Subject: [NEXTEL-1] IEEE 802.11b Wireless Networking Overview

From: Marcel <[email-masked]>
Date: Fri, 07 Feb 2003 04:58:20 -0500

IEEE 802.11b Wireless Networking Overview

By The Cable Guy

Wireless Networking Components
IEEE 802.11b Operating Modes
IEEE 802.11b Operation Basics
IEEE 802.11 Security
Windows XP Support for IEEE 802.11b
For More Information

In September of 1999, the Institute of Electrical and Electronic
Engineers (IEEE) ratified the specification for IEEE 802.11b,
also known as Wi-Fi. IEEE 802.11b defines the physical layer
and media access control (MAC) sublayer for communications
across a shared, wireless local area network (WLAN).

At the physical layer, IEEE 802.11b operates at the radio
frequency of 2.45 gigahertz (GHz) with a maximum bit rate of
11 Mbps. It uses the direct sequence spread spectrum (DSSS)
transmission technique. At the MAC sublayer of the Data Link
layer, 802.11b uses the carrier sense multiple access with
collision avoidance (CSMA/CA) media access control (MAC)
protocol.

A wireless station with a frame to transmit first listens on the
wireless medium to determine if another station is currently
transmitting (this is the carrier sense portion of CSMA/CA). If
the medium is being used, the wireless station calculates a
random backoff delay. Only after the random backoff delay
elapses can the wireless station again listen for a transmitting
station. By instituting a random backoff delay, multiple stations
that are waiting to transmit do not end up trying to transmit at
the same time (this is the collision avoidance portion of
CSMA/CA).

Collisions can occur and, unlike with Ethernet, they might not
be detected by the transmitting nodes. Therefore, 802.11b uses
a Request to Send (RTS)/Clear to Send (CTS) protocol with
an Acknowledgment (ACK) signal to ensure that a frame is
successfully transmitted and received.

Wireless Networking Components

IEEE 802.11b wireless networking consists of the following
components:

Stations

A station (STA) is a network node that is equipped with
a wireless network device. A personal computer with a
wireless network adapter is known as a wireless client.
Wireless clients can communicate directly with each
other or through a wireless access point (AP). Wireless
clients are mobile.
Wireless APs

A wireless AP is a wireless network node that acts as a
bridge between STAs and a wired network. A wireless
AP contains:
At least one interface that connects the wireless
AP to an existing wired network (such as an
Ethernet backbone).
A wireless network device with which it creates
wireless connections with STAs.
IEEE 802.1D bridging software, so that it can act
as a transparent bridge between the wireless and
wired networks.

The wireless AP is similar to a cellular phone network's
base station. Wireless clients communicate with both
the wired network and other wireless clients through the
wireless AP. Wireless APs are not mobile and act as
peripheral bridge devices that extend a wired network.
Ports

A port is a channel of a device that can support a single
point-to-point connection. For IEEE 802.11b, a port is an
association, a logical entity over which a single wireless
connection is made. A typical wireless client with a
single wireless network adapter has one port and can
support only one wireless connection. A typical wireless
AP has multiple ports and can simultaneously support
multiple wireless connections. The logical connection
between a port on the wireless client and the port on a
wireless AP is a point-to-point bridged LAN
segment—similar to an Ethernet-based network client
that is connected to an Ethernet switch.

IEEE 802.11b Operating Modes

IEEE 802.11 defines two operating modes: Ad hoc mode and
Infrastructure mode.

In ad hoc mode, also known as peer-to-peer mode, wireless
clients communicate directly with each other (without the use
of a wireless AP). Two or more wireless clients who
communicate using ad hoc mode form an Independent Basic
Service Set (IBSS). Ad hoc mode is used to connect wireless
clients when a wireless AP is not present.

In infrastructure mode, there is at least one wireless AP and
one wireless client. The wireless client uses the wireless AP to
access the resources of a wired network. The wired network
can be an organization intranet or the Internet, depending on
the placement of the wireless AP.

A single wireless AP that supports one or multiple wireless
clients is known as a Basic Service Set (BSS). A set of two or
more wireless APs that are connected to the same wired
network is known as an Extended Service Set (ESS). An ESS
is a single logical network segment (also known as a subnet),
and is identified by its Service Set Identifier (SSID). If the
available physical areas of the wireless APs in an ESS overlap,
then a wireless client can roam, or move from one location
(with a wireless AP) to another (with a different wireless AP)
while maintaining Network layer connectivity.

IEEE 802.11b Operation Basics

When a wireless adapter is turned on, it begins to scan across
the wireless frequencies for wireless APs and other wireless
clients in ad hoc mode. Assuming that the wireless client is
configured to operate in infrastructure mode, the wireless
adapter chooses a wireless AP with which to connect. This
selection is made automatically by using an SSID and signal
strength and frame error rate information. Next, the wireless
adapter switches to the assigned channel of the selected
wireless AP and negotiates the use of a port. This is known as
establishing an association.

If the signal strength of the wireless AP is too low, the error
rate too high, or if instructed by the operating system (in the
case of Windows XP), the wireless adapter scans for other
wireless APs to determine whether a different wireless AP
can provide a stronger signal or lower error rate. If such a
wireless AP is located, the wireless adapter switches to the
channel of that wireless AP and negotiates the use of a port.
This is known as reassociation.

