Curso de Telefonía IP para el MTC. Sesión 2 Requerimientos principales. Mg. Antonio Ocampo Zúñiga

Transcription

1 Curso de Telefonía IP para el MTC Sesión 2 Requerimientos principales Mg. Antonio Ocampo Zúñiga

2 Factors Affecting Audio Clarity Fidelity: Audio accuracy or quality Echo: Usually due to impedance mismatch Jitter: Variation in the arrival of voice packets Delay: Time it takes for the signal to propagate from one end to the other end of the conversation

3 Factors Affecting Audio Clarity Packet loss: Loss of packets on the network Side tone: Allows speakers to hear their own voice Background noise: Low-volume noise heard at the far end of the conversation

4 Jitter in IP Networks Steady Stream of Packets Time Same Packet Stream After Congestion or Improper Queuing

5 Sources of Delay Router 64 kb/s E1 Packet Flow E1 64 kb/s Router Fixed Coder Delay Fixed: Serialization Delay Fixed: Switch Delay Fixed: Switch Delay Fixed: Switch Delay Fixed Dejitter Buffer Fixed: Packetization Delay Variable: Output Queuing Delay

6 Acceptable Delay: G.114 Range in Milliseconds Description Acceptable for most user applications Above 400 Acceptable, provided that administrators are aware of the transmission time and its impact on the transmission quality of user applications Unacceptable for general network planning purposes (However, it is recognized that in some exceptional cases, this limit will be exceeded.)

7 Effect of Packet Loss Lost Audio Packet 1 Lost Packet 2 Packet 3

8 MOS and PSQM MOS Mean opinion score Defined in ITU-T Recommendation P.800 Results in subjective measures Scores from 1 (worst) to 5 (best); 4.0 is toll quality

9 MOS and PSQM PSQM Perceptual Speech Quality Measurement Defined in ITU Standard P.861 Automated in-service measurement Scores from 6.5 (worst) to 0 (best)

10 Voice Quality Measurement Comparison Feature MOS PSQM Test method Subjective Objective End-to-end packet loss test Inconsistent No End-to-end jitter test Inconsistent No

11 QoS Mechanisms for VoIP Header compression Frame Relay traffic shaping (FRTS) FRF.12 PSTN fallback IP RTP Priority and Frame Relay IP RTP Priority

12 QoS Mechanisms for VoIP IP to ATM class of service (CoS) Low Latency Queuing (LLQ) Multilink PPP (MLP) Resource Reservation Protocol (RSVP)

13 Objectives of QoS Support dedicated bandwidth Improve loss characteristics Avoid and manage network congestion Shape network traffic Set traffic priorities across the network

14 Applying QoS In the Output Queue In the WAN VoIP QoS In Conjunction with IP

15 Transporting Modulated Data over IP Networks Fax and modem traffic consists of digital data modulated into high-frequency tones. In contrast to voice, packet loss is much more critical for fax and modem communications. VoIP compression algorithms are designed for voice, not for fax or modem data frequencies.

16 Transporting Modulated Data over IP Networks Methods to transmit fax and modem over IP networks: Terminating and transmitting the data on the gateway (fax relay) Sending the data in-band into the RTP stream (fax pass-through) Receiving and converting faxes to files using T.37 (store-and-forward)

17 Pass-Through Topology G kb/s G kb/s Encoding Decoding IP Network Analog Data Analog Data Tunnelled Through 64 kb/s VoIP Analog Data End-to-End Connection

18 Fax Pass-Through Considerations Works only when the configured codec is G.711 or clear channel. Some gateways have limited port numbers for simultaneous use. VAD and echo cancellation are disabled. Supported under the following call control protocols: H.323, SIP and MGCP

19 Modem Pass-Through Considerations Works only when the configured codec is G.711 or clear-channel. VAD and echo cancellation need to be disabled.

20 Modem Pass-Through Considerations Modem pass-through over VoIP performs these functions: Represses processing functions Issues redundant packets Provides static jitter buffers Differentiates modem signals from voice and fax signals Reliably maintains a modem connection across the packet network

21 Relay Topology DSP Demodulates DSP Modulates IP Network Analog Data TCP Transmission of Data Packets Analog Data Connection 1 Connection 2 Connection 3

22 Fax Relay Considerations T.38 fax relay includes these features: Fax relay packet loss concealment MGCP-based fax (T.38) and DTMF relay SIP T.38 fax relay T.38 fax relay for T.37/T.38 fax gateway T.38 fax relay for VoIP H.323

23 Modem Relay Considerations Modem relay includes these features: Modem tone detection and signaling Relay switchover Payload redundancy Packet size Dynamic and static jitter buffers

24 Store-and-Forward Fax On-ramp receives faxes that are delivered as attachments. PSTN Fax Off-ramp sends standard messages that are delivered as faxes. Fax PSTN

25 Fax and Modem Pass-Through G3 Fax Initiates the Call Original Gateway Terminating Gateway G3 Fax IP Network VoIP Call T.30 CED tone Call Control Issues NSE NSE Accept Change codec VoIP Call Change codec

26 Fax Relay G3 Fax Initiates the Call Gateway Gateway G3 Fax IP Network T.30 VoIP Call T.30 CED Tone DIS Message Fax Relay Switchover (PT96) Send Codec ACK (PT97) Download Codec Download Codec Codec Download Done (PT96) Codec Download ACK (PT97) Fax Relay Established

27 H.323 T.38 relay G3 Fax Initiates the Call T.38 Gateway T.38 Gateway G3 Fax IP Network T.30 VoIP Call T.30 CED Tone DIS Message Mode Request Mode Request ACK Close VoIP and Open T.38 Channels T.38 UDP Packets

28 SIP T.38 Relay G3 Fax Initiates the Call T.38 Gateway T.38 Gateway G3 Fax IP Network T.30 VoIP Call T.30 CED Tone DIS Message INVITE (T.38 in SDP) 200 OK ACK T.38 UDP Packets

29 Gateway Signaling Protocols with Fax and Modem Pass-Through and Relay MGCP T.38 fax relay provides two modes of implementation: Gateway-controlled mode: Gateways negotiate fax relay transmission by exchanging data in SDP messages. Allows the use of MGCP-based T.38 fax without the necessity of upgrading the call agent software.

30 Gateway Signaling Protocols with Fax and Modem Pass-Through and Relay Call agent-controlled mode: Call agents instruct gateways to process fax traffic. Call agent can instruct gateways to revert to gateway-controlled mode if it can not handle fax control.

31 DTMF Support DTMF tones are distorted when gateways use compression on slower WAN links. DTMF relay addresses this problem. S0 256 kb/s G.729 Codec being Used S1 256 kb/s

32 Summary Because of the nature of IP networking, voice packets sent via IP are subject to certain transmission problems. Several methods may be used to determine audio quality in a VoIP network. QoS is used to meet the strict requirements concerning packet loss, delay, and jitter in a VoIP network. There are some challenges to transporting modulated data, including fax and modem calls, over IP networks.

33 Summary There are some challenges to transporting modulated data, including fax and modem calls, over IP networks. These features support fax and modem traffic: Fax and modem pass-through Fax and modem relay Store-and-forward fax

34 Summary (Cont.) T.38 pass-through and relay use special protocol enhancements on H.323, SIP, and MGCP. DTMF support is provided by gateways.