medium access control for wireless links l.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
Medium Access Control for Wireless Links PowerPoint Presentation
Download Presentation
Medium Access Control for Wireless Links

Loading in 2 Seconds...

play fullscreen
1 / 86

Medium Access Control for Wireless Links - PowerPoint PPT Presentation


  • 162 Views
  • Uploaded on

Medium Access Control for Wireless Links. CS 515 Mobile and Wireless Networking Ibrahim Korpeoglu Computer Engineering Department Bilkent University. Outline. What we have see so far? PHY layer functions and parameters General Wireless System Architecture Media Access Control

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Medium Access Control for Wireless Links' - moana


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
medium access control for wireless links

Medium Access Control for Wireless Links

CS 515

Mobile and Wireless Networking

Ibrahim Korpeoglu

Computer Engineering Department

Bilkent University

outline
Outline
  • What we have see so far? PHY layer functions and parameters
  • General Wireless System Architecture
  • Media Access Control
    • Classes of MAC protocols
    • Simplex and Duplex Channels
  • Coordinated MAC Schemes
    • FDMA
    • TDMA
      • Capacity of TDMA systems and which factors affect the capacity.
    • Spread Spectrum Access Methods
      • FHMA
        • Case study: Bluetooth
      • CDMA
      • Hybrid Spread Spectrum Schemes.
  • Random MAC Schemes
    • CSMA
    • MACA and MACAW
    • Case Study: IEEE 802.11 MAC

© Ibrahim Korpeoglu

what we have seen so far
What we have seen so far?
  • Physical layer functions
    • Get stream of bits and transport them to the other end.
      • Modulation/Demodulation
    • We have seen that this is not an easy task
        • Large-scale path loss and Small-scale fading and multipath effects causes the received power at the receiver to
          • Fluctuate (hard to decode the symbols (bits))
          • To decrease (Affects of interfering sources increases)
        • Received signal power level affect the quality of the signals (information) that is transported.
          • Received signal power level defines the Signal-to-Noise (SNR) ratio

© Ibrahim Korpeoglu

what we have seen so far4
What we have seen so far?
  • We have seen
    • That SNR and bandwidth of a channel affects
      • Datarate – bps) of the wireless channel by Shannon limit
      • The bit error rate (BER) on the channel.
    • That multipath fading results in a wireless channel error model that changes states between good (low-error rate) and bad (high error-rate)
    • Large-scale path loss defines the range of stations for different environments (LOS, urban,…)
  • The above factors are important channel characteristics that affect the design of wireless systems architectures and design of the protocols and applications for wireless links/networks
  • In short, we have seen so far some of Fundamental Concepts of Wireless Communication.

© Ibrahim Korpeoglu

what we will do now
What we will do now?
  • We will look now to the protocols, algorithms, schemes that are developed over this wireless channels.
        • How can we share a wireless channel:
          • Results in Wireless Media Access Control Protocols
        • How we can change base stations: Results in Handoff algorithms and protocols
        • How can we seamlessly support mobile applications over wireless links:
          • Results in mobility protocols like Mobile IP, Cellular IP, etc.
        • How can we design efficient transport protocols over wireless links:
          • Results in solutions like SNOOP, I-TCP, M-TCP, etc.
        • How different wireless networks/systems are designed?
          • Bluetooth, IEEE 802.11, GSM, etc.

© Ibrahim Korpeoglu

wireless system architecture and functions
Wireless System Architecture and Functions

Applications

TCP/IP

Neighbor Discovery and Registration,

Multicasting, Power Saving Modes, Address

Translation (IP-MAC), Routing, Quality of Services,

Subnet Security

Wireless Subnet

Controller

Medium Access Control, MAC level Scheduling,

Link Layer Queueing, Link Layer Reliability – ACKs,

NACKs, ….

Wireless

Link Layer

(Layers 1 and 2 in ISO/OSI Network Reference Model)

Link Controller

Transceiver

Frame Controller

Framing and frame synchronization, error control,

CRC, bit scrambling, widening, ….

Carrier frequency, channel bandwidth, carrier detect,

Captude detect, channel data rate, modulation,

Received signal strength (RSSI), transmit power,

Power control, …

Physical Radio

© Ibrahim Korpeoglu

medium access control
Medium Access Control
  • Wireless spectrum (frequency band) is a very precious and limited resource.
      • We need to use this resource very efficiently
  • We also want our wireless system to have high user capacity
      • A lot of (multiple) users should be able to use the system at the same time.
  • For these reasons most of the time, multiple users (or stations, computers, devices) need to share the wireless channel that is allocated and used by a system.
      • The algorithms and protocols that enables this sharing by multiple users and controls/coordinates the access to the wireless channel (medium) from different users are called MEDIUM ACCESS, or MEDIA ACCESS or MULTIPLE ACCESS protocols, techniques, schemes, etc…)

© Ibrahim Korpeoglu

wireless media access control
Wireless Media Access Control
  • Random Schemes (Less-Coordinated)
      • Examples: MACA, MACAW, Aloha, 802.11 MAC,…
      • More suited for wireless networks that are designed to carry data: IEEE 802.11 Wireless LANs
  • Coordinated Schemes
      • Examples: TDMA, FDMA, CDMA
      • More suited for wireless networks that are designed to carry voice: GSM, AMPS, IS-95,…
  • Polling based Schemes
      • Examples: Bluetooth, BlueSky,…
      • Access is coordinated by a central node
      • Suitable for Systems that wants low-power, aims to carry voice and data at the same time.

