J

.jobs - Pronounced dot jobs.The generic Top Level Domain (gTLD) reserved exclusively for job seekers and companies seeking employees. It is not now permissible for third parties (i.e., corporate recruiters) to use .jobs.This domain was created in 2005 under the sponsorship of an alliance between Employ Media, the Society for Human Resource Management (SHRM),Verisign, and ICANN. See also gTLD, Internet, and sponsored domain.

.jpg - The file extension that identifies files in the Joint Photographic Experts Group (JPEG) format. See also JPEG.

J (Joule) - See joule.

J-1 - The first level of the Japanese J-carrier digital hierarchy. J-1 mimics the North American T1 system. A J-1 system comprises circuit terminating equipment in the form of a combination of a channel service unit (CSU) and a data service unit (DSU) that jointly serve to interface a device to a full duplex (FDX) four-wire digital circuit and to perform various signal formatting, signal timing, monitoring, and diagnostic functions. J-1 operates at a signaling rate of 1.544 Mbps, which supports a frame rate of 8,000 frames per second (fps), with each frame comprising a framing bit followed by 192 bits of user payload.The framing bits are used for synchronization and, in some cases, for monitoring, diagnostic, and other network management purposes. The 192 bits of user payload are organized into 24 time-division multiplexed (TDM) time slots, each of which is 8 bits wide.At a rate of 8,000 fps, each time slot is repeated 8,000 times per second, which translates into a DS-O channel at 64 kbps (8 bits × 8,000 per second = 64,000 bps). Taken together, the 24 8-bit TDM channels at 8,000 fps yields an aggregate payload transmission rate of 1.536 Mbps.Adding the 8,000 framing bits (one per frame) per second, yields the aggregate signaling rate of 1.544 Mbps. J-1 was initially designed to operate over a physical four-wire twisted-pair copper circuit, but is considered medium independent and will run over coaxial cable, optical fiber, microwave, satellite, and free space optics (FSO) just as well. J-1 generally is used in local loops and other short-haul applications. In long-haul applications, J-3 and other, higher speed, standards generally are employed. See also carrier, CSU, DS-0, DSU, FDX, four wire, frame, framing bit, J-3, J-carrier, payload, signaling rate, synchronization, T1, T-carrier, TDM, time slot, and transmission rate.

J-2 - The second level in the Japanese J-carrier digital hierarchy. J-2 runs at two signaling rates. At 6.312 Mbps, J-2 mimics the North American T2, comprising four J-1s at 1.544 Mbps each, plus 132 kbps of overhead and justification, or bit stuffing, to adjust for variations in the clocking rates of the incoming J-1s. Multiplexing is performed by M12 (Multiplex J1-to-J2) terminals that multiplex four J-1 signals, which yields 96 DS-0 channels at 64 kbps per channel. At 7.786 Mbps, J-2 supports 120 DS-0 channels, which maps J-2 into E-2, the European version. Despite similarities in terms of signaling rates and channel capacities, J-carrier is not compatible with either E-carrier or T-carrier. See also bit stuffing, digital signal hierarchy, DS-0, E-2, E-carrier, J-carrier, T2, and T-carrier.

J-3 - The third level in the Japanese J-carrier digital hierarchy. J-3 runs at a signaling rate of 32.064 Mbps in support of 480 DS-0 channels, which maps J-3 into E-3, the European version. Despite similarities in terms of signaling rates and channel capacities at various levels, J-carrier is not compatible with either E-carrier or T-carrier. See also digital signal hierarchy, DS-0, E-3, E-carrier, J-carrier, and T-carrier

J-4 - The fourth level in the Japanese J-carrier digital hierarchy. J-4 runs at a signaling rate of 97.728 Mbps in support of 1,440 DS-0 channels. See also digital signal hierarchy, DS-0, and J-carrier.

J-5 - The fifth level in the Japanese J-carrier digital hierarchy. J-5 runs at a signaling rate of 565.148 Mbps in support of 7,680 DS-0 channels which maps J-5 into E-5, the European version. Despite similarities in terms of signaling rates and channel capacities at various levels, J-carrier is not compatible with either E-carrier or T-carrier. See also digital signal hierarchy, DS-0, E-5, E-carrier, J-carrier, signaling rate, and T-carrier.

