Telecommunications for the
future - 2
Rob Parker
CERN IT Division
Fixed (cabled) links
Transmission
network
“long” distance
users
users
Distribution network
“short” distance
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Fixed (cabled) links
• Transmission
– Modulation
– Multiplexing
– Cross-connection / switching
• Distribution
the “Local loop”
– Distribution to distribution frame
– Last “mile”
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Modulation
•
•
•
•
Frequency Modulation
Phase Modulation
Amplitude Modulation
Pulse Modulation
All of these have various versions
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Multiplexing & Demultiplexing
• Multiplexing:
Combining several different information streams into
one
• Demultiplexing
Restoring the multiple information streams from the
single one
MULTIPLEXOR
DEMULTIPLEXOR
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Types of Multiplexing
• Frequency Division
• Time Division
– PDH (Plesiochronous Digital Hierarchy)
– SDH (Synchronous Digital Hierarchy)
– ATM (Asynchronous Transfer Mode)
• Wavelength Division (for optical cables)
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PDH
• the data sources are nominally synchronous (to
within a few 10s of ppm of the nominal rate)
• this makes the multiplexing process very
complicated because of bit stuffing and
stripping….and prone to transmission errors
• every new data rate in the hierarchy needs a
completely new multiplexing definition
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PDH Hierarchy
Japan
Europe
397200 kbit/s
564992 kbit/s
N. America
x4
x4
97728 kbit/s
274176 kbit/s
x3
x3
x6
32064 kbit/s
139254 kbit/s
x4
44736 kbit/s
x5
34368 kbit/s
x4
x7
6312 kbit/s
8448 kbit/s
x3
x4
primary rate
x4
1544 kbit/s
2048 kbit/s
x24
x30
64 kbit/s
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SDH
• the data sources are precisely synchronous
• the multiplexing process is relatively simple
• lower data rate “tributaries” can be extracted
from the data stream without total
demultiplexing (and similarly for inserting a
tributary)
• can easily make “self-healing” rings
• the specification is “future proof”
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SDH Hierarchy
9953.28 Mbit/s
STM-64
x4
2488.32 Mbit/s
STM-16
x4
622.08 Mbit/s
STM-4
x4
155.52 Mbit/s
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STM-1
10
Two fibre unidirectional line switched ring
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Two fibre unidirectional path switched ring
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Wavelength Division
Multiplexing
• uses different wavelengths on the same fibre
• is totally protocol independent (SDH, ATM,
Ethernet…)
• known as Dense Wavelength Division Multiplex
(DWDM) when the wavelengths are close (a
few nm.)
• for DWDM, 40 or more wavelengths can be
used on one fibre
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DWDM principle
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DWDM system
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DWDM components
•
•
•
•
•
•
Tunable lasers
Wavelength adaptors
Diffraction gratings
Thin film filters
Bragg gratings
Waveguide gratings
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SDH & DWDM combined
• SDH and DWDM are complementary
• SDH provides:
– flexibility
– resilience in case of failure
• DWDM provides:
– very high bandwidth
CONCLUSION: BANDWIDTH IS NO LONGER A PROBLEM
ON LONG-DISTANCE TRANSMISSION LINKS
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Examples of SDH/DWDM
systems
• TAT-14 (transatlantic cable)
– 8 fibre, dual bi-directional ring with protection ring
– 16 wavelengths of STM-64 per fibre pair
– 2.4 Tbit/s total capacity if fully equipped
• FA-1: Flag Atlantic 1 (transatlantic cable)
–
–
–
–
six fibres
40 wavelengths per fibre
10 Gbit/s SDH per wavelength
2.4 Tbit/s total capacity if fully equipped
(NB: 2.4 Tbit/s can carry 10,000,000 telephone circuits)
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Distribution technologies
• CATV
Community Access (or Cable) TV
• ISDN
Integrated Services Digital Network
• ADSL
Asymmetric Digital Subscriber Line
• Optical fibre
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CATV
– a “cable modem” can provide 10 Mbit/s of
bandwidth, BUT:
– the medium is shared, so performance is
variable
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ISDN
• uses existing telephone distribution
cabling
• 2 * 64 kbit/s + 16 kbit /s to the user
• is widely available worldwide
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ADSL
• uses existing telephone distribution
cabling
• “Asymmetric”: the line speed is different
to and from the subscriber, because:
– data requirements are generally less in the
direction “subscriber to network” than the
reverse
– to reduce crosstalk at the exchange, where
many ADSL lines may arrive bundled
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ADSL principles
• uses a filter to separate the frequency
range 0-4 kHz which leaves the analog
telephone connection unchanged
• uses the frequencies above 4 kHz (to
about 1 MHz) to provide digital
connection to the telephone exchange
• at the telephone exchange, connection is
made to the ISP
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ADSL data rates
• different rates can be used, depending
on:
– the distance to the telephone exchange
– the quality of the cable
• maximum rate 6 Mbit/s to subscriber, 600
kbit/s to telephone exchange
• European offerings are generally < 1
Mbit/s
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Typical distribution cabling
“local loop”
TELEPHONE EXCHANGE
multi-pair cable
~ 5 km
distribution frame
single pair cable
subscribers
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Fibre in the local loop
• FTTK
Fibre to the kerb
– Fibre goes to a distribution frame, at the
limit virtually outside the house
• FTTH
Fibre to the home
– Fibre goes all the way to the user’s premises
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Fibre in the Local Loop (FITL)
TELEPHONE EXCHANGE
multi-pair cable
~ 5 km FTTK
distribution frame
single pair cable
FTTH
subscribers
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