© Ibrahim Korpeoglu

duplexing
Duplexing
  • It is sharing the media between two parties.
  • If the communication between two parties is one way, the it is called simplex communication.
  • If the communication between two parties is two- way, then it is called duplex communication.
  • Simplex communication is achieved by default by using a single wireless channel (frequency band) to transmit from sender to receiver.
  • Duplex communication achieved by:
      • Time Division (TDD)
      • Frequency Division (FDD)
      • Some other method like a random access method

© Ibrahim Korpeoglu

duplexing10
Duplexing
  • Usually the two parties that want to communication in a duplex manner (both send and receive) are:
      • A mobile station
      • A base station
  • Two famous methods for duplexing in cellular systems are:
      • TDD: Time Division Duplex
      • FDD: Frequency Division Duplex

© Ibrahim Korpeoglu

duplexing fdd
Duplexing - FDD

F

  • A duplex channel consists of two simplex channel with different carrier frequencies
      • Forward band: carries traffic from base to mobile
      • Reverse band: carries traffic from mobile to base

M

B

R

Base

Station

Mobile

Station

Reverse

Channel

Forward

Channel

frequency

fc,,F

fc,R

Frequency separation

Frequency separation should be carefully decided

Frequency separation is constant

© Ibrahim Korpeoglu

duplexing tdd
Duplexing - TDD
  • A single radio channel (carrier frequency) is shared in time in a deterministic manner.
      • The time is slotted with fixed slot length (sec)
      • Some slots are used for forward channel (traffic from base to mobile)
      • Some slots are used for reverse channel (traffic from mobile to base)

M

B

Mobile

Station

Base

Station

Slot number

0 1 2 3 4 5 6 7 …

F

R

F

R

F

R

F

R

….

channel

Reverse

Channel

Forward

Channel

time

Ti+1

Ti

Time separation

© Ibrahim Korpeoglu

duplexing tdd versus fdd
Duplexing – TDD versus FDD
  • FDD
        • FDD is used in radio systems that can allocate individual radio frequencies for each user.
          • For example analog systems: AMPS
        • In FDD channels are allocated by a base station.
        • A channel for a mobile is allocated dynamically
        • All channels that a base station will use are allocated usually statically.
        • More suitable for wide-area cellular networks: GSM, AMPS all use FDD
  • TDD
        • Can only be used in digital wireless systems (digital modulation).
        • Requires rigid timing and synchronization
        • Mostly used in short-range and fixed wireless systems so that propagation delay between base and mobile do not change much with respect to location of the mobile.
          • Such as cordless phones…

© Ibrahim Korpeoglu

multiple access coordinated
Multiple Access - Coordinated
  • We will look now sharing the media by more than two users.
  • Three major multiple access schemes
    • Time Division Multiple Access (TDMA)
      • Could be used in narrowband or wideband systems
    • Frequency Division Multiple Access (FDMA)
      • Usually used narrowband systems
    • Code Division Multiple Access
      • Used in wideband systems.

© Ibrahim Korpeoglu

narrow and wideband systems
Narrow- and Wideband Systems
  • Narrowband System
    • The channel bandwidth (frequency band allocated for the channel is small)
        • More precisely, the channel bandwidth is large compared to the coherence bandwidth of the channel (remember that coherence bandwidth is related with reciprocal of the delay spread of multipath channel)
        • AMPS is a narrowband system (channel bandwidth is 30kHz in one-way)
  • Wideband Systems
    • The channel bandwidth is large
        • More precisely, the channel bandwidth is much larger that the coherence bandwidth of the multipath channel.
        • A large number of users can access the same channel (frequency band) at the same time.

© Ibrahim Korpeoglu

slide16
Narrowband Systems
      • Could be employing one of the following multiple access and duplexing schems
        • FDMA/FDD
        • TDMA/FDD
        • TDMA/TDD
  • Wideband systems
      • Could be employing of the following multiple access and duplexing schemes
        • TDMA/FDD
        • TDMA/TDD
        • CDMA/FDDCDMA/TDD

© Ibrahim Korpeoglu

cellular systems and mac
Cellular Systems and MAC

© Ibrahim Korpeoglu

frequency division multiple access
Frequency Division Multiple Access

B

  • Individual radio channels are assigned to individual users
  • Each user is allocated a frequency band (channel)
      • During this time, no other user can share the channel
  • Base station allocates channels to the users

fN,F

f1,F

f2,F

f2,R

f1,R

fN,R

M

M

M

© Ibrahim Korpeoglu

features of fdma
Features of FDMA
  • An FDMA channel carries one phone circuit at a time
  • If channel allocated to a user is idle, then it is not used by someone else: waste of resource.
  • Mobile and base can transmit and receive simultaneously
  • Bandwidth of FDMA channels are relatively low.
  • Symbol time is usually larger (low data rate) than the delay spread of the multipath channel (implies that inter-symbol interference is low)
  • Lower complexity systems that TDMA systems.

© Ibrahim Korpeoglu

capacity of fdma systems
Capacity of FDMA Systems

Frequency spectrum allocated for FDMA system

Guard

Band

channel

Guard

Band

Bt : Total spectrum allocation

Bguard: Guard band allocated at the edge of the spectrum band

Bc : Bandwidth of a channel

AMPS has 12.MHz simplex spectrum band, 10Khz guard band, 30kHz

channel bandwidth (simplex): Number of channels is 416.

