A primer on the network

Dennis Couture

Director – Rural Markets

coutured@nortel.com

Congressional staff briefing June 7, 2005

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The Telecommunications Network

> Our telecommunications network allows us to transmit “information” electronically virtually anywhere.

> “Information” takes many forms, each with different characteristics• Voice • Email• Broadcast video • Real-time video • Gaming

Mixing traffic requires solid engineering

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Traffic types and characteristics

Bandwidth Delay Tolerance

Error Tolerance

Voice Low Low Medium

Email Low / Medium High Zero

Broadcast video High High Medium

Real-time video Medium / High Low Medium

Gaming Low / Medium Low Low

The more stringent the demands,the more expensive the network is to build and

to maintain.

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What is voice?

> Voice is audible human communication• Content is concentrated at frequencies below 1KHz• Nuance and voice uniqueness reside above 1KHz• Traditional telephony transmits a maximum of 4KHz (NOT Hi-Fi)

> Other characteristics of voice:• Voice communication is usually half duplex (“wasteful”)• Voice is sensitive to delay

• Echo• Delayed “half duplex” speech (“Over to you, Bill!)

• Voice is tolerant of interference• The human ear/brain combination is a great receiver• Analog noise is filtered out and ignored• Digital bit errors have virtually no impact on intelligibility

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Sound Waves

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A real speech signal

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Analog Telephony

• Electrical signal mirrors the voice signal• Objective is to accurately transport this signal• 4KHz signal allows speaker recognition• Common impairments include:

– Loss– Noise– Distortion– Delay

• Impairments can accumulate t

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> Analog signal is “sampled”• 8,000 samples of 8 bits every second• 64Kbps required to carry normal voice

> Samples are transmitted digitally

> Signal is reconstructed by connecting the dots

> Impairments are minimized• Loss• Noise• Distortion• Delay

> Impairments do not accumulate

Digital Telephony

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At this point, the voice has become a stream of data!!!

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Digital Formats

> This 64 Kb format is called Pulse Code Modulation (PCM)• 64 Kb is a slow bit rate

> Multiple 64 Kb channels can be combined• This is called Time Domain Multiplexing (TDM)• Typically 24 channels are sequenced in a 1.544 Mb signal• These are called DS-1 or T-1 circuits• Think of loading a conveyor belt into pre-assigned slots

> Multiple DS-1 channels can then be combined • DS-3 carries 28 DS-1 channels or 672 voice channels• Optical systems go much higher• OC-192 carries 129,024 voice channels

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Let’s talk about transport

> Transmission facilities carry communications from point to point• Access facilities reach out to end users• Interoffice facilities (trunks) connect internal parts of the

network

> A tremendous amount of telephone company investment lies in transport (much more than switching)• Procurement and installation of cable facilities• Terminal equipment at each end of the facility• Ongoing maintenance costs

> These facilities deliver high bandwidths in a reasonably secure manner

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Fiber Transmission

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Fiber Optics

> Light is pulsed on and off very rapidly• Think naval flashing lights!!!

> Fiber optic systems provide:• Very high capacity

• FASTER - Bit rates are very high• LONGER - Distance between repeaters is increasing• WIDER - Multiple “colors” can ride on the same fiber

• Secure transmission• Excellent reliability• Excellent economics

Fiber optic systems carry the vast majority of long distance voice and data traffic

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Increasing optical bandwidth with Wave Division Multiplexing (WDM)

WDMSimilar to Adding Lanes

TDMExploits Higher Speed

Up to 72 “colors” of light share the same fibercapable of 9,289,728 simultaneous calls!

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• DWDM Upgrade (Dense Wave Division Multiplexing)

•Supports OC-48 and/or 192 line rates

•Provides “effective” line rates of Terabits per second.

Add more lanes to the highway

Channel 2

Channel 3

Channel n

Channel 1

OpticalFiber

DWDM

Coupler

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Enough of the preliminaries…..

How does the network work?

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Network topology

> The network architecture physically resembles a highway or train system• It consists of network nodes (cities and towns) interconnected

with transport facilities (roads or tracks)

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A SONET Network in the D.C. areaSynchronous Optical NETwork

Looks a lot like the highway system!!!

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Meanwhile at the edge of the network…..

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Increasing looplength

> Loops connect your telephone to the network

> Analog transmission

> Long loops (> 18 Kft.) deteriorate the signal• Loss, Frequency Distortion, Noise

> Load coils can correct this distortion• Cut off high frequencies• Preclude use of DSL

Telephone LoopsTwisted Pairs

^4 Khz

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> Short loops (no load coils) are now standard • Less costly to build and maintain• Allows higher frequencies, so as to carry data• DSL can be provisioned in a CSA

> High speed links connect the Digital Loop Carrier (DLC) to the central office• May be copper or fiber fed

Carrier Serving Areas

CarrierServingAreas

DLC

DLCCentral Office

Local Serving Area

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$$

Today’s investment supports future services.

