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Computer Networks and Internets, 5e
By Douglas E. Comer
Lecture PowerPointsAdapted from notes
By Lami Kaya, [email protected]
2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.
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Chapter 12
Accessand
Interconnection Technologies
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Topics Covered
12.1 Introduction
12.2 Internet Access Technology: Upstream and Downstream
12.3 Narrowband and Broadband Access Technologies
12.4 The Local Loop and ISDN
12.5 Digital Subscriber Line (DSL) Technologies
12.6 Local Loop Characteristics and Adaptation
12.7 The Data Rate of ADSL
12.8 ADSL Installation and Splitters
12.9 Cable Modem Technologies
12.10 The Data Rate of Cable Modems
12.11 Cable Modem Installation
12.12 Hybrid Fiber Coax
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Topics Covered
12.13 Access Technologies That Employ Optical Fiber
12.14 Head-End and Tail-End Modem Terminology
12.15 Wireless Access Technologies
12.16 High-Capacity Connections at the Internet Core
12.17 Circuit Termination, DSU/CSU, and NIU
12.18 Telephone Standards for Digital Circuits
12.19 DS Terminology and Data Rates
12.20 Highest Capacity Circuits (STS Standards)
12.21 Optical Carrier Standards
12.22 The C Suffix
12.23 Synchronous Optical NETwork (SONET)
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12.1 Introduction
This chapter
concludes the discussion of data communications by examining two
facilities used in the Internet
discusses access technologies, such as dialup, DSL, and cable
modems
considers high-capacity digital circuits used in the core of the Internet expands the discussion of the telephone system multiplexing
hierarchy
gives examples of circuits that common carriers offer to businesses
and ISPs
focuses on the data communications aspects of the technologies byconsidering multiplexing and data rates
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12.2 Internet Access Technology:Upstream and Downstream
Internet access technologyrefers to a data communications
system that connects an Internet subscriber to an ISP such as a telephone company(DSL)or cable company
How is access technology designed?
Most Internet users follow an asymmetricpattern
a subscriber receives more data from the Internet than sending a browser sends a URL that comprises a few bytes
in response, a web server sends content
Upstreamto refer to data traveling from a subscriber to an
ISP
Downstreamto refer to data traveling from an ISP in the
Internet to a subscriber
Figure 12.1 illustrates the definitions
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12.2 Internet Access Technology:
Upstream and Downstream
7
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12.3 Narrowband and Broadband AccessTechnologies
A variety of technologies are used for Internet access
They can be divided into two broad categories based on the
data rate they provide
Narrowband
Broadband
In networking terms, network bandwidthrefers to data rate
Thus, the terms narrowbandand broadbandreflect industry
practice
12.3.1 Narrowband Technologies
12.3.2 Broadband Technologies
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12.3 Narrowband and Broadband AccessTechnologies
12.3.1 Narrowband Technologies
refers to technologies that deliver data at up to 128 Kbps
For example, the maximum data rate for dialup noisy phone lines is
56 Kbps and classified as a narrowband technology
Figure 12.2 (below) summarizes the main narrowband access
technologies
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12.3 Narrowband and Broadband AccessTechnologies
12.3.2 Broadband Technologies
generally refers to technologies that offer high data rates, but the
exact boundary between broadband and narrowband is blurry
many suggest that broadband technologies deliver more than 1 Mbps
but this is not always the case, and may mean any speed higher than dialup
Figure 12.3 (below) summarizes the main broadband access
technologies
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Narrowband vs Broadband
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Internet connectionnarrow or broadband
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ISP Hierarchy
customer
Local loopor Last mile
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Local Loop Technologies
Electric local loop(POTSlines): Voice, ISDN, DSL
Optical local loop: Fiber Optics services such as FiOS
Satellite local loop: communications satelliteand cosmos
Internet connections of satellite television(DVB-S)
Cable local loop: Cablemodem
Wireless local loop(WLL): LMDS, WiMAX, GPRS, HSDPA,
DECT
12 4 Th L l L d ISDN
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12.4 The Local Loop and ISDN Local loop describes the physical connection between a
telephone company Central Office (CO) and a subscriber
consists of twisted pair and dialup call with4KHz of bandwidth
It often has much higher bandwidth; a subscriber close to a CO may be able
to handle frequencies above 1MHz
Integrated Services Digital Network(ISDN)
ISDN offers three separate digital channels
designated B, B, and D(usually written 2B + D)
The 2Bchannels (each 64Kbps) are intended to carry
digitized voice, data, or compressed video
Both of the Bchannels can be combined or bonded to produce a
