THOMSON Gateway Configuration Manual

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Thomson Gateway

Wireless Configuration Guide

Thomson Gateway

Wireless Configuration Guide

Distribution and copying of this document, use and communication of its contents is not permitted without written authorization from THOMSON. The content of this document is furnished for informational use only, may be subject to change without notice, and should not be construed as a commitment by THOMSON. THOMSON assumes no responsibility or liability for any errors or inaccuracies that may appear in this document.

Thomson Telecom Belgium Prins Boudewijnlaan, 47 B-2650 Edegem Belgium

http://www.thomson-broadband.com

Trademarks

The following trademarks are used in this document:

> DECT is a trademark of ETSI. > Bluetooth® word mark and logos are owned by the Bluetooth SIG, Inc. > Ethernet™ is a trademark of Xerox Corporation. > Wi-Fi® and the Wi-Fi logo are registered trademarks of the Wi-Fi Alliance. "Wi-Fi CERTIFIED", "Wi-Fi ZONE", "Wi-Fi Alli-

ance", their respective logos and "Wi-Fi Protected Access" are trademarks of the Wi-Fi Alliance.

> UPnP™ is a certification mark of the UPnP™ Implementers Corporation. > Microsoft®, MS-DOS®, Windows® and Windows NT® are either registered trademarks or trademarks of Microsoft Corpo-

ration in the United States and/or other countries.

> Apple® and Mac OS® are registered trademarks of Apple Computer, Incorporated, registered in the United States and

other countries.

> UNIX® is a registered trademark of UNIX System Laboratories, Incorporated. > Adobe®, the Adobe logo, Acrobat and Acrobat Reader are trademarks or registered trademarks of Adobe Systems, Incor-

porated, registered in the United States and/or other countries.

Other brands and product names may be trademarks or registered trademarks of their respective holders.

Document Information

Status: v2.0 (April 2007) Reference: E-DOC-CTC-20060609-0001 Short Title: Config Guide: WLAN R6.2 (and higher)

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Contents

About this Wireless Configuration Guide ................................ 1

1 Introducing Wireless Networking .............................................. 3

1.1 Introduction .................................................................................................... 3

1.2 WLAN Components and Terminology ............................................................. 5

2 802.11 Standards ....................................................................... 11

2.1 MAC Sublayer ............................................................................................... 13

2.2 802.11a ......................................................................................................... 16

2.3 802.11b ......................................................................................................... 19

2.4 802.11g ......................................................................................................... 22

3 Security....................................................................................... 25

3.1 Disabling SSID Broadcasting ........................................................................ 27

3.2 MAC Address Filtering .................................................................................. 28

3.3 Wired Equivalent Privacy (WEP) .................................................................... 29

3.4 Wi-Fi Protected Access (WPA)....................................................................... 31

3.5 WPA2 ............................................................................................................ 35

4 Wi-Fi Multi Media (WMM)......................................................... 37

5 Wireless Distribution System (WDS)....................................... 39

6 Virtual Access Points................................................................. 41

6.1 What is a Virtual Access Point? .................................................................... 42

6.2 Multiple SSIDs .............................................................................................. 44

6.3 Architectural Elements ................................................................................. 46

7 Thomson Gateway Wireless Configuration ............................ 47

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7.1 Basic Configuration ...................................................................................... 48

7.1.1 Connecting Wireless Stations for the First Time ............................................................................... 49

7.1.2 Configuring Wireless Stations.............................................................................................................51

7.1.3 Configuring the Thomson Gateway Access Point ............................................................................. 54

7.1.4 Connecting Additional Wireless Stations...........................................................................................61

7.1.5 Configuring your Thomson Gateway with WDS ...............................................................................63

7.1.6 Reset to Factory Defaults .....................................................................................................................65

7.2 Expert Configuration..................................................................................... 67

7.2.1 Access Point Settings...........................................................................................................................69

7.2.2 Security .................................................................................................................................................72

7.2.3 Associated Stations..............................................................................................................................77

7.2.4 Networks ...............................................................................................................................................78

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Used symbols

Typographical conventions

Following typographical convention is used throughout this manual:

> Sample text indicates a hyperlink to a Web site.

