9.4 MAN and WAN Services

Over the past few years , the network services industry has undergone a boom and bust cycle. As a result, many new service providers entered the market, but some already have gone out of business. What's left behind, though, in the United States and other parts of the world, is a glut of installed but unused fiber- optic cable. At the same time, wave division multiplexer vendors continue to improve their products to support more and more logical circuits on the cable that's already in use, further expanding the supply of circuits.

While this boom-and-bust cycle has thrown the networking industry into a state of confusion, the resulting fiber-optic cable surplus has helped to drive down the monthly lease prices paid by subscribers ” especially in unregulated areas. In regulated areas, the service provider simply maintains prices and leaves the unused fiber unlit. But even in those areas, new technologies, such as fixed wireless and line-of-sight optical, now offer cost-effective alternatives.

The last mile and local loop are terms sometimes used in reference to the local connection between the customer premises and the serving central office ( CO ) . For residential dial services, the local loop almost exclusively is metallic twisted-pair cables. With DSL technology, it is possible to transmit a few megabits per second of data over a two- or three-mile long metallic circuit.

Nonresidential subscribers typically have more options for the local network connection. In densely populated urban areas, there could be many potential subscribers in a single building, so service providers may install fiber-optic cable to the building before they have an order, speculating that they can recover their installation costs by landing multiple customers later.

Those installations typically include a SONET or SDH multiplexer that serves as the network access point for all of the subscribers. The mux may be located within the building, but usually not on any customer's premises, since around-the-clock access is required by the service provider and because subscribers don't want their data passing through a potential competitor's facilities. Service providers sometimes refer to this as customer located equipment ( CLE ) to distinguish it from CPE or CO equipment.

In a high-rise office building, the CLE is linked to the subscriber's offices with a cable that passes through a riser conduit or elevator shaft. The demarcation point is the end of the cable that terminates on the customer's premises. Today, the riser cable typically is fiber optic, but it also may be coaxial cable for DS3 and E3 services, or twisted-pair metallic cable for voice and lower speed data services.

In some instances, the service provider may install the multiplexer in a street cabinet or an underground telecommunications vault, which can serve as the network access point for subscribers in many adjacent office buildings . If an existing cabinet or vault has room and a suitable environment for the mux, the service provider can use it as a quicker and cheaper alternative to leasing CLE facilities inside the building.

Office buildings in suburban areas generally have no fiber-optic cable installed unless the previous tenant paid for its installation. Still, it may be only a few blocks away or even passing in front of the building, which would reduce the cost and time required to extend it to a new subscriber. Figure 9-10 shows some of the various methods used by service providers for connecting to their data networking subscribers.

Figure 9-10. Service provider connections vary by subscriber density and location.

With telecom deregulation and reforms in the 1980s and 1990s, the distinctions among categories of service providers have blurred. In simple terms, local exchange carriers (LECs) handle metropolitan and regional services, and interexchange carriers (IXCs) handle long-distance services. But now it's possible for the LECs and IXCs to get into each other's businesses. Competitive local exchange carriers (CLECs) and ELECs generally compete only with the LECs, but some of them have national links between metropolitan areas.

Any of these service providers also could be an Internet service provider ( ISP ) . There are many independent ISPs as well. While the traditional service providers generally specialize in layer-one services, such as dial-up and high-speed data connections, ISPs handle layer-three (IP) data. As previously defined, layer-three packets are sent over layer one facilities or, as in the case of Ethernet services, over layers one and two. If there are multiple service providers involved, the ISPs typically split off their Internet-bound data at the COs, send it to their own backbone facilities or an ISP peering point, and leave the remaining network traffic (such as voice and non-Internet data) for the other service provider to handle.

As described previously in this chapter, shared SONET and SDH network subscribers each are assigned fixed-bandwidth point-to-point channels that are logically separated from all other subscribers' data. By comparison, in layer-two or layer-three networks, such as the Internet, subscribers share a multipoint network, each using a variable portion of a large pool of bandwidth as needed, typically making its bandwidth utilization much greater.

This inherent ability to pool bandwidth among layer-three network subscribers enables ISPs to offer a range of prices, according to the amount of oversubscription used and the resulting service quality. Residential Internet services may be economical, but most of them include no commitments by the ISPs to limit the network latency or deliver a specified minimum amount of bandwidth. At the other end of the range, ISPs may offer more costly premium services, including service level agreements (SLAs), which are aimed at business customers.

Virtual private networks (VPNs) can be used to prevent eavesdropping by other subscribers in a shared public network, such as the Internet. VPN software or dedicated VPN switches encrypt the IP packet and wrap it in another unencrypted IP packet whose address, but not its payload, can be read by the Internet routers. The process then is reversed at the destination and the original packet is delivered intact. VPNs also can be used in private IP networks to isolate and protect the data from eavesdropping by other users within the organization. VPN technology is explained in greater detail in the following section of this chapter.

The more costly premium Internet services are used for most storage networking data, since its volume and criticality typically surpass that of any other type of data. Or more likely, the storage networking data is sent instead over private shared IP networks. In either case, data compression can be used between sites to reduce the bandwidth required, thereby cutting the bandwidth cost or improving the performance of the applications.

Some IP storage switches and most IP routers now offer data compression, so the storage data can be compressed before it's sent to the router, or it can be compressed along with all other data by the router itself. Depending on the repetitiveness of the data, the compression ratios can be as great as 5:1 or better. For backup applications that span long distances or require large volumes of data to be transmitted, data compression can substantially reduce one of the biggest budget items ”the cost of bandwidth.

Another possible source of cost savings ”especially for relatively short distances ”is some form of local loop bypass. Dedicated fiber was mentioned as an alternative previously in this chapter, but its installation cost may be prohibitively high. However, less conventional alternatives recently have become available and probably merit investigation for use where fiber-optic service is costly or not readily available.

Fixed-wireless devices now support data rates of tens of megabits per second over distances of a few miles. Radio-frequency interference and the requirement for rooftop installation sometimes can present insurmountable challenges, but the signal quality is getting better and the size of the equipment is being reduced, which makes it suitable for more applications.

Free-space optical systems also may be a workable alternative for local bypass. They do not suffer from radio interference, but the signal can be affected by physical interference, such as buildings, birds, or fog. Assuming that other buildings can be avoided and a line-of-sight link can be established, the impact of some of the other hazards can be reduced by installing backup links so the signal may be able to find a better alternative path .

Performances of hundreds of megabits per second over distances of a mile or so now are possible for some of these optical systems. Another advantage is that the transceivers can be installed indoors, behind many types of office windows , so it may be possible to avoid the hassle of installing the units on rooftops.