Wireless LAN Technology

Deepak John

SJCET-Pala

Wireless LAN is a wireless local area network that uses radio waves as its carrier.

Advantages

very flexible within the reception area

Ad-hoc networks without previous planning possible

(almost) no wiring difficulties

more robust against disasters like, e.g., earthquakes, fire - or users

pulling a plug...

Disadvantages

typically very low bandwidth (1-10 Mbit/s)

products have to follow many national restrictions

key application areas:

i. LAN extension

ii. cross-building interconnect

iii. nomadic access

iv. ad hoc networking

Single Cell LAN Extension

LAN Extension

wireless LAN will be linked into a wired LAN on the same premises.

Multi Cell LAN Extension

Cross-Building Interconnect

connect LANs in nearby buildings

point-to-point wireless link

Devices connected are typically bridges or routers.

Used where cable connection not possible (e.g. across a street)

Nomadic Access

Wireless link between LAN hub and mobile data terminal equipped

with antenna

also useful in extended environment such as campus or cluster of

buildings

users move around with portable computers

Temporary peer-to-peer network set up to meet immediate need

Ad Hoc Networking

b) Ad hoc LAN

Wireless LAN Requirements

throughput - efficient use wireless medium

no of nodes - hundreds of nodes across multiple cells

connection to backbone LAN - using control modules

service area - 100 to 300 m

low power consumption - for long battery life on mobiles

transmission robustness and security

license-free operation

handoff/roaming

dynamic configuration - aaddition, deletion, and relocation of end

systems without disruption to users

generally categorized according to the transmission technique that

is used. They are:

i. Infrared (IR) LANs

ii. Spread spectrum LANs

iii. Narrowband microwave

Wireless LAN Technology

Infrared LANs

constructed using infrared portion of spectrum

strengths

spectrum virtually unlimited hence high rates possible

unregulated spectrum

infrared shares some properties of visible light

reflection covers room, walls isolate networks

inexpensive and simple

weaknesses

background radiation, e.g. sunlight, indoor lighting

power limited by concerns for eye safety and power consumption

Transmission Techniques

directed-beam IR

point-to-point links

range depends on power and focusing

for indoor use can set up token ring LAN

omnidirectional

single base station with line of sight to other stations

acts as a multiport repeater

other stations use directional beam to it

diffused configuration

stations focused / aimed at diffusely reflecting ceiling

Spread Spectrum LAN Configuration

usually use multiple-cell arrangement

adjacent cells use different center frequencies

configurations:

hub

connected to wired LAN

connect to stations on wired LAN and in other cells

may do automatic handoff

peer-to-peer

no hub

MAC algorithm such as CSMA used to control access

for ad hoc LANs

Transmission Issue

Three microwave bands have been set aside by FCC which doesn’t

need a license if the equipments operates under 1W power

They are:

902-928 MHz (915 MHz band)-Industrial Band

2.4-2.4835 GHz (2.4 GHz band)-Scientific Band

5.725-5.825 GHz (5.8 GHz band)- Medical Band

Commonly known as ISM band ,it is used by Wireless LAN with

spread spectrum technology

Narrowband Microwave LANs

Use of a microwave radio frequency band for signal transmission

i. Licensed

ii. Unlicensed

1. Licensed Narrowband RF

Microwave radio frequencies are licensed within specific geographic

areas to avoid potential interference.

Each geographic area has a radius of 28 km and can contain five

licenses, with each license covering two frequencies.

Uses cell configuration(18GHz)

One advantage of the licensed narrowband LAN is that it guarantees

interference-free communication

Unlicensed Narrowband RF

Radio LAN introduced narrowband wireless LAN in 1995 which

uses the unlicensed ISM spectrum

Used at low power (0.5 watts or less)

Operates at 10 Mbps in the 5.8-GHz band

Range = 50 m to 100 m

The RadioLAN product makes use of a peer-to-peer configuration.

