Chapter 6

Modern Telecommunications Systems

Introduction

• Telematique – the integration of computers and telecommunications systems

• Computers are changing roles from computing machines into communications machines

Telecommunications

• The science and technology of communication by electronic transmission of impulses through telegraphy, cable, telephony, radio, or television either with or without physical media

• Tele is Greek for distance

• Communicate has its roots in the Latin word “to impart”

Voice Networks

• Interactive - Bidirectional networks that provide on-demand communication

• The first telephone networks were deployed widely following World War II

• By the late 1950s in the United States, telephones were a permanent fixture in most homes

Circuit Switched Networks

• Telephone networks use circuit switching that creates a complete, dedicated, end to end connection before voice data begins to flow

• Circuit creation results in exclusive allocation of specific data transmission resources for the duration of the call

Circuit Switching

• Guarantees that each successful connection owns all the resources necessary to deliver a high quality link

• When the call ends, the circuit is torn down, and the resources are freed; these resources can then be utilized for a new connection

Switched Network

• It is the capacity of the network to interconnect any two endpoints

Legacy

• The telephone network is one of the largest legacy systems ever created and maintained

• Phone handsets over 50 years old can still interoperate seamlessly with current equipment

• Some basic design specifications date back to the early 1900s

Telephone Signals

• Original telephone specifications were based on analog signal technology

• Analog signals vary in amplitude (signal strength) and in frequency (pitch)

• The telephone handset converts sound into continuously varying electrical signals with the microphone

• The speaker at the other end converts electrical signals back to sound

Analog Signal

Digital Signals

• These signals are discrete and discontinuous

• They exist in predetermined states

• Binary signals are digital signals limited to only two states, 0 and 1

Digital Signal

Multiplexing

• Multiplexing is subdividing the physical media into two or more channels

• Telephone lines use frequency multiplexing to carry both voice and DSL signals simultaneously

• The frequencies between 0 and 4000 Hz carry voice, and those between 25 kHz and 1.5 MHz carry DSL

Digitizing Voice Signals

• By converting analog voice signals into a digital format, voice can then be processed like other digital data by computers

• The economies of Moore’s law and semiconductor economics can be brought to bear on voice applications

Pulse Amplitude and Pulse Code Modification

Analog to Digital Conversion

• Generally a two step process– First, the analog signal is sampled at regular

intervals; measurements taken at these periods are converted to a discrete value

– Second, the discrete values are converted to a binary format; this is called pulse code modulation

Fidelity

• Translating a signal from analog to digital format results in loss of data. By increasing the number of discrete values produced per second (sampling more often) and increasing the range of discrete values produced by sampling, the digitized waveform more closely represents the analog original. This is fidelity.

Nyquist’s Theorem

• A mathematical formula that will quantify the fidelity of the signal given the rate and resolution of sampling

• For a 4000 Hz signal, fidelity will be acceptable if the signal is sampled 8000 times per second with a resolution of 8 bits per sample

• A 4000 Hz signal is equivalent to a 64000 bit per second data stream

The Digital Telephone

• When a voice signal enters the local switch, it is digitized

• The local switch is located physically close to the end users of the telephone line (usually within 10000 ft)

• The switch is capable of handling 500 to 1000 copper lines

• It is connected via high speed digital links back to the central office

Central Office

• Handles the telephone traffic for a number of small communities or a small city

• Commonly central offices are responsible for 100000 lines

Central Office Network Configuration

Customer Premise Equipment

• CPE is the device found at the customer termination of a telephone connection (fax, telephone, modem, etc.)

