Cisco Presentation Guide - Main Page

Cisco Presentation Guide - Main Page

IEEE 802.11 Networks




Lecture 10

What are IEEE 802.11 Networks ?

Wireless networks operating in a license-free

Instrumental-Scientific-Medicine (ISM) band

2.4 GHz

5 GHz

In European environment, ETSI requires dynamic channel selection and automatic transmit power adaptation

Optimized for indoor usage

Outdoor use is also common in CZ connection of multiple separated buildings, P2P connections

Defines link and physical layer

Multiple options of physical layer implementation


Spread-Spectrum Transmission

Standardization bodies require to spread the transmitted power over a wide frequency range

Max transmitted power is 100 mW

The wider frequency range than is really necessary is used

The information is transmitted on multiple frequencies (redundancy)

Signal may be reconstructed even if the noise is present

Coexistence of multiple (uncoordinated) systems


Channels of the 2.4 GHz ISM Band

Europe: channels 1-13, USA: channels 1-11

Adjacent-channel spacing 5 MHz, channel bandwidth 22 MHz

Every channel overlaps with 4 neighboring channels

To completely avoid overlaps, we need to choose channels that provide 25 MHz spacing

Only 3 channels may be utilized simultaneously without interference


Channels of the 5 GHz ISM Band

Non-overlapping channels

Different for various countries/regions

Europe (ETSI) – 11 non-overlapping channels


Structure of IEEE 802.11



Selected IEEE 802.11 Recommendations

802.11 (the core standard, current: 802.11-2012)

Original 802.11-1997 standard uses only 1 & 2 Mb/s

Former 802.11a (802.11-2012, clause 18)

5 GHz (USA), 6,9,12,18,24,36,48,54 Mb/s

Former 802.11b (802.11-2012, clause 17)

2.4 GHz (USA, Europe)

Extends 802.11 by DSSS 1,2,5.5 and 11 Mb/s

Former 802.11g (802.11-2012, clause 19)

2.4 GHz (USA, Europe)

Modulations similar to 802.11a (but 2.4GHz band)

Former 802.11h (integrated into 802.11-2012)

5 GHz (Europe)

Similar to 802.11a, but requires dynamic power adjustment and automatic channel selection


You may download 2012 standard at:

Related 802.11-2012 Recommendations

802.11i – security+QoS (integrated in 802.11-2012)

QoS moved here from 802.11e

802.11f – fast roaming between APs.

Superseded by 802.11k – Radio Resource

Management & 802.11r – Fast roaming (integrated in 802.11-2012)

Avoids the need of reoccurring authentication during handover between APs

Useful for latency-critical mobile application

(IP telephony, multimedia transfers, …)

802.11n – 2.4 & 5 GHz (802.11-2012, clause 20)

MIMO, wider bandwidth (20/40 MHz), speeds up to

144.4 Mb/s for 20 MHz & 300 Mb/s for 40 MHz.


Other Related Recommendations

802.11ac – 5 GHz

Multi-station WLAN throughput >= 1 Gb/s, single station 500 Mb/s, 80/160 MHz range, up to 8 streams, up to QAM-256.

802.11ad – 60 GHz, WiGig theoretical maximum throughput up to 7 Gbit/s.

will be used in a new wireless USB specification.



„Wireless Fidelity“

Logo provided by a consortium that focuses on

802.11 products interoperability testing


Components of IEEE 802.11

Wireless Network

Access Point

Wireless clients (stations)

Distribution System

Wireless medium (frequency band)


WiFi Usage Alternatives


Used for temporary wireless interconnection of 2 or more nearby computers

Needs to be configured manually


Permanent wireless network infrastructure using access point (AP)

AP may act as a bridge to the wired network

Often also provides other functions, like DHCP server, NAT and other „PnP“ functions


Architectures of WiFi Networks

Independent Basic Service Set (IBSS)

A group of directly communicating stations

No frame relaying

Not interconnected with a wired network

Basic Service Set (BSS)

Utilizes access point, stations communicate using AP

Extended Service Set (ESS)

Interconnects multiple BSS using the distribution system

The distribution system is out of scope of the IEEE

802.11 standard


Various (proprietary) solutions of Wireless Distribution

Systems (WDS)







