CSIS 6251 CSIS 625 Week 13 Presentation Layer, Application Layer, Wireless, cell, etc. Copyright 2001 - Dan Oelke For use by students of CSIS 625 for purposes.

CSIS 625 1

CSIS 625 Week 13

Presentation Layer,

Application Layer,

Wireless, cell, etc.Copyright 2001 - Dan Oelke

For use by students of CSIS 625 for purposes of this class only.

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Overview• Presentation Layer topics

– ASN.1 & BER– Network Management– Encryption– Authentication

• Cell Phone – What is a cell– Analog vs. Digital– TDMA, CDMA, etc.

• Fixed Wireless– LMDS, MMDS

• Optical Wireless• Final Exam

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ASN.1 & BER

• ASN.1 - Abstract Syntax Notation One– A formal language for describing messages that go

between computer systems.• BER - Basic Encoding Rules

– The method by which messages using ASN.1 are arranged into bits for transmission.

• Many systems today use ASN.1 with BER to define their message structure.

• BER typically uses a header for each field that defines what it is, the length, and then the data

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ASCII - The anti-ASN.1 system

• Many protocols on the Internet today use ASCII based encoding mechanisms– HTTP, SMTP, FTP control, etc.

• ASN.1 encoded messages can not be decoded by just looking at them on a terminal, while ASCII based messages can

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Network Management

• SNMP - Simple Network Management– Uses ASN.1 encoded messages to get/put values in a

table type structure– messages are sent over UDP/IP – Requests are only simple set’s and gets.– More complex operations can take significant work– Simplicity allows for very simple (and cheap) devices

to implement this protocol. (cheap Ethernet switches for example)

– Everything is a table in SNMP• Can be a limitation for more complex devices – requires

multiple tables that reference one another• Makes life simpler for the devices implementing SNMP

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CMISE - Common Management Information Service Elements

• Uses an object oriented view of the system• Many layers of protocols• A very rich filtering and selection system.• Promoted and standardized mostly through

Bellcore/Telcordia – Driven by phone companies desire to have a common

management system for everything

• Set of objects is “standardized” but every vendor has their own extensions so the management system must adopt to these extensions.

• Mostly dead system • So bloated it takes seconds to do a single query• Requires many megabytes of RAM on managed systems.

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Encryption

• Encryption is a method by which information is modified so that others can not understand it.– Scrambling of the data

– aka Cryptography

• Stenography is a method by which information is hidden from others.– Hiding of the data using “noise”

– Least significant bits in pictures or audio

• Encryption has 2 major branches– Asymmetric Encryption (Public Key)

– Symmetric Encryption

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Encryption - the players

• Instead of using A sends a message to B, cryptography books have taken to using some relatively standard names for the nodes communicating

• Alice, Bob, Carol, Dave - participants in an communication

• Eve - the eavesdropper - listens in on communication, but doesn’t alter the communication.

• Mallory - a malicious active attacker• Peggy - a prover• Victor - a verifier

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Symmetric Key Encryption

• Both the sender and the receiver know some common secret.

• The secret is the key to decoding the message• The secrecy of the key is important• Transporting and securing the key between the

Alice and Bob is difficult, because it must be done through a secure mechanism.

• One time pad - the key is as big as the message. The message is xor’d with the key.– The only truly unbreakable encryption system.– Most products that claim one-time-pad are not.

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Symmetric Key encryption types

• DES - Data Encryption Standard– Uses a 56 bit key

– All 256 keys can be tested in < 24 hours with a $250k machine

• 3DES - Use of DES three times over – Gives 3x56 or 168 bits of keyspace

• AES - Advanced Encryption Standard– Rijndael is the new chosen standard– 128-256 bit key

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Asymmetric Encryption

• Commonly called Public Key encryption• Two numbers (secrets) are created. • One of these keys is called the public key and

given to everyone.• One of these keys is called the private key and is

kept secret.• To send a message, the public key is used to

encrypt the data. After that, only someone with the private key can decode the message.

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Public Key Encryption types

• RSA - Ron Rivest, Adi Shamir, and Leonard Adleman.– An algorithm that picks two large prime numbers and multiplies

them. It is assumed that it is very very difficult to factor the resulting number.

– The bigger the numbers the harder it is to break the encryption

• Elliptic Curve

• Many practical systems use public key encryption to encrypt a symmetric key that is then used to encrypt the rest of the message– Public Key encryption tends to use compute expensive algorithms.

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Key Size

• Comparing key size between different algorithms is not easily done.

• A typical 128bit symmetric key encryption method might take as long to break as a 1024 bit asymmetric key encryption.

• Don’t get into “my key is bigger” battles.

• What is important is how strong the overall system is.– Key size is one factor

– Algorithm choice is another

– Use of proven algorithms is best

– Implementation is often the biggest problem

– Beware – a lot of people are selling snake oil.

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Authentication

• Authentication is the process of making sure that someone is who they say they are.

• It is also the process of making sure that a message has been transported without being modified.– Much more than error detection– Mallory might intercept the message, change it and

change the CRC

• Secure hash– An algorithm that creates a big hashed number and it is

very difficult to produce another message with the same number.

– SHA-1, MD5, RIPEMD

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Authentication & Encryption

• Authentication often uses many of the same public key encryption algorithms as encryption.

• Message tampering detection– Create a secure hash, and then encrypt using the private

key.

– Anyone can then decrypt the hash using a public key and compare the result with their own copy of the public key.

