Mobile computing- Multiplexing-Spread spectrum-MAC protocol-FDMA-TDMA-CDMA

1. RAMCO INSTITUTE OF TECHNOLOGY
Department of Computer Science and Engineering
Academic Year: 2019- 2020 (Even Semester)
Degree, Semester & Branch: VI Semester B.E. CSE.
Course Code & Title: CS8601 Mobile Computing.
Name of the Faculty member: Dr.M.Kaliappan, Associate Professor/CSE
----------------------------------------------------------------------------------------------------------------
UNIT-I INTRODUCTION 9
Introduction to Mobile Computing – Applications of Mobile Computing- Generations of
Mobile Communication Technologies- Multiplexing – Spread spectrum -MAC Protocols –
SDMA- TDMA- FDMA- CDMA
----------------------------------------------------------------------------------------------------------------
Credit: Jochen Schiller, “Mobile Communication”, PHI, Prasant Kumar Pattnaik, Rajib
Mall, “Fundamentals of Mobile Computing”, PHI
---------------------------------------------------------------------------------------------------------------
Objective:
 To enable the students to understand the basic concepts of mobile computing.
Outcome:
 Explain the basics of mobile computing and MAC protocols
----------------------------------------------------------------------------------------------------------------
1. Mobile computing (2 marks)
 Mobile computing is a technology in which computation performed remotely while
on move
 People can access information from anywhere at any time
 Mobility
 Change location while communicating to invoke computing service
 Computing
 Carryout process related to service invocations on a remote computer
1.1 Basics of Communication Technologies (4 marks)
 Mobile handset
 Individual users with handset directly communicate with each other over a
radio link or wired link formed through intermediaries such as base station and
fixed line.
 When all intermediaries are located in ground, the communication system is
called terrestrial radio system.
 If at least one of the intermediaries is satellite, then it is referred as satellite
radio system
 Cell phone system
 Provide data services like SMS,MMS, email and web browsing
 Current location of the user is maintained by mobile telecommunication
networks while move
 Types of communication networks (2 marks)
 Voice networks
 Data networks
 Voice networks
 Analog signal is modulated on a carrier signal for transmission
 Ex: Traditional Telephonic network-Circuit switching

2.  Data networks
 Data network is a telecommunications network that allows computers to
exchange digital data.
 Data refers to any information like text, documents, picture, movie, sound
 Example: packet switching
 Computer networks
 Controller area networks(CAN)
 LAN
 Internet
 Controller area networks(CAN) (2 marks)
 Very small networks used to connect different components of embedded
controller
 Less than 50 meters
 Ex : Automatic doors, coffee machines
 LAN (2 marks)
 Used to connect no. of computers connected within organization to share data
and other resources like printers, fax.
 Typically deployed in a building or a campus
 Operate from 10mbps to 1Gbps
 Now 100Gigabit Ethernet

3.  Internetworks
 Several LAN interconnected using switches
 Network of networks
 A node in LAN communicates with a node in another LAN using packet
switching
1.2 Components of wireless communication system (4 marks)
Transmitter
 It modulate or encode data, voice, video into higher frequency carrier signal, then it
can be radiated and propagated use of radio frequency spectrum
Receiver
 It receive the modulated signals and reverse the function of transmitter and thereby
recover the transmitted signal
Antenna
 It convert the electrical signal from transmitter to a electromagnetic RF wave or
conversely, to convert RF wave to an electrical signals
Filters
 It used to reject interfering signal lying outside the band of receiver and transmitter.
Amplifier
 Amplifies the strength of a signal. It minimize the noise
Mixer
 Used to achieve frequency conversion at the transmitters and receivers
1.3 Architecture of Mobile Telecommunication system
WLAN
 Connectivity between computers over short distance using wireless medium.
 Educational institute, office building

4. 1.4 Mobile Computing vs Wireless networking (2 marks)
Mobile Computing Wireless Networking
It is a technology that access data
through wireless network
It is a network that Connectivity between
computers over short distance using wireless
medium
It refers to a device performing
computation that is not always
connected to a central network
It refers to the data communication without the use
of a landline. Eg. Cellular Telephone, Two way
radio, Satellite, Wireless Connection
It denotes accessing information and
remote computational services while
on the move
It provides the basic communication infrastructure
necessary for mobile computing
It refers to computing devices that are
not restricted to a desktop. Eg: Smart
Phone, PDA, Laptop etc
It is a method/technology of transferring
information between a computing devices such as
PDA & data sources without a physical connection
1.5 Applications of mobile computing (2 marks)
 Emergency services
 Stock Broker
 Vehicles
 Estate Agents
 courts
 companies
 Stock Information Collection/Control
 Credit Card Verification
 Taxi/Truck Dispatch
 Electronic Mail/Paging
1.6 Disadvantages of Mobile computing (2 marks)
 Battery consumption hindrance
 Interference
 Inefficient bandwidth in transmission.
 Connection losses over entire network.

