networking ch 8

Wireless frequency ranges:

*2.4-GHz band is used to send and receive wireless signals but is also used by cordless telephones and other types of signals that interfere with wireless *New wireless LAN (WLAN) technologies use the 5-GHz band which is also used by weather and military radar communications *WLAN equipment using this band must be able to detect radar signals and then switch to a different frequency

802.11 WIRELESS LAN ACCESS METHOD pt2

*802.11 contains a mechanism whereby a station can reserve the transmission media (AP) for its use (ex. when a large amount of data needs to be transmitted) *Station sends a signal known at Request to Send (RTS) to the access point *If access point is able to give exclusive use to this station, the AP (1) suspends communications with other stations and (2) gives requesting station a Clear to Send (CTS) signal *RTS/CTS protocol is not highly used since it favors one transmission but decreases overall network efficiency by locking out other stations *Station maintains exclusive use of the medium until the transmission is complete

Wireless transmission are susceptible to:

*Fading is loss of signal strength as a result of reflection, diffraction and scattering *Attenuation *Noise resulting from EMI (sources include cellular phones, mobile phone and overhead lights) can have an especially strong effect on wireless transmission

band signals

*Narrowband-concentrates signal energy at a single frequency or a very small range of frequency thereby limiting bandwidth and the ability to support multiple channels *Broadband- uses a wide band of frequency (more of the spectrum) and supports wider bandwidth and multiple channels *Spread-spectrum- is constantly switching from one frequency to another using a defined pattern known only to the sender or receiver -Provides a more secure form of communications since an unauthorized listener with not be likely to be able to keep up with the frequency changes -As explained in a previous chapter, the more levels that a signal can be measured results in each signal carrying more data

multipath signals

*reflection-the wave encounters an obstacle and reflects or bounces back toward its source. *diffraction-a wireless signal splits into secondary waves when it encounters an obstruction. *Scattering-is the diffusion, or the reflection in multiple different directions, of a signal. Scattering occurs when a wireless signal encounters an object that has small dimensions compared with the signal's wavelength.

802.11 wireless LAN association

Association is the communication that occurs between a wireless device and an access point to enable the device to connect to the network via the access point Association can be: *In active scanning, a wireless device transmits a frame known as a probe on all available channels within its frequency range and waits for a response from an access point that is available to connect to the network *In passive scanning, a wireless device listens on all channels within its frequency range for a signal known as a beacon frame which is sent by access points that are available to connect to a network

802.11 WIRELESS LAN ACCESS METHOD

CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance)Station checks for wireless activity before attempting to send; if activity is detected, stations waits before retrying *If no activity is detected, station waits a little longer for activity (a randomly determined time period) and if no further activity is detected, the transmission is sent *Since collisions or other forms of interference can still cause the transmission to fail, 802.11 uses an ACK/Timeout mechanism to determine if the transmission needs to be resent *Less throughput than CSMA/CD *802.11 techniques can either transmit or receive, but cannot do both simultaneously (half duplex)

DSSS (direct-sequence spread spectrum)

In DSSS, a signal's bits are distributed over an entire frequency band at once.

802.16 (WiMAX)

WiMAX is a wireless digital communications system, that is intended for wireless "metropolitan area networks" *30 miles (50 km) for fixed stations, and 3 - 10 miles (5 - 15 km) for mobile stations but throughput decreases as a function of distance *Throughput up to 70 Mbps WiMAX supports: *Home or small office implementation through use of a fixed antenna *Mobile installation through use of a WiMAX transceiver (typically in a PCMCIA slot) *WiMAX issues include cost and limited availability

FHSS transmission

a signal jumps between several different frequencies within a band in a synchronization pattern known only to the channel ' s receiver and transmitter.

Antennas

are required to send and receive and must match the power output, frequency and radiation pattern defined for the service

Fixed

in which the location of the transmitter and the receiver does not move

Mobile

in which the receiver is located anywhere within the transmitter's range

access point (a.k,a, AP, wireless access point or WAP)

is a device that accepts wireless signals from multiple nodes and retransmits these signals to the rest of the network *Access point can be a stand-alone device or can be imbedded in wireless switches or wireless routers (typically on home networks) *Access point can typically handle 10 to 100 connections *Number and placement (a WLAN can have multiple APs) is dependent upon the environment and where wireless access is required

Bluetooth

is a standard for low power, short distance wireless communications between many different types of devices (ex. PDA, cell phones & headsets, and cordless telephones)

wireless characteristics

obstacle stands in the signal's way, the signal might pass through the obstacle, be absorbed by the obstacle or have one of the following occur: Reflection, diffraction and scattering can cause signals to follow multiple paths Scattering & reflection sometimes give a signal a better change to reach their destination (ex. in a building)

Reassociation

occurs when a station moves out of the range of an access point and must associate with a different access point

Cellular networks

provide data services to mobile clients over packet-switched networks. Though there are many types of cellular networks, all share an infrastructure in which coverage areas are divided into cells that are serviced by antennas and base stations.

Satellite Internet Access

utilizes satellites to provide internet access *Satellites used for communications are usually in a geosynchronous orbit *move at a speed that is equivalent to the earth's rotation. Therefore the satellite is always at the same location relative to a point on the earth *Downlink is encrypted or otherwise secured to prevent unauthorized access to the transmission

The beacon frame

The beacon frame contains the information that the wireless device needs in order to associate with the access point The wireless device then determines whether or not is wishes to associate with this access point

IEEE 802.11

The most popular standard wireless standard for LANs is IEEE 802.11 (a.k.a. Wi-Fi) which implements the Datalink and Physical layers of the OSI model (TCP/IP Network Interface layer) Uses 48 bit physical address (same as MAC address) which is a factor in allowing 802.11 to co-exist with 802.3 (wired Ethernet)

directional antennas

Used when location of devices is fixed and allows all power to be focused in one direction Ex. Satellite connection to an ISP

Omni-directional antennas

Used when location of devices is not known or devices are mobile (ex. wireless LANs). Power is disbursed in all directions resulting in lower powered signals