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D-5
Cisco ONS 15454 Installation and Operations Guide
November 2001
Appendix D Glossary
EIA
Electrical Interface Assemblies. Provides connection points for the ONS 15454 and DS-1, DS-3, or EC-1
units.
EMI
Electromagnetic Interference. Interference by electromagnetic signals that can cause reduced data
integrity and increased error rates on transmission channels.
Envelope
The part of messaging that varies in composition from one transmittal step to another. It identifies the
message originator and potential recipients, documents its past, directs its subsequent movement by the
Message Transfer System (MTS), and characterizes its content.
EOW
Express Orderwire. A permanently connected voice circuit between selected stations for technical
control purposes.
Ethernet switch
An Ethernet data switch. Ethernet switches provide the capability to increase the aggregate LAN
bandwidth by allowing simultaneous switching of packets between switch ports. Ethernet switches
subdivide previously-shared LAN segments into multiple networks with fewer stations per network.
External timing reference
A timing reference obtained from a source external to the communications system, such as one of the
navigation systems. Many external timing references are referenced to Coordinated Universal Time
(UTC).
F
Falling threshold
A falling threshold is the counterpart to a rising threshold. When the number of occurrences drops below
a falling threshold, this triggers an event to reset the rising threshold. See also rising threshold.
FDDI
Fiber Distributed Data Interface. LAN standard, defined by ANSI X3T9.5, specifying a 100-Mbps
token-passing network using fiber optic cable, with transmission distances of up to 2 km. FDDI uses a
dual-ring architecture to provide redundancy.
Frame
Logical grouping of information sent as a data link layer unit over a transmission medium. Often refers
to the header and trailer, used for synchronization and error control that surrounds the user data
contained in the unit.
Free run synchronization mode
Occurs when the external timing sources have been disabled and the ONS 15454 is receiving timing from
its Stratum 3 level internal timing source.