These ppt are the part 2 of mobile computing concepts. These ppt defines the following things
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.
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Mobile Computing (Part-2)
1. • 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 )
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
3. 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.
4. 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.
5. 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.
6. 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.
8. 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
9. 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.
11. 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
12. 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
13. 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
15. 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
16. 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
17. 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
18. 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
19. 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
20. 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
21. 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
22. 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
23. 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
24. 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)
25. 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”
26. 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
27. 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
28. 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
29. 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
35. 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. Hidden Terminal Problem
36
How about increasing carrier sense range ??
E will defer on sensing carrier no collision !!!
CB D
Data
A
E
CTS
RTS
F
37. Hidden Terminal Problem
37
But what if barriers/obstructions ??
E doesn’t hear C Carrier sensing does not help
CB D
Data
A
E
F
CTS
RTS
38. Exposed Terminal
38
B should be able to transmit to A
RTS prevents this
CA B
E
D
CTS
RTS
39. Exposed Terminal
39
B should be able to transmit to A
Carrier sensing makes the situation worse
CA B
E
D
CTS
RTS
40. 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
41. 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)
42. 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
43. 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.
44. 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.
45. 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.
46. 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.
47. 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.
48. 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.
50. 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)
51. 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.
52. 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
53. 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
54. 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.
55. Applications of Data Broadcasting
Applicable in remote area
Electronic news paper
Advertising
Trading networks
Distance learning programs
Radio & television broadcasting
59. Bluetooth - overview
59
Goals of the Bluetooth SIG
• Wireless standard (unification) for the
interconnection of computing and
communication devices.
• Inexpensive
• Short range
• Wireless radios
60. Bluetooth - overview
60
Change in Bluetooth original scope
From : Getting rid of cables between
devices
To: Becoming more like a wireless
LAN
61. Bluetooth - overview
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. Bluetooth - overview
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. Bluetooth - overview
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)
64. 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. Bluetooth - architecture
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. Bluetooth - architecture
66
Node range
Slave nodes need to be within 10 meters of the
master node.
Why design such a short range?
67. Bluetooth - architecture
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. Bluetooth - architecture
68
How can such a limited range architecture really
provide competition for 802.11 (WiFi)?
69. Bluetooth - architecture
69
How can such a limited range architecture really
provide competition for 802.11 (WiFi)?
Answer : Scatternets
71. 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
72. Bluetooth – protocol stack architecture
Tanenbaum, Andrew S, Computer Networks 4th Ed. figure 4-37
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
73. Bluetooth – protocol stack architecture
Tanenbaum, Andrew S, Computer Networks 4th Ed. figure 4-37
Link Manager
logical channels between devices
power management
authentication
quality of service
73
74. BlueTooth – protocol stack architecture
Tanenbaum, Andrew S, Computer Networks 4th Ed. figure 4-37
Logical link control adaptation protocol (L2CAP)
shields upper layers from the details of transmission
somewhat analogous to 802 LLC sub layer
74
75. BlueTooth – protocol stack architecture
Tanenbaum, Andrew S, Computer Networks 4th Ed. figure 4-37
Middleware layer
RFcomm : serial communications, mouse, keyboard
…
Telephony : speech oriented protocol
Service Discovery : locate services in the network
75
76. Bluetooth – protocol stack architecture
Audio
controls audio, applications have direct access
Control
a control protocol, applications have direct access76
77. Bluetooth – protocol stack architecture
Tanenbaum, Andrew S, Computer Networks 4th Ed. figure 4-37
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
79. 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.
80. 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)
81. 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)
82. The Bluetooth Frame Structure
82
Image taken from Class Textbook – Tanenbaum, Andrew S. Computer Networks 4th Ed. p.310-17.
83. 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. 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.
85. 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.
86. 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.
87. 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.
88. 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.
89. 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.
90. 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
92. 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).
93. 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.
94. TCP : Congestion
Congestion is temporary overload at some
node.
Example:
buffer getting full in a router.
In that case, the packets get lost.
95. 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.
96. 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.
97. 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.
98. 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.
99. .. 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.
100. Indirect TCP
mobile host
access point
(foreign agent) wired Internet
“wireless” TCP standard TCP
Step/Session 1Step/Session 2
101. I-TCP socket and state migration
mobile host
access point2
Internet
access point1
socket migration
and state transfer
102. 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
103. 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.
104. Snooping TCP I
“wired” Internet
buffering of data
end-to-end TCP connection
local retransmission correspondent
hostforeign
agent
mobile
host
snooping of ACKs
105. 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)
106. 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
107. 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
108. 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
109. 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
110. 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
111. 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
112. 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
114. 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
115. 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?
116. 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
117. 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
118. 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)
119. 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
120. 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
121. 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
123. 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
124. 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)
131. 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, ..
133. 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.
134. CNP134/50
Mobile IP does not use a new packet
type for agent solicitation;
it uses the router solicitation packet
of ICMP.
Agent Solicitation
140. CNP140/50
The movement of the mobile host
is transparent to
the rest of the Internet.
Key Objective of MIP
141. 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.
142. 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
144. 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
145. 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)
146. 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…
147. 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
149. What is WAP?
A set of protocols which allow data exchange for
mobile cellular systems
It is device independent
It is network independent
150. 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:
151. 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 ?
152. 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.
155. 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)
156. 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
159. 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 :
160. 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.
161. 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
162. 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
163. 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
164. 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
165. 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.
166. 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
167. 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
168. 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.
169. 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