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Cell Phone Technology

• Based on the idea of offering mobile phone service

• Reference:– Much of this material taken from:– http://www.ee.washington.edu/class/498/sp98/final/marsha/final.html

– http://www.iec.org/online/tutorials/cell_comm/

– http://www.iec.org/online/tutorials/tdma/

• There were early radio to telephone type systems, but cellular technology didn’t start until the early 1980’s.– Limited number of channels in a large area

– 1981the FCC approved the use of a larger number channels.

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Cell phone overview

• Based on the idea of using radio frequencies in small areas called cells.– 806-890 MHz and 1850-1990 MHz

• Cells are often laid out in a honeycomb type of topology.– Frequencies can then be re-used in non-adjacent cells.

– A cell is 1-2 miles in radius in urban areas

– up to 20 miles in radius in rural area.

– Micro-Cell and Pico-Cell are used in very small very high density areas

• Such as inside a sports stadium

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Cellular system components

• PSTN - Public Switched Telephone System– The “normal” phone system

• MTSO - Mobile Telephone Switch Office– The switch that controls all of the cell sites

– This switch coordinates handoffs between cells

• Mobile Base Station - The cell site– The antenna, radios, interface equipment, etc.

• MSU - Mobile Subscriber Units– The cell phone you carry around.

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Cell phone - Analog vs. Digital

• Analog technology uses FDM with FM analog modulation– call uses a fixed frequency in its cell for the

duration

– Older scheme - more coverage in united states than digital technology

– Any scanner can listen in on these conversations• Newt Gingrich knows this well• Congress made it illegal to listen in on these

conversations, but it is very difficult to enforce unless you come out with a tape

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Cell phone - Analog vs. Digital

• Digital technology – Uses PCM to get a digital stream

– Then uses various audio compression techniques to get voice down to rates as low as 8kbps.

• Compression may reduce the quality of the voice

– New technology with good coverage in high population density areas.

– Allows many more calls for a given bandwidth

– May use “silence suppression” to further increase capacity• Silence suppression - not sending data when there is no-one talking

• Effectively doubles the amount of data that can be sent.

• Need to have comfort noise added

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Cell - Modulation techniques

• FDMA - Frequency Division Multiple Access– used for analog phones

– The same thing as Frequency Division Multiplexing

• TDMA - Time Division Multiple Access– Divides a frequency into multiple timeslots

– Each call uses one timeslot in one frequency

– Increases the number of calls that may be present

– Used with digital technology

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Cell - Modulation techniques (cont.)

• CDMA - Code Division Multiple Access– Use of entire time and frequency

– Each call has minimal interference with one another.

– Not a hard limit on the number of calls possible • Each call just adds a little more interference

• GSM - Global System for Mobile Communications– TDMA based system used in Europe.

– Voice compressed to 13kbps

• PCS - Personal Communications Services– use of 1900 MHz frequencies

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Cell handoff

• As a phone moves from one cell to another, the call must be handed off with minimal interruption.– When signal strength decreases sufficiently, base station

notifies switch– The switch queries the other base stations to determine

which has the strongest signal from the phone.– The switch then notifies the new base station to take over

the call– Normally less voice is dropped for only 10-100ms

• CDMA can use a “soft” handoff with no interruption – The use of the entire frequency spectrum means that there

is no other channel to use

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LMDS - Local Multipoint Distribution System

• A high speed wireless access technology using bands in the 28-31Ghz frequency spectrum.

• Speeds in the 100’s of Mbps depending on the amount of spectrum given to a given connection.

• Is a point-to-multipoint technology– One central site

– Many subscriber sites

• Started in 1998 when FCC auctioned off the spectrum for LMDS– Must own a license to use a set of frequencies.

• Or pay the current license holder for the privilege of using it

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LMDS - continued

• May use TDMA or FDMA or combination• Encoding techniques of PSK, QPSK, 8PSK, 4-

QAM, 16-QAM, or 64-QAM• Limited Rage

– 3-5 Mile radius

– Caused by extremely high frequencies and need to have strong signal to noise ratio for higher QAM methods.

• Reference:– http://www.iec.org/online/tutorials/lmds/

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MMDS - Microwave Multipoint Distribution System

• Also called Multi-channel Multi-point Distribution System

• Fixed wireless technology• Uses 33 channels of 6MHz each (2.5 - 2.7Ghz)• Originally set up for poor man’s cable, now used

to send digital data• Sprint is the major license holder in the US

– Have offered Internet access up to 1Mbps using this technology

• Up to 35mile radius

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Optical Wireless

• Also called Free Space Optics

• Use of lasers sent through air instead of fiber optics cable.

• Can be point to point or point to multipoint systems

• Speeds from 155Mbps up to (claimed) 10Gbps

• Issues to deal with– Movement of buildings

– Pigeons and other flying objects

– Fog and Rain

– Perceived Safety issues

• Limited Range - 2-4km

• Reference:– http://www.lightreading.com/document.asp?doc_id=1207

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OFDM

• Orthogonal Frequency Division Multiplexing

• A technique used by 802.11a and some other wireless systems to send a large amount of data over radio waves.

• Divides the bandwidth into several frequencies.

• The bandwidth of each frequency has a few bits modulated into it using QPSK or QAM.

• When all the bits from all the frequencies are added together it can achieve a very high bit rate.

• Often some of the bits are redundant so that some type of FEC can be employed.

• More info:– http://www.sss-mag.com/ofdm.html

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Final Exam

• Non Comprehensive– All material since the Mid-term

• Similar format to the Mid-term– Some vocabulary

– Some short answer

• Open book, Open note• 2 hour time limit