Example: For more information, visit us at www.thomson-broadband.com

> Sample text indicates an internal cross-reference.

Example: If you want to know more about guide, see “1 Introduction” on page 7”.

> Sample text indicates an important content-related word.

Example: To enter the network, you must authenticate yourself.

> Sample text indicates a GUI element (commands on menus and buttons, dialog box elements, file

names, paths and folders).

Example: On the File menu, click Open to open a file.

> Sample text indicates a CLI command to be input after the CLI prompt.

Example: To obtain a list of all available command groups, type

help at the top level.

> Sample text indicates input in the CLI interface. > Sample text indicates comment explaining output in the CLI interface.

Example:

Documentation and software updates

THOMSON continuously develops new solutions, but is also committed to improving its existing products.

For suggestions regarding this document, please contact documentation.speedtouch@thomson.net

For more information on THOMSON's latest technological innovations, documents and software releases, visit us at http://www.thomson-broadband.com

A note provides additional information about a topic.

A caution warns you about potential problems or specific precautions that need to be taken.

=> language list

CODE LANGUAGE VERSION FILENAME en* english 4.2.0.1 <system>

Only one language is available

Output

Input

Comments

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About this Wireless Configuration Guide

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Chapter 1

Introducing Wireless Networking

1 Introducing Wireless Networking

1.1 Introduction

IEEE 802.11

In the early ‘90s a lot of wireless systems were developed because people wanted to connect their laptop computers to the network (and Internet) when entering the office. The problem was that none of these systems was compatible with the other. Finally, the IEEE association elaborated a standard for Wireless Local Area Networks (WLAN). This standard was referred to as 802.11 or Wi-Fi (Wireless Fidelity). As Ethernet had become the actual standard for LAN, the WLAN standard was designed to be compatible with Ethernet above the data link layer. As a result, IP packets can be sent over a WLAN in the same way that they are sent over Ethernet.

Overview of wireless standards

This is an extensive alphabetical list of existing popular wireless standards:

> AMPS: Advanced Mobile Phone System. AMPS is the first analog cellular standard in the U.S. Although

AMPS is still in use, it is anticipated to be replaced by the United States Digital Cellular (USDC) standard.

> Bluetooth: Bluetooth is an industrial specification for Wireless Personal Area Networks (WPANs).

Bluetooth provides a way to connect and exchange information between devices such as mobile phones, laptops, PCs, printers, digital cameras and video game consoles via a secure, globally unlicensed shortrange radio frequency. IEEE 802.15.1 has derived a WPAN standard based on the Bluetooth v1.1 specifications. It includes a medium access control and physical layer specification.

> CDPD: Cellular Digital Packet Data is a digital standard for packet data services. CDPD was designed to

overlay with existing cellular infrastructure, thereby permitting simple and inexpensive installation.

> CEBus: The Consumer Electronics Bus (CEBus) standard was created by the Electronic Industries

Association (EIA). CEBus is an engineering standard for home automation products. It supports carrier current, RF, IR, coaxial cable, twisted pair, and fibre optic cable.

> DECT: Digital Enhanced (formerly European) Cordless Telecommunications is a universal cordless

telephone standard developed by the European Telecommunications Standard Institute (ETSI). DECT offers services for both voice and data communications.

> GSM: Global System for Mobile Communications. The GSM standard was developed in Europe to

standardize cellular communications among European countries. GSM has proven to be one of the most successful standards of the last decades and continues as one of the world`s most popular standards for new cellular radio and personal communications equipment.

> HIPERLAN: HIgh PErformance Radio LAN is a WLAN standard. It is a European alternative for the

IEEE 802.11 standards. It is defined by ETSI. In ETSI, the standards are defined by the BRAN (Broadband Radio Access Networks) project.

> IEEE 802.11 a,b,g: This is the IEEE standard for WLANs. The goal of the IEEE 802.11 committee is to

standardize WLAN development in the ISM (Industrial, Scientific and Medical) band. The standard focuses on the Media Access Control (MAC) and the physical (PHY) protocol levels. The IEEE 802.11 standard is still under development, but is anticipated to become the WLAN standard.

