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==Perhubungan media==
Jaringan komputer boleh dikelaskan menurut teknologi berkait hardware dan software yang digunakan bagi saling menyambung perandi individual di jaringan, seperti kabel eletrik ([[HomePNA]], [[komunikasi talian kuasa]], [[G.hn]]), [[fiber optik]], dan [[gelombang radia]] ([[LAN tanpa wayar]]). Bagi [[model OSI]], ini terletak pada paras 1 dan 2.
Salah satu ''keluarga'' media perhubungan yang terkenal adalah secara keseluruhannya dikenali sebagai [[Ethernet]]. Ia ditakrifkan oleh [[IEEE 802]] dan menggunakan pelbagai piwaian dan media yang membolehkan perhubungan antara peranti. Teknologi LAN tanpa wayar direka bagi menyambung peranti tanpa pendawaian. Peranti ini menggunakan isyarat gelombang radio atau [[IrDA|infra]] sebagai perantaraan pemancaran.
==Teknologi dengan wayar ==
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Computer networks:
; Facilitate communications : Using a network, people can communicate efficiently and easily via email, instant messaging, chat rooms, telephone, video telephone calls, and video conferencing.
; Permit sharing of files, data, and other types of information: In a network environment, authorized users may access data and information stored on other computers on the network. The capability of providing access to data and information on shared storage devices is an important feature of many networks.
; Share network and computing resources: In a networked environment, each computer on a network may access and use resources provided by devices on the network, such as printing a document on a shared network printer. [[Distributed computing]] uses computing resources across a network to accomplish tasks.
; May be insecure: A computer network may be used by computer hackers to deploy [[computer virus]]es or [[computer worm]]s on devices connected to the network, or to prevent these devices from normally accessing the network ([[Denial-of-service attack|denial of service]]).
; May interfere with other technologies: [[Power line communication]] strongly disturbs certain forms of radio communication, e.g., amateur radio.<ref> [http://www.arrl.org/broadband-over-powerline-bpl The National Association for Amateur Radio: Broadband Over Powerline]</ref> It may also interfere with [[last mile]] access technologies such as [[ADSL]] and [[VDSL]].<ref>{{cite web|title=The Likelihood and Extent of Radio Frequency Interference from In-Home PLT Devices|url=http://stakeholders.ofcom.org.uk/binaries/research/technology-research/pltreport.pdf|publisher=Ofcom|accessdate=18 June 2011}}</ref>
; May be difficult to set up: A complex computer network may be difficult to set up. It may also be very costly to set up an effective computer network in a large organization or company.
==Communication media==
Computer networks can be classified according to the hardware and associated software technology that is used to interconnect the individual devices in the network, such as [[Cable|electrical cable]] ([[HomePNA]], [[power line communication]], [[G.hn]]), [[optical fiber]], and [[radio waves]] ([[wireless LAN]]). In the [[OSI model]], these are located at levels 1 and 2.
A well-known ''family'' of communication media is collectively known as [[Ethernet]]. It is defined by [[IEEE 802]] and utilizes various standards and media that enable communication between devices. Wireless LAN technology is designed to connect devices without wiring. These devices use [[radio waves]] or [[IrDA| infrared]] signals as a transmission medium.
===Wired technologies===
===Repeaters and hubs===
A [[repeater]] is an [[Electronics|electronic]] device that receives a [[signal (information theory)|signal]], cleans it of unnecessary noise, regenerates it, and [[retransmission (data networks)|retransmit]]s it at a higher power level, or to the other side of an obstruction, so that the signal can cover longer distances without degradation. In most twisted pair Ethernet configurations, repeaters are required for cable that runs longer than 100 meters. A repeater with multiple ports is known as a [[Network hub|hub]]. Repeaters work on the Physical Layer of the OSI model. Repeaters require a small amount of time to regenerate the signal. This can cause a [[propagation delay]] which can affect network communication when there are several repeaters in a row. Many network architectures limit the number of repeaters that can be used in a row (e.g. Ethernet's [[5-4-3 rule]]).configur
Today, repeaters and hubs have been made mostly obsolete by switches (see below).