© Ibrahim Korpeoglu

time division multiple access
Time Division Multiple Access
  • The allocated radio spectrum for the system is divided into time slots
    • In each slot a user can transmit or receive
    • A user occupiess a cyclically repeating slots.
    • A channel is logically defined as a particular time slot that repeats with some period.
  • TDMA systems buffer the data, until its turn (time slot) comes to transmit.
      • This is called buffer-and-burst method.
  • Requires digital modulation

© Ibrahim Korpeoglu

tdma concept
TDMA Concept

Downstream Traffic: Forward Channels: (from base to mobiles)

1

2

3

N

1

2

3

….

N

Logical forward channel to a mobile

Base station broadcasts to mobiles on each slot

Upstream Traffic: Reverse Channels: (from mobile to base)

1

2

3

N

1

2

3

….

N

Logical reverse channel from a mobile

A mobile transmits to the base station in its allocated slot

Upstream and downstream traffic uses of the two different carrier frequencies.

© Ibrahim Korpeoglu

tdma frames
TDMA Frames
  • Multiple, fixed number of slots are put together into a frame.
  • A frame repeats.
  • In TDMA/TDD: half of the slots in the frame is used for forward channels, the other is used for reverse channels.
  • In TDMA/FDD: a different carrier frequency is used for a reverse or forward
      • Different frames travel in each carrier frequency in different directions (from mobile to base and vice versa).
      • Each frame contains the time slots either for reverse channels or forward channel depending on the direction of the frame.

© Ibrahim Korpeoglu

general frame and time slot structure in tdma systems
General Frame and Time Slot Structure in TDMA Systems

One TDMA Frame

Preamble

Information

Trail Bits

Slot 1

Slot 2

Slot 3

Slot N

Guard

Bits

Sync

Bits

Control

Bits

Information

CRC

One TDMA Slot

A Frame repeats in time

© Ibrahim Korpeoglu

a tdma frame
A TDMA Frame
  • Preamble contains address and synchronization info to identify base station and mobiles to each other
  • Guard times are used to allow synchronization of the receivers between different slots and frames
      • Different mobiles may have different propagation delays to a base station because of different distances.

© Ibrahim Korpeoglu

efficiency of a frame tdma system
Efficiency of a Frame/TDMA-System
  • Each frame contains overhead bits and data bits.
      • Efficiency of frame is defined as the percentage of data (information) bits to the total frame size in bits.

bT: total number of bits in a frame

Tf: frame duration (seconds)

bOH: number of overhead bits

Number of channels in a TDMA cell:

m: maximum number of TDMA users supported in a radio channel

© Ibrahim Korpeoglu

slide27
TDMA
  • TDMA Efficiency
    • GSM: 30% overhead
    • DECT: 30% overhead
    • IS-54: 20% overhead.
  • TDMA is usually combined with FDMA
    • Neighboring cells be allocated and using different carrier frequencies (FDMA). Inside a cell TDMA can be used. Cells may be re-using the same frequency if they are far from each-other.
    • There may be more than one carrier frequency (radio channel) allocated and used inside each cell. Each carrier frequency (radio channel) may be using TDMA to further multiplex more user (i.e. having TDMA logical channels inside radio channels)
        • For example: GSM uses multiple radio channels per cell site. Each radio channel has 200KHz bandwidth and has 8 time slots (8 logical channels). Hence GSM is using FHMA combined with TDMA.

© Ibrahim Korpeoglu

contemporary tdma systems
Contemporary TDMA Systems

© Ibrahim Korpeoglu

features of tdma
Features of TDMA
  • Enables the sharing of a single radio channel among N users
  • Requires high data-rate per radio channel to support N users simultaneously.
    • High data-rate on a radio channel with fixed bandwidth requires adaptive equalizers to be used in multipath environments (remember the RSM delay spread s parameter)
  • Transmission occurs in bursts (not continues)
      • Enables power saving by going to sleep modes in unrelated slots
      • Discontinues transmission also enables mobile assisted handoff
  • Requires synchronization of the receivers.
      • Need guard bits, sync bits. large overhead per slot.
  • Allocation of slots to mobile users should not be uniform.
      • It may depend on the traffic requirement of mobiles.
      • This brings extra flexibility and efficiency compared to FDMA systems.

© Ibrahim Korpeoglu

capacity of tdma systems
Capacity of TDMA Systems
  • Capacity can be expressed as
    • System Capacity (the capacity of the overall system covering a region)
        • Depends on:
          • Range of cells
          • Whether the system can support macro-cells, micro-cells or pico-cells.
    • Cell Capacity
        • Depends on the radio link performance between a base-sation and mobiles:
          • The lowest C/I (carrier-to-interence) ratio the system can operate for example quality of transmission. This in turn depends on the speech coding technique, desired speech quality, etc.
          • Data-rate over the channel which dependsmodulation efficiency (bits_per_second/Hz) and channel bandwidth.
          • The frequency re-use factor

© Ibrahim Korpeoglu

system capacity
System Capacity:

Cluster: 7 cells constitute a cluster.

Cluster size = 7

C

B

A

y

B

D

x

A

G

G

E

A

z

F

Frequency reuse factor is 1/7: same frequency is used every 7 cell.

A is one set of frequencies, B is an other, etc.

A mobile in cell x receives carrier signal from base x and interferences from

base stations at cells y and z. The carrier signal strength of all combined signal

strengthfrom interfering base stations is called C/I or S/I ratio.

