Chapter4

Report
Computer Networks
Digital Access Technologies
How Computer Networks are Built?
 LANs (Local Area Networks) are relatively cheap
and easy to built.
 WANs are expensive and difficult to built
 PSTN (Public Switched Telephone Network) is
”an older brother” to the computer networks.
 Computer still need to use infrastructure built by
PSTN, esspecially when it comes to WAN
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Core, Distribution and Access Network
 Core Network
 Combination of switching centers and transmission
systems connecting switching centers.
 Distribution Network
 Network in between the access and core network
 Access Network
 The portion of public network that connects individuals or
companies to some access node through which they can
reach the core network (directly or via some distribution
network)
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Example
Core
Distribution
ISP1
Fiber-optic
Access
OC12 DPT
HFC
Regional
Core
HFC
OC48
SONET
Switch
ISP2
HFC
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Various Access Options
 Access at the customer premises
 Narrow band
 PSTN based access
 ISDN based access
 Broad band
 xDSL
 Cable modem
 Fiber to the curb
 Distribution network
 E1, E2, E3, SDH (T1, T2, T3, SONET)
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PSTN Based Access
 PSTN (Public Switched Telephone Network) is acronym
for the telephone network that uses circuit switching
 When using PSTN access to a computer network, the modems are
needed at both ends of the connection
 Circuit switching is used although inconvenient for data traffic
 Data are transmitted in bursts and therefore the bandwidth is not
used all the time during the connection
 The bit rate is limited with the bandwidth (usually 4KHz for voice)
and the Shannon’s theorem
 Different modulation techniques (QAM, multiple PSK) can improve
the bit rate
 Due to the known S/N for voice channel these techniques cannot do
much more then 33600bps
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Local Loop Based Access
 Local loop describes connection from telephone
office to home
 Also known as local subscriber line
 Most local loops use analog signals
 Sometimes called POTS (Plain Old Telephone Service)
 Legacy infrastructure is copper
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Access through Dial-up or Leased Line
PSTN
Dial-up or leased-line modem
 Dial-up
 The connection goes through the switch (telephone exchange (the line is
released after disconnection)
 The customer is charged by the time it uses the connection
 Leased line (Dedicated line)
 The connection does not go through the switch (the line is dedicated all
the time to the connection)
 The customer is charged with fixed monthly rate
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Connecting to ISP through Local Loop
Up to 56 Kbps
from ISP to the
subscriber
Up to 33.6 Kbps
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ISDN Based Access
 ISDN stands for Integrated Services Digital Network
 Provides digital service on existing local loop copper
 Establishes a digital pipe between the customer and the telephone company
 Allows access to multiple services through a single access (digital
telephone, digital terminal, digital facsimile machine)
 Attempt to replace the analog telephone system with digital one
(for voice and data traffic)
 Obsolete for many reasons
 Too expansive
 Charged by time
 Almost equivalent to analog modems (64 Kbps)
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ISDN Architecture
Customer’s office
ISDN
PBX
Customer’s office
Carrier’s office
PRA
BRA
NT1
ISDN
Exchange
NT1
LAN Gateway
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ISDN Standardized Channels
 BRA/BRI (Basic Rate Access/Basic Rate Interface)
 2B+D
 2 x 64 Kbps + 16 Kbps = 144 Kbps (not including overhead)
 designed to operate using the average local copper pair
 PRA/PRI (Primary Rate Access/Primary Rate Interface)





23 B + D
23 x 64 Kbps + 64 Kbps = 1.536 Mbps (not including overhead)
Designed to operate using DS-1/E1
In Europe 30 B + D
Optional backup D channel.
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Access with xDSL
 xDSL stands for a family of DSL (Digital
Subscriber Loop) technologies
 ADSL (Asymmetric DSL) is most popular for
residential access
 Higher speed into home than out of home
 More bits flow in ("downstream") than out
("upstream")
 The maximum speed depends on the length and
quality of the copper in the subscriber loop
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ADSL - Configuration
Network
Interface
Device
Access
Multiplexer
 Takes advantage of higher frequencies on most local loops
 Can be used simultaneously for POTS (Plain Old Telephone Service)
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ADSL Frequency Spectrum
 Divides the bandwidth into 256 x 4.3K channels
 1 (ch 0) POTS, 5 (ch 1-5) not used, 1 upstream control, 1 downstream
control
 Typical 6-30 for upstream, rest for downstream
 Each 4.3K channel 4K baud sample, V.34 QAM modulation, up to 15
bits per baud
4K * 15 = 60 Kbps per channel
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Other DSL Technologies
 SDSL (Symmetric DSL) divides frequencies evenly
 HDSL (High-rate DSL) provides DS1 bit rate in both
directions
 Short distances
 Four wires
 VDSL (Very high bit rate DSL) provides up to 52 Mbps
 Very short distance
 Requires Optical Network Unit (ONU) as a relay
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Access with Cable Modem
 Cable TV already brings high bandwidth coaxial cable into
the houses
 Cable modems encode and decode data from cable TV
coaxial cable
 Instead of a traditional cable box, the splitter is installed in the
home (directs the TV bands to the TV set and the Internet access
bands to the PC)
 Bandwidth dedicated to the Internet is multiplexed among
all users
 Usually the rate is asymmetrical (500 Kbps to 1 Mbps from PC to
Internet and 3 to 10 MHz in the oposite direction
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Cable Network Configuration
Traditional
cable TV
network
Hybrid fibercoax (HFC)
cable TV
network
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Cable Frequency Spectrum







TV band: 54 – 550 MHz, 6MHz per channel
Downstream : 550 – 750 MHz
6MHz QAM-64 (6bit) == 36 Mbps (gross), 27 Mbps (net)
Total effective downstream bandwidth 200 / 6 * 27 = 891 Mbps
Upstream : 5 – 42 MHz
6MHz QPSK (2bit) == 12 Mbps (gross), 9 Mbps (net)
Total effective upstream bandwidth 37 / 6 * 9 = 54 Mbps
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Access through Fiber to the Curb
Infrastructure with fiber can be build especially for Internet access
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Distribution with Optical Hierarchies
 SONET (Synhronious Optical NETwork)
 A standard for TDM used in United States
 SDH (Synhronous Digital Hierarchy)
 A standard for TDM in Europe
 Both use synhronous communication
 Digital telephony systems use clocking for
synchronous data delivery
 Synchronous network moves data at a precise rate
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Optical Hierarchies - Characteristics
 SONET and SDH are based on the principal of direct
synchronous multiplexing.
 Provide advanced network management and maintenance
features.
 Both SONET and SDH can transport signals for all the
networks in existence today and it has the flexibility to
accommodate any networks defined in the future.
 Can be used in the three traditional telecommunications
areas: long-haul networks, local networks and loop carriers.
It can also be used to carry CATV video traffic.
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