13.7 WDM Network Implementations
555
ranges from 10 to 40 km in circumference and typically contains three or four nodes,
whereas a maximum of 32 users located up to 20 km away can be attached to a single
PON. Within the access network ring, either a fixed OADM or a ROADM provides
the capability to add or drop multiple wavelengths to local users or to other regional
networks. A degree-4 OXC provides the connection from the access ring to a metro
network.
In contrast to the stringent performance specifications imposed on wideband longhaul DWDM systems, the shorter spans in metro and access networks relax some of
the requirements. In particular, if CWDM technology is employed, the 20-nm spectral
band tolerances allow the use of optoelectronic devices that are not temperaturecontrolled. However, other requirements unique to metro applications arise, such as
the following:
• A high degree of connectivity is required to support meshed traffic in which various
wavelengths are inserted and extracted at different points along the path;
• A modular and flexible switching device such as a ROADM is needed because the
wavelength add/drop patterns and link capacities vary dynamically with various
levels of new and completed service demands from different nodes;
• Because the add/drop functions can change dynamically from node to node,
special components, such as a variable optical attenuator (VOA), are needed
to equalize the power levels of newly added wavelengths with those that already
are on the fiber;
• Optical amplifiers optimized for metro network use are needed, because interconnection losses can be fairly high and the power level of express wavelengths can
change as the light signal passes through successive nodes.
13.7.3 Data Center Networks
The tremendous worldwide growth of dynamic data communications and cloudbased services created a need to move the information products of computation intensive functions from localized devices such as personal computers, cell phones, security cameras, and environmental monitors to large centralized storage and computer
clusters. These facilities are known as data centers (also spelled “datacenters”) and
can contain thousands of servers, storage units, and network equipment that need to
be interconnected inside the data centers and also must be linked to metro and wide
area networks. The interconnections form a data center network (DCN) [50–55].
Figure 13.39 presents a high level snapshot of a traditional DCN architecture,
which consists of a three-layer tree switching topology. A typical data center contains
a large number of equipment racks that each generally hold up to 48 vertically stacked
servers. These servers are all interconnected through an edge switch that is mounted
at the top of the rack and hence is referred to as Top-of-Rack (ToR) switch. The
ToR switch is called an edge switch because it collects traffic flows from various
sources at the periphery of the DCN. As such, the ToR switches function as the
access point to other racks in the data center. At the next higher level numerous
555
ranges from 10 to 40 km in circumference and typically contains three or four nodes,
whereas a maximum of 32 users located up to 20 km away can be attached to a single
PON. Within the access network ring, either a fixed OADM or a ROADM provides
the capability to add or drop multiple wavelengths to local users or to other regional
networks. A degree-4 OXC provides the connection from the access ring to a metro
network.
In contrast to the stringent performance specifications imposed on wideband longhaul DWDM systems, the shorter spans in metro and access networks relax some of
the requirements. In particular, if CWDM technology is employed, the 20-nm spectral
band tolerances allow the use of optoelectronic devices that are not temperaturecontrolled. However, other requirements unique to metro applications arise, such as
the following:
• A high degree of connectivity is required to support meshed traffic in which various
wavelengths are inserted and extracted at different points along the path;
• A modular and flexible switching device such as a ROADM is needed because the
wavelength add/drop patterns and link capacities vary dynamically with various
levels of new and completed service demands from different nodes;
• Because the add/drop functions can change dynamically from node to node,
special components, such as a variable optical attenuator (VOA), are needed
to equalize the power levels of newly added wavelengths with those that already
are on the fiber;
• Optical amplifiers optimized for metro network use are needed, because interconnection losses can be fairly high and the power level of express wavelengths can
change as the light signal passes through successive nodes.
13.7.3 Data Center Networks
The tremendous worldwide growth of dynamic data communications and cloudbased services created a need to move the information products of computation intensive functions from localized devices such as personal computers, cell phones, security cameras, and environmental monitors to large centralized storage and computer
clusters. These facilities are known as data centers (also spelled “datacenters”) and
can contain thousands of servers, storage units, and network equipment that need to
be interconnected inside the data centers and also must be linked to metro and wide
area networks. The interconnections form a data center network (DCN) [50–55].
Figure 13.39 presents a high level snapshot of a traditional DCN architecture,
which consists of a three-layer tree switching topology. A typical data center contains
a large number of equipment racks that each generally hold up to 48 vertically stacked
servers. These servers are all interconnected through an edge switch that is mounted
at the top of the rack and hence is referred to as Top-of-Rack (ToR) switch. The
ToR switch is called an edge switch because it collects traffic flows from various
sources at the periphery of the DCN. As such, the ToR switches function as the
access point to other racks in the data center. At the next higher level numerous