> No VoIP in this picture

> Common facilities to the remote unit• Mixed TDM and IP traffic

> Loop carries analog voice plus IP

> Could add Broadband Voice via the PC or an IP Phone

> Analog phone service survives power outages

Mixed Service Delivery With a typical DLC

NGDLC

DSLAM

PSTN

DSL

TDM GR-303

IP

IP

AnalogVoice

RTP/IP

TCP/IP

TDMSwitch

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Mixed Service Delivery With BBDLCand a TDM switch

> Common facilities to the remote unit carry only IP traffic> Loop carries analog voice plus IP> BBDLC converts analog voice to IP> Could add Broadband Voice via the PC or an IP Phone > Analog phone service survives power outages

IPGateway

PSTN

IP

IP

AnalogVoice

DSL

RTP/IP

TCP/IP

BBDLC

TDMSwitch

VoIPMGCP

MEGACO/H.248

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Triple Play> Why do Triple Play????

• One method of extracting more revenue from a DSL line• Trying to improve a marginal business case for Internet Access

• A significant defensive move against the cable threat

> Triple play is a big step• Content acquisition is an important issue• Own or share a head end?• Buildout of access network can be costly

> Impact of HDTV must be considered• Bandwidth requirements / channel capacity• MPEG-4 equipment

> Video on Demand is a significant revenue opportunity

The demand for more advanced video-related capabilities will continue to rise.

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Ah, but where do the roads go?

> Roads interconnect towns and cities

> Access facilities connect users to their local central office

> Trunk facilities connect central offices to each other

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Switching Systems

> A digital switch:• Converts voice into a digital format• Interprets the dialed digits• Routes the call to its destination by:

• Completing local calls• Transferring all other calls to another switch for completion

Central Office

Central Office

DLC

Trunks

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North American Numbering Plan

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Numbering Issues

> In the wireline world, an area code defines a geographic area• Mobile phones and some VoIP services break this model• This is the source of the E911 problem for both VoIP and

cellular

> Geographic and Overlay area codes• 7 vs. 10 digit dialing

> Toll free increase (888, 877 etc.)

> Depletion of number inventory• Explosion of cell phones, pagers, fax machines, 2nd lines• Recovery program underway (000’s block pooling)

> Local Number Portability• Support of a competitive marketplace

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> The method of controlling the routing of a call• Uses in-band tones and computer communications

> Dual Tone Multifrequency (DTMF) i.e. TouchTone® in-band signaling

> SS7 (Signaling System 7) (out of band)• Very fast and reliable• Sets up calls via Signal Transfer Points• Performs special data retrieval

From service Control Points• 800 number translation

> Call Progress Tones• Busy (60 IPM)• Reorder (120 IPM)• Special Information Tones (SIT)

> VoIP essentially uses in-band signaling• Security and reliability issue

Signaling

Central Office

Central Office

STP

SCP SCP

STP

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Old-Fashioned long distance

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555 ... ...

...

...

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EndOffices

TandemOffices

AT&TLong Lines

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Modern long distance> The network is “flatter”, with multiple LD Carriers

> The Local Access and Transport Area defines a Bell Company’s operating area.

> Rural telephone companies may connect directly to LD carriers but often connect to the closest Bell tandem switch

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55 ...

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AT&T

MCI

Sprint

:

LATA LATA5

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Operator Services

> Human intervention assists in call processing• Collect calls• Bill to 3rd party• International calling assistance• Directory Assistance• Emergency call transfer

> Dial “O” for local operators

> Dial “OO” for LD operators

TOPS*Tandem

EndOffice

EndOffice

* TOPS – Traffic Operator Position System – a Nortel product

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Let’s talk Wireless!!!!> Wireless communications are booming

> Most of the world utilizes the GSM radio format

> In North America, multiple radio formats in use:• CDMA, GSM, AMPS, TDMA• Impacts the development of wireless devices• GSM and CDMA formats are incompatible

((( )))

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Base Station

Controller Cell sites

Mobility Switching

System

Only a small portion of the call is carried over wireless

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Why is it called “cellular”?> A cellular radio has limited range

> As a user moves out of range, the call transfers to a different tower (a ‘handoff” that is often the source of dropped calls)

> The area served by a tower is a “cell”

> Cells are laid out in a honeycomb pattern and can be subdivided into new cells as traffic increases

> Adjacent cells use unique frequencies to avoid interference requiring the phone to change channels

