Mobile Computing (Part-2)

• Wireless Networking
• Wireless LAN Overview: IEEE 802.11
• Wireless applications
• Data Broadcasting
• Bluetooth
• TCP over wireless
• Mobile IP
• WAP: Architecture, protocol stack, application environment,
applications.
Mobile Computing
( Part – 2 )

Wireless Networking
 A technology that enables two or more entities to communicate
without wires
 Any type of computer network that is wireless
 For information transformation, it uses electromagnetic waves
such as radio waves at the level of physical layer of the network

Types of Wireless Connections
 Wireless PAN: interconnects devices within small area. For example,
Bluetooth, infrared etc. provides WPAN among the mobile handsets,
laptops etc.
 Wireless LAN: links devices using wireless distributed method (e.g.
spread spectrum, orthogonal frequency division multiplexing (OFDM)
radio). It provides a connection through an access point to the wider
internet. User can move within a local coverage area & still be
connected to the network for example Wi-Fi. It is used to connect the
networks in two or more buildings together.
 Wireless MAN: connects several wireless LANs. Wi-Max is a type of
WMAN.
 Wireless WAN: these networks can be used to connect branch offices
of a business or public internet system. A typical system contains base
station, gateways, access points and wireless bridging relays.

Wireless LAN
 A wireless LAN or WLAN is a wireless local area network that uses
radio waves as its carrier.
 WLAN is restricted in buildings, college campus etc. & operated by
individuals.
 The last link with the users is wireless, to give a network connection to
all users in a building or campus.
 Goal of WLAN is to replace office cabling and introduce higher flexibility
for communication at public places, meetings etc.
 The backbone network usually uses cables
 It is based on IEEE 802.11 standard.
 Wireless interface card is a primary component of WLAN.
 This interface card can be connected to mobile units as well as to a
fixed network.

Common Topologies: Infrastructure & Ad-hoc
networks
The wireless LAN connected to a wired LAN (Infrastructure networks)
 There is a need of an access point that bridges wireless LAN traffic into the
wired LAN.
 The access point (AP) can also act as a repeater for wireless nodes,
effectively doubling the maximum possible distance between nodes.

Common Topologies: Infrastructure & Ad-hoc
networks
Complete Wireless Networks (Ad-hoc networks)
 The physical size of the network is determined by the maximum reliable
propagation range of the radio signals.
 Referred to as ad hoc networks
 Are self-organizing networks without any centralized control
 Suited for temporary situations such as meetings and conferences.

infrastructure
network
ad-hoc network
AP
AP
AP
wired network
AP: Access Point
Source: Schiller

Wireless LANs: Characteristics
 Types
 Infrastructure based
 Ad-hoc
 Advantages
 Flexible deployment
 Minimal wiring difficulties
 More robust against disasters (earthquake etc)
 Historic buildings, conferences, trade shows,…
 Disadvantages
 Low bandwidth compared to wired networks (1-10 Mbit/s)
 Proprietary solutions
 Need to follow wireless spectrum regulations

Components/Architecture
 Station (STA) - Mobile node
 Access Point (AP) - Stations are connected to
access points.
 Basic Service Set (BSS) - Stations and the AP
with in the same radio coverage form a BSS.
 Extended Service Set (ESS) - Several BSSs
connected through APs form an ESS.

IEEE 802.11 in OSI Model
10
Wireless

802.11 Scope & Modules
11
MAC
Sublayer
MAC Layer
Management
PLCP Sublayer
PMD Sublayer
PHY Layer
Management
LLC
MAC
PHY
To develop a MAC and PHY spec for wireless
connectivity for fixed, portable and moving stations
in a local area

PHY Sublayers
12
 Physical layer convergence protocol (PLCP)
 Provides common interface for MAC
 Offers carrier sense status & CCA (Clear channel
assesment)
 Performs channel synchronization / training
 Physical medium dependent sublayer (PMD)
 Functions based on underlying channel quality and
characteristics
 E.g., Takes care of the wireless encoding

PLCP
13
 PLCP has two structures.
 All 802.11b systems have to support Long preamble.
 Short preamble option is provided to improve efficiency when
trasnmitting voice, VoIP, streaming video.
 PLCP Frame format
 PLCP preamble
 SFD: start frame delimiter
 PLCP header

PLCP (802.11b)
14
long
preamble
192us
short
preamble
96us
(VoIP, video)

MAC management
 Synchronization
- finding and staying with a WLAN
- synchronization functions
 Power Management
- sleeping without missing any messages
- power management functions
 Roaming
- functions for joining a network
- changing access points
- scanning for access points
 Management information base

MAC Management Layer
16
 Synchronization
 Finding and staying with a WLAN
 Uses TSF timers and beacons
 Power Management
 Sleeping without missing any messages
 Periodic sleep, frame buffering, traffic indication map
 Association and Reassociation
 Joining a network
 Roaming, moving from one AP to another
 Scanning

Synchronization
17
 Timing Synchronization Function (TSF)
 Enables synchronous waking/sleeping
 Enables switching from DCF to PCF
 Enables frequency hopping in FHSS PHY
 Transmitter and receiver has identical dwell interval at each center
frequency
 Achieving TSF
 All stations maintain a local timer.
 AP periodically broadcasts beacons containing timestamps,
management info, roaming info, etc.
 Not necessary to hear every beacon
 Beacon synchronizes entire BSS
 Applicable in infrastructure mode ONLY
 Distributed TSF (for Independent BSS) more difficult

Power management
18
 Battery powered devices require power efficiency
 LAN protocols assume idle nodes are always ON and thus ready
to receive.
 Idle-receive state key source of power wastage
 Devices need to power off during idle periods
 Yet maintain an active session – tradeoff power Vs throughput
 Achieving power conservation
 Allow idle stations to go to sleep periodically
 APs buffer packets for sleeping stations
 AP announces which stations have frames buffered when all
stations are awake – called Traffic Indication Map (TIM)
 TSF assures AP and Power Save stations are synchronized
 TSF timer keeps running when stations are sleeping
 Independent BSS also have Power Management
 Similar in concept, distributed approach

