CushionTheBlow (구 BluemoonSoft)

In the mathematical field of graph theory, a spanning tree T of a connected, undirected graph G is a tree composed of all the vertices and some (or perhaps all) of the edges of G. Informally, a spanning tree of G is a selection of edges of G that form a tree spanning every vertex. That is, every vertex lies in the tree, but no cycles (or loops) are formed. On the other hand, every bridge of G must belong to T.

A spanning tree of a connected graph G can also be defined as a maximal set of edges of G that contains no cycle, or as a minimal set of edges that connect all vertices.

In certain fields of graph theory it is often useful to find a minimum spanning tree of a weighted graph. Other optimization problems on spanning trees have also been studied, including the maximum spanning tree, the minimum tree that spans at least k vertices, the minimum spanning tree with at most k edges per vertex (MDST), the spanning tree with the largest number of leaves (closely related to the smallest connected dominating set), the spanning tree with the fewest leaves (closely related to the Hamiltonian path problem), the minimum diameter spanning tree, and the minimum dilation spanning tree.

Reference:
http://en.wikipedia.org/wiki/Spanning_tree_%28mathematics%29

Jitter is an unwanted variation of one or more characteristics of a periodic signal in electronics and telecommunications. Jitter may be seen in characteristics such as the interval between successive pulses, or the amplitude, frequency, or phase of successive cycles. Jitter is a significant factor in the design of almost all communications links (e.g. USB, PCI-e, SATA, OC-48).

Jitter can apply to a number of signal qualities (e.g. amplitude, phase, pulse width or pulse position), and can be quantified in the same terms as all time-varying signals (e.g. RMS, or peak-to-peak displacement). Also like other time-varying signals, jitter can be expressed in terms of spectral density (frequency content). Jitter period is the interval between two times of maximum effect (or between two times of minimum effect) of a jitter characteristic, for a jitter that varies regularly with time. Jitter frequency, the more commonly quoted figure, is its inverse. Generally, very low jitter frequency is not of interest in designing systems, and the low-frequency cutoff for jitter is typically specified at 1 Hz.

Reference:
http://en.wikipedia.org/wiki/Jitter

Botnet is a jargon term for a collection of software robots, or bots, that run autonomously and automatically. They run on groups of zombie computers controlled remotely. This term can also refer to the network of computers using distributed computing software.

While the term "botnet" can be used to refer to any group of bots, such as IRC bots, the word is generally used to refer to a collection of compromised computers (called zombie computers) running programs, usually installed via worms, Trojan horses, or backdoors, under a common command and control infrastructure. The majority of these computers are running Microsoft Windows operating systems, but other operating systems can be affected. A botnet's originator (aka "bot herder") can control the group remotely, usually through a means such as IRC, and usually for nefarious purposes. Individual programs manifest as IRC "bots". Often the command and control takes place via an IRC server or a specific channel on a public IRC network. A bot typically runs hidden, and complies with the RFC 1459 (IRC) standard. Generally, the perpetrator of the botnet has compromised a series of systems using various tools (exploits, buffer overflows, as well as others; see also RPC). Newer bots can automatically scan their environment and propagate themselves using vulnerabilities and weak passwords. Generally, the more vulnerabilities a bot can scan and propagate through, the more valuable it becomes to a botnet controller community. The process of stealing computing resources as a result of a system being joined to a "botnet" is sometimes referred to as "scrumping".

Botnets have become a significant part of the Internet, albeit increasingly hidden. Due to most conventional IRC networks taking measures and blocking access to previously-hosted botnets, controllers must now find their own servers. Often, a botnet will include a variety of connections, ranging from dial-up, ADSL and cable, and a variety of network types, including educational, corporate, government and even military networks. Sometimes, a controller will hide an IRC server installation on an educational or corporate site, where high-speed connections can support a large number of other bots. Exploitation of this method of using a bot to host other bots has proliferated only recently, as most script kiddies do not have the knowledge to take advantage of it.

Several botnets have been found and removed from the Internet. The Dutch police found a 1.5 million node botnet and the Norwegian ISP Telenor disbanded a 10,000-node botnet. Large coordinated international efforts to shut down botnets have also been initiated. It has been estimated that up to one quarter of all personal computers connected to the internet may become part of a botnet.

Reference:
http://en.wikipedia.org/wiki/Botnet

A distance-vector routing protocol is one of the two major classes of routing protocols used in packet-switched networks for computer communications, the other major class being the link-state protocol. A distance-vector routing protocol uses the Bellman-Ford algorithm to calculate paths.

Examples of distance-vector routing protocols include RIPv1 and 2 and IGRP. EGP and BGP are not pure distance-vector routing protocols but their concepts are the same. In many cases, EGP and BGP are considered DV (distance-vector) routing protocols.

A distance-vector routing protocol requires that a router informs its neighbors of topology changes periodically and, in some cases, when a change is detected in the topology of a network. Compared to link-state protocols, which requires a router to inform all the nodes in a network of topology changes, distance-vector routing protocols have less computational complexity and message overhead.

Reference:
http://en.wikipedia.org/wiki/Distance-vector_routing_protocol

Orthogonal frequency-division multiplexing (OFDM) — essentially identical to Coded OFDM (COFDM) and Discrete multi-tone modulation (DMT) — is a frequency-division multiplexing (FDM) scheme utilized as a digital multi-carrier modulation method. A large number of closely-spaced orthogonal sub-carriers are used to carry data. The data are divided into several parallel data streams or channels, one for each sub-carrier. Each sub-carrier is modulated with a conventional modulation scheme (such as quadrature amplitude modulation or phase shift keying) at a low symbol rate, maintaining total data rates similar to conventional single-carrier modulation schemes in the same bandwidth.

