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Voice Over IP (VoIP). Mayoor Savla Vitaliy Zavesov. What is VoIP?. VoIP is a term used in IP telephony to describe a set of facilities for managing the delivery of voice information using the Internet Protocol.

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Voice Over IP (VoIP)

Mayoor Savla

Vitaliy Zavesov


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What is VoIP?

  • VoIP is a term used in IP telephony to describe a set of facilities for managing the delivery of voice information using the Internet Protocol.

    • This means sending voice information in digital form in discrete packets rather than in the circuit committed protocols of the Public Switched Telephone Network (PSTN).


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Components of a VoIP System (1)

  • Speech is an analog signal that is converted to a digital signal at the sender using encoding schemes such as PCM.

    • Signal alternates between talkspurts and silence periods

    • CELP based encoders provider rate reduction

  • Encoded Speech is packetized into packets of equal size


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Components of the VoIP System (2)

  • Packets are sent over an IP network using a UDP Protocol

    • TCP is usually too heavy for voice applications

  • A playout buffer is used to smooth playout at the receiver

  • Content of received voice packets is delivered to the decoder which reconstructs the speech signal

    • May implement various packet loss concealment techniques to replace lost packets


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Technical Advantages of VoIP

  • With circuit-switched technology, capacity is allocated for the length of the call, regardless if voice is being transported at any time. VoIP technology uses bandwidth more efficiently

  • VoIP is perceived to be open and flexible, allowing providers to take advantage of equipment and technology at a higher level of productivity and cost savings

  • Offer customers exciting new phone features

    • Unified Messaging

    • Personal Portals

    • Caller ID on TV set

    • Point, Click and call personal directories

    • Talking email

  • Need a single line to talk on the phone and surf the Internet at the same time


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Business Advantages of VoIP

  • Cost Reduction: There can be a real savings in long distance telephone costs which is extremely important to most companies – especially those with International markets

  • Regionalize functions and equipment associated with delivering phone service – and spread costs across multiple markets

  • Simplification: Integrated Voice/Data Network allows more standardization and reduces total equipment needs.

    • Telecom providers can look to leverage their experience and infrastructure (i.e., existing nationwide backbone network)

  • Consolidation: Consolidation of accounting systems and combining operations leads to efficiency

  • Expand phone services into new markets (developing nations – Asia, Latin America)

    • No existing telephone/cable network and Costs are too high

    • VoIP Over Satellite - Use of VSATs


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Quality of Voice Issues(1)

  • Transmission of voice packets over a network is subject to packet loss due to network elements - causing degradation in voice quality at the receiver

    • Additional loss is incurred in the playout buffer at the receiver caused by network delay jitter

  • Interactivity between the communicating parties is affected by the delays incurred in the network

    • Large delay may lead to collisions whereby participants can talk in turns

    • Should be maintained below a certain maximum – NTE 150ms – possibly shorter for conversations with stringent interactivity delays

    • No control over how the packets are routed to reach their destination


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Quality of Voice Issues (2)

  • Voice Encoding affects the Quality of Speech

  • Presence of echo - a major source of quality degradation in voice communication

    • Reflection of signals at the four to two wire hybrids (combination of VoIP segment and a circuit segment)

    • PC-based phones – microphone at remote end picks up the voice played on the loud-speakers and echoes it back to the speaker


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Packet Loss

  • Loss Concealment Techniques

    • Insert Silence, Noise or a previously received packet

    • Interpolate, regenerate based on structure of codec and exploit decoder state

  • <5 consecutive packets

    • Increase in background noise as long as percentage of speech loss remains relatively low

    • Use of loss concealment techniques to mitigate packet loss

  • > ~20 consecutive packets

    • Cannot be concealed due to loss of intelligibility

    • Improve Network Reliability and decrease network configuration time when failures occur


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Packet Delay

  • Delay variations (Jitter)

    • Use of a playout buffer at the receiver to achieve a smooth playback of speech

    • Fixed Scheduling of packet playback – constant end-to end delay on all packets.

      • packets exceeding target delay are dropped

    • Adaptive Scheduling of packet playback – delay constant within a talkspurt but varies from one talkspurt to another.

  • Schemes are ineffective as it is impossible to have an apriori determination of variation in delay

    • Pattern of packet loss

    • Magnitude of delay variations

    • Rate at which variations take place


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Present Day Commercial Deployment

  • Presently used in Intranets to support full-duplex, real-time voice communications since they have more predictable bandwidth available than public network

  • Corporations limit their Internet voice traffic to half-duplex asynchronous applications such as voice messaging

  • Enterprise positions a VoIP device at a gateway


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VoIP Gateways

  • A gateway converts telephone conversation into the correct format as data packets to enable it to travel across a data network.

  • Gateways can be used with standard phone and fax equipment, connected to it through a PBX (Private Branch Exchange - private telephone switchboard)

  • Gateways contain such devices as signal translators, protocol translators, fault isolators, and other devices needed to implement VoIP communication.

  • Current gateway implementations include cable, DSL, wireless, and satellite (VSAT) gateways.


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Drawbacks of Current Internet Telephony Solutions

  • Voice Transmission are treated the same as data transmissions and providers have little control over the quality of the transmissions once they hit the public Internet

  • Internet Telephony does not offer emergency 911, operator services or QoS guarantees

  • Lack of standardized protocols imply that Internet Telephony products do not interoperate with each other or with PSTN


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Potential Future Markets for VoIP

  • Equipment developers and manufacturers see a window of opportunity to innovate and compete. They are busy developing new VoIP-enabled equipment attempting to break into the market in time.

    • 3Com NBX Solutions

    • Cisco Unity Bridge

    • Avaya ECLIPSE product suite

    • SysMaster VoiceMaster products

    • Alloptic GEAR family of products

  • Internet service providers see the possibility of competing with PSTN for customers

  • Users are interested in the integration of voice and data applications in addition to cost savings


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Issues for VoIP to be commercialized

  • Technology is not fully developed to the point where it can replace the services and quality provided by PSTN

  • Must be clear that VoIP is indeed cost-effective.

    • Protect its investment in circuit switched telecom operations since VoIP would be complementary to its existing technology

    • Significant costs to setup networks and other pieces of transport architecture

    • There must be significantly lower total cost of operation compared to today’s PSTN

  • Service Providers are awaiting the development of the remaining pieces of technology that will ensue quality transport in the last mile

    • Connection from homes and businesses to the IP back-bone


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References

  • Assessing the Quality of Voice Communications over Internet Backbones by A. Markopoulou, F. Tobagi, M. Karam

  • Is the Internet ready for VoIP by F. Tobagi, A. Markopoulou, M. Karam

  • Assessment of VoIP Service Availability in the Current Internet by W. Jiang and H. Schulzrinne

  • Whitepaper: Preparing for the Promise of Voice-over Internet Protocol (VoIP) – Cox Communications

  • http://www.nwfusion.com/research/voip.html


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