Roaming & Scanning
19
 Stations switch (roam) to different AP
 When channel quality with current AP is poor
 Scanning function used to find better AP
 Passive Scanning  Listen for beacon from different Aps
 Active Scanning  Exchange explicit beacons to determine best AP
 Station sends Reassociation Request to new AP
 If Reassociation Response successful  Roaming
 If AP accepts Reassociation Request
 AP indicates Reassociation to the Distribution System
 Distribution System information is updated
 Normally old AP is notified through Distribution System

MAC management frame
20
 Beacon
 Timestamp, Beacon Interval, Capabilities, ESSID, Supported
Rates, parameters
 Traffic Indication Map
 Probe
 ESSID, Capabilities, Supported Rates
 Probe Response
 Timestamp, Beacon Interval, Capabilities, ESSID, Supported
Rates, parameters
 same for Beacon except for TIM
 Association Request
 Capability, Listen Interval, ESSID, Supported Rates
 Association Response
 Capability, Status Code, Station ID, Supported Rates

MAC Management Frame
21
 Reassociation Request
 Capability, Listen Interval, ESSID, Supported Rates, Current
AP Address
 Reassociation Response
 Capability, Status Code, Station ID, Supported Rates
 Disassociation
 Reason code
 Authentication
 Algorithm, Sequence, Status, Challenge Text
 Deauthentication Reason

Security
22
 Range of attacks huge in wireless
 Easy entry into the network
 Jamming, selfish behavior, spatial overhearing
 Securing the network harder than wired networks
 Especially in distributed environments
 WEP  symmetric 40 or 128-bit encryption
 WPA: Wi-Fi protected access
 Temporal key integrity protocol (TKIP) – better
 User authentication
 IEEE 802.11i – Efforts toward higher security

Collision Detection
23
 What is the aim of collision detection ?
It’s a transmitter’s job:
To determine if the packet was
successfully received without
explicitly asking the receiver

Wireless LAN: Motivation
 Can we apply media access methods from fixed
networks?
 Example CSMA/CD
 Carrier Sense Multiple Access with Collision
Detection
 send as soon as the medium is free, listen into the
medium if a collision occurs (original method in
IEEE 802.3)

 Medium access problems in wireless networks
 signal strength decreases proportional to the square
of the distance
 sender would apply CS and CD, but the collisions
happen at the receiver
 sender may not “hear” the collision, i.e., CD does
not work
 CS might not work, e.g. if a terminal is “hidden”

The Emergence of MACA, MACAW, & 802.11
26
 Wireless MAC proved to be non-trivial
 1992 - research by Karn (MACA)
 1994 - research by Bhargavan (MACAW)
 Led to IEEE 802.11 committee
 The standard was rectified in 1999

Difference Between Wired and
Wireless
 If both A and C sense the channel to be idle at the
same time, they send at the same time.
 Collision can be detected at sender in Ethernet.
 Half-duplex radios in wireless cannot detect collision
at sender.
A B C
A
B
C
Ethernet LAN Wireless LAN

Hidden Terminal Problem
 Hidden terminals
 A and C cannot hear each other.
 A sends to B, C cannot receive A.
 C wants to send to B, C senses a “free” medium (CS
fails)
 Collision occurs at B.
 A cannot receive the collision (CD fails).
 A is “hidden” for C.
 Solution?
 Hidden terminal is peculiar to wireless (not found in
wired)
 Need to sense carrier at receiver, not sender!
BA C

802.11 MAC (DCF)
29
 CSMA/CA based protocol
 Listen before you talk
 CA = Collision avoidance (prevention is better than cure !!)
 Robust for interference
 Explicit acknowledgment requested from receiver
 for unicast frames
 Only CSMA/CA for Broadcast frames
 Optional RTS/CTS offers Virtual Carrier Sensing
 RTS/CTS includes duration of immediate dialog
 Addresses hidden terminal problems

IEEE 802.11
31
CTS = Clear
To Send
RTS = Request
To Send
D
Y
S
M
K
RTS
CTS
X

IEEE 802.11
32
D
Y
S
X
M
K
silenced
silenced
silenced
silenced
Data
ACK

802.11 Steps
33
 Receiver replies with CTS
 Also contains (DATA + ACK) duration.
 Neighbors update NAV again
 Tx sends DATA, Rx acknowledges with ACK
 After ACK, everyone initiates remaining countdown
 Tx chooses new R = rand (0, CW_min)
 If RTS or DATA collides (i.e., no CTS/ACK returns)
 Indicates collision
 RTS chooses new random no. R1 = rand (0, 2*CW_min)
 Note Exponential Backoff Ri = rand (0, 2^i * CW_min)
 Once successful transmission, reset to rand(0, CW_min)

RTS/CTS
35
 Does it solve hidden terminals ?
 Assuming carrier sensing zone = communication zone
C
F
A B
E
D
CTS
RTS
E does not receive CTS successfully  Can later initiate transmission to D.
Hidden terminal problem remains.
CTS

36
 How about increasing carrier sense range ??
 E will defer on sensing carrier  no collision !!!
CB D
Data
A
E
CTS
RTS
F

37
 But what if barriers/obstructions ??
 E doesn’t hear C  Carrier sensing does not help
CB D
Data
A
E
F
CTS
RTS

Exposed Terminal
38
 B should be able to transmit to A
 RTS prevents this
CA B
E
D
CTS
RTS

Exposed Terminal
39
 B should be able to transmit to A
 Carrier sensing makes the situation worse
CA B
E
D
CTS
RTS

Thoughts !
40
 802.11 does not solve HT/ET completely
 Only alleviates the problem through RTS/CTS and
recommends larger CS zone
 Large CS zone aggravates exposed terminals
 Spatial reuse reduces  A tradeoff
 RTS/CTS packets also consume bandwidth
 Moreover, backing off mechanism is also wasteful
The search for the best MAC protocol is still on.
However, 802.11 is being optimized too.
Thus, wireless MAC research still alive

