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 NetworkingA technology that enables two or more entities to

communicate without wiresAny type of computer network that is wirelessFor 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.

http://www.rococosoft.com/products/images/image_1.gif

infrastructure network

ad-hoc network

APAP

AP

wired network

AP: Access Point

Source: Schiller

Wireless LANs: CharacteristicsTypes

Infrastructure basedAd-hoc

AdvantagesFlexible deploymentMinimal wiring difficultiesMore robust against disasters (earthquake etc)Historic buildings, conferences, trade shows,…

DisadvantagesLow bandwidth compared to wired networks (1-

10 Mbit/s)Proprietary solutionsNeed to follow wireless spectrum regulations

Components/ArchitectureStation (STA) - Mobile nodeAccess 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

10Wireless

802.11 Scope & Modules

11

MAC Sublayer

MAC LayerManagement

PLCP Sublayer

PMD Sublayer

PHY LayerManagement

LLC

MAC

PHY

To develop a MAC and PHY spec for wirelessconnectivity for fixed, portable and moving stationsin 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 characteristicsE.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 formatPLCP preamble

SFD: start frame delimiterPLCP header

PLCP (802.11b)

14

longpreamble

192us

shortpreamble

96us(VoIP, video)

MAC managementSynchronization - 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/sleepingEnables switching from DCF to PCFEnables frequency hopping in FHSS PHY

Transmitter and receiver has identical dwell interval at each center frequency

Achieving TSFAll 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 BSSApplicable in infrastructure mode ONLY

Distributed TSF (for Independent BSS) more difficult

Power management

18

Battery powered devices require power efficiencyLAN 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 periodsYet maintain an active session – tradeoff power Vs throughput

Achieving power conservationAllow idle stations to go to sleep periodicallyAPs buffer packets for sleeping stationsAP 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 APWhen channel quality with current AP is poor

Scanning function used to find better APPassive Scanning Listen for beacon from different ApsActive 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 wirelessEasy entry into the networkJamming, selfish behavior, spatial overhearing

Securing the network harder than wired networksEspecially in distributed environments

WEP symmetric 40 or 128-bit encryptionWPA: Wi-Fi protected access

Temporal key integrity protocol (TKIP) – betterUser 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: MotivationCan 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 networkssignal strength decreases proportional to the

square of the distancesender would apply CS and CD, but the

collisions happen at the receiversender may not “hear” the collision, i.e., CD

does not workCS 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 committeeThe 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 terminalsA 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!“virtual carrier sensing”: Sender “asks”

receiver whether it can hear something. If so, behave as if channel busy.

BA C

802.11 MAC (DCF)

29

CSMA/CA based protocolListen before you talkCA = Collision avoidance (prevention is better than

cure !!)

Robust for interference Explicit acknowledgment requested from receiver

for unicast framesOnly CSMA/CA for Broadcast frames

Optional RTS/CTS offers Virtual Carrier SensingRTS/CTS includes duration of immediate dialogAddresses hidden terminal problems

802.11 MAC (DCF)

30

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

Ksilenced

silenced

silenced

silencedData

ACK

802.11 Steps

33

Receiver replies with CTSAlso contains (DATA + ACK) duration.Neighbors update NAV again

Tx sends DATA, Rx acknowledges with ACKAfter ACK, everyone initiates remaining countdownTx chooses new R = rand (0, CW_min)

If RTS or DATA collides (i.e., no CTS/ACK returns)Indicates collisionRTS 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)

34

But is that enough?

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 DData

A

E

CTS

RTSF


37

But what if barriers/obstructions ??E doesn’t hear C Carrier sensing does not

help

CB DData

A

EF

CTS

RTS

Exposed Terminal

38

B should be able to transmit to ARTS prevents this

CA B

E

D

CTSRTS

Exposed Terminal

39

B should be able to transmit to ACarrier sensing makes the situation worse

CA B

E

D

CTSRTS

Thoughts !

40

802.11 does not solve HT/ET completelyOnly alleviates the problem through RTS/CTS

and recommends larger CS zone

Large CS zone aggravates exposed terminals Spatial reuse reduces A tradeoffRTS/CTS packets also consume bandwidthMoreover, 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/CTSUseful? 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/CTSInfratructured wireless networks (EWLAN)

Hybrid Channel Access

42

The optimization timeline

T RRTS

CTS

Data

ACK

RTS

CTS

Data

ACK

T RRTS

CTS

Data

RTS

CTS +ACK

Data

T RRTS

CTS

Data

Poll +ACK

Data

RTS

CTS +ACK

Bac

kof

f

Bac

kof

fB

ack

off

Bac

kof

f

Poll +ACK

Data

Bac

kof

fB

ack

off

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 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

56

BlueTooth

Bluetooth - overview

57

Motivation

In 1994 the L.M. Ericsson company wanted to connect mobile phones to other deviceswithout using cables.