© Ibrahim Korpeoglu

c i affect on capacity
C/I affect on capacity
  • C/I ratio affects the cluster size, hence the frequency reuse factor.
      • Frequency_reuse_factor = 1 / cluster_size
      • Cluster size can be 3, 7, 12, 13, …
  • Cluster size affects the cell capacity
    • (it affects the maximum number of frequencies that can be used in a cell)
  • A low C/I requirement for appropriate quality enables smaller cluster sizes, hence larger frequency reuse factor, meaning that larger cell capacities

© Ibrahim Korpeoglu

comparing systems
AMPS Parameters

Channel bandwidth = 30Khz

Required C/I: 18 dB

Frequency re-use factor: 1/7 (cluster size=7)

Required bandwidth per user = 30kHz.

GSM Parameters

Channel banwidth: 200 KHz

Required bandwidth per user = 200/8 = 25 Khz.

Required minimum C/I: 9dB

Frequency re-use factor: 1/3 (cluster size=3)

Comparing Systems

© Ibrahim Korpeoglu

spread spectrum access
Spread Spectrum Access
  • SSMA uses signals that have transmission bandwidth that is several orders of magnitued larger than minimum required RF bandwidth.
  • Provides
    • Immunity to multipath interference
    • Robust multiple access.
  • Two techniques
    • Frequency Hopped Multiple Access (FHMA)
    • Direct Sequence Multiple Access (DSMA)
        • Also called Code Division Multiple Access – CDMA

© Ibrahim Korpeoglu

frequency hopping fhma
Frequency Hopping (FHMA)
  • Digital muliple access technique
  • A wideband radio channel is used.
      • Same wideband spectrum is used
  • The carrier frequency of users are varied in a pseudo-random fashion.
        • Each user is using a narrowband channel (spectrum) at a specific instance of time.
        • The random change in frequency make the change of using the same narrowband channel very low.
  • The sender receiver change frequency (calling hopping) using the same pseudo-random sequence, hence they are synchronized.
  • Rate of hopping versus Symbol rate
      • If hopping rate is greather: Called Fast Frequency Hopping
        • One bit transmitted in multiple hops.
      • If symbol rate is greater: Called Slow Frequency Hopping
        • Multiple bits are transmitted in a hopping period
        • GSM and Bluetooth are example systems

© Ibrahim Korpeoglu

case study bluetooth
Case Study - Bluetooth
  • Uses Frequency Hopping in cell (piconet) over a 79 MHz wideband radio channel.
  • Uses 79 narrowband channels (carrier frequencies) to hop through.
    • Freq (f) = 2402+k MHz, k = 0,...,78
    • Channel spacing is 1 MHz (narrowband channel bandwidth)
    • Wideband spectrum width = 79 MHz.
    • Hopping Rate = 1600 Hops/Second
    • Hopping sequence is determined by Bluetooth Hardware address and Clocks that are syncrozied between sender and receiver

79 MHZ

0

1

2

3

.....

77

78

79-Hop System

1 MHZ

A hop sequence could be: 7,1,78,67,0, 56,39,.......

© Ibrahim Korpeoglu

case study bluetooth p iconet and fhss
Case Study: Bluetooth – Piconet and FHSS

Each node is classified as master or slave.

Master defines a piconet (a cell). Maximum 7 slaves can be connected to

a master. Master coordinates access to the the media.

All traffic has to go over master.

Slaves can not talk to each-other

directly.

Picocell

S

Range = 10m

Raw Data-rate: 1 Mbps/piconet

Radio channel used by devices in

a piconet is 79MHz channel, which

Is frequency hopped: hopping

though 789 channels.

Hoprate = 1600 hops/sec

FHSS

M

S

S

All slaves and the master hops according to the same hopping sequence.

The hopping sequence is determined by the clock and BT_address of the master.

© Ibrahim Korpeoglu

case study bluetooth scatternet and fhss
Case Study: Bluetooth – Scatternet and FHSS

Piconet

S

S

Piconet can be combined

into scatternets.

Red slave acts as a

bridge between two

piconets.

M2

Piconet

S

FHSS

S

FHSS

M1

Each piconet uses FHSS with different

hopping sequences (masters are different).

This prevents interference between piconets.

S

S

© Ibrahim Korpeoglu

case study bluetooth media access in a piconet
Case Study: Bluetooth - Media access in a piconet

Inside a piconet, access to the

frequency hopped radio channel

is coordinated using time

division multiple access: TDMA/TDD.

Slot duration = 1/1600 sec = 625ms

Piconet

S1

FHSS

M

In an even slot, master transmits to a

slave.

In an odd slot, the slave that is addressed

in the previous master-to-slave slot transmits.

S3

S2

0 1 2 3 4 5 6 7 …..

M-S1

S1-M

M-S2

S2-M

M-S3

S3-M

M-S1

S1-M

……

slot time=625ms

© Ibrahim Korpeoglu

code division multiple access cdma
Code Division Multiple Access (CDMA)
  • In CDMA, the narrowband message signal is multiplied by a very large bandwidth signal called spreading signal (code) before modulation and transmission over the air. This is called spreading.
  • CDMA is also called DSSS (Direct Sequence Spread Spectrum). DSSS is a more general term.
  • Message consists of symbols
      • Has symbol period and hence, symbol rate
  • Spreading signal (code) consists of chips
      • Has Chip period and and hence, chip rate
      • Spreading signal use a pseudo-noise (PN) sequence (a pseudo-random sequence)
      • PN sequence is called a codeword
      • Each user has its own cordword
      • Codewords are orthogonal. (low autocorrelation)
      • Chip rate is oder of magnitude larger than the symbol rate.
  • The receiver correlator distinguishes the senders signal by examining the wideband signal with the same time-synchronized spreading code
  • The sent signal is recovered by despreading process at the receiver.

