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. • 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).
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
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
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
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
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
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
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
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
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
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.
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
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
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
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