Zyxel P-660HN-T1A Router Manual PDF (Setup & Configuration Guide)

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P-660HN-TxA User’s Guide

291

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 160

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

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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.

Figure 161

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.

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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.

Figure 162

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

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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.

Figure 163

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.

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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.

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 ZyXEL Device 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

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