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Introduction to VoIP

Cisco Networking Academy Program

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Requirements of Voice in an IPInternetwork

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IP Internetwork

IP is connectionless.

IP provides multiple paths from source todestination.

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Packet Loss, Delay, and Jitter

Packet loss

Loss of packets severely degrades the voice application.

Delay

VoIP typically tolerates delays up to 150 ms before thequality of the call degrades.

Jitter

Instantaneous buffer use causes delay variation in thesame voice stream.

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Consistent Throughput

Throughput is the amount of data transmittedbetween two nodes in a given period.

Throughput is a function of bandwidth, error

performance, congestion, and other factors.

Tools for enhanced voice throughput include:

Queuing

Congestion avoidance

Header compression

RSVP

Fragmentation

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Reordering of Packets

IP assumes packet-ordering problems.

RTP reorders packets.

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Reliability and Availability

Traditional telephony networks claim 99.999%uptime.

Data networks must consider reliability and

availability requirements when incorporating voice.

Methods to improve reliability and availabilityinclude:

Redundant hardware

Redundant links

UPS

Proactive network management

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Analog vs. Digital

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Gathering the Requirements

Is an analog or digital gateway required?

What is the required capacity of the gateway?

What type of connection is the gateway going to use? Is ForeignExchange Office (FXO), FXS, E&M, T1, E1, PRI, or BRI signalingrequired?

What signaling protocol is used? H.323, Media Gateway ControlProtocol (MGCP), or session initiation protocol (SIP)?

Is voice compression a part of the design? If so, which type?

Are direct inward dialing (DID), calling line identification (CLID),

modem relay, or fax relay required?

Is the device acting only as gateway or as gateway androuter/LAN switch? Is inline power for IP Phones required?

Is remote site survivability required?

To which country is the hardware shipped?

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Enterprise Gateway ConsiderationsRemote Site

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Enterprise Gateway ConsiderationsCentral Site

Dial plan integration

Voice-mail integration

Gateway for PBX interconnect

Inline power requirements for IP Phones

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Encapsulating Voice in IP Packets

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Major VoIP Protocols

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VoIP Protocols and the OSI Model

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Real-Time Transport Control Protocol

Monitors the quality of the data distribution andprovides control information

Provides feedback on current network conditions

Allows hosts involved in an RTP session toexchange information about monitoring andcontrolling the session

Provides a separate flow from RTP for UDP

transport use

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RTP Header Compression

RTP header compression saves bandwidth bycompressing packet headers across WAN links.

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When to Use RTP Header Compression

Narrowband links

Slow links (less than 2 Mbps)

Need to conserve bandwidth on a WAN interface

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Calculating Bandwidth Requirements

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Bandwidth Implications of Codec

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Impact of Voice Samples

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Data Link Overhead

Ethernet

18 bytes overhead

MLP

6 bytes overhead

Frame Relay

6 bytes overhead

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Security and Tunneling Overhead

IPSec

50 to 57 bytes

L2TP/GRE

24 bytes

MLPPP

6 bytes

MPLS4 bytes

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Specialized Encapsulations

X.25 over TCP/IP

IPv6 over IPv4

L2F

Others

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Total Bandwidth Required

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Effect of VAD

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