> IrDA: The Infrared Data Association (IrDA) was formed to develop a standard for wireless communication

using infrared (IR) technology. Some of the main goals of the committee are to develop a standard that permits low cost, low power, point-to-point user communications using IR as the transmission medium.

> IS-54: Interim Standard 54. See USDC.

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> IS-95: IS-95 is a digital cellular standard from the United States that uses a Code Division Multiple Access

(CDMA) scheme. In a CDMA system, users share time and frequency resources simultaneously. This occurs through assigning a distinct digital code to each user. This code is added to the information data and modulated onto the carrier, using spread spectrum techniques. Although it anticipates providing significant capacity improvement and increased interference rejection over other digital cellular standards, IS-95 remains somewhat controversial because of its wide bandwidth requirements.

> PHS: The Personal Handphone System standard was developed in Japan specifically for indoor and

microcell usage.

> UMTS: Universal Mobile Telecommunications System. UMTS is one of the third-generation (3G) mobile

phone technologies. The currently most common form uses W-CDMA as the underlying air interface and is standardized by the 3GPP. UMTS is the European answer to the ITU IMT-2000 requirements for 3G cellular radio systems and was designed to succeed GSM.

> USDC: United States Digital Cellular, also known as IS-54 (Interim Standard 54), was developed to replace

the AMPS standard, particularly in urban areas where AMPS did not provide adequate channel capacity. USDC allows the co-existence of AMPS so that providers can gradually phase out AMPS as needed.

> WiMAX: WiMAX is a wireless industry coalition whose members organized to advance IEEE 802.16

standards for broadband wireless access (BWA) networks. WiMAX 802.16 technology is expected to enable multimedia applications with wireless connection and, with a range of up to 30 miles, enable networks to have a wireless last mile solution.

Wi-Fi

The Wi-Fi Alliance is a global, non-profit organization that is responsible for testing and certifying interoperability of wireless devices.

The Wi-Fi Alliance controls the Wi-Fi Certified logo which is permitted only on compliant equipment, indicating that the device is interoperable with any other product also showing the logo. The following illustration shows the Wi-Fi certified logo.

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1.2 WLAN Components and Terminology

Access point

A WLAN base station or Access Point (AP) behaves as a networking hub, allowing to interconnect several devices wirelessly to the local WLAN.

WLAN topologies

A WLAN consists of several devices. The logical grouping of devices belonging to a particular WLAN is called a service set. Depending on the architecture, the following topologies can be determined:

> Independent Basic Service Set (IBSS) or ad-hoc network > Basic Service Set (BSS) or infrastructure network > Extended Service Set (ESS)

Independent Basic Service Set (IBSS) or ad-hoc network

This is a peer-to-peer WLAN, because wireless stations communicate directly with one another. Communication does not happen via an AP. Wireless stations communicate with each other via 802.11 Network Interface Cards (NIC). This kind of WLAN is usually small and very temporary (usually they last until the sharing of information is accomplished). The following example illustrates an IBSS:

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Basic Service Set (BSS) or infrastructure network

This WLAN topology requires a specialized station, called an AP. Wireless stations do not communicate directly with one another, but all communication is passed to the destination via this central AP. APs can be connected to a wired network via an uplink port. The following example illustrates a BSS:

Extended Service Set (ESS)

When several BSSs are connected to each other via a Distribution System (very often an Ethernet switch), the WLAN is called an ESS. The distribution system can be either wired or wireless. The following example illustrates an ESS:

Wireless Access Point

Wireless Stations

Distribution System

BSS2BSS1

BSS3

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Basic Service Set Identifier (BSSID)

The Basic Service Set Identifier (BSSID) uniquely identifies each BSS. The BSSID is a 48-bit address with the same format as an IEEE 802 MAC address. The value of all 1s is used to indicate the broadcast BSSID.

The station that starts the BSS determines the BSSID of that BSS, depending on the topology:

> In case of an ad-hoc network, the BSSID is determined by the use of a 46-bit random number generator.

