D. DIFFRACTION PASSES OR BENDS AROUND AN OBSTACLE. REFLECTION BOUN...

20. D. Diffraction passes or bends around an obstacle. Reflection bounces off a smooth sur-face, refraction passes through, and scattering bounces off an uneven surface.

Chapter

5 Access Methods,

Architectures, and

Spread Spectrum

Technology

The following CwTS exAM objeCTiveS

Are Covered in ThiS ChApTer:

Define concepts which make up the functionality of RF Ûand spread spectrum technologyOFDM & HR/DSSS channels

Û

N

Co-location of HR/DSSS and OFDM systemsAdjacent-channel and co-channel interferenceWLAN / WPAN co-existenceCSMA/CA operation – half duplexDefine and differentiate between the following physical layer wireless technologiesHR/DSSSOFDMMIMO

of information. Two common access methods are CSMA/CD and CSMA/CA. The type

of medium in use—wired or wireless—will determine which of these two access methods

is best suited for the application. We saw in previous chapters that wireless LANs use radio

frequency with the communication medium. This chapter will discuss the various techniques

and methods used to get digital computer data from one device to another using spread spectrum

and modulation technologies. It is important to understand that different spread spectrum tech-

nologies such as FHSS, DSSS, and others will allow for various data rates. The spread spectrum

technology in use will determine the maximum amount of data transfer as well as the resil-

ience to noise and other interfering sources. We will look at the number of channels avail-

able, channel spacing, and the frequencies of unlicensed radio frequency band used. We will

also discuss the implications of overlapping channels and interference. Finally, this chapter

will discuss co-location of different technologies used in various amendments to the stan-

dard as well as interference from wireless LANs and other sources, including WPANs.

Network Access Methods

Network access methods allow devices connected to a common infrastructure the oppor-

tunity to communicate and transmit data across a network medium from one device to

another. Several different types of network access methods are used in computer networks.

The two types we will discuss in this chapter are:

Carrier Sense Multiple Access/Collision Detection (CSMA/CD)Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA)

Because Ethernet networks have the capability to detect collisions, 802.3 Ethernet net-

works use CSMA/CD as the access method. Devices on a wireless LAN do not have the

capability to detect collisions; therefore 802.11 wireless LANs use CSMA/CA for the access

method. Since multiple wireless devices can use an access point at the same time, wireless

devices that connect to an AP are competing to share the medium; therefore, it is important

to control the medium in order to minimize collisions. The CSMA/CA process provides this

control.

Network Access Methods 133

Detecting Network Traffic Collisions with CSMA/CD