Local Loop

• Also known as the access line

• Identified by the last four digits of the telephone number

• It is the physical connection between the CPE and the local switch

• The first three digits of a seven digit telephone number identify the local switch to the central office

Local Switch

• A local switch is a smart router. It can independently connect calls from any two lines terminating directly into it.– This helps to keep local calls confined to the

local switch

• It identifies and routes outbound calls quickly to the central office

Topology

• Topology is the configuration of elements in a network

• The local exchange (local switch and all attached CPE and trunks) form a switched star network

• This is an effective arrangement when most of the lines are idle at any one time

• At peak hours 15% of a given set of lines are in use

Regional Connections

• A Central Office is connected to other Central Offices by high speed links; it also has connections to other higher level centers and long distance networks

• These links in the US form a network of 150 million lines

Regional Telephone Switching Networks

Call Setup

• When the handset is raised, the local switch issues a dial tone

• When the user inputs the destination phone number, the local exchange uses it to set up the circuit

• A leading 1 signals the local switch that the call is long distance and routes the call immediately to the Central Office

T-Services

• T-services are high speed digital links using time-division multiplexing (TDM) to move multiple signals

• TDM successively allocates time segments on a transmission medium to different users

• It combines multiple low speed streams into one high speed stream

T-1

• The T-1 line is capable of carrying 1.544 Mbps

• The T-1 frame is composed of 24 time slices. Each time slice is a channel. Each channel is capable of carrying one phone circuit.

Time-Division Multiplexing and the T-1 Frame

T-1 Frame

• Multiplexing equipment aggregates the incoming individual channels and constructs a frame

• Each channel can transmit 8 bits per frame• Each frame contains 24 channels and one

“framing” or start bit• 8000 frames are transmitted per second

yielding 1.544 Mbps

The T-Service Hierarchy

• The T-1 connection is composed of 24 channels called B channels

• They are able to carry the digitized audio data for one voice circuit

• A T-1 connection can carry 24 Bs

• A T-3 connection can carry 672 Bs (45 Mbps)

T-Services

E-Services

• Europeans use a slightly different standard called the E series

• 8000 frames per second with each frame composed of 32 channels

• Only 30 of the channels can be used for data, the other two are reserved for signaling information and signaling the framing start sequence

• Carries 2.048 Mbps

Corporate Use of T-Services

• T-services are available to customers

• T-lines can be configured to create a high speed private point-to-point network

• Internally, data and voice can be mixed, so that a T-1 line can be provisioned to carry 12 voice circuits and 12 data circuits

• T-1s allow rapid connection of fixed locations with high speed private links

Data Communication Networks

• Voice networks have hard requirements for network latency (the amount of time needed for data to move from one end to the other)

• Data that arrives late or out of order is worthless

• Pure data networks have looser time constraints opening the door to different topologies and technologies

Packet Switching

• In traditional voice networks, circuits are established that provide for a continuous stream of data; packet switching takes outgoing data and aggregates it into segments called packets

• Packets carry up to 1500 bytes at a time

• Packets have a header prepended onto the front of the packet that contains the destination address and sequence number

Packet Routing

• In circuit switched networks, the entire data pathway is created before data transmission commences; in packet networks, the packet travels from router to router across the network

• At each router, the next hop is chosen, slowly advancing the packet toward its destination

Packet Routing

• Given moment to moment changes in network loading and connections, packets may or may not take the same route

• In taking different routes, packets may arrive in a different order than the order they were transmitted

• The destination uses the sequence number in the header to reassemble the incoming data in the correct order

Local Area Networking

• Until the 1990s, local area networking used vendor specific protocols that made interoperability difficult

• With widespread deployment of personal computers, networking to the desktop became more imperative for companies, so that they could fully leverage their IT infrastructure investments

Metcalfe’s Law

• Robert Metcalfe is the patent holder for Ethernet networking

• He asserted that the value of a network increases as a square function to the number of attached nodes

OSI Model

• OSI was the Open System Interconnection model that attempted to modularize and compartmentalize networking interfaces

• The result was a seven layer model• As data passes down from layer 7 to layer 1 it

is broken into smaller pieces and encapsulated with wrappers of additional information used at the corresponding layer by the recipient to reconstruct the original data and destination

