Even if your company is very small, it will not take long before you have employees or branch offices that cannot be connected with fast and relatively inexpensive local area network (LAN) equipment and cabling. This is unfortunate, because “fast and inexpensive” are great terms to hear in computer networking. Thankfully, there have been many advancements in wide area networking (WAN) that make it more tolerable and affordable. Whenever you hear WAN, think about distances beyond the floor of your office building. You might even want to think of the global Internet itself, and don’t forget about those satellites beaming signals from space!
WAN connections that require a high bandwidth capacity or the capability to span a large distance might use fiber-optic cabling. In addition to the massively long distances that are supported, fiber-optic cabling provides great immunity from electromagnetic interference (EMI).
Synchronous Optical Network (SONET) is a Layer 1 technology that uses fiber-optic cabling as its media. Because SONET is a Layer 1 technology, it can be used to transport various Layer 2 encapsulation types, such as Asynchronous Transfer Mode (ATM). And because SONET uses fiber-optic cabling, it offers high data rates, typically in the 155 Mbps to 10 Gbps range, and long-distance limitations, typically in the 20 km to 250 km range. Optical carrier transmission rates, such as OC3 (close to 155 Mbps) and OC12 (close to 622 Mbps) are examples of specifications for digital signal transmission bandwidth.
The term SONET is often used synonymously with the term Synchronous Digital Hierarchy (SDH), which is another fiber-optic multiplexing standard. Although these standards are similar, SONET is typically utilized in North America, whereas SDH has greater worldwide popularity.
A SONET network can vary in its physical topology. For example, devices can connect as many as 16 devices in a linear fashion (similar to a bus topology) or in a ring topology. A metropolitan-area network (MAN) often uses a ring topology. The ring might circumnavigate a large metropolitan area. Sites within that metropolitan area could then connect to the nearest point on the SONET ring.
A SONET network uses a single wavelength of light, along with time-division multiplexing (TDM) to support multiple data flows on a single fiber. This approach differs from dense wavelength division multiplexing (DWDM), which is another high-speed optical network commonly used in MANs. DWDM uses as many as 32 light wavelengths on a single fiber, and each wavelength can support as many as 160 simultaneous transmissions using more than eight active wavelengths per fiber. Coarse wavelength division multiplexing (CWDM) uses fewer than eight active wavelengths per fiber. Current standards make even more transmissions possible.