dtmf and kpml

7/27/2019 Dtmf and Kpml

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12/1/2008

Dual Tone Multi Frequencyand

Key Press Markup Language

VoIP Security (COMS W4995)

Prof. Henning Schulzrinne

Aniruddha Niranjan

Computer Science Department


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Overview

DTMF

Introduction

Architecture

Encoding

Transmission

Decoding Security

KPML

Introduction

Operation

Key Concepts

DRegex

Security

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DTMF

Method to instruct the telephone switching

system of number to be dialed

To issue commands to switching system

or related telephony systems

Numbers and commands issued from a

keypad

Used mostly in interactive response

systems

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Architecture

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Three modules

Generation at keypad

Transmission using RTP

Decoding

DecodingTransmissionGeneration


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Keypad Frequencies

Set of low

frequencies

Set of high

frequencies

Button click

generates acombination

8 => S(852, 1336)

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Payload format designated as telephone-

event

Media type is audio/telephone-event

Carried as part of audio stream

Considered a very highly compressed

audio codec

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Timestamp and marker bit of RTP

header used

Payload format

Number between 0

and 255 identifies

specific telephony

event

End of

event

Reserved,

set to 0

Duration of event or

segment

Power level

of the tone

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Decoding

Firstly checks are performed based on

Frequency groups

Inter-digit interval

Signal strength

Goertzel algorithm for decoding

Uses Discrete Fourier Transforms

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Security

Telephone event payload highly

compressed

Very high sensitivity to even small

changes in bit values

Secure Real Time Protocol must be

implemented

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KPML

Capturing DTMF tones that are

supplemental or mid-call key presses

SIP event notification package

SUBSCRIBE and NOTIFY methods used

Goal is to optimize memory usage and

processing time

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SIP Based OperationCaller CalleeApplication Server

(1) INVITE

(2) 200 OK

(3) ACK

(4) SUBSCRIBE

(5) 200 OK

(6) NOTIFY

(7) 200 OK

(9) NOTIFY (Digits)

(10) 200 OK

(8)

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Key Concepts

Subscription

Duration

PersistentNon-persistent

Single NOTIFY Continuous NOTIFY

Timers

Inter-Digit

Critical

Extra

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Key Concepts

Pattern matches

Digit suppression

User input buffer behavior

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DRegex

Digit Regular expression mapping of

POSIX extended regular expressions

POSIX not used directly

Mapping used

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DRegex

A few example expressions15


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DRegex Formal Syntax

Augmented Backus-Naur Form

DRegex = 1*( DRegexPosition [ RepeatCount ] )

DRegexPosition = DRegexSymbol / DRegexSet

DRegexSymbol = [ "L" ] DRegexCharacter

DRegexSet = "[" 1*DRegexSetList "]"

DRegexSetList = DRegexCharacter [ "-" DRegexCharacter ]DRegexCharacter = DIGIT / "A" / "B" / "C" / "D" / R / "*" / "#" /

"a" / "b" / "c" / "d" / r

RepeatCount = "." / "{" RepeatRange "}"

RepeatRange = Count / ( Count "," Count ) /

( Count "," ) / ( "," Count )

Count = 1*DIGIT

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KPML Security

Sensitive information may be typed in

through keypad

Unauthenticated subscriptions can create

problems too

Use of mechanisms such as TLS, sips,

S/MIME recommended

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Conclusion

DTMF maps keypad characters to a

combination of predefined frequencies

Transported as an RTP payload and

decoded using Goertzel algorithm

KPML enables transport of mid-call key

press information

A SIP-centric model that makes use of

DRegex

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References

http://nemesis.lonestar.org/reference/telec

om/signaling/dtmf.html

http://www.corp.att.com/history/milestone

_1963.html

http://ltl13.exp.sis.pitt.edu/WebSite/Gloss

ary/Letters/D.htm

RFC 4733; H. Schulzrinne, M.Taylor

RFC 4730; E. Burger, M. Dolly19

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