Download Session 13

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
• 3G
– 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