introduction to voipwireless.ictp.it/school_2006/lectures/steve/voip-ebook.pdf · introduction to...
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Introduction to VOIP Stephen Okay
Abdus Salam Int’l Center for Theoretical PhysicsTrieste, Italy, February 21, 2006
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Intro to VOIP• Classic Telephony• Data Networks(Review)• VOIP
• What it is
• Protocols
• Hardware
• Software
• Examples
• Web Links
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Classic Telephony in 1 slide
• Classic Telephony• Calls happen by electro-mechanical manipulation
of voltage levels between the telco network and end-user phones.
• voice “payload” and transport signals must be sent together over the same line/circuit.
• “Modern” equipment is electronic, but must work w/ older equipment.
• This makes upgrades and enhancements difficult and expensive.
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VOIP in 1 slide• Manipulate bits not volts
• everything can now happen inside a PC
• Can be all-digital or a mix of digital and analog equipment
• Telephone Company/PTT not required
• Can use low-power, commodity hardware instead of expensive, dedicated gear
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Intro to VOIP• Review of TCP/IP• “voice chat over IP” applications vs. VOIP protocols
• VOIP protocols• H323
• SIP
• VOIP hardware
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TCP/IP Review• 4-layer stack• Packet-switched• Error correcting• Reliable delivery• Designed in 1960s/1970s for use over
slow, unreliable analog phone networks• Application layer key to VOIP and other
commonly used protocols
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“Voice chat”• point-to-point or server-based.• works even with very low bandwidth• often proprietary, closed systems run as a
business rather than a public utility. • Often no routing between PSTN or other
networks.• Profit motive can hamper scalability,
availability and end-users
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“Voice Chat”• Examples:
• Skype• iChatAV• Yahoo!IM• MBONE vic/vat tools
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VOIP
• Uses formal protocol suites to provide:• Call routing, forwarding, voicemail,
etc.• Compatibility w/ legacy PSTN/POTS
systems• Quality of Service(QOS)
• Done mostly w/ software• Can still do point-to-point calls
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Major VOIP benefits• It’s all just bits.
• VOIP is just another application running over a data network(usually TCP/IP)
• Service expansion/integration often just a matter of writing the code, not tearing down a switching center.
• Per-call costs very low, often free• Equipment is often commodity priced
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VOIP Concerns• Regulatory issues• Tax revenue issues• Market exploitation and control • Quality & Reliability of Infrastructure
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VOIP Protocols• H.323• SIP• MGCP• IAX• Others
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H.323• Specification defined by ITU
• Wide support among all telecom providers & manufacturers all over the world.
• Industry moving away from using it in new products.
• Meant to provide gateway for telephony devices into the PSTN
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SIP• “Session Initiation Protocol”
• Designed by Cisco Systems, Inc.
• Stand-alone computer-to-computer protocol
• Does not presume a PSTN
• Calls routed/managed by a SIP Server
• No real “official” version, so there are lots of different implementations.
• Rapidly becoming dominant protocol
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Protocol details• Most VOIP protocols split signalling from
voice data, unlike POTS• potential firewall issues
• QOS/capacity checked before calls are initiated
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VOIP Hardware• PSTN Gateways• Telephone adapters• Handsets• Softswitch/PBX• (Much of this can actually be done on a PC)
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PSTN Gateways
• Provides a bridge between VOIP networks and the PSTN.
Phone Jacks Ethernet Serial Console
Office/Telecom SystemPCI Card
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Analog Telephone Adapter
• Lets you use regular phone on VOIP network • Avoid dedicated or “locked” adapters if
possible• $50-200 US
Phone Jacks Ethernet
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VOIP Phones
• Looks / acts like regular phone, but really an embedded computer with VOIP client software
• Ethernet or wireless(802.11b) models available
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Soft Phones• Telephony functions completely in
software• Run on desktop, laptop and embedded
systems. • Some common softphone clients
• SJPhone• XLite• KPhone• OhPhone
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VOIP Networks• Examples
• PC-to-PC• PC-to-PSTN• VOIP-based POTS replacement• VOIP PBX using Asterisk
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Asterisk• Open Source GPL-Licensed PBX • Runs under most popular versions of
UNIX (Linux, BSD, OS X)• Can replace a traditional office PBX• Supports soft phones, SIP handsets,
wireless phones and
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PC-PC VOIP Network with Analog phones
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VOIP Network Using Satellite connected to Internet and PSTN
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Local Village Wireless Network w/ SIP clients and Asterisk Server
PC running Asterisk PBX
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Multi-Village Network w/ connection to PSTN
Remote Villages
Relay/Bridge
Tower For mounting Receiver
AsteriskServer Voice
Gateway Local PSTN
Dial-up Internet, Satellite BB
Town
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VOIP Vendors/Services• Skype• Vonage• FreeWorldDialup• LOTS of small, “microbrew” VOIP startups
popping up all over the world• You??
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Open Source VOIP projects• Asterisk(SIP-based PBX)
• www.asterisk.org• OpenH323(H.323)
• www.openh323.org• VOCAL (H.323,SIP, MGCP)
• www.vovida.org
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VOIP Information Links• VOIPInfo - www.voipinfo.org• Voxilla - www.voxilla.org