VoIP

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VoIP

• Voice Transmission Over Data Network

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What is VoIP?

• A method for
• Taking analog audio signals
• Turning audio signals into digital data
• Digital data can be transmitted over the Internet
  

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More Definition

• A technology used to
• transmit voice conversations
• over a data network
• using IP

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Brief Explanation of Voice Transmission

• Audio is digitizing by using sampling circuits
• Digital audio is compressed
• Therefore, voice transmission with less bandwith

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Types of Voice Transmission

• Through data network without IP
• TDM, FR or ATM
• Directly transmission
• VoDSL, VoTDM, VoFR or VoATM
• Network dependency

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Types of Voice Transmission

• IP Packets
• VoIP
• Independent of data network

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Types of VoIP

• IP to IP (Computer to computer)
• Typically by using server-client based programs
• Connect to same server

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Types of VoIP

• IP to PSTN (Computer to Phone)
• Need media gateway
• Compatible with circuit switching and packet
  switching

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Types of VoIP

• PSTN to PSTN (Computer to Phone)
• Through internet
• For long distance voice transmission
• Decrease bandwidth cost

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Circuit Switching

• The path is decided before data transmission
• Route is dedicated and exclusive

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Packet Switching

• IP breaks data into packet
• Data load and IP address of source and
  destination
• Reassemble packets
• No predetermined path, decide own path
• Find its way using the info

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Comparision

• PSTN
• Old and expensive
• Circuit dedication
• More reliable (Dedicated for only one session)

• VoIP
• More modern
• No circuit dedication
• Using the same path with other people at the same
  time
• Cost is shared
• Delay and packet loss

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Codecs

• Stands for coder-decoder
• Coder converts audio signal into a compressed
  digital form for transmission
• Decoder returns back into an uncompressed audio
  signal for replay

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Codecs

• Sample
• Sort
• Compress
• Packetize audio data

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Soft Switches

• Translate phone numbers to IP addresses
• Find the current IP address of requested number
• Called mapping process handled by central call
  processor running a softswitch

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Soft Switches

• A specialized database/mapping program
• Connects endpoints
• Include
• Where the endpoint is on the network
• What phone number is associated with that
  endpoint
• The current IP address assigned to that endpoint

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Protocols

• To communicate efficiently between completely
  different pieces of software and hardware

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H.323

• Comprehensive and complex protocol
• Originally designed for video conferencing
• Provide specifications for
• Real time applications
• Interactive videoconferencing
• Data sharing
• Audio applications such as VoIP
• Not specific to VoIP

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SIP(Session Initiation Protocol)

• Smaller, flexible and more efficient
• Specifically developed for initiating, building
  and terminating multimedia applications
• More suitable for VoIP

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Factors that Affect Call Quality in VoIP

• Packet Loss
• For real time applications, requesting packet
  again is impossible.
• Limited packet loss is omitted.

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Factors that Affect Call Quality in VoIP

• Latency
• Duration is related to connection type between
  source and destination
• To minimize, priority for VoIP packets.
• Coders also results in latency.

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Factors that Affect Call Quality in VoIP

• Jitter
• Variation in delay times
• Connection state and density vary.
• Packet delay time change
• To prevent, use RTP protocol

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RTP( Real-time Transport Protocol)

• Calculate average delay time of packet
• Postpone packets that comes early
• Try to equalize delay time

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VoIP Packet Headers

• For real time applications, no tolerance to error
  check and retransmission of loss packet mechanism
  UDP
• UDP header info lt TCP header info
• However, no packet order info on UDP
• Again, RTP

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RTP

• RTP add its own header to packets
• RTP includes the order number of packets

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VoIP Packet

• Coded audio (data) 20 to 150 byte
• IP Header (20 bytes)
• UDP Header (8 bytes)
• RTP Header (12 bytes)
• UDPRTPltTCP

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Advantages of VoIP

• Low cost for operators
• Low or no price for customers
• Flexibility
• Efficiency
• Less bandwidth

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Disadvantages of VoIP

• Reliability
• Latency, jitter, packet loss
• Dependent on wall power
• No integration to other systems like digital
  video recorders, digital tv home security systems
• Attacked by worms and viruses, hacking
• No universal and specific VoIP standards