There are several kinds of hardware that may be used to implement a WiFi wireless network:
WiFi 무선 네트워크를 구현하기 위해 사용되는 하드웨어의 종류는 여러가지가 있다.
- Wireless adaptersornetwork interface controllers(NICs for short) are network cards with the 802.11 standard which let a machine connect to a wireless network. WiFi adapters are available in numerous formats, such asPCIcards,PCMCIAcards,USBadapters, andCompactFlashcards. Astationis any device that has such a card.
- 무선 어댑터또는 네트워크 인터페이스 컨트롤러 (NIC)는 머신이 무선 네트워크에 연결될 수 있도록 하는802.11 표준네트워크 카드이다. WiFi 어댑터들은 다양한 포맷들이 존재한다. PCI 카드, PCMCIA 카드, USB 어댑터, Compact Flash 카드. 하나의 스테이션은 그러한 카드를 가지고 있는 장치이다.
- Access points(APfor short; sometimes calledhotspots) can let nearby wifi-equipped stations access a wired network to which the access point is directly connected.
- 엑세스 포인트 (AP, 때론 hotspot이라 부른다)는 주변의 와이파이-장착된 스테이션들이직접 연결된유선 네트워크에 접근하는 것을허용한다.
The 802.11 standard defines two operating modes:
802.11 표준은 2개 동작 모드를 정의한다.
- Infrastructure mode, in which wireless clients are connected to an access point. This is generally the default mode for 802.11b cards.
- 인프라스트럭쳐 모드, 무선 클라이언트들은 엑세스 포인트(AP)에 접속된다. 이것은 일반적으로 802.11b 카드들의 기본 모드입니다.
- Ad hocmode, in which clients are connected to one another without any access point.
- 애드훅(Adhoc) 모드, 클라이언트들은 임의 엑세스 포인트 없이 연결된다.
Infrastructure mode (인프라스트럭쳐 모드)
Inmode infrastructure, each station computer (STAfor short) connects to an access point via a wireless link. The set-up formed by the access point and the stations located within its coverage area are called thebasic service set, orBSSfor short. They form one cell. Each BSS is identified by a BSSID, a 6-byte (48-bite) identifier. Ininfrastructuremode, the BSSID corresponds to the access point'sMAC address.
It is possible to link several access points together (or more precisely several BSS's) using a connection called adistribution system(DSfor short) in order to form anextended service setorESS. The distribution system can also be a wired network, a cable between two access points or even a wireless network.
An ESS is identified with anESSID(Extended Service Set Identifier), a 32-character identifier (inASCIIformat) which acts as its name on the network. The ESSID, often shortened toSSID, shows the network's name, and in a way acts a first-levelsecuritymeasure, since it is necessary for a station to know theSSIDin order to connect to the extended network.
When a roaming user goes from one BSS to another while moving within the ESS, his or her machine's wireless network adapter is able to switch access points depending on the quality of the signal it receives from different access points. Access points communicate with one another using a distribution system in order to trade information about the stations and, if necessary, to transmit data from mobile stations. This feature which lets stations move "transparently" from one access point to another is calledroaming.
Communicating with the access point
When a station joins a cell, the cell sends aprobe requeston each channel. This request contains the ESSID that the cell is configured to use, as well as the traffic volume that its wireless adapter can support. If no ESSID is set, the station listens to the network for an SSID.
Each access point broadcasts at regular intervals (about ten times a second) a signal called abeacon, which gives information on its BSSID, its characteristics, and, if applicable, its ESSID. The ESSID is automatically broadcast by default, but it is possible (and recommended) to disable this option.
Whenever a probe request is received, the access point checks the ESSID and the traffic volume request found in the beacon. If the given ESSID matches that of the access point, the acces point sends a response containing synchronization data and information on its traffic load. This way, the station that receives the response can check the quality of the signal being sent by the access point in order to determine how far away it is. Generally speaking, the closer an access point is, the higher its data transfer capacity is.
So a station within range of multiple access points (which have the same SSID) maychoosethe access point offering the best balance of capacity and current traffic load.
|When a station is within range of several access points, the station chooses which one to connect to.|
Ad hoc mode
Inad hoc mode, wireless client machines connect to one another in order to form a peer-to-peer network, i.e. a network in which every machine acts as both a client and an access point at the same time.
The set-up formed by the stations is called theindependent basic service set, or IBSS for short.
An IBSS is a wireless network which has at least two stations and uses no access point. The IBSS therefore forms a temporary network which lets people in the same room exchange data. It is identified by an SSID, just like an ESS in infrastructure mode.
In an ad hoc network, the range of theindependent BSSis determined by each station's range. That means that if two of the stations on the network are outside each other's range, they will not be able to communicate, even if they can "see" other stations. Unlike infrastructure mode, ad hoc mode has no distribution system that can send data frames from one station to another. An IBSS, then, is by definition a restricted wireless network.
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