Service Set = logical group of stations

BSS=Basic Service Set

SSID = Service Set Identifier

BSS ID = BSS Identifier = AP MAC address


Physical Layer


Relations between IEEE 802.11



Sublayers of IEEE 802.11 Physical


Physical Layer Convergence Procedure


appends its own header to L2 frames specifies the particular modulation method

Detects the state of the channel for MAC sublayer of the link layer busy/free

Physical Medium Dependent (PMD)

Takes care of the physical transmission


Physical Layer Convergence

Procedure Sublayer

Different for various modulation methods

Long (128b) and optional short (56b) preamble

(for real-time transfers)

PLCP frame format (differs for various PMD):

Preamble: synchronization sequence + Start

Frame Delimiter (different for individual PMDs)

PLCP header

Data Rate (8b)

MAC PDU length: 16b

Header CRC (16b)

For backward compatibility, PLCP frames are always transmitted using 1Mb/s BPSK

=> increases overhead


Functional Blocks of the Physical


Scrambling – „randomizes“ transmitted data

(random data are supposed by transmission facilities)

XOR with pseudo-random sequence at transmitter and receiver side

Encoding (Flyby)

Interleaving – protects against impulse noise

Mapping of groups of bits into modulation symbols


PMD Sublayer Options


Frequency Hopping Spread Spectrum

Developed for military purposes (concealment of tr.)

Frequency band is divided into 75 1-MHz channels

Transmitter alters channels (max 400 ms per channel)

Every station has a unique channel hopping pattern

Hopping patterns assigned by AP

3 alternative hopping patterns are defined in Europe

Allows coexistence of multiple independent systems in the same frequency band

According to 802.11, 26 systems may theoretically coexist in 2.4 GHz frequency band

Simpler manufacturing

Does not use sophisticated calculations

Less power consumption

Problem: as the hopping patterns are fixed, even the noised channels are used for transmission

802.11 defines 12Mbps and 2 Mb/s FHSS scheme

Direct Sequence Spread Spectrum

Defined in 802.11 for 1Mb/s and 2 Mb/s

The transmitted information is encoded so that it is spread into 22 MHz band

Three 22MHz bands are available

Spreading is accomplished using s chip sequence

Binary 1= chip sequence is transmitted, binary 0 bit=negation of chip sequence is transmitted

Individual stations use unique chip sequences

Mutually orthogonal, expressed in +1, -1 values

Knowing the chip sequence of a particular transmitter, the receiver may reconstruct just that transmitter's data from the signal that is a mixture of multiple transmissions inner product of the received signal with the transmitter chip sequence

A 1MHz bit stream is converted to a 11MHz chip sequence (1b=11 chips).


High-Rate DSSS

Enhanced DSSS

Defined in 802.11b for 5.5 and 11 Mb/s


Orthogonal Frequency Division

Multiplexing (OFDM)

A frequency band is divided into multiple narrow-band channels with smaller bitrates (similar to ADSL)

Signal of each single low-speed channel is more immune max. 54 Mb/s

All channels use the same number of bits expressed by a modulation symbol

As the initial media measurement does not make sense on the wireless media – propagation conditions may change very rapidly

Shorter reach especially in the rugged terrain

Originally utilized in 802.11a standard (5 GHz), later also in 802.11g (2.4 GHz)


Data rates in OFDM

Defined by modulation & coding scheme index (MCS)


OFDM is used with

20 MHz channel - 52+4 subcarriers

40 MHz channel

Modulation: BPSK, QPSK, 16-QAM, 64-QAM

Coding rate – the forward error correction (how much bits encode data): 1/2, 2/3, 3/4, 5/6

Guard interval: 400/800 ns

Number of parallel spatial streams: 1-4

Combination 802.11a/g – more MCS streams


Adds 256-QAM (FEC 3/4 and 5/6)

80/160 MHz channels


Signal Propagation Problems

Multi-path reflections

Multiple reflected signals that traveled along different paths sum up on the receiver

Delay differences may not exceed 500 ns

Hidden node problem

Station S detects a free channel in its neighborhood, but it is not free in the receiver's neighborhood (typically AP) – the source of the signal is not visible from station S

Caused by a limited signal reach and obstacles in the signal path

May be solved by RTS-CTS mechanism


Wireless Media Access Methods

Distributed Coordination Function (DCF)


Point Coordination Function (PCF) for real-time applications (QoS may be implemented)

