1© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
Understanding Voice over IP Protocols
Cisco Systems—Service Provider Solutions Engineering
February, 2002
222© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
Topics to Discuss
• History of VoIP
• VoIP—Early Adopters
• VoIP—Standards and Standards Bodies
• VoIP—Making Sense of the Protocols
• “The Great Voice Myth”
• VoIP—Protocol Challenges
• Summary
333© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
Why Move to VoIP?
• Cost savings—toll bypass
• Open standards—H.323, SIP, MGCP
• Multi-vendor interoperability
• Integrated IP voice and data networks
444© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
Cisco Packet Voice Architecture
TDM/Circuit Switch
TDM/Circuit Switch
Digital Trunk Digital Trunk SubsystemSubsystem
Line Line ConcentrationConcentration
AdministrationAdministrationMaintenanceMaintenance
BillingBilling
Call ControlCall ControlConnection ControlConnection Control
FeaturesFeatures
Common ChannelCommon ChannelSignaling ComplexSignaling Complex
Sw
itch
ing
Net
wo
rk
StandardsStandards--Based Based Packet Infrastructure LayerPacket Infrastructure Layer
(IP, ATM)(IP, ATM)
Open Call Control LayerOpen Call Control Layer(SIP, H.323, MGCP, etc.)(SIP, H.323, MGCP, etc.)
Open Service Open Service Application LayerApplication Layer
(JAIN, AIN, TAPI,(JAIN, AIN, TAPI,JTAPI, XML etc.)JTAPI, XML etc.)
Open/Standard Interface
Open/Standard Interface
555© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
Topics to Discuss
• History of VoIP
• VoIP—Early Adopters
• VoIP—Standards and Standards Bodies
• VoIP—Making Sense of the Protocols
• “The Great Voice Myth”
• VoIP—Protocol Challenges
• Summary
666© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
Early Adopters—Advanced Services and Toll-Bypass
• Regulatory opportunities allowed for toll-bypass
• PC-to-phone, calling-card and international fax services
• Cisco-based carriers used standard protocols, but not all carriers implemented standards
• Inter-carrier connections had protocol interoperability challenges
777© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
Topics to Discuss
• History of VoIP
• VoIP—Early Adopters
• VoIP—Standards and Standards Bodies
• VoIP—Making Sense of the Protocols
• “The Great Voice Myth”
• VoIP—Protocol Challenges
• Summary
888© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
Making the Rules for VoIP
• IETF (Internet Engineering Task Force)The community of engineers that standardizes the protocols that define how the Internet and Internet Protocols work. http://www.ietf.org/
• ITU (International Telecommunications Union)An international organization within the United Nations System where governments and the private sector coordinate global telecom networks and services. http://www.itu.int/home/index.html
999© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
Defining the VoIP Protocols
• H.323An ITU Recommendation that defines “Packet-based multimedia communications systems”. H.323 defines a distributed architecture for creating multimedia applications, including VoIP
• SIPDefined as IETF RFC 2543. SIP defines a distributed architecture for creating multimedia applications, including VoIP
• MGCPDefined as IETF RFC 2705. MGCP defines a centralized architecture for creating multimedia applications, including VoIP
• H.248An ITU Recommendation that defines “Gateway Control Protocol”. H.248 is the result of a joint-collaborate with the IETF. H.248 defines a centralized architecture, and is also known as “Megaco”
• MegacoDefined as IETF RFC 2885. Megaco defines a centralized architecture
101010© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
Topics to Discuss
• History of VoIP
• VoIP—Early Adopters
• VoIP—Standards and Standards Bodies
• VoIP—Making Sense of the Protocols
• “The Great Voice Myth”
• VoIP—Protocol Challenges
• Summary
111111© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
H.324Terminal
H.324Terminal
H.323Gatekeeper
H.323Gatekeeper
Packet Network
H.323Terminal
H.323Terminal
H.323Gateway
H.323Gateway
H.323MCUH.323MCU Scope of
H.323
PSTN ISDN
V.70Terminal
V.70Terminal
SpeechTerminalSpeechTerminal
H.320Terminal
H.320Terminal
SpeechTerminalSpeechTerminal
H.323 Components
e
GK
121212© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
Scope of H.323 Recommendation
Video CodecH.261, H.263
Call ControlH.225.0
System ControlSystem Control
Audio CodecG.711, G.722,G.723, G.728,
G.729
RAS ControlH.225.0
RTP
RTCP
H.245 Control
Audio I/OEquipment
User Data Applications
T.120, etc.
