The Electromagnetic Spectrum

The electromagnetic spectrum and its uses for communication.

ISM Unlicensed Frequency Bands

ExtremelyLow

VeryLow

Low Medium High VeryHigh

UltraHigh

SuperHigh

Infrared VisibleLight

Ultra-violet

X-Rays

AudioAM Broadcast

Short Wave Radio FM BroadcastTelevision Infrared wireless LAN

Cellular (840MHz)NPCS (1.9GHz)

902-928 MHz26 MHz

5 GHz(IEEE 802.11)

HyperLANHyperLAN2

2.4 – 2.4835 GHz83.5 MHz

(IEEE 802.11)

102210161014

1012

900 MHz vs. 2.4 GHz vs. 5GHz

900 MHz 2.4 GHz

PROs

CONs

Greater Range than 2.4 GHz ( For in- Building LANs)

Maximum Data Rate 1 Mbps

Limited Bandwidth

Crowded Band

Global Market

IEEE 802.11b/g

Higher Data Rates (10+ Mbps)

Less Range than

900 MHz (For In-Building LANs)

5 GHz

Global Market

IEEE 802.11a

Higher Data Rates (20+ Mbps, up to 50M)

Much Less Range than 900 or 2.4GHz

Higher Cost RF Components

Large Antenna required

The 802.11 Standard

199719971–2 Mbps1–2 Mbps

2.4G2.4G

1999199954 Mbps54 Mbps

5G5GOrthogonal Orthogonal

FDMFDM

1999199911 Mbps11 Mbps

2.4G2.4G

2001200154 Mbps54 Mbps

2.4G2.4G

Power Management (awake, doze)Timing Management (beacon, sync within 4usec)

Frequency Hopping

• 79 Channels, 1 MHz Each• Maximum time on any one freq is .4 sec in any 30 sec period• Changes frequency (Hops) at least every 0.4 seconds (dwell

time – adjustable), Lost packets are re-transmitted on next hop

• Synchronized hopping sequences required by same random number generators in every station

• 26 hopping patterns in three different sets with minimum interference with each other – called orthogonal patterns

2.400 GHz 2.483 GHz

Tim

e

12

34

56

78

9

Direct Sequence

Similar to CDMA 22 MHz wide stationary channels (11 chips) Each data bit becomes a string of chips (Barker sequence) transmitted

in parallel across a wide frequency range, data may be decoded from redundant bits

• 3 non-overlapping channels, can move to an alternate channel to avoid interference

• 3 Access Points can occupy same area• 802.11b

• DSSS 1 M baud data rate with 1 bit (1Mbps), 2 bit (2 Mbps)• HR-DSSS 1.375 M baud, 4 bit (5.5M), 8 bit (11 Mbps)

IEEE 802.11 Architecture

ap

apap

Ad-Hoc Mode

Infrastructure Mode

Ad-Hoc Mode

Computers are brought together to form a network. There is no structure to the network; there are no fixed points; and usually every node is able to communicate with every other node.

No administration and pre configuration

IETF MANET (Mobile Ad hoc Networks) working group

Ad-Hoc Mode

Infrastructure Mode

It uses fixed network access points (AP) with which mobile nodes can communicate. These network APs are connected to wired network to widen the LAN's capability by bridging wireless nodes to other wired nodes.

All communications between mobiles and wired network clients go through the AP.

Mobiles can roam between APs and seamless wide area coverage is possible.

ap

apap

Infrastructure Mode

Wireless LANs Issues (CSMA)

The range of a single radio may not cover the entire system Hidden station problem (A->B, C->B since C does not hear A,

collision) Exposed station problem (B->A, C hears B, C won’t send to D,

reduced efficiency)

Multiple Access with Collision Avoidance

IDEA: having a short frame transmitted from both sender and receiver before the actual transfer

A sending a short RTS (30 bytes) to B with length of LB responding with a CTS to A, whoever hears CTS shall remain silent

for the duration of LA sends data (length L) to BFurther optimization as MACAW (MACA for Wireless)

