Introduction to VoIP

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Introduction to VoIP

• 3UET
• Session 3

2
Introduction

• Lesson objectives
• Explain difference between Circuit Switched and
  Packet Switched
• Explain advantages of VoIP
• Define what are Codecs

3
Analogue Systems

• Traditional broadcast media such as radio,
  television, and PSTN use analogue technology,
• Represented as a series of varying sine waves.
• The term analogue can be traced to the similarity
  between the actual ?uctuations of the human voice
  and the analogous, or comparable, modulation of
  a carrier wave.

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Introduction to VoIP

• Analogue - electromagnetic signals that
  continuously vary in their strength and speed.
• e.g. voice

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Analogue Systems

• The human voice occupies the 20Hz to 20KHz range,
  with most energy in the 3003300Hz range

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Basic Telephone System Operation

• Telephone systems use analogue-switched lines to
  provide voice communications by converting sound
  waves, vibrations that move in the air, into
  electrical signals.
• When a person speaks into a telephone handset,
  acoustical energy vibrations caused by the
  persons voice apply varying amounts of pressure
  to the diaphragm.

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Basic Telephone System Operation

• In response to the natural rise and fall of human
  speech, the diaphragm in turn converts this
  pressure into different amounts of current or
  electrical energy.
• This variation in the current is in effect an
  electrical representation of the human voice
• The resulting output of this process is an
  analogue electrical signal

8
PSTN

• The job of the Public Switched Telephone Network
  (PSTN) is to reliably facilitate telephone
  conversations at anytime
• PSTN combines analogue, digital, and
  electromechanical data links to make sure every
  time you pick up a phone receiver you hear a
  dial-tone

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PSTN

• The PSTN is a switched network
• In switched networks links between endpoints
  dont need to be permanent because they arent
  needed constantly
• The only time a link between two endpoints is
  needed is when a call is in progress between
  them.
• The rest of the time is idle
• Switching is a method whereby links are
  established and removed as needed.

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SS7

• Signaling System 7 (SS7) is the PSTNs signaling
  component
• A second network that runs alongside the PSTN
• Its purpose is coordinating communication
  between switches, telephone company databases and
  billing systems
• Toll-free calling and long distance call routing
  are both functions of SS7

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Circuit Switched vs Packet Switched

• Circuit Switching
• Circuit switching used by telephone networks for
  more than 100 years
• Connection is made for the duration of the call
• When the dialled party answers the call, a
  circuit is established connecting the two phones
  together via a series of switches
• Circuit is dedicated to that call for the
  duration
• Until 1960 or so, every call a dedicated wire
  stretching from one end of the call to the other
  for the duration of the call

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Circuit Switched vs Packet Switched

• Today your voice is digitized and combined with
  others onto a single fiber optic cable for much
  of the journey
• There's still a dedicated piece of copper wire
  going into your house
• An exchange/switch with 50 circuits can only
  handle 50 calls at any one time
• Limits capacity
• Expensive

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Circuit Switched vs Packet Switched

• Think about...
• Voice is transmitted at a fixed rate of 64Kbps in
  each direction
• Translates to 16 KB each second (960 KB every
  minute)
• 10-minute conversation, the total transmission is
  9,600 KB (roughly 10 megabytes)
• In a typical phone conversation, much of this
  data is waste

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Circuit Switched vs Packet Switched

• Silence
• While you are talking, the other party is
  listening, generally means that only half of the
  connection is in use at any given time
• Dead Air where neither party talks
• If we could remove the silence, we transmit less
  data
• Instead of sending a continuous stream of bytes
  (both silent and noisy), what if we sent the
  bits?
• Hence we arrive at packet switching, i.e.
  non-dedicated resources

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Circuit Switched vs Packet Switched

• Packet Switching
• On the internet a dedicated connection is not
  maintained between client and server
• Data networks send/receive as required
• The sending computer chops data into small chunks
  (known as packets) and adds an address to each
  one telling the network devices where to send
  them

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Circuit Switched vs Packet Switched

• A router picks it up and moves it a little closer
  to its destination
• Another router does the same and so forth until
  it reaches its destination
• Instructions contained within the packets are
  used to reassemble the data into its original
  state

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Circuit Switched vs Packet Switched

• Individual packets in a stream may travel
  entirely different routes from A to B
• A brief connection is opened just long enough to
  send a small chunk of data
• Very efficient - the network routes the packets
  along the least congested and cheapest lines

