1. Voice Over IP Fundamentals
BAI 613

2. Course Objectives PSTN Fundamentals Voice over IP Technology
Quality of Service
VoIP Signaling Protocols
Enterprise VoIP

3. The beginning of PSTN The beginning of the PSTN
First voice transmission in 1876
Used a ring-down circuit
Only one way
Bi-directional evolution
Required
Carbon microphone
Battery
Electromagnet
Iron diaphragm
Cable between each location
N x (N – 1)/2
If 10 locations, 45 pairs of lines needed to run into calling location
Impractical
A central switch allowed location to location connection without multiple lines running to each location
Operated by human operators

4. PSTN Basics (1/15) Analog and Digital Signaling
All sound that you hear is in analog form
Telephony networks originally based on analog infrastructure
Analog communication
Time and amplitude
Requires amplification over long distances
Susceptible to line noise
Amplifiers also amplified line noise
Known as cumulative noise
Digital communication
Based on 1’s and 0’s
Line noise less of an issue
Use repeaters to amplify
Repeaters clean signal to original condition (1 or 0)
When the benefits of this became evident, telephony migrated to PCM (Pulse Code Modulation)

5. PSTN Basics (2/15) Digital Voice Signals
PCM most common encoding of analog signal to digital
PCM uses the Nyquist Theorem
Sampling at twice the highest frequency on a voice line results in good quality voice transmission.
PCM Process
Analog waveform filtered to remove anything greater than 4000 Hz.
Filtered signal is sampled at 8000 times per second
Converted to discrete digital form.
8 bits * 8000 = 64,000 bits per second
Basis for the telephone infrastructure is 64Kbps
2 PCM variations
u-law - America
a-law – Europe

6. PSTN Basics (3/15) Loops, Trunks, and Interswitch Comms Local Loop
Pair of copper wires running to the demark point (eg Your home, business, etc.)
Physically connects simple phone to the central office switch
Class 5 switch or end office switch
Trunk
Communication path between several central office switches
Interswitch Communications
Central office switches interconnect through trunks to tandem switches (class 4 switches)
Higher-layer tandem switches connect local tandem switches

7. PSTN Basics (4/15)

8. PSTN Basics (5/15) PSTN Signaling User-to-network
When using twisted pair, a user connects to the PSTN via analog, ISDN, or T1 carrier
Most common method is Dual Tone Multi-Frequency (DTMF)
DTMF is in-band signaling

9. PSTN Basics (6/15) User-to-Network (cont.) ISDN Out-of-band signaling
Uses a separate channel for signalling
B channel (bearer)
Voice, data, fax
D channel (data or control)
Signaling
Basic Rate Interface (BRI)
Two 64 Kbps B channels
One 16 Kbps D channel
Primary Rate Interface (PRI)
Twenty three 64 Kbps B channels
One 64 Kbps D channel

10. PSTN Basics (7/15) Network-to-network Signaling Usually carried via
T1/E1 over twisted pair
T1 is Mbps, used in North America
E1 is Mbps, used in Europe
T3/E3, T4 over coaxial cable
T3 carries 28 T1’s or Kbps connections at Mbps
E3 carries 16 E1’s or Kbps connections at Mbps
T4 carries 168 T1’s or Kbps connections at Mbps
T3, T4 over microwave link
Synchronous Optical Network (SONET) across fiber
OC-3 – Mbps
OC-12 – Mbps
OC-48 – Gbps

11. PSTN Basics (8/15) Network-to-network (cont.)
Includes in-band signaling methods such as Multi-Frequency (MF) and Robbed Bit Signaling (RBS)
Signaling System 7 (SS7) is most common
Out-of-band
Reduced post-dialing delay
Increased call completion
Connection to the IN (Intelligent Network)

12. PSTN Basics (9/15) PSTN Services and Applications
Common custom calling features
Call waiting
Call forwarding
Three-way calling
CLASS features (as a result of SS7)
Call display
Call blocking
Calling line ID blocking
Automatic callback
Call return (*69)
Inter-exchange features
Circuit-switched long distance
Calling cards
800/888/877 numbers
VPNs
Private leased lines
Virtual cicuits (Frame Relay or ATM)

13. PSTN Basics (10/15) PSTN Numbering Plans ITU-T NANP
International Numbering Plan
NANP
North American Numbering Plan
NPA-NXX-XXXX
NPA – Numbering Plan Area (area code)
NXX – Central Office Code
N is a value between 2-9
X is a value between 0-9
XXXX – Station Number
Some places in the US and Canada require 1+10 digit calling for local calls

14. PSTN Basics (11/15) Drivers Behind Convergence of Voice and Data
Drawbacks to PSTN
Data has taken over as primary traffic on many networks build for voice
PSTN cannot create and deploy features quickly enough
Data/Voice/Video (D/V/V) cannot converge on PSTN as currently built
Architecture built for voice not flexible enough to carry data
Packet Telephony Drivers
Circuit Switching model is breaking into an open standards layered model
Standards-Based Packet Infrastructure Layer
Open Call-control Layer
Open Service Application Layer