single channel with an effective data rate of 128 Kbps
The Dchannel (16Kbps) is used as a control channel
Newer local loop technologies provide higher data rates at
lower cost, relegatingISDN to a few special cases
ISDN has little niche market in US, pretty popular in Japan &Europe
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12.5 Digital Subscriber Line (DSL)
Technologies DSLis one of the main technologies used to provide high-speed data
communication services over a local loop Figure 12.4 (below) lists DSL variants
Because the names differ only in the first word, the set is collectively referredto by the acronym xDSL
Currently, ADSL is most popular
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12.5 Digital Subscriber Line (DSL)
Technologies ADSL is the most widely deployed variant
and the one that most residential customers use
ADSL uses FDM to divide the bandwidth of the local loop
into three regions
one of the regions corresponds to traditional analog phone service,
which is known as Plain Old Telephone Service (POTS) and two regions provide data communication
Figure 12.5 (below) illustrates how ADSL divides bandwidth
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12.6 Local Loop Characteristics and
Adaptation ADSL technology is complex
because no two local loops have identical electrical characteristics
ADSL is adaptive
That is, when a pair of ADSL modems are powered on, they probe
the line between them to find its characteristics
agree to communicate using techniques that are optimal for the line
ADSL uses Discrete Multi Tone modulation (DMT)
that combines frequency division multiplexing andinverse
multiplexingtechniques
FDM in DMT is implemented by dividing the bandwidth into286separate frequencies called sub-channels
255sub-channels allocated for downstreamdata transmission
31allocated for upstreamdata transmission
12 6 L l L Ch i i d
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12.6 Local Loop Characteristics and
Adaptation Two of the upstream channels are reserved for control
information There is a separate modemrunning on each sub-channel,
which has its own modulated carrier Carriers are spaced at 4.1325KHz intervals to keep the signals from
interfering with one another
To guarantee that its transmissions do not interfere withanalog phone signals ADSL avoids using the bandwidth below 26KHz
Two ends assess the signal quality at each frequency
Use the quality to select a modulation scheme If a particular frequency has a highsignal-to-noise ratio
ADSL selects a modulation scheme that encodes many bits per baud
If the quality on a given frequency is low ADSL selects a modulation scheme that encodes fewer bits per baud
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12.7 The Data Rate of ADSL
How fast can ADSL operate? ADSL can achieve
a downstream rate of 8.448Mbps on short local loops
and an upstream rate of 640Kbps
Network control channel requires 64Kbps
The effective upstream rate for user data is 576Kbps
ADSL2can download at close to 20Mbps
Adaptation has an interesting property ADSL does not guarantee a data rate
ADSL can only guarantee to do as well as line conditions allow
Those farther(physical distance) from a CO (or local loop passes near
sources of interference) lower data rates than subscribers who live near
the CO (or a local loop does not pass near sources of interference) thus
the downstream rate varies from 32Kbps to 8.448Mbps
the upstream rate varies from 32 to 640 Kbps
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12.8 ADSL Installation and Splitters
Analog phones operate at frequencies below4 KHz
lifting a receiver can generate noise that interferes with DSL signals
ADSL uses an FDM device known as a splitter
It divides the bandwidth by passing low frequencies to one output
and high frequencies to another
A splitter is passive; it does not require power A splitter is usually installed at the location where the local loop
enters a residence or business
Figure 12.6 illustrates the connection
A variation of ADSL wiring (DSL-lite) has become popular it does not require a splitter to be installed on the incoming line
a subscriber can install DSL by plugging a splitter into a wall jack
and plugging a telephone into the splitter
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12.8 ADSL Installation and Splitters
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12.9 Cable Modem Technologies
A variety of wireless and wired technologies have been
developed for use in the local loop
An alternative access technology that uses the wiring
already in place for cable television
It is also known as Community Antenna TeleVision (CATV)
It uses FDM to deliver TV signals over coaxial cable
CATV is not available in all countries
Coaxial cable has high bandwidth and is less susceptible to
electromagnetic interference than twisted pair
CATV systems use FDM to deliver many channels
In CATV the bandwidth is insufficient to handle a FDM scheme that
extends a channel to each user
Using a separate channel per subscriber does not scale
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Cablemodem connection
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12.10 The Data Rate of Cable Modems
How fast can a cable modem operate?
In theory, a cable system can support data rates of 52Mbps
downstream and 512Kbps upstream.