OFDM has developed into a popular scheme for wideband digital communication, whether wireless or over copper wires, used in applications such as digital television and audio broadcasting, wireless networking and broadband internet access.

The primary advantage of OFDM over single-carrier schemes is its ability to cope with severe channel conditions — for example, attenuation of high frequencies in a long copper wire, narrowband interference and frequency-selective fading due to multipath — without complex equalization filters. Channel equalization is simplified because OFDM may be viewed as using many slowly-modulated narrowband signals rather than one rapidly-modulated wideband signal. The low symbol rate makes the use of a guard interval between symbols affordable, making it possible to handle time-spreading and eliminate intersymbol interference (ISI). This mechanism also facilitates the design of single-frequency networks, where several adjacent transmitters send the same signal simultaneously at the same frequency, as the signals from multiple distant transmitters may be combined constructively, rather than interfering as would typically occur in a traditional single-carrier system.

Reference:
http://en.wikipedia.org/wiki/OFDM

Radio jamming is the (usually deliberate) transmission of radio signals that disrupt communications by decreasing the signal to noise ratio. Unintentional jamming occurs when an operator transmits on a busy frequency without checking that it is in use first, or without being able to hear distant stations on the same frequency. Another form of unintentional jamming occurs when equipment accidentally radiates a signal, such as a cable TV plant that accidentally emits on an aircraft emergency frequency.

Distinction between "jamming" and "interference"
Originally the terms were used interchangeably but nowadays most radio users use the term "jamming" to describe the deliberate use of radio noise or signals in an attempt to disrupt communications (or prevent listening to broadcasts ) whereas the term "interference" is used to describe unintentional forms of disruption (which are far more common). However the distinction is still not universally applied. For inadvertent disruptions, see electromagnetic compatibility.

Reference:
http://en.wikipedia.org/wiki/Radio_jamming

A frame check sequence (FCS) refers to the extra checksum characters added to a frame in a communication protocol for error detection and correction.

The sending host computes a checksum on the entire frame and appends this as a trailer to the data. The receiving host computes the checksum on the frame using the same algorithm, and compares it to the received FCS. This way it can detect whether any data was lost or altered in transit. It may then discard the data, and request retransmission of the faulty frame.

The FCS is often transmitted in such a way that the receiver can compute a running sum over the entire frame, including the trailing FCS, and expect to see a fixed result (such as zero) when it is correct. Such an FCS generally appears immediately before the frame-ending delimiter.

By far the most popular FCS algorithm is a cyclic redundancy check, used in the Ethernet, X.25, HDLC, Frame Relay, Point-to-Point Protocol (PPP) and other data link layer protocols.

Reference:
http://en.wikipedia.org/wiki/Frame_Check_Sequence

A virtual LAN, commonly known as a VLAN, is a group of hosts with a common set of requirements that communicate as if they were attached to the same wire, regardless of their physical location. A VLAN has the same attributes as a physical LAN, but it allows for end stations to be grouped together even if they are not located on the same LAN segment. Network reconfiguration can be done through software instead of physically relocating devices.

VLANs are created to provide the segmentation services traditionally provided by routers in LAN configurations. VLANs address issues such as scalability, security, and network management. Routers in VLAN topologies provide broadcast filtering, security, address summarization, and traffic flow management. By definition, switches may not bridge IP traffic between VLANs as it would violate the integrity of the VLAN broadcast domain.

Virtual LANs are essentially Layer 2 constructs, whereas IP subnets are Layer 3 constructs. In a LAN employing VLANs, a one-to-one relationship often exists between VLANs and IP subnets, although it is possible to have multiple subnets on one VLAN or have one subnet spread across multiple VLANs. Virtual LANs and IP subnets provide independent Layer 2 and Layer 3 constructs that map to one another and this correspondence is useful during the network design process.

Reference:
http://en.wikipedia.org/wiki/Virtual_LAN

요약
알레르겐(먼지 ·꽃가루 ·동물의 털 따위)을 콧속에 흡입함으로써 생기는 급성 비염.

본문
코 알레르기 또는 알레르기성 비염이라고도 한다. 증세는 갑자기 연속적으로 재채기를 하고, 수양성 비즙(맑은 콧물)이 다량으로 나오고 코가 막힌다. 또한 동시에 머리가 무겁고 눈물이 나오기도 한다. 증세가 아주 같으면서 항원이 분명하지 않은 것을 혈관운동신경성 비염이라 한다. 즉, 하루로 보면 아침에 일어나서 몸이 일시적으로 식었을 때에 앞에서 말한 바와 같은 증세가 일어났다가 수시간 만에 낫는 것도 있으며, 1년 중 환절기라든지 추운 계절에 많다. 흔히 코감기로 혼동되는데 감기와는 다르다. 천식이나 두드러기를 합병하는 수가 많다. 치료는 항원이 분명할 때는 탈감작요법(脫感作療法)을 행하고, 기타 약물요법, 수술적 요법, 이학적(理學的) 요법 등이 있는데 완전히 낫기는 어렵다.

Reference:
http://100.naver.com/100.nhn?docid=81650