Communication and Networks Assignment
Communications and Networks
version 1.0
Diploma in Information Technology
Copyright © 2020 by Singapore Institute of Management Pte Ltd. All rights reserved.
Lesson 15: Interconnection Technologies
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Lesson 15 Learning Outcomes
Explain upstream and downstream
Explain narrowband, broadband and DSL technologies
Describe the characteristics of ADSL and local loops
Explain the evolution of access technologies
Describe SONET
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Lesson 15 Outline
Broadband and Narrowband
Cable Modem Technologies
Core Technologies
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Internet Access Technology
Internet access technology: a data communications system that connects an Internet subscriber to an ISP
Most Internet users follow an asymmetric pattern
Subscriber receives more data than sending
Browser sends a URL that comprises a few bytes
Web server can respond with content that are more than a few bytes
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Downstream and Upstream
Downstream: data traveling from an ISP in the Internet to a subscriber
Upstream: data traveling from a subscriber to an ISP
downstream
upstream
Source: Douglas, C (2016) Computer Networks and Internets
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Narrowband and Broadband
Technologies are used for Internet access can be divided into two broad categories based on data rate
Narrowband
Broadband
But network bandwidth refers to data rate
Hence, narrowband and broadband are used to reflect industry practice
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Narrowband Technologies
Narrowband: technologies that deliver data at up to 128 Kbps
Max data rate for dialup phone lines is 56 Kbps and hence classified as narrowband technology
Source: Douglas, C (2016) Computer Networks and Internets
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ISDN
Integrated Services Digital Network (ISDN) offers three separate digital channels:
B, B, and D (D is a control channel supports 16Kbps)
Usually written 2B + D
The 2 B channels (each 64 Kbps) are intended to carry digitised voice, data, or compressed video
B channels can be combined or bonded to produce single channel with data rate of 128 Kbps
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Broadband Technologies
Broadband: technologies that offer higher data rates than dialup
Source: Douglas, C (2016) Computer Networks and Internets
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Local Loop
Local loop: physical connection between a telephone company Central Office (CO) and a subscriber
Consists of twisted pair and dialup call with 4KHz of bandwidth
Subscriber close to CO may be able to handle frequencies above 1MHz
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Digital Subscriber Line (DSL)
DSL: one of the main technologies used to provide high-speed communication services over local loop
Source: Douglas, C (2016) Computer Networks and Internets
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Asymmetric DSL
ADSL most widely deployed variant and most common for residential use
uses FDM to divide bandwidth of the local loop into three regions
one region corresponds to analog phone service, known as Plain Old Telephone Service (POTS)
Source: Douglas, C (2016) Computer Networks and Internets
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ADSL Properties
Complex: as no two local loops have identical electrical characteristics
Adaptive: when a pair of ADSL modems are powered on, they probe the line between them to find its characteristics
Use techniques that are optimal
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Discrete Multi Tone Modulation
ADSL uses Discrete Multi Tone modulation (DMT)
Combines frequency division multiplexing and inverse multiplexing techniques
FDM in DMT is implemented by dividing the bandwidth into 286 sub-channels
255 sub-channels for downstream
31 sub-channel for upstream
2 upstream are reserved as control channel for control information
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Control Channels (1/2)
Each control channel has a separate modem with its own modulated carrier
Carriers are spaced at 4.1325 KHz intervals to keep the signals from interfering with one another
To guarantee that its transmissions do not interfere with analog phone signals
ADSL avoids bandwidth below 26 KHz
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Control Channels (2/2)
Two ends assess the signal quality at each frequency
Use the quality to select a modulation scheme
If a frequency has a high signal-to-noise ratio
selects a modulation scheme that encodes many bits per baud
If the quality on a given frequency is low
selects a modulation scheme that encodes fewer bits per baud
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ADSL Data Rate
ADSL can achieve:
8.448 Mbps downstream on short local loops
64 Kbps upstream for control channel
576 Kbps upstream for user data
Adaptation property of ADSL does not guarantee a data rate
Only do as well as line conditions allow
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ADSL Local Loop Distance
Subscribers far from CO or local loop passes near sources of interference have lower data rates
Subscribers near the CO or local loop does not pass near sources of interference have better data rates
Data rates can vary:
Downstream 32Kbps to 8.448Mbps
Upstream 32Kbps to 640 Kbps
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ADSL Splitter
Analog phones operate at frequencies below 4KHz
lifting a receiver can generate noise that interferes with DSL signals
ADSL uses an FDM device known as a splitter
Divides bandwidth by passing low frequencies to one output and high frequencies to another
Usually installed at the location where local loop enters a residence or business
Passive: does not require power
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ADSL Installation
Source: Douglas, C (2016) Computer Networks and Internets
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DSL-Lite
A variation of ADSL wiring (DSL-lite)
Does not require a splitter to be installed on the incoming line
Subscriber can install DSL by plugging a splitter into a wall jack and plugging a telephone into the splitter
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Practice 15.1
How are access technologies classified under broadband and narrowband? Give an example of each.