The used mechanism provides a high probability of selecting a unique BSSID.

> In case of an infrastructure network, the BSSID is the same as the MAC address of the AP. All wireless

stations communicating to the AP send to the BSSID.

Service Set Identifier (SSID) or Network Name

Wireless stations communicate with each other through the air, which is a shared medium. As no physical connection exists between the APs and the wireless stations, a name must be given to allow unique identification of your WLAN. This is called the Service Set Identifier (SSID) or Network Name. Wireless stations must be part of a specific SSID environment in order to communicate with the other stations belonging to the same WLAN.

The SSID has a length between 0 and 32 octets. A length equal to 0 octets indicates the broadcast SSID. This SSID is included in the SSID Information Element (IE), which is part of management frames such as beacon frames, probe request/response frames and association/reassociation request frames. The following illustration depicts the format of the SSID IE:

Two types of SSIDs are defined, depending on the topology:

> In case of an ad-hoc network, the SSID is also called the Independent Basic Service Set Identifier (IBSS

ID).

> In case of an infrastructure network, the term Basic Service Set Identifier (BSS ID) or Extended Service

Set Identifier (ESS ID) can be used instead of SSID.

The following example illustrates the use of the BSSID and SSID:

In many cases both types of SSID are referred to as SSID or Network Name.

The use of the term Basic Service Set Identifier can cause confusion with the BSSID defined as the MAC address of the AP.

Element

1 byte 1 byte 0-32 bytes

Length SSID

Distribution System

BSS2

ESS

BSS1

BSSID1

BSSID2

SSID

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Beacon frame

Beacon frames are transmitted periodically by APs to let wireless stations identify the wireless APs nearby. They inform the wireless stations in the BSSs (and thus a possible ESS) about the existence of a wireless network.

Beacon frames are transmitted on all channels (in the regulatory allowed spectrum) and contain among other things the BSSID, the SSID and a set of capabilities, e.g. the supported data rate, the supported security mechanism,...

Standards overview

The 802.11 standard defines a set of different physical layer technologies to be used in combination with

802.11 MAC. The standard has evolved over the years. The different technologies primarily vary in frequency bands and applied modulation techniques (resulting in different transmission rates). A short overview:

> 802.11

The first standard was released in 1997. It operated at a data transmission rate of 1 or 2 Mbps, which was much too slow for most applications, and was transmitted at 2.4 GHz. This standard is now often referred to as 802.11 legacy.

> 802.11b

This standard was ratified in 1999. It uses the same frequency band as the original 802.11, but uses a different modulation technique, so that a transmission rate of 11 Mbps is achieved.

> 802.11a

At the same time that 802.11b was ratified, 802.11a was ratified.This standard uses the 5 GHz band and has a data transmission rate up to 54 Mbps.

> 802.11g

This standard was ratified in 2003. 802.11g is backward compatible with 802.11b and also operates in the

2.4 GHz band. Because of the use of a different modulation technique, data transmission rate can go up to 54 Mbps.

For further details on each of these standards, please refer to “2 802.11 Standards” on page 11.

The following list contains an exhaustive overview of all existing 802.11 standards:

Standard Description

802.11 Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications: the original 1 Mbps and 2 Mbps, 2.4 GHz RF and IR standard (1997).

802.11a High-speed Physical Layer in the 5 GHz band: 54 Mbps, 5 GHz standard (1999).

802.11b Higher speed Physical Layer extension in the 2.4 GHz band: enhancements to 802.11 to support 5.5 and 11 Mbps (1999).

802.11c Bridge operation procedures; included in the IEEE 802.1D standard (2001).

802.11d Specification for Operation in Additional Regulatory Domains (2001).

802.11e Enhancements: QoS, including packet bursting (2005).

802.11F Recommended Practice for Multi-Vendor Access Point Interoperability via an Inter-Access Point Protocol Across Distribution Systems Supporting IEEE 802.11 (2003). Withdrawn in February 2006.

