Computer Networks and Internets

Report
Ch 12 Long-Distance and Local Loop
Digital Technologies
1
Motivation
Connect computers across
Large
geographic distance
Public right-of-way
 Streets
 Buildings
 Railroads
2
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

3
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
4
Illustration of Digitized Signal
coded digitally as
000-010-100-100-111-001-001-001-001-001…
Pick neatest
digital value for each sample
5
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

6
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
7
Equipment for Leased Connections
Analog circuit
Modem
required at each end
Digital Circuit
DSU
/ CSU required at each end
8
Digital Circuit and DSU/CSUs
Needed because telephone industry digital
encoding differs from computer industry
digital encoding
9
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
10
Telephone Standards
Cost
of digital circuit depends on
Distance
Capacity
11
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
12
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
13
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)
14
Highest Capacity Digital Circuits
15
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
17
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
18
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
19
Illustration of ADSL Wiring
20
Illustration of ADSL Wiring
21
Illustration of ADSL Billing
22
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)
23
Cable Modem Technology
Source: IEC On-Line Education
24
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
25
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
26
Hybrid Fiber Coax (HFC)
Neighborhood
area
Trunk
FDM
TDM
27
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
28
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
29
Fiber To The Curb (FTTC)
(人行道的)路緣
Source: Bell Labs Technique Journal
30
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
31
Broadband Access Technologies
Source: TEN-Telecom Workshop
32
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
33
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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