Various network topologies are used to meet the needs of high-speed traffic in the access network: hub and spoke, multidrop, ring, and mesh. In hub-and-spoke networks, data can be aggregated and sent point to point using either single-channel or multichannel techniques. Each method and its characteristics are presented here.

Aggregated Point to Point Using a Single Channel per Optical Fiber

• The cost of channel cards with interfaces to CPE or PoP/CO equipment can be reduced in the absence of WDM links (to which incremental cost is attached).

• Management costs can be reduced through such techniques as software-configured rates and management-initiated diagnostics.

• Where WDM can provide some benefit, either because of fiber conservation needs or because longer distances increase the cost of leasing fiber, wide WDM can be an option for single-fiber links.

Figure 1. Aggregated Point to Point

Aggregated Multichannel Point to Point

Coarse or wide WDM links reduce cost of optics and provide a perfectly satisfactory multichannel solution for access network links where high-density wavelength multiplexing is not required. It is highly likely that multichannel systems will be intermixed with single-channel, point-to-point links.

Figure 2. Aggregated Multichannel Point to Point

Spatially Distributed WDM

• This is most often evident in multidrop configurations.

• It is generally appropriate for campus and riser applications.

Figure 3. Spatially Distributed WDM

Arbitrary Mesh

• Inevitably, as requirements evolve, nodes in the access network will need to be linked, to connect segments of customer networks.

• Both WDM and non–WDM links will be required.

• While this stretches the meaning of “access” as defined here, it must be recognized that enterprise networking is designed to meet real customer needs—sometimes straying beyond the bounds of convenient technology definitions.

Figure 4. Arbitrary Mesh

Link Protection

• Many practical network applications will require some form of redundancy or protection switching.

• The industry regards the ability to switch from one link to a backup within 50 ms as a de facto standard, even though for many data applications there is no firm basis for this figure. In practice, even shorter switching times are possible.

Figure 5. Link Protection