VoIP over Internet Protocol
Voice
VoIP
• What is VoIP?
• Types of VoIP services?
• How does it Work?
• Benefits & Limitations?
VoIP=
Voice over Internet Protocol
• Voice communication transmitted over
a digital/computer network
• Voice transmitted using the same
protocols used on the Internet
POTS, PSTN & VoIP
- Analog vs. Digital
VoIP is an evolution of the old reliable
plain old telephone service (POTS)
• Switch board operators
• Mechanical switches
• Digital switches (70s & 80s)
• IP telephony
VoIP vs. PSTN
PSTN – Public Switched Telephone Network
(the worldwide telephone network)
4 Elements of the PSTN
1. Customer Premises Equipment
2. Access System
3. Transport Core
4. Signaling
PSTN – 4 Elements
Fig. 6-1 (Panko 2004)
VoIP vs. PSTN
• VoIP - A method for converting analog
audio signals to digital data to be
transmitted using IP
• VoIP does not rely on the PSTN
infrastructure but will interface with it
Who uses VoIP?
• CARRIER NETWORKS used by PSTN to
improve bandwidth efficiency
• PRIVATE NETWORKS used by large
corporation to bypass PSTN (reduce cost)
• PUBLIC INTERNET used by consumer
and small business markets
Why? – usually because of cost
Circuit Switching vs. Packet Switching
PSTN
based on circuit switching
VoIP
uses packet switching
Circuit Switching - PSTN
2-way connection with reserved capacity
• 2 systems wanting to communicate
establish a circuit before they transmit any
information
• That circuit remains open for the duration
of the exchange
Circuit Switching - PSTN
Works well for voice transmission
• Voice traffic fairly constant (someone talking
most of the time)
Not good for data transmission
• Data traffic has short high speed bursts of
data separated by long silences
• Reserved capacity of circuit wasteful
Packet Switching
No dedicated end-to-end connection
• Breaks voice conversation into pieces
• Transmits the pieces (pieces find their own
way through the network)
• Reassembles the pieces back into voice
conversation
Fig. 2-1 (Doherty 2006)
http://www.pbs.org/opb/nerds2.0.1/geek_glossary/packet_switching_flash.html
How does VoIP work?
Signaling (how communication is handled between switches
and phones)
VoIP uses a TERMINAL ADAPTER
• connects home handset to broadband Internet Acts as a
translator (converts analog to digital)
Terminal Adapter to SOFTSWITCH
• digital messages received by broadband phone service
provider’s softswitch
• Softswitch is a specialized database/mapping program. The
Softswitch knows where endpoints are on the network, what
number is associate with that endpoint, and the current IP
address assigned to that endpoint. - routes calls
Fig. 2-4 (Doherty 2006)
Fig. 2-3 (Doherty 2006)
Types of VoIP Service
• ATA – analog telephone adapter
connects a standard phone to computer/Internet
connection for use with VoIP
• IP PHONES – has technology needed to encode
voice for digital transfer and to send and receive
packets over IP (Built-in NIC, RJ-45 adapter) (also WiFi wireless
Internet phones)
• COMPUTER-to-COMPUTER – software
(softphone), microphone, speakers and Internet
connection. Free or low-cost
VoIP conversation
• Human voice = analog sound
• Digital telephony
• Human voice converted to digital stream by a transmitter
(re-created on receiving end)
• Analog-to-Digital conversion accomplished by SAMPLING
• SAMPLING
• Analog sound wave measured thousands of times per second
• Voltage levels converted to digital bytes
Voice Packetization
Fig. 2-6 (Doherty 2006)
UDP & RTP – for IP telephony transport
VoIP uses IP (based on IP address not phone number)
• IP is not a reliable protocol (no error correction)
• Not feasible to correct errors by retransmitting
data (this delay would cause jitter)
• If packet lost – receiver replays previous packet’s
sound or interpolates based on data of earlier and
later packets
UDP – User Datagram Protocol
RTP – Real Time Protocol (contains sequence
#’s and time stamps get things in order)
Other protocols used…
With VoIP – many different types of hardware and
software need to communicate
(any combination of analog, softphone or IP phone, ATAs (to
handle digital to analog conversion), and softswitches (to map calls) ).
Several protocols currently used
(for signaling (supervision))
• H.323 (complex – designed for video conferencing)
• SIP – Session Initiation Protocol (more streamlined – designed
specifically for VoIP)
• MGCP – Media Gateway Control Protocol (focused on
endpoint control (call waiting,etc))
Benefits of VoIP
Efficiency
Cost
Flexibility
• VoIP available anywhere there is an
Internet connection
Limitations of VoIP
Limitations to 911 services (ex: WiFi Internet
phone not compatible with E911 – routed instead to
emergency call center)
Power with VoIP phones
• Terminal adapter provides power to your
handset
• Terminal adapters plugged into electrical
outlet
Services