© Ibrahim Korpeoglu

cdma advantages
CDMA Advantages
  • Low power spectral density.
      • Signal is spread over a larger frequency band
      • Other systems suffer less from the transmitter
  • Interference limited operation
      • All frequency spectrum is used
  • Privacy
      • The codeword is known only between the sender and receiver. Hence other users can not decode the messages that are in transit
  • Reduction of multipath affects by using a larger spectrum
  • Random access possible
      • Users can start their transmission at any time
  • Cell capacity is not concerete fixed like in TDMA or FDMA systems. Has soft capacity
  • Higher capacity than TDMA and FDMA
  • No frequency management
  • No equalizers needed
  • No guard time needed
  • Enables soft handoff

© Ibrahim Korpeoglu

cdma principle
CDMA Principle

Represent bit 1 with +1

Represent bit 0 with -1

One bit period (symbol period)

1

1

Data

0

PN-Code

(codeword)

1 1 1 0 1 0 1 1

1 1 1 0 1 0 1 1

Coded

Signal

Chip period

Input to the modulator (phase modulation)

© Ibrahim Korpeoglu

processing gain
Processing Gain
  • Main parameter of CDMA is the processing gain that is defined as:

Gp: processing gain

Bspread: PN code rate

Bchip: Chip rate

R: Data rate

  • IS-95 System (Narrowband CDMA) has a gain of 64. Other systems have gain between 10 and 100.
    • 1.228 Mhz chipping rate
    • 1.25 MHz spread bandwidth

© Ibrahim Korpeoglu

near far problem and power control
Near Far Problem and Power Control

B

pr(M)

  • At a receiver, the signals may come from various (multiple sources.
      • The strongest signal usually captures the modulator. The other signals are considered as noise
      • Each source may have different distances to the base station
  • In CDMA, we want a base station to receive CDMA coded signals from various mobile users at the same time.
      • Therefore the receiver power at the base station for all mobile users should be close to eacother.
      • This requires power control at the mobiles.
  • Power Control: Base station monitors the RSSI values from different mobiles and then sends power change commands to the mobiles over a forward channel. The mobiles then adjust their transmit power.

M

M

M

M

© Ibrahim Korpeoglu

dsss transmitter
DSSS Transmitter

Baseband

BPF

sss(t)

Message

+

m(t)

Transmitted

Signal

p(t)

PN Code

Generator

Oscillator

fc

Chip Clock

© Ibrahim Korpeoglu

dsss receiver
DSSS Receiver

Phase Shift Keying

Demodulator

IF Wideband

Filter

Received

Data

Received

DSSS Signal

at IF

Synchronization

System

PN Code

Generator

© Ibrahim Korpeoglu

spectra of received signal
Spectra of Received Signal

Spectral Density

Spectral Density

Interference

Signal

Interference

Signal

Frequency

Frequency

Output of Correlator after

dispreading,

Input to Demodulator

Output of Wideband filter

© Ibrahim Korpeoglu

cdma example
CDMA Example (*)

R

Receiver (a base station)

Data=1011…

Data=0010…

A

B

Transmitter

Transmitter (a mobile)

Codeword=101010

Codeword=010011

Data transmitted from A and B is multiplexed using CDMA and codewords.

The Receiver de-multiplexes the data using dispreading.

(*) This example is adapted from the CDMA example of Prof. Randy Katz at UC-Berkeley.

© Ibrahim Korpeoglu

cdma example transmission from two sources
CDMA Example – transmission from two sources

1 0 1 1

A Data

0 1 0 0 1 1

0 1 0 0 1 1

0 1 0 0 1 1

0 1 0 0 1 1

A

Codeword

1 0 1 1 0 0

0 1 0 0 1 1

1 0 1 1 0 0

1 0 1 1 0 0

A Signal

0 0 1 0

B Data

1 0 1 0 1 0

1 0 1 0 1 0

1 0 1 0 1 0

1 0 1 0 1 0

B

Codeword

1 0 1 0 1 0

0 1 0 1 0 1

1 0 1 0 1 0

1 0 1 0 1 0

B Signal

Transmitted

A+B

Signal

© Ibrahim Korpeoglu

cdma example recovering signal a at the receiver
CDMA Example – recovering signal A at the receiver

A+B

Signal

received

A

Codeword

at

receiver

Integrator

Output

Comparator

Output

0 1 0 1

Take the inverse of this to obtain A

© Ibrahim Korpeoglu

cdma example recovering signal b at the receiver
CDMA Example – recovering signal B at the receiver

A+B

Signal

received

B

Codeword

at

receiver

Integrator

Output

Comparator

Output

1 1 0 1

Take the inverse of this to obtain B

© Ibrahim Korpeoglu

cdma example using wrong codeword at the receiver
CDMA Example – using wrong codeword at the receiver

A+B

Signal

received

Wrong

Codeword

Used at

receiver

Integrator

Output

Comparator

Output

X 0 1 1

Noise

Wrong codeword will not be able to decode the original data!

