Architecture of WDM:

i. The general architecture of wavelength routing mesh network is as shown in figure 4.24:

ii. The different WDM network elements like Optical Line Terminals (OLT), Optical Add Drop Multiplexer (OADM), and Optical Cross Connects (OXC) etc. are shown in the architecture.

iii. OLTs are placed either at the end of links or in point-to-point configurations. OADMs are used at places where some fractions of the wavelengths need to be terminated and others need to be added and are typically in linear or ring topologies.

iv. OXCs enable mesh topologies and switching of wavelengths. Clients of these networks can be ATM, SONET, IP switches using the optical layer.

v. Network supports a variety of client types, such as IP routers, ATM switches, and SONET terminals and ADMs.

Fig4.24: Architecture of wavelength Routing mesh

Optical Line Terminals (OLT):

i. They are used at either end of a point-to-point link to multiplex and demultiplex wavelengths.

Fig 4.24(a): Optical Line Terminal

ii. Three functional elements inside an OLT: transponders, wavelength multiplexers, and optionally, optical amplifiers.

iii. A transponder adapts the signal coming in from a client of the optical network into a signal suitable for use inside the optical network.

iv. Likewise, in the reverse direction, it adapts the signal from the optical network into a signal suitable for the client.

v. Interface between the client and the distance and loss between the client and the transponder.

vi. The most common interface is the SONET/SDH short - reach (SR).

Optical Add/Drop Multiplexers (OADM):

i. Optical add/drop multiplexers (OADMs) provide a cost-effective means for handling pass-through traffic in both metro and long-haul networks. „

ii. OADMs may be used at amplifier sites in long-haul networks but can also be used as stand-alone network elements, particularly in metro networks.

Fig4.25: Three node linear network examples

iii. In figure 4.25, three node linear network examples to illustrate the role of optical add/drop multiplexers.

iv. Three wavelengths are needed between nodes A and C, and one wavelength each between nodes A and B and between nodes B and C.

4.25(a): Solution using point-to-point WDM systems.

4.25(b): Solution using an optical add/drop multiplexer at node B.

Parallel OADM Architecture:

Fig4.26 (a): Parallel OADM Architecture

i. No constraints on what λ-s can be adropped (minimal constraints on planning lightpaths).

ii. Loss is fixed 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.

Modular Parallel OADM Architecture:

Fig 4.26(b): Modular Parallel OADM Architecture

i. Implies constraints on what λ-s can be adropped. Cost effective also if adropping small number of λ-s.

ii. The tolerance of lasers/filters can be higher. Loss is fixed (adropping additional channels is easy). Loss is not uniform for all λ-s.

Serial OADM Architecture:

Fig 4.26(c): Serial OADM Architecture

i. A single channel is adropped (SC-OADM). To drop multiple channels, SC-OADMs can be cascaded.

ii. Adding additional SC-OADMs disrupts existing channels for a short while, thus planning is needed ahead of time.

iii. Highly modular (cost is low for less λ-s). Loss increases with λ-s to be adropped which may require additional OLAs.

Band-drop OADM Architecture:

Fig 4.26(d): Band-drop OADM Architecture

Fixed group of λ-s is adropped and undergo a further level of demultiplexing. Adropping additional λ-s does not affect loss (if this λ-s are in the group).

Optical Cross connects (OXC):

i. OXCs are required to handle mesh topologies (OADMs have only two ports making them available for ring and point-to-point only).

ii. OXC are also key elements for reconfigurability. Some ports are connected to other OXCs and some are terminated by optical layer client equipment (SONET, ATM, etc.). OXCs usually do not contain OLTs (separate products).

Fig 4.27: A Typical OXC