1. ITCV 1033 Intro to Voice & Data Networking
VoIP
ITCV 1033
Intro to Voice & Data Networking
Glenn Jones 2015

2. Introduction to VoIP
Voice conversations have typically been carried across the PSTN
PSTN is a WAN (wide area network)
WAN’s use different technolgies than LAN’s
By digitizing and packetizing voice, it can be carried across a data network, such as Internet
A voice application over IP is called VoIP
Glenn Jones 2015

3. Review Question
Explain the meaning of VoIP
Glenn Jones 2015

4. Voice Over the Internet
At first, data was modulated to look like voice
Now, voice is being packetized to look like data
75-90 % of a voice conversation is idle time
Wasted bandwidth and revenue for telco
PSTN boasts % uptime!
This will be difficult to approach with VoIP
Glenn Jones 2015

5. Review Question
What was one of the major accomplishments of the PSTN in its 100+ year history?
Glenn Jones 2015

6. Voice Over Internet Protocol
The PSTN boasts % uptime
It will be difficult to achieve this reliability with VoIP
Fax appears to the PSTN as an analog device
Data transmission via facsimile FoIP is very popular in many industries, such as health care
Many companies use VoIP for internal calls
QoS can be achieved across internal data networks
POTS will continue to be used for many years
Glenn Jones 2015

7. Review Question
Define and describe FoIP
Glenn Jones 2015

8. Common PSTN Features CLID—calling line identification
3-way and conference calling
Call transfer
Voice mail
Voice to text
VoIP must at least match these features
Glenn Jones 2015

9. Quality of Service Voice traffic cannot tolerate significant delays
Voice traffic must be given priority over data
Methods of achieving acceptable QoS
Int-Serv using RSVP (resource reservation protocol); each flow is maintained; not scalable
Diff-Serv using DS bits on each datagram; scalable
IEEE 802.1p standard for layer 2 LAN’s
Glenn Jones 2015

10. Review Question Define QoS and explain its significance in VoIP
Glenn Jones 2015

11. Real-Time Transfer Protocol
RTP carries the data (20ms of packetized voice)
RTCP (control) monitors flow QoS on different port
20ms of voice is the RTP payload (application)
RTP hdr—time stamp, sequence, compression info
UDP header contains source & destination ports
IP header contains source and dest IP addresses
Ethernet header contains source & dest MAC addr
Glenn Jones 2015

12. Review Question
Approximately how much voice conversation is carried in each RTTP packet?
Glenn Jones 2015

13. Delay Max 50ms (1/20th second) acceptable in PSTN
VoIP packets may be buffered until all arrive
Jitter = variation in datagram arrival times
Because of buffering, more delay is acceptable with VoIP than with POTS; up to 150 ms
Glenn Jones 2015

14. Review Questions Compare acceptable delay ~ PSTN & VoIP
Describe jitter
Glenn Jones 2015

15. Voice Gateways Interfaces analog POTS devices with IP networks
PSTN
IP Network
Interfaces analog POTS devices with IP networks
Works for analog telephones and analog faxes
AKA voip gateway, broadband gateway, IP gateway
AKA media gateway (may include video media)
AKA analog telephone adapter (ATA)
VoIP phones have internal gateway functionality
Glenn Jones 2015

16. Review Question
What is a voice gateway?
Glenn Jones 2015

17. VoIP Signaling
Just as in-band and SS7 signaling are used with PSTN, VoIP calls must be set up & managed by
MGCP—Media Gateway Control Protocol
MEGACO—new name for MGCP
Emphasis on standardizing industry equipment
Developed by ITU and IETF
Includes H.248 protocol
H.323
SIP
Glenn Jones 2015

18. VoIP Signaling Protocols
Service providers are “in control” with:
MGCP & Megaco
Protocols developed by / for telco
Master/slave; call agent or softswitch is required
End users are “in control” with these protocols
H.323—dev by ITU in 1990’s; multimedia, robust
SIP—developed by IETF in late 1990’s for voice
SIP & H.323 are peer-to-peer protocols
Glenn Jones 2015

19. Review Question Compare Megaco with H.323 and SIP protocols
Glenn Jones 2015

20. H.323 Application Protocol
Circuit switched PSTN calls have 64kbps
VoIP datagrams are compressed to conserve BW
Silence is removed, results in <10kbps bandwidth
“H” series standards are from the ITU-T
H.323 developed for multimedia—voice & video
Other H protocols deal w/ signaling, security, etc.
Based on session oriented TCP (overhead & delay)
Uses RSVP based on flows (conversations)
Glenn Jones 2015

