Lancom 1821n Wireless User manual

110752/0310
LANCOM Systems GmbH
Adenauerstr. 20/B2
52146 Würselen
Germany
E-Mail: [email protected]
Internet www.lancom.eu
LANCOM 1811n Wireless
LANCOM 1821n Wireless
LANCOM 1811n Wireless 쮿LANCOM 1821n Wireless
쮿Handbuch
쮿Manual
...connecting your business
110752_LC-18x1n-MANUAL-cover_REV1 1110752_LC-18x1n-MANUAL-cover_REV1 1 19.03.2010 13:39:2619.03.2010 13:39:26

LANCOM 1811n Wireless
LANCOM 1821n Wireless

© 2010 LANCOM Systems GmbH, Wuerselen (Germany). All rights reserved.
While the information in this manual has been compiled with great care, it may not be deemed an assurance of product
characteristics. LANCOM Systems shall be liable only to the degree specified in the terms of sale and delivery.
The reproduction and distribution of the documentation and software supplied with this product and the use of its contents
is subject to written authorization from LANCOM Systems. We reserve the right to make any alterations that arise as the
result of technical development.
Windows®, Windows Vista™, Windows NT® and Microsoft® are registered trademarks of Microsoft, Corp.
The LANCOM Systems logo, LCOS and the name LANCOM are registered trademarks of LANCOM Systems GmbH. All other
names or descriptions used may be trademarks or registered trademarks of their owners.
Subject to change without notice. No liability for technical errors or omissions.
Products from LANCOM Systems include software developed by the OpenSSL Project for use in the OpenSSL Toolkit (http:/
/www.openssl.org/).
Products from LANCOM Systems include cryptographic software written by Eric Young ([e-mail]).
Products from LANCOM Systems include software developed by the NetBSD Foundation, Inc. and its contributors.
Products from LANCOM Systems contain the LZMA SDK developed by Igor Pavlov.
LANCOM Systems GmbH
Adenauerstr. 20/B2
52146 Wuerselen
Germany
www.lancom.eu
Wuerselen, März 2010
11
0
75
2
/
03
1
0

LANCOM 1811n Wireless – LANCOM 1821n Wireless
Preface
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Preface
Thank you for your confidence in us!
With LANCOM 1811n Wireless and LANCOM 1821n Wireless you have chosen
a powerful wireless router that possesses integrated DSL respectively ADSL
and ISDN interfaces by default as well as an integrated 4-port switch.
LANCOM wireless routers provide the comprehensive functions of an access
point, professional firewall and high-quality VPN gateway in a single, com-
pact device. They thus combine most important functions as a reliable solution
for small and mid-sized enterprises, home and branch offices.
The wireless routers operate either in the 2.4 GHz or in the 5 GHz frequency
band. The 5 GHz band is ideal as the backbone for cost-effective, high-
bandwidth transmission that is free of interference and highly secure. The
LANCOM Wireless Router series models can operate in stand-alone mode, in
managed mode or in client mode. The access point can be used in managed
mode with a LANCOM WLAN Controller without any additional software
upgrades.
The LANCOM 1811n Wirelessand LANCOM 1821n Wireless provide a
maximum wireless LAN performance of up to 300 Mbps thanks to the support
of the IEEE 802.11n standard. The 802.11n standard includes many new
mechanisms—such as the use of MIMO, 40-MHz channels, packet
aggregation and block acknowledgement—in order to increase the
bandwidth available for user applications significantly. This allows a more
than fivefold increase in speed over 802.11a/g networks with physical data
rates of up to 300 Mbps.
MIMO (multiple input multiple output) technology allows the LANCOM
Wireless Router to transfer several data streams in parallel and thus
significantly improve data throughput. MIMO uses several transmit/receive
units for both the transmitter and the receiver. The separate data streams are
identified by unique characteristics that result from the different paths that
the data take. By processing multiple data streams MIMO achieves not just
higher data throughput but also better coverage (fewer "radio black spots")
and better stability. These are the most important arguments for 802.11n for
commercial customers in particular.
Model variants
This documentation is intended for LANCOM Wireless Router users. The
following models are available:

