VoIPVoIP
• What is VoIP• Background & Benefit• VoIP Concepts• What is H.323• Another VoIP Protocol SIP• Considerations
• What is VoIP• Background & Benefit• VoIP Concepts• What is H.323• Another VoIP Protocol SIP• Considerations
Contents
1. What is VoIP?
PSTN
Router RouterIP Network
PBX PBX
①
②
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Voice is transported over Internet with compression of Audio data.
Offices are located everywhere, cost grows heavier.
2. Background & Benefit
Data망
Voice망
PBXVoice
Data SystemData System
PBXVoice
Business IP LAN/WAN grown rapidly, and Ethernet network is everywhere.
Data SystemVoice
& Data Data System
PBX PBXVoice Voice
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International Standards Organization’s Open System Interconnection 7-layer Reference Model.
Defines functional layers needed for communication between 2 or more network end-points.
Message delivery requires each function layer.
7 Application Interactive Voice (VoIP)
6 Presentation Codec
5 Session RTP, RTCP
4 Transport TCP & UDP
3 Network IP, Diffserv
2 Data Link Ethernet
1 Physical UTP Cat 5
3. VoIP Concepts OSI-7 layer
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VoIP standards define protocol messages and packet structure/content for each layer.
“Protocol Stack” sum of protocol layers in OSI model
Several VoIP standards: IEEE developed H.323 IETF developed SIP MGCP used by DOCSIS (Cable TV), disassociated Gateway IEEE/IETF Megaco/H.248 IETF/IEEE disassociated Gateway
3. VoIP Concepts
General
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Layer 7
Layer 6
Layer 5
Layer 4
Layer 3
Layer 2
Layer 1
Layer 7
Layer 6
Layer 5
Layer 4
Layer 3
Layer 2
Layer 1
SenderSender ReceiverReceiver
Message
Defines functions for sharing physical medium and media access.
Ethernet, 10/100 Base T LAN IEEE 802.
Uses Layer 1 as UTP Cat 5 cable.
MAC (Media Access Control) 6 byte address unique for each device.
Sometimes called MAC layer
3. VoIP Concepts
Layer 2 Data Link
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IP (Internet Protocol) actually lower layer neutral.
IP Packet Network, IP defines packet content w/Sender, Receiver address and IPv4 header.
IPv4 Packet Header 20 bytes (five 32-bit words) include: Type of Service byte (Diffserv code point) Header & Packet size Sequence Fragmentation flag, Offset Type of transport (TCP/UDP) Check sums and DLL CRC
Router protocols exist at this layer.
3. VoIP Concepts Layer 3 Network
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Layer 4 defines controls reliability and other route characteristics.
For VoIP TCP (Transport Control Protocol) & UDP (Uniform Datagram Protocol) employed.
TCP “reliable” includes retransmit, used for VoIP network call signaling.
UDP relies on application for transport reliability, Real-time nature of voice suggests use of unreliable transport, UDP for voice packets.
3. VoIP Concepts Layer 4 Transport
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Each session (call or request) carries a unique ID.
RTP (real-time protocol) & RTCP (real-time control protocol), session statistics.
Receiver requests, application processes, e.g. for excessive delay, app could request change codec.
Call (session) control signaling messages: request alerting connect release
ReSerVation Protocol (RSVP), Quality of Service protocol.
3. VoIP Concepts Layer 5 Session
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To conserve LAN/WAN bandwidth, codec applies compression at encoding, common Codecs: G.711, PCM codec, 64 kbps G.726, ADPCM, 16 to 32 kbps G.723.1, MP-MLQ/ACELP 5.3/6.4 kbps G.729A, CS-ACELP, 8 kbps
Codec is selected from “Capability” message, may be predefined.
Silence suppression using VAD (Voice Activity Detection) and CNG (Comfort Noise Generation).
3. VoIP Concepts Layer 6 Presentation
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Inter-active Voice communications: IP Phone call Office to Office FAX IP Conference Calling Voice Mail Unified Messaging, TTS “reads” e-mails
3. VoIP Concepts Layer 7 Application
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Multimedia communication(audio, data & video) over an unreliable network, aimed at IP and UDP.
Application Layer control protocol.
An umbrella standard employs many other protocols by reference and Annex.
