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A

PPENDIX

D

Wireless LANs

Wireless LAN Topologies

This section discusses ad-hoc and infrastructure wireless LAN topologies.

Ad-hoc Wireless LAN Configuration

The simplest WLAN configuration is an independent (Ad-hoc) WLAN that connects a set of

computers with wireless adapters (A, B, C). Any time two or more wireless adapters are within

range of each other, they can set up an independent network, which is commonly referred to as an

ad-hoc network or Independent Basic Service Set (IBSS). The following diagram shows an example

of notebook computers using wireless adapters to form an ad-hoc wireless LAN.

Figure 172

Peer-to-Peer Communication in an Ad-hoc Network

BSS

A Basic Service Set (BSS) exists when all communications between wireless clients or between a

wireless client and a wired network client go through one access point (AP).

Intra-BSS traffic is traffic between wireless clients in the BSS. When Intra-BSS is enabled, wireless

client

A

and

B

can access the wired network and communicate with each other. When Intra-BSS is

disabled, wireless client

A

and

B

can still access the wired network but cannot communicate with

each other.

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Figure 173

Basic Service Set

ESS

An Extended Service Set (ESS) consists of a series of overlapping BSSs, each containing an access

point, with each access point connected together by a wired network. This wired connection

between APs is called a Distribution System (DS).

This type of wireless LAN topology is called an Infrastructure WLAN. The Access Points not only

provide communication with the wired network but also mediate wireless network traffic in the

immediate neighborhood.

An ESSID (ESS IDentification) uniquely identifies each ESS. All access points and their associated

wireless clients within the same ESS must have the same ESSID in order to communicate.

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Figure 174

Infrastructure WLAN

Channel

A channel is the radio frequency(ies) used by wireless devices to transmit and receive data.

Channels available depend on your geographical area. You may have a choice of channels (for your

region) so you should use a channel different from an adjacent AP (access point) to reduce

interference. Interference occurs when radio signals from different access points overlap causing

interference and degrading performance.

Adjacent channels partially overlap however. To avoid interference due to overlap, your AP should

be on a channel at least five channels away from a channel that an adjacent AP is using. For

example, if your region has 11 channels and an adjacent AP is using channel 1, then you need to

select a channel between 6 or 11.

RTS/CTS

A hidden node occurs when two stations are within range of the same access point, but are not

within range of each other. The following figure illustrates a hidden node. Both stations (STA) are

within range of the access point (AP) or wireless gateway, but out-of-range of each other, so they

cannot "hear" each other, that is they do not know if the channel is currently being used. Therefore,

they are considered hidden from each other.

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Figure 175

RTS/CTS

When station

A

sends data to the AP, it might not know that the station

B

is already using the

channel. If these two stations send data at the same time, collisions may occur when both sets of

data arrive at the AP at the same time, resulting in a loss of messages for both stations.

RTS/CTS

is designed to prevent collisions due to hidden nodes. An

RTS/CTS

defines the biggest

size data frame you can send before an RTS (Request To Send)/CTS (Clear to Send) handshake is

invoked.

When a data frame exceeds the

RTS/CTS

value you set (between 0 to 2432 bytes), the station

that wants to transmit this frame must first send an RTS (Request To Send) message to the AP for

permission to send it. The AP then responds with a CTS (Clear to Send) message to all other

stations within its range to notify them to defer their transmission. It also reserves and confirms

with the requesting station the time frame for the requested transmission.

Stations can send frames smaller than the specified

RTS/CTS

directly to the AP without the RTS

(Request To Send)/CTS (Clear to Send) handshake.

You should only configure

RTS/CTS

if the possibility of hidden nodes exists on your network and

the "cost" of resending large frames is more than the extra network overhead involved in the RTS

(Request To Send)/CTS (Clear to Send) handshake.

If the

RTS/CTS

value is greater than the

Fragmentation Threshold

value (see next), then the

RTS (Request To Send)/CTS (Clear to Send) handshake will never occur as data frames will be

fragmented before they reach

RTS/CTS

size.

Note: Enabling the RTS Threshold causes redundant network overhead that could

negatively affect the throughput performance instead of providing a remedy.

Fragmentation Threshold

A

Fragmentation Threshold

is the maximum data fragment size (between 256 and 2432 bytes)

that can be sent in the wireless network before the AP will fragment the packet into smaller data

frames.

A large

Fragmentation Threshold

is recommended for networks not prone to interference while

you should set a smaller threshold for busy networks or networks that are prone to interference.

If the

Fragmentation Threshold

value is smaller than the

RTS/CTS

value (see previously) you

set then the RTS (Request To Send)/CTS (Clear to Send) handshake will never occur as data frames

will be fragmented before they reach

RTS/CTS

size.

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Preamble Type

Preamble is used to signal that data is coming to the receiver. Short and long refer to the length of

the synchronization field in a packet.

Short preamble increases performance as less time sending preamble means more time for sending

data. All IEEE 802.11 compliant wireless adapters support long preamble, but not all support short

preamble.

Use long preamble if you are unsure what preamble mode other wireless devices on the network

support, and to provide more reliable communications in busy wireless networks.

Use short preamble if you are sure all wireless devices on the network support it, and to provide

more efficient communications.

Use the dynamic setting to automatically use short preamble when all wireless devices on the

network support it, otherwise the AMG1302/AMG1202-TSeries uses long preamble.

Note: The wireless devices MUST

use the same preamble mode in order to communicate.

IEEE 802.11g Wireless LAN

IEEE 802.11g is fully compatible with the IEEE 802.11b standard. This means an IEEE 802.11b

adapter can interface directly with an IEEE 802.11g access point (and vice versa) at 11 Mbps or

lower depending on range. IEEE 802.11g has several intermediate rate steps between the

maximum and minimum data rates. The IEEE 802.11g data rate and modulation are as follows:

Wireless Security Overview

Wireless security is vital to your network to protect wireless communication between wireless

clients, access points and the wired network.

Wireless security methods available on the AMG1302/AMG1202-TSeries are data encryption,

wireless client authentication, restricting access by device MAC address and hiding the AMG1302/

AMG1202-TSeries identity.

Table 105

IEEE 802.11g

DATA RATE (MBPS)

MODULATION

1

DBPSK (Differential Binary Phase Shift Keyed)

2

DQPSK (Differential Quadrature Phase Shift Keying)

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CCK (Complementary Code Keying)

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OFDM (Orthogonal Frequency Division Multiplexing)