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ISM band

WLANs operate in the ISM band. The Industrial, Scientific and Medical (ISM) radio bands were originally reserved internationally for non-commercial use of RF electromagnetic fields for industrial, scientific and medical purposes.

The ISM bands are defined by the ITU-R in 5.138 and 5.150 of the Radio Regulations. The channels and their allocations are governed by regulatory bodies and can differ due to variations in national radio regulations. However, many countries have similar spectrum regulations. The ETSI has the regulatory control over the wireless spectrum in Europe, the Federal Communications Commission (FCC) in the United States, the MPHPT in Japan.

Radiocommunication services operating within these bands must accept harmful interference, which may be caused by these applications.

The different ISM bands are:

> 900 MHz band (902 - 928 MHz) > 2.4 GHz band (2.4 - 2.5 GHz) > 5.8 GHz band (5.725 - 5.875 GHz) > 24 GHz band (24 - 24.25 GHz)

802.11g Further Higher-Speed Physical Layer Extension in the 2.4 GHz Band: 54 Mbps, 2.4 GHz standard (backwards compatible with b) (2003).

802.11h Spectrum and Transmit Power Management Extensions in the 5 GHz band in Europe: Spectrum Managed 802.11a for European compatibility (2004).

802.11i Medium Access Control (MAC) Security Enhancements: enhanced security (2004).

802.11j 4.9 GHz–5 GHz Operation in Japan: extensions for Japan (2004).

802.11k Radio resource measurement enhancements.

802.11m Maintenance of the standard.

802.11n Higher throughput improvements: aims for a data transmission rate of 540 Mbps.

802.11p WAVE - Wireless Access for the Vehicular Environment: to support Intelligent Transportation Systems (ITS) applications.

802.11r Fast BSS transitions.

802.11s ESS Mesh Networking.

802.11T Wireless Performance Prediction (WPP) - test methods and metrics.

802.11u Interworking with non-802 networks (for example cellular).

802.11v Wireless network management.

802.11w Protected Management Frames.

802.11y Contention Based Protocol: defines 3.65 - 3.7 GHz operation in USA.

Not all standards are ratified yet.

Standard Description

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IEEE 802.11b/g wireless Ethernet operates on the 2.4 GHz band, IEEE 802.11a operates on the 5 GHz band. The following illustration shows the different ISM bands.

U-NII band

U-NII stands for the Unlicensed National Information Infrastructure. The FCC has made 300 MHz of spectrum available for U-NII devices that will provide short-range, high speed wireless digital communications. The UNII band includes following bands:

> U-NII 1 band or U-NII indoor (5.15 - 5.25 GHz) > U-NII 2 band or U-NII low (5.25 - 5.35 GHz) > U-NII 3 band or U-NII ISM ((5.725 - 5.825 GHz)

What is antenna diversity?

Antenna diversity is a function included in most WLAN equipment that has two antennas.

In simple terms, diversity monitors the signal from each antenna and automatically switches to the one with the better signal. The user usually has no control over this function.

Use of directional antennas

The wireless coverage area should be fit to the desired area. Directional antennas can be used at the perimeter directing their broadcasting inward. Some APs offer attenuation levels to be set via their webbased setup utility.

902-928

2400-2500

5725-5875

24000-24125

f (MHz)

5,15-5,25

5.725-5.825

5.25-5,35

f (MHz)

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2 802.11 Standards

802.11 protocol stack

The 802.11 protocol stack is very similar to the other 802 variants (such as Ethernet). The physical layer corresponds to the OSI physical layer and the data link layer is split into two sublayers:

> The MAC sublayer determines how the channel is allocated. > The LLC sublayer interfaces the different 802 variants to the network layer.

The 802.11 protocol defines a number of standards which differ on the physical layer level. Depending on the transmission technique, the following standards are defined in the 802.11 protocol stack:

History

The 1997 802.11 standard specifies a single MAC sublayer that interacts with three transmission techniques:

> Infrared > Frequency Hopping Spread Spectrum (FHSS) > Direct Sequence Spread Spectrum (DSSS)

These transmission techniques operate at 1 or 2 Mbps and with low power.