Feature Protocol
Call Signaling H.225 (Q.931)
Media Control H.245
Audio Codecs G.711, G.723, G.729
Video Codecs H.261, H.263
FAX T.38
Media Transport RTP/RTCP
Security H.235
Supplementary SVC H.450
4. What is H.323? General
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Four “entities”:
Gatekeeper
Zone controller,
RAS channel (register, status, admit)
Gateway
Provides access between disparate networks
Terminal (IP Phone)
End-point, user interface device
MCU (Multi-point Control Unit)
Conference controller & audio processor
Internet
Zone
GatewaysTerminals
Gatekeeper W/MCU
PSTN PSTN
4. What is H.323? Elements
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RTP
G.711G.729G.723.1
H.261H.263
AudioApps
VideoApps
H.225.0Call
signaling
TCPUDP
IP
Link Layer 802.3
RTCPH.225.0
RASH.245
Controlsignalin
g
T.120Data
Terminal control and management
H.323 StackH.323 Stack
4. What is H.323? Protocol Architecture
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H.3
23 e
nd
poin
t
H.3
23 e
nd
poin
t
Gate
keep
er
setup
ARQACF
ARQ
ACF
alerting
connect
end sessionend session
Release complete
Endpoint gets GK permission (ARQ/ACF)
ARQ request for Fast start to GK with destin
ation & media.
Setup request between endpoints, includes
media and codec.
Other messages (alerting, connect) then ope
n audio channel.
Close audio channel (either endpoint)
Disconnect, return to idle advise to GK.
Audio
DRQ
DCF
DRQ
DCF
4. What is H.323? Call Flow
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RAS = blackH.225 = blueH.245 = green
SIP: Establish multi-media communications sessions IETF’s RFC-2543.
Text based protocol, similar to HTTP, SIP URLs SIP:[email protected].
Low layer neutral, Reliable (TCP) or Unreliable (UDP), packet or byte netwo
rk.
Client/Server Application layer control protocol.
SIP Entities include User Agents and proxy, registrar, location servers, redir
ect server.
Session has unique Call ID (call leg id) with CSeq (command sequence) for
each transaction controlled by a Call Agent.
5. Another VoIP Protocol - SIP General
SIP (Session Initiation Protocol)
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RTP/RTCP
G.711G.729G.723.1
H.261H.263
AudioApps
VideoApps
UDP
IP
Link Layer, 802.3
SIP
Terminal control and management
SIP StackSIP Stack
TCP
SAP/Q.931SDP
5. Another VoIP Protocol - SIP Protocol Architecture
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5. Another VoIP Protocol - SIP Call Flow
INVITE+SDP
200+SDP
ACK
RTP/RTCP
BYE
200
User
Ag
en
t A U
se r A
gen
t B
Proxy
Requests from the User Client include: Invite Ack Cancel Bye Option Register
Response message has 3-digit “Status Code”: 1xx Information 2xx Success 3xx Redirect 4xx Client error 5xx Server error 6xx Global failure
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IP a “best effort” network, packet may be delayed or discarded at router/switch.
Codec, highly compressed voice requires significant processing power and time, DSPs common. 723.1 delay 37.5 msec (30 msec frame + 7.5 msec look-ahead) 729A delay 15 msec (10 msec frame + 5 msec look ahead)
Bandwidth Required = (Packet header + payload). IP/UDP/RTP packet headers = 40 bytes Adding header & payload, 723.1 needs 17Kbps
DiffServ prioritizes packets at layer 3 with TOS byte.
802.1 p/Q, priority & queuing at layer 2, with TOS bit in Ethernet packet.
6. Considerations QoS
QoS (Quality of Service)
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NAT server controls use of scarce Public IP addresses.
NAT generally not compatible with standard VoIP protocols.
H.323 NAT assigns public IP address and port for outgoing request changing ad
dress & port in original packet Far end sends H.245 request to open a logical second port on same IP a
ddress. NAT does not recognize the request on a different port and discards packet as illegal.
6. Considerations NAT
NAT (Network Address Translation)
Internet
LANRouter running NAT
IP = xxx.xxx.xxx.xxx
IP = yyy.yyy.yyy.yy1IP = yyy.yyy.yyy.yy2
IP = yyy.yyy.yyy.yyn
Private IP Address
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Firewalls Prevent unauthorized incoming access from WAN to LAN. Prevent unauthorized outgoing access to WAN from LAN. Typically use Allow/Deny Table of address/port. IP address can be allowed use of defined ports. IP address can be allowed full I/O access. Certain applications, ftp, telnet, etc. use fixed ports.
Most Firewalls support H.323/SIP traffic
6. Considerations Firewalls
InternetLocal Network
Firewall
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Thank youThank you