LANCOM 1811n Wireless – LANCOM 1821n Wireless
Preface
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LANCOM 1811n Wireless
LANCOM 1821n Wireless
Model
restrictions
Passages applying only to certain models are identified either in the text itself
or by a comment in the margin.
Otherwise the documentation refers to all models collectively as the LANCOM
Wireless Router series.
Security settings
To maximize the security available from your product, we recommend that you
undertake all of the security settings (e.g. firewall, encryption, access protec-
tion) that were not already activated when you purchased the product. The
LANconfig Wizard 'Security Settings' will help you with this task. Further infor-
mation is also available in the chapter 'Security settings'.
We would additionally like to ask you to refer to our Internet site www.lan-
com.eu for the latest information about your product and technical develop-
ments, and also to download our latest software versions.
Components of the documentation
The documentation of your device consists of the following parts:
Installation Guide
User manual
Reference manual
Menu Reference Guide
You are now reading the user manual. It contains all information you need to
put your device into operation. It also contains all of the important technical
specifications.
The Reference Manual is to be found as an Acrobat document (PDF file) at
www.lancom.eu/download or on the CD supplied. It is designed as a supple-
ment to the user manual and goes into detail on topics that apply to a variety
of models. These include, for example:
The system design of the operating system LCOS
Configuration
Management
Diagnosis
Security
Routing and WAN functions

LANCOM 1811n Wireless – LANCOM 1821n Wireless
Preface
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Firewall
Quality of Service (QoS)
Virtual Private Networks (VPN)
Virtual Local Networks (VLAN)
Wireless networks (WLAN)
Voice communication in computer networks with Voice over IP (VoIP)
Backup solutions
LANCAPI
Further server services (DHCP, DNS, charge management)
The Menu Reference Guide (also available at www.lancom.eu/download or on
the CD supplied) describes all of the parameters in LCOS, the operating system
used by LANCOM products. This guide is an aid to users during the configu-
ration of devices by means of WEBconfig or the telnet console.
This documentation was created by …
... several members of our staff from a variety of departments in order to
ensure you the best possible support when using your LANCOM product.
Should you find any errors, or if you would like to suggest improvements, ple-
ase do not hesitate to send an e-mail directly to:
[email protected]
Our online services www.lancom.eu are available to you around the
clock if you have any questions on the content in this manual, or if you
require any further support. The area 'Support' will help you with
many answers to frequently asked questions (FAQs). Furthermore, the
knowledgebase offers you a large reserve of information. The latest
drivers, firmware, utilities and documentation are constantly available
for download.
In addition, LANCOM Support is available. For telephone numbers
and contact addresses for LANCOM Support, please refer to the enc-
losed leaflet or the LANCOM Systems Web site.

LANCOM 1811n Wireless – LANCOM 1821n Wireless
Preface
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Information symbols
Very important instructions. Failure to observe these may result in damage.
Important instruction that should be observed.
Additional information that may be helpful but is not essential.

LANCOM 1811n Wireless – LANCOM 1821n Wireless
Contents
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Contents
1 Introduction 10
1.1 What is a wireless LAN? 10
1.1.1 Modes of operation of wireless LANs and access points
11
1.2 Wireless LANs in accordance with 802.11n 11
1.2.1 Advantages of 802.11n 12
1.2.2 Compatibility with other standards 13
1.2.3 The physical layer 13
1.2.4 The MAC layer 19
1.3 What can your LANCOM Wireless Router do? 21
2 Installation 25
2.1 Package contents 25
2.2 System requirements 26
2.2.1 Configuring the LANCOM devices 26
2.2.2 Operating access points in managed mode 26
2.3 Status displays and interfaces 26
2.3.1 Device connectors 33
2.4 Hardware installation 35
2.5 Software installation 37
2.5.1 Starting the software setup 37
2.5.2 Which software should I install? 38
3 Basic configuration 39
3.1 Details you will need 39
3.1.1 TCP/IP settings 39
3.1.2 Configuration protection 41
3.1.3 Settings for the wireless LAN 42
3.1.4 Settings for the ISDN Connection 43
3.1.5 Charge protection 43
3.2 Instructions for LANconfig 43
3.3 Instructions for WEBconfig 45
3.4 TCP/IP settings for PC workstations 49