5. 1.7 Characteristics of Mobile computing (8 marks)
• Ubiquity
• Location awareness
• Adaptation
• Broadcast
• Personalization
Ubiquity
• Ability of user to perform computation from everywhere at any place
• Ex: Business executive receives notification and issue transaction in the
coverage area
Location awareness
• Handheld device equipped with GPS that provide current location of the users.
• Many application from strategic to personalized service require location based
service
• Ex: to locate the shop in nearby place
• Traffic control
• Fleet management
• Emergency service
Adaptation
• Ability of system to adjust to bandwidth fluctuation without inconveniencing
the user
• Intermittent disconnection and bandwidth fluctuation occur due to handoff,
obstacle and environment noise
Broadcast
• Delivery of data simultaneously to hundreds of mobile users
• Ex: users at specific location near a railway station may ne sent advertising
information of taxi
Personalization
• Services in mobile environment can be personalized according to user profile
1.8 Cellular Mobile communication (8 marks)
• Partition the region into smaller regions called cells.
• Each cell gets at least one base station or tower
• Users within a cell talks to the tower
• BS keep track of the calls
Properties of Cell structure
• Typical Cell sizes
– tens of meters in buildings
– some cites few hundred meters
– country side few tens of kilometers

6. • Advantages of cell structures:
– more capacity due to frequency reuse
– less transmission power needed
– more robust, tolerate failures
– deals interference, transmission area locally
• Problems:
– fixed network needed for the base stations
– handover (changing from one cell to another) necessary
– interference with other cells
Other common channel
Channel Reuse
• Cell structure can reuse frequency only when certain distance is maintained between
cells that use the same channels.
• Fixed Allocation schemes:
– certain frequencies are assigned to a certain cell
– problem: different traffic load in different cells
• Dynamic Allocation schemes:
– base station chooses frequencies depending on the frequencies already used in
neighbor cells

7. 1.9. Generation of Mobile computing (8 marks)
• 1G - First generation (Analog and FM)
• 2G - Second generation (Digital, TDMA, CDMA)
• 3G - Third generation (Multi-media)
• 4G - Fourth generation (LTE)
• 5G - Fifth Generation(WWWW)
1G
• These phones were the first mobile phones to be used, which was introduced in 1982
and completed in early 1990.
• It was used for voice services
• It based on Advanced Mobile Phone System (AMPS) technology.
• Analog signals are modulated on higher frequency
• Signals transmitted by separate channels
– Incoming signal-25MHz
– Outgoing signal-25 MHz
• Frequency spectrum was split into sub channels, each used by different users
• 1G frequency band split into five band sub-band1, sub-band2, sub-band3, sub-band4,
sub-band5
• Each channel to be adjacent to each other, and separated from adjacent channel about
30khz called guard band
• Guard band reduce number of simultaneous calls in cell
• 1G Standards
– USA- AMPS(Advanced Mobile phone system)
– Europe-NMT450(Nordic Mobile Telephone)
– UK-TACS(Total access communication system)
• 1G follows multiple access type , once a caller hanged up, another caller use the same
frequency(FDMA)
• Call was dropped during handoff when another call use the channel in another cell
• Hand off or what happens when a user is mobile? (2 marks)
Especially, when crossing a cell boundary while continuing the call. Handoff
strategy is invoked. It finds a new base station and higher priority over new
call invocation.
– Hard handoff
• Mobile user is passed between disjoint towers that assign different
frequency or adapt different air-interface technology
– Soft handoff
– Mobile user communicates to two towers simultaneously and the signal is
treated as a multipath signal
Drawback of 1G
• Poor voice quality and battery life
• Large phone size
• Poor handoff reliability
• It has no security
• Offered very low level of spectrum efficiency
• The analogue signals are more to suffer interference problems
2G
• It uses digital signals for voice transmission, and provides services to deliver
text (SMS) and picture message at low speed (in kbps).
• It use the bandwidth of 30 to 200 KHz

8. • It reduction in call cost
• 2G follows some principles of 1G,
• 2G use CDMA and TDMA modulation techniques as channel access technology
• North America used IS-95 standard (CDMA), can multiplex up to 64calls per
channel in 800MHZ
• Europe used GSM, which used TDMA, can multiplex up to 8calls per channel
in 900 and 800MHz
• Advantages
– Provides better quality and capacity
– Provide higher voice quality
– Provide data service like SMS and email
– digital calls are free of static & background noise
– 2G technology offers improved privacy
• Disadvantages
– Unable to handle complex data such as videos.
– Required strong digital signals to help mobile phones work.
2.5G
• It used GPRS(General packet Radio service) , extension of GSM
• It based on packet switching technique
• Deployment began in 2000 followed by EDGE(Enhanced Data GSM Environment)
• Advantages and Disadvantages
• User remains connected to the internet without additional cost.
• Faster internet browsing
• Use streaming application
• Camera phones
• Did not offer multi-megabit data rate
• Cell towers had a limited coverage area.
• Built mainly for voice services and slow data
3G
• First 3G adopted in Japan in 2001 as DoCoMo
• It referred as IMT-2000(International Mobile Tele communication)
• It support purely data network, use packet switching
• UMTS (Universal Mobile Telephone system) is one of the 3G mobile system for
GSM networks.
• UMTS support various cell size from Pico cell to global cell like satellite
• Europe adopted UMTS, while the USA uses CDMA2000
• Advantages
• Higher data transmission rate
• Offered increased bandwidth
• High quality traditional voice call
• Provide cheaper calls
• Support applications like email, live-video buffering location based service,
video telephony, multimedia gaming
4G
• It make mobile broadband internet access
• It has high speed internet access from smart phone to laptop with USB wireless
modem
• It uses WiMAX and LTE standard
• 4G including IP telephony, video conferencing and 3D television
• Advantages