What is the purpose of control channels? What can be done to prevent their signal from interfering with one another?
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Lesson 15 Outline
Broadband and Narrowband
Cable Modem Technologies
Core Technologies
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Cable Modem Technologies
Community Antenna TeleVision (CATV): an alternative access technology that uses the wiring already in place for cable television
uses FDM to deliver TV signals over coaxial cable
CATV systems use FDM to deliver many channels
But bandwidth is insufficient to handle FDM scheme that extends a channel to each user
Using a separate channel per subscriber does not scale
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Cable Modem Data Rate
In theory, cable system can support:
52 Mbps downstream
512 Kbps upstream.
In practice, the data rate can be much less
Data rate only pertains to communication between the local cable office and the subscriber's site
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Sharing Bandwidth
Bandwidth of cable system is shared with N subscribers
size is controlled by the cable provider
Effective data rate available to each individual subscriber varies over time
if N subscribers share a single frequency
amount of capacity available to an individual subscriber will be 1/N
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Cable Modem Installation
Cable modem installation is straightforward
Cable modems attach to the cable wiring directly
FDM hardware in existing cable boxes and cable modems guarantees:
data and entertainment channels will not interfere with one another
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Hybrid Fiber Coax (HFC)
HFC provides high-speed data communications
Fiber to connect to the central facilities
Coax to connect to individual subscribers
It is hierarchical
Uses fiber for parts that require highest bandwidth
Uses coax for parts that can tolerate lower data rates
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Trunk and Feeder Circuit
Trunk: the high-capacity connections between the cable office and each neighborhood area
can be up to 24km long
Feeder circuit: the connection to an individual subscriber
Feeder circuits are usually less than 1.6km
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HFC Illustration
Source: Douglas, C (2016) Computer Networks and Internets
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Fiber Access Technologies
A variety of technologies employs optical fiber in a hybrid system
or deploy optical fiber all the way to each subscriber
Source: Douglas, C (2016) Computer Networks and Internets
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Fiber to Curb and Building
Fiber To The Curb (FTTC) uses fiber for high capacity trunks
Idea is to run fiber close to subscriber
Use copper for the feeder circuits
Uses two media in each feeder circuit to provide additional service like voice
Fiber To The Building (FTTB) use fiber to allow high upstream data rates for businesses
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Fiber to Home and Premises
Fiber To The Home (FTTH) uses fiber to deliver higher downstream for residential subscribers
Emphasis is on many channels of entertainment and video
Fiber To The Premises (FTTP) is a generic term that encompasses both FTTB and FTTH
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Head-end & Tail-end Modem
Two types of modem based on location:
Head-end modem: modem used at the CO
Cable Modem Termination System (CMTS): set of head-end modems used by cable providers.
Tail-end modem: modem used at the subscriber
Data Over Cable System Interface Specifications (DOCSIS): specifies format of data that can be sent and messages that are used to request services like pay-per-view)
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Wireless Access Technologies
Imagine a farm or remote village:
telephone wiring to such locations exceeds the max distance for technologies like ADSL
Also unlikely to have cable TV
Local loop may not work on all types of lines
Need wireless access technologies
Source: Douglas, C (2016) Computer Networks and Internets
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Lesson 15 Outline
Broadband and Narrowband
Cable Modem Technologies
Core Technologies
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What is a T1 Line?