===Bridges===
A [[network bridge]] connects multiple [[network segment]]s at the [[data link layer]] (layer 2) of the [[OSI model]]. Bridges broadcast to all ports except the port on which the broadcast was received. However, bridges do not promiscuously copy traffic to all ports, as hubs do, but learn which [[MAC Address|MAC addresses]] are reachable through specific ports. Once the bridge associates a port and an address, it will send traffic for that address to that port only.
Bridges learn the association of ports and addresses by examining the source address of frames that it sees on various ports. Once a frame arrives through a port, its source address is stored and the bridge assumes that MAC address is associated with that port. The first time that a previously unknown destination address is seen, the bridge will forward the frame to all ports other than the one on which the frame arrived.
Bridges come in three basic types:
*Local bridges: Directly connect LANs
* Remote bridges: Can be used to create a wide area network (WAN) link between LANs. Remote bridges, where the connecting link is slower than the end networks, largely have been replaced with routers.
* Wireless bridges: Can be used to join LANs or connect remote stations to LANs.
===Switches===
A [[network switch]] is a device that forwards and filters [[OSI layer 2]] [[datagrams]] (chunks of data communication) between ports (connected cables) based on the MAC addresses in the packets.<ref>
{{cite web
|url=http://www.webopedia.com/TERM/s/switch.html
|title=Define switch.
|publisher=www.webopedia.com
|accessdate=April 8, 2008
}}</ref>
A switch is distinct from a hub in that it only forwards the frames to the ports involved in the communication rather than all ports connected. A switch breaks the collision domain but represents itself as a broadcast domain. Switches make forwarding decisions of frames on the basis of MAC addresses. A switch normally has numerous ports, facilitating a star topology for devices, and cascading additional switches.<ref>
{{cite web
|url=http://networkbits.net/lan-components/local-area-network-lan-basic-components/
|title=Basic Components of a Local Area Network (LAN)
|publisher=NetworkBits.net
|accessdate=April 8, 2008
}}</ref>
Some switches are capable of routing based on Layer 3 addressing or additional logical levels; these are called multi-layer switches. The term ''switch'' is used loosely in marketing to encompass devices including routers and bridges, as well as devices that may distribute traffic on load or by application content (e.g., a Web [[Uniform Resource Locator|URL]] identifier).
===Routers===
A [[Router (computing)|router]] is an internetworking device that forwards [[Packet (information technology)|packets]] between networks by processing information found in the datagram or packet (Internet protocol information from [[Osi_model#Layer_3:_Network_Layer|Layer 3 of the OSI Model]]). In many situations, this information is processed in conjunction with the routing table (also known as forwarding table). Routers use routing tables to determine what interface to forward packets (this can include the "null" also known as the "black hole" interface because data can go into it, however, no further processing is done for said data).
===Firewalls===
A [[Firewall (computing)|firewall]] is an important aspect of a network with respect to security. It typically rejects access requests from unsafe sources while allowing actions from recognized ones. The vital role firewalls play in network security grows in parallel with the constant increase in 'cyber' attacks for the purpose of stealing/corrupting data, planting viruses, etc.'''
==Network performance==
{{main|network performance}}
'''Network performance''' refers to the [[service quality]] of a telecommunications product as seen by the customer. It should not be seen merely as an attempt to get "more through" the network.
The following list gives examples of Network Performance measures for a circuit-switched network and one type of packet-switched network, viz. ATM:
*Circuit-switched networks: In [[circuit switched]] networks, network performance is synonymous with the [[grade of service]]. The number of rejected calls is a measure of how well the network is performing under heavy traffic loads.<ref>[http://www.com.dtu.dk/teletraffic/handbook/telenook.pdf ITU-T Study Group 2, Teletraffic Engineering Handbook] (PDF), Retrieved on February 13, 2005.</ref> Other types of performance measures can include noise, echo and so on.