Takes on 802.11
41
 Role of RTS/CTS
 Useful? No?
 Is it a one-fit-all? Where does it not fit?
 Is ACK necessary?
 MACA said no ACKs. Let TCP recover from losses
 Should Carrier Sensing replace RTS/CTS?
 New opportunities may not need RTS/CTS
 Infratructured wireless networks (EWLAN)

Hybrid Channel Access
42
 The optimization timeline
T R
RTS
CTS
Data
ACK
RTS
CTS
Data
ACK
T R
RTS
CTS
Data
RTS
CTS +ACK
Data
T R
RTS
CTS
Data
Poll +ACK
Data
RTS
CTS +ACK
Backoff
BackoffBackoff
Backoff
Poll +ACK
Data
BackoffBackoff
802.11 Implicit ACK Hybrid Channel Access

Infra red vs radio transmission
 WLAN uses two basic transmission technologies: infra red
transmission and radio transmission.
 Infra red technology uses diffuse light reflected at walls, furniture etc.
 Senders can be simple LEDs or laser diodes. Photodiodes acts as a
receiver.
 Advantages of Infra red technology:
 Simple & extremely chief.
 PDA, laptops, mobile phones, etc have an Infra red association interface.
 No license is needed
 Electrical devices do not interfere with Infra red transmission.
• Disadvantages of Infra red technology:
• Low bandwidth
• Can not penetrate walls or other obstacles.
• Coverage is limited to 10 meters.

Infra red vs radio transmission
 Advantages of Radio transmission:
 Higher bandwidth
 Covers wide area networks and mobile cellular phones.
 Can penetrate walls or other obstacles
• Disadvantages of Radio transmission:
 Radio transmission is permitted in only certain frequency bands.
 Electrical devices can interfere & destroy data transmitted.
 Very limited license free bands are available.

Advantages of WLANs
 Flexibility: nodes can communicate within radio coverage. Senders &
receivers can be placed anywhere, as radio waves can penetrates walls
etc.
 Easy to implement: WLANs are easy to setup, relocate, change &
manage. WLANs can operate in locations where implementation of
wiring may not be possible.
 Reliability: failures in wired network in commonly due to cable faults.
WLAN is free from such type of failures.
 Planning: wiring plans (as in wired networks) are not required.
 Robustness: wireless networks can survive disasters e.g. earthquakes,
flood etc. wired networks will breakdown.
 Low implementation cost: WLANs have low implementation cost as
they are easy to setup, relocate, change & manage. Adding additional
user to WLAN do not increases the cost.

Disadvantages of WLANs
 Quality of service: WLANS offer lower quality than their wired
counterparts due to lower bandwidth, higher error rates due to
interference and higher delay due to error correction and detection
mechanisms.
 Safety & Security: radio waves might interfere with other hi-tech
equipment e.g. in hospitals. Hacking in open wireless LAN is much
easier than in fiber optics. All wireless standards must offer encryption,
privacy mechanism etc. to secure them from hacking.
 Restrictions: all wireless products have to comply with national
regulations. Several government and nongovernment institutions word
wide regulate the operation and restrict the frequencies to minimize the
interference.
 Proprietary solutions: many companies have come up with proprietary
solutions offering standardized functionality plus many enhanced
features. However these enhanced features only work when all
adopters from same vendors are used for all wireless nodes.

Design Issues for WLANs
 Global operation: WLAN products are sold in all counties, so national &
international frequency regulations must be considered.
 Low power: devices communicating in WLAN are running on battery power.
Special power saving modes & functions must be applied in designing the
WLAN.
 License free operation: LAN operator do not need a special license to be
able to use the product. The equipment must operate in a license free band.
 Robust transmission technology: electrical devices can interfere the radio
transmission. WLAN transceivers can not be adjusted for perfect
transmission. Antennas are omnidirectional. Senders & receivers may move.
Robust transmission technology must be used to handle the above issues.
 Easy to use: WLANs are made for simple to use. They should not require
complex management, rather work on plug & play basis.
 Safety & security: WLANs must be safe to operate. Encryption mechanism
& users privacy must be taken into account. Users must not be able to read
personal data during transmission.

Applications of WLAN
 Office/campus environment: it is very useful in office and building with
big campus. People can move between floors, rooms , indoors &
outdoors. In office, persons can move with his laptop to meetings etc.
and continue working. In university, students & faculties can move from
one place to other and continue working.
 Homes: in homes, WLAN can be used for networking of different home
devices like phones, computers & other appliances.
 Public places: it includes airports, railway stations or places where
many people assembles and need to access information
 War/defense sites: WLAN helps to access the network during the war
at defense sites. Major research is going on, on mobile and ad hoc
network for defense establishment. It can be very useful in natural
disaster or at an accidental site as well.
 Workgroup environment: WLAN can be very useful where a
workgroup or team works together in a building etc.

Data Broadcasting
 Data Broadcasting is method of making data available to a
group of persons.
 Broadcast systems are unidirectional systems.
 Broadcast system provides one way communication in which
user or device can only receive the information e.g. radio station,
television station, news paper etc.
 Broadcast systems are classified into two categories:
 Digital Audio Broadcasting (DAB)
 Digital Video Broadcasting (DVB)

Digital Audio Broadcasting (DAB)
 DAB system used single frequency networks (SFN)
 SFN is frequency efficient as a single radio station needs
one frequency throughout country
 It used following two basic transport mechanism:
 Main Service Channel (MSC): consists of common
interleaved frames (CIF) data fields that are sent in every
24 ms.
 Fast Information Channel: contains fast information
blocks and carries all control information which is
required interpreting the contens of MSC.

