Wireless Local Area Network (WLAN)

The IEEE 802.11 standard, which is similar in scope and functionality to IEEE 802.3 (Ethernet), is a common basis for wireless LAN operation

As with 802.3, the 802.11 standard defines a common Media Access Control (MAC) and multiple physical layers, such as 802.11a, 802.11b, and 802.11g

The initial 802.11 wireless LAN standard, ratified in 1997, specifies the use of both direct sequence spread spectrum (DSSS) and frequency hopping spread spectrum (FHSS) for delivering 1- and 2-Mbps data rates in the 2.4-GHz frequency band

Wireless Local Area Network (WLAN)

To provide higher data rates when operating in the 2.4-GHz band, the 802.11 group ratified the 802.11b physical layer in 1999, enhancing the initial DSSS physical layer to include additional 5.5- and 11-Mbps data rates

Also in 1999, the 802.11 group ratified the 802.11a standard, which offers data rates up to 54 Mbps in the 5-GHz band using orthogonal frequency division multiplexing (OFDM)

802.11g, ratified in 2004, is the most recent 802.11 physical layer, which further enhances 802.11b to include data rates up to 54 Mbps in the 2.4-GHz band using OFDM

Wireless Local Area Network (WLAN)

LAN Extension

Hub

Server Switch

Internet

Access PointHub

Wireless LAN (WLAN) as an extension to wired LAN

Work Group Bridge

WLAN Topology

Access Point

Wireless “Cell”

Channel 6

Wireless Clients

LAN Backbone

Channel 1

Access Point

Wireless “Cell”

Wireless Clients

WLAN Topology The basic service area (BSA) is the area of RF

coverage provided by an access point, also referred to as a “microcell.” To extend the BSA, or to simply add wireless devices and extend range of an existing wired system, an Access Point can be added

The Access Point attaches to the Ethernet backbone and communicates with all the wireless devices in the cell area

The AP is the master for the cell, and controls traffic flow, to and from the network. The remote devices do not communicate directly with each other; they communicate to the AP

WLAN Topology If a single cell does not provide enough

coverage, any number of cells can be added to extend the range. This is known as an extended service area (ESA)

It is recommended that the ESA cells have 10-15% overlap to allow remote users to roam without losing RF connections

Bordering cells should be set to different non-overlapping channels for best performance

Association ProcessSteps to Association:

AP sends probe response. Client evaluates AP response, selects best AP.

Client sends probe.

Client sends authenticationrequest to selected AP (B).

AP B confirms authenticationand registers client.

Client sends associationrequest to selected AP (B).

AP B confirms associationand registers client.

Access Point

B

Access Point A

Initial connection to an access point

Roaming / Re-Association

Steps to Re-association:

Roaming from Access Point A to Access Point B

Access Point

B

Access Point A

Adapter listens for beaconsfrom APs. Adapter evaluates AP-beacons, selects best AP.

Adapter sends associationrequest to selected AP (B).

AP B confirms associationand registers adapter.

AP B informs AP A of re-association with AP B.

AP A forwards buffered packets to AP B and de-registers adapter.

RF Channels Each 802.11 physical layer defines a set of RF

channels. For example, the 802.11b/g standard defines 14 RF channels in the 2.4-GHz band

In the case of 802.11b/g, these channels overlap with each other

As a result, companies installing 802.11b/g wireless LANs should set adjacent access points (where their radio cells overlap) to non-conflicting channels, such as channels 1, 6, and 11

Other 802.11 standards, such as 802.11a, define separate RF channels that do not overlap

802.11 DSSS

(14) 22 MHz wide channels 3 non-overlapping channels (1, 6,11) 11 Mbps data rate

1 2 6 113 4 5 7 8 9 12 13 1410

2.402 GHz 2.483 GHz

Channels

Channel SetupSite Survey Channel ExampleSite Survey Channel Example

Channel 1

Channel 6

Channel 11

Channel 1

Channel 6

Channel 11

Channel 11

Channel 1

Channel 6

Channel 11

RTS/CTS

Request-to-send/Clear-to-send (RTS/CTS) is an optional function of 802.11 to regulate the transmission of data on the wireless LAN

In most cases, the RTS/CTS function is helpful in counteracting collisions between hidden nodes

