Transcript Computer Networks and Internets

Ch 12 Long-Distance and Local Loop
Digital Technologies
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Motivation
Connect computers across
Large
geographic distance
Public right-of-way
 Streets
 Buildings
 Railroads
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Digital Telephone
Analog signals degrade as they pass over
copper wires
 Amplifier along a path distorts the signal
slightly and introduce noise
 Digitalization: convert an analog signal to
digital form
 Analog-to-digital (AD) converter

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Digital Telephone
Designed for
use in voice system
Analog
audio from use’s telephone converted to
digital format
Digital format sent across network
Digital format converted back to analog audio
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Illustration of Digitized Signal
coded digitally as
000-010-100-100-111-001-001-001-001-001…
Pick neatest
digital value for each sample
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Illustration of Digitized Signal

Nyquist’s Sampling Theorem

If a continuous signal is sampled at a rate greater
than twice the highest significant frequency, the
original signal can be reconstructed from the
samples
Human voice up to 4k Hz
 Telephone standard known as Pulse Code
Modulation (PCM)

Sample rate 8k Hz
 Sample value in range 0 to 255

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Long-Distance Transmission
Technologies
General solution: lease
transmission facilities
from telephone company
Point-to-point
topology
NOT part of conventional telephone system
Copper, fiber microwave, or satellite channels
available
Customer chooses analog or digital
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Equipment for Leased Connections
Analog circuit
Modem
required at each end
Digital Circuit
DSU
/ CSU required at each end
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Digital Circuit and DSU/CSUs
Needed because telephone industry digital
encoding differs from computer industry
digital encoding
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Digital Circuit and DSU/CSUs
Channel Service
Handles
Unit (CSU) portion
line termination and diagnostics
 Line
connection test
 Loop-back capacity when installing and testing
 Prohibits excessive consecutive 1’s (non-zero voltage for
1’s) to prevent excessive current over the copper
 Balanced encoding scheme: success 1 bits alternating
between +3V and -3V
Data
Service Unit (DSU) portion
Translates
data between two encodings
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Telephone Standards
Cost
of digital circuit depends on
Distance
Capacity
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Common Data Rates
Most
common in North America
T1
circuit
T3 circuit (28 times T1)
Also available
Fractional
T1 (e.g., 56 Kbps circuit) since T1 is too
expensive to private individual
The phone company uses TDM to subdivide a T1
circuit
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Inverse Multiplexing
 Combines
two or more circuits to produce
intermediate capacity circuit
 A DSU/CSU is required at the ends of each T1 circuit
 How to efficiently use the bandwidth?
 Proxy
 Load
balancing
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Highest Capacity Digital Circuits
Also available from
phone company as the
major trunk connections across the country
Use optical fiber
Electrical standards called Synchronous
Transport Signal (STS)
Optical standards called Optical Carrier (OC)
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Highest Capacity Digital Circuits
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Local Subscriber Loop
Telephone terminology
Refers
to connection between residences /
businesses and the phone company central
office (CO)
Crosses public right-of-way
Originally analog
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Local Subscriber Loop
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Digital Local Loop Technologies
Integrated Services Digital Network
(ISDN)
Provides
digitalized voice and data over
conventional TP local loop wires
Provides simultaneous voice / data services
Basic Rate Interface (BRI) offers three separate
(2B+D) digital channels
B
channel: 64k bps for voice/data
 D channel: 16k bps for controlling purpose
ISDN
now is an expensive alternative that offers
little throughput
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Digital Local Loop Technologies
Asymmetric
Digital Subscriber Line (ADSL)
Runs
over conventional POTS wiring
Provides simultaneous voice / data services
Higher capacity, but asymmetric
 Downstream
can reach 6.14 Mbps
 Upstream can reach 640 Kbps
Optimized
for typical users who receive much
more information than they send
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Illustration of ADSL Wiring
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Illustration of ADSL Wiring
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Illustration of ADSL Billing
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Other DSL Technologies
Symmetric Digital Subscriber Line (SDSL)
Uses
a different encoding scheme than ADSL
Can operate over local loops for which ADSL is
inappropriate
Business policy
High-Rate Digital Subscriber Line (HDSL)
Provides
1.544M bps in two directions
Short distance limitation
Requires two independent TPs, but can tolerate
failure gracefully (i.e., operate at one-half rate)
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Cable Modem Technology
Source: IEC On-Line Education
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Cable Modem Technology
Wiring (e.g., TPs) places an
upper bound on
how fast data can be transferred
Deliver over existing CATV coaxial cable
Cable system has unused bandwidth
Group of subscribers in neighborhood share
the downstream bandwidth (up to 36 Mbps) by
FDM
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Cable Modem Technology

Upstream communication
Dual path approach in dial-up modem without
changing the existing cable system
 Modification to the basic infrastructure (e.g., HFC)


VOD services beyond computer network connections
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Hybrid Fiber Coax (HFC)
Neighborhood
area
Trunk
FDM
TDM
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Hybrid Fiber Coax (HFC)
 Allow
2-way communication across a cable system
 Optical fiber
 Highest
bandwidth
 Replaces trunk from CO to neighborhood concentration
points
 Coaxial
cable
 Less
bandwidth
 Extends from neighborhood concentration point to individual
subscribers (feeder circuit)
 Replaces
all amplifier / modem with 2-way devices
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Hybrid Fiber Coax (HFC)
 Combination
of FDM & TDM
 5-50
MHz : upstream
 50-450 MHz : analog TV(6M/ch)
 450-750 MHz : downstream
 The
group shares one carrier frequency with TDM
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Fiber To The Curb (FTTC)
(人行道的)路緣
Source: Bell Labs Technique Journal
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Fiber To The Curb (FTTC)
 Differs from HFC
Run
OF closer to the end subscribers
Uses two media in each feeder circuit to provide an
additional service
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Broadband Access Technologies
Source: TEN-Telecom Workshop
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Summary
Technologies exist that
span long distances
Leased
analog lines (require modems)
Leased digital circuits (require DSU / CSUs)
Digital circuits
Available
from phone company
Cost depends on distance and capacity
Popular capacities called T1 and T3
Fractional T1 also available
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Summary (continued)
High capacity circuits available
Popular
capacities known as OC-3, OC-12
Local loop refers
to connection between
central office and subscriber
Local loop technologies include
DSL
(especially ADSL)
Cable modems
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