The Optical Edge

Today, SDH/SONET channels are used for voice and time-division-multiplexed private-line traffic where the equipment typically involves ADMs and digital cross-connects (see Figure 11.11). Optical networks today are also used to support Layer 2 services, such as ATM, Frame Relay, and Ethernet, and switch equipment. In addition, they are used to support Layer 3 services, such as routed IP; IP routers are the most typical equipment type at Layer 3.

Figure 11.11. The optical edge

Vast changes are occurring at the optical edge. As discussed in the following sections, three categories of optical access products (i.e., specialized access equipment) are being introduced at the optical edge: next-generation digital loop carrier systems, PONs, and MSPPs.

Next-Generation Digital Loop Carriers

The main feature of a next-generation digital loop carrier is its support for SDH/SONET fiber as well as copper connections to the local exchange. These devices allow the integration of DSL modem support without requiring the installation of DSL access multiplexers (DSLAMs). In other words, the DSLAM functionality is built into the next-generation digital loop carrier. Also, the new digital loop carriers can support ATM multiplexing for DSL services along with the traditional TDM interfaces for voice. Voice channels are multiplexed in the traditional TDM style and sent over the PSTN, and data traffic is combined and sent on one shared ATM trunk, with each customer assigned a separate ATM virtual circuit.

Along with support for plain old telephone service (POTS), ISDN, and DSL interfaces, new interfaces are also being added to support a wider range of services. These include 10Mbps and 100Mbps Ethernet in support of emerging metro area Ethernet services, PON headends in support of PON service, and OADMs that support the provisioning of wavelength services.

Ultimately, the biggest issue facing the deployment of next-generation digital loop carriers is regulation. Regulation adds a level of complexity due to disputes over which portions of the local loop the incumbent local exchange carriers (ILECs) are required to wholesale to their competitors.

PONs

A PON is basically a fiber-to-the-premises (FTTP) arrangement in which a single optical fiber serves multiple (up to 32) premises. It is, in essence, a point-to-multipoint configuration that reduces the amount of fiber required. Taking advantage of WDM, PONs allow for two-way traffic on a single fiber pair by making use of one wavelength for downstream traffic and another for upstream traffic. PONs work by using passive (i.e., unpowered) optical splitters to split the power of the optical signal and route it to multiple subscribers. (Figure 12.8 in Chapter 12, "Broadband Access Alternatives," shows an example of a PON.)

A PON shares one fiber channel among up to 32 customers to deliver voice, data, and potentially video. PONs reduce costs by distributing costs across more endpoints and replacing expensive ADMs or DWDM nodes with optical splitters and couplers at each fiber connection in the network. Downstream signals, broadcast to each premise by using a shared fiber, are encrypted to prevent eavesdropping. Upstream bandwidth is allocated by assigning a time slot to each subscriber when the user has traffic to send.

PONs have started to be of more interest to local telcos because they significantly reduce the cost of provisioning fiber to the subscriber compared to approaches such as fiber-to-the-curb or fiber-to-the-home. (Chapter 12 provides a comprehensive discussion of fiber-to-the-x and PONs.)

MSPPs

MSPPs are specialized optical access systems that enable carriers to get a quick start in offering a full range of services. Typically, MSPPs reside at the carrier's POP or at the customer's site. They incorporate WDM, allowing service providers and customers to take advantage of the fact that a fiber can carry much more than the single wavelength that SDH/SONET fibers today carry. With MSPPs, carriers can also offer customers different grades of service, as well as telephony and other multimedia offerings.

Today's MSPPs are based largely on proprietary technology, so they vary from provider to provider, but they all share some basic characteristics, including the following:

• Fiber access Optical interfaces can use SDH/SONET, Gigabit Ethernet (GigE), or 10GigE, and they can include an automatic restoration capability.
• Digital multiplexing MSPPs can use TDM, a form of packet access, or a combination of both.
• Support for WDM Many MSPPs incorporate CWDM, so a single fiber pair can support multiple customers. Coarse wavelength division multiplexers were developed for metro and campus environments. They use a less powerful laser source, reducing their capabilities but also reducing their costs and making it easier to justify using them for the type of short-haul applications metro and campus environments represent. Because the laser sources are less powerful with CWDM, the channels need to be spaced further apart (at 100GHz or 200GHz intervals), and one result of this is that fewer channels can be derived. Generally, CWDM is limited to no more than 64 channels. But because of the lower cost of the light sources, CWDM devices are much less costly to build and can be more justified in their deployment in the metro network.