RadioLAN product automatically elects one node as the Dynamic

Master.

IEEE 802.11 IEEE has defined the specifications for a wireless LAN, called IEEE

802.11, which covers the physical and data link layers.

Defines standard for WLANs using the following four technologies

Frequency Hopping Spread Spectrum (FHSS)

Direct Sequence Spread Spectrum (DSSS)

Infrared (IR)

Orthogonal Frequency Division Multiplexing (OFDM)

Versions: 802.11a, 802.11b, 802.11g, 802.11e, 802.11f, 802.11i

Distribution System

Portal

802.x LAN

Access

Point

802.11 LAN

BSS2

802.11 LAN

BSS1

Access

Point

802.11 - Architecture of an infrastructure network Station (STA)

terminal with access mechanisms to the wireless medium and radio contact to the access point

Basic Service Set (BSS)

group of stations using the same radio frequency

Access Point

station integrated into the wireless LAN and the distribution system

Portal

bridge to other (wired) networks

Distribution System

interconnection network to form one logical network

STA1

STA2 STA3

ESS

802.11 - Architecture of an ad-hoc network

Direct communication within a

limited range

Station(STA):terminal with

access mechanisms to the

wireless medium

Basic Service Set (BSS):

group of stations using the same

radio frequency

802.11 LAN

BSS2

802.11 LAN

BSS1 STA1

STA4

STA5

STA2

STA3

Services Distribution of Messages

Distribution service (DS):Used to exchange MAC frames from

station in one BSS to station in another BSS

Integration service: Transfer of data between station on IEEE 802.11

LAN and station on integrated IEEE 802.x LAN

Association Related Services

Association: Establishes initial association between station and AP.

Re-association :Enables transfer of association from one AP to

another, allowing station to move from one BSS to another.

Disassociation: Association termination notice from station or AP

Access and Privacy Services

Authentication: Establishes identity of stations to each other.

De-authentication: Invoked when existing authentication is

terminated

Privacy: Prevents message contents from being read by unintended

recipient

IEEE standard 802.11 protocol stack

mobile terminal

access point

server

fixed terminal

application

TCP

802.11 PHY

802.11 MAC

IP

802.3 MAC

802.3 PHY

application

TCP

802.3 PHY

802.3 MAC

IP

802.11 MAC

802.11 PHY

LLC

infrastructure network

LLC LLC

Medium Access Control

MAC layer covers three functional areas

reliable data delivery

access control

Security

Reliable Data Delivery

Loss of frames due to noise, interference, and propagation effects.

To ensure reliable data delivery IEEE 802.11 includes a frame

exchange protocol .

Two frame exchange

Source station transmits data

Destination responds with acknowledgment (ACK)

If source doesn’t receive ACK, it retransmits frame

Four frame exchange for enhanced reliability

Source issues request to send (RTS)

Destination responds with clear to send (CTS)

Source transmits data

Destination responds with ACK

The RTS alerts all stations that are within reception range of the

source that an exchange is under way

Similarly, the CTS alerts all stations that are within reception range

of the destination that an exchange is under way

Access Control

Medium access control is based on distributed control and centralized

control.

Uses a MAC algorithm called DFWMAC (distributed foundation

wireless MAC).

It provides a distributed access control mechanism with an optional

centralized control.

IEEE 802.11 defines two MAC sub layers: the distributed

coordination function (DCF) & Point coordination Function (PCF).

1. Distributed Coordination Function(DCF)

The lower sub layer of the MAC layer.

