Network Computingwww.networkcomputing.com

SONET is not only fast; it's versatile. Voice calls from one office to another can be multiplexed and placed on a SONET fiber along with data. With the bandwidth SONET affords, compression or encapsulation into an IP packet is unnecessary. A single OC-3 connection can carry more than 2,000 uncompressed simultaneous voice calls, and all data types can be multiplexed right beside the calls. Frame relay signals can be taken directly from DS-0 to DS-3 and placed into a SONET frame along with ATM cells from DS-1 up. You won't need separate networks carrying varied data types from office to office, because they can be combined into SONET frames.

Because many of SONET's advantages are unmatched by other Layer 1 protocols--such as the capability to combine data types along with voice calls over a single fiber--installing a SONET network is a smart choice for many businesses. And whether you are calling your national carrier or LEC (local exchange carrier) to provide fiber or calling out the backhoes to run your own, it is important to understand everything involved in how SONET works.

SONET offers bandwidth up to OC-192 (9,953.28 Mbps) and can carry a wide variety of information. SONET also offers exceptional BERs (bit-error rates) of 1x10-10 (1 error in 10 billion bits) compared with copper transmission methods of 1x10-6 (1 error in 1 million bits). Even with these pluses, SONET networks need to be carefully designed to facilitate growth and protection based on your needs.

Backhoe Not Required

Before you start tearing up the street, note that SONET services can be provided by a carrier or LEC that may also help you design your network efficiently (see "No Competition Among Local Providers," page 136, for more information on leasing these services). Everything starts with a pair of fibers. Fiber is usually available from national carriers or LECs in either lit or dark form. For the purpose of this article, lit fiber is connected to a provider's network and has a signal that is generated by the provider's equipment. Dark fiber connects nodes without attaching to the provider's network. If lit fiber is installed, your choice of equipment will be limited. Usually, the provider will supply the equipment or at least offer a list of devices you can use.

Although SONET is a standard, vendors use a few bytes as they please. Most vendors, however, maintain code that will allow interoperability between their equipment and someone else's. Using lit fiber lets you move voice and data over the provider's network to PSTN or Internet services.

Dark fiber has the advantage of being yours to manipulate, within reason. You can place any brand equipment on the line at possibly any OC speed, as long as no repeaters are on the line. Repeaters placed on the line by the carrier or LEC will be designed for whatever data rate you specify but won't be able to repeat the data on the line if you begin sending data at a higher rate. Dark fiber can be installed as point-to-point connections between buildings in a campus environment or between offices within the same city.

Building a Network

SONET networks can be built in one of three fashions: linear, ring or a combination of the two. Linear networks are just as they sound: SONET switches are connected in a linear fashion with two end points and a number of switches in between. The best example of a linear network is a 10BASE-2 Ethernet network. Ring networks connect SONET switches in a round-robin fashion similar to a token-ring network.

Information can travel over two types of fiber: single mode or multimode. Multimode fiber has a larger diameter and uses infrared light (or diodes) at the transmitting end. Because of the infrared signal's wavelength, it can travel only less than a mile before it needs to be regenerated. It is therefore used primarily in LANs or short-hop campus connections. If your data needs to travel a longer distance, then single-mode fiber is the way to go. The fiber is thinner and more fragile than multimode, so it should be handled carefully.

In a simple SONET linear network, PTE (path-terminating equipment) sits at each end. This equipment takes subrate signals and multiplexes them into a SONET frame. Between the PTE devices may be several LTE (line-terminating equipment) devices. An LTE is an ADM (add/drop multiplexer); instead of demultiplexing the entire signal, as the PTE does, it pulls out only the subrate services that it needs and inserts different signals down the line. If your network is long enough, STE (section-terminating equipment) will be in the fiber somewhere. These repeaters or regenerators take the incoming signal from one side and rebroadcast it out the other side. Placement of STE in the network depends on the strength of the signal being transmitted down the fiber.

Single-mode fiber can have three or more strengths of lasers broadcasting the signal: LH (long haul), MH (medium haul) or SH (short haul). LH is the most powerful in this example and lets a signal travel approximately 90 miles before regeneration. MH has a maximum transmission distance of approximately 15 miles to 25 miles, and SH is rated for 1 mile to 2 miles. Because the lasers' strengths are tuned to the distances they need to travel, you should make sure your equipment matches your distance needs. You would burn out the receivers on both ends if you connected two single-mode LH switches with only 10 feet of fiber.

Linear SONET networks are the easiest to design and install. They don't offer the redundancy available with ring networks but are good if point-to-point, noncritical connections are needed. Two fibers are typically used--a transmit and receive--at the end points (PTE). By comparison, ADMs (LTEs) in the network will have four fibers: one transmit and receive pair from both the upstream and downstream. Although ring networks also use four fibers (one pair each from the upstream and downstream), they should never be called "transmit" and "receive." Because the information travels in a circle, a working fiber and a backup or protection fiber are connected to the upstream and downstream switches. Ring networks require two fibers for redundancy; with only one fiber all data transmission would cease if the fiber were cut.