Reassociation with a different wireless AP can occur for
several reasons. The signal can weaken as either the wireless
adapter moves away from the wireless AP or the wireless AP
becomes congested with too much traffic or interference. By
switching to another wireless AP, the wireless adapter can
distribute the load to other wireless APs, increasing the
performance for other wireless clients. You can achieve
contiguous coverage over large areas by placing your wireless
APs so that their signal areas overlap slightly. As a wireless
client roams across different signal areas, it can associate and
reassociate from one wireless AP to another, maintaining a
continuous logical connection to the wired network.

IEEE 802.11 Security

The IEEE 802.11 standard defines the following mechanisms
for wireless security:

Authentication through the open system and shared key
authentication types
Data confidentiality through Wired Equivalent Privacy
(WEP)

Open system authentication does not provide authentication,
only identification using the wireless adapter's MAC address.
Open system authentication is used when no authentication is
required. Some wireless APs allow the configuration of the
MAC addresses of allowed wireless clients. However, this is
not secure because the MAC address of a wireless client can
be spoofed.

Shared key authentication verifies that an authenticating
wireless client has knowledge of a shared secret. This is
similar to preshared key authentication in Internet Protocol
security (IPsec). The 802.11 standard currently assumes that
the shared key is delivered to participating STAs through a
secure channel that is independent of IEEE 802.11. In practice,
this secret is manually configured for both the wireless AP and
client. Because the shared key authentication secret must be
distributed manually, this method of authentication does not
scale to a large infrastructure mode network (for example,
corporate campuses and public places, such as malls and
airports). Additionally, shared key authentication is not secure
and is not recommended for use.

Inherent in the nature of wireless networks, securing physical
access to the network is difficult. Because a physical port is
not required, anyone within range of a wireless AP can send
and receive frames, as well as listen for other frames being
sent. Without WEP, eavesdropping and remote packet sniffing
would be very easy. WEP is defined by the IEEE 802.11
standard and is intended to provide the level of data
confidentiality that is equivalent to a wired network.

WEP provides data confidentiality services by encrypting the
data sent between wireless nodes. WEP encryption uses the
RC4 symmetrical stream cipher with either a 40-bit or 104-bit
encryption key. WEP provides data integrity from random
errors by including an integrity check value (ICV) in the
encrypted portion of the wireless frame.

However, one significant problem remains with WEP. The
determination and distribution of WEP keys are not defined
and must be distributed through a secure channel that is
independent of 802.11. In practice, this is a text string that
must be manually configured (using a keyboard) for both the
wireless AP and wireless clients. Obviously, this key
distribution system does not scale well to an enterprise
organization.

Additionally, there is no defined mechanism to change the
WEP key—either per authentication or at periodic intervals
over the duration of an authenticated connection. All wireless
APs and clients use the same manually configured WEP key
for multiple connections and authentications. With multiple
wireless clients sending large amounts of data, it is possible for
a malicious user to remotely capture large amounts of WEP
cipher text and use cryptanalysis methods to determine the
WEP key.

The lack of WEP key management, to both automatically
determine a WEP key and change it frequently, is a principal
limitation of 802.11 security, especially with a large number of
wireless clients in infrastructure mode. The lack of automated
authentication and key determination services also effects
operation in ad hoc mode, where users might want to engage in
peer-to-peer collaborative communication (for example, in
areas such as conference rooms).

The combination of a lack of both adequate authentication
methods and key management for encryption of wireless data
has led the IEEE to adopt the IEEE 802.1X Port-Based
Network Access Control standard for wireless connections.
The details of IEEE 802.1X will be described in a future
TechNet Cable Guy article.

Windows XP Support for IEEE 802.11b

Windows XP provides built-in support for IEEE 802.11b. An
installed wireless adapter scans for available wireless networks
and passes that information to Windows XP. Next, the
Windows XP Wireless Zero Configuration service configures
wireless connectivity–without the need for user configuration
or intervention. If no wireless APs are found, Windows XP
configures the wireless adapter to use ad hoc mode. The
Windows XP Wireless Zero Configuration service is integrated
with support for IEEE 802.1X authentication.

While the Windows XP Wireless Zero Configuration service
provides automatic configuration for most wireless situations,
you can manually configure IEEE 802.11b settings on the
Wireless Networks tab, which is available in the properties
of a connection that corresponds to an installed wireless
adapter in the Network Connections folder. The Wireless
Networks tab only appears for wireless adapters.

On the Wireless Networks tab, you can view and configure
the following:

Whether you want Windows XP to automatically
configure your wireless settings through the Wireless
Zero Configuration service. You can disable this service
if you have third-party wireless configuration software.
The list of wireless networks that are within association
range of your computer.
The ordered list of preferred wireless networks with
which the wireless client computer attempts to associate
and authenticate.
Whether the wireless client computer operates in ad hoc
mode, infrastructure mode, or both. By default, a
Windows XP wireless client operates in both modes, but
infrastructure mode and connections to wireless APs is
preferred.
For each wireless network that is found or manually
added, you can configure the wireless network name,
whether WEP is enabled, the type of authentication,
whether the wireless network is in ad hoc or
infrastructure mode, and whether the WEP key is
automatically determined or manually configured.

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