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4 Setting up Internet access 50
4.1 The Internet Connection Wizard 52
4.1.1 Instructions for LANconfig 52
4.1.2 Instructions for WEBconfig 53
5 Connecting two networks 54
5.1 Which details are necessary? 55
5.1.1 General information 55
5.1.2 Settings for the TCP/IP router 57
5.1.3 Settings for NetBIOS routing 58
5.2 Instructions for LANconfig 59
5.3 1-Click-VPN for networks (site-to-site) 60
5.4 Instructions for WEBconfig 61
6 Providing dial-in access 62
6.1 Which details are necessary? 62
6.1.1 General information 63
6.1.2 Settings for TCP/IP 64
6.1.3 Settings for NetBIOS routing 65
6.2 Settings on the dial-in computer 65
6.2.1 Dialing-in via VPN 65
6.2.2 Dialing-in via ISDN 65
6.3 Instructions for LANconfig 66
6.4 1-Click-VPN for LANCOM Advanced VPN Client 66
6.5 Instructions for WEBconfig 68
7 Sending faxes with LANCAPI 69
7.1 Installation of the LANCOM CAPI Faxmodem 70
7.2 Installation of the MS Windows fax service 71
7.3 Sending a fax 72
7.3.1 Send a fax with any given office application 72
7.3.2 Send a fax with the MS Windows fax service 72

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8 Security settings 74
8.1 Security in the wireless LAN 74
8.1.1 Encrypted data transfer 74
8.1.2 802.1x / EAP 77
8.1.3 LANCOM Enhanced Passphrase Security 77
8.1.4 Access control by MAC address 78
8.1.5 IPSec over WLAN 78
8.2 Tips for the proper treatment of keys and passphrases 78
8.3 Security settings Wizard 79
8.3.1 LANconfig Wizard 79
8.3.2 WEBconfig Wizard 80
8.4 The security checklist 80
9 Options and accessories 85
9.1 Optional AirLancer Extender antennas 85
9.1.1 Antenna diversity 86
9.1.2 Polarization diversity 86
9.1.3 MIMO 86
9.1.4 Installing the AirLancer Extender antennas 86
9.2 LANCOM Public Spot Option 88
10 Advice & assistance 90
10.1 No WAN connection can be established 90
10.2 Slow DSL transmission 90
10.3 Unwanted connections under Windows XP 91
10.4 Cable testing 91
11 Appendix 93
11.1 Performance data and specifications 93
11.2 Connector wiring 95
11.2.1 Ethernet interface 10/100Base-TX 95
11.2.2 ADSL interface 95
11.2.3 DSL interface 96
11.2.4 ISDN-S0interface 96
11.2.5 Configuration interface (outband) 97
11.3 CE declaration of conformity 97
12 Index 98

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1Introduction
The models of the LANCOM Router series offer each a DSL or ADSL connector
and also an ISDN connector. The ISDN line can be used as back-up for the DSL
connection, for remote management of the router or as basis for the office
communication via LANCAPI.
In addition to their function as routers between LAN and the Internet, all
models of the LANCOM Router series operate also as base stations for wireless
networks. With the base station you link wireless PCs and notebooks to a net-
work, connect these devices to the existing wired LAN and enable also the
wireless devices to access the Internet.
1.1 What is a wireless LAN?
The following sections describe the functionality of wireless networks
in general. You can see from the table 'What your LANCOM can do'
further below which functions your device supports. Please refer to
the reference manual for further information on this topic.
A wireless LAN connects individual end-user devices (PCs and mobile compu-
ters) to form a local network (also called – Local Area Network). In contrast
to a traditional LAN, communication takes place over a wireless connection
and not over network cables. For this reason it is called a Wireless Local Area
Network (WLAN).
A wireless LAN provides the same functionality as a cable-based network:
Access to files, servers, printers etc. as well as the integration of individual
work stations into a corporate mail system or access to the Internet.
There are obvious advantages to wireless LANs: Notebooks and PCs can be
installed where they are needed—problems with missing connections or
structural changes are a thing of the past with wireless networks.
WLAN Router
Notebook
SERVER
NTBA
ADSL
TAE
INTERNET
ISDN Phone
ISDN
PC