9. • Capable of provide 10Mbps-1Gbps speed
• High quality streaming video
• Combination of Wi-Fi and Wi-Max
• High security
• Provide any kind of service at any time as per user requirements anywhere
• Expanded multimedia services
• Low cost per-bit
• Battery uses is more
• Disadvantages
• Hard to implement and Need complicated hardware
• Expensive equipment required to implement next generation network
Comparison of Generation of cell phones or mobile computing (2 marks)
5G
• 5G will not only enable better usage of smartphones, but it will also start the era of
driverless smart cars and smart homes.
• In the smartphone sector, 5G is expected in the field of AR (Augmented Reality) and VR
(Virtual Reality)
• The main focus of 5G will be on world-Wireless World Wide Web (WWWW).
• It is a complete wireless communication with no limitations.
• Provides large broadcasting of data up to 20 Gbps.
• Multi-media newspapers, watch TV programs with the clarity(HD Clarity)
• Large phone memory, dialing speed, clarity in audio/video
• T-Mobile's LTE/LAA network(License Assisted Access)
Plan of 5G
• Although 5G will first launch in the U.S. probably by the end of 2018
• China will start their pre-commercial use from 2019.
• In India, we can expect the launch of 5G network by the end of 2019.
• By 2020 ,we can use the network commercially

10. 1.10 Structure of Mobile Computing applications (8 Marks)
Functionalities provided by each tier structure of a mobile computing application
Presentation tier
 It describes the presentation of the information.
 Topmost level - Concerns the user interface
 Facilitates the users to issue requests and present the results to them meaningfully
 Programs run on the client‟s computer.
 Includes
 web browsers
 customized client programs for dissemination of information and
 For collection of data from users
Application Tier
 It responsible for making logical decisions and performing computation
 Moves and processes data between the presentation and data tier
 “engine” of an automobile
 It processing of user input, obtaining information and then making decisions
 Implemented using technologies like Java, .NET services, cold fusion, etc.
Data tier
• Responsible for basic facilities of data storage, access and manipulation
• Often contains a database
• Information is stored and retrieved from this database (file system can also be used to
store small amount of data)

11. • Implemented on a fixed server
1.10 Multiplexing (16 Marks)
Multiplexing (2 Marks)
• Multiplexing describes how several users can share a medium with minimum or no
interference.
• The task of multiplexing is to assign space, time, frequency, and code to each
communication channel with a minimum of interference and a maximum of medium
utilization.
• Example, highways with several lanes.
– Many users (car drivers) use the same medium (the highways) with hopefully
no interference (i.e., accidents).
– This is possible due to the provision of several lanes (space division
multiplexing) separating the traffic.
– In addition, different cars use the same medium (i.e., the same lane) at
different points in time (time division multiplexing).
1.10.1 Types of Multiplexing
• Space division multiplexing
• Frequency division multiplexing
• Time division multiplexing
• Code division multiplexing
Space division multiplexing (8 Marks)
– The term communication channel refers to an association of sender(s) and
receiver(s) who want to exchange data.
– Figure shows six channels ki and introduces a three dimensional coordinate
system such as code c, time t and frequency.
• In space division multiplexing (SDM), the space si is represented as circles
indicating the interference range
• The channels k1to k3 can be mapped onto the three „spaces‟ s1 to s2 which
clearly separate the channels and prevent the interference ranges from
overlapping.
• Guard space (2 Marks)
– The space between the interference ranges is sometimes called guard space.