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Introduction to VoIP

• Introduction to VoIP
• VoIP - Voice over Internet Protocol
• Any technology that transmits a conversation over
  a data network

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Introduction to VoIP

• Analogue Signals
• Digitised, chopped up into small pieces
  (packets)
• Transmitted across a data network (e.g. the
  internet)

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Introduction to VoIP

• Free phone calls?
• Some VoIP providers offer free calls between
  customers or other VoIP providers
• Not quite free - Cost of internet connection,
  possibly higher bandwidth than otherwise required
• Cheaper Phone Calls?
• NodePhone (13/5/07)
• Free to other NodePhone customers
• 18c untimed calls to fixed lines anywhere in
  Australia
• From 5c/min International calls
• Engin - Engin National Plan (13/5/07)
• Free Engin to Engin
• Free National Calls (fixed lines)
• International calls from 3.5c/min

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Introduction to VoIP

• Activity
• Look up some other VoIP providers
• What plans do they offer?
• Lookup http//www.whirlpool.net.au gt discussion
  forum gt Voice over IP gt Voice over IP (VoIP)
  Information Here
• Look at the VoIP Providers

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Introduction to VoIP

• Advantages of VoIP
• Multiple Lines on a Physical Link - More than one
  call on a single broadband connection
• Location Independence - Incoming calls find your
  phone regardless of where you are connected to
  the network
• Staff can work from anywhere with a sufficiently
  fast and stable Internet connection
• Many VoIP packages include features that most
  telcos charge extra for e.g. 3-way calling, call
  forwarding, automatic redial, and caller ID.
• Integrate with other services - e.g. CRM

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Introduction to VoIP

• VoIP Challenges
• Routing VoIP traffic through firewalls and
  address translators
• Network Traffic Issues - Delay/Network Latency,
  Packet loss, Jitter, Echo
• The principal cause of packet loss is congestion,
  which can be controlled by congestion management
  and avoidance
• Variation in delay is called jitter
• Security
• Majority of consumer VoIP devices do not support
  encryption yet
• Easy to eavesdrop on VoIP calls and change their
  content
• Sniffing tools commonly available
• Skype uses encryption which is transparent to the
  user

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Introduction to VoIP

• Reliability
• Power supplied to analogue phones via phone
  company exchange
• Back-up generators or batteries located at the
  telephone exchange
• VoIP hardware powered by household electricity
• PSTN has been matured over decades and extremely
  reliable
• Broadband networks - even the best are still
  subject to intermittent outages
• SLAs on ADSL

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Introduction to VoIP

• Computer Hardware
• Phone system is dependant on individual PCs of
  varying specifications and power. A call can be
  affected by processor drain
• Difficulty Sending faxes/modem calls
• Sending faxes over VoIP difficult
• Voice codecs are not designed for fax transmission

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Introduction to VoIP

• Emergency calls
• Unable to locate network geographically
• Fixed line phones - fixed location
• Mobile phones - triangulate based on cell towers
• Calls cannot easily be routed to a nearby call
  center
• 000 calls impossible on some VoIP systems
• In the event that the caller is unable to give an
  address, emergency services may be unable to
  locate them in any other way
• One large VoIP carrier requires the registration
  of the physical address where the VoIP line will
  be used
• Some also maintain their own emergency call
  center that will take emergency calls

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Introduction to VoIP

• VoIP Phones
• Several types of phone
• ATA (Analog Telephone Adapter)
• Connects a standard phone for use with VoIP
• Analog-to-digital converter
• Some providers bundling ATAs free with their
  service
• Ordinary Phone ---- ATA ---- Ethernet ---- Router
  ---- Internet ---- VOIP Service Provider

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Introduction to VoIP
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Introduction to VoIP

• VoIP Phones
• IP Phones
• Look just like normal phones
• IP phones have an Ethernet connector
• Wired or WiFi
• Onboard software handles IP calls
• Also known as Hardphone
• IP Phone ----- Ethernet ----- Router ----
  Internet ---- VOIP Service Provider

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Introduction to VoIP

• VoIP Phones
• Softphones
• Phone implemented in software running on a PC or
  PDA
• Use PC speakers/microphone or headset
• e.g. Skype, X-Lite, SJPhone

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Codecs

• Codecs
• Short for coder/decoder
• Any technology for compressing and decompressing
  data
• Can be implemented in software, hardware, or both
  in combination
• Audio Codecs - WAV, MP3, AAC, WMA

