Zyxel NBG4604 Router Manual PDF (Setup & Configuration Guide)

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Appendix D Wireless LANs

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241

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.

Figure 163

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

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242

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 164

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

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

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several intermediate rate steps between the maximum and minimum data rates.

The IEEE 802.11g data rate and modulation are as follows:

IEEE 802.1x

In June 2001, the IEEE 802.1x standard was designed to extend the features of

IEEE 802.11 to support extended authentication as well as providing additional

accounting and control features. It is supported by Windows XP and a number of

network devices. Some advantages of IEEE 802.1x are:

•

User based identification that allows for roaming.

•

Support for RADIUS (Remote Authentication Dial In User Service, RFC 2138,

2139) for centralized user profile and accounting management on a network

RADIUS server.

•

Support for EAP (Extensible Authentication Protocol, RFC 2486) that allows

additional authentication methods to be deployed with no changes to the access

point or the wireless stations.

RADIUS

RADIUS is based on a client-server model that supports authentication,

authorization and accounting. The access point is the client and the server is the

RADIUS server. The RADIUS server handles the following tasks:

• Authentication

Determines the identity of the users.

• Authorization

Determines the network services available to authenticated users once they are

connected to the network.

• Accounting

Keeps track of the client’s network activity.

RADIUS is a simple package exchange in which your AP acts as a message relay

between the wireless station and the network RADIUS server.

Table 86

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)

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Types of RADIUS Messages

The following types of RADIUS messages are exchanged between the access point

and the RADIUS server for user authentication:

• Access-Request

Sent by an access point requesting authentication.

• Access-Reject

Sent by a RADIUS server rejecting access.

• Access-Accept

Sent by a RADIUS server allowing access.

• Access-Challenge

Sent by a RADIUS server requesting more information in order to allow access.

The access point sends a proper response from the user and then sends another

Access-Request message.

The following types of RADIUS messages are exchanged between the access point

and the RADIUS server for user accounting:

• Accounting-Request

Sent by the access point requesting accounting.

• Accounting-Response

Sent by the RADIUS server to indicate that it has started or stopped accounting.

In order to ensure network security, the access point and the RADIUS server use a

shared secret key, which is a password, they both know. The key is not sent over

the network. In addition to the shared key, password information exchanged is

also encrypted to protect the network from unauthorized access.

Types of Authentication

This appendix discusses some popular authentication types:

EAP-MD5

,

EAP-TLS

,

EAP-TTLS

,

PEAP

and

LEAP

.

The type of authentication you use depends on the RADIUS server or the AP.

Consult your network administrator for more information.

EAP-MD5 (Message-Digest Algorithm 5)

MD5 authentication is the simplest one-way authentication method. The

authentication server sends a challenge to the wireless station. The wireless

station ‘proves’ that it knows the password by encrypting the password with the

challenge and sends back the information. Password is not sent in plain text.

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