© Ibrahim Korpeoglu

hybrid spread spectrum techniques
Hybrid Spread Spectrum Techniques
  • FDMA/CDMA
      • Available wideband spectrum is frequency divided into number narrowband radio channels. CDMA is employed inside each channel.
  • DS/FHMA
      • The signals are spread using spreading codes (direct sequence signals are obtained), but these signal are not transmitted over a constant carrier frequency; they are transmitted over a frequency hopping carrier frequency.

© Ibrahim Korpeoglu

hybrid spread spectrum techniques54
Hybrid Spread Spectrum Techniques
  • Time Division CDMA (TCDMA)
      • Each cell is using a different spreading code (CDMA employed between cells) that is conveyed to the mobiles in its range.
      • Inside each cell (inside a CDMA channel), TDMA is employed to multiplex multiple users.
  • Time Division Frequency Hopping
      • At each time slot, the user is hopped to a new frequency according to a pseudo-random hopping sequence.
      • Employed in severe co-interference and multi-path environments.
          • Bluetooth and GSM are using this technique.

© Ibrahim Korpeoglu

random access
Random Access
  • Packet Radio Protocols
      • Multihop radio network that carries packets
          • Not circuit oriented like GSM, CDMA, etc.
      • Example Protocols
        • Pure Aloha
        • Slotted Aloha
  • CSMA Protocols
    • 1-persistent CSMA
    • non-persistent CSMA
    • p-persistent CSMA
    • CSMA/CD
  • Reservation Protocols
    • Reservation Aloha
    • PRMA
  • Others
    • MACA, MACAW
    • IEEE 802.11 MAC

© Ibrahim Korpeoglu

pure aloha
Pure Aloha

Algorithm:

A mobile station transmits immediately whenever is has data.

It then waits for ACK or NACK.

If ACK is not received, it waits a random amount of time and retransmits.

Ignoring the propagation delay between mobiles

and base station:

B

The time difference between the time

a mobile send the first bit of packet and the

time the base station receives the last bit of

the packet is given by 2T.

T = C/P

T: packet time.

C: channel data rate (bps)

P: packet length (bits)

Ack/Nack

Data

M3

M1

M2

During this 2T period of time, the packet may collide

with someone elses packet.

© Ibrahim Korpeoglu

throughput of aloha
Throughput of Aloha

Normalized

Throughput

~0.185

0.5

Normalized

Channel Occupancy

© Ibrahim Korpeoglu

csma carrier sen s e multiple access
CSMA: Carrier Sense Multiple Access
  • Aloha does not listen to the carrier before transmission.
  • CSMA listen to the carrier before transmission and transmits if channel is idle.
  • Detection delay and propagation delay are two important parameters for CSMA
        • Detection delay: time required to sense the carrier and decide if it is idle or busy
        • Propagation delay: distance/speed_of_ligth. The time required for bit to travel from transmitter to the receiver.

© Ibrahim Korpeoglu

csma variations
CSMA Variations
  • 1-persistent CSMA:
      • A station waits until a channel is idle. When it detects that the channel is idle, it immediately starts transmission
  • Non-persistent CSMA:
      • When a station receives a negative acknowledgement, it waits a random amount of time before retransmission of the packet altough the carrier is idle.
  • P-persistent CSMA
      • P-persistent CSMA is applied to slotted channels. When a station detects that a channel is idle, it starts transmission with probability p in the first available timeslot.
  • CSMA/CD
      • Same with CSMA, however a station also listen to the carrier while transmitting to see if the transmission collides with someone else transmission.
          • Can be used in listen-while-talk capable channels (full duplex)
          • In single radio channels, the transmission need to be interrupted in order to sense the channel.

© Ibrahim Korpeoglu

maca medium access with collision avoidance
MACA – Medium Access with Collision Avoidance
  • CSMA protocols sense the carrier, but sensing the carrier does not always releases true information about the status of the wireless channel
      • There are two problems that are unique to wireless channels (different than wireline channels), that makes CSMA useless in some cases. These problems are:
          • Hidden terminal problem
          • Exposed terminal problem.

© Ibrahim Korpeoglu

maca hidden terminal problem
MACA – Hidden Terminal Problem

C’s cell

A’s cell

A

B

C

Hidden

terminal

  • A is transmitting to B.
  • C is sensing the carrier and detects that it is idle (It can not hear A’s transmission).
  • C also transmits and collision occurs at B.
  • A is hidden from C.

© Ibrahim Korpeoglu

maca exposed terminal problem
MACA – Exposed Terminal Problem

B’s cell

C’s cell

A

B

C

D

Exposed

terminal

  • B is transmitting to A. C is hearing this transmission.
  • C now wants to transmit to D. It senses the existence of carrier signal and
  • defers transmission to D.
  • However, C can actually start transmitting to D while B is transmitting to A,
  • Since A is out of range of C and C’s signals can not be heard at A.
  • C is exposed to B’s transmission.

© Ibrahim Korpeoglu

maca solution concept
MACA Solution Concept

Ali, lets talk! I am available.

Can

Can, I want to talk to you!

Can, I want to talk to you!

Biltepe

Mountain

Ali

Veli

© Ibrahim Korpeoglu

maca protocol
MACA Protocol
  • When a station wants to transmit data
      • It sends an RTS (Ready-to-Send) packet to the intended receiver
          • The RTS packet contains the length of the data that needs to be transmitted
          • Any station other than the intended recipient hearing RTS defers transmission for a time duration equal to the end of the corresponding CTS reception
      • The receiver sends back CTS (Clear-to-Send) packet back to sender if it is available to receive.
          • The CTS packet contains the length of the data that original sender wants to transmit
          • Any station other than the original RTS sender, hearing CTS defers transmission until the data is sent.
      • The original sender upon reception of the CTS, starts transmitting.