To gain access to the shared wireless medium, a station can only transmit if no other station is transmitting

RTS/CTS can be set in the access point or a radio card individually, or on both devices at the same time

Hidden-Node Problem in Wireless LANs

Hidden-Node Problem

The problem is that Station A might be in the middle of transmitting a frame to the access point when Station B wants to send a frame

Station B will listen to the medium to determine whether another station is already transmitting

Because Station B cannot hear Station A, Station B starts transmitting the frame

A collision then occurs at the access point, which destroys both frames

Both stations will have to retransmit their respective frames, which will likely result in another collision

RTS/CTS

The RTS/CTS function is a handshaking process that minimizes the occurrence of collisions when hidden nodes are operating on the network

In addition, protection mechanisms can use RTS/CTS to avoid collisions between 802.11b and 802.11g radio cards

If hidden nodes are not causing significant retransmissions or hidden nodes are not present, then RTS/CTS is generally not necessary

RTS/CTS

RTS/CTS works by enabling each station to explicitly request a time slot for data transmission

A will first send an RTS frame to the access point before attempting to transmit a data frame

The access point receives the RTS frame and responds with a CTS frame

Both stations receive the CTS frame. This gives clearance for Station A to transmit a data frame

The CTS frame carries a duration value that informs all other stations, including Station B, to not transmit during the specified time interval

Fragmentation

A radio card or access point can be set to optionally use fragmentation, which divides 802.11 data frames into smaller pieces (fragments) that are sent separately to the destination

Each fragment consists of a MAC layer header, frame check sequence (FCS), and a fragment number indicating its ordered position within the frame

Because the source station transmits each fragment independently, the receiving station replies with a separate acknowledgement for each fragment

Fragmentation

An 802.11 station applies fragmentation only to frames having a unicast destination address

To minimize overhead on the network, 802.11 does not fragment broadcast and multicast frames

The destination station re-assembles the fragments into the original frame using fragment numbers

After ensuring that the frame is complete, the station hands the frame up to higher layers for processing

Even though fragmentation involves more overhead, its use can result in better performance if you tune it properly

Fragmentation Fragmentation can increase the reliability of frame

transmissions when significant RF interference is Present When transmitting smaller frames, collisions are less

likely to occur Frames that do encounter errors can be retransmitted

faster because they are smaller The fragment size value can typically be set between

256 and 2048 bytes, although this value is user-configurable

Fragmentation is activated by setting a particular frame size threshold (in bytes)

If the frame that the access point is transmitting is larger than the threshold, it will trigger fragmentation

Data Rates

The default data rate setting on access points is generally auto, which allows radio cards to use any of the data rates of the given physical layer

For example, 802.11b allows data rates of 1, 2, 5.5, and 11 Mbps

The 802.11g standard extends these data rates up to 54 Mbps

The radio card usually attempts to send data frames at the highest supported rate, such as 11 Mbps for 802.11b stations and 54 Mbps for 802.11g stations

Data Rates

When set to auto, the radio card automatically rate shifts to the highest data rate that the connection can support

A lower data rate might be necessary if the radio card encounters too many retransmissions

It is possible to set the access point to a specific data rate, such as 1 Mbps, which forces the access point to send all frames at 1 Mbps

In general, a radio card is able to communicate successfully with lower data rates over longer ranges

Data Rates

The access point data rate setting does not affect the data rate of the radio cards

If the radio card is set to auto data rates (the default setting), then the radio card can still use the highest possible data rate when sending frames to the access point

To maximize the range with fewer retransmissions, set the radio cards to lower, fixed data rates

These data rate settings impact only the transmit data rate. The radio card will still receive frames at higher data rates if necessary

Transmit Power

Most access points and radio cards allow the setting of transmit power

The highest value is generally 100 mW (0.1 W), with increments of lower power available

Some devices enable settings as low as 1 mW In most cases, it is best to set all wireless LAN

devices to the highest transmit power, which is generally the default setting

To configure a wireless LAN for optimum capacity, you can set the transmit power to a lower value, which effectively reduces the size of the radio cells surrounding each access point and radio card

Transmit Power

More access points are necessary to cover an entire facility, as compared to using higher transmit power levels

Fewer wireless users will then associate with each access point

The result is better performance due to fewer users competing for access to the medium