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Chapter 1: Introduction
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Apart from that, wireless LANs can also be used for connections over longer
distances. Expensive leased lines and the associated construction measures
can be saved.
LANCOM Wireless Routers and LANCOM Access Points can be opera-
ted either as self-sufficient Access Points with their own configuration
(WLAN modules in "Access Point mode“) or as components in a WLAN
infrastructure, which is controlled from a central WLAN-Controller
("managed mode").
Split management can be used to separate the WLAN configuration
from the rest of the router configuration. This allows router settings
and VPN settings to be adjusted locally, for example in a branch office
or home office installation, and the WLAN configuration is regulated
by a LANCOM WLAN Controller at the main office.
Please observe the corresponding notices to this in this documenta-
tion or in the LCOS reference manual.
1.1.1 Modes of operation of wireless LANs and access points
Wireless LAN technology and access points in wireless LANs are used in the
following modes of operation:
Simple, direct connection between terminal devices with an access point
(ad-hoc mode)
Extensive wireless LANs, possibly connected to a LAN, with one or more
access points (infrastructure network)
Transmission of VPN-encrypted connections with VPN pass through
Establishing access to the Internet
Connecting two LANs over a wireless link (point-to-point mode)
Connecting devices with an Ethernet interface via an access point (client
mode)
Extending an existing Ethernet network with a wireless LAN (bridge mode)
Central administration using a LANCOM WLAN Controller
1.2 Wireless LANs in accordance with 802.11n
The new wireless LAN standard IEEE 802.11n—ratified as „WLAN Enhance-
ments for Higher Throughput“ in september 2009—features a number of

LANCOM 1811n Wireless – LANCOM 1821n Wireless
Chapter 1: Introduction
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technical developments that promise up to six-times the performance in wire-
less LANs.
Some of the improvements refer to the physical layer (PHY), which describes
the transmission of individual bits over the physical medium—in this case the
air represents the physical medium. Other additions are concerned with the
MAC (medium access control) that among other things governs access to the
transmission medium. The two areas are treated separately below.
You can find additional information on this subject in the LCOS refe-
rence manual or in the technical papers relating to this topic.
1.2.1 Advantages of 802.11n
The new technology includes the following advantages:
Higher effective data throughput
The 802.11n standard includes a number of new mechanisms to signifi-
cantly increase available bandwidth. Current wireless LAN standards
based on 802.11a/g enable physical data rates (gross data rates) of up to
54 Mbps, which turn out to be approx. 22 Mbps net. Networks based on
802.11n currently achieve a gross data throughput of up to 300 Mbps
(in reality approx. 120 to 130 Mbps net) – theoretically the standard defi-
nes up to 600 Mbps with four data streams. For the first time, maximum
speeds exceed the 100 Mbps of cable-based Fast Ethernet networks,
which are currently standard in most workplaces.
Improved and more reliable wireless coverage
The new 802.11n technologies do not just increase date throughput but
bring about improvements in the range and reduce the wireless dead
spots in existing a/b/g installations.
This results in better signal coverage and improved stability for signifi-
cantly better utilization of wireless networks, in particular for users in pro-
fessional environments.
Greater range
Data throughput generally decreases when the distance between receiver
and transmitter increases. The overall improved data throughput allows
wireless LANs based on 802.11n to achieve greater ranges, as a signifi-
cantly stronger wireless signal is received by the Access Point over a given
distance than in 802.11a/b/g networks.

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1.2.2 Compatibility with other standards
The 802.11n standard is backwardly compatible to previous standards
(IEEE 802.11a/b/g). However, some of the advantages of the new technology
are only available when, in addition to the access points, the wireless LAN cli-
ents are also compatible with 802.11n.
In order to allow the co-existence of wireless LAN clients based on 802.11a/
b/g (called "legacy clients") 802.11n access points offer special mechanisms
for mixed operation, where performance increases over 802.11a/b/g are not
as high. Only in all-802.11n environments is the "greenfield mode" used,
which can exploit all the advantages of the new technology. In greenfield
mode both access points and wireless LAN clients support the 802.11n stan-
dard, and access points reject connections with legacy clients.
1.2.3 The physical layer
The physical layers describes how data must be transformed in order for them
to be transmitted as individual bits over the physical medium. In this process
the following steps are performed in a wireless LAN device:
Modulation of digital data into analog carrier signals
Modulation of the carrier signal into a radio signal in the selected fre-
quency band, which for a wireless LAN is either 2.4 or 5 GHz.
The second modulation step in IEEE 802.11n occurs in the same way as in
conventional wireless LAN standards and is therefore not covered here.
However, there are a number of changes in the way digital data are modula-
ted into analog signals in 802.11n.
Improved OFDM modulation (MIMO-OFDM)
Like 802.11a/g, 802.11n uses the OFDM scheme (Orthogonal Frequency Divi-
sion Multiplex) as its method of modulation. This modulates the data signal
not on just one carrier signal but in parallel over several. The data throughput
that can be achieved with OFDM modulation depends on the following para-
meters, among other things:
Number of carrier signals: Whereas 802.11a/g uses 48 carrier signals,
802.11n can use a maximum of 52.

