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13 Fiber Optic Communication Networks
A variety of ROADM architectures are possible. This section shows three example
architectures based on wavelength blockers, arrays of small switches, and wavelength-selective switches. Section 13.6 describes the concept of an optical crossconnect, which uses a more complex architecture of multiple wavelength-selective
switches. Each ROADM type can have a number of alternative configurations and
different operational characteristics. The design of a particular ROADM depends
on factors such as cost, reliability, technology maturity, desired network operating
flexibility, and upgrade capability of the equipment. ROADM characteristics include:
• Wavelength dependence. If the architectures are wavelength-dependent they are
said to be colored or have colored ports. When the ROADM operation is independent of wavelength, it is referred to as being colorless or having colorless
ports.
• ROADM degree. The degree of a ROADM refers to the number of bidirectional
multiple-wavelength interfaces the device supports. Thus a degree-2 ROADM
has two bidirectional WDM interfaces and a degree-4 ROADM can support up
to four bidirectional WDM interfaces, for example, in north, south, east and west
directions.
• Remote reconfiguration. The ability to change the ROADM configuration from
a remotely located network management workstation is an important feature,
because it greatly reduces operations costs by eliminating the need for a service
person to visit the ROADM site in order to manually upgrade the device.
• Express channels. The ability to have express channels that allow a selected
set of wavelengths to pass through the node without the need for optical-toelectrical-to-optical conversion saves the expense of having optical transceivers
at the ROADM for those wavelengths.
• Modular expansion. To avoid an initial high setup cost to connect transmitters
and receivers to each add/drop port, service providers usually first activate the
minimum number of ports needed to support current traffic and then later add
more channels as the service demand increases. This is known popularly as the
pay-as-you-grow approach.
• Minimum optical impairment. Having an express wavelength feature requires
a careful engineering design to avoid the accumulation of optical signal impairments as a particular wavelength set passes through several sequential ROADMs.
These include effects such as crosstalk between channels, wavelength-dependent
attenuation, ASE noise, and polarization-dependent loss.
Wavelength Blocker Configuration
Figure 13.21 shows the simplest ROADM configuration, which uses a broadcastand-select approach. In this degree-2 architecture a passive optical coupler splits the
incoming lightwave signal power into two paths. One branch is an express path and
the other branch is diverted to a drop site. Located in the express path is a device called
a wavelength blocker that can be configured to block those wavelengths that are to
be received at the node. The drop segment contains a 1 × N optical power splitter,
which divides and directs the light signal into N tunable filters that allow selection of
13 Fiber Optic Communication Networks
A variety of ROADM architectures are possible. This section shows three example
architectures based on wavelength blockers, arrays of small switches, and wavelength-selective switches. Section 13.6 describes the concept of an optical crossconnect, which uses a more complex architecture of multiple wavelength-selective
switches. Each ROADM type can have a number of alternative configurations and
different operational characteristics. The design of a particular ROADM depends
on factors such as cost, reliability, technology maturity, desired network operating
flexibility, and upgrade capability of the equipment. ROADM characteristics include:
• Wavelength dependence. If the architectures are wavelength-dependent they are
said to be colored or have colored ports. When the ROADM operation is independent of wavelength, it is referred to as being colorless or having colorless
ports.
• ROADM degree. The degree of a ROADM refers to the number of bidirectional
multiple-wavelength interfaces the device supports. Thus a degree-2 ROADM
has two bidirectional WDM interfaces and a degree-4 ROADM can support up
to four bidirectional WDM interfaces, for example, in north, south, east and west
directions.
• Remote reconfiguration. The ability to change the ROADM configuration from
a remotely located network management workstation is an important feature,
because it greatly reduces operations costs by eliminating the need for a service
person to visit the ROADM site in order to manually upgrade the device.
• Express channels. The ability to have express channels that allow a selected
set of wavelengths to pass through the node without the need for optical-toelectrical-to-optical conversion saves the expense of having optical transceivers
at the ROADM for those wavelengths.
• Modular expansion. To avoid an initial high setup cost to connect transmitters
and receivers to each add/drop port, service providers usually first activate the
minimum number of ports needed to support current traffic and then later add
more channels as the service demand increases. This is known popularly as the
pay-as-you-grow approach.
• Minimum optical impairment. Having an express wavelength feature requires
a careful engineering design to avoid the accumulation of optical signal impairments as a particular wavelength set passes through several sequential ROADMs.
These include effects such as crosstalk between channels, wavelength-dependent
attenuation, ASE noise, and polarization-dependent loss.
Wavelength Blocker Configuration
Figure 13.21 shows the simplest ROADM configuration, which uses a broadcastand-select approach. In this degree-2 architecture a passive optical coupler splits the
incoming lightwave signal power into two paths. One branch is an express path and
the other branch is diverted to a drop site. Located in the express path is a device called
a wavelength blocker that can be configured to block those wavelengths that are to
be received at the node. The drop segment contains a 1 × N optical power splitter,
which divides and directs the light signal into N tunable filters that allow selection of