12. • For the remaining channels (k4 to k6), three additional spaces would be needed.
• EX: Telephone Exchange system
– Each subscriber is given a separate pair of copper wires to the local exchange
• Disadvantages (2 Marks)
– SDM implies a separate sender for each communication channel that clearly
represents a waste of space
Frequency division multiplexing (FDM) (8 Marks)
• It subdivides the frequency dimension into several non-overlapping frequency bands.
• Each channel ki is now allotted its own frequency band.
• Senders using a certain frequency band continuously.
• Guard spaces are needed to avoid frequency band overlapping or interference.
• Ex: This scheme is used for radio stations
– Within the same region, where each radio station has its own frequency.
Advantages (2 Marks)
• This scheme does not need coordination between sender and receiver: the receiver only
has to tune in to the specific sender
Disadvantages (2 Marks)
• While radio stations broadcast 24 hours a day, mobile communication typically takes
place for only a few minutes at a time.
• Assigning a separate frequency for each communication scenario would be a waste of
(scarce) frequency resources.
• Additionally, the fixed assignment of a frequency to a sender inflexible and limits the
number of senders.
Time division multiplexing (8 Marks)
In time division multiplexing (TDM), a channel ki is given the whole bandwidth for a
certain amount of time, i.e., all senders use the same frequency but at different points in
time.
Guard spaces
• Guard spaces represent time gaps, have to separate the different periods when the
senders use the medium.
• In our highway example, this would refer to the gap between two cars. If two
transmissions overlap in time, this is called co-channel interference.

13. To avoid interference, precise synchronization between senders is necessary.
Disadvantages (2 Marks)
• It needs complex coordinating system to avoid interference. In which all senders
need precise clocks or, distribute a synchronization signal to all senders.
• A receiver listens at exactly the right point in time.
Advantages (2 Marks)
• This scheme is flexible as one can assign more sending time to senders with a
heavy load and less to those with a light load
Frequency and time division multiplexing (4 Marks)
It can be combined, i.e., a channel ki can use a frequency band for a certain amount of
time.
Guard space
• Guard spaces are needed both in the time and in the frequency dimension.
Advantages
• This scheme is more robust against frequency selective interference efficiently.
Disadvantages (2 Marks)
• Need necessary coordination mechanism between different senders.
• Two senders will interfere when they select the same frequency at the same time.
• Interference in a certain small frequency band.
• A channel may use this band only for a short period of time.
• EX: GSM
• The mobile phone standard GSM uses this combination of frequency and time
division multiplexing for transmission between a mobile phone and a base station

14. Code division multiplexing (8 Marks)
Code division multiplexing (CDM) is a new scheme in commercial communication
systems.
First used in military applications due to its inherent security features it now features in
many civil wireless transmission scenarios.
All channels ki use the same frequency at the same time for transmission.
Separation is achieved by assigning each channel its own „code‟,
Guard spaces
• Guard spaces are realized by using codes. e.g., orthogonal codes.
• Advantages (2 Marks)
It gives good protection against interference and tapping. Different codes have to be
assigned.
Assigning individual codes to each sender does not usually cause problems.
• Disadvantages (2 Marks)
• A receiver has to know the code and must separate the channel with user data from the
background noise composed of other signals and environmental noise.
• Additionally, a receiver must be synchronized with the transmitter to apply the
decoding correctly.
1.11 Spread spectrum (16 marks)
Spread spectrum (2 marks)
Spread spectrum techniques involve spread across a wide bandwidth needed to
transmit data
i) The following figure shows an idealized narrowband signal from a sender of user data
(here power density dp/df versus frequency f).

15. ii) The sender now spreads the signal in which, narrowband signal converts into a broadband
signal
iii) During transmission, narrowband and broadband interference add to the signal
iv) The sum of interference and user signal is received. The receiver is converting the spread
user signal into a narrowband signal again.
v) The receiver applies a bandpass filter to cut off frequencies left and right of the
narrowband signal. Finally, the receiver can reconstruct the original data
• Example
• Six different channels use FDM for multiplexing, which means that each channel has
its own narrow frequency band for transmission.
• Between each frequency band a guard space is needed to avoid adjacent channel
interference.
• Channels 1, 2, 5, and 6 could be received while the quality of channels 3 and 4 is too
bad to reconstruct transmitted data. Narrowband interference destroys the
transmission of channels 3 and 4.

16. • All narrowband signals are now spread into broadband signals using the same
frequency range.
• CDM is now used instead of FDM. Spreading of a narrowband signal is achieved
using a special code
• Each channel is allotted its own code, which the receivers have to apply to recover the
signal. Without knowing the code, the signal cannot be recovered and behaves like
background noise.
• Spread spectrum now allows in license-free bands such as 850 MHz that use for TDM
and FDM systems
• Advantages of Spread Spectrum (2 marks)
– Cross-talk elimination
– Better output with data integrity
– Better security
– Reduction in noise
– Not easy to demodulate/decode
• Disadvantage (2 marks)
• Increased complexity of receivers that have to despread a signal.
• The large frequency band that is needed due to the spreading of the signal.
• Spread signals appear more like noise; they still raise the background noise
level.
Types of spread spectrum (2 marks)
• Frequency Hopping Spread Spectrum(FHSS)
• Direct Sequencing Spread Spectrum(DSSS)
Frequency Hopping Spread Spectrum (FHSS) (8 marks)
o In frequency hopping spread spectrum (FHSS) systems, the total available
bandwidth is split into many channels of smaller bandwidth plus guard spaces
between the channels.
o Transmitter and receiver stay on one of these channels for a certain time and
then hop to another channel. This system implements FDM and TDM.
o The pattern of channel usage in the frequency hopping spread spectrum is
called the hopping sequence,
o Dwell time (2 marks)
 The time spend on a channel with a certain frequency in the frequency
hopping spread spectrum is called the dwell time

