1. Medium Access Control for Wireless Links
CS 515
Mobile and Wireless Networking
Ibrahim Korpeoglu
Computer Engineering Department
Bilkent University

2. Outline Random MAC Schemes
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 MAC
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3. 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
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4. 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.
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5. 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 , GSM, etc.
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6. 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
Wireless
Link Layer
(Layers 1 and 2 in ISO/OSI Network Reference Model)
Link Controller
Medium Access Control, MAC level Scheduling,
Link Layer Queueing, Link Layer Reliability – ACKs,
NACKs, ….
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
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7. 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…)
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8. Wireless Media Access Control
Random Schemes (Less-Coordinated)
Examples: MACA, MACAW, Aloha, MAC,…
More suited for wireless networks that are designed to carry data: IEEE 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.
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9. 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
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10. 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
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11. 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
fc,R
fc,,F
frequency
Frequency separation
Frequency separation should be carefully decided
Frequency separation is constant
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12. 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) B M
Base
Station
Mobile
Station
Slot number …
channel F R F R F R F R ….
Reverse
Channel
Forward
Channel
time Ti
Ti+1
Time separation
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13. Duplexing – TDD versus 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…
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14. 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.
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15. 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.
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16. Narrowband Systems Wideband 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
CDMA/FDD CDMA/TDD
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17. Cellular Systems and MAC
Multiple Access Technique
AMPS
FDMA/FDD
GSM
TDMA/FDD
USDC (IS-54 and IS-136)
PDC
CT2 Cordless Phone
FDMA/TDD
DECT Cordless Phone
US IS-95
CDMA/FDD
W-CDMA
CDMA/TDD
cdma2000
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18. Frequency Division Multiple AccessB
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
f1,F
f2,F
fN,F
f1,R
f2,R
fN,R … M M M
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19. 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.
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20. 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.
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21. 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
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22. 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.
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23. 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.
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24. 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
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25. 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.
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26. 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
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27. TDMA TDMA Efficiency TDMA is usually combined with FDMA
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.
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28. Contemporary TDMA Systems
GSM
(Europa)
IS-54
(USA)
PDC
(Japan)
DECT
(European Cordless)
Bit Rate
270.8 Kbps
48.6 Kbps
42 Kbps
1.152 Mbps
Bandwidth
200 KHz
30 KHz
25 KHz
1.728 MHz
Time Slot
0.577 ms
6.7 ms
0.417 ms
Upstream slots
per frame 8 3 12
Duplexing
FDD
TDD
Efficiency of Time Slots
73 %
80 %
67 %
Modulation
GMSK
p/4 DQPSK
Adaptive Equalized
Mandatory
Optional
None
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29. 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.
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30. 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 depends modulation efficiency (bits_per_second/Hz) and channel bandwidth.
The frequency re-use factor
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31. System Capacity: Cluster: 7 cells constitute a cluster. C B
Cluster size = 7 A y B D A x 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
strength from interfering base stations is called C/I or S/I ratio.
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32. 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
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33. Comparing Systems GSM Parameters AMPS 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)
AMPS Parameters
Channel bandwidth = 30Khz
Required C/I: 18 dB
Frequency re-use factor: 1/7 (cluster size=7)
Required bandwidth per user = 30kHz.
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34. 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
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35. 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
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36. 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 1 2 3
..... 77 78
79-Hop System
1 MHZ
A hop sequence could be: 7,1,78,67,0, 56,39,
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37. 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.
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38. 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
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39. 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
…..
M-S1
S1-M
M-S2
S2-M
M-S3
S3-M
M-S1
S1-M ……
slot time=625ms
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40. 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.
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41. 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
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42. CDMA Principle Represent bit 1 with +1 Represent bit 0 with -1 1 1
One bit period (symbol period) 1 1
Data
PN-Code
(codeword)
Coded
Signal
Input to the modulator (phase modulation)
Chip period
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43. 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
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44. Near Far Problem and Power Control
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. B
pr(M) M M M M
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45. DSSS Transmitter Message Baseband sss(t) + BPF m(t) Transmitted Signal
p(t)
PN Code
Generator
Oscillator fc
Chip Clock
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46. DSSS Receiver Phase Shift Keying IF Wideband Demodulator Filter
Received
Data
Received
DSSS Signal
at IF
PN Code
Generator
Synchronization
System
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47. 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
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48. CDMA Example (*) R Receiver (a base station) Data=1011… Data=0010… A B
Transmitter (a mobile)
Transmitter
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.
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49. CDMA Example – transmission from two sources
A Data A
Codeword
A Signal
B Data B
Codeword
B Signal
Transmitted
A+B
Signal
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50. CDMA Example – recovering signal A at the receiver
A+B
Signal
received A
Codeword at
receiver
Integrator
Output
Comparator
Output
Take the inverse of this to obtain A
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51. CDMA Example – recovering signal B at the receiver
A+B
Signal
received B
Codeword at
receiver
Integrator
Output
Comparator
Output
Take the inverse of this to obtain B
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52. CDMA Example – using wrong codeword at the receiver
A+B
Signal
received
Wrong
Codeword
Used at
receiver
Integrator
Output
Comparator
Output X
Noise
Wrong codeword will not be able to decode the original data!
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53. 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.
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54. 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.
CS 515
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55. Random Access Packet Radio Protocols CSMA 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 MAC
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56. 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 M1 M3 M2
During this 2T period of time, the packet may collide
with someone elses packet.
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57. Throughput of Aloha ~0.185 0.5 Normalized Throughput Normalized
Channel Occupancy
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58. 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.
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59. CSMA Variations 1-persistent CSMA: Non-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.
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© Ibrahim Korpeoglu