DCF sub layer uses CSMA /CA

if station has frame to send it listens to medium

if medium idle, station may transmit

else waits until current transmission complete

To ensure the smooth and fair functioning of CSMA, the MAC

frame transmissions are separated by a time gap called IFS

Access Control

2. Point Coordination Function (PCF)

polling by centralized polling master (point coordinator)

uses PIFS when issuing polls

point coordinator polls in round-robin to stations configured for

polling

when poll issued, polled station may respond using SIFS

if point coordinator receives response, it issues another poll using

PIFS

if no response during expected turnaround time, coordinator issues

poll

IEEE 802.11 MAC Frame Format

Control Frames

Power Save-Poll (PS-Poll)

Request to Send (RTS)

Clear to Send (CTS)

Acknowledgment (ACK)

Contention-Free (CF)-end

CF-End + CF-Ack

Management Frames

used to manage communications between stations and Aps

such as management of associations

requests, response, reassociation, dissociation, and authentication

Data Frames

eight data frame subtypes, in two groups

1. Data Carrying

carry upper-level data

2. Not Data Carrying

do not carry user data

Null Function

carries no data, polls, or acknowledgments

carries power mgmt bit in frame control field to AP

indicates station is changing to low-power state

IEEE-802.11 Addressing There are four address fields, each 6 bytes long.

The IEEE 802.11 addressing mechanism specifies four cases, defined

by the value of the two flags in the FC field, To DS and From DS.

The interpretation of the four addresses (address 1 to address 4) in the

MAC frame depends on the value of these flags

IEEE 802.11 Physical Layer

The PHY is the interface between the MAC and wireless media,

which transmits and receives data frames over a shared wireless

medium.

The physical layer is further subdivided into sub layers:

Physical Layer Convergence Procedure (PLCP) sub layer:

Reformats data received from MAC layer into frame that PMD

sub layer can transmit

Physical Medium Dependent(PMD) Sub layer:

Takes the binary bits of information from PLCP-PDU (PPDU)

and transform them into RF signals

defines method for transmitting and receiving data

IEEE 802.11 DSSS

Operating in the 2.4-GHz ISM band, at data rates of 1 Mbps and 2

Mbps.

Up to three non overlapping channels, each with a data rate of 1

Mbps or 2 Mbps, can be used in the DSSS scheme.

Each channel has a bandwidth of 5 MHz.

The encoding scheme that is used is DBPSK (differential binary

phase shift keying) for the 1 Mbps rate and DQPSK(differential

Quadrature phase shift keying )for the 2 Mbps rate.

IEEE-802.11 FHSS

FHSS system makes use of multiple channels,

Data transmission over the media is controlled by the FHSS PMD sub

layer as directed by the FHSS PLCP sub layer.

PMD takes the binary bits of and transforms them into RF signals for

the wireless media by using carrier modulation and FHSS technique

IEEE-802.11b HR-DSSS The IEEE 802.11b PHY is one of the PHY layer extensions of IEEE

802.11 and is referred to as high rate direct sequence spread

spectrum (HR/DSSS).

Providing data rates of 5.5 and 11 Mbps.

IEEE 802.11b defines two physical-layer frame formats, which

differ only in the length of the preamble

IEEE-802.11a OFDM

Makes use of the frequency band called the Universal Networking

Information Infrastructure (UNII), which is divided into three parts.

UNII-1 band is intended for indoor use

UNII-2 band be used either indoor or outdoor,

UNII-3 band is for outdoor use.

The IEEE 802.11a PHY adopts orthogonal frequency division

multiplexing (OFDM) instead of spread spectrum techniques

OFDM splits a single high-speed digital signal into several slower

signals running in parallel

Provides rates of 6, 9 , 12, 18, 24, 36, 48, 54 Mbps

IEEE-802.11g OFDM

extends data rates above 20 Mbps, up to 54 Mbps.

operates in the 2.4-GHz.

offers a wider array of data rate and modulation schemes.

provides compatibility with 802.11 by specifying the same modulation and framing schemes as these standards for 1,2,5.5, and 11 Mbps.

BLUE TOOTH

IEEE 802.15

is a wireless LAN technology using short-range radio links, intended

to replace the cable(s) connecting portable and/or fixed electronic

devices.

is an ad hoc network where devices can automatically find each other,

establish connections, and discover what they can do for each other.

range 10-100 mtrs.

features are robustness, low complexity, low power and low cost.

uses a 2.4-GHz ISM band divided into 79 channels of 1 MHz each

A Bluetooth device has a built-in short-range radio transmitter.

