NACK or NAK (negative acknowledgment)
A message that says there was an error in transmission and the previous block needs to be re-sent before anything else can happen.

NADC (North American Digital Cellular)
A TIA standard that is specified as IS-54 and was adopted in 1992. It has since been updated, and it is now called IS-136. It uses TDMA and TDD schemes, and it offers a total of three time slots. It operates on the 800MHz frequency band, uses AMPS for signaling to reserve resources, and transfers speech in digital form; therefore, it is a digital overlay that is interoperable with analog AMPS infrastructure. UWC-136, an advancement of the U.S. TDMA (IS-136) standard, uses EDGE technology.

nailed-up connection
A slang term for a permanent, dedicated path through a switch; often used for lengthy, regular data transmission going through a PBX.

name resolution
The process of mapping a name into a corresponding address. The domain name system provides a mechanism for naming computers in which programs use remote name servers to resolve machine names into IP addresses for those machines.

nanosecond
One billionth of a second.

NAP (network access point)
The point where backbones interconnect to exchange traffic between providers. Bottlenecks at NAPs greatly affect the ability to roll out new time-sensitive, loss-sensitive applications, such as Internet telephony, VoIP, VPNs, streaming media, and TV over Internet.

narrowband
A service occupying low bandwidth (64Kbps or below).

narrowcast
Transmission of specific programming to predetermined users of a telecommunications network. Only some users of the network are receiving the same information.

NCP (Network Control Program)
A program that resides in a communications controller that controls the operation of the communications controller.

near-end crosstalk
Unwanted energy transferred from one circuit usually to an adjoining circuit. It occurs at the end of the transmission link where the signal source is located, with the absorbed energy usually propagated in the opposite direction of the absorbing channel’s normal current flow. Usually caused by high-frequency or unbalanced signals and insufficient shielding.

NetBIOS (Network Basic Input/Output System)
A session-layer interface that is widely used in PC networks.

NetBIOS extended user interface
A transport-layer protocol designed to support NetBIOS over 802.2 LANs.

network
(1) A collection of devices connected by communication lines for data processing or information interchange. (2) A series of points connected by communications channels. (3) A network of telephone lines normally used for dialed telephone calls. (4) A group of computers and peripherals that are interconnected so that they can communicate with each other.

network address translation
An Internet standard that enables a LAN to use one set of IP addresses for internal traffic and a second set of addresses for external traffic. A NAT box located where the LAN meets the Internet makes all necessary IP address translations.

network architecture
The philosophy and organizational concept for enabling communications between data processing equipment at multiple locations. The network architecture specifies the processors and terminals, and defines the protocols and software that must be used to accomplish accurate data communications.

network control
In a network, the establishment, authorization, and maintenance of logical and physical connections between stations and applications, plus the synchronization, routing, integrity, and recovery of data transmitted during the established connections.

network layer
In the OSI model, the logical network entity that services the transport layer. It is responsible for ensuring that data passed to it from the transport layer is routed and delivered through the network.

network node
A point on the network where communications lines interface. Thus, a network node might be a PBX, a local exchange, a multiplexer, a modem, a host computer, or one of several other devices.

network redundancy
Including in communication pathway additional links in order to connect all nodes.

network topology
The physical and logical relationship of nodes in a network; the schematic arrangement of the link and nodes of a network, typically either a star, ring, tree, or bus topology, or some hybrid combination thereof.

networking
Communication between stations in a network.

next-generation gateway switch
A switch that is designed to support a wide variety of traffic?data, voice, fax, multimedia, and other emerging sensory forms?over a data backbone. It provides seamless interoperability between the circuits that network the PSTN and packet-switching networks.

next-generation network
A high-speed packet- or cell-based network that’s capable of transporting and routing a multitude of services, including voice, data, video, and multimedia. It is a common platform for applications and services that the customer can assess across the entire network, as well as outside the network.

NIC (network interface card)
A component that connects a station to a network (e.g., LAN). Also called a network adapter card.

NIOD (Network Inward/Outward Dialing)
A system that provides the capability for dialing both ways between a toll network and a local network.

N-ISDN (Narrowband ISDN)
A network architecture and set of standards introduced in 1983 for an all-digital network. It was intended to provide end-to-end digital service using the public telephone networks worldwide and, therefore, to provide high-quality, error-free transmission.

NIST (National Institute of Standards and Technology)
A group that prior to 1988, was called the National Bureau of Standards.

NLSP (NetWare Link Services Protocol)
A link-state protocol that offers low network overhead and fast convergence.

NMC (network management center)
A center used for control of a network. May provide traffic analysis, call detail recording, configuration control, fault detection and diagnostics, and maintenance.

NMT (Nordic Mobile Telephone System)
A Scandinavian wireless system that originally operated at 450MHz, offered around 220 channels, and had a very large coverage area.

node
Any device on a network that can independently send or receive information and that has a network address; also the point at which the device is linked to the network.

noise
Unwanted electrical signals, introduced by circuit components or natural disturbances, that tend to degrade the performance of a communications channel.

nonblocking
Describes a switch where a through traffic path exists for each attached station. Generically, a switch or switching environment designed to never experience a busy condition due to traffic volume.

nonswitched line
In data communication, a permanent connection between computers or devices that does not have to be established by dialing.

nonvolatile storage
A storage medium whose contents are not lost when the power is removed.

NPA (numbering plan area)
A geographic subdivision of the territory covered by a national or integrated numbering plan. An NPA is identified by a distinctive area code.

NRZ (nonreturn to zero)
A digital signaling technique in which the signal is at a constant level for the duration of time.

NSP (network service provider)
A very large, global backbone carrier that owns its own infrastructures (for example, AT&T, WorldCom, UUnet, Sprint, Verizon, Cable & Wireless, and Qwest).

NTSC (National Television Systems Committee)
A television broadcasting system that uses 525 picture lines and a 60Hz field frequency. It was developed by the committee, and is used primarily in the United States, Canada, Mexico, and Japan.

NTU (network terminating unit)
The part of the network equipment that connects directly to the data terminal equipment.

null attached
The operation of an FDDI concentrator without being attached to the backbone network. This configuration establishes a small, autonomous, single-ring FDDI network consisting of a limited number of directly connected single attached stations.

NXX
The current exchange numbering plan, in which N is any digit from 2 to 9 and X is any digit from 0 to 9.