15. PSTN Basics (12/15) Packet Telephony Drivers (cont.)
Standards-Based Packet Infrastructure Layer
Based on IP
RTP (Real-Time Transport Protocol)
UDP
Used for transporting real-time traffic
To date all VoIP signaling protocols utilize RTP/UDP/IP
Uses time stamps to determine when a packet is expected, if it was in order, or if it was received
Time stamping helps end stations tune settings to mask potential network problems such as delay, jitter, and packet loss.
Jitter – the variation of interpacket arrival time, or the difference when a packet is supposed to be received and when it is actually received.
Self-Healing
Traffic has multiple paths due to dynamic routing protocols

16. PSTN Basics (13/15) Packet Telephony Drivers (cont.)
Open Call-Control Layer
The process of making a routing decision about where a call needs to go and making the call happen.
Similar to PSTN Call Signaling
VoIP call-control protocols
SIP
H.323
MGCP
H.248/Megaco
Open Service Application Layer
Vendors can release APIs to the products to allow rapid development of applications.
E.g. Displaying stocks and weather information

17. PSTN Basics (14/15) VoIP Call Control Protocols H.323
An ITU-T recommendation that specifies how multimedia traffic is carried over packet networks.
Complex protocol – not created for simple application development
Created to enable multimedia applications to run over unreliable networks
MGCP (Evolution from SGCP and IPDC)
SGCP and MGCP were developed to enable a central device (Media Gateway Controller) to control endpoints.
SIP
Described by RFC 3261
Application-layer control protocol for creating, modifying, and terminating sessions with one or more participant

18. PSTN Basics (15/15) VoIP Call Control Protocols (cont.) H.248/MEGACO
Joint effort of IETC and ITU-T Study Group 16
Exploded H.323’s gatekeeper model and removed signaling from the gateway, putting it in a media gateway controller (MGC)

19. Enterprise Telephony Today (1/2)
Similarities Between PSTN and ET
Circuit Switching
Common Infrastructure Model
Local Loop
Common Services
Differences between PSTN and ET
Signaling
PSTN uses signaling interfaces developed by industry bodies
PBX manufacturers use proprietary protocols to enable features on their equipment
Services Offered
ET requirements are much greater than typical residential users

20. Enterprise Telephony Today (2/2)
Common ET and PSTN Internetworking
ET must eventually interconnect with the PSTN
5 common designs
Simple business line
Uses a line directly from the PSTN as a business line
PBX
A Private Branch Exchange provides many advanced features
Usually connects to the PSTN via a T1 or E1 cicuit
Key-System
Similar to a PBX, with less features
Used typically for 50 people or less
Centrex Line
Provided and managed by the LEC (Local Exchange Carrier) or CLEC (Competitive Local Exchange Carrier)
Virtual Private Networks
The PSTN contains a private dial plan for the business

21. Basic Telephony Signaling (1/4)
Direct Current Signaling
Relies on DC to signal the end switch or office
Toggles on or off the flow of DC
Uses two arrangements
Subscriber Loop
When subscriber goes off-hook, CD -48V flows across the line or loop between telephone and CO
When subscriber goes on-hook, the capacitor in the telephone blocks the flow of current
E & M (recEive and transMit)
Uses a form of DC signaling to indicate state changes on trunks or tie-lines

22. Basic Telephony Signaling (2/4)
In-Band and Out-of-Band Signaling
Single Frequency
Used for interoffice trunks
Has two states:
On-hook or idle
Off-hook or busy
Multi Frequency
Used by interoffice trunks to indicate events
Seizure
Release
Answer
Acknowledge
Transmit information such as calling party number
DTMF
Used to transmit telephone number digits from the subscriber to the local office

23. Basic Telephony Signaling (3/4)
Local-Start and Ground-Start Signaling
Local-Start
Simplest and least intelligent
Works same way as the telephone and local end office
Initiates and closes a call by creating and closing a loop
Glare can occur
Two end points try to seize the line at the same time, resulting in two people being connected unknowingly
Ground-Start
Preferred method for PBX
Provides positive recognition of connects and disconnects
Current detection determines which end initiated a call
Reduces Glare

24. Basic Telephony Signaling (4/4)
Channel-Associated-Signaling (CAS) & Common-Channel Signaling (CCS)
CAS carries signaling info in the voice path itself
Robs a bit from the voice transmission channel (Robbed Bit Signaling)
CCS uses a separate signaling path
Faster and more flexible than CAS
Method used today is known as Signaling System 7 (SS7) or in Canada as CCS

25. PSTN Services (1/3) POTS (Plain Old Telephone Service)
Standard Telephony Service
Dial Tone
Access to national and international carriers
911 service
Custom calling features
Call waiting, call forwarding, etc.
Voice mail
Custom Local Area Signaling Service (CLASS)
Call trace
Auto-callback
Caller ID
Call number blocking

26. PSTN Services (2/3) Business Services Virtual Private Voice Networks
Alternative to Tie Lines
Centrex Services
The Telco contains, maintains, and managing the equipment and all the services
Provides many features found in PBX
Less expensive than maintaining own PBX for small company
Call Center Services
Automatic call distribution (ACD)
Service Provider Services
Database Services
800 number services
900 number services
Calling Card services
Authorization services

27. PSTN Services (3/3) Service Provider Services (cont.)
Operator Services
Toll and Assistance
Directory Assistance
Billing Services

28. Reading Chapter 1 Chapter 2 Chapter 3 Chapter 5
Exclude detailed E&M Signaling
Page 50 – 54
Exclude Page 55-77
Chapter 5