© Ibrahim Korpeoglu

maca solution for hidden terminal problem
MACA Solution for Hidden Terminal Problem

A is transmitting to B.

C’s cell

A’s cell

CTS(n)

RTS(n)

RTS(n)

X

A

B

C

CTS(n)

C defers transmission

for duration of n bytes of

data transmission. Node A

is no longer hidden from C effectively.

X defers transmission

until expected CTS

reception time by RTS

sender.

Data(n)

Waiting time of node X is much smaller than waiting time of node C.

© Ibrahim Korpeoglu

maca solution for exposed terminal problem
MACA Solution for Exposed Terminal Problem

B is transmitting to A

B’s cell

C’s cell

RTS(n)

RTS(n)

A

B

C

D

RTS(m)

CTS(n)

CTS(m)

Data(n)

Data(m)

  • C defers transmission upon hearing B’s RTS until B could get CTS from A.
  • After that C can start transmission to D. For that it first sends an RTS.
  • C is not longer exposed to the data transmission of B.

© Ibrahim Korpeoglu

case study ieee 802 11b mac
Case Study: IEEE 802.11b MAC
  • IEEE 802.11b: High Data-rate Wireless LAN standard.
  • Operates in 2.4-2483 MHz ISM RF Band.
      • 83 MHz spectrum width
  • Max data-rate: 11Mbps simplex.
  • Spectrum Usage: FHSS or DHSS
  • Modulation Technique: CCK with QPSK
  • For 11Mbps:
      • Symbol rate = 1,375 MSps
      • Number of symbol states = 8
          • One symbol can encode 3 bits of information.
  • Range: around 100m.

© Ibrahim Korpeoglu

802 11b
802.11b
  • Works in Two Operational Modes
      • Infrastructure Mode
      • Ad-Hoc Mode

Infrastructure Mode

Access

Point

Access Point

Wireless Link

Wireless Link

Wireless Link

Mobile

Station

Extended Service Set (ESS)

Basic Service Set (BSS)

All traffic has to go through access points

Access point provides connectivity to the wired backbone

© Ibrahim Korpeoglu

802 11b69
802.11b

Ad-Hoc Mode

Independent Basic Service Set (IBSS)

Mobile Stations can talk directly with each-other. All stations in an IBSS

need to be in the range of each-other.

© Ibrahim Korpeoglu

802 11b mac sublayer
802.11b MAC Sublayer
  • Support two different MAC modes depending on the operational mode of the Wireless LAN
      • 1) DCF: Distributed Coordination Function
          • Based on CSMA/CA
          • Carrier Sensing: Physical and Virtual.
      • 2) PCF: Point Coordination Function
          • Connection oriented
          • Contention free service
          • Polling based

© Ibrahim Korpeoglu

802 11b phy layer
802.11b PHY Layer
  • Can support data rates at: 1,2,5.5,11 Mbps

© Ibrahim Korpeoglu

slide72
FHSS
  • 2.4 GHz band is divided into 75 one-MHz subchannels. The sender and receiver hops through this 75 channels in a synchronized manner using a hopping pattern.
  • Can not support more than 2 Mbps data-rate.

© Ibrahim Korpeoglu

slide73
DSSS
  • Divides the 2.4 GHz band into 14 twenty-two MHz channels
  • Adjacent channels can overlap partially.
  • 3 of 14 channels are completely non-overlapping
  • Data is sent over one 22 MHz channel without hoppling using DSSS technique (chipping and code words are used like CDMA)
        • Each access point uses a different 22 MHz channel if possible.
        • All mobiles in the coverage of the access point uses the channel that is used by the access point. 802.11b MAC is used to coordinate the access to the shared 22 MHz channel.
        • Original 802.11 systems use 11 bit chipping (code words of length 11).
        • Later 802.11b systems use 8 bit chipping (code words of length 8 bits). Defines 64 different codewords from a space of 256.

© Ibrahim Korpeoglu

dsss channels
DSSS Channels

25

MHz

25

MHz

2.412

GHz

2.437

GHz

2.462

GHz

Channel 1

22 MHz

Channel 11

22 MHz

Channel 6

22 MHz

2.400

GHz

2.484

GHz

Spectrum Allocated for 802.11b

Channel 1, 6, and 11 are non-overlapping.

© Ibrahim Korpeoglu

channel assignment and registration
Channel Assignment and Registration.
  • In multi-access environment, the operator should try to allocate non-overlapping channels to the physically adjacent channels.
  • If adjacent access points use overlapping channels, then interference can be high.
  • A mobile station periodically tunes to all channels and evaluates the signal strength received over each channel
      • Depending on the signal strength received over the channels, a mobile selects an access point and registers with that provided that the access points accepts the mobile. This is also called association.
      • Re-association with a new access point occurs when
        • the mobile moves away from the current access point.
        • When the signal conditions changes between the mobile and current access point.
        • When there are a lot of users associated with the current access point.

© Ibrahim Korpeoglu

re association at the phy layer
Re-association at the PHY layer.

Access

Point (AP)

A

Access

Point (AP)

B

Signal from A

Signal from B

Associated withAccess Point B

Associated withAccess Point A

Mobile tunes to the channel of AP B when it moves into its range.

© Ibrahim Korpeoglu

an example 3 cell reuse scheme for wlan deployment
An example 3-cell Reuse scheme for WLAN deployment

1

11

11

6

6

1

1

1

11

11

6

6

An access point is located in the center of each hexagon.

