Wireless Lan(WLAN )Components


Wireless Access Points:
An access point connects wireless clients (or stations) to the wired LAN. Client devices do not typically communicate directly with each other; they communicate with the AP. In essence, an access point converts the TCP/IP data packets from their 802.11 frame encapsulation format in the air to the 802.3 Ethernet frame format on the wired Ethernet network.
In an infrastructure network, clients must associate with an access point to obtain network services. Association is the process by which a client joins an 802.11 network. It is similar to plugging into a wired LAN. Association is discussed in later topics.
An access point is a Layer 2 device that functions like an 802.3 Ethernet hub. RF is a shared medium and access points hear all radio traffic. Just as with 802.3 Ethernet, the devices that want to use the medium contend for it. Unlike Ethernet NICs, though, it is expensive to make wireless NICs that can transmit and receive at the same time, so radio devices do not detect collisions. Instead, WLAN devices are designed to avoid them.
CSMA/CA:
Access points oversee a distributed coordination function (DCF) called Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). This simply means that devices on a WLAN must sense the medium for energy (RF stimulation above a certain threshold) and wait until the medium is free before sending. Because all devices are required to do this, the function of coordinating access to the medium is distributed. If an access point receives data from a client station, it sends an acknowledgement to the client that the data has been received. This acknowledgement keeps the client from assuming that a collision occurred and prevents a data retransmission by the client.
RF signals attenuate. That means that they lose their energy as they move away from their point of origin. Think about driving out of range of a radio station. This signal attenuation can be a problem in a WLAN where stations contend for the medium.
Imagine two client stations that both connect to the access point, but are at opposite sides of its reach. If they are at the maximum range to reach the access point, they will not be able to reach each other. So neither of those stations sense the other on the medium, and they may end up transmitting simultaneously. This is known as the hidden node (or station) problem.
One means of resolving the hidden node problem is a CSMA/CA feature called request to send/clear to send (RTS/CTS). RTS/CTS was developed to allow a negotiation between a client and an access point. When RTS/CTS is enabled in a network, access points allocate the medium to the requesting station for as long as is required to complete the transmission. When the transmission is complete, other stations can request the channel in a similar fashion. Otherwise, normal collision avoidance function is resumed.
Wireless NICs
You may already use a wireless network at home, in a local coffee shop, or at the school you attend. Have you ever wondered what hardware components are involved in allowing you to wirelessly access the local network or Internet? In this topic, you will learn which components are available to implement WLANs and how each is used in the wireless infrastructure.
To review, the building block components of a WLAN are client stations that connect to access points that, in turn, connect to the network infrastructure. The device that makes a client station capable of sending and receiving RF signals is the wireless NIC.
Like an Ethernet NIC, the wireless NIC, using the modulation technique it is configured to use, encodes a data stream onto an RF signal. Wireless NICs are most often associated with mobile devices, such as laptop computers. In the 1990s , wireless NICs for laptops were cards that slipped into the PCMCIA slot. PCMCIA wireless NICs are still common, but many manufacturers have begun building the wireless NIC right into the laptop. Unlike 802.3 Ethernet interfaces built into PCs, the wireless NIC is not visible, because there is no requirement to connect a cable to it.
Other options have emerged over the years as well. Desktops located in an existing, non-wired facility can have a wireless PCI NIC installed. To quickly set up a PC, mobile or desktop, with a wireless NIC, there are many USB options available as well.
Wireless Routers
Wireless routers perform the role of access point, Ethernet switch, and router. For example, the Linksys WRT300N used is really three devices in one box. First, there is the wireless access point, which performs the typical functions of an access point. A built-in four-port, full-duplex, 10/100 switch provides connectivity to wired devices. Finally, the router function provides a gateway for connecting to other network infrastructures.
The WRT300N is most commonly used as a small business or residential wireless access device. The expected load on the device is low enough that it should be able to manage the provision of WLAN, 802.3 Ethernet, and connect to an ISP.

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