A Study on Quality of Service A Study on Quality of Service Issues in Internet TelephonyIssues in Internet Telephony

IP Telephony – Applications and ServicesIP Telephony – Applications and ServicesAdvantages and benefits of Voice over IPAdvantages and benefits of Voice over IPTechnical Challenges – QoS issues Technical Challenges – QoS issues Proposed SolutionsProposed Solutions

VOIP- Applications and ServicesVOIP- Applications and Services

Integration of Data, Voice and FaxIntegration of Data, Voice and Fax Sound GradingSound Grading Unified messagingUnified messaging Video telephonyVideo telephony Web-based call centersWeb-based call centers Low-cost voice callsLow-cost voice calls Remote teleworkingRemote teleworking

Advantages and BenefitsAdvantages and Benefits

Benefits put in three categories asBenefits put in three categories as Cost ReductionCost Reduction SimplificationSimplification ConsolidationConsolidation

– Efficient usage of already existing network resourcesEfficient usage of already existing network resources– Reduced number of access linksReduced number of access links– No per-minute distance sensitive charges.No per-minute distance sensitive charges.– No bandwidth limitation.No bandwidth limitation.

Technical Challenges- QoS IssuesTechnical Challenges- QoS Issues

Packet LossPacket Loss Packet DelayPacket Delay Network JitterNetwork Jitter

Packet LossPacket Loss

Packet loss in IP networks affect time Packet loss in IP networks affect time sensitivity of voice transmission.sensitivity of voice transmission.

Possible solutionsPossible solutions Noise SubstitutionNoise Substitution Packet repetitionPacket repetition Packet InterpolationPacket Interpolation Frame interleavingFrame interleaving Network upgradeNetwork upgrade Forward Error correctionForward Error correction

Packet DelayPacket Delay

- Codec delayCodec delay- Encoding delay ( frame processing delay + lookahead delay)Encoding delay ( frame processing delay + lookahead delay)- Decoding delay is half the encoding delayDecoding delay is half the encoding delay- Higher compression achieved at the price of longer delaysHigher compression achieved at the price of longer delays

- Serialization delaySerialization delay- Longer frames result in higher delay in transmitting the packetLonger frames result in higher delay in transmitting the packet- Higher speed lines reduce serialization delaysHigher speed lines reduce serialization delays

- Queuing delayQueuing delay- Occurs at the switching and transmission points of the networkOccurs at the switching and transmission points of the network- Can be reduced using mechanisms such as differentiated services and Can be reduced using mechanisms such as differentiated services and

Resource Reservation Protocol ( RSVP)Resource Reservation Protocol ( RSVP)

- Other sources of delayOther sources of delay- Delays caused by modems in dial up networks, delays due to inefficient Delays caused by modems in dial up networks, delays due to inefficient

operating systems and sound card delaysoperating systems and sound card delays- Can be avoided by using digital lines and using gateway cards with Can be avoided by using digital lines and using gateway cards with

specialized Digital signal processorsspecialized Digital signal processors

Delays Encountered in IP Telephony

Network JitterNetwork Jitter

Variance in the inter-frame arrival time at the receiver is Variance in the inter-frame arrival time at the receiver is called jittercalled jitter

Jitter occurs due to variability of queuing delays in the Jitter occurs due to variability of queuing delays in the networknetwork

Can be reduced by using Jitter buffersCan be reduced by using Jitter buffers– To allow for variable packet arrival times and still achieve steady stream of To allow for variable packet arrival times and still achieve steady stream of

packets, the receiver holds the first packet in a jitter buffer , before playing packets, the receiver holds the first packet in a jitter buffer , before playing it out.it out.

Selection of Jitter buffer is crucial to IP telephony systems.Selection of Jitter buffer is crucial to IP telephony systems. Cisco, Hypercom and Netrix offer intelligent buffers that Cisco, Hypercom and Netrix offer intelligent buffers that

adjust automatically according to network availability.adjust automatically according to network availability.