In practice, the rate can be much less
The data rate of a cable modem only pertains to
communication between the local cable office and thesubscriber's site
The bandwidth is shared among a set of Nsubscribers (the
size of the set is controlled by the cable provider)
sharing the bandwidth with other subscribers can be a disadvantage because the effective data rate available to each individual subscriber
varies over time
if Nsubscribers share a single frequency
the amount of capacity available to an individual subscriber will be 1/N
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Cable Infrastructure
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12.11 Cable Modem Installation
Cable modem installation is straightforward
Cable modems attach to the cable wiring directly
The FDM hardware in existing cable boxes and cable
modems guarantees that data and entertainment channels
will not interfere with one another
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12.12 Hybrid Fiber Coax (HFC)
HFC can provide high-speed data communications
a HFC system uses a combination of optical fibers and coaxial cables
fiber used for the central facilities and coax used for connections to
individual subscribers
An HFC system is hierarchical
It uses fiber optics for the portions that require the highest bandwidth and it uses coax for parts that can tolerate lower data rates
Trunkto refer to the high-capacity connections between the
cable office and each neighborhood area
Feeder circuit to refer to the connection to an individualsubscriber
Trunk connections can be up to 15miles long
Feeder circuits are usually less than a mile
12 12 H b id Fib C
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12.12 Hybrid Fiber Coax
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12 13 Access Technologies That Employ
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12.13 Access Technologies That Employ
Optical Fiber There are available a variety of technologies that either
employ optical fiber in a hybrid system or deploy optical fiberall the way to each subscriber
Figure 12.8 summarizes names of key technologies
12 13 Access Technologies That Employ
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12.13 Access Technologies That Employ
Optical Fiber Fiber To The Curb (FTTC)
it uses optical fiber for high capacity trunks
the idea is to run optical fiber close to the end subscriber
and then use copper for the feeder circuits
it uses two media in each feeder circuit to allow the cable system to
provide an additional service, such as voice Fiber To The Building (FTTB)
it will use optical fiber to allow high upstream data rates for businesses
Fiber To The Home (FTTH)
uses optical fiber to deliver higher downstream data rates toresidential subscribers
The emphasis is on many channels of entertainment and video
Fiber To The Premises (FTTP)
A generic term, FTTP, encompasses both FTTB and FTTH
12 14 Head End and Tail End Modem
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12.14 Head-End and Tail-End ModemTerminology
An access technology requires a pair of modems with one at the subscriber's site and one at the provider's site
Head-end modem to refer to a modem used at the CO
Head-end modems are not individual devices
Instead, a large set of modems is built as a unit that can beconfigured, monitored, and controlled together
A set of head-end modems used by a cable provider is known as a
Cable Modem Termination System (CMTS)
Tail-end modem to refer to a modem used at the subscriber
Data Over Cable System Interface Specifications (DOCSIS)
specifies both the format of data that can be sent as well as the
messages that are used to request services (e.g., pay-per-view)
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12.15 Wireless Access Technologies
How to provide access in rural areas?
Imagine a farm or remote village many miles from the nearest city
The twisted pair wiring used to deliver telephone service to such
locations exceeds the maximum distance for technologies like ADSL
Rural areas are least likely to have cable television service
Even in suburban areas, technologies like ADSL may havetechnical restrictions on the type of line they can use
it may be impossible to use high frequencies on telephone lines that
contain loading coils, bridge taps, or repeaters
Local loop technology may not work on all lines
To handle special cases, a variety of wireless access technologies
have been explored
Figure 12.9 lists a few examples
12 15 Wi l A T h l i
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12.15 Wireless Access Technologies
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S t llit ti
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Satellite connection
12 16 High Capacity Connections at the
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12.16 High-Capacity Connections at theInternet Core
Access technologies handle the last mile(sometimes called
as First mile)problem
where the last mile is defined as the connection to a typical
residential subscriber or a small business
An access technology provides sufficient capacity for a
residential subscriber or a small business the term Small Office Home Office (SOHO) is used to refer them
Connections to large businesses or connections among
providers require substantially more bandwidth
Corerefers to connections at the backbone of Internet Core technologies refers to high-speed technologies
Figure 12.10 shows the aggregate traffic from the Internet to
the provider
12 16 High Capacity Connections at the
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12.16 High-Capacity Connections at the
Internet Core
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12 16 High Capacity Connections at the
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12.16 High-Capacity Connections at theInternet Core
What technology can a provider use to move data a long
distance at a rate of 10Gbps? The answer lies in a point-to-pointdigital circuitleased from a
