Why haven't you moved to fiber?

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Metro Networks

A primer on fiber-optic access in the metropolitan-area network

CORINNA CORNEJO, Larscom Inc.

Demand for bandwidth is growing exponentially. It's not just the exotic applications like video-on-demand and peer-to-peer file sharing that are leading to this dramatic increase in demand; it's also the seemingly simple, yet ubiquitous, transportation of voice and data, like e-mail, that is fueling this growth in demand for bandwidth.

Industry analyst RHK Inc. (San Francisco) reported that worldwide demand for bandwidth grew about 200% between 1998 and 2000. RHK expects demand to continue growing. They estimate another 400% growth in demand between 2000 and 2002.

How can you evolve your network to satisfy this ever-increasing demand for bandwidth? By incorporating fiber-optic access devices into the topology of your network to link to the dark fiber in the metropolitan-area network.

The advantages of migrating to fiber network access are widely publicized: significantly greater access speeds, unlimited scalability, transmission reliability, and futureproofing the network topology. Yet, many organizations hesitate to make the migration. Concerns about "losing" their investment in legacy network access equipment and the cost of migrating to fiber keep many organizations from making the move. These concerns are turning out to be largely unfounded.

It is a fallacy to assume that a complete change-out of legacy network equipment (i.e., multiplexers, inverse multiplexers, and routers) is necessary to take advantage of fiber speeds. Whether you're considering the migration of voice via private branch exchange (PBX), fractional T1 (1.554 Mbits/sec) to full T3 (44.736 Mbits/sec) routers, or even video equipment, current legacy access equipment can connect with fiber equipment. Legacy equipment can be connected either using media converters or directly to a fiber transceiver with 10/100Base-T Ethernet.

Having legacy network access equipment coexist with fiber equipment can be a long- or short-term arrangement. Traditional technologies are expected to coexist with SONET, ATM, frequency-division multiplexing (FDM), and WDM in fiber networks for the foreseeable future. The reality of today's network is that much of the local traffic is still moving at T1 speeds. So legacy multiplexers, inverse multiplexers, and routers still serve a useful role. Migrating to fiber does not require a complete change-out of all the equipment in your network. Both copper and fiber can effectively coexist on the same network.

Market forces are joining to lower both the initial costs of migrating to fiber and the ongoing costs of subscribing to fiber access. Beyond that, technological advancements are keeping the costs of running a fiber network below that of copper networks.

Carriers and utilities have a stockpile of "dark" fiber already installed but not provisioned. This dark fiber is serviceable today for OC-3 (155-Mbit/sec) and OC-12 (622-Mbit/sec) connections. Phillips Group reported in "Dark Fiber USA" that dark fiber pricing has remained relatively stable and predicted dark fiber prices will decline gradually as the total fiber capacity increases in some markets.

Alternative carriers, like power utility companies and railroads, are exploiting their rights-of-way to enter the fiber access market. These alternative carriers are generating new revenue streams for their parent companies by cashing in on fiber resources that have been already capitalized. These new sources of competition are pressuring traditional carriers to keep subscription prices down.

The Tolly Group found fiber to be a cost-effective alternative to copper and urged enterprises to consider fiber on this basis. The group's study examined network design and active and passive equipment associated with both fiber and copper networks and concluded that fiber-based networks can cost as much as 15%-22% less than copper.

Additional operation savings can be realized due to the lower network maintenance costs and faster troubleshooting associated with fiber technologies. Fiber networking equipment can be centrally deployed, reducing the need for telecommunications rooms and their associated real estate, power, temperature control, and ventilation costs.

Fiber data and voice transmission offers error-free transmission over long and short distances. Unlike copper, fiber is immune to electromagnetic and radio-frequency interference. Fiber is also immune to crosstalk.

Fiber offers a secure pipe for data. It cannot be tapped into or otherwise intercepted in the same manner as copper wire and wireless transmissions. That's a critically important consideration when transmitting sensitive data.

Fiber supports high data transmission rates, essentially futureproofing your network in the face of ever-increasing demand for bandwidth. With fiber, your network can evolve to support high-bandwidth applications, including video and audio transmission, as well as data backup.

