Zyxel P-2602HW-D1A Router Manual PDF (Setup & Configuration Guide)

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P-2602H(W)(L)-DxA Series User’s Guide

Appendix E Wireless LANs

361

A

PPENDIX

E

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 stations (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 213

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

BSS

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

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

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

wireless station A and B can access the wired network and communicate with each other.

When Intra-BSS is disabled, wireless station A and B can still access the wired network but

cannot communicate with each other.

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Appendix E Wireless LANs

Figure 214

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 stations within the same ESS must have the same ESSID in order to

communicate.

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Appendix E Wireless LANs

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Figure 215

Infrastructure WLAN

Channel

A channel is the radio frequency(ies) used by IEEE 802.11a/b/g wireless devices. Channels

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

region) so you should use a different channel than 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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Appendix E Wireless LANs

Figure 216

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.

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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.

Preamble Type

A preamble is used to synchronize the transmission timing in your wireless network. There are

two preamble modes:

Long

and

Short

.

Short preamble takes less time to process and minimizes overhead, so it should be used in a

good wireless network environment when all wireless stations support it.

Select

Long

if you have a ‘noisy’ network or are unsure of what preamble mode your wireless

stations support as all IEEE 802.11b compliant wireless adapters must support long preamble.

However, not all wireless adapters support short preamble. Use long preamble if you are

unsure what preamble mode the wireless adapters support, to ensure interpretability between

the AP and the wireless stations and to provide more reliable communication in ‘noisy’

networks.

Select

Dynamic

to have the AP automatically use short preamble when all wireless stations

support it, otherwise the AP uses long preamble.

Note:

The AP and the wireless stations 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:

Table 147

IEEE 802.11g

DATA RATE (MBPS)

MODULATION

1

DBPSK (Differential Binary Phase Shift Keyed)

2

DQPSK (Differential Quadrature Phase Shift Keying)

5.5 / 11

CCK (Complementary Code Keying)

6/9/12/18/24/36/48/54

OFDM (Orthogonal Frequency Division Multiplexing)

Rate

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