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Payload data rate: Airborne data transmission is fundamentally unreli-
able. Even small glitches in the WLAN system can result in errors in data
transmission. Check sums are used to compensate for these errors, but
these take up a part of the available bandwidth. The payload data rate
indicates the ratio between theoretically available bandwidth and actual
payload. 802.11a/g can operate at payload rates of 1/2 or 3/4 while
802.11n can use up to 5/6 of the theoretically available bandwidth for
payload data.
These two features increase the maximum useable bandwidth of 54 Mbps for
802.11a/g to 65 Mbps for 802.11n. This increase is not exactly spectacular,
but it can be further improved by using the following features:
MIMO technology
MIMO (multiple input multiple output) is the most important new technology
contained in 802.11n. MIMO uses several transmitters and several receivers
to transmit up to four parallel data streams on the same transmission channel
(currently only two parallel data streams have been implemented). The result
is an increase in data throughput and improved wireless coverage.
20 MHz 20 MHz
IEEE 802.11a/b/g:
48 carrier signals
IEEE 802.11n:
52 carrier signals
Gross bandwidth
Payload rate for 802.11a/b/g: 1/2
Checksum Payload data
Payload rate for 802.11a/b/g: 3/4
Maximum payload rate for 802.11n:5/6

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Chapter 1: Introduction
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For example, the Access Point splits the data into two groups which are then
sent simultaneously via separate antennas to the WLAN client. Data through-
put can therefore be doubled using two transmitting and receiving antennas.
But how can several signals be transmitted on a single channel simultane-
ously? This was considered impossible with previous WLAN applications.
Let us consider how data is transmitted in "normal" wireless LAN networks:
Depending on antenna type, an Access Point's antenna broadcasts data in
several directions simultaneously. These electromagnetic waves are reflected
by the surrounding surfaces causing a broadcast signal to reach the WLAN cli-
ent's antenna over many different paths; this is also referred to as "multipath
propagation". Each of these paths has a different length meaning that indivi-
dual signals reach the client with a different time delay.
These time-delayed signals interfere with each other at the WLAN client and
significantly weaken the original signal. For this reason, conventional WLAN
networks should always have a direct line of sight (LOS) between transmitter
and receiver in order to reduce the influence of reflections.
MIMO technology transforms this weakness in WLAN transmission into a
strength that allows an enormous increase in data throughput. As mentioned
MIMO AP 802.11n MIMO Client 802.11n
Access Point WLAN-Client

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above, it is virtually impossible to transmit different signals on the same chan-
nel simultaneously as the receiver cannot distinguish between them. MIMO
uses the reflection of electromagnetic waves and the associated spatial aspect
to obtain a third criterion for identifying the signals.
A signal sent by transmitter A and received by receiver 1 follows a different
path than a signal from transmitter B to receiver 2. Due to the different reflec-
tions and changes in polarization that both signals experience along their
paths, each of these paths takes on its own characteristics. When data trans-
mission starts, a training phases records the characteristics of the path by
transmitting standardized data. Subsequently, the data received here is used
to calculate which data stream the signals belong to. The receiver decides for
itself which of the incoming signals is to be processed, thus avoiding loss from
interference.
MIMO thus allows the simultaneous transmission of several signals over one
shared medium, such as the air. Individual transmitters and receivers must be
positioned a minimum distance apart from one another, although this is just
a few centimeters. This separation results in differing reflections and signal
paths that can be used to separate the signals.
Generally speaking, MIMO can provide up to four parallel data streams, which
are also called "spatial streams". However, the current generation of chips can
only implement two parallel data streams as the separation of data streams
based on characteristic path information demands high levels of computing
power, which consumes both time and electricity. The latter tends to be unde-
sirable particularly for WLAN systems, where attempts are often made to
achieve independence from power sockets at the WLAN client or when using
PoE as the electricity supply for the Access Point.
MIMO AP 802.11n MIMO Client 802.11n
A
B
1
2