58

Special Interest Group (SIG)L.M. EricssonIBMIntelNokiaToshiba


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 scopeFrom : 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 802.15V 1.0 is a complete specification from

application layer to physical layer802.15 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 node1 to 7 active slave nodes0 to 255 parked nodes


65

Parked nodea slave device in a low power state to

conserve the drain on the devices batteriesIn this state the device can only respond to

the beacon from the master node


66

Node rangeSlave nodes need to be within 10 meters of

the master node.Why design such a short range?


67

CommunicationOnly possible between master and slave

nodesPiconet 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

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

Bluetooth – protocol stack architectureTanenbaum, 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 implementation71

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


Link Managerlogical channels between devicespower managementauthenticationquality of service

73

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 transmissionsomewhat analogous to 802 LLC sub layer

74

BlueTooth – protocol stack architecture Tanenbaum, Andrew S, Computer Networks 4th Ed. figure 4-37

Middleware layerRFcomm : serial communications, mouse,

keyboard …Telephony : speech oriented protocolService Discovery : locate services in the

network

75

Bluetooth – protocol stack architecture

Audio controls audio, applications have direct

accessControl

a control protocol, applications have direct access

76


The top layer is for application and profiles.each application need only use the necessary

subset of the protocol stack to accomplish it’s task

77

More Bluetooth protocol layers

78

A. Radio LayerB. Base band LayerC. 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 layerIt is the physical layer of the BluetoothEach frame is transmitted over a logical

channel called a linked between master slave

Two kinds of linksAsynchronous Connection less (ASL)Synchronous Connection Oriented (SCO)

Logical Link Control and Adaptation Protocol (L2CAP)

81

There are 3 major functionsI. 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 thatwas 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 willbecome 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 hoppingA 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

TCP Issues in Wireless

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.

SolutionAll other TCP connections experiencing the

congestion slows down the transmission and the congestion is soon resolved.

Slow startTCP 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 RetransmitThe 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 networksfading, shadowinghand 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 hostaccess 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 migrationand state transfer

Indirect TCP IIAdvantages

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

Disadvantagesloss 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 TCPIndirect TCP 2 TCP sessions.Snooping TCPOne 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 correspondenthostforeign

agent

mobilehost

snooping of ACKs

Transparent extension of TCP within the foreign agent

buffering of packets sent to the mobile hostlost 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 semanticsNo change to correspondent nodeNo major state transfer during handover

ProblemsSnooping TCP does not isolate the wireless link wellMay need change to MH to handle NACKsSnooping 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 hostno caching, no retransmissionmonitors 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

Disadvantagesloss on wireless link propagated into fixed networkadapted 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 retransmitas 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

Advantagesimple 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 advanceMAC can inform TCP layer of upcoming loss of connectionTCP stops sending, but does not assume a congested link MAC layer signals again if reconnected

Advantagescheme 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 retransmissionTCP 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 solutionRFC2018 allows for acknowledgements of single packets,

not only acknowledgements of in-sequence packet streams without gaps

sender can now retransmit only the missing packetsAdvantage: much higher efficiencyDisadvantage

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, respectivelythus, even short messages need a minimum of 7 packets!

Transaction oriented TCPRFC1644, T-TCP, describes a TCP version to avoid this

overheadconnection setup, data transfer and connection release can

be combinedthus, only 2 or 3 packets are needed

Advantageefficiency

Disadvantagerequires changed TCPmobility no longer transparent

Revision of TCP in wirelessTCP in wired: problem & solutionIn wireless: Problem & solutionIndirectSnoopingMobileFast retransmission/fast recoveryTransmission/time-out freezingSelective RetransmissionTransaction oriented TCP

Mobile IP

Outline Intro to mobile IPOperationProblems with mobility

Portable Networking Technology

CS 640114

Cellular systemsCellular Digital Packet Data (CDPD)3GLTE (3.9G)*

BluetoothLow cost, short range radio links between

mobile devicesWireless Ethernet (802.11)

Widely used wireless MAC layer technology

Mobility and Standard IP Routing

CS 640115

IP assumes end hosts are in fixed physical locationsWhat happens if we move a host between networks?