AP assigns the bandwidth to individual stations


Commonly combined together with DCF

Contention-Free Period and Contention Period


Not implemented and utilized very often today

IEEE 802.11e (802.11i)

8 priorities, IFS has to be proportional to the priority value admission control (distributed/centralized)

Timeslot reservation 29

Collisions on the Wireless Media

The detection of the collision is problematic on the wireless media

The same antenna cannot be utilized for transmission and reception at the same time

Station may not hear all the other stations well

Hidden node problem

We may detect the free medium, but not the collision

A solution is to acknowledge all received frames


CSMA/CA: Carrier Sense Multiple Access with Collision Avoidance


A free channel has to be detected before a transmission may start

A pause of the duration (DIFS+ random interval) follows

Avoids collision of the acknowledgment frames

After a frame is transmitted, a timeslot is reserved for transmission of an acknowledgment

The frame header carries the duration of the frame transmission (including time necessary for acknowledgment)

An acknowledgment is sent in short inter-frame space – SIFS

If a transmitter does not receive frame acknowledgment in a specified timeout, it waits for a random time and makes a new attempt

Broadcasts and multicasts are not acknowledged


Hidden Node Problem


Solution of the Hidden Node

Problem (1)

A station that wants to transmit sends RTS frame first

RTS identifies source, destination and duration of the intended transmission

Duration of RTS+CTS+data+ACK, always with SIFS

The destination station sends CTS with the intended transmission time copied just the remaining part of the interaction

All stations that hear RTS or CTS have to treat the medium as busy for the advertised transmission duration interval

The result is that all station in both the transmitter and receiver signal range are informed about the ongoing transmission


Solution of the Hidden Node

Problem (2)


Advantages of RTS-CTS


Only RTS frames may collide

As RTS is very short, the impact to the efficiency is limited

Efficient only for long data frames (much longer than RTS frames)

A threshold of the data frame length for usage of

RTS-CTS mechanism may be configured

Commonly switched off completely on point-topoint links as there are no hidden nodes


Link Layer


Responsibilities of Link Layer

CRC calculation and checking

Fragmentation of frames

As there is a high probability of interference, sending a frame by a multiple shorter fragments decreases overhead in case of a retransmission

A threshold of a length of frame that will be fragmented may be defined

From MAC protocol perspective, all fragments of a frame are sent as a single burst broadcast a multicast fragments are never fragmented

Frame header informs about a modulation used to transmit (the rest of) the frame

Header is always sent on the basic bitrate

Several lengths of interframe spaces are defined

Used in MAC protocol and traffic prioritization.


CSMA/CA Timing


IEEE 802.11 Frame Types

Data frames contains the useful information passed between transmitter and receiver

Control frames control information that accompanies the data frames

Management frames

Connection into WLAN, authentication


Management Frames

Association Request, Association Response,

Reassociation Request, Reassociation

Response, Disassociation

Probe Request, Probe Response

Authentication, Deauthentication

Beacon (transmitted periodically by an AP or by a dedicated station in IBSS)

Announcement Traffic Indication Frame (ATIM)

Management Frames contain the fixed parts

(different for various frame types) and optional

Information Elements (IE)


Examples of IEs


Supported Rates – multiplies of 500 kbps

Frequency hopping parameter set dwell time, hopping pattern

Direct Sequencing (DS) parameter set channel number

Contention Free (CF) parameter set

CFPMaxDuration, CFPDurationRemaining, time to nearest CFP, interval between CFPs

TIM (Traffic Indication Map) informs about frames buffered for individual stations in power-save mode

Interval between timeslots for sending of broadcast and multicast frames

Control Frames

Request to Send (RTS), Clear to Send (CTS)

Acknowledgment (ACK)

PS-Poll (Power Save Poll)

CF-End (Contention Free Period End)

CF-End+CF-Ack (Contention Free Period End +

ACK of last data frame)


Data Frames





Null data

CF-ACK (without data)

CF-Poll (without data)

CF-Ack+CF-Poll (without data)


Discovery of Access Point

Initiated by a station


Probe Request on all channels (involves station's

SSID and all supported rates)

AP responds with Probe Response contains SSID, supported rates, parameters of the physical layer assigned to a requesting station

(hopping pattern for FHSS, chip sequence for DSSS)


Listening for beacon frames (on all channels)

