uter and other services on a network. Layer 6, the presentation layer, formats information SO that a software application can read it. Layer 5, the session layer, supports connections between sessions and handles administrative tasks and security. Layer 4, the transport layer, corrects transmission errors and ensures that the information is delivered reliably. Layer 3, the network layer, identifies computers on a network and determines how to direct information transfer over that network. Layer 2, the data link layer, groups data into con- tainers to prepare that data for transfer over a network. Layer 1, the physical layer, defines how a transmission medium connects to a computer, as well as how electrical or optical information is transferred on the transmission medium. OSP (online service provider) A provider that organizes online content and provides intu- itive user navigation. OSPF (Open Shortest Path First) A routing protocol used on TCP/IP networks in which routers maintain an internal map of the network and exchange information about the current state of each network link. OSPF's features include least-cost routing, multipath routing, and load balancing. Glossary OSS (operational support system) OSS (operational support system) A set of programs that help a communications service provider monitor, control, analyze, and manage problems with a telephone or computer network. OTDR (optical time domain reflectometer) A device that can be used in fiber networks to detect potential leaks that could be the result of unwanted intrusion. OTN (Optical Transport Network) A network that is composed of a set of optical net- work elements connected by optical fiber links and is able to provide functionality of transport, multiplexing, routing, management, supervision, and survivability of optical channels carrying client signals, according to the requirements given in ITU-T Recommendation G.872. out-of-band management A system in which management data is communicated through a link, outside the network, typically throu