FHSS and DSSS use the 2.4 GHz ISM band. Both techniques are also referred to as 802.11 legacy.

All of the three standards are now outdated and replaced.

To achieve higher bandwidth two new techniques were introduced in 1999:

> Orthogonal Frequency Division Multiplexing (OFDM), used in the 802.11a standard, operating at up to

54 Mbps.

> High-Rate DSSS (HR-DSSS), used in the 802.11b standard, operating at up to 11 Mbps.

In 2001 an enhanced version of the 802.11b, namely 802.11g, was released. It also operates at up to 54 Mbps, applying OFDM. 802.11g is backward compatible with 802.11b.

Upper Layers

Data Link Layer

Physical Layer

MAC Sublayer

802.11g OFDM

802.11b

HR-DSSS

802.11a OFDM

802.11 DSSS

802.11 FHSS

802.11

Infrared

Logical Link Control

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Overview

The MAC sublayer and the three contemporary transmission techniques are described in the following chapter:

Topi c Page

“2.1 MAC Sublayer” 13

“2.2 802.11a” 16

“2.3 802.11b” 19

“2.4 802.11g” 22

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2.1 MAC Sublayer

MAC architecture

The MAC architecture uses the following two access methods:

> Distributed Coordination Function (DCF): this is the fundamental access method of the MAC sublayer.

The DCF is also known as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). This access method is implemented in all wireless stations, for use within both ad-hoc network and infrastructure network configurations.

> Point Coordination Function (PCF): this is an optional access method of the MAC sublayer. The PCF can

only be used within infrastructure network configurations. This access method uses a point coordinator (PC), which operates at the AP of the BSS, to determine which station has the right to transmit at a given time.

The DCF and the PCF can coexist in a way that allows both to operate simultaneously within the same BSS. When a PC is operating in a BSS, the two access methods alternate.

Carrier Sense Multiple Access (CSMA)

CSMA is a listen before talk mechanism. A wireless station that wants to transmit a frame must first sense the medium. The wireless station senses the medium using two mechanisms:

> A physical carrier sense mechanism: this mechanism is provided by the physical layer. A station can

check the physical layer and detect whether the medium is in use. The wireless medium is in use if another station is transmitting.

> A virtual carrier sense mechanism: this mechanism is provided by the MAC sublayer. Even if none of the

stations is transmitting, the medium might still be reserved by a station via the Network Allocation Vector (NAV). The NAV of a station gives a prediction of future transmissions on the medium. It is based on the duration information in the 802.11 frames. The NAV is a timer that is decremented at a uniform rate. A station will not try to transmit until the NAV has decremented to 0.

If one of the mechanisms indicates that the medium is in use, then a station must postpone its transmission. If the medium is not in use, then a station is allowed to transmit.

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Distributed Coordination Function (DCF)

The following components are important to understand the operation of the DCF:

> Interframe space: in the DCF, a wireless station that wants to transmit a data frame must wait a specific

amount of time after the station senses that the medium is not in use. This amount of time is known as the DCF Interframe Space (DIFS).

> Random backoff algorithm: several stations will sense at the same time that the medium is not in use. As

a result, there is a high probability that several stations will try to transmit simultaneously, causing a collision. To avoid this situation, DCF uses a random backoff algorithm.

> Positive acknowledgements: a station acknowledges the correct receipt of a data frame by sending an

acknowledgement frame back to the sending station. The receiving station is allowed to skip the random backoff algorithm and waits only a short interval before transmitting the acknowledgement frame. The short interval is known as the Short Interframe Space (SIFS).

The exchange of a data frame (Data) and an acknowledgement frame (ACK) between sender and receiver is illustrated in the following figure:

Interframe space (IFS)

The time interval between frames is called the Interframe Space (IFS). Different IFSs are defined to provide priority levels for accessing the wireless medium:

> Short Interframe Space (SIFS): this is the shortest of the interframe spaces. A SIFS is used to separate

transmissions belonging to a single dialogue, e.g. between a data frame and an acknowledgement frame.