The use of lower power settings and a greater number of access points is beneficial for supporting voice-over- Wi-Fi applications, assuming that roaming delays between the access points is kept to a minimum by careful system design

Power-Save Mode

Most radio cards employ an optional 802.11 power-save mode that users can enable

Access points do not implement power-save mode, except for the buffering functions necessary to support power saving functions of the radio cards

If power-save mode is enabled, the radio card enters sleep mode, which draws much less current than when the card is operating actively

Power-save mode can conserve batteries on mobile devices by 20 to 30 percent

Power-Save Mode

Before switching to power-save mode, the radio card notifies the access point by setting the Power Management bit in the Frame Control field of an upstream frame

The access point receives this frame and starts buffering applicable data frames

The buffering takes place until the radio card awakens and requests that the access point send the saved frames to the radio card

After entering sleep mode, the radio card keeps track of time and wakes up periodically to receive each beacon coming from the access point

Power-Save Mode

The use of power-save mode can make batteries last longer in user devices

Throughput decreases for data moving from the access point to the user device. The radio card will awaken immediately and send data going from the user device to the access point, however

As a result, upstream throughput remains unchanged in low-power mode.

SSID

The service set identifier (SSID) is an alphanumeric value set in access points and radio cards to distinguish one wireless LAN from another

The SSID provides a name for the wireless LAN. The beacon frame includes the SSID

Microsoft Windows extracts the SSID from the radio card, which obtains SSIDs from the beacon frames

Windows displays a list of available wireless networks (by SSID) to the user

If the user chooses to connect to one of the wireless LANs, Windows initiates the association process

Infrastructure Mode Configuration

An infrastructure wireless LAN, offers a means to extend a wired network

Each access point forms a radio cell, also called a basic service set (BSS)

Infrastructure Mode Configuration

With partial overlap users are able to roam throughout the facility

The co-located radio cell configuration is useful if a company needs greater capacity than what a single access point can deliver

Infrastructure Mode Operation

Infrastructure mode operation, includes Scanning Connecting with a network Data transfer Roaming

Scanning

Each radio card implements a scanning function to find access points

Scanning occurs after booting the user device, and periodically afterward to support roaming

The 802.11 standard defines two scanning methods: Passive scanning Active scanning

Passive Scanning1. The radio card automatically tunes to each RF channel,

listens for a period of time, and records information it finds regarding access points on each channel

2. By default, each access point transmits a beacon frame every 100 milliseconds on a specific RF channel, which the administrator configures

3. While tuned to a specific channel, the radio card receives these beacon frames if an access point is in range and transmitting on that channel

4. The radio card records the signal strength of the beacon frame and continues to scan other channels

5. After scanning each of the RF channels, the radio card makes a decision about the access point with which it will associate

Active Scanning1. The radio card sends probe request frames on

each RF channel2. If able to do so, any Access Point receiving the

probe request sends a probe response3. The radio card uses the signal strength and

possibly other information corresponding to the probe response frame to make a decision as to the access point to which it will associate

The probe response is similar to a beacon frame Active scanning enables the radio card to

receive information about nearby access points in a timely manner, without waiting for beacons

Connecting with a Network

After performing the authentication handshake, radio card sends an association request frame to the access point

This request contains information about the radio card, including the service set identifier (SSID) and the radio card’s supported data rates

SSID must match the one configured in the access point The access point replies to the radio card with an

association response frame containing an association identifier (AID), which is a number that represents the radio card’s association

At this point, the radio card is considered associated, and can then begin sending data frames to the access point

Data Transfer

The exchange of data in an 802.11 network is bidirectional between the radio card and access point

A radio card or access point (802.11 station) having the destination MAC address of the data frame replies with an acknowledgement (ACK) frame

This adds significant overhead to a wireless LAN Wireless LANs perform error detection and error

correction at Layer 2 If an 802.11 station sending a data frame does

not receive an ACK after a specific period of time, the station retransmits the frame

Data Transfer

These retransmissions occur up to a particular limit, which is generally three to seven times

After that, higher-layer protocols, such as Transmission Control

Protocol (TCP), must provide error recovery To allow for extended range, 802.11 includes

automatic data rate shifting For example, an 802.11 station generally lowers its

transmission data rate if a retransmission is necessary

Access points support multiple data rates to facilitate this kind of operation, where different remote stations might transmit data upstream at different rates

