Telecommunications for the future - 2

Rob Parker

CERN IT Division

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Fixed (cabled) links

Transmission

network

“long” distance

Transmission

network

“long” distance

usersusers

Distribution network

“short” distance

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Fixed (cabled) links

• Transmission– Modulation– Multiplexing– Cross-connection / switching

• Distributionthe “Local loop”– Distribution to distribution frame– Last “mile”

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Modulation

• Frequency Modulation

• Phase Modulation

• Amplitude Modulation

• Pulse ModulationAll 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 Hierarchy397200 kbit/s

97728 kbit/s

32064 kbit/s

6312 kbit/s

1544 kbit/s

64 kbit/s

2048 kbit/s

8448 kbit/s

34368 kbit/s

139254 kbit/s

564992 kbit/s

274176 kbit/s

44736 kbit/s

x4x4

x4

x4

x4x4

x3x3

x6

x7x5

x3

x30x24

Japan

N. America

Europe

primary rate

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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

2488.32 Mbit/s

622.08 Mbit/s

155.52 Mbit/s

x4

x4

x4

STM-1

STM-4

STM-64

STM-16

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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

subscribers

multi-pair cable

single pair cable

distribution frame~ 5 km

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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

subscribers

multi-pair cable

single pair cable

distribution frame~ 5 km

FTTH

FTTK