> Cells can also be “sectorized” to increase capacity

((( )))

((( )))

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Wireless to the world> Mobile calls are wireless only at the edge

> The remainder is handled exactly like a wireline call

Base Station

Controller

Cell sites

LD #1

LD #2

LD #3

:

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LATA

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Mobility Switching

System

…

Today’s network Wireless network

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Wireless Communications> More than just PCS/Cellular Service

> Wireless data is becoming more and more important• WiFi technology is spreading rapidly• Spokane and Philadelphia city-wide• Walla Walla rural network (1500 square miles)• Small hot spots (McDonald’s, Starbucks)• Home networking

> WiMax will emerge to compete with WiFi• More than just range extension• Adds Quality of Service (QoS) assurance• Terminal pricing will be critical

> Security is an issue – war chalking

Convergence of wired and wireless on an IP network is the future!

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What is the Internet?> The Internet is a large computer network

• Originally designed to allow communications in times of nuclear war – DARPA design

• Largely a self-healing network

> All data transmission is packetized• Utilizes IP (Internet Protocol) routing• Packets are switched by routers

> The most popular applications are:• Email• The World Wide Web

• Access the web using Modems, LANs, xDSL, cable etc.• Web content is stored in servers at web sites

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What is IP?• Internet Protocol is the grammar of the Internet

• A standard method of computer communication• Utilizes a unique 4-byte addressing scheme (e.g. 47.11.97.255)• Sends variable length packets

• Packets are delivered on a “best efforts” basis• Packets are often lost• Packet delivery times are relatively slow and quite variable

• Error detection and correction are the responsibility of end users• TCP (Transmission Control Protocol) does this• Error correction is accomplished by retransmitting missing or

damaged packets• Real-time applications typically do not correct errors

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The Internet’s dirty little secrets

> The Internet was never designed as a real-time delivery system.• The transit time for individual packets can vary substantially• This variance is called jitter and effectively adds delay to real-

time applications – e.g. VOICE

> Traffic capacity is relatively poorly engineered, particularly at the edges• IP doesn’t handle congestion well• Delay increases the jitter limits• IP will drop packets under congested conditions

Delay and packet loss are the two major causes of voice quality deterioration in VoIP calls

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Packetizing voice (VoIP)

> The format for carrying voice is changing to a packetized format, primarily using IP• Voice can be transmitted much like data is over the Internet

> Maintaining voice quality is a big issue• Control of delay is essential• Packet loss must be minimized• Compression has some disadvantages

> “Voice over packet” and “Voice over the Internet” are not exactly the same

> Traditional TDM voice will be around for years

> Interworking of VoIP and TDM is mandatory

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The role of VoIP> VoIP is many things to many people

> To the Enterprise:• A vehicle for major expense reduction• A key to improving productivity through feature integration

> To the Carrier:• A vehicle to lower LD costs, enhance CLEC reach and build unique

customer networks• Eventually, the means to operate one network, eliminating TDM

> To the Consumer:• Today, primarily a method of buying cheaper telephone service

It will take many years for VoIP to totally replace TDM; but high value subscribers will adopt the

technology sooner.

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How does IP telephony work?(IP in the middle)

GatewayGateway

“Gatekeeper”

PSTN LocalExchange

PSTN LocalExchange

“Gatekeeper”

IPIP

SS7SS7Gateway

TDMTDMTDMTDM

ParisNew York LegendSS7 ProtocolBearer PathInterworking ProtocolIP Control Protocol

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How does IP telephony work?(Totally IP)

GatewayGateway

“Gatekeeper”

PSTN LocalExchange

PSTN LocalExchange

“Gatekeeper”

IPIP

SS7SS7Gateway

TDMTDMTDMTDM

VoIP lineVoIP line

LegendSS7 ProtocolBearer PathInterworking ProtocolIP Control Protocol

DSL over existing copper loops

DSL over existing copper loops

Telco provides

the link to the Internet

Telco provides

the link to the Internet

Telco provides

the link to the Internet

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How does IP telephony work?(IP to PSTN)

GatewayGateway

“Gatekeeper”

PSTN LocalExchange

PSTN LocalExchange

“Gatekeeper”

IPIP

SS7SS7Gateway

TDMTDMTDMTDM

Sydney

VoIP line

LegendSS7 ProtocolBearer PathInterworking ProtocolIP Control Protocol

VoIP line

Common Transport

Telephone company

terminates the call

Telco provides

the link to the Internet

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An architecture for evolution

IPGateway

PSTN

IP

IP

AnalogVoice

DSL

RTP/IP

TCP/IP

BBDLC

Hybrid Switch

Legacylines

SIPTA

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Eventually the hybrid becomes a softswitch.

ApplicationServer

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