What would an example of adding fiber access to the metro network look like? Consider Company A has just acquired Company B, which is located at the other end of the metro area. Company A will maintain the two physical campuses after the acquisition is completed-Company A's campus will be headquarters and Company B's campus will become the technical development campus.Th 0106lwspr18f1

Figure 1. A fiber-optic connection can be deployed between two campuses, creating a high-speed connection for data transmission and online collaboration.

Both campuses have multiple established connections to the WAN and the public-switched telephone network (PSTN). Multiple connections provide each campus the safety of redundant access to these networks. Neither campus has a direct connection to the other. The company wants to deploy a link between the two campuses so high-speed data transmission and online collaboration, including videoconferencing, can take place between engineering teams and other departments located in both sites (see Figure 1).

The simplest way to build the high-speed connection between campuses would be to install a point-to-point fiber connection with a fiber access device at either end. That would satisfy Company A's desire to have a high-speed link between the two campuses. It would also allow the company to save some access costs by decommissioning the direct connections at one of the campuses to the WAN and PSTN.

While a point-to-point fiber connection is a good solution, it does not address all the communication concerns of the company and its campuses. In addition to a high-speed connection between the campuses, Company A also wants to increase the access speeds to the WAN. Redundancy in access is an important consideration. In a simple point-to-point link, one or the other campus would remain vulnerable to interruptions in its network access should the link between the campuses go down.Th 0106lwspr18f2

Figure 2. By linking directly to the fiber metropolitan-area network, both campuses realize the advantages of high-speed access along with the added security of a self-healing connection to the dual-ring-metro and wide-area networks.

The solution is to install fiber access devices at both campuses and use them to link directly to the fiber metro network (see Figure 2). That will allow both campuses to have direct high-speed access through the metro-network ring to the WAN and PSTN. Since access to the metro ring is shared among a number of customers, the subscription costs will be lower than a dedicated line.

By accessing the metro ring directly, both campuses will enjoy the security of self-healing access to the dual-ring-metro and wide-area networks, thus avoiding vulnerability to access interruptions presented by a point-to-point link. The company also stands to save access costs in this scenario because a single point of access to the fiber metro ring on each campus will replace the multiple access connections currently provisioned at both campuses.

When making the migration to fiber with the future intention of a full fiber migration, look for scalable fiber network access equipment that will allow you to invest for today's traffic and tomorrow's speeds in one device. Today, it's reasonable to provision a SONET-based OC-3 or OC-12 connection. In the mid-term, expect demand for bandwidth to grow to justify an OC-48 (2.5-Gbit/sec) connection.

A flexible fiber-optic multiplexer for network access will allow you to customize the traffic capacity as your needs evolve. Look for an access device that can support the multiplexing of a number of different input/output (I/O) modules at various access speeds: T1 through OC-12 and beyond. Such a flexible access device should also be capable of being configured to support various combinations of access speeds to support multiple T1 lines, for example, or a mix of T1, DS-3 (44.736-Mbit/sec), and OC-3 lines.

A key differentiator among fiber-optic access multiplexers is the soft backplane. This software-defined backplane enables maximum flexibility in the installation of I/O modules and allows the multiplexer to accept the highest-speed I/O modules without physically changing the backplane.

Traditional time-division multiplexing (TDM) backplanes limit the speed and type of I/O module that can be installed in any given slot on the multiplexer. A soft backplane allows any I/O module to be installed in any slot. Additionally, the traditional TDM backplane requires that the backplane itself be physically replaced when the speed of an I/O module exceeds the speed for which the backplane was defined. With a soft backplane, there is no need to change out the backplane itself to accommodate higher access speeds because the backplane is defined logically, not hard-wired.

A fiber-optic network access device having the ability to "grow" with your organization will also minimize interruptions on your network by enabling migration to faster speeds without requiring a truck-roll upgrade. An additional advantage is that once familiar with the operation and user interface for such a scalable access device, you will avoid the learning curve inherent in moving to a new platform.

Fiber-optic access devices exist today that can both support your legacy equipment and provide the capability to migrate more completely to fiber in the future. Flexibility and scalability in I/O modules and a software-defined backplane are essential capabilities to look for in fiber access equipment that will take your network from the T1 speeds of today to the OC-48 speeds of tomorrow.


Corinna Cornejo is marketing communications manager for Larscom Inc. (Milpitas, CA). She can be reached via the company's Website, www.larscom.com.

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