Digital Video Broadcasting (DVB)
 Introduces digital television broadcasting using satellite
transmission, cable technology etc.
 A set top box (integrated reciever decoder) is connected to high
resolution monitor.
 Set top box receive signals via satellite, ISDN, terrestrial receiver
etc.
 A user can send data such as channel selection, authentication,
shopping list etc.
 Different types of TVs with different resolutions are:
 Standard Definition TV (SDTV)
 High Definition TV (HDTV)
 Enhanced Definition TV (EDTV) or LCD or LED

Cyclic Repetition of Data Blocks (Broadcast Disk)
As sender does not know when a receiver starts to listen, therefore
transmission of important information has to repeated after certain time
interval. Following three broadcast patterns are used for this purpose:
 Flat Disk: All blocks are repeated one after another. Every block is
transmitted for an equal time.
 Multidisk: Blocks are transmitted for equal time but the blocks having
high priority are repeated more frequently.
Skewed Disk: Blocks are repeated as per their priorities. High priority blocks
are repeated more frequently than the low priority ones.
Example: consider three data blocks A, B & C, A with high priority.
Flat Disk
Multi Disk
Skewed Disk
A B C A B C
A B A C A B
A A B C A A

Advantages of Data Broadcasting
 No wired connection is needed e.g. TV systems.
 Reliable, efficient and cost effective method
 Especially suited for instantaneous transmission of information
 Large number of users are supplied with the same information
simultaneously
 Multiple type of data can be supported e.g. text, audio, video, news,
entertainment, sports, business Data Broadcasting etc.

Applications of Data Broadcasting
 Applicable in remote area
 Electronic news paper
 Advertising
 Trading networks
 Distance learning programs
 Radio & television broadcasting

Bluetooth - overview
57
Motivation
In 1994 the L.M. Ericsson company wanted
to connect mobile phones to other devices
without using cables.

58
Special Interest Group (SIG)
L.M. Ericsson
IBM
Intel
Nokia
Toshiba

59
 Goals of the Bluetooth SIG
• Wireless standard (unification) for the
interconnection of computing and
communication devices.
• Inexpensive
• Short range
• Wireless radios

60
 Change in Bluetooth original scope
From : Getting rid of cables between
devices
To: Becoming more like a wireless
LAN

61
 In 1999 the Bluetooth SIG published a 1500 page
document describing V 1.0.
 IEEE assigned designation 802.15 and used the
V 1.0 specification as its basis. Then they began
to modify parts of it.

62
 Differences between V 1.0 and 802.15
 V 1.0 is a complete specification from application
layer to physical layer
 802.15 is only standardizing (modifying) the
physical and data link layers.

63
 The Bluetooth 802.15 specification not only
creates competition for other wireless
technologies, namely 802.11, but the two occupy
most of the same 2.4 GHz spectrum and thus
interfere with each other.
(More on this later)

Bluetooth - architecture
64
 Piconet – the basic unit of a Bluetooth
system.
 1 Master node
 1 to 7 active slave nodes
 0 to 255 parked nodes

65
 Parked node
 a slave device in a low power state to conserve the
drain on the devices batteries
 In this state the device can only respond to the
beacon from the master node

66
 Node range
 Slave nodes need to be within 10 meters of the
master node.
 Why design such a short range?

67
 Communication
 Only possible between master and slave nodes
 Piconet uses centralized Time Division Multiplexing.
 The master node controls the clock and determines
which devices occupy which time slot.

68
 How can such a limited range architecture really
provide competition for 802.11 (WiFi)?

69
 How can such a limited range architecture really
provide competition for 802.11 (WiFi)?
 Answer : Scatternets

Bluetooth – architecture
70
Tanenbaum, Andrew S., Computer Networks 4th Ed. figure 4-35

Bluetooth – protocol stack architecture
Tanenbaum, Andrew S, Computer Networks 4th Ed. figure 4-37
 Does not fit any known models :
 OSI, TCP/IP, including the 802 model
 Radio layer corresponds to the physical layer
 deals with radio transmission and modulation
 focuses on inexpensive implementation
71

 Base band layer
 Combines part of the typical physical and data link layer
roles.
 Specifically the MAC sub-layer of the data link layer.
 Focuses on how the master controls the time slots and how
the slots are grouped into frames.
72

 Link Manager
 logical channels between devices
 power management
 authentication
 quality of service
73

BlueTooth – protocol stack architecture
 Logical link control adaptation protocol (L2CAP)
 shields upper layers from the details of transmission
 somewhat analogous to 802 LLC sub layer
74

BlueTooth – protocol stack architecture
 Middleware layer
 RFcomm : serial communications, mouse, keyboard
…
 Telephony : speech oriented protocol
 Service Discovery : locate services in the network
75

 Audio
 controls audio, applications have direct access
 Control
 a control protocol, applications have direct access76

 The top layer is for application and profiles.
 each application need only use the necessary
subset of the protocol stack to accomplish it’s task77

More Bluetooth protocol layers
78
A. Radio Layer
B. Base band Layer
C. L2CAP Layer

Radio Layer
79
 Low power system can operated up to 10
meter with the frequency in the 2.4 GHz ISM
band
 Lowest defined layer of the Bluetooth
specification
 Band is divided into 79 channels with 1 MHz
each
 Frequency hopping spread spectrum at 1600
hops/sec.

The Bluetooth Base band Layer
80
 It lies on top of the Bluetooth radio layer
 It is the physical layer of the Bluetooth
 Each frame is transmitted over a logical
channel called a linked between master slave
 Two kinds of links
 Asynchronous Connection less (ASL)
 Synchronous Connection Oriented (SCO)

Logical Link Control and Adaptation
Protocol (L2CAP)
81
 There are 3 major functions
 I. It accepts packets up to 64 kB from the upper layers &
breaks them into frames for transmission
 II. It handles the multiplexing and demultiplexing of
multiple packet source
 Packet has been reassemble the L2CAP layer can determines which upper-
layer protocol to handle ( RFcomm or telephony)
 III. The third major function is to handles the quality of service
requirements when both links are established and during normal
operation ( not all devices can handle the 64 kB maximum packet)

The Bluetooth Frame Structure
82
Image taken from Class Textbook – Tanenbaum, Andrew S. Computer Networks 4th Ed. p.310-17.

Bluetooth Frame Structure
83
 Access code (72 bits)
 Header (54 bits)
 Address field, type field, flow bit, acknowledge bit,
sequence bit, and checksum
 Data ( 0-2744 bits)

84
Conclusion
Bluetooth is a wireless technology that
was developed to be very low cost in hopes
that it would become widely used.
It is still yet to be seen whether or not it will
become the standard that the initial SIG
had hoped it would become.