SystemControl
UserInterface
Video I/OEquipment
H.225Layer
UDP
TCP
UDP
IP
ReceivePain
Delay(Sync)
131313© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
Media (UDP)RTP StreamRTP StreamRTCP StreamRTCP Stream
H.323Endpoint A
H.245 (TCPDynamic Port)Open Logical Channel
H.225 (TCP Port 1720)Setup
Alerting / Connect
Open Logical Channel Acknowledge
Capabilities Exchange / MSD
RTP StreamRTP Stream
H.323Endpoint B
VVVV
H.323 Signaling
141414© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
Gatekeeper A Gatekeeper B
RRQ/RCF
ARQ
RRQ/RCF
LRQ
IP Network
Phone A
Gateway A Gateway B
H.225 (Q.931) Setup
H.225 (Q.931) Alert and Connect
H.245
RTP
ACF
LCF
VV
Basic H.323 Call
VV
ARQ
ACF
Phone B
151515© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
MidwestZoneMidwestZone
ChicagoGWChicagoGW
LocalPSTNLocalPSTN
Deploying H.323 Networks
WestZoneWestZone
• Addition of new rate centers
• Minimizes GK configuration• Addition of new zones
DGK• Addition of new NPAs
LAGW #1
LAGW #1
RateCenter #1
RateCenter #1
RateCenter #1
RateCenter #1
LAGW #2
LAGW #2
Intra-LATAToll
Intra-LATAToll
GK GK GK
EastZoneEastZone
NYGWNYGW
LocalPSTNLocalPSTN
161616© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
MGCP/H.248/Megaco—Architectures
PSTN
Call Agent
SS7
AccessGateway
MGCP / H.248 / Megaco
IMT
PRI
RTP
Call Agent
PSTN
PSTN
171717© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
Deploying MGCP/H.248/Megaco Networks
Modem Dial-up TrafficTraditional TDM Traffic
CA
MGCPand ISDNBackhaul
Service Provider's
TDMNetwork
Service Provider's
PacketNetwork
MGMG
TDMVoice
NAS/VoIP
ISDN/PRI
OSSOSS
Billing and MeasurementServer
Billing and MeasurementServer
SLT
VoIP
MG
SS7Backhaul
SS7
STP
PSTNIMTs
181818© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
SIP Architecture
Legacy PBX
SIP User Agents (UA)
Application Services
eMail LDAP Oracle XML
SIPSIP
RTP(Media)
SIP
CPLCPL3pcc
PSTN
CAS or PRI
INTELLIGENT
SERVICES
SIP Proxy, Registrar & Redirect Servers
191919© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
SIP VoIP Network
Calling Party
PSTN
Called Party
PSTN
SignalingSignaling
Bearer Or Media
Bearer Or Media
SIP Signaling
Media (UDP)
INVITE
100 Trying
180 Ringing
100 Trying
RTCP StreamRTCP Stream
INVITE
200 OK200 OK
180 Ringing
ACK ACK
SIP Signaling and SDP Signaling(UDP or TCP)
202020© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
SIP Servers/ServicesSIP Servers/Services
SIP User Agents
Registrar RedirectLocationDatabase
SIP Proxy
SIP Servers/Services
REGISTER“Here I am”
INVITE“I want to talk to another UA
Proxied INVITE“I’ll handle it for
you”
“Where is this name/phone#?”
3xx Redirection“They moved,
try this address”
SIP User Agents SIP-GW
212121© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
Central ZoneCentral Zone
ChicagoPOPChicagoPOP
West ZoneWest Zone
SFPOPSFPOP
PSTN312
East ZoneEast Zone
NYPOPNYPOP
PSTN212
PSTN415
Deploying SIP Networks
IP Network
222222© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
Topics to Discuss
• History of VoIP
• VoIP—Early Adopters
• VoIP—Standards and Standards Bodies
• VoIP—Making Sense of the Protocols
• “The Great Voice Myth”
• VoIP—Protocol Challenges
• Summary
232323© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
Voice Myths
• Networks can only be built one way
• Networks will only use one protocol
• All networks will converge
• VoIP allows centralized or distributed architectures
• H.323, SIP, MGCP and H.248/Megaco will all be present in VoIP networks
• Networks will converge to IP
Myths Facts
242424© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
Topics to Discuss
• History of VoIP
• VoIP—Early Adopters
• VoIP—Standards and Standards Bodies
• VoIP—Making Sense of the Protocols
• “The Great Voice Myth”
• VoIP—Protocol Challenges
• Summary
252525© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
Interconnecting VoIP Networks
??SIPSIPH.323
H.323
MGCPMGCPH.248H.248
MegacoMegaco
262626© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
Connecting VoIP to SS7/C7 Networks
INVITEACKSDPSDP
H.323
MGCPMGCP
SIPSIP
IAM
CRCXACKSDPSDP
H.225 Setup (ANI,DN)Proceeding
H.245H.245
272727© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
VoIP Interworking Issues
• Service interworkingE.g.: H.450 <-> SIP <-> MGCP
• Media interworkingEnd-to-end codec negotiation
• Bearer interworkingEnd-to-end fax, modem, DTMF
282828© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
VoIP Interworking
• Bearer levelModem (relay/passthru)
Fax (relay/passthru)
T.38
T.37
DTMF (relay/passthru)
• Media levelCodec (negotiation, selection)
• Service translation issues
Call deflection
Park/hold
• Signal issuesSDP
H.245
292929© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
Fax and Modem Passthru Mechanisms
• Modem and fax are control mechanisms based on PLL (Phase Locked Loops)
• They are both time sensitive
• Highly sensitive to packet network impairments: Jitter
Packet loss
Delay
• Susceptible to clock slew (clock sync differences between gateways)
303030© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
IPCloud
Voice Gateway
Voice Gateway
G.711µ
G.729
G.729
G.711µ
PCMG.711µ
PCMG.711µ
Passthru Simplified
DSPDSP
DSPDSP
313131© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
What Is Modem Passthru?