17. Hopping pattern: F2-F5-F3-F6-F1-F6-F4
Types of FHSS (2 marks)
• Slow hopping
• Fast hopping
– Slow hopping
– In slow hopping, the transmitter uses one frequency for several bit periods.
– Figure shows five user bits with a bit period tb.
– Performing slow hopping, the transmitter uses the frequency f2 for
transmitting the first three bits during the dwell time td. Then, the transmitter
hops to the next frequency f3.
– Slow hopping systems are typically cheaper and have relaxed tolerances. It is
an option for GSM
– Fast hopping
– For fast hopping systems, the transmitter changes the frequency several times
during the transmission of a single bit.
– In the example, the transmitter hops three times during a bit period.
– Fast hopping systems are more complex to implement because the transmitter
and receiver have to stay synchronized within same points of time.
Example
– FHSS system is used in Bluetooth

18. – Bluetooth performs 1,600 hops per second and uses 79 hop carriers equally
spaced with 1 MHz in the 2.4 GHz ISM band
Transmitter
Receiver
Applications of FHSP or spread spectrum (2 marks)
– Military use
– Bluetooth
– Walkie-Talkies
– Other radios
• Ex YouTube Video: https://www.youtube.com/watch?v=CkhA7s5GIGc
Direct sequence spread spectrum (8 Marks)
• In Direct sequence spread spectrum (DSSS), a user bit stream XOR with a
sequence pattern or spreading factor. It is called chipping sequence
• While each user bit has a duration tb, the chipping sequence consists of smaller
pulses, called chips, with a duration tc.
• If the chipping sequence is generated properly that is called pseudo-noise sequence.
• The spreading factor s = tb/tc determines the bandwidth of the resulting signal.
• If the original signal needs a bandwidth w, the resulting signal needs s·w after
spreading

19. • Civil applications use spreading factors between 10 and 100, military applications
use factors of up to 10,000.
• Wireless LANs standard IEEE 802.11 the sequence 10110111000 that is called
Barker code.
• The DSSS Barker codes exhibit a good robustness against interference and
insensitivity to multi-path propagation.
• Other known Barker codes are 11, 110, 1110, 11101, 1110010, and 1111100110101
DSSS transmitter
Digital modulation (2 Marks)
• The spreading of the user data with the chipping sequence.
Radio modulation (2 Marks)
• The spread signal is then modulated with a radio carrier
• The receiver has to know the original chipping sequence generated from the
transmitter.
• The receiver only has to perform the inverse functions of the two transmitter
modulation steps.

20. • However, noise and multi-path propagation require additional mechanisms to
reconstruct the original data.
• The first step in the receiver involves demodulating the received signal. This is
achieved using the same carrier as the transmitter reversing the modulation.
• Additional filtering can be applied to generate this signal.
DSSS receiver
• During a bit period, which also has to be derived via synchronization, an integrator
adds all these products. Calculating the products of chips and signal, and adding the
products in an integrator are also called correlation, the device a correlator.
• Finally, in each bit period a decision unit samples the sums generated by the
integrator and decides if this sum represents a binary 1 or a 0
Example
• User data 01
• Applying the 11-chip Barker code 10110111000
• The results in the spread „signal‟ 1011011100001001000111.
• On the receiver side, this „signal‟ is XORed bit-wise after demodulation with the same
Barker code as chipping sequence.
• This results in the sum of products equal to
– 0 for the first bit
– 11 for the second bit.
• The decision unit map the first sum (=0) to a binary 0, the second sum (=11) to a
binary 1. This constitutes the original user data
What happens in case of multi-path propagation? (2 marks)
• Several paths with different delays exist between a transmitter and a receiver.
Additionally, the different paths may have different path losses.
• In this case, rake receivers are used to provide a possible solution. It take advantage of
the multi-path propagation by combining the different paths.
• A rake receiver uses n correlators for the n strongest paths. Each correlator is
synchronized to the transmitter plus the delay on that specific path.
• As soon as the receiver detects a new path which is stronger than the currently
weakest path, it assigns this new path to the correlator with the weakest path.
– The output of the correlators are then combined and fed into the decision unit.
Comparison between FHSS and DSSS (2 marks)
FHSS DSSS
Multiple frequencies are used Single frequency is used
Frequency reuse is allowed Frequency reuse is not allowed