It uses Frequency Hop Spread Spectrum (FHSS) to avoid any

interference.

Applications

Automatic synchronization between mobile and stationary devices

Connecting mobile users to the internet using Bluetooth-enabled

wire-bound connection ports

Dynamic creation of private networks

Types of Bluetooth Wireless Technology

Depending on the power consumption and range of the device,

there are 3 Bluetooth Classes as:

1. Class 1: Max Power – 100mW ; Range – 100 m

2. Class 2: Max Power – 2.5mW ; Range – 10 m

3. Class 3: Max Power – 1mW ; Range – 1 m

Protocol Architecture

Bluetooth is a layered protocol architecture

Core protocols

Cable replacement and telephony control protocols

Adopted protocols

Core protocols

Radio

Baseband

Link manager protocol (LMP)

Logical link control and adaptation protocol (L2CAP)

Service discovery protocol (SDP)

Cable replacement protocol

RFCOMM

Telephony control protocol

Telephony control specification – binary (TCS BIN)

Adopted protocols

TCP/UDP/IP

OBEX

WAE/WAP

HCI

Bluetooth Radio

Baseband

Audio Link Manager (LMP)

L2CAP

RFCOMM TCS SDP

TCP/UDP

PPP

AT

Co

mm

an

ds

OB

EX

Application

Radio Layer

The bottom layer in protocol stack, equivalent to the physical layer

of the Internet model.

It deals with radio transmission and modulation.

The Radio layer defines the requirements for a Bluetooth transceiver

operating in the 2.4 GHz ISM band.

divided into 79 channels of 1 MHz each.

Support data rate: 1Mbps (Basic Rate) / 3 Mbps (Enhanced Data

Rate).

Uses a technique called frequency hopping, for establishing radio

links with other Bluetooth devices

Baseband layer

is roughly equivalent to the MAC sub layer in LANs.

It is responsible for constructing ,encoding and decoding packets,

and managing error correction, encrypting and decrypting for secure

communication etc..

The primary and secondary communicate with each other using time

slots.

Two types of links can be established between primary and

secondary:

Synchronous connection-oriented (SCO) links:

used when avoiding latency (delay in data delivery) is more

important than integrity (error-free delivery).

used for voice transmission.

Asynchronous connectionless (ACL) links:

used when data integrity is more important than avoiding latency.

used for data transmission.

L2CAP

The Logical Link Control and Adaptation Protocol, is roughly equivalent to the LLC sublayer in LANs.

used for data exchange on an ACL link; SCO channels do not use L2CAP.

This layer has four major functions:

• First, it accepts packets of up to 64 KB from the upper layers and breaks them into frames for transmission.

• Second, it handles the multiplexing and de-multiplexing of multiple packet sources.

• Third, L2CAP handles Segmentation and reassembly

• Finally, L2CAP enforces quality of service requirements between multiple links.

Audio: interfaces directly with the baseband. Each voice

connection is over a 64Kbps.uses PCM encoding.

Host Controller Interface: provides a uniform method of access to

the baseband, control registers, etc through USB, PCI, or UART.

Service Discover Protocol (SDP): protocol of locating services

provided by a Bluetooth device.

Telephony Control Specification (TCS): defines the call control

signaling for the establishment of speech and data calls between

Bluetooth devices.

RFCOMM: provides emulation of serial links (RS232). Upto 60

connections

Bluetooth defines two types of network topology:

Piconet

Scatternet

PICONET

known as small net, have up to eight stations.

One primary, the rest are secondary.

Communication can be one-to-one or one-to-many.

Each of the active slaves has an assigned 3-bit Active Member

address.

An additional eight secondary's can be in the “parked state.

