Voice Over IP Fundamentals

BAI 613

Course Objectives

• PSTN Fundamentals• Voice over IP Technology• Quality of Service• VoIP Signaling Protocols• Enterprise VoIP

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

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)

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

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

PSTN Basics (4/15)

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

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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

PSTN Basics (7/15)– Network-to-network Signaling

• Usually carried via– T1/E1 over twisted pair

» T1 is 1.544 Mbps, used in North America» E1 is 2.048 Mbps, used in Europe

– T3/E3, T4 over coaxial cable» T3 carries 28 T1’s or 672 64 Kbps connections at 44.736

Mbps» E3 carries 16 E1’s or 512 64 Kbps connections at 35.368

Mbps» T4 carries 168 T1’s or 4032 64 Kbps connections at

274.176 Mbps– T3, T4 over microwave link– Synchronous Optical Network (SONET) across fiber

» OC-3 – 155.52 Mbps» OC-12 – 622.08 Mbps» OC-48 – 2.488 Gbps

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)

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)

PSTN Basics (10/15)• PSTN Numbering Plans

– ITU-T• 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» X is a value between 0-9

• Some places in the US and Canada require 1+10 digit calling for local calls

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

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

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

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

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)

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

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

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

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

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

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

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

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

PSTN Services (3/3)– Service Provider Services (cont.)

• Operator Services– Toll and Assistance– Directory Assistance– Billing Services

Reading

• Chapter 1• Chapter 2• Chapter 3

– Exclude detailed E&M Signaling•Page 50 – 54

– Exclude Page 55-77

• Chapter 5