> Distributed Interframe Space (DIFS): this is used by a station that wants to start a new transmission. > Extended Interframe Space (EIFS): this is used by a station that has received a frame that it could not

understand. This is needed to prevent the station from colliding with a future frame belonging to the current dialogue.

Data

ACK

Contention Window

DIFS

SIFS

DIFS

Delay Access Backoff After Delay

Receiver

Sender

Other

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Random backoff algorithm

Backoff is a well known method to avoid collisions between several stations that want to access the medium. Each station must select a random number between 0 and a given value, called the Contention Window (CW). This random number is the number of 802.11 slot times that the station must wait before it is allowed to transmit. The station always checks whether another station has accessed the medium at the beginning of the previous slot.

Each time the station decides to transmit and a collision occurs, it increases the value of the CW. The value of the CW is a moving ceiling starting at CW

min

and stopping at a maximum value known as CW

max

. The

following figure illustrates the CW

min

and CW

max

values for binary random backoff:

The random backoff algorithm must be executed in the following situations:

> When the station senses the medium before the first transmission of a frame and the medium is in use. > After each retransmission. > After a successfull transmission.

This algorithm is not used when a station decides to transmit a new frame and the medium has not been in use for more than a DIFS.

Positive acknowledgements

The correct receipt of a data frame, requires the receiving station to respond with an ACK. This technique is known as Positive Acknowledgement. If no ACK is received by the sending station, it assumes that an error has occurred. If no ACK is received:

1 The sending station updates its retry counter.

2 The sending station doubles the value of the Contention Window.

3 A retransmission of the data frame is scheduled by the sender.

Initial Attempt

First Retransmission

Second Retransmission

Third Retransmission

max

min

255 255

127

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2.2 802.11a

Technical specifications

The 802.11a standard operates in the U-NII band and applies Orthogonal Frequency Division Multiplexing (OFDM) as modulation technique. The data transfer rate can be up to 54 Mbps, but will be scaled back to 48, 36, 24, 18, 12, 9 or 6 Mbps (this is known as Adaptive Rate Selection) when the signal quality becomes an issue. 802.11a allows 64 users per access point.

There is no compatibility with either 802.11b or 802.11g.

U-NII band 802.11a channel allocation

The following table summarizes the 802.11a channels in the U-NII band. The assigned channels can differ

from country to country.

The regulatory information in this section is given for information only. This information is subject to change by the regulatory bodies. The FCC is the regulatory body in the U.S., the ETSI is the regulatory body in Europe.

Channel

Identifier

Centre

Frequency

(GHz)

Regulatory Domain

America ETSI Japan Tai wa n Singapore

34 5.170 - - Y - -

36 5.180 Y Y - - Y

38 5.190 - - Y - -

40 5.200 Y Y - - Y

42 5.210 - - Y - -

44 5.220 Y Y - - Y

46 5.230 - - Y - -

48 5.240 Y Y - - Y

52 5.260 Y Y - Y -

56 5.280 Y Y - Y -

60 5.300 Y Y - Y -

64 5.320 Y Y - Y -

100 5.500 - Y - - -

104 5.520 - Y - - -

108 5.540 - Y - - -

112 5.560 - Y - - -

116 5.580 - Y - - -

120 5.600 - Y - - -

124 5.620 - Y - - -

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Spectral mask

The spectral mask or Power Spectral Density (PSD) mask for the 802.11a standard is shown in the following figure:

Available channels

In the U-NII 1 and 2 bands, eight non-overlapping channels are available. Bear in mind that the centre frequencies of the edge channels are 30 MHz from the band edge and the spacing between the centre frequencies is 20 MHz.

In the U-NII 3 band, four non-overlapping channels are available. In contrast to the other bands, the centre frequencies are only 20 MHz from the edge band. The spacing between the other centre frequencies remains 20 MHz.

128 5.640 - Y - - -

132 5.660 - Y - - -

136 5.680 - Y - - -

140 5.700 - Y - - -

149 5.745 - - - - -

153 5.765 - - - - -

157 5.785 - - - - -

161 5.805 - - - - -

Users are responsible for ensuring that the channel set configuration is in compliance with the regulatory standards of the country they are residing.

