Wireless LAN
(網多實驗專題報告)
第一組
電機四
林長寬
B90901093
Introduction
The concept of wireless is widely used in our daily life. From remote controllers,
notebooks with wireless interface card, to even out cell phone, with the mobile
communication provided by wireless devices, life is sure to be much easier.
Elements and architectures in a wireless network
Wireless Host
As in the case of wired networks, hosts are the end-system devices that run
applications. A wireless host might be a laptop, palmtop, PDA, phone, or desktop
computer. The host themselves may or may not be mobile.
Base Station
The base station is a key part of the wireless network infrastructure, which seems to
have no obvious counterpart in a wired network. A base station is responsible for
sending and receiving data to and from a wireless host that is associated with the base
station. Cell towers and access points in an 802.11 wireless LAN are examples of base
stations.
Basic Service Set (BSS)
The fundamental building block of a wireless network architecture is the basic
service set, which contains one or more wireless stations and a central base station (in
infrastructure mode), known as AP in 802.11 parlances.
Network Infrastructure
(1) Infrastructure Mode
Hosts associated with a base station are often referred to as operated in
infrastructure mode, since all traditional network services are provided by the network
to which a host is connected via base station.
(2) Ad hoc Mode
In ad hoc networks, wireless hosts have no such infrastructure with which to
connect. In the absence of such infrastructure, the hosts themselves must provide for
services such as routing, address assignment, DNS-like name translation, etc.
CSMA/CA protocol (Carrier Sense Multiple Access/Collision Avoidance)
Once a wireless station is associated with an AP, it can start sending and receiving
data frames to and from the access point. But because multiple stations may want to
transmit data frames at the same time over the same channel, a multiple access
protocol is needed to coordinate the transmission. CSMA/CA means that each station
senses the channel before transmitting, and refrains from transmitting when the
channel is sensed busy.
DIFS
If initially the station senses the channel idle, it transmits its frame after a short
period of time known as Distributed Inter-frame Space (DIFS).
SIFS
As shown in the figure above, when the destination station receives a frame of data,
it waits a short period of time known as the Short Inter-frame Spacing (SIFS) and then
sends the following response packet.
RTS & CTS
RTS and CTS are the tricks used in 802.11 MAC protocol to prevent from the
problem of hidden terminal and fading. The 802.11 protocol allows a station to use a
short Request to Send (RTS) control frame and a short Clear to Send (CTS) control
frame to reserve access to the channel. When a sender wants to send a DATA frame, it
can first send an RTS frame to the AP, indicating the total time required to transmit
the DATA frame and the acknowledgement frame. When the AP receives the RTS
frame, it responds by broadcasting a CTS frame. This CTS frame has the purpose of
both sending explicit permission to send and also instructing the other stations no to
send for the reserved duration.
Other Transmission Techniques
DSSS & FHSS
Direct sequence and frequency hopping spread spectrum are the most popular
spread spectrum techniques. Such techniques are originally developed by the U.S.
army for the military communication. During the process of spread spectrum, a series
of pseudo noise code is multiplied into either directly to the signal or into the
frequency band, so that the transmitted signal is quite efficiently hidden in the given
channel. The details of the spread spectrum theory can be referred in the following
book:
Communication System 4th edition, by Simon Haykin, ISBN 0-471-17859-1
Wireless Standards
IrDA
The Infrared Data Association (IrDA) is a voluntary organization of various
manufacturers working together to ensure that the infrared communications between
different computers, PDAs, printers, digital cameras, remote controls, etc. are all
compatible with each other regardless of brand. The term is also often used to
designate an IrDA compliant infrared communications port on a device. Informally, a
device able to communicate via IrDA compliant infrared is sometimes simply said to
"have IrDA". IrDA ports support roughly the same transmission rates as traditional
parallel ports. The only restriction on their use is that the two devices must be within a
few feet of each other and there must be a clear line of sight between them.
802.11a/b/g
802.11a/b/g is a family of specifications for wireless local area networks (WLANs)
developed by a working group of the Institute of Electrical and Electronics Engineers
(IEEE). The original specification provides for an Ethernet Media Access Controller
(MAC) and several physical layer (PHY) options, the most popular of which uses
GFSK modulation at 2.4GHz, enabling data rates of 1 or 2Mbps. Since its inception,
two major PHY enhancements have been adopted and become "industry standards".
802.11b adds CCK modulation enabling data rates of up to 11Mbps, and 802.11a
specifies OFDM modulation in frequency bands in the 5 to 6GHz range, and enables
data rates up to 54Mbps.
HomeRF
The Home Radio Frequency Working Group developed a single specification
(Shared Wireless Access Protocol-SWAP) for a broad range of interoperable
consumer devices. SWAP is an open industry specification that allows PCs,
peripherals, cordless telephones and other consumer devices to share and
communicate voice and data in and around the home without the complication and
expense of running new wires. The SWAP specification provides low cost voice and
data communications in the 2.4GHz ISM band. Unlike other wireless LAN standards,
the HomeRF protocol provides high quality, multi-user voice capabilities. HomeRF
combines the best of broadband wireless data networking technology with the most
prevalent digital cordless telephony standard in the world.
HiperLAN
High Performance Radio LAN. Generic title for a series of ETSI standards
designed to provide high-speed wireless connectivity within a LAN or between LANs
(fixed wireless access). HiperLAN1 was designed to provide 24 Mbps connections in
the 5GHz frequency band, but hasn't been commercialized as yet. HiperLAN2 is
touted as the leading European standard for wireless LAN's operating in the 5GHz
range, offering 54 Mbps data rates and incorporating enhanced Security, QoS, DFS
and other features.