Frequently Asked Questions (FAQ)

1:	What is the difference between SONET and SONET Digital Hierarchy (SDH)?
A1:	The first level in the SDH hierarchy is Synchronous Transport Mode 1 (STM-1 ) and has a line rate of 155.52Mb/s. This is equivalent to SONET's STS-3c, STM-4 at 622.08Mb/s, and STM-16 at 2488.32Mb/s. The other difference is in the overhead bytes, which are defined differently for SDH. A common misconception is that STM-Ns are formed by multiplexing STM-1s. STM-1s, STM-4s and STM-16s that terminate on a network node are broken down to recover the VCs they contain. The outbound STM-Ns are then reconstructed with new overhead.
2:	What is "dark fiber"?
A2:	Dark fiber refers to unused fiber-optic cable. Often companies lay more fiber lines than they need so that the company does not have to incur the cost of having to lay new fiber-optic cable. The dark strands can be leased to organizations that want to establish optical connections among their own locations. In the case of a dark fiber deployment, the fiber is neither controlled by nor connected to the network service provider. Instead, the customer provides the necessary components to make the fiber functional; the network service provider provides only the path, or wavelength, to the organization.
3:	What is "customer-owned dark fiber"?
A3:	With customer-owned dark fiber networks, the end customer owns and controls the actual fiber and decides to which service provider they want to connect for services such as telephony, cable TV, and Internet. Professional third-party companies who specialize in dark fiber systems manage the installation and maintenance of the fiber on behalf of the customer.
4:	What is Cisco COMET?
A4:	Cisco COMET is considered the next step in the evolution of IP+Optical networking by providing the following: Complete optical restoration, topology, and transmission support to maximize service velocity Multiservice delivery to maximize service density Edge support for all optical interfaces to maximize service variety Transport that is scalable and efficient to maximize service capacity throughout the network