Channel

Identifier

Centre

Frequency

(GHz)

Regulatory Domain

America ETSI Japan Tai wa n Singapore

0 dBr

-30 dBr

MHzfc+11 fc+22fc-22 fc-11

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Maximum power output

The U-NII band is split into three working domains, with different power outputs allowed:

> U-NII 1 band: maximum power output of 50 mW. > U-NII 2 band: maximum power output of 250 mW. > U-NII 3 band: maximum power output of 1 W (esp. for outdoor applications).

The following table indicates the maximum power levels and antenna gains allowed for each IEEE 802.11a regulatory domain:

Range

Typical indoor range is 10 m at 54 Mbps and 60 m at 6 Mbps. By using directional antennas, you can enlarge the range of your network.

Regulatory Domain Maximum Power Level (mW) with

6 dBi Antenna Gain

America (160 mW EIRP maximum on channels 36-48, (800 mW EIRP maximum on channels 52-64)

Japan (10 mW/MHz EIRP maximum)

Singapore (100 mW EIRP maximum)

Ta iw an (800 mW EIRP maximum)

The use of directional antennas is regulated country by country. Check the local regulations before using directional antennas.

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802.11 Standards

2.3 802.11b

Technical specifications

The 802.1b standard operates in the 2.4 GHz band and applies HR-DSSS with Complementary Code Keying (CCK), or optionally Packet Binary Convolutional Coding (PBCC) as modulation technique. The data transfer rate can be up to 11 Mbps, but will be scaled back to 5.5, 2 or 1 Mbps when the signal quality becomes an issue. 802.11b allows 32 users per access point.

802.11b is backward compatible with 802.11 legacy.

ISM band 802.11b channel allocation

The following table summarizes the 802.11b channels in the 2.4 GHz ISM band. The assigned channels can differ from country to country.

Channel

Identifier

Centre

Frequency

(GHz)

Regulatory Domain

America EMEA

1. EMEA: stands for Europe, Middle East and Africa.

Japan Israel China

12.412Y Y Y - Y

2 2.417 Y Y Y - Y

32.422Y Y Y - Y

4 2.427 Y Y Y - Y

52.432Y Y Y Y Y

6 2.437 Y Y Y Y Y

72.442Y Y Y Y Y

8 2.447 Y Y Y Y Y

92.452Y Y Y - Y

10 2.457 Y Y Y - Y

11 2.462 Y Y Y - Y

12 2.467 - Y Y - -

13 2.472 - Y Y - -

14 2.484 - - Y - -

Users are responsible for ensuring that the channel set configuration is in compliance with the regulatory standards of the country in which they are residing.

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802.11 Standards

Spectral mask

The spectral mask for the 802.11b standard is shown in the following figure:

Available channels

An important concept regarding channel assignments is that the channel actually represents the centre frequency that the transceiver within the radio and access point uses (for example 2.412 GHz for channel 1 and 2.417 GHz for channel 2). There is only 5 MHz separation between the centre frequencies, and an 802.11b signal occupies approximately 30 MHz of the frequency spectrum. The signal falls within about 15 MHz of each side of the centre frequency. As a result, an 802.11b signal overlaps with several adjacent channel frequencies. You can tell that, despite there being eleven channels allocated (for the United States), there are actually only three non-overlapping channels: 1, 6, and 11.

Maximum power output

The maximum power output for the 802.11b standard is 100 mW.

However, keep in mind that an improper combination of power level and antenna gain can result in Equivalent Isotropic Radiated Power (EIRP) above the amount allowed per regulatory domain. The following table indicates the maximum power levels and antenna gains allowed for each IEEE 802.11b regulatory domain.

0 dBr

+ 9

-30 dBr

-50 dBr

MHzf

+20 fc+30fc-30 fc-20 f

Regulatory Domain Antenna Gain (dBi) Maximum Power Level (mW)

America (4 W EIRP maximum)

2.2 100

5.2 100

6 100

8.5 100

12 100

13.5 100

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