A secondary in a “parked state” is synchronised with the primary

but cannot take part in communication until it is moved from the

“parked state”

Scatternet

formed by the combinations of piconet.

A secondary station in one piconet can be the primary in another

piconet.

This station can receive messages from the primary in the first piconet

(as a secondary) and acting as a primary, deliver them to secondaries in

the second piconet .

States of a Bluetooth Device

STANDBY

inquiry page

connected transmit

PARK HOLD SNIFF

unconnected

connecting

active

low power

ACTIVE (connected/transmit): the device is uniquely identified by a

3bits AM_ADDR and is fully participating.

SNIFF state: participates in the piconet only within the SNIFF

interval.

HOLD state: no data transfer, master can put slaves on HOLD state.

PARK state (low-power): releases AM_ADDR but stays synchronized

with master

Bluetooth Link Security

Elements:

Authentication – verify claimed identity

Encryption – privacy

Key management and usage

Security algorithm parameters:

Unit address

Secret authentication key (128 bits key)

Secret privacy key (4-128 bits secret key)

Random number

Virtual LAN

A virtual local area network (VLAN) is a logical group of

workstations, servers and network devices that appear to be on the

same LAN despite their geographical distribution.

All workstations and servers used by a particular workgroup share

the same VLAN, regardless of the physical connection or location.

The group membership in VLANs is defined by software, not

hardware.

A VLAN is a broadcast domain created by one or more switches.

802.1Q

Without

VLANs

With

VLANs

Each switch port could be assigned to a different VLAN.

Ports assigned to the same VLAN share broadcasts.

Ports that do not belong to that VLAN do not share these broadcasts.

VLAN operation

1. VLANs are assigned on the switch port. There is no “VLAN”

assignment done on the host (usually).

2. In order for a host to be a part of that VLAN, it must be assigned an

IP address that belongs to the proper subnet.

Remember: VLAN = Subnet

3. Assigning a host to the correct VLAN is a 2-step process:

1. Connect the host to the correct port on the switch.

2. Assign to the host the correct IP address depending on the

VLAN memebership

VLAN Membership

1.Static VLAN

are called port-based and port-centric membership VLANs.

Ports on a switch are manually assigned to a VLAN.

This is the most common method of assigning ports to VLANs.

As a device enters the network, it automatically assumes the VLAN

membership of the port to which it is attached.

2. Dynamic VLAN

allow membership based on the MAC address of the device connected

to the switch port.

As a device enters the network, it queries a database within the switch

for a VLAN membership.

membership is configured using a special server called a VLAN

Membership Policy Server (VMPS).

Configuration

Network administrators are responsible for configuring VLANs both

manually and statically.

Each switch must know about which station belongs to which VLAN and the membership of stations connected to other switches.

Three methods have been devised for this purpose:

i. Table maintenance

ii. Frame tagging

iii. Time-division multiplexing

Communication

i. Table Maintenance

when a station sends a broadcast frame to its group members, the

switch creates an entry in a table and records station membership.

The switches send their tables to one another periodically for

updating

ii. Frame Tagging

when a frame is traveling between switches, an extra header is

added to the MAC frame to define the destination VLAN.

The frame tag is used by the receiving switches to determine the

VLANs to be receiving the broadcast message.

iii. Time-Division Multiplexing (TDM)

the connection (trunk) between switches is divided into timeshared

channels

IEEE 802.1Q: Features

Allows up to 4095 VLANs

Allows port based and MAC address based,

Upward compatible with existing VLAN-unware hubs and bridges

Supports both shared-media and switched LANs.

Retains plug and play mode of current LAN bridges.

Allows priority associated with each VLAN.

Supports static and dynamic configurations for each VLAN

Advantages & Disadvantage

Disadvantage:

Costly

Software based

Human labor to program

Depending on variety switches

Management complexity

Advantages:

More Security

Ease of administration

Broadcast control

Reduction in network traffic