Open System Interconnection Model

OSI is a Model

• OSI was intended to be the final structure and framework for global networking

• Widespread implementation of the entire OSI model has never taken place– It took years to develop– It was the product of a committee– It was extremely rigid

ARPANET

• In the early 1970s, the Department of Defense saw the need to make heterogeneous networks of information systems communicate seamlessly

• They needed networks that were self healing and had a distributed intelligence

• ARPA (Advanced Research Projects Agency) took the OSI layering concept and built an operational system with layers 3, 4, and 5 only

The Internet

• From this nucleus of networked machines grew the Internet

• ARPA called the OSI layer 4 protocol TCP (Transmission Control Protocol) and layer 3 IP (Internet Protocol), hence the Internet networking standard TCP/IP

• This has become the de facto global standard, and OSI has been relegated to a reference model

Internetworking Technology

• The Internet Protocol Suite is a group of helper applications that standardizes interactions between systems and assists users in navigating the Internet

• These helper applications work at many different levels of the OSI model from seven all the way down to two

Internet Protocol Suite

• Layer seven applications include– FTP – File Transfer Protocol– HTTP – HyperText Transfer Protocol– SMTP – Simple Mail Transfer Protocol

• Layer two protocols include– ARP – Address Resolution Protocol

Internet Protocol

• The Layer three protocol is responsible for the standard dotted decimal notation used for computer addressing– Each machine has a unique address specified

by a set of four numbers ranging from 0 to 255

– These numbers are separated by decimal points in the format 216.39.202.114

DNS

• Domain Name System– A distributed database that contains the

mappings between IP numbers and human readable naming

– DNS is also a Internet Protocol Suite helper application

– DNS takes a request for www.yahoo.com and returns the corresponding IP address

Domain Names

• Composed of a hierarchical naming database

• Moves from general to specific in a right to left manner

• The rightmost element of the name is called the Top Level Domain (TLD)

• TLDs can be country codes, organizations (.org), commercial (.com), and others

Communication Between Networks

• Layers 1 and 2 are used for the transmission of data packets between routers

• Layer 1 – The Physical Layer– Specifies voltage parameters, timing signaling

rates, and cable specifications

• Layer 2 – The Data Link Layer– Describes how data is formatted for transmission

across a specific type of Physical Layer link

Physical Layer Technologies

• Transmission links can be built using either conducting or radiating media– Conducting media create a direct physical

connection between network components like copper wire or fiber optics

– Radiating media uses radio waves to link stations together

10 Base T

• The most common Ethernet based wiring standard

• Uses 8 stranded wire links• These wires are similar in size to telephone

wire and use slightly larger modular plugs• Carries data signals at 10 Mbps to 1000

Mbps over distances up to several hundred meters

Coaxial Cable

• Useful to carry signals over distances up to several miles

• Diameter of coax ranges from 1/4th inch to one inch

• Inner wire surrounded by a foam insulator, wrapped by a metal shield and covered with an external insulator

Coaxial Cable Construction

Optical-Fiber Media

• Used in new installations instead of coax

• Capable of carrying extremely high rates of data over distances exceeding 100 miles

• Constructed of a glass core covered with plastic cladding and bundled with a tough external sheath

Construction of Optical-Fiber Cable

Transmission Modes

• Multimode – uses internal reflectivity of the cladding to propagate the signal down the fiber

• Graded Index – the glass’s refractive index varies from the center to the edge, causing the light to bend back toward the center

• Single Mode – no reflection or refraction, light travels down the center of the fiber like a wave guide

Wavelength Division Multiplexing

• Multiple different data streams are sent at the same time down the same fiber. Each stream is on a distinct color of light.