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Codecs

• Discussion
• Have you ever compressed files with zip or rar?
• What happens?
• Compression - Smaller file size
• When you decompress the file do you get your
  original document back?
• Yes - lossless compression
• Do some files compress better than others?
• Yes - internal redundancy

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Codecs

• Discussion
• Have you ever created a jpeg image using a tool
  that allows you to set the quality?
• What happens to the image at lower quality
  settings?
• Smaller file size, becomes blurry (information
  loss)
• Have you ever ripped CDs?
• Do you use MP3, AAC, WMA, or Ogg?
• Why?
• Some are proprietary formats, limited playback
  options
• Ogg is open but not well supported
• Differences in quality on the resultant output
• What happens when ripping CDs?
• Translation from one codec to another
• Compression

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Codecs

• Does your ripping program let you change the bit
  rate?
• What effect does that have?
• File size, perceived quality
• Does music ripped in AAC or WMA sound
  better/worse than MP3?
• Yes, different psycoacoustic modelling
• If you burn MP3s to an Audio CD and rip it again
  does it sound the same?
• No, data loss every time there's a lossy encode
• When authoring audio/video use lossless formats
  for masters, lossy for final distribution

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Codecs

• Compression/Encoding
• Different codecs use different methods to encode
  data
• Generally incompatible with each other
• Different codecs are used for different tasks, a
  video codec can't be used for Audio
• All things being equal, different codecs will
  deliver differences in audio/video quality

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Codecs

• Bit rate
• Information per unit of time
• Usually expressed as bits per second
• Higher numbers more data
• Bit rate is often an indicator of sound/video
  quality, however different codecs will achieve
  different perceived quality at different bit
  rates
• e.g. 64Kbps WMA would sound similar to 128Kbps
  MP3
• Higher bit rates - more information, more effort
  to decode, more disk space required
• Some devices will require lower bit rates, e.g.
  hand held's

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Codecs

• Loss
• Two main approaches to compression
• Lossless
• Compresses data, and the decompressed version is
  identical to the original
• Often used for documents, program files and other
  data where similar to the original won't do
• Lossless algorithms are usually generic, can
  compress anything
• Often less space saved
• e.g. zip, rar

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Codecs

• Lossy
• If we accept there's data in the original file
  that we don't need, we can throw it away
• e.g. sounds we don't hear, shades of colours
  don't see
• Terms like
• psychoacoustic (the way the brain interprets
  sound) and
• psycovisual used to describe techniques which
  determine what we can see/hear and removing
  everything else
• e.g. shout and tap fingers at the same time, will
  hear shouting but not finger tapping

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Codecs

• Video - 20 frames per second often little change
  between frames
• Locating and cutting out unchanged segments
  called statistical data redundancy
• Slightly corrupt video files and digital TV often
  pixelate in areas with movement
• Every time you save your file in a lossy file
  format, it discards more of the data
• Move to a lossy format only as the very final
  step in your project.

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Codecs

• Quality Factors for VoIP
• Bandwidth - More bandwidth, higher
  bitratesbetter sound quality
• Codec - Different codecs have different audio
  characteristics
• Hardware - Better hardware in general results in
  higher quality sound
• Network Factors - Better networking better
  sound

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VoIP Codecs

• ULAW - Best sound for big connections
• ALAW - Supposedly good for Fax.
• G726.1 (g726) - Compromise
• ILBC - Often a default for Softphones (Xten etc).
• G729 - Quite lossy, good for low bandwidth
  connections
• GSM - Slightly better quality then g729
• Recommended Speeds
• 1.5/256 or Higher ULAW or ALAW
• 512/128 g726.1 or ULAW (If your connection is
  of good quality)
• 256/64 ILBC, GSM or G729
• Note If you are on a higher speed you can use
  the lower speed codecs. This will allow you to
  make more simultaneous calls.

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• The following table shows bandwidth requirements
  for many common codecs.Codec.................Band
  width Usage (Up/Down)G.711 (64 Kbps).......87.2
  KbpsG.729 (8 Kbps)........31.2 KbpsG.723.1 (6.3
  Kbps)....21.9 KbpsG.723.1 (5.3 Kbps)....20.8
  KbpsG.726 (32 Kbps).......55.2 KbpsG.726 (24
  Kbps).......47.2 KbpsG.728 (16 Kbps).......31.5
  KbpsGSM (7 or kbps).......lowILBC
  (low)............lowMore Information
  compare.ozvoip.com/codecs.php

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VoIP Codecs

• Review
• http//www.ozvoip.com/frequently-asked-questions/
  codec