© Ibrahim Korpeoglu

802 11b phy layer78
802.11b PHY Layer

MAC

PLCP

PLCP: Physical Layer Convergence Protocol

PMD: Physical Medium Dependent Sublayer

PHY Layer

PMD Sublayer

PLCP Frame Format

SYNC(128)

SFD(16)

Signal(8)

Service(8)

Length(16)

CRC(16)

MPDU(Variable Length)

SYNC: Synchronization fieldSFD: Start frame deliminerSignal: Indicated how fast the data will be transmittedService: ReservedLength: MAC Protocol Data Unit (MPDU) lengthCRC: used for error detecting on the frame

© Ibrahim Korpeoglu

802 11b mac sublayer79
802.11b MAC Sublayer
  • Supports both infrastructure and ad-hoc modes of operation.
  • CRC is added to each MAC frame
  • Packet fragmentation is supported to chop large higher layer (IP) packets into small pieces. Has advantages:
      • Probability a packet gets corrupted increases with the packet size.
      • In case of corruption, only a small fragment needs to be re-transmitted.

© Ibrahim Korpeoglu

inter frame space ifs
Inter-frame Space (IFS
  • 4 types of Inter-frame spaces:
      • Short IFS (SIFS): period between completion of packet transmission and start of ACK frame
      • Point Coordination IFS (PIFS): SIFS plus a slot time.
      • Distributed IFS (DIFS): PIFS plus a slot time.
      • Extended IFS (EIFS): longer IFS used by a station that has received a packet that it could no longer understand. Needed to prevent collisions.

© Ibrahim Korpeoglu

mac protocol
MAC Protocol
  • 802.11b uses CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) MAC protocol.
  • CSMA/CA is the protocol to implement the distributed coordination function (DCF) of the MAC sub-layer.
      • RTS/CTS is used to avoid collisions.
      • Use of RTS/CTS can be enabled or disabled depending on the traffic load (probability of collisions).

© Ibrahim Korpeoglu

csma transmission of mpdu data without use of rts cts
CSMA – Transmission of MPDU (Data) without use of RTS/CTS

DIFS

Data

Source

SIFS

ACK

Destination

Contention Window (Slot Times)

DIFS

Data

Others

Defer Access

Backoff afterDefer

A station backoffs a random number of slot times.

© Ibrahim Korpeoglu

csma ca transmission of mpdu data using rts cts
CSMA/CA – Transmission of MPDU (Data) using RTS/CTS

DIFS

RTS

DATA

Source

SIFS

SIFS

SIFS

CTS

ACK

Destination

Others

DIFS

Defer Access for NAV(RTS)

Defer Access for NAV(CTS)

Backoff afterDefer

Defer Access for NAV(Data)

© Ibrahim Korpeoglu

csma ca collision avoidance
CSMA/CA Collision Avoidance

RTS/CTS is used to reserve channel forthe duration of the packet transmission. This prevents hidden and exposed terminalproblems

ACK is required to understand if the packet is correctly received (without any collisions ) at the receiver. Ethernet does not require ACK to be sent, since the transmitter can detect the collision on the channel (cable) without requiring an explicit feedback from the receiver.

A wireless transmitter can not detect collision, because:1) Transmit power is much larger than the received power: received signal is regarded as noise (not collision). 2) There could be a hidden terminal

Access Point

Mobile

RTS

CTS

DATA

ACK

© Ibrahim Korpeoglu

802 11b frame format
802.11b Frame Format

IEEE 802.11b MAC Frame Format

FC(2 bytes)

ID(2)

Add1(6)

Add2(6)

Add3(6)

SC(2)

Add4(6)

Data(0-2312 bytes)

CRC(4)

Frame Control Format (2 bytes)

Protocol(2 bits)

Type(2)

Subtype(4)

To DS(1)

From DS(1)

More Frag(1)

Retry(1)

Pw Mgt(1)

More Data(1)

WEP(1)

Order(1)

Protocol Version: version of 802.11 standardType: Management. Control, Data frameSubtype: RTS, CTS, ACK frameTo DS: 1 if frame is sent to Distribution System (DS)From DS: 1 if frame is received from Distribution SystemMore fragment: 1 if there are more fragments belonging to the same frame following the current frame. Retry: indicates that is fragment is retransmission of previously transmitted fragment. Power Management: the type of power management mode that the station will be after the transmission of the frame. More Data: indicates that there are more frames buffered at the sender for this station. WEP: indicates that frame body is encrypted according to WEP. Order: indicates that the frame is sent using the strictly-ordered service class.

Frame Control (FC): protocol version and frame typeDuration/ID (ID): power-save poll message frame type and for NAV calculationAddress Fields: contains up-to 4 MAC addressesSequence Control: fragmentation and sequence number. Data: higher layer data that is maximum 2312 bytes. CRC: 32 bit cyclic redundancy check for detecting error on the frame.

© Ibrahim Korpeoglu

mobility
Mobility
  • What happens when a station moves between access points
      • Re-association function of the PHY layer associates a mobile with a new access point.
      • Some vendor specific, layer-2 (datalink layer) solutions solves the mobility at layer.
      • Solutions like Mobile IP needed to provide seamless mobility to higher layers (transport and application layers).
        • DHCP is also a method but not as convenient as Mobile IP.
  • We will see in the forthcoming classes how Mobile IP works.

© Ibrahim Korpeoglu