RTS, RTS, not CTSnot CTSTx okTx ok

CTS, CTS, not RTSnot RTS

Keep silentKeep silent

CTS, CTS, RTSRTS

Keep silentKeep silent

Medium Access ControlDistributed Coordination Function

Distributed Control, Ethernet-like CSMACSMA/CA (collision avoidance)

• Physical channel sensing• Sense channel, transmit entire frame, retry if necessary

• Virtual channel sensing (MACAW)• Add ACK frame

Network Allocation Vector Network Allocation Vector (quiet time)(quiet time)

Short frame (30B)Short frame (30B)Contains data Contains data

lengthlength

data length data length copied from RTScopied from RTS

Fragmentation for Throughputfragment burst

Unreliable ISM bandsError rate p = 10-4,

success rate for full Ethernet frame (12,144 bit) <30%, (1-p)**n

Error rate p = 10-6, 1% will be damaged.

Medium Access ControlPoint Coordination Function (PCF)

• Central Control• Base polls other stations

• Broadcast a beacon frame periodically (10ms to 100ms) with system parameters (hopping sequence, dwell time, clock synchronization)

• Base determines the transmission priority• QoS guarantee

• Can Coexist with DCF

Short Short InterFrame InterFrame

SpacingSpacing

RTS/CTS/ACKRTS/CTS/ACK Fragment burstFragment burst

The 802.11 Data Frame Structure

WEPWEP

More More FrameFrame

ssOrdered Ordered framesframes

Data Data ControControlMgmtlMgmt

To APTo AP

RTSRTSCTSCTSACKACK

Intercell Intercell traffic traffic

addressesaddresses

Sleep / Sleep / AwakeAwake

Frame length Frame length plus ack plus ack

(used for NAV)(used for NAV)

Intercell Intercell traffic traffic

addressesaddresses

Fragment Fragment sequencesequence

802.11 AP Services

Distribution Services• Association

• station reports identity, data rate, power

• Disassociation• Reassociation

• handover

• Distribution• routing

• Integration• format conversion

IntraCell Services• Authentication

• conducted after association

• Deauthentication• Privacy

• Wired-Equivalent Privacy WEP RC4

• Data Deliver

Steps to Association:

Client evaluates APresponse, selects best AP.

AP sends Probe ResponseAccess Point A

Access Point

B

Initial connection to an Access Point

Client sends probe

Client sends authenticationrequest to selected AP (A).

AP A confirms authenticationand registers client.

Client sends associationrequest to selected AP (A).

AP A confirms associationand registers client.

Association Process-- Passive Scanning

Steps to Re-association:

Adapter listens for beaconsfrom APs.

Adapter evaluates APbeacons, selects best AP.

Adapter sends associationrequest to selected AP (B).

AP B confirms associationand registers adapter.

Access Point

A

Access Point

B

Roaming from Access Point A to Access Point B

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

AP A forwards buffered packetsto AP B and de-registers adapter.

Re-association Process

Cellular and 802.11b

Cellular True mobility Secure Roaming & Handoff Integration with SP Voice

networks Data to 384Kbps with 2.5G Max at 2Mbps with 3GPP Higher network costs/user Licensed spectrum

802.11b 11Mbps today Integration with Enterprise

data network Unlicensed spectrum Higher client costs/user Limited mobility/roaming QoS and Security, voice in

development

Many Enterprises see the need for both solutions in the medium-term

Bluetooth

Frequency: 2.4 GHz ISM Band - FHSS 2.4 - 2.48G, 79Mhz = 79x1Mhz, 1600 hops per sec

Range: 10m (100m), Omni-directional, save power Low Power: 1mW (100mW with amplifier) Speed: 1Mbps gross Network: 8 devices (1master+7slave) in a piconet 3 Simultaneous voice plus data Data -- asymmetric @723.2k+57.6k, or symmetric @433.9k 10 piconet can form a scatternet Low Cost Ericsson, IBM, Nokia, Intel, Toshiba, etc. Founded Feb 98, www.bluetooth.com