Roaming Periodically, each radio card performs scanning, either

active or passive, to update its access point list If the associated access point signal becomes too weak,

then the radio card will implement a re-association process The radio card sends a re-association frame to the new

access point and a disassociation frame to the old access point

802.11 does not require the authentication frame handshake when re-associating

If the old access point has buffered data frames destined to the radio card, then the old access point will forward them to the new access point for delivery to the radio card

Ad Hoc Mode Configuration

802.11 standard allows users to optionally connect directly to each other

No need for access points Peer-to-peer connectivity

Ad hoc mode is beneficial when a user needs to send a file to another user within the same room, and no other networking is practical

Both users can enable ad hoc mode on their radio cards

Ad Hoc Mode Operation

There are no access points; therefore, the radio cards must send beacons

The ad hoc mode of operation transpires as follows:1. After a user switches to ad hoc mode, the radio card

begins sending beacons if one is not received within a specific period of time

2. After receiving a beacon, each radio card waits a random period of time

3. If a beacon is not heard from another station in this time, then the station sends a beacon. The random wait period causes one of the stations to send a beacon before any other station. Over time, this distributes the job of sending beacons evenly across all 802.11 stations

Ad Hoc Mode Operation

With ad hoc networks, there is no direct connection to a wired network

A user, however, can configure an 802.11-equipped device as an ad hoc station, such as a PC, to provide a shared connection to a wired network

Thus, with specialized software or functions within the PC operating system, the PC can offer functions similar to those of an access point

All of the other ad hoc stations needing to reach devices on the wired network funnel their packets through the PC’s connection to the network

Wireless Medium Access

Before transmitting frames, a station must first gain access to the medium

The 802.11 standard defines two forms of medium access: Distributed coordination function (DCF) Point coordination function (PCF)

DCF is mandatory and based on the carrier sense multiple access with collision avoidance (CSMA/CA) protocol

802.11 stations contend for access and attempt to send frames when there is no other station transmitting

If another station is sending a frame, stations are polite and wait until the channel is free

Wireless Medium Access

The following are details on how DCF works: As a condition of accessing the medium, the MAC layer

checks the value of its network allocation vector (NAV), (which is a counter resident at each station)

The NAV must be zero before a station can attempt to send a frame

Prior to transmitting a frame, a station calculates the amount of time necessary to send the frame based on the frame’s length and data rate

The station places a value representing this time in the Duration field in the header of the frame

When other stations receive the frame, they examine this Duration field value and use it as the basis for setting their corresponding NAVs

This process reserves the medium for the sending station

Wireless Medium Access

An important aspect of the DCF is a random Back-off timer that a station uses if it detects a busy medium

If the channel is in use, the station must wait a random period of time before attempting to access the medium again

This ensures that multiple stations do not transmit at the same time

The random delay causes stations to wait different periods of time, which avoids the situation in which all the stations sense the medium at exactly the same time, find the channel idle, transmit, and collide with each other

The Back-off timer significantly reduces the number of collisions and corresponding retransmissions, especially when the number of active users increases

Wireless Medium Access

With radio-based LANs, a transmitting station cannot listen for collisions while sending data, because the station cannot have its receiver on while transmitting the frame

As a result, the receiving station needs to send an acknowledgement if it detects no errors in the received frame

If the sending station does not receive an ACK after a specified period of time, it assumes that there was a collision (or RF interference) and retransmits the frame

To support time-bounded delivery of data frames, the 802.11 standard defines the optional point coordination function (PCF), which enables the access point to grant access to an individual station to the medium by polling the station during the contention-free period

Stations cannot transmit frames unless the access point polls them first

Wireless Medium Access

The period of time for PCF-based data traffic (if enabled) occurs alternately between contention (distributed coordination function [DCF]) periods

The access point polls stations according to a polling list, and then switches to a contention period when stations use DCF

This process enables support for both synchronous (for example, video applications) and asynchronous (for example, e-mail and web-browsing applications) modes of operation

No known wireless NICs or access points on the market today, however, implement PCF

Without effective quality of service (QoS), the existing version of the 802.11 standard does not optimize the transmission of voice and video

802.11e task group refined the 802.11 MAC layer to improve QoS for better support of audio and video