and AAL2 for VBR
    • AAL5 proposed by computer industry in contrast to telecommunication industry that proposed AAL1 – AAL3/4  for IP datagrams
structure of the aal
Convergence sublayer (service specific part)

Covergence sublayer (common part)

Segmentation reassembly sublayer

ATM layer

ATM physical layer

Structure of the AAL
  • AAL has two parts:
    • Convergence sublayer
      • Interfaces with application for framing and error detection
      • Two parts: service-specific part and common part
    • Segmentation And Reassembly (SAR) sublayer
      • Adds headers and trailers to data units given by convergence layer to form cell payloads
convergence and sar layer operations
Convergence and SAR Layer Operations
  • Convergence sublayer adds its header/trailer to the message
  • Message is broken into 44-48 byte units, which are passed to SAR
  • SAR adds its own header/trailer and passes each piece to ATM layer
  • Some AAL protocols have null header/trailer
    • Above figure represents the most general case
ip over atm
ATM is widely used as Internet backbone

Permanent VCs between each pair of entry/exit point

Permanent VCs avoid having to establish dynamic VCs for transiting cells

Fro n entry points, n(n-1) permanent VCs are needed

Routers have 2 addresses:

An IP address

An ATM (LAN) address

ATM network needs to transit datagram to the exit router

Uses permanent VC

Uses AAL5

IP over ATM
practice problem 1
Practice Problem # 1
  • Q: Consider a CSMA/CD network running at 1 Gbps over a 1 km cable with no repeaters. The signal speed in the cable is 200,000 km/sec. What is the minimum frame size?
  • A:
    • For a 1 km cable, the one-way propagation time is 5 msec or 2t = 10 msec. Shortest frame should take more than this time to transmit to allow the sender to identify any collisions in the worst case.
    • At 1Gbps, the number of bits that should be transmitted during 10 msec = 10,000 bits = 1250 bytes.
    • Thus, the frame should not be shorter than 1250 bytes.
practice problem 2
Practice Problem # 2
  • Q:A 4-Mbps token ring has a token holding timer value of 10 msec. What is the longest frame that can be sent on this ring?
  • A:
    • At 4 Mbps, a station can transmit 40,000 bits or 5000 bytes in 10 msec.
    • This is an upper bound on frame length.
    • From this amount, some overhead bytes must be subtracted, giving a slightly lower limit for the data portion.
practice problem 3
Practice Problem # 3
  • Q:At a transmission rate of 5 Mbps and a propagation speed of 200 m/msec, to how many meters of cable is the 1-bit delay in a token ring interface equivalent?
  • A:
    • At 5 Mbps, a bit time is 200 nsec.
    • In 200 ns, the signal travels 40 m.
    • Thus, insertion of one new station adds as much delay as insertion of 40 meters of cable.
practice problem 4
Q:A very heavily loaded 1-km long, 10 Mbps token ring has a propagation speed of 200 m/msec. There are 50 stations uniformly spaced along the ring. Data frames are 256 bits, including 32 bits of overhead. Acknowledgements are piggybacked onto the data frames are are thus included as spare bits within the data frames and are effectively free. The token is 8 bits. Is the effective data rate of this ring higher or lower than the effective data rate of 10 mbps CSDM/CD network?

A:

Measured from the time of token capture, it takes 25.6 msec to transmit a packet.

Additionally, a token must be transmitted, taking 0.8 msec

Token must propagate 20 meters taking 0.1 msec.

Thus we have sent 224 bits in 26.5 msec, which results in an effective data rate of 8.5 Mbps. This is more than the effective bandwidth for the Ethernet (4.7 Mbps(why?)) under the same parameters.

Practice Problem # 4
practice problem 5
Practice Problem # 5
  • Q:Ethernet frame must be at least 64 bytes long to ensure that the transmitter is still going in the event of a collision at the far end of the cable. Fast Ethernet has the same 64 byte minimum frame size but can get the bits out ten times faster. How is it possible to maintain the same minimum frame size?
  • A:The maximum wire length in Fast Ethernet is 1/10 as long as in the regular Ethernet.
practice problem 6
Practice Problem # 6
  • Q:A large FDDI ring has 100 stations and a token rotation time of 40 msec. The token holding time is 10 msec. What is the maximum achievable efficiency of the ring?
  • A:
    • With a rotation time of 40 msec and 100 stations, the time for the token to move between stations is 40/100=0.4 msec.
    • A station may transmit for 10 msec, followed by a 0.4 msec gap while the token moves to the next station.
    • The best case efficiency is then 10/10.4=96%.
power management1
Power management
  • Idea: switch the transceiver off if not needed
  • States of a station: sleep and 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?)
power saving with wake up patterns infrastructure1
T

D

awake

TIM

DTIM

data transmission

to/from the station

B

p

d

broadcast/multicast

PS poll

Power saving with wake-up patterns (infrastructure)

TIM interval

DTIM interval

D

B

T

T

d

D

B

access

point

busy

busy

busy

busy

medium

p

d

station

t

power saving with wake up patterns ad hoc1
A

transmit ATIM

Power saving with wake-up patterns (ad-hoc)

ATIM

window

beacon interval

B1

A

D

B1

station1

B2

B2

a

d

station2

t

B

D

beacon frame

random delay

transmit data

a

d

awake

acknowledge ATIM

acknowledge data

802 11 roaming1
802.11 - 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
wlan ieee 802 11b1
Frequency

On certain frequencies in the free 2.4 GHz ISM-band

Security

Limited, WEP insecure, SSID

Cost

100€ adapter, 250€ base station, dropping

Availability

Many products, many vendors

Special Advantages/Disadvantages

Advantage: many installed systems, lot of experience, available worldwide, free ISM-band, many vendors, integrated in laptops, simple system

Disadvantage: heavy interference on ISM-band, no service guarantees, slow relative speed only

What’s new?

Define a new PHY layer. All the MAC schemes, management procedures are the same

User data rate max. approx. 6 Mbit/s

WLAN: IEEE 802.11b
channel selection non overlapping
Channel selection (non-overlapping)

Europe (ETSI)

channel 1

channel 7

channel 13

2400

2412

2442

2472

2483.5

[MHz]

22 MHz

US (FCC)/Canada (IC)

channel 1

channel 6

channel 11

2400

2412

2437

2462

2483.5

[MHz]

22 MHz

wlan ieee 802 11a
Frequency

US 5 GHz: free 5.15-5.25, 5.25-5.35, 5.725-5.825 GHz ISM-band

Connection set-up time

Connectionless/always on

Security

Limited, WEP insecure, SSID

Availability

Some products, some vendors

Quality of Service

Typ. best effort, no guarantees (same as all 802.11 products)

Special Advantages/Disadvantages

Advantage: fits into 802.x standards, free ISM-band, available, simple system, uses less crowded 5 GHz band

Disadvantage: stronger shading due to higher frequency, no QoS

WLAN: IEEE 802.11a
operating channels for 802 11a us u nii
Operating channels for 802.11a / US U-NII

channel

36

40

44

48

52

56

60

64

5150

5180

5200

5220

5240

5260

5280

5300

5320

5350

[MHz]

16.6 MHz

center frequency =

5000 + 5*channel number [MHz]

149

153

157

161

channel

5725

5745

5765

5785

5805

5825

[MHz]

16.6 MHz

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