• A wavelength is also called a lambda– Multiplexing hundreds or thousands of

wavelengths down a single fiber is called Dense Wavelength Division Multiplexing (DWDM)

Advanced Fiber Transport

• Due to low installation costs and high data capacity, optical fiber is the medium of choice for new buildings

• Fiber has the flexibility to carry voice, data, and video with no change to the installed fiber base

FDDI

• OSI layer 1 and 2 specification

• Used when building high speed redundant metropolitan area data networks

• Employs two unidirectional rings so that any cable cut can be “healed” by looping data back onto the other ring

FDDI Network Configuration

SONET

• Synchronized Optical NETwork

• Set of standard rates for high speed data transmission

• STS stands for Synchronous Transport Signal (SONET over copper)

• OC stands for Optical Connection (SONET over fiber)

• STS-1 and OC-1 rates are identical

OC Line Rates

OC-1 SONET Framing

• A SONET frame is made up of a 9 bit x 90 byte block of data (6,480 bits total)

• The frame rate is 8000 per second yielding a data rate of 51.84 Mbps

• For higher OC or STS levels, the frame rate is multiplied by the trailing number (i.e. OC-3 is 8000 x 3, OC-12 is 8000 x 12)

Frame Relay, ATM, and Gig-E

• These technologies represent newer frame based networking standards that are able to deliver high speed, low latency connections

• Use frame-based protocols and star topologies

ATM Cells and Frame Relay Packets

The Last Mile

• High speed global networks are of little value if individual access is unavailable

• WANs terminate locally at POPs (Points of Presence)

• For businesses, T-1 connections are a common solution to the last mile; T-1s are expensive to setup and require long term contracts

Digital Subscriber Lines

• DSL enables regional phone providers to deliver digital connectivity to customers over existing copper connections

• At the local switch, an additional network unit is installed called a DSLAM (Digital Subscriber Local Access Multiplexer)

• The DSLAM injects and extracts the DSL information into the copper line

DSL

• On the customer side, a modem/router is attached to the line, injecting and extracting the DSL signals

• DSL connections from the customer to the local switch is limited to 3.5 miles

• 80% of phone subscribers in the US are currently within these boundaries

Digital Cable

• 60% of US homes and businesses are accessible to cable broadcasters

• Cable initially was designed for one way content delivery

• In the 1990s, systems were upgraded to deliver interactive programming and digital data access

Digital Cable

• The highest margin, fastest growth sector of the cable industry is cable-based Internet access

• Cable providers piggyback a 5 – 10 Mbps digital backbone onto existing broadcast spectrum

• Home users attach specially constructed “Cable Modems” (routers) to interface home systems to the cable data feed

Voice Over Cable

• Cable operators want to bundle more services for customers

• Delivery of telephone connectivity over cable systems is an additional service they can provide

• This service will require additional capital outlays to provision customers at a time when “growth at any cost” is not a viable business strategy

Wireless Systems

• Licensed wireless – Includes cellular voice and data networks

• Unlicensed wireless – ad hoc networking technologies like 802.11b and 802.11g

• Both these technologies enable consumers to have untethered, mobile connectivity bringing networking to the consumer instead of making the consumer find the network

Licensed Wireless

• Cellular service first began in the early 1980s

• It has grown at a 30% compounded rate over the last decade with penetration of 50% across the US

• Cellular systems are dense networks of low power broadband radio transmitters and receivers

Cellular Network Architecture

Cellular Standards

• 1 G Systems– AMPS – Advanced Mobile Phone System

• 2 G Systems– CDMA – Code Division Multiple Access– TDMA – Time Division Multiple Access– GSM – Global System for Mobile Communications

• 3 G– W-CDMA– IMT-2000

Unlicensed Wireless

• 802.11.b – An Ethernet networking standard that replaces layers 1 and 2 with a wireless equivalent

• 11 Mbps network connectivity over a 50m radius

• No transmitter license is necessary so it is inexpensive for consumers with little setup or administration costs

Summary

• Advances in semiconductor technology have enabled enormous advances in telecommunications systems

• Rapid change is occurring in this field, and seems set to change how individuals and organizations grow, act, and react