Network Support for QoSNetwork Support for QoS

Providing controlled networking environmentProviding controlled networking environment

Using management tools to configure network nodes , monitor Using management tools to configure network nodes , monitor performance and manage capacity and flow on a dynamic basisperformance and manage capacity and flow on a dynamic basis

– Traffic prioritized by protocol, location and application typeTraffic prioritized by protocol, location and application type– Queuing mechanisms manipulated to reduce delaysQueuing mechanisms manipulated to reduce delays

Adding control protocols such as RTP, RTCP, RSVP to provide greater Adding control protocols such as RTP, RTCP, RSVP to provide greater assurance of controlled QoS within the networkassurance of controlled QoS within the network

Other Networking tools to provide QoS includeOther Networking tools to provide QoS include– Congestion Management ( Weighted fair queuing)Congestion Management ( Weighted fair queuing)– Qos Signaling (IP precedence and RSVP)Qos Signaling (IP precedence and RSVP)– Packet ResidencyPacket Residency– RTP header compressionRTP header compression– Generic traffic shapingGeneric traffic shaping– Weighted Random Early detectionWeighted Random Early detection

Existing Service models and Existing Service models and mechanismsmechanisms

Two keymodels: Intserv and Diffserv. THE INTEGRATED SERVICE MODEL

– Guaranteed service for applications requiring a fixed delay bound– Controlled-load service for application requiring reliable and

enhanced best-effort service THE IETF DIFFERENTIATED SERVICES FRAMEWORK

– The first approach specifies the QoS in deterministically or statistically quantitative terms of throughput, delay, jitter, and/or loss. Such approach is called quantitative Diffserv.

– The second approach specifies the services in terms of some relative priority of access to network resources and is called prioritybased Diffserv.

Existing Solutions The CISCO Solution : Enterprise IP Telephony LUCENT Gateway Solution for Service Provider networks

The Cisco data and IP Telephony The Cisco data and IP Telephony Network ArchitectureNetwork Architecture

Lucent IP and PSTN ArchitectureLucent IP and PSTN Architecture

ReferencesReferences

1. G. A. Thom, “H.323: The Multimedia Communications Standard for 1. G. A. Thom, “H.323: The Multimedia Communications Standard for Local Area Networks,” IEEE Commun. Mag., Dec. 1996.Local Area Networks,” IEEE Commun. Mag., Dec. 1996.

2. ITU Rec. H.323, “Visual Telephone Systems and Equipment for Local 2. ITU Rec. H.323, “Visual Telephone Systems and Equipment for Local Area Networks which Provide a Non-Guaranteed Quality of Service,” Area Networks which Provide a Non-Guaranteed Quality of Service,” Nov. 1996.Nov. 1996.

3. Samir Mohamed, Francisco Cervantes-pérez, Hossam Afifi, "Integrating 3. Samir Mohamed, Francisco Cervantes-pérez, Hossam Afifi, "Integrating networks measurements and speech quality subjective scores for networks measurements and speech quality subjective scores for control purposes", IEEE INFOCOM 2001 - The Conference on Computer control purposes", IEEE INFOCOM 2001 - The Conference on Computer Communications, no. 1, April 2001 pp. 641-649 Communications, no. 1, April 2001 pp. 641-649

4. Goodman, O. Lockart, and W. Wong, “Waveform Substitution 4. Goodman, O. Lockart, and W. Wong, “Waveform Substitution Techniques for Recovering Missing Speech Segments in Packet Techniques for Recovering Missing Speech Segments in Packet Voice Communications,” IEEE Trans. Acoustics, Speech and Sig. Voice Communications,” IEEE Trans. Acoustics, Speech and Sig. Processing, Dec.1986, vol. ASSP-34, no.Processing, Dec.1986, vol. ASSP-34, no. 6, pp. 1440–48.6, pp. 1440–48.

5. IEEE Communication Society Library. 5. IEEE Communication Society Library. http://dl.comsoc.org/cocoon/comsoc/servlets/GetPublication;jsessionid=http://dl.comsoc.org/cocoon/comsoc/servlets/GetPublication;jsessionid=01C893DE28A4EBF38E5B9DFFBD461325?id=1214901C893DE28A4EBF38E5B9DFFBD461325?id=12149