60. 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.
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61. MACA – Hidden Terminal Problem
A’s cell
C’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.
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62. 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.
CS 515
© Ibrahim Korpeoglu

63. Ali, lets talk! I am available.
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
CS 515
© Ibrahim Korpeoglu

64. 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.
CS 515
© Ibrahim Korpeoglu

65. MACA Solution for Hidden Terminal Problem
A is transmitting to B.
A’s cell
C’s cell
RTS(n)
RTS(n)
CTS(n) X A B C
X defers transmission
until expected CTS
reception time by RTS
sender.
CTS(n)
C defers transmission
for duration of n bytes of
data transmission. Node A
is no longer hidden from C effectively.
Data(n)
Waiting time of node X is much smaller than waiting time of node C.
CS 515
© Ibrahim Korpeoglu

66. 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.
CS 515
© Ibrahim Korpeoglu

67. Case Study: IEEE b MAC
IEEE b: High Data-rate Wireless LAN standard.
Operates in 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.
CS 515
© Ibrahim Korpeoglu

68. 802.11b Infrastructure Mode Works in Two Operational Modes
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
CS 515
© Ibrahim Korpeoglu

69. 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.
CS 515
© Ibrahim Korpeoglu

70. 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
CS 515
© Ibrahim Korpeoglu

71. 802.11b PHY Layer 802.11b Data Rate Specifications
Can support data rates at: 1,2,5.5,11 Mbps
802.11b Data Rate Specifications
Data rate
Code Length
Modulation
Symbol Rate
Bits/Symbol
1 Mbps
11 (Barker Sequence)
BPSK
1 MSps 1
2 Mbps
QPSK 2
5.5 Mbps
4 (CCK)
1.375 MSps 4
11 Mbps
8 (CCK) 8
CS 515
© Ibrahim Korpeoglu

72. 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.
CS 515
© Ibrahim Korpeoglu

73. 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 b MAC is used to coordinate the access to the shared 22 MHz channel.
Original systems use 11 bit chipping (code words of length 11).
Later b systems use 8 bit chipping (code words of length 8 bits). Defines 64 different codewords from a space of 256.
CS 515
© Ibrahim Korpeoglu

74. Spectrum Allocated for 802.11b
DSSS Channels 25
MHz 25
MHz
2.412
GHz
2.437
GHz
2.462
GHz
Channel 1
22 MHz
Channel 6
22 MHz
Channel 11
22 MHz
2.400
GHz
2.484
GHz
Spectrum Allocated for b
Channel 1, 6, and 11 are non-overlapping.
CS 515
© Ibrahim Korpeoglu

75. 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.
CS 515
© Ibrahim Korpeoglu

76. Re-association at the PHY layer.
Access
Point (AP) A
Access
Point (AP) B
Signal from A
Signal from B
Associated with Access Point B
Associated with Access Point A
Mobile tunes to the channel of AP B when it moves into its range.
CS 515
© Ibrahim Korpeoglu

77. An example 3-cell Reuse scheme for WLAN deployment1 11 11 6 6 1 1 1 11 11 6 6
An access point is located in the center of each hexagon.
CS 515
© Ibrahim Korpeoglu

78. MPDU (Variable Length)
802.11b PHY Layer
MAC
PLCP: Physical Layer Convergence Protocol
PMD: Physical Medium Dependent Sublayer
PLCP
PHY Layer
PMD Sublayer
PLCP Frame Format
SYNC (128)
SFD (16)
Signal (8)
Service (8)
Length (16)
CRC (16)
MPDU (Variable Length)
SYNC: Synchronization field SFD: Start frame deliminer Signal: Indicated how fast the data will be transmitted Service: Reserved Length: MAC Protocol Data Unit (MPDU) length CRC: used for error detecting on the frame
CS 515
© Ibrahim Korpeoglu

79. 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.
CS 515
© Ibrahim Korpeoglu

80. 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.
CS 515
© Ibrahim Korpeoglu

81. 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).
CS 515
© Ibrahim Korpeoglu

82. CSMA – Transmission of MPDU (Data) without use of RTS/CTS
DIFS
Source
Data
SIFS
Destination
ACK
Contention Window (Slot Times)
DIFS
Others
Data
Defer Access
Backoff after Defer
A station backoffs a random number of slot times.
CS 515
© Ibrahim Korpeoglu

83. CSMA/CA – Transmission of MPDU (Data) using RTS/CTS
DIFS
Source
RTS
DATA
SIFS
SIFS
SIFS
Destination
CTS
ACK
Others
DIFS
Defer Access for NAV(RTS)
Defer Access for NAV(CTS)
Backoff after Defer
Defer Access for NAV(Data)
CS 515
© Ibrahim Korpeoglu

84. CSMA/CA Collision Avoidance
RTS/CTS is used to reserve channel for the duration of the packet transmission. This prevents hidden and exposed terminal problems
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
CS 515
© Ibrahim Korpeoglu

85. 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 ( 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 standard Type: Management. Control, Data frame Subtype: RTS, CTS, ACK frame To DS: 1 if frame is sent to Distribution System (DS) From DS: 1 if frame is received from Distribution System More 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 type Duration/ID (ID): power-save poll message frame type and for NAV calculation Address Fields: contains up-to 4 MAC addresses Sequence Control: fragmentation and sequence number. Data: higher layer data that is maximum bytes. CRC: 32 bit cyclic redundancy check for detecting error on the frame.
CS 515
© Ibrahim Korpeoglu

86. 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.
CS 515
© Ibrahim Korpeoglu