voice over ip friday science seminar 02/16/07 southern oregon university
TRANSCRIPT
Voice Over IP
Friday Science Seminar
02/16/07
Southern Oregon University
Objectives
A technical overview of the devices and protocols that enable Voice over IP (VoIP)
Demo Packet8 and Skype
Discuss network administrator concerns
Who Am I?
Priscilla Oppenheimer
SOU Adjunct Faculty in Computer Science
27 years experience developing data communications and networking systems
MS in Information Science from the University of Michigan
What Is VoIP?
Voice over the Internet Protocol (IP)
Sending voice (telephone) conversations over the Internet Or over any internetwork or
intranet that uses IP
Also known as IP telephony
Why Is This Cool?
Saves you money!
You can choose your area code
Mobility
Voice mail to email notification
Lots of other features
Many VoIP Implementations
1. Enterprises are merging their voice and data networks
2. Vonage, Packet 8, BroadVoice, and others offer VoIP to broadband home users
3. Software applications such as Skype and Gizmo implement peer-to-peer VoIP
Enterprise VoIP
Source: Cisco Systems
VoIP at Home with Phone Adapter
Source: BroadVoice
Peer-to-Peer VoIP
Skype
Gizmo
Software that runs on PCs, Macs, and Unix
The caller logs into an authentication server, finds target, and then the voice is sent directly to the callee
Our Telephony Legacy
On-Hook to Off-Hook
Source: Cisco Systems
The weight of the receiver opens a spring-loaded switch hook inside the phone thereby disconnecting the idle phone from the telephone line.
Lifting the receiver closes the switch hook and allows current to flow which causes the phone switch to send the dial tone signal.
Dialing and Switching
Source: Cisco Systems
Ringing
Source: Cisco Systems
Talking
Source: Cisco Systems
VoIP Emulates Our Legacy System
Handles: On-hook Off-hook Dial tone Dialing Switching Ringing Voice
Legacy: from Latin legatus1. A gift by will especially of money or
other personal property2. Something transmitted by or received
from an ancestor or predecessor
VoIP with Analog Phones
Router 1 Configurationvoice-port 1/0/0
ring-frequency 30
ring cadence pattern01
!
dial-peer voice 1 pots
destination-pattern 5551234
port 1/0/0
!
dial-peer voice 10 voip
destination-pattern 5555678
session target ipv4:10.5.6.7
codec g711ulaw
Source: Cisco Systems
Digital IP Phones Carry Legacy into the Future!
Have an IP address
Have Ethernet interface(s)
Have an analog-to-digital converter
Have sex appeal!
VoIP Functionality Realms
Digitizing and packetizing voice
SignalingOff-hook, dial tone, dialed digits, ring,
ringback Call setup and teardown
Public Switched Telephone Network (PSTN) integration
VoIP Protocol Suites
Sending digitized voice Realtime Transport Protocol (RTP)
Signaling Session Initiation Protocol (SIP) H.323
PSTN Integration Media Gateway Control Protocol (MGCP) Electronic Number Mapping System (ENUM)
Digitizing an Analog Signal
Source: Forouzan, "Data Communications and Networking"
Digitizing Voice: Nyquist Theorem
The sampling rate must be at least two times the highest frequency contained in the analog signal The highest frequency for voice is 4000 Hz Sample 8000 times per second Store in an 8-bit byte 64,000 bps bandwidth required
Quantizing
Mapping a continuum of possible amplitudes into a finite number of discrete values
Can be uniform or non-uniform
Non-uniform (picture on right) uses smaller step functions at lower amplitudes
Encoding
Wave form coders Non-uniform approximation of wave form G.711 = ITU-T standard for non-uniform
representation of 64 Kbps Pulse Code Modulation (PCM)
Predictive algorithms (encode differences between samples)
Vocoders (synthesize voice)
Conjugate Structure Code-Excited Linear Prediction
Uses codebook and feedback to learn and predict voice wave form
ITU-T G.729 best-known exampleWorks on 10-millisecond audioGenerates an 80-bit payloadUses 8 Kbps bandwidthSupports silence suppression
Packetizing the Digitized Voice
Place two 80-bit payloads in one RTP/UTP/IP packet
Realtime Transport Protocol (RTP)
Developed by the IETF (RFC 1889)
Carries realtime audio and video
Runs above UDP/IP
Adds sequence number and timestamp
Uses a UDP even port number The RTP Control Protocol (RTCP) uses the next
higher odd port number Ports 16384-32767 are used
Session Initiation Protocol (SIP)
Developed by the IETF (RFC 3261)
Most common VoIP signaling protocol on the Internet
Call setup, teardown, ring, ringback, etc.
A SIP address is similar to an email address sip:[email protected]
SIP Protocol Behavior
Can use TCP or UDP port 5060Other ports seen in the wild
Request/response protocol with ASCII text messagesREGISTER, INVITE, BYE, etc.Similar to HTTPShares some of HTTP's status codes
200 OK 404 Not Found
SIP Invite
Media Gateway Control Protocol (MGCP)
Developed by the IETF (RFC 3435)
Protocol and architecture
Source: NCTT
ENUM
Developed by the IETF (RFC 3761)
Maps E.164 telephone number to Domain Name System (DNS) name
Retrieves an NAPTR record stored in a DNS database
$ORIGIN 2.1.2.1.5.5.5.0.7.7.1.e164.arpa.
IN NAPTR 100 10 "u" "sip+E2U" "!^.*$!sip:[email protected]!i" .
IN NAPTR 102 10 "u" "smtp+E2U" "!^.*$!mailto:[email protected]!i" .
VoIP Network Design
Voice is sensitive toDelay and jitterDropped packets
Use TestYourVoIP from Brix Networks to test your network performance
Test Your VoIP with G.711
From AFN to Boston From Boston to AFN
Test Your VoIP Signaling Delay
From AFN to Boston From Boston to AFN
Test Your VoIP with G.729
From AFN to Boston From Boston to AFN
VoIP Concerns
Local number portability (LNP)
Power outages
9-1-1 Does your VoIP provider know where you
are?
Legal (and illegal?) "wire tapping" by law enforcement
More VoIP Concerns
Security Confidentiality Integrity Availability Authentication
Rerouting calls through a service that is less expensive Is Phreaking back?
And More VoIP Concerns
VoIP may not work with firewalls
Some VoIP applications don't "play by the rules" and cause concerns for security and privacy experts Skype behavior is hard to distinguish from a
hacker's attack Skype also causes some nodes to be "super
nodes" without their knowledge
VoIP Summary
Can be implemented many ways
Emulates our legacy telephone system
Uses RTP for voice transport
Uses SIP for call setup and teardown
It's cool and can save you money!
Time to SIP some beers at Standing Stone?