21. H.323 Networks Required components: Optional components:
H.323 terminal—computer, phone, etc.
H.323 gateway—protocol converter (POTS IP)
Gateway function may be included in intelligent terminals
Optional components:
H.323 gatekeeper—authentication, management
Aka softswitch, voice server
H.323 multipoint control unit; conferencing >2 users G W G W
Term
Term
IP Network
Glenn Jones 2015

22. H.323 Networks With optional components:
Gatekeeper—deals with management functions offloaded from gateways in larger H.323 networks
MCU for audio / video conferencing
Gatekeeper / softswitch / voice server
MCU G W G W
Term
IP Network
Term
Intelligent Terminal
Intelligent Terminal
Glenn Jones 2015

23. SIP Application Protocol
Session Initiation Protocol
Developed by IETF—I’net engineering task force
Developed as a multimedia alternative to H.323
Less complicated than H.323
SIP is a peer-to-peer (client / server) protocol
End devices are UAC (user agent client) or UAS (server)
Depends on which device initiated the call
Can use TCP or UDP at layer 4
Glenn Jones 2015

24. SIP Devices UA (user agent)—IP phone, softphone
UAC (client) initiates (requests) the call
UAS (server) accepts (responds to) the call
Proxy server—makes requests for others
Aka softswitch, voice server
Redirect server—maps and returns addresses
Registrar server—location database
Glenn Jones 2015

25. SIP Devices UA (user agent)—IP phone, softphone
UAC (client) initiates (requests) the call
UAS (server) accepts (responds to) the call
Proxy server—makes requests for others
Aka softswitch, voice server
Redirect server—maps and returns addresses
Registrar server—location database
SIP redirect server
SIP registrar server UA
IP Network
Proxy server / softswitch / voice server UA
Glenn Jones 2015

26. Review Question
What are 2 other names for the gatekeeper (H.323) / Proxy server (SIP)
Glenn Jones 2015

27. RTSP Real Time Streaming Protocol
Allows listening to downloads on the fly
No need to wait until entire file is transferred
Glenn Jones 2015

28. Call Quality and Integrity
Lost data packet can be retransmitted
Lost voice packet will leave space in audio
VoIP devices will need security updates
Authentication may be accomplished by
Encryption—resource intensive; slows process
IPSec
Challenge-response based authentication
Glenn Jones 2015

29. VoIP Security
Firewalls filter packets based on addresses, ports, content, etc.
Firewalls close unused ports to prevent attacks
Corporate firewalls must open the correct ports
VoIP traffic can use multiple, varying ports
Allowing VoIP traffic can be very complex
Glenn Jones 2015

30. BW & Compression Standards
G.711—uncompressed PCM at 64kbps
Codec—coder / decoder
Converts analog to compressed digital signal
G.723.1—compressed to 5.3kbps
G.729—compressed to 8kbps
Overhead is required
Bandwidth depends on compression & overhead
Glenn Jones 2015

31. Review Questions What is the data rate for uncompressed digital voice?
Why is it desirable to compress voice packets?
Glenn Jones 2015

32. Traffic Engineering Analogy—Municipal highway system VoIP
Traffic flows freely most of the time
Busy hours 8AM and 5PM
More lanes are better, but at what cost?
VoIP
Plenty of network bandwidth most of the time
Congestion during busy hours
Design system for optimum performance
Glenn Jones 2015

33. Mean Opinion Score & Delay5
Perfect 4
Fair (PSTN quality)
G.711’s MOS=4.3 3
Annoying
G.723.1’s MOS=3.9 2
Horrible 1
Impossible to communicate
MOS developed by ITU to grade sound quality
Up to 150 ms delay is acceptable with VoIP
Sources of delay include
Processing , algorithmic, packetization, buffering, switching, etc.
De-jitter delay; wait until all packets are in buffer
Glenn Jones 2015

34. MOS—Mean Opinion Scores
5 - Perfect. Like face-to-face conversation or radio reception.
4 - Fair. Imperfections can be perceived, but sound still clear. This is (supposedly) the range for cell phones.
3 - Annoying.
2 - Very annoying. Nearly impossible to communicate.
1 - Impossible to communicate
Glenn Jones 2015

35. Review Questions Define MOS Give an example of a conversation w MOS 5?
What is the MOS of most cell phones?
What is the MOS of analog PSTN conversations?
Glenn Jones 2015

36. Business Considerations
ROI (return on investment) depends on
Hard benefits—cost savings
Soft benefits—increased productivity
Expenses
Hardware and software
IP phones or softphones
Network upgrades
Labor—installation, training, maintenance, contracts
Glenn Jones 2015