Bluetooth Architecture
 Core Specification -
Deals with the lower
layers of the architecture
and describes how the
technology works.
 Profile Specification -
Focuses on how to build
interoperating devices
using the core
technology.

RF Layer
 The Radio (layer) is the lowest defined
layer of the Bluetooth specification.
 It defines the requirements of the
Bluetooth transceiver device operating
in the 2.4GHz ISM band.

 In order to minimize interference the nominal
antenna power is 1 mW which can be
extended to 100mW.
 The low power limits the range to about 10
centimeters to 10 meters. With higher power
of 100mW range of 100meters can be
achieved.
 It uses a packet switching protocol based on a
technology called spread-spectrum
frequency hopping to spread the energy
across the ISM band.

Spread-Spectrum frequency hopping
 A device will use 79 individual randomly chosen
frequencies within a designated range, changing from
one to another on a regular basis.
 The designated range is from 2.402GHz to 2.480GHz,
in steps of 1MHz.
 The frequency hopping is done at a rate of 1600 times
a second.
 This allows more devices to use the limited time slice
and secondly reduces the chance of two transmitters
being on the same frequency at the same time.

 Baseband layer – This layer defines the
timing, framing, packets and flow control on
the link.
 Link Manager – Responsible for managing
connection states(authentication &
encryption), enforcing fairness among slaves
& power mangt.
 Logical Link Layer – Handles multiplexing,
segmentation and reassembly of large
packets and device discovery.
 Audio – The audio data is directly mapped to
the baseband layer.

Bibliography
90
 Computer Network by Andrew S. Tanenbaum, 4th Edition p.310-17.
 Bluetooth Revealed by Brent A. Miller and Chatschik Bisdikian, Ph.D.,
2nd Edition.
 http://www.palowireless.com/infotooth/tutorial/radio.asp

What is TCP?
 TCP provides reliable, ordered delivery
 a program on one computer to another program on
another computer.
 In terms of Layer:
 TCP provides a communication service
 an intermediate level between an application
program and the Internet Protocol (IP).

What TCP does?
 A large chunk of data across the Internet using IP, the software
issues a single request to TCP and
TCP breaks data into packets
 Due to network congestion and other unpredictable network
behavior, IP packets can be lost, duplicated, or delivered out of
order.
 TCP
 detects these problems,
 requests retransmission of lost packets,
 rearranges out-of-order packets, and
 even helps minimize network congestion to reduce the occurrence
of the other problems.
 TCP reassembles a perfect copy of the data originally transmitted
and passes that datagram to the application program.

TCP : Congestion
Congestion is temporary overload at some
node.
Example:
buffer getting full in a router.
In that case, the packets get lost.

Congestion control
 If a packet is dropped, the sender notices the
absence of ack of a packet. Ack by receiver
confirms no loss of any packet.
 If the ack is not received, the TCP assumes that
there is traffic congestion and reduces the
transmission rate to avoid loss.
Solution
 All other TCP connections experiencing the
congestion slows down the transmission and the
congestion is soon resolved.

Slow start
 TCP behavior after detection of congestion is
called slow start.
 TCP calculates a congestion window for a
receiver.
 TCP first sends one segment (TCP packet). If ack
is received for this then sends 2 packets next
time. If Ack is proper for this too, this sends 4
segments next time.
 This is called the exponential growth of the
window in Slow start mechanism.
 This size grows exponentially till Congestion
threshold is reached.

Fast Retransmit
 The sender retransmits missing packets (if it is not a
case of congestion) before the timer expires.
 If there is time-out due to missing acknowledge then
instead of Fast Retransmit, Slow start method is used
in traditional TCP.

Issues in TCP over wireless
 TCP assumes, packet loss :due to congestion only.
 Error rates in fixed or wired networks are negligible.
 This assumption fails in wireless networks as error rates in
wireless networks
 fading, shadowing
 hand off, and other radio effects, that cannot be considered
congestion
 Hence, when packets are dropped or corrupted on the
wireless link, the congestion control mechanism on the sender
comes into action which slows down data rate. As a result the
throughput decreases drastically.

.. Issues in TCP over wireless
 Packet loss may be due to mobility of device
itself. There could be still some packets in transit
to old foreign agent. This problem is due to re-
routing traffic.
 TCP reacting with SLOW START even in case of
Time-out (which may be due to handover in
wireless) results in severe performance
degradation.

Indirect TCP
mobile host
access point
(foreign agent) wired Internet
“wireless” TCP standard TCP
Step/Session 1Step/Session 2

I-TCP socket and state migration
mobile host
access point2
Internet
access point1
socket migration
and state transfer

Indirect TCP II
 Advantages
 no changes in the fixed network necessary, no changes for
the hosts (TCP protocol) necessary, all current
optimizations to TCP still work
 transmission errors on the wireless link do not propagate
into the fixed network
 simple to control, mobile TCP is used only for one hop
between, e.g., a foreign agent and mobile host
 therefore, a very fast retransmission of packets is possible,
the short delay on the mobile hop is known
 Disadvantages
 loss of end-to-end semantics, an acknowledgement to a
sender does not any longer mean that a receiver really got
a packet, foreign agents might crash
 higher latency possible due to buffering of data within the
foreign agent and forwarding to a new foreign agent

Snooping TCP
Indirect TCP
 2 TCP sessions.
Snooping TCP
 One TCP session.
 The access point snoops into the traffic and
buffers packets for fast re-transmission.

