Code : STM#220

1. Distribution ED01 English Code : STM#220 IP Telephony System Error Handling & Management Samsung Electronics Co., Ltd.

2. Contents Overview Delay/Latency Jitter Echo Packet Loss Voice Compression

3. Overview • This book details various issues facing Voice over IP (VoIP) and explains how they can affect packet networks • The issues of IP Telephony • Delay/latency • Jitter • Echo • Packet Loss • Voice Compression

4. Delay/Latency

5. Delay/Latency • Definition • The amount of time it takes for speech to exit the speaker’s mouth and reach the listener’s ear • VoIP technologies impose a fundamental transmission delay due to packetization and the buffering of received packets before playout at the receiving endpoint • The Type of Delay • Handling delay • Processing delay • actual packetization, compression, packet switching • Queuing delay • Packets are held in a queue because more packet are sent out than the interface can handle at a given interval • Propagation delay • Speed of light in fiber or copper-based networks • Being almost Imperceptible to the human ear • Serialization delay

6. Sender Receiver PBX PBX Network First Bit Transmitted Last Bit Received A A Network Transit Delay t Processing Delay Processing Delay End-to-End Delay Delay/Latency

7. Delay/Latency • Effect of Delay on Voice Quality • 0 to 150 ms: Acceptable for most user applications • 50 to 300 ms: Acceptable provided that Administrations are aware of the transmission time impact on the transmission quality of user applications • above 300 ms: Unacceptable for general network planning purposes; however, it is recognized that in some exceptional cases this limit will be exceeded. • End to end transmission time Encode + Packet + Queuing + Transmission + Decode + Jitter Buffer

8. Jitter

9. Jitter • Jitter • The variation in the delay of received packets • Due to network congestion, improper queuing, or configuration errors, this steady stream can become lumpy, or the delay between each packet can vary instead of remaining constant. • Jitter can cause missing syllables or some parts of word

10. Sender Receiver Network B C A Sender Transmits t D3 = D2 A B C Sink Receives D1 D2 = D1 t Jitter

11. Jitter • Jitter Buffer • Conceals interarrival packet delay variation • The Jitter Buffer adds to the end-to-end Delay • The more jitter, the larger jitter buffer needs to be compensate for the unpredictable nature of the packet network

12. V V Sender Receiver IP Network RouterB RouterA 20ms 20ms B C A 50 10 30 20ms 20ms RTP Timestamp From Router A Interframe gap of 20ms A B C 30 50 10 20ms 80ms A C B RTP Timestamp From Router A Delitter Buffer removes Variation 10 30 50 RTP Timestamp From Router A Variable Interframe Gap (Jitter) Jitter • Jitter Buffer

13. Echo

14. Tx Sender’s voice Rx Voice Network Tx Rx Receiver’s voice sender Receiver Echo of sender’s voice Echo • What? • In a phone conversation, you hear your own voice repeated • The audible leak-through of your own voice into your own receive (return) path. • Cause • Echo is normally occurred by impedance mismatch • Two basic characteristics of echo • The louder the echo (echo amplitude), the more annoying it is • The longer the round-trip delay (the “later” the echo), the more annoying it is • Echos must be delayed by at least 25 ms to be audible • Echos arriving after very short delays(25 ms) are masked by the physical and electrical sidetone signal

15. Receiver sender FXO:FXS FXO:FXS PBX PBX GW GW WAN E&M E&M Analog (echo signal returns too quickly to be audible) Analog (Tail circuit) (good candidates for echo sources) Digital (long delay, >30 ms each direction) Echo • Locating an Echo • Leak-through happens only in the analog portion of the network • Analog signals can leak from one path to another, or acoustically trough the air from a loudspeaker to the a microphone • Voice traffic in the digital portions of the network dose not leak from one path into another • The analog signals that represent bits can tolerate a lot of distortion

16. Echo • Effects of Network Elements on Echo • Loudness • The loudness contributes to echo • Hybrid Transformers • Echo sources are points of signal leakage between analog transmit and receive paths • Hybrid transformers are often prime culprits for this signal leakage • Analog signals can be reflected in the hybrid transformer in the tail circuit • Ensure that output and input impedances are matched between the hybrid and the terminating device • Telephones • Extending the digital transmission segments closer to the actual telephone will decrease the potential for echo • Routers • Network delay increase user annoyance for an echo of equal strength • Adding router does not cause echo; it exacerbates existing echo problem

17. Packet Loss

18. Packet Loss • Packet Loss • Packet loss in data networks is both common and expected • When putting critical traffic on data networks, it is important to control the amount of packet loss in that network • ping • Ping plotter • chariot • When putting voice on data network, it is important to build a network that can successfully transport voice in a reliable and timely manner • The suggested rate of packet loss that is allowed for VoIP communication is 3% or less The Rate of Packet loss

19. Voice Compression

20. Voice Compression • Digitizing Voice: PCM Waveform Encoding • Nyquist Theorem: sample at twice the highest frequency • Voice frequency range: 300-3400 Hz • Sampling frequency = 8000/sec (every 125us) • Bit rate: (2 x 4 Khz) x 8 bits per sample = 64,000 bits per second (DS-0) • By far the most commonly used method PCM 64 Kbps = DS-0

21. Voice Compression • Voice Compression • Objective: reduce bandwidth consumption • Compression algorithms are optimized for voice • Drawbacks/tradeoffs • Quantization distortion • Tandem switching degradation • Delay (echo) • Voice coding standards

22. Voice Compression • Voice Compression Technologies Unacceptable Business Quality Toll Quality 64 * PCM (G.711) (Cellular) Bandwidth (Kbps) 32 * ADPCM 32 (G.726) 24 * ADPCM 24 (G.726) 16 * ADPCM 16 (G.726) * LDCELP 16 (G.728) 8 * CS-ACELP 8 (G.729) * LPC 4.8 0 Quality

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