Source: https://www.youtube.com/watch?v=o5zSxG-Atsc
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Last Mile Problem
Access technologies handle last mile problem
Last mile: the connection to a typical residential subscriber or a small business
Access technology provides sufficient capacity for a residential subscriber or a small business
Small Office Home Office (SOHO)
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Core Technologies
Connections among providers and enterprises require substantially more bandwidth
Core: connections at the backbone of Internet
Core technologies: high-speed technologies
Source: Douglas, C (2016) Computer Networks and Internets
Last mile
Core
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Point-to-Point Digital Circuit
Point-to-point high-capacity digital circuit can be leased from a telephone company
Can be used to transfer data
Monthly fee depends on data rate of the circuit and the distance spanned
Telephone companies have the authority to install wiring that crosses municipal streets
Between two buildings, across a city, from one city to another
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Leasing a Digital Circuit
Subscribers must follow the rules of telephone system and adhere to standards for transmitting digitised voice
Computer industry and the telephone industry developed independently
Need a hardware interface to work between computer and digital circuit
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DSU and CSU
Data Service Unit/Channel Service Unit (DSU/CSU): hardware interface for a computer to a digital circuit
Two parts combined into a chassis
CSU handles line termination and diagnostics
DSU handles digital format translation for data
Between format used on circuit and computer
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CSU Test Facility
A CSU contains a loopback test facility
To transmit a copy of all data that arrives across the circuit back to sender
Excessive 1s can cause excessive current on the cable due to voltage levels
To prevent problems, can use an encoding that guarantees a balance like differential encoding
Or use a technique known as bit stuffing
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Interface Standard
Interface standard used on the computer depends on rate that circuit operates
If < 56 Kbps, the computer can use RS-232
If > 56 Kbps, use RS-449 or V.35 standards
One additional piece of equipment may be used
Network Interface Unit (NIU) or Smartjack
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Network Interface Unit
NIU forms a boundary between equipment owned by the telco and equipment provided by subscriber
Telco refers to the boundary as the demarc
Digital circuit needs DSU/CSU at each end
To translate between digital representation used by phone and digital representation used by computer
Digital circuit from a telco follows same transmission standards as digital phone calls
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Digital Circuit Standards
In US, standards for digital circuits consist of the letter T followed by a number like T1
Many small businesses use a T1 circuit
But T-standards are not universal
Japan adopted a modified version of the T-series standards
Europe uses the letter E
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Digital Circuit Capacity
Source: Douglas, C (2016) Computer Networks and Internets
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T-standards Data Rate
Data rates of T standards have been chosen to handle multiple voice calls
Capacity of circuits does not increase linearly with their numbers; T3 is much more than 3x T1
Telcos may lease circuits with lower capacity than those listed in the figure
Known as fractional T1 circuits
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STS Standards
Telephone companies use trunk to denote a high-capacity circuit,
Synchronous Transport Signal (STS) standards specify the details of high-speed connections
Data rates for STS-24 and above are > 1 Gbps
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List of STS & OC Standards
Source: Douglas, C (2016) Computer Networks and Internets
STS standards: electrical signals used in the digital circuit interface (i.e., over copper)
Optical Carrier (OC) standards: optical signals that propagate across the fiber
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Concatenated Circuits
STC and OC allows for an optional suffix of the letter C, which stands for concatenated
denotes a circuit with no inverse multiplexing
OC-3 can consist of 3x OC-1 or single circuit that operates at 155.520 Mbps
Single circuit vs multiple circuits
Generally, single circuit provides more flexibility and eliminates need for inverse multiplexing equipment
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Digital Transmission Standards
Two major digital transmission standards:
US, Synchronous Optical NETwork (SONET)
Europe Synchronous Digital Hierarchy (SDH)
SONET specifies details like:
How data is framed
How lower-capacity circuits are multiplexed into a high-capacity circuit
How synchronous clock information is sent along with data
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SONET Frame on STS-1 Circuit
Source: Douglas, C (2016) Computer Networks and Internets
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Practice 15.2
With cable modem technologies, why is there still a need for core technologies?
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Reading
Douglas, C. (2016). Computer Networks and Internets, Global Edition (6th ed.). Pearson Education. ISBN: 978-1292061177 Chapter 12
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End of Lesson
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