Voice Quality (Special Topic) ISSUE1.0 Fixed Network Technical Support Dept.

HUAWE Introduction Please enumerate the voice quality faults during the deployment maintenance and describe briefly the troubleshooting.

HUAWE References Support website—engineers documentation Guide to NGN Voice, Fax and Modem Configuration B.doc

HUAWE NGN Bearer Network Grade and Definition of VoIP Quality Network GradeUnidirectional Delay (ms)Packet Loss RatioJitter (ms) Good (customized)0~400~0.1%0~10 Relatively poor*40~1000.1%~1%10~20 Bad*100~4001%~5%20~60 * Indicates that a grade is defined based on the communication industry standard YD/T Technical Requirements for IP Telephone Gateway Equipment GradeMOSUsers’ Satisfaction Superior Very good, able to be heard clearly, low delay, smooth communication. Good Slightly poor, able to be heard clearly, low delay, not smooth communication, some noise Medium Medium, unable to be heard clearly, with certain delay; communication is possible. Poor Poor, unable to be heard clearly, great delay, repeated communication. Bad0--1.5Bad, unable to be understood, great delay, not smooth communication.  Definition of bearer network quality grade  Definition of VoIP Voice Quality According to the communication industry standard YD/T Technical Requirements for IP Telephone Gateway Equipment

HUAWE Service Performance in Different Network Conditions Service Type Good Network Service Performance Poor Network Service Performance Bad Network Service Performance Modem Transparent transmission AvailableUnavailable Fax Transparent transmission AvailableUnavailable T.38Available Unavailable Voice G.711 a/uSuperiorGoodMedium G.729 a/bGood Poor G.723GoodClose to goodMedium Video384KNo deterioration, availableSlight deterioration, available Apparent deterioration, unavailable Two-stage dialing RFC2833 Available, good voice quality or better Available, voice quality close to good Available, as poor voice quality as medium MessageAvailable The red part with oblique lines as the background is the service performance unable to satisfy operation requirements

HUAWE Network delay: Delay means that the less time difference for end-to-end packet transmission, the better. Unit: ms. Generally, if delay exceeds 100ms, we will feel in our conversation that the peer does not speak naturally and reacts slowly; if delay exceeds 250ms, we will feel unbearable in our conversation. Delay results from the following: 1)End-to-end voice delay = encoding delay+compression & packetization delay+network transmission delay+ unpacking & decompression delay +decoding delay. 2) Buffer (jitterbuf) set to eliminate network jitter. Key Factor Which Affects Voice Quality—Network Delay

HUAWE Network jitter: Jitter refers to delay offset. Unit: ms. Network jitter refers to change of IP packet transmission time. Jitter indicates how steady packet transmission is. The transmission mode of IP network is packet-based store-and-forward and packets are transmitted at a fixed rate. Besides, voice is sensitive to delay and those voice packets with great jitter will generally be discarded. In this case, jitter may bring about voice distortion. Network jitter is normally distributed. The jitter value we generally call is the root-mean-square value of a normal distribution curve and the maximum actual jitter is much greater than the root- mean-square value. Therefore, if the network jitter reported via RTCP is 60ms, voice quality will not be improved until jitter buffer is increased to 120ms and good voice quality will not be achieved until jitter buffer is increased to 160~180ms. Key Factor Which Affects Voice Quality—Network Jitter

HUAWE Network packet loss ratio: Packet loss ratio refers to the ratio of lost packets to all packets and is represented with x% When any packet loss occurs to network, the lost-packet compensation algorithm will be employed to avoid any influence brought about by packet loss. According to the correlation between previous and subsequent voice information, lost frames are reconstructed during the decoding to ensure acoustic effects of received voice; it is stipulated in ITU—T that the maximum compensation time of PLC should be 60ms. If packet loss exceeds the compensation range of a device, noise, voice distortion or call drop will occur. Key Factor Which Affects Voice Quality—Network Packet Loss

HUAWE Network bandwidth: It is an index used to measure network flow. If different encoding/decoding methods are used, bandwidth occupied will be different. If there is heavy call traffic, but insufficient network bandwidth in the system, delay and jitter will occur and the packet loss index will sharply decrease. This will have a big, even severe, influence upon voice quality. Key Factor Which Affects Voice Quality—Network Bandwidth

HUAWE CODEC: One of the key technologies of VoIP service is the voice coding & compression technology. The encoding/decoding algorithm and compression technology will have a direct influence upon voice quality of VoIP service (voice encoding & compression is implemented in a gateway. In this process, digital coding is first needed and converted into PCM codes. Then, a special DSP chip will be used for data compression. Finally, IP packet data are formed so as to adapt to transmission bandwidth over IP network. Relationship between encoding/decoding rate and voice quality: the higher rate, the better voice quality. Key Factor Which Affects Voice Quality—Selected CODEC

HUAWE EC: Echo indicates that a speaker’s voice goes through network equipment and loops back to himself. Features of echo: echo affects the perception quality of the speaker instead of that of the receiver; If IP is interconnected with PSTN and either of both communication parties uses a two-wire telephone, which involves a 2-/4-wire conversion circuit with coils, echo will be heard; echo can be perceived by people in two conditions: strength: some signals should be fed back (leakage, sound wave reflection) and there should be certain amplitude; delay: there should be enough delay. ITU--T G.131 and ITU--T G.161 point out that when the echo delay exceeds 25ms, echo can be perceived by people. Therefore, an Echo Controller (EC) should be equipped. Generally based on the DSP technology, the EC employs an Finite Impact Response (FIR) filter to simulate an echo path and generate echo signals, so as to subtract actual echo signals and offset echo. Key Factor Which Affects Voice Quality--EC

HUAWE Countermeasures for network bandwidth: 1. Give consideration to equipment capacity in network design, select proper encoding/decoding and as great bandwidth as possible at the egress. 2. When there are both voice service and data service on an IAD, the IAD may be configured with the QoS strategies of voice service to ensure voice service bandwidth. 3. Adopt Voice Activity Detection and Comfort Noise Generation on the device to reduce bandwidth occupancy. Countermeasure for delay: 1. The bearer network itself reduces network delay. Countermeasure for jitter: 1. The bearer network itself reduces network jitter; 2. Set the jitterbuf on the device Countermeasure for packet loss: 1. Employ the bearer network to reduce packet loss; 2. Equip the device with the lost-packet compensation algorithm. Handling of the Above-mentioned Key Factors

HUAWE Questions What factors will affect voice quality of NGN? Give examples of how to check the quality of a bearer network.

HUAWE Summary Different kinds of service (voice, modem, and fax) present different requirements for network. In handling a corresponding problem, we should know clearly about the requirements of various kinds of service for network quality.