WDM Network Elements - Electrical and Computer Engineering - Lecture Slides, Slides of Electrical Engineering

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WDM Network Elements

Architectural Aspects ofNetwork Elements^!

Optical Line Terminals (OLT) – widelydeployed today

Optical Add-Drop Multiplexers (OADM)– some deployment has been done

Optical Crossconnects (OXC) –deployment just starting

WDM Network Architecture^!

OLTs are placed either at the end of links orin point-to-point configurations

OADMs are used at places where somefraction of the wavelengths need to beterminated and others need to be added andare typically in linear or ring topologies.

OXCs enable mesh topologies and switchingof wavelengths.

Clients of these networks can be ATM,SONET, IP switches using the optical layer.

Important Features of WDMArchitecture^!

Each link can support a number of wavelengths (physicallimitations!)

!

Wavelength Reuse:

Multiple lightpaths can use the same

wavelength in the network as long as they do not overlap onthe same link.

!

Wavelength conversion:

lightpaths may undergo conversion

along the lightpath for better utilization/adaptation of signals.

!

Transparency:

optical layer is protocol insensitive.

!

Circuit Switching:

lightpath establishment on demand. (no

packet switching at the optical layer!)

!

Survivability:

in event of link/node failures lightpaths can be

rerouted (resiliency!)

!

Lightpath topology:

graph representation of nodes and

links/lightpaths between them (the view of the higher layer)

Elements inside OLTs^!

Transponders

Wavelength multiplexers (demultiplexers)

(Optical amplifiers)

λ

1

,

IP router

SONETSONET

O/E/OO/E/O

Laser

Receiver

Non ITU

λ

Non ITU

λ

ITU

λ

3

ITU

λ

2

ITU

λ

1

λ

1

λ

2

λ

3

λ

OSC

λ

OSC

Optical line terminal

Transponder

Mux/demux

λ

1

,

IP router

SONETSONET

O/E/OO/E/O

Laser

Receiver

Non ITU

λ

Non ITU

λ

ITU

λ

3

ITU

λ

2

ITU

λ

1

λ

1

λ

2

λ

3

λ

OSC

λ

OSC

Optical line terminal

Transponder

Mux/demux

Transponders

Adapts a signal to be transmitted in the WDM network.My include a simple OEO conversion or opticalwavelength conversion (research labs). The interfacebetween the client and the transponder may vary(depending on: bit-rate, distance, loss, etc.)

Most likely SONET i-face is short-reach (SR) but can bea very-short range (VSR) interface for >=10Gbps.

The signal generated by the transponder shoguld(optimally) conform to ITU standards.

Transponders may add networking functionality suchas: overhead for management purposes or forwarderror correction (FEC – OEO required!).

May Monitor BER of signal.

Optical Multiplexers^!

Any multiplexing technology can beused

Optical amplifiers may be used to boostsignals in both directions (for receptionas well as transmission).

Supervisory Channel in OLTs^!

Optical Supervisory Channel (OSC) iscarried on a separate wavelength

OSC is used to monitor the performanceof amps on the links as well as formanagement functions (performance,fault, configuration, security,accounting).

Optical Line Amplifiers^!

Placed in the “middle” of optical fibre with a distanceof about 80-120km.

EDFA is currently the most used amp.

Typical amps cascade two or more gain blocks withmid-stage access.

In the mid-stage compensating elements can be put(e.g., chromatic dispersion compensators, flat gaincompensators, maybe OADMs).

Amplifiers also contain gain control and performancemonitoring capabilities.

Employment of Raman amplification has just started,where a laser pumps light in the opposite direction ofthe signal.

Optical Line Amplifiers^!

The optical supervisory channel is terminated at theinput and re-injected at the output (OEO conversion,electronic processing).

In a system using both C and L band, bands areseparated and employ separate EDFAs.

OADM

Raman

pump

laser

Receiver

λ

OSC

Gain stage

λ

1

λ

2

λ

W

Gain stage

Laser

Dispersioncompensator

OADM

Raman

pump

laser

Receiver

λ

OSC

Gain stage

λ

1

λ

2

λ

W

Gain stage

Laser

Dispersioncompensator

OADMs^!

May be used in OLAs (seen previously).

Can be used as stand alone networkelements.

OADMs can save on costs significantly, byreducing the number of point-to-pointconnections (terminations), thus reducing thenumber of OLTs (and transponders, thatgenerate most of the cost).

In the first of the following pictures - at nodeB six out of eight transponders are connectedback-to-back – what a waste!

OADM vs. OLT

Note, that transponders can be skipped in the first picture, if those OLTsAre engineered in that way. (remember power levels and required SNR!)

Optical passthrough

Add/Drop Add/Drop

Transponder

OLT

OADM

Node A

Node A

Node B Node B

Node C Node C

(a) (b)

Optical passthroughOptical passthrough

Add/Drop Add/Drop

Transponder

OLT

OADM

Node A

Node A

Node B Node B

Node C Node C

(a) (b)

Parallel OADM Architecture^!

No constraints on what

λ

-s can be adropped (minimal constraints on planning

lightpaths).

!

Loss is fixed (adropping additional channels is easy).

!

Not cost effective if adropping small number of

λ

-s.

!

Since all

λ

-s are always re-multiplexed, the tolerance of lasers/filters must be

stringent.

1

2

W

1

2

W

W

2

1

Drop

Add

Demux

Mux

1

2

W

1

2

W

W

2

1

Drop

Add

Demux

Mux

Modular Parallel OADMArchitecture^!

Implies constraints on what

-s can be adropped.

!

Cost effective also if adropping small number of

-s.

!

The tolerance of lasers/filters can be higher

!

Loss is fixed (adropping additional channels is easy).

!

Modular

multistage

approaches are also used today

!

Loss is not uniform for all

-s.

λ

1

,

λ

2

,…,

λ

W

λ

1

,

λ

2

,…,

λ

W

Band 4

Drop

Add

Demux

Mux

Band 3Band 2

Band 1

λ

1

,

λ

2

λ

1

,

λ

2

,…,

λ

W

λ

1

,

λ

2

,…,

λ

W

Band 4

Drop

Add

Demux

Mux

Band 3Band 2

Band 1

λ

1

,

λ

2