Snooping TCP I
“wired” Internet
buffering of data
end-to-end TCP connection
local retransmission correspondent
hostforeign
agent
mobile
host
snooping of ACKs

Transparent extension of TCP within the foreign agent
 buffering of packets sent to the mobile host
 lost packets on the wireless link (both directions!)
will be retransmitted immediately by the mobile host
or foreign agent, respectively (so called “local”
retransmission)
 the foreign agent therefore “snoops” the packet flow
and recognizes acknowledgements in both
directions, it also filters ACKs
 changes of TCP only within the foreign agent (+min.
MH change)

Snooping TCP II Data transfer to the mobile host
 FA buffers data until it receives ACK of the MH, FA detects packet loss
via duplicated ACKs or time-out
 fast retransmission possible, transparent for the fixed network
 Data transfer from the mobile host
 FA detects packet loss on the wireless link via sequence numbers, FA
answers directly with a NACK to the MH
 MH can now retransmit data with only a very short delay
 Advantages:
 Maintain end-to-end semantics
 No change to correspondent node
 No major state transfer during handover
 Problems
 Snooping TCP does not isolate the wireless link well
 May need change to MH to handle NACKs
 Snooping might be useless depending on encryption schemes

Mobile TCP Special handling of lengthy and/or frequent disconnections
 M-TCP splits as I-TCP does
 unmodified TCP fixed network to supervisory host (SH)
 optimized TCP SH to MH
 Supervisory host
 no caching, no retransmission
 monitors all packets, if disconnection detected
 set sender window size to 0
 sender automatically goes into persistent mode
 old or new SH reopen the window
 Advantages
 maintains semantics, supports disconnection, no buffer
forwarding
 Disadvantages
 loss on wireless link propagated into fixed network
 adapted TCP on wireless link

Fast retransmit/fast recovery
 Change of foreign agent often results in packet loss
 TCP reacts with slow-start although there is no congestion
 Forced fast retransmit
 as soon as the mobile host has registered with a new foreign
agent, the MH sends duplicated acknowledgements on
purpose
 this forces the fast retransmit mode at the communication
partners
 additionally, the TCP on the MH is forced to continue sending
with the actual window size and not to go into slow-start after
registration
 Advantage
 simple changes result in significant higher performance
 Disadvantage
 further mix of IP and TCP (to know when there is a new
registration), no transparent approach

Transmission/time-out freezing
 Mobile hosts can be disconnected for a longer time
 no packet exchange possible, e.g., in a tunnel, disconnection
due to overloaded cells or mux. with higher priority traffic
 TCP disconnects after time-out completely
 TCP freezing
 MAC layer is often able to detect interruption in advance
 MAC can inform TCP layer of upcoming loss of connection
 TCP stops sending, but does not assume a congested link
 MAC layer signals again if reconnected
 Advantage
 scheme is independent of data and TCP mechanisms
(Ack,SN) => works even with IPsec
 Disadvantage
 TCP on mobile host has to be changed, mechanism depends
on MAC layer

Selective retransmission
 TCP acknowledgements are often cumulative
 ACK n acknowledges correct and in-sequence receipt of
packets up to n
 if single packets are missing quite often a whole packet
sequence beginning at the gap has to be retransmitted (go-
back-n), thus wasting bandwidth
 Selective retransmission as one solution
 RFC2018 allows for acknowledgements of single packets,
not only acknowledgements of in-sequence packet streams
without gaps
 sender can now retransmit only the missing packets
 Advantage: much higher efficiency
 Disadvantage
 more complex software in a receiver, more buffer needed at
the receiver

Transaction oriented TCP
 TCP phases
 connection setup, data transmission, connection release
 using 3-way-handshake needs 3 packets for setup and
release, respectively
 thus, even short messages need a minimum of 7 packets!
 Transaction oriented TCP
 RFC1644, T-TCP, describes a TCP version to avoid this
overhead
 connection setup, data transfer and connection release can be
combined
 thus, only 2 or 3 packets are needed
 Advantage
 efficiency
 Disadvantage
 requires changed TCP
 mobility no longer transparent

Revision of TCP in wireless
 TCP in wired: problem & solution
 In wireless: Problem & solution
 Indirect
 Snooping
 Mobile
 Fast retransmission/fast recovery
 Transmission/time-out freezing
 Selective Retransmission
 Transaction oriented TCP

Mobile IP
Outline
Intro to mobile IP
Operation
Problems with mobility

Portable Networking Technology
CS 640114
 Cellular systems
 Cellular Digital Packet Data (CDPD)
 3G
 LTE (3.9G)*
 Bluetooth
 Low cost, short range radio links between mobile
devices
 Wireless Ethernet (802.11)
 Widely used wireless MAC layer technology

Mobility and Standard IP Routing
CS 640115
 IP assumes end hosts are in fixed physical locations
 What happens if we move a host between networks?
 IP addresses enable IP routing algorithms to get
packets to the correct network
 Each IP address has network part and host part
 This keeps host specific information out of routers
 DHCP is used to get packets to end hosts in networks
 This still assumes a fixed end host
 What if a user wants to roam between networks?
 Mobile users don’t want to know that they are moving
between networks
 Why can’t mobile users change IP when running an
application?

Mobile IP
CS 640116
 Mobile IP was developed as a means for transparently
dealing with problems of mobile users
 Enables hosts to stay connected to the Internet regardless
of their location
 Enables hosts to be tracked without needing to change
their IP address
 Requires no changes to software of non-mobile
hosts/routers
 Requires addition of some infrastructure
 Has no geographical limitations
 Requires no modifications to IP addresses or IP address
format
 Supports security
 Could be even more important than physically connected routing

Mobile IP Entities
CS 640117
 Mobile Node (MN)
 The entity that may change its point of attachment from network to
network in the Internet
 Detects it has moved and registers with “best” FA
 Assigned a permanent IP called its home address to which other hosts
send packets regardless of MN’s location
 Since this IP doesn’t change it can be used by long-lived applications as
MN’s location changes
 Home Agent (HA)
 This is router with additional functionality
 Located on home network of MN
 Does mobility binding of MN’s IP with its COA
 Forwards packets to appropriate network when MN is away
 Does this through encapsulation