J-band - The range of radio frequencies from 10 GHz to 20 GHz in the super high frequency (SHF) range of the electromagnetic spectrum.The J-band is used for satellite communications and radar applications. See also electromagnetic spectrum, radar, satellite, and SHF.

J-carrier (Japanese-carrier) A hierarchy of standards for digital transmission that essentially is the Japanese version of the United States T-carrier digital carrier system.At levels one and two, J-carrier mimics T-carrier with respect to the signaling rates, but diverges at level three and beyond. J-carrier also uses a different PCM companding technique (A-law rather than µ-law). As J-carrier is medium-independent, it can be provisioned over any of the transmission media, at least at the J-1 rate of 1.544 Mbps. (At the J-3 rate of 32.064 Mbps, twisted pair is unsuitable due to issues of signal attenuation.) The fundamental building block of J-carrier is a 64-kbps channel, referred to as DS-0 (Digital Signal level Zero).Through time division multiplexing (TDM), J-carrier interleaves DS-0 channels at various signaling rates to create the services that comprise the Japanese digital hierarchy.

J-carrier Level	Data Rate	64-kbps Channels (DS-0s)	Equivalent J1s
0	64 kbps	1	Not applicable
J-1	1.544 Mbps	24	1
J-2	6.312 Mbps 7.786 Mbps	96 120	4 5
J-3	32.064 Mbps	480	20
J-4	97.728 Mbps	1,440	60
J-5	565.148 Mbps	7,680	320

Japanese Digital Hierarchy: J-carrier

See digital signal hierarchy for a side-by-side comparison of the North American, European, and Japanese digital hierarchies. See also A-law, carrier, channel, companding, digital, DS-0, E-carrier, J-1, J-2, J-3, J-4, J-5, µ-law, PCM, signaling rate, T-carrier, and TDM.

Jabber - An XML-based instant messaging (IM) system written as an open source application by Jeremie Miller in 1998. See also IM, open source, and XML.

jabber - Meaningless data introduced into a local area network (LAN), usually as the result of a malfunctioning transceiver, transceiver cable, or network interface card (NIC). See also LAN, NIC, and transceiver.

jack - A female connector or outlet with receptacles designed to receive the pins of a male plug in a plugand-jack connection. See also connector, plug, and RJ.

jacket - See cable jacket.

Jailbreak - See voice mail jail.

jamming signal - 1. A signal that intentionally introduces interference into a communication channel, either to intentionally prevent error-free reception or as a means of advising stations of some event. 2. In local area networks (LANs), employing the carrier sense multiple access with collision detection (CSMA/CD) protocol, a station that detects a signal collision sends a jamming signal over a subcarrier frequency to advise all stations of that fact. See also CSMA/CD, frequency, LAN, signal, and subcarrier.

Japanese Digital Cellular (JDC) - A Japanese 2G digital cellular radio standard now known as Personal Digital Cellular (PDC). See also PDC.

Japanese Total Access Communications System (JTACS) - See JTACS.

Java Telephony Application Programming Interface (JTAPI) - A cross-platform, multivendor computer telephony (CT) solution based on Java, JTAPI uses highly efficient applets, which are small sets of application program code, for network-based CT operation. JTAPI added Internet/intranet functionality to CT, thereby enabling the creation of Web-based applications that integrate browser applications with call center functionality. JTAPI was developed jointly by Sun Microsystems, Lucent Technologies, IBM, and Nortel.

JBOD (Just a Bunch Of Disks) - A derogatory term for spanning, which is the use of a number of external physical hard drives organized into a single logical drive. JBOD is a simple storage technology that allows a computer to write to a large storage medium comprising multiple smaller drives. Unlike a redundant array of independent disks (RAID), JBOD does not provide any advantages in terms of redundancy or performance. See also RAID.