*ATM: In an [[Asynchronous Transfer Mode]] (ATM) network, performance can be measured by line rate, [[quality of service]] (QoS), data throughput, connect time, stability, technology, modulation technique and modem enhancements.<ref>[http://www.telecommagazine.com Telecommunications Magazine Online], Americas January 2003, Issue Highlights, Online Exclusive: Broadband Access Maximum Performance, Retrieved on February 13, 2005.</ref>
There are many different ways to measure the performance of a network, as each network is different in nature and design. Performance can also be modelled instead of measured; one example of this is using state transition diagrams to model queuing performance in a circuit-switched network. These diagrams allow the network planner to analyze how the network will perform in each state, ensuring that the network will be optimally designed.<ref>{{cite web| url=http://cne.gmu.edu/modules/os_perf/std.t.html |title = State Transition Diagrams |accessdate =July 13, 2003}}</ref>
==Network security==
{{main|network security}}
In the field of networking, the area of '''network security'''<ref>{{cite journal | doi = 10.1007/978-3-540-30176-9_41 | last = Simmonds | first = A | coauthors = Sandilands, P; van Ekert, L|title = An Ontology for Network Security Attacks | journal = Lecture Notes in Computer Science | volume = 3285 | pages = 317–323 | year = 2004 | series = Lecture Notes in Computer Science | isbn = 978-3-540-23659-7 }}</ref> consists of the provisions and [[policies]] adopted by the [[network administrator]] to prevent and monitor [[unauthorized]] access, misuse, modification, or denial of the computer network and network-accessible resources. Network security is the authorization of access to data in a network, which is controlled by the network administrator. Users are assigned an ID and password that allows them access to information and programs within their authority. Network Security covers a variety of computer networks, both public and private that are used in everyday jobs conducting transactions and communications among businesses, government agencies and individuals.
==Network resilience==
{{main|resilience (network)}}
In computer networking: “'''Resilience''' is the ability to provide and maintain an acceptable level of [[Service (systems architecture)|service]] in the face of [[Fault (technology)|faults]] and challenges to normal operation.”<ref>The [http://wiki.ittc.ku.edu/resilinets_wiki/index.php/Definitions#Resilience ResiliNets] Research Initiative definition of '''resilience'''.</ref>
==Views of networks==
Users and network administrators typically have different views of their networks. Users can share printers and some servers from a workgroup, which usually means they are in the same geographic location and are on the same LAN, whereas a Network Administrator is responsible to keep that network up and running. A [[community-of-interest network|community of interest]] has less of a connection of being in a local area, and should be thought of as a set of arbitrarily located users who share a set of servers, and possibly also communicate via [[peer-to-peer]] technologies.
Network administrators can see networks from both physical and logical perspectives. The physical perspective involves geographic locations, physical cabling, and the network elements (e.g., [[Router (computing)|router]]s, [[Network bridge|bridges]] and [[Application-level gateway|application layer gateways]]) that interconnect the physical media. Logical networks, called, in the TCP/IP architecture, [[subnetwork|subnets]], map onto one or more physical media. For example, a common practice in a campus of buildings is to make a set of LAN cables in each building appear to be a common subnet, using [[Virtual LAN| virtual LAN (VLAN)]] technology.
Both users and administrators will be aware, to varying extents, of the trust and scope characteristics of a network. Again using TCP/IP architectural terminology, an [[intranet]] is a community of interest under private administration usually by an enterprise, and is only accessible by authorized users (e.g. employees).<ref name="RFC2547">RFC 2547</ref> Intranets do not have to be connected to the Internet, but generally have a limited connection. An [[extranet]] is an extension of an intranet that allows secure communications to users outside of the intranet (e.g. business partners, customers).<ref name="RFC2547"/>
Unofficially, the Internet is the set of users, enterprises, and content providers that are interconnected by [[Internet Service Providers]] (ISP). From an engineering viewpoint, the [[Internet]] is the set of subnets, and aggregates of subnets, which share the registered [[IP address]] space and exchange information about the reachability of those IP addresses using the [[Border Gateway Protocol]]. Typically, the [[human-readable]] names of servers are translated to IP addresses, transparently to users, via the directory function of the [[Domain Name System]] (DNS).
Over the Internet, there can be [[Business-to-business| business-to-business (B2B)]], [[Business-to-consumer| business-to-consumer (B2C)]] and [[Consumer-to-consumer electronic commerce|consumer-to-consumer (C2C)]] communications. Especially when money or sensitive information is exchanged, the communications are apt to be '''secured''' by some form of [[communications security]] mechanism. Intranets and extranets can be securely superimposed onto the Internet, without any access by general Internet users and administrators, using secure [[Virtual Private Network]] (VPN) technology.
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== Lihat juga ==
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