Cvoice 6.0 S01 L03

Specifying Requirements for VoIP Calls

Introducing Voice over IP

© 2008 Cisco Systems, Inc. All rights reserved.

CVOICE v6.0—1-1

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  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

© 2008 Cisco Systems, Inc. All rights reserved.

CVOICE v6.0—1-2

Jitter in IP Networks Steady Stream of Packets

Time

Same Packet Stream After Congestion or Improper Queuing

© 2008 Cisco Systems, Inc. All rights reserved.

CVOICE v6.0—1-3

Sources of Delay

Packet Flow

64 kb/s

64 kb/s

Router

Router E1 E1 Fixed Coder Delay

Fixed: Packetization Delay

Fixed: Serialization Delay

Fixed: Switch Delay

Fixed: Switch Delay

Fixed: Switch Delay

Fixed Dejitter Buffer

Variable: Output Queuing Delay

© 2008 Cisco Systems, Inc. All rights reserved.

CVOICE v6.0—1-4

Acceptable Delay: G.114

Range in Milliseconds 0–150

150–400

Above 400

© 2008 Cisco Systems, Inc. All rights reserved.

Description Acceptable for most user applications 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.)

CVOICE v6.0—1-5

Effect of Packet Loss

Lost Audio

Packet 1

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Lost Packet 2

Packet 3

CVOICE v6.0—1-6

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

© 2008 Cisco Systems, Inc. All rights reserved.

CVOICE v6.0—1-7

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

© 2008 Cisco Systems, Inc. All rights reserved.

CVOICE v6.0—1-8

QoS Mechanisms for VoIP  Header compression  Frame Relay traffic shaping (FRTS)  FRF.12  PSTN fallback  IP RTP Priority and Frame Relay IP RTP Priority  IP to ATM class of service (CoS)  Low Latency Queuing (LLQ)  Multilink PPP (MLP)  Resource Reservation Protocol (RSVP)

© 2008 Cisco Systems, Inc. All rights reserved.

CVOICE v6.0—1-9

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

© 2008 Cisco Systems, Inc. All rights reserved.

CVOICE v6.0—1-10

Applying QoS In the Output Queue

In the WAN

VoIP QoS

In Conjunction with IP

© 2008 Cisco Systems, Inc. All rights reserved.

CVOICE v6.0—1-11

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.  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) © 2008 Cisco Systems, Inc. All rights reserved.

CVOICE v6.0—1-12

Pass-Through Topology 0110011

G.711 64 kb/s Encoding

G.711 64 kb/s Decoding

0110011

IP Network

Analog Data

Analog Data Tunnelled Through 64 kb/s VoIP

0110011

Analog Data 0110011

End-to-End Connection

© 2008 Cisco Systems, Inc. All rights reserved.

CVOICE v6.0—1-13

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 – MGCP

© 2008 Cisco Systems, Inc. All rights reserved.

CVOICE v6.0—1-14

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

© 2008 Cisco Systems, Inc. All rights reserved.

CVOICE v6.0—1-15

Relay Topology 0110011

0110011 DSP Demodulates

DSP Modulates

IP Network

Analog Data

TCP Transmission of Data Packets

Analog Data

0110011

0110011 Connection 1

© 2008 Cisco Systems, Inc. All rights reserved.

Connection 2

Connection 3

CVOICE v6.0—1-16

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

© 2008 Cisco Systems, Inc. All rights reserved.

CVOICE v6.0—1-17

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

© 2008 Cisco Systems, Inc. All rights reserved.

CVOICE v6.0—1-18

Store-and-Forward Fax  On-ramp receives faxes that are delivered as e-mail attachments. Fax

E-Mail

PSTN

 Off-ramp sends standard e-mail messages that are delivered as faxes. E-Mail

Fax

PSTN

© 2008 Cisco Systems, Inc. All rights reserved.

CVOICE v6.0—1-19

Fax and Modem Pass-Through

Original Gateway

G3 Fax Initiates the Call

Terminating Gateway

G3 Fax

IP Network VoIP Call T.30 CED tone Call Control Issues NSE NSE Accept Change codec

© 2008 Cisco Systems, Inc. All rights reserved.

VoIP Call

Change codec

CVOICE v6.0—1-20

Cisco Fax Relay Gateway

G3 Fax Initiates the Call

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

© 2008 Cisco Systems, Inc. All rights reserved.

CVOICE v6.0—1-21

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

© 2008 Cisco Systems, Inc. All rights reserved.

CVOICE v6.0—1-22

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

© 2008 Cisco Systems, Inc. All rights reserved.

CVOICE v6.0—1-23

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.  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.

© 2008 Cisco Systems, Inc. All rights reserved.

CVOICE v6.0—1-24

DTMF Support  DTMF tones are distorted when gateways use compression on slower WAN links.  DTMF relay addresses this problem.

S0 256 kb/s

© 2008 Cisco Systems, Inc. All rights reserved.

G.729 Codec Being Used

S1 256 kb/s

CVOICE v6.0—1-25

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.

© 2008 Cisco Systems, Inc. All rights reserved.

CVOICE v6.0—1-26

Summary (Cont.)  These features support fax and modem traffic: – Fax and modem pass-through – Fax and modem relay – Store-and-forward fax  T.38 pass-through and relay use special protocol enhancements on H.323, SIP, and MGCP.  DTMF support is provided by Cisco IOS gateways.

© 2008 Cisco Systems, Inc. All rights reserved.

CVOICE v6.0—1-27

© 2008 Cisco Systems, Inc. All rights reserved.

CVOICE v6.0—1-28