21. Sender need not wait Sender has to wait if the spectrum is busy
Power strength of the signal is high Power strength of the signal is low
It is never affected by interference It can be affected by interference
It is cheaper It is expensive
1.12 MAC protocol (2 Marks)
• It enforces the discipline in the access of shared medium when multiple user content
to access that channel.
• The objectives of MAC protocol are maximization of the utilization of the channel
and minimization of the average latency of the transmission.
• MAC (Medium access control) protocol is sub layer of data link layer, it directly
invoke the physical layer.
Properties of MAC protocol (2 Marks)
• It helps maximize the utilization of channel
• Channel allocation need to be fair
• Support different types of traffic having different and average bitrate
• It should be robust when equipment failure and change network condition
• It enforce the discipline in the access of shared medium from multiple user
• 802.11 is the standard for MAC protocol
• –EX: 802.11 network card, router
Functionalities of MAC protocol (4 Marks)
• Roaming
o •Provide uninterrupted service when user walk from one place to another
within wireless station
• Point Coordination function
o •Provide guarantees on reduce access delay and minimum transmission
bandwidth and other QoS
• power conservation
o •Provide power management
o •Transceiver must be switched off whenever carrier sensing is not needed
o •Two state : sleep and wake
• Authentication
o •recognizing a user‟s identity. It is the mechanism of associating an incoming
request with a set of identifying credentials
Wireless Networks
• Infrastructure Based
• Infrastructure less
Infrastructure Based
o Access points are connected to existing wired network
o It interact with wireless node as well as existing wired node
o It is bridge between other network and provide authentication mechanism

22. Infrastructure less network
• The network set up on the fly at any place. Nodes networked among themselves.
Nodes communicate directly each other, or forward message through intermediate
nodes. It is called scatternet.
• Difficulty in Collision detection and other Problems
• Piconet : Set up personal area network using blutooth technology
1.8 MAC Protocol Issues (8 Marks)
Identify the specific reasons as to why the MAC protocols designed for
infrastructure-based wireless networks may not work satisfactorily in infrastructure
less environment. Because, collisions detect scheme is difficult to implement in a wireless
environment since collisions are hard to be detected by the transmitting nodes. Hidden and
Exposed terminal problems makes MAC protocols inefficient.
• Hidden terminal problem
• Exposed terminal problem
Hidden terminal problem (2 Marks)
• Transmission of A reaches B, but not C
• Transmission of C reaches B, but not A

23. • However, the radio signal of B reaches both A and C in the range of B
• The effect is A can not detect C and C can not detect A
• A starts sending to B, C does not receive this transmission, C also wants send to B
and sense the medium. To C the medium appears to be free, Thus C starts sending
to B that cause collision.
• But now A cannot detect collision and continue with its transmission
• A is hidden for C and vice versa
• Exposed terminal problem (2 Marks)
• The radio Transmission of A reaches B, and C
• The radio Transmission of C reaches D, and A
• A want to communicate B , A stars sending data to B
• C wants to communicate with D, C sense the carrier and finds that A is talking
to B
• C has to wait till A finishes with B, However D is outside of A waiting is not
necessary
• In fact, A,B and C,D can communicate with each other in parallel with out any
collision, but according to protocol that is not possible.
• A and S are exposed terminals
Solutions
• Collision detection mechanism are good idea on wired LAN , but they cannot used
in wireless LAN
• Reason
– CD mechanism requires the implementation of Full duplex technique for
transmitting and receiving at the same time
– In wireless LAN, we cannot assume that all the stations will be able to
receive signal from each other(hidden terminal problem)
– Collision Avoidance mechanism is a solution

24. 1.9 Taxonomy of MAC protocols (2 Marks)
• Fixed assignment schemes
– Circuit switched method
– Resources are assigned for entire duration of the call
• Random assignment schemes
– Connection less Packet switched technique
– The node start to transmit as soon as they have a packet to send
• Reservation based assignment schemes
– Connection based Packet switched technique
– A node make a explicit reservation of the channel for a call before
transmitting
– Suitable for varying traffic
1.9.1 Fixed assignment schemes (16 Marks)
– Frequency division multiple access(FDMA)
– Time division multiple access(TDMA)
– Code division multiple access(CDMA)
1.9.1.1 Frequency division multiple access(FDMA) (8 Marks)
– The available bandwidth (frequency) is divided in to many frequency band called
channel(coaxial cable or microwave beam)
– Full duplex communication is take place, each user is allocated a forward
channel for communication from handset to base station and reverse
channel(Backward) for communicating from BS to hand set
– Each user is allocated two unique frequency channel, one for transmitting , other
for receiving
– When a call is underway, no other user would be allocated the same frequency
band
– Unused transmission time occurs when the user pauses between the transmission, it
does not achieve the high channel utilization
– It is used in satellite communication systems and telephone trunklines.(First
Generation)
The two frequencies: uplink and Downlink
– Uplink
From mobile station to base station or from ground control to satellite
– Downlink
From base station to mobile station or from satellite to ground control