• It is the transport of modem signals (modulation, error correction and compression) through a packet network using PCM encoded packets
323232© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
Modem Passthru (Cont.)
• Modem tone detection (<= V.90)
• Switchover signaling
• No VAD
• EC off
• RTP payload redundancy (10ms packetization) RFC2198 (optional)
333333© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
Modem Passthru Issues
• Consecutive packet drops (loss) cause retrain
• Consecutive drops during retrain causes disconnect
• Variation of delay (jitter) has quite an effect
• Jitter (at 10%) is a conservative estimate—Since jitter mostly impacts performance with packet loss
343434© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
What Is Modem Relay?
• Modem relay involves demodulating the modem signal at ingress gateway
• Passing this data as packet data to terminating gateway
• Re-modulating the data and passes it to the receiving modem
353535© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
Fax Relay—T.38
• Real-time
• Also called demod/remod
• Can be used in H.323/MGCP/SIP signaling
• Delivers fax data over UDP streams (uses same RTP port)—reuses voice UDP ports
• Fallback to proprietary mode
• Method of encoding the T.30 and T.4 into packets
PSTNPSTNPSTNPSTN
IP
T.30 T.30UDP
363636© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
DTMF
• What is DTMF
• Why is it required? and where is it used?
• How do you transport it in IP?
• DTMF implementation
373737© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
DTMF (Cont.)
• In TDM world, all voice traffic is sent as uncompressed 64Kbs PCM streams; anything sent on that circuit is an untouched stream of bits; (e.g., voice speech, modem tones, fax tones, and DTMF digits)
• DSP codecs designed to interpret human speech, can distort DTMF tones (machine-tones)
• High b/w codecs less likely to distort
• Distortion causes problems with voicemail and IVR systems
383838© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
DTMF Schemes with VoIP Protocols
H.323H.323MGCP, H.248,
Megaco
MGCP, H.248,
MegacoSIPSIP
In-BandIn-Band In-BandIn-Band In-BandIn-Band In-BandIn-Band
Out-of-BandOut-of-Band
Cisco RTP, H.245 Alphanum,H.245 Signal, AVT Tones RFC2833
Cisco RTP, H.245 Alphanum,H.245 Signal, AVT Tones RFC2833
Cisco RTP,NSE, NTE,RFC2833
Cisco RTP,NSE, NTE,RFC2833 RFC2833RFC2833
393939© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
Topics to Discuss
• History of VoIP
• VoIP—Early Adopters
• VoIP—Standards and Standards Bodies
• VoIP—Making Sense of the Protocols
• “The Great Voice Myth”
• VoIP—Protocol Challenges
• Summary
404040© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
Summary
• Understand the possibilities and the issues
• Avoid protocol/product based bias
• Decide on application
• Consider market and business drivers
• Deploy what’s possible today
• Choose signaling protocol depending on services intended to be offered
• Many possibilities—stay tuned
414141© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
Crystal Ball on VoIP
• All three protocols (or its variations) are here for the long run
• Changes/enhancements will be made
• IP will be the core
424242© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1
Reference URLs
• ITU: www.itu.org
• IETF: www.ietf.org
• SIP: www.cs.columbia.edu/~hgs/sip/
• H.323: www.packetizer.com/iptel/h323/
• MGCP:www.softswitch.org/asp/techlibrary_protocol.asp?page=techlibrary
434343© 2002, Cisco Systems, Inc. All rights reserved.4426_02_2002_c1