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Even if the aim of four spatial streams has not yet been achieved, the use of
two separate data connections results in a doubling of data throughput,
which represents a true technological leap in the area of WLAN systems. Com-
bined with the improvements in OFDM modulation, the data throughput that
can be attained increases to 130 Mbps.
The short description "transmitter x receiver" expresses the actual number of
transmitting and receiving antennas. 2x2 MIMO describes two transmitting
and two receiving antennas.
MIMO in outdoor use
Outdoor 802.11n applications cannot use natural reflections since signal
transmission usually takes place over the direct path between directional
antennas. In order to transmit two data streams in parallel, special antennas
are employed that use polarization channels turned through 90° to each
other. These so-called "dual-slant" antennas are really two antennas in one
housing. Since a third signal does not offer additional reliability, outdoor
applications generally use as many antennas (or polarization channels) as
there are data streams for transmission.
40 MHz channels
As the above explanation of OFDM modulation states, data throughput rises
with an increasing number of carrier signals because this allows several sig-
nals to be transmitted simultaneously. If a channel with a bandwidth of
20 MHz supports no more than 48 (802.11a/g) or 52 (802.11n) carrier signals,
the obvious choice would be to use a second channel with additional carrier
signals.
This method was used in the past by a number of manufacturers (including
LANCOM Systems) and was referred to as "turbo mode", allowing data rates
of up to 108 Mbps. Turbo mode does not form part of the official IEEE stan-
MIMO AP 802.11n POLARIZATION
DIVERSITY
Building
MIMO AP 802.11n
POLARIZATION
DIVERSITY
Building

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dard but is frequently employed on point-to-point connections, for example,
because compatibility to other manufacturers tends to play a secondary role.
However, the success of the underlying technology has lead to its incorpora-
tion into 802.11n. IEEE 802.11n uses the second transmission channel in a
way that maintains compatibility to IEEE 802.11a/g devices. 802.11n trans-
mits data over two contiguous channels. One of these assumes the task of a
control channel that, among other things, handles the administration of data
transmission. Concentrating these basic tasks into the control channel means
that devices supporting a transmission at 20 MHz only can also be connected.
The second channel is an extension that only comes comes into effect if the
remote client also supports data transmission at 40 MHz. The use of the
second channel remains optional throughout, with transmitter and receiver
deciding dynamically whether one or two channels should be employed.
As the implementation of 40 MHz with separate control and extension chan-
nels is more efficient in the 802.11n standard than in the conventional turbo
mode, more than double the amount of carrier signals can be obtained (108
in total). The maximum data throughput when using improved OFDM modu-
lation and two parallel data streams thus rises to 270 Mbps.
Short guard interval
The final improvement of the 802.11n standard is the improvement in the
chronological sequence of data transmission. A signal that is to be transmit-
ted in a WLAN system is not broadcast at a distinct point in time but is "held
up" for a certain, constant transmission period. In order to prevent interfe-
rence at the receiving end, a short break is made following the transmission
period before the transmission of the next signal commences. The entire dura-
tion of transmission period and break are referred to in WLAN terminology as
"symbol length" and the break itself is known as the "guard interval".
IEEE 802.11a/g uses a symbol length of 4 μs: the information transmitted on
the carrier signal changes following transmission of 3.2 μs and a break of
20 MHz 20 MHz
Control channel Extension channel

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0.8 μs. 802.11n reduces the break between transmissions to the so-called
"short guard interval" of only 0.4 μs.
Transmitting data in shorter intervals thus increases the maximum data
throughput when using improved OFDM modulation, two parallel data
streams and transmission at 40 MHz to 300 Mbps.
1.2.4 The MAC layer
Frame aggregation
The improvements in the physical layer brought about by the new 802.11n ini-
tially describe only the theoretical data throughput of the physical medium.
However, the share of this theoretical bandwidth that is actually available for
payload data is limited by two factors:
in addition to the actual payload data, each data packet in a wireless LAN
system contains additional information such as a preamble and MAC
address information.
Time is lost to the management events that occur when the transmission
medium is actually accessed. Thus the transmitter must negotiate access
authorization with the other receivers before transmitting each data
packet (frame); further delays are caused by data packet collisions and
other events.
This loss, referred to as "overhead", can be reduced by combining several data
packets together to form one large frame and transmitting them together. In
this process, information such as the preamble are only transmitted once for
all the combined data packets and delays due to negotiating access to the
transmission medium only occur at longer intervals.
The use of this method, known as frame aggregation, is subject to certain
restrictions:
3,2 μs 0,8 μs
0,4 μs
OFDM Symbol
3,2 μs
Payload data