25. Advantages (2 Marks)
• Reduces the bit rate information
• It reduces the cost and lowers the inter symbol interference
• An FDMA system can be easily implemented.
• Since the transmission is continuous, less number of bits are required for
synchronization and framing
Disadvantages (2 Marks)
• The maximum flow rate per channel is fixed and small
• Guard bands lead to a waste of capacity.
• Unused transmission time occurs when the user pauses between the transmission, it
does not achieve the high channel utilization
1.9.1.2 Time division multiple access (8 Marks)
• Multiple nodes are allotted different time slots to access the channel
• Time line is divided into fixed time slot and these are allocated multiple users
• All users use the same channel, but take turn in transmitting
• Time slots are allocated in round robin manner
• Each user assigned one time slot per frame, unused time slots goes ideal, leading low
channel utilization
• The fundamental unit of time is called a burst period and it lasts for approximately
0.577 ms.
• Eight of these burst periods are grouped into what is known as a TDMA frame
• A 25 MHz frequency range holds 124 single bandwidth of each kHz; each of these
frequency channels contains 8 TDMA conversation channels
Advantages of TDMA (2 Marks)
– Permits flexible data rates.
– No guard band required for the wideband system.
Disadvantages (2 Marks)
– Call time is needed in each slot to accommodate time to inaccuracies (due to
clock instability).
– Electronics operating at high bit rates increase energy consumption.
– Complex signal processing is required to synchronize within short slots.
1.9.1.3 Code Division Multiple Access (CDMA) (8 Marks)
• CDMA is a sort of multiplexing that facilitates multiple users to occupy a single
transmission channel. It optimizes the use of available bandwidth.
• Multiple users are allotted different code(sequence of 0 and 1) to access same channel
• Special coding scheme is used that allows signal from multiple users to be
multiplexed over the channel
• Multiple users use the same frequency at the same time

26. • CDMA employs analog-to-digital conversion (ADC) in combination with spread
spectrum technology
• It used m-bit pseudo noise (PN) code sequence to generate different code.
• Using m-bits, 2 m
-1 different codes can be obtained. From this code , each user will
use one code
• CDMA used in second-generation (2G) and third-generation
(3G) wireless communications
• The technology is commonly used in ultra-high-frequency (UHF) cellular
telephone systems, bands ranging between the 800-MHz and 1.9-GHz.
CDMA code generation
• A code for a user is orthogonal that refers the vector inner product is zero
• Two vector: p=(2,5,0), q=(0,0,17)
• Inner product p*q=(2*0 + 5*0 + 0*17) =0
• Bipolar notation: 0 represented as -1, 1 represented as +1
• Sender
– Unique random number key
– XOR the signal with random number key
• Receiver
– Knows the random number key
– Demodulate the signal to get data
• Sender
– X_data = 1, X_key = 010011
– Bipolar representation of X_key = -1,+1,-1,-1,+1,+1
– Calculated signal at sender Xs= X_data * X_key
– +1* X_key = +1 * (-1,+1,-1,-1,+1,+1)= (-1,+1,-1,-1,+1,+1)
– Y_data = 0, Y_key = 110101
– Bipolar representation of X_key = +1,+1,-1,-1,+1 ,-1,+1
– Calculated signal at sender Ys= Y_data * Y_key
– -1* Y_key = -1 * (+1,+1,-1,-1,+1 ,-1,+1)= (- 1,-1,+1,+1,-1 ,+1,-1)
• Receiver
– Xs+Ys= (-1,+1,-1,-1,+1,+1) + (- 1,-1,+1,+1,-1 ,+1,-1)
– (-2, 0, 0, -2, +2, 0)
• Z wants to get information of sender X and sender Y
• Z * X_key = (-2, 0, 0, -2, +2, 0) * (-1,+1,-1,-1,+1,+1)
= (2 +0+0+2+2+0)=6 > 0 (positive)
X_data = 1
• Z * Y_key = (-2, 0, 0, -2, +2, 0) * (+1,+1,-1,-1,+1 ,-1,+1)
= (-2 +0+0-2-2+0)=-6 < 0 (negative)
Y_data = 0

27. Advantages (2 Marks)
• CDMA provide a power control
• Flexible transfer may be used.
• Reduces interference.
Disadvantages (2 Marks)
• The code length must be carefully selected. A large code length can induce delay or
may cause interference.
• Time synchronization is required.
Space Division Multiple Access (SDMA) (4 Marks)
• It is used for allocating a separated space to users in wireless networks.
• Application
– Application involves assigning an optimal base station to a mobile phone user.
The mobile phone receives several base stations with different quality. A MAC
algorithm decide which base station is best, taking into account which
frequencies (FDM), time slots (TDM) or code (CDM) are still available.
– SDMA is always in combination with .one or more other schemes
– The SDMA algorithm is formed by cells and antennas which constitute the
infrastructure implementing space division multiplexing (SDM).
– A new application of SDMA comes up with beam-forming antenna arrays.
Single users are separated in space by individual beams.
– This improves the overall capacity of a cell
– e.g., measured in bit/s/m2 or voice calls/m2.
1.9.2 Random assignment schemes (16 Marks)
• Pure ALOHA
• Slotted ALOHA
• CSMA
• CSMA/CD
• CSMA/CA
Pure ALOHA
• Developed at university of Hawai
• If a node has data to send , it begins to transmit
• It does not check whether the channel is busy before transmitting
• If the frame successfully reaches the destination , it send next frame
• If the frame fails at destination , it is resent after a random amount of time to
reduce the probability of re-collision
• Suitable for small network, not suitable for large network. It leads collision.

