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Introducing Voice over IP

SpecifyingRequirements forVoIP Calls

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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 tothe 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 theconversation

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Jitter in IP Networks

Time

Steady Stream of Packets

Same Packet Stream After Congestion or Improper Queuing

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Sources of Delay

Router

64 kb/s

64 kb/sRouter

Packet Flow

E1

E1

FixedDejitterBuffer

FixedCoderDelay

Fixed:Packetization

DelayVariable:Output

QueuingDelay

Fixed:Serialization

Delay

Fixed:SwitchDelay Fixed:

SwitchDelay

Fixed:SwitchDelay

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Acceptable Delay: G.114

Unacceptable for general network planningpurposes (However, it is recognized that in someexceptional cases, this limit will be exceeded.)

Above 400

Acceptable, provided that administrators are awareof the transmission time and its impact on thetransmission quality of user applications

150400

Acceptable for most user applications0150

DescriptionRange in Milliseconds

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Effect of Packet Loss

Packet 1 Packet 3Lost Packet 2

Lost Audio

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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)

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Voice Quality Measurement Comparison

NoInconsistentEnd-to-end jitter test

NoInconsistentEnd-to-end packet loss test

ObjectiveSubjectiveTest method

PSQMMOSFeature

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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)

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Objectives of QoS

Support dedicated bandwidth

Improve loss characteristics

Avoid and manage network congestion

Shape network traffic Set traffic priorities across the network

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Applying QoS

VoIPQoS

In the WANIn the Output Queue

In Conjunction with IP

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Transporting Modulated Data over IPNetworks

Fax and modem traffic consists of digital data modulated intohigh-frequency tones.

In contrast to voice, packet loss is much more critical for fax andmodem communications.

VoIP compression algorithms are designed for voice, not for faxor 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)

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Analog Data

Analog Data Tunnelled

Through 64 kb/s VoIP

G.711 64 kb/sEncoding

IP Network

G.711 64 kb/sDecoding

Analog Data

0110011 0110011

0110011 0110011

End-to-End Connection

Pass-Through Topology

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Works only when the configured codec is G.711 orclear 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

Fax Pass-Through Considerations

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Works only when the configured codec is G.711 orclear-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 packetnetwork

Modem Pass-Through Considerations

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Analog Data

DSPDemodulates

DSPModulates

Analog Data

0110011 0110011

0110011 0110011

IP Network

TCP Transmission

of Data Packets

Connection 1 Connection 2 Connection 3

Relay Topology

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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

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Modem Relay Considerations

Modem relay includes these features:

Modem tone detection and signaling

Relay switchover

Payload redundancy

Packet size

Dynamic and static jitter buffers

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Store-and-Forward Fax

On-ramp receives faxes that are delivered as e-mail attachments.

PSTN

PSTN

Off-ramp sends standard e-mail messages that are deliveredas faxes.

Fax E-Mail

FaxE-Mail

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Fax and Modem Pass-Through

IP Network

G3 FaxInitiates the Call

G3 FaxOriginalGateway

TerminatingGateway

Call Control Issues NSE

Change codec Change codec

VoIP Call

T.30

CED tone

NSE Accept

VoIP Call

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Cisco Fax Relay

IP Network

G3 FaxInitiates the Call

G3 FaxGateway Gateway

DIS Message

VoIP Call

Send Codec ACK (PT97)

Fax Relay Switchover (PT96)

Fax Relay Established

Download Codec

T.30 T.30

CED Tone

Codec Download ACK (PT97)

Codec Download Done (PT96)Download Codec

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H.323 T.38 relay

IP Network

G3 FaxInitiates the Call

G3 FaxT.38

GatewayT.38

Gateway

CED Tone

T.30 T.30VoIP Call

DIS Message

Mode Request

Mode Request ACK

Close VoIP and Open T.38 Channels

T.38 UDP Packets

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SIP T.38 Relay

IP Network

G3 FaxInitiates the Call

G3 FaxT.38

GatewayT.38

Gateway

CED Tone

DIS Message

T.30 T.30VoIP Call

INVITE (T.38 in SDP)

200 OK

ACK

T.38 UDP Packets

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Gateway Signaling Protocols with Fax andModem Pass-Through and Relay

MGCP T.38 fax relay provides two modes ofimplementation:

Gateway-controlled mode:

Gateways negotiate fax relay transmission by exchanging datain SDP messages.

Allows the use of MGCP-based T.38 fax without the necessityof upgrading the call agent software.

Call agent-controlled mode:

Call agents instruct gateways to process fax traffic.

Call agent can instruct gateways to revert togateway-controlled mode if it can not handle fax control.

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DTMF Support

DTMF tones are distorted when gateways use compression onslower WAN links.

DTMF relay addresses this problem.

S0256kb/s

G.729 CodecBeing Used

S1256kb/s

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Summary

Because of the nature of IP networking, voice packets sent via IPare subject to certain transmission problems.

Several methods may be used to determine audio quality in aVoIP network.

QoS is used to meet the strict requirements concerning packetloss, delay, and jitter in a VoIP network.

There are some challenges to transporting modulated data,including fax and modem calls, over IP networks.

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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.

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