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8/6/2019 2.Intro to VoIP
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1 2005 Cisco Systems, Inc. A ll rights reserved. Cisco PublicIP Telephony
Introduction to VoIP
Cisco Networking Academy Program
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2 2005 Cisco Systems, Inc. A ll rights reserved. Cisco PublicIP Telephony
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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