28. Slotted ALOHA
• The time divided into equal size slots in which a packet can be sent, the size of the
packet is restricted
• A node wants to send a packet, start transmission at beginning of the slot
• The slotted ALOHA employs beacon signal that are sent at precise interval that mark
the beginning of the slot at which point the nodes can start to transmit
• Transmission time is broken into slots equivalent to the transmission time of a single
packet. Stations are only allowed to transmit at slot boundaries
• Does not work well in large network
Problem of Slotted ALOHA (2 Marks)
– The random retransmission of the packet will have an effect on the delay
associated with successful packet delivery.
– If the limit is too short, the probability of re-collision is high.
– If the limit is too long the probability of re-collision less but there is
unnecessary delay in the retransmission.
CSMA (Carrier sense Multiple Access)
• A node senses the medium before transmit, It the medium is busy, wait before
transmitting
– CSMA/CD
– CSMA/CA
– CSMA/CD does not work well in wireless network, it difficult for a
transmitting node to detect collision since received signal from other node
would be too feeble
– Only destination node notice the corrupted frame after it computes checksum.
This leads to retransmission and wastage of channel utilization
– In contrast, in wired network when node detects a collision , it immediately
stop transmission, thereby minimize the channel wastage
Do you agree with the following statement: “In CSMA/CD protocol, when two nodes
transmit on a shared medium, a collision can occur only when two nodes start
transmitting exactly at the same time instant”, Explain your answer.
• Yes. A collision may occur only when two nodes start transmitting exactly at the
same time instant because a node senses the medium before transmit, It the medium
is free, it sends data immediately
Why are collision detection based MAC protocols not suitable for wireless networks? (2
Marks)
 CD mechanism requires the implementation of Full duplex technique for transmitting
and receiving at the same time
 In wireless LAN, we cannot assume that all the stations will be able to receive signal
from each other(hidden terminal problem)

29.  Collision Avoidance mechanism is a solution
CSMA/CA (8 Marks)
• Idea is prevent collision
• When node is transmitting on the channel, other nodes might be waiting to
transmit until channel become free
• When transmitting node complete its transmission , the waiting nodes start
transmitting at a same time,
• To overcome collision, all the nodes are forced to wait for a random time and
then sense the medium again
• The chances of two nodes starting to transmit at the same time is greatly
reduced
• Non persistent
• 1-persistent
• P-Persistent
• Non persistent
• Channel transmit frame if it is ideal, otherwise it wait a random time and again
sense line
1-persistent
• When the station detects an idle-channel, it immediately transmits the frame with
probability 1
P-Persistent
• If time slot <= propagation time slot, send data if channel is ideal
• With the probability q=l-p, the station then waits for the beginning of the next time
slot

30. 1.9.3 Reservation based scheme (8 Marks)
• It is the RTS/CTS scheme
• A sender transmits an RTS(Request to send) packet to receiver before transmission
• The receiver sends a CTS (clear to send) packet, the data transmission commences
only after that.
• When the other node senses the CTS packet, they wait until the transmission is
complete.
• Reservation is achieved by sending RTS/CTS packets between sender and receiver,
other nodes defer its transmission in specified period to avoid collision.
• RTS/CTS based MAC protocol
– MACA, MACAW, MACA-BI, PAMAS,DBTMA,MARCH,S-MAC –
designed for wireless sensor networks
• MACA
– Multiple access collision avoidance
– MACA solves the hidden/exposed terminal problem
– A node requests to use the medium by sending RTS to receiver. Since radio
signals propagate Omni directional , every node within the senders range will
hear RTS and then refrain from transmitting
– The receiver is ready to receive data, it sends CTS
Solution for Hidden terminal problem (2 Marks)
• RTS contain sender‟s name and receiver‟s name as well as length of the future
transmission
• CTS contain sender‟s name and receiver‟s name as well as length of the planned
transmission
• CTS hear by C also, the medium is reserved by A with specified time, C refrains from
transmitting for time indicated in CTS
• The collision cannot occur at B during data transmission
• Issues

31. • Collision can occur during the sending of RTS. A and C could send RTS at the same
time. But an RTS occurs over a very small time compared to data transmission. The
probability of collision occur less
Solution for exposed terminal problem (2 Marks)
Issues
• Collision can occur during the sending of RTS. A and C could send RTS at the same
time. But an RTS occurs over a compared to data transmission. Thevery small time
probability of collision occur less
Application oriented questions
1 Consider a user data 101. What is the spreading signal when applying the 11-chip
Barker code 11110111001?