telephone company
High-capacity digital circuits are available for a monthly fee, and can
be used to transfer data
Telephone companies have the authority to install wiring
that crosses municipal streets
A circuit can extend between two buildings, across a city, or
from a location in one city to a location in another
The fee charged depends on the data rate of the circuit and the
distance spanned
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One of International Backbones
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One of International Backbones
12 17 Circuit Termination DSU/CSU
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12.17 Circuit Termination, DSU/CSU,
and NIU To use a leased digital circuit, one must agree to follow the
rules of the telephone system including adhering to the standards that were designed for
transmitting digitized voice
Computer industry and the telephone industry developed
independently Standards for telephone system digital circuits differ from those used
in the computer industry
A special piece of hardware is needed to interface a computer to a
digital circuit provided by a telephone company
Known as a Data Service Unit/Channel Service Unit (DSU/CSU) Device contains two functional parts, usually combined into a single chassis
The CSU portion of the DSU/CSU device handles line termination
and diagnostics
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CSU/DSU for T1 connection
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CSU/DSU for T1 connection
(T1)
12 18 Telephone Standards for Digital
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12.18 Telephone Standards for Digital
Circuits A digital circuit leased from a telco follows the same digital
transmission standards that the telco uses to transportdigital phone calls
In the USA, standards for digital telephone circuits were
given names that consist of the letter Tfollowed by a
number One of the most popular is known as T1
Many small businesses use a T1circuit to carry data
T-standards are not universal
Japan adopted a modified version of the T-series standards Europe chose a slightly different scheme; it uses the letter E
Figure 12.11 lists the data rates of several digital circuit
standards
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12.18 Telephone Standards for Digital
Circuits
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12 19 DS T i l d D t R t
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12.19 DS Terminology and Data Rates
The data rates of Tstandards have been chosen so they
can each handle multiple voice calls
Note that the capacity of circuits does not increase linearly with their
numbers
For example, the T3standard defines a circuit with much more than
three times the capacity of T1
Telcos may lease circuits with lower capacity than thoselisted in the figure
they are known as fractional T1 circuits
To multiplex multiple phone calls onto a single connection
known as Digital Signal Level standards or DS standards
For example, DS1denotes a service that can multiplex 24
phone calls onto a single circuit
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12.20 Highest Capacity Circuits
(STS Standards)
Telephone companies use the term trunk to denote a high-capacity circuit, and have created a series of
standards for digital trunk circuits
Synchronous Transport Signal(STS) standards specify the
details of high-speed connections
Figure 12.12 summarizes the data rates associated with
various STS standards
All data rates in the table are given in Mbps, for easy comparison
Note that data rates for STS-24and above are greater than 1Gbps
12 20 Hi h t C it Ci it
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12.20 Highest Capacity Circuits
(STS Standards)
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12 21 O ti l C i St d d
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12.21 Optical Carrier Standards
Telcos define an equivalent set of Optical Carrier (OC)
standards
Figure 12.12 gives the names for optical standards as well
as for copper standards
One should observe a distinction between the STS and OC
terminology: the STS standards refer to the electrical signals used in the digital
circuit interface (i.e., over copper)
the OC standards refer to the optical signals that propagate across
the fiber
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12.22 The C Suffix
The STC and OC terminology described above has one
additional feature not shown in Figure 12.12 an optional suffix of the letter C, which stands for concatenated
The suffix denotes a circuit with no inverse multiplexing
an OC-3 circuit can consist of three OC-1 circuits operating at 51.840
Mbps each
or it can consist of a single OC-3C (STS-3C) circuit that operates at
155.520Mbps
Is a single circuit operating at full speed better than multiple
circuits operating at lower rates?
The answer depends on how the circuit is being used
In general, having a single circuit operating at full capacity
provides more flexibility
and eliminates the need for inverse multiplexing equipment
12.23 Synchronous Optical NETwork
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12.23 Synchronous Optical NETwork
(SONET) Telcos defined a broad set of standards for digital
transmission In North America, the standards are known by the term Synchronous
Optical NETwork (SONET)
In Europe they are known as the Synchronous Digital Hierarchy (SDH)
SONET specifies some details, such as how data is framed
how lower-capacity circuits are multiplexed into a high-capacity circuit
how synchronous clock information is sent along with data
Figure 12.13 shows the SONET frame format used on anSTS-1 circuit
12 23 Synchronous Optical NETwork
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12.23 Synchronous Optical NETwork
(SONET)
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SONET
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