Mobile IP Entities contd.
CS 640118
 Foreign Agent (FA)
 Another router with enhanced functionality
 If MN is away from HA the it uses an FA to send/receive data to/from HA
 Advertises itself periodically
 Forward’s MN’s registration request
 Decapsulates messages for delivery to MN
 Care-of-address (COA)
 Address which identifies MN’s current location
 Sent by FA to HA when MN attaches
 Usually the IP address of the FA
 Correspondent Node (CN)
 End host to which MN is corresponding (eg. a web server)

MIPv4: Overview
CNP119/50
 MIPv4 Nodes
 MN (Mobile Node): Host
 CN (Correspondent Node): Host
 HA (Home Agent): Router
 FA (Foreign Agent): Router
 MIPv4 Address
 HoA (Home Address): MN
 CoA (Care-of-Address): FA

Mobile IP Support Services
(Steps)
CS 640120
 Agent Discovery
 HA’s and FA’s broadcast their presence on each network to which they
are attached
 Beacon messages via ICMP Router Discovery Protocol (IRDP)
 MN’s listen for advertisement and then initiate registration
 Registration
 When MN is away, it registers its COA with its HA
 Typically through the FA with strongest signal
 Registration control messages are sent via UDP to well known port
 Encapsulation – just like standard IP only with COA
 Decapsulation – again, just like standard IP

Mobile IP Operation
CS 640121
 A MN listens for agent advertisement and then initiates
registration
 If responding agent is the HA, then mobile IP is not necessary
 After receiving the registration request from a MN, the HA
acknowledges and registration is complete
 Registration happens as often as MN changes networks
 HA intercepts all packets destined for MN
 This is simple unless sending application is on or near the same
network as the MN
 HA masquerades as MN
 There is a specific lifetime for service before a MN must re-register
 There is also a de-registration process with HA if an MN returns home

Registration Process
CS 640122

Tables maintained on routers
CS 640123
 Mobility Binding Table
 Maintained on HA of MN
 Maps MN’s home address with
its current COA
 Visitor List
 Maintained on FA serving an
MN
 Maps MN’s home address to its
MAC address and HA address

Mobile IP Operation contd.
CS 640124
 HA then encapsulates all packets addressed to MN and
forwards them to FA
 IP tunneling
 FA decapsulates all packets addressed to MN and
forwards them via hardware address (learned as part of
registration process)

Mobile IP Tunneling
CS 640125
Across Internet

Some more details…
MIP addressing & working
CS 640126

CNP127/50
Home Address (HoA) and Care-of Address (CoA)

CNP128/50
The home address is permanent;
the care-of address changes as the mobile
host moves from one network to another.
Note:

MIPv4 Agents
CNP129/50
 Home Agent (HA) & Foreign Agent (FA)

Protocols Operation
CNP130/50
 Agent Discovery
 Registration
 Data Transfer

MIPv4: Control Operations
CNP131/50
 Agent Discovery
 MN  FA (CoA)
 ICMP Agent Solicitation & Advertisement
 Registration to HA (via FA)
 MN  FA  HA
 Over UDP (destination port 434)
 Data Tunneling
 CN => HA (HoA) => FA (CoA) => MN
 IP-in-IP Tunneling, ..

MIPv4: Control & Data Flows
CNP132/50

Agent advertisement
CNP133/50
 MIP does not use a new packet type for agent
advertisement;
 it uses the router advertisement packet of ICMP,
and
 appends an agent advertisement message.

CNP134/50
Mobile IP does not use a new packet
type for agent solicitation;
it uses the router solicitation packet
of ICMP.
Agent Solicitation

CNP135/50
Registration request and reply

CNP136/50
A registration request or reply is sent
by UDP using the well-known port 434.
Note:

CNP137/50
Registration request format

CNP138/50
Registration reply format

CNP140/50
The movement of the mobile host
is transparent to
the rest of the Internet.
Key Objective of MIP

Security in Mobile IP
CS 640141
 Authentication can be performed by all parties
 Only authentication between MN and HA is required
 Keyed MD5 is the default
 Replay protection
 Timestamps are mandatory
 Random numbers on request reply packets are
optional
 HA and FA do not have to share any security
information.

Problems with Mobile IP
CS 640142
 Suboptimal “triangle” routing
 What if MN is in same subnetwork as the node to which it
is communicating and HA is on the other side of the world?
 It would be nice if we could directly route packets
 Solution: Let the CN know the COA of MN
 Then the CN can create its own tunnel to MN
 CN must be equipped with software to enable it to learn the COA
 Initiated by HA who notifies CN via “binding update”
 Binding table can become stable

MIP: Triangular Routing Problem
CNP143/50

Other Mobile IP Problems
CS 640144
 Single HA model is fragile
 Possible solution – have multiple HA
 Frequent reports to HA if MN is moving
 Possible solution – support of FA clustering
 Security
 Connection hijacking, snooping…
 Many open research questions

Mobile IP and its Variants
CNP145/50
 Mobile IPv4 (MIPv4)
 MIPv4
 Low-Latency Handover for MIPv4 (FMIPv4)
 Regional Registration for MIPv4 (HMIPv4)
 Mobile IPv6 (MIPv6)
 MIPv6
 Fast Handover for MIPv6 (FMIPv6)
 Hierarchical MIPv6 (HMIPv6)

Mobility in IPv6
CS 640146
 Route Optimization is a fundamental part of Mobile
IPv6
 Mobile IPv4 it is an optional set of extensions that may
not be supported by all nodes
 Foreign Agents are not needed in Mobile IPv6
 MNs can function in any location without the services of
any special router in that location
 Security
 Nodes are expected to employ strong authentication and
encryption
 Other details…

Mobile IPv6 (MIPv6)
CNP147/50
 MIPv6 = MIPv4 + IPv6
 Major Differences from MIPv4
 FA in MN
 No FA for MIPv6
 CoA: IP address of MN
 By DHCPv6 or IPv6 Stateless Auto-Configuration
 Route Optimization
 To solve the “Triangular Routing” Problem
 Provided by default
 MN  CN

WAP
Wireless Applications Protocol

What is WAP?
 A set of protocols which allow data exchange for
mobile cellular systems
 It is device independent
 It is network independent

Current Constraints of
 Less Bandwidth
 High Latency
 Less Stable Connections
 Less Predictable Availability
 Diverse range of network
standards
Less CPU Power
Less Memory and Storage
Restricted Power Consumption
Small / Variable Sized Displays
Variable Input Types
(Keypad, Pen, etc,)
Wireless Interfaces: Mobile Devices:

Wireless networks and phones
– have specific needs and requirements not
addressed by existing Internet technologies.
WAP enables any data transport
– TCP/IP, UDP/IP, IS-135/6, SMS, or USSD.
The WAP architecture
– has several modular entities which together form
a fully compliant Internet entity
– all WML content is accessed via HTTP 1.1
requests.
Why Use WAP ?

