Voice Over IP (VoIP)

Mayoor SavlaVitaliy Zavesov

04/19/23 2

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)

Sender

Receiver

Encoder

Packetizer

Talk Silence

talkspurt

silenceNetwork

Depacketizer &playout buffer

Decoder &Concealment

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