Takes a longer time




Station authenticates to the AP

Open and Shared Key Mode

Open = no authentication

Shared Key = authentication using the shared WEP key challenge-response protocol

Authentication frame used for both request and response


Association with Access Point (1)

During association, AP maps its logical port to a particular station

Association Request (from station) involves SSID, supported rates, listen (wakeup) interval for frame reception from the AP (in power save mode)

Association Response (from AP) status code, Association ID, supported rates

Association ID used for the future management of the communication on the radio channel


Association with Access Point (2)

A station may be associated only to a single AP at the same time

If a station looses the signal, it tries to make a handover to different AP

The conditions that trigger the handover and the way the station chooses the new AP are dependent on the wireless card driver implementation signal quality, number of collisions in the BSSretry counters,…

AP commonly supports tens of associations

Number of stations associated with AP is much more limited by the available bandwidth


Power-Save Mode

Allows to save station's battery while maintaining the possibility of reception of frames destined for the station

AP buffers unicast frames for stations in their sleeping interval

Station periodically awakes to receive the beacon frame from the AP (wakeup interval) beacon frame informs which stations have the frame buffered on the AP (utilizes Association ID)

After an awaken station hears that AP has a frame buffered for it, it requests the frame using a Poll frame

A wakeup interval is defined by AP administrator and propagated in beacon frames

Broadcast/multicast frames have to be buffered if there is at least one power-save station in the BSS


IEEE 802.11 Link Layer Frame


Special Networking Devices of the

Wireless Infrastructure


Repeater, Bridge

Repeater AP

Purely wireless bridge (not connected to a wired network)

Sends and receives on the same channel – divides the throughput by 2

Repeater is associated as a client of another AP

Workgroup Bridge

Connect a working group of stations equipped with Ethernet

NIC to a wireless network acts as Ethernet switch/hub with WiFi NIC

Encapsulates Ethernet frames into WiFi frames, the receiving AP has to decapsulate them accordingly

Wireless Bridge

A variation of the workgroup bridge

Provides a point-to-point wireless interconnection of two



Usage of the Repeater


Usage of the Point-to-Point Bridge


Usage of the Point-to-Multipoint



Interoperability of Special

Wireless Infrastructure Devices

The functionality of special devices is NOT standardized

Nomenclature is not unified

Uncertain interoperability between vendors

Not all APs support the „client mode“, i.e. the ability of being a client of another AP


WiFi Security (1)


Open + Shared key


WEP – Wired-Equivalent Privacy

Shared key (64/128B), used simultaneously for encryption and authentication

Easy to crack (if the attacker has enough time and sufficient traffic on the channel)

Matter of tens of minutes

Freely available cracking software

It is recommended to use together with a mechanism of periodic change of encryption keys


WiFi Security (2)

WiFi Protected Access – WPA

Repairs main WEP drawbacks

Problems of the initialization vector

Temporal Key Integrity Protocol (TKIP)


better encryption algorithm (AES)

Still not implemented in some old WiFi devices


Intelligent Wireless Networks and Wireless Mesh Networks


Basic Features of Intelligent WiFi

Networks (1)

Centralized WLAN management

WLAN Controller

A coverage by a wireless signal is accomplished using

„lightweight“ APs managed by a WLAN controller

AP download their configuration from WLAN controller

Increases security, easier centralized management

L2/L3 traffic tunneling to WLAN controller (LWAPP)

Secure operation of multiple different kinds of users on a shared wireless infrastructure

Support for L2 and L3 roaming


Basic Features of Intelligent WiFi

Networks (2)

Coordinated physical layer management

Dynamic channel assignment (according to noise conditions, AP interferences, load on individual channels)

Control of the transmission power (compensation of

AP outages)

Detection of rogue APs + enforcing of client reassociation to the authorized AP

Balancing of client association to individual APs


Wireless Mesh

Outdoor wireless network solution

Combines 2.4 and 5 GHz bands

5 GHz Wireless distribution system -

2.4 GHz hot-spots

Roof APs (RAP) and Mesh APs (MAP)

Traffic may be bridged into wired infrastructure by RAP or tunneled to a WLAN controller

Path over the wireless mesh is determined using a special protocol

Adaptive Wireless Path protocol

Hybrid Wireless Mesh Protocol

Some other protocol, e.g. OLSR or B.A.T.M.A.N


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