Why Use WAP ? (cont..)
WAP utilizes standard Internet markup language
technology (XML)
Optimizing the content and airlink protocols
The WML UI components map well onto existing
mobile phone user interfaces
– no re-education of the end-users
– leveraging market penetration of mobile devices
WAP utilizes plain Web HTTP 1.1 servers
– leveraging existing development methodologies
– CGI, ASP, NSAPI, JAVA, Servlets, etc.

Web Server
Content
CGI
Scripts
etc.
WMLDecks
withWML-Script
WAP Gateway
WML Encoder
WMLScript
Compiler
Protocol Adapters
Client
WML
WML-
Script
WTAI
Etc.
HTTPWSP/WTP
The WAP Architecture

More Acronyms to learn
WAE - Wireless Application Environment
WSP - Wireless Session Protocol
WTP - Wireless Transaction Protocol
WTLS - Wireless Transport Layer Security
WDP - Wireless Datagram Protocol

WAP Developers
An incomplete list of corporations currently
developing WAP products and/or services:
 AT&T
 Bell Atlantic Mobile
 GTE
 Sprint PCS
 US West
 Nextel
 France Telecom
 Telenor (Norway)
 Bell Mobility (Canada)
 Japan Telecom
 Nokia
 Ericsson
 Motorola
 Qualcomm
 Samsung
 Palm Computing (3Com)
 IBM
 Phone.com (Unwired Planet)

Tag-based browsing language:
– Screen management (text, images)
– Data input (text, selection lists, etc.)
– Hyperlinks & navigation support
XML-based language
Inherits technology from HTML
WML
Wireless Markup Language

WAP Network Example
Web
Server
Wireless
Network
WAP
Proxy
HTML
Filter
WTA
Server
WAP
Client
WML
WML
HTML

WWW Protocol Stack
TCP/IP
UDP/IP
TLS - SSL
HTTP
HTML
Java Script

Components of WAP Architecture
Other Services
And Applications
Transport Layer (WDP)
Security Layer (WTLS)
Transaction Layer (WTP)
Session Layer (WSP)
Application Layer (WAE)
GSM CDMA PHS IS-136 CDPD PDC-P FLEX Etc…
Bearers :

Wireless Application Environment
(WAE)
 General-purpose application environment based on a
combination of WWW and mobile telephony
technologies.
 It defines the user interface on the phone. It contains
WML and WTA (Wireless Telephony Application).
 Primary objective – interoperable environment.

Wireless Application Environment
(Contd.)
 WAE includes a micro-browser (Client software
designed to overcome challenges of mobile handheld
devices that enables wireless access to services such
as Internet information in combination with a suitable
network) server environment which provides
 WML
 WML script
 WTA
 Content formats

Wireless Session Protocol (WSP)
 The WAP session protocol (WSP) layer provides
a lightweight session layer to allow efficient
exchange of data between applications.
 Application layer with a consistent interface for
two session services
 Connection-oriented service that operates above the
transaction layer protocol (WTP)
 Connectionless service that operates above a secure
or non-secure datagram service (WDP)
 Optimized for low-bandwidth bearer networks
with long latency

Wireless Transaction Protocol (WTP)
o Runs on top of a datagram service
 The WAP transaction protocol (WTP) layer
provides transaction support, adding reliability to
the datagram service provided by WDP.
 Light weight transaction-oriented protocol
 Three classes of transaction services
 Unreliable one-way requests
 Reliable one-way requests
 Reliable two-way request-reply transactions

Wireless Transport Layer Security
(WTLS)
Based on industry-standard Transport Layer
Security (TLS) protocol
 Optimized for use over narrow-band
communication channels
 Features:
 Data integrity
 Privacy
 Authentication
 Denial-of-service protection

Wireless Datagram Protocol
(WDP)
The WAP datagram protocol (WDP) is the
Transport layer that sends and receives messages
via any available bearer network, including SMS,
USSD, CSD, CDPD, IS–136 packet data, and
GPRS.
Operates above the data capable bearer services
supported by various network types.
Provides a common interface to the upper layer
protocols and hence they function independent of
the underlying wireless network.

Bearers
 Differing levels of quality of service with respect to
throughput, error rate, and delays
 WAP protocols are designed to compensate for or
tolerate these varying levels of service
 WDP specification lists the bearers that are supported
and techniques used to allow WAP protocols to run
over each bearer

Sample WAP Stacks
No layer
IP Non-IP
UDP WDP
WTP
WSP/B
WAE
WTLS
WAE
User Agents
No layer
IP Non-IP
UDP WDP
WTP
WTLS
Applications over
transactions
No layer
IP Non-IP
UDP WDP
WTLS
Applications over
Datagram Transport
WAP Technology
Outside of WAP

Mobile-Originated Example of WAP
Architecture
The request from the mobile device is sent as a URL
through the operator's network to the WAP gateway, which
is the interface between the operator's network and the
Internet as shown in the figure below.

HTML
JavaScript
HTTP
TLS - SSL
TCP/IP
UDP/IP
Wireless Application Protocol
Wireless Application
Environment (WAE)
Session Layer (WSP)
Security Layer (WTLS)
Transport Layer (WDP)
Other Services and
Applications
Transaction Layer (WTP)
SMS USSD CSD IS-136 CDMA CDPD PDC-P Etc..
Bearers:
Comparison between Internet and WAP Models
Internet
WAP Architecture

Mobile Computing (Part-2)

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