JDC (Japanese Digital Cellular) - A Japanese 2G digital cellular radio standard now known as Personal Digital Cellular (PDC). See also PDC.

jitter - 1. Uncertain variation in the timing of a received signal as compared to the timing of the transmitted signal.All signals experience some amount of delay, or latency, as they propagate across a circuit as, even at the speed of light, it takes some amount of time to travel the distance from one point to another. The timing of the signal elements remains consistent, however, barring changes in the length or other physical characteristics of the circuit caused by variations in temperature or other external forces. As devices are added to a circuit, even relatively simple devices such as amplifiers and repeaters performing relatively simple processes, additional delay is introduced, and the potential for variability in delay is increased because of factors such as fluctuations in power sources and faulty internal components. Should variability exceed specified tolerances, the timing of the pulses can be unacceptably irregular, and the receiving device may be unable to interpret the received signal correctly. 2. Variability in latency of a block, cell, frame, packet, or other message unit. Data message units can suffer jitter not only due to issues of signal jitter, but also because they may encounter different levels of congestion, which may cause them to spend different amounts of time in queues.These factors, and others, contribute to jitter. Some applications, such as e-mail, are tolerant of jitter, while other applications, such as real-time, uncompressed voice, are highly intolerant of jitter. 3. Undesirable rapid or jumpy movement of images, such as those displayed on a television or computer monitor. Jitter can be caused by circuit instability or faulty system components. See also latency, propagation delay, and velocity of propagation (Vp).

Joan/Eleanor - The code name for the first walkie talkie system, which comprised a ground unit, Joan, and an airborne unit, Eleanor. The system allowed agents of the Office of Strategic Services (OSS), the predecessor to the Central Intelligence Agency (CIA), behind enemy lines to communicate with aircraft in a manner that virtually defied detection at the time. See also walkie talkie.

job security - [This space intentionally left blank.] See also offshoring and outsourcing.

Joint Photographic Experts Group (JPEG) - See JPEG.

joule (J) - The work done when a force of one newton applied to a point moves that application point one meter in the direction of application. Joule is named for James Prescott Joule (1818–1889), a British physicist. See also newton.

JPEG (Joint Photographic Experts Group) - A graphics file format for editing still images, as well as color facsimile, desktop publishing, graphic arts, and medical imaging. A symmetrical compression technique, JPEG is equally expensive, processor-intensive, and time consuming in terms of both compression and decompression. JPEG is a joint standard of the International Telecommunications Union (ITU-T T.81) and the International Organization for Standardization (ISO 10918-1). JPEG involves a lossy compression mechanism using discrete cosine transform (DCT). Compression rates of 100:1 can be achieved, although the loss is noticeable at that level. Compression rates of 10:1 or 20:1 yield little degradation in image quality. See also compression, DCT, GIF, ISO, ITU-T, lossy compression, PNG, and symmetric.

JTACS (Japanese Total Access Communications System) - A Japanese version of the TACS 1G analog cellular radio technology developed for use in the United Kingdom. See 1G, analog, cellular radio, narrowband, and TACS.

JUGHEAD (Jonzys Universal Gopher Hierarchy Excavation And Display) - An Internet browser similar in operation to Veronica, but limited to keyword searches in directory titles on a specific site in Gopherspace. Jughead indexes the keywords in directory titles, but does not index the files within the directories. Playing off Archie and Gopher, subsequent developers of search mechanisms tried to stay with the Archie comic book/rodent theme, proving once and for all that even acronyms can be fun. Unfortunately, there is not even a hint of humor reflected in the chosen names of more recent and more powerful browsers. See also Archie, browser, Gopher, Gopherspace, Internet, and Veronica.

jumbo payload - In Internet Protocol version 6 (IPv6), a user data field that exceeds the IPv4 limit of 65,535 octets.A jumbo payload is indicated in the payload length field of the IPv6 header. See also header, IPv4, IPv6, octet, and payload.

jumper - A short wire or optical fiber used to make semi-permanent connections between circuits on a distribution frame or patch panel.

Junk Fax Protection Act - In the United States, a modification (2005) to the Telephone Consumer Protection Act (1991) and Federal Communications Commission (FCC) rules that defines an existing business relationship (EBR), codifies an existing EBR exemption to the prohibition to sending unsolicited fax advertisements, requires the sender to provide details as to how the recipient can opt out of future faxes, and specifies the circumstances under which a request to opt out complies with the Act. See also fax, FCC, and Telephone Consumer Protection Act.

junk mail - See spam.

just a bunch of disks (JBOD) - See JBOD.

justification - See bit stuffing.