IP addresses enable IP routing algorithms to get packets to the correct networkEach IP address has network part and host part

This keeps host specific information out of routersDHCP is used to get packets to end hosts in networks

This still assumes a fixed end hostWhat 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 usersEnables hosts to stay connected to the Internet

regardless of their locationEnables hosts to be tracked without needing to

change their IP addressRequires no changes to software of non-mobile

hosts/routersRequires addition of some infrastructureHas no geographical limitationsRequires no modifications to IP addresses or IP

address formatSupports 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 InternetDetects 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 locationSince this IP doesn’t change it can be used by long-lived

applications as MN’s location changesHome Agent (HA)

This is router with additional functionalityLocated on home network of MNDoes mobility binding of MN’s IP with its COAForwards 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 functionalityIf MN is away from HA the it uses an FA to send/receive data

to/from HAAdvertises itself periodicallyForward’s MN’s registration requestDecapsulates messages for delivery to MN

Care-of-address (COA)Address which identifies MN’s current locationSent by FA to HA when MN attachesUsually 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): HostCN (Correspondent Node): HostHA (Home Agent): RouterFA (Foreign Agent): Router

MIPv4 AddressHoA (Home Address): MNCoA (Care-of-Address): FA

Mobile IP Support Services (Steps)

CS 640120

Agent DiscoveryHA’s and FA’s broadcast their presence on each network to

which they are attachedBeacon messages via ICMP Router Discovery Protocol (IRDP)

MN’s listen for advertisement and then initiate registrationRegistration

When MN is away, it registers its COA with its HATypically through the FA with strongest signal

Registration control messages are sent via UDP to well known port

Encapsulation – just like standard IP only with COADecapsulation – again, just like standard IP

Mobile IP Operation

CS 640121

A MN listens for agent advertisement and then initiates registrationIf 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 completeRegistration happens as often as MN changes networks

HA intercepts all packets destined for MNThis is simple unless sending application is on or near the same

network as the MNHA masquerades as MNThere is a specific lifetime for service before a MN must re-registerThere 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 TableMaintained on HA of MNMaps MN’s home address

with its current COA

Visitor ListMaintained on FA serving an

MNMaps 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 FAIP 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 DiscoveryMN FA (CoA)ICMP Agent Solicitation & Advertisement

Registration to HA (via FA)MN FA HAOver UDP (destination port 434)

Data Tunneling CN => HA (HoA) => FA (CoA) => MNIP-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

CNP139/50

Data transfer

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 partiesOnly authentication between MN and HA is

requiredKeyed MD5 is the default

Replay protectionTimestamps are mandatoryRandom numbers on request reply packets

are optionalHA and FA do not have to share any

security information.

Problems with Mobile IP

CS 640142

Suboptimal “triangle” routingWhat 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 MNThen the CN can create its own tunnel to MNCN must be equipped with software to enable it to learn the

COAInitiated 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 fragilePossible solution – have multiple HA

Frequent reports to HA if MN is movingPossible solution – support of FA clustering

SecurityConnection hijacking, snooping…

Many open research questions

Mobile IP and its Variants

CNP145/50

Mobile IPv4 (MIPv4)MIPv4Low-Latency Handover for MIPv4 (FMIPv4)Regional Registration for MIPv4 (HMIPv4)

Mobile IPv6 (MIPv6)MIPv6Fast 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 nodesForeign Agents are not needed in Mobile IPv6

MNs can function in any location without the services of any special router in that location

SecurityNodes are expected to employ strong

authentication and encryptionOther details…

Mobile IPv6 (MIPv6)

CNP147/50

MIPv6 = MIPv4 + IPv6Major Differences from MIPv4

FA in MN No FA for MIPv6

CoA: IP address of MNBy DHCPv6 or IPv6 Stateless Auto-Configuration

Route Optimization To solve the “Triangular Routing” ProblemProvided by defaultMN 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

CGIScriptsetc.

WML D

ecks

with

WML-Script

WAP Gateway

WML Encoder

WMLScriptCompiler

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 DevelopersAn 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

WirelessNetwork

WAPProxy

HTMLFilter

WTAServer

WAPClient

WML

WMLHTML

WWW Protocol Stack

TCP/IPUDP/IP

TLS - SSL

HTTP

HTMLJava Script

Components of WAP Architecture

Other ServicesAnd 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 providesWMLWML scriptWTAContent 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 servicesConnection-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 requestsReliable one-way requestsReliable 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 integrityPrivacyAuthenticationDenial-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

WAEUser 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 ArchitectureThe 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.

HTMLJavaScript

HTTP

TLS - SSL

TCP/IPUDP/IP

Wireless Application Protocol

Wireless ApplicationEnvironment (WAE)

Session Layer (WSP)

Security Layer (WTLS)

Transport Layer (WDP)

Other Services andApplications

Transaction Layer (WTP)

SMS USSD CSD IS-136 CDMA CDPD PDC-P Etc..

Bearers:

Comparison between Internet and WAP Models

Internet

WAP Architecture

Thank you

Engineering

Mobile Computing (Part-2)