Transcript The Electromagnetic Spectrum The electromagnetic spectrum and its uses for communication.

The Electromagnetic Spectrum
The electromagnetic spectrum and its uses for communication.
ISM Unlicensed Frequency Bands
Short Wave Radio
AM Broadcast
Audio
Extremely Very Low
Low
Low
FM Broadcast
Infrared wireless LAN
Television
Cellular (840MHz)
NPCS (1.9GHz) 1014
1016
1022
Medium High Very Ultra
High High
902-928 MHz
26 MHz
1012
Super Infrared Visible Ultra- X-Rays
High
Light violet
2.4 – 2.4835 GHz
83.5 MHz
(IEEE 802.11)
5 GHz
(IEEE 802.11)
HyperLAN
HyperLAN2
900 MHz vs. 2.4 GHz vs. 5GHz
900 MHz
PROs
Greater Range
than
2.4 GHz ( For
in- Building LANs)
Maximum Data
Rate 1 Mbps
CONs
Limited Bandwidth
Crowded Band
2.4 GHz
5 GHz
Global Market
Global Market
IEEE 802.11b/g
IEEE 802.11a
Higher Data Rates
(10+ Mbps)
Higher Data Rates
(20+ Mbps, up to
50M)
Less Range than
900 MHz (For InBuilding LANs)
Much Less Range
than 900 or 2.4GHz
Higher Cost RF
Components
Large Antenna
required
The 802.11 Standard
Power Management (awake, doze)
Timing Management (beacon, sync within 4usec)
1999
54 Mbps
5G
Orthogonal
FDM
1997
1–2 Mbps
2.4G
1999
11 Mbps
2.4G
2001
54 Mbps
2.4G
Frequency Hopping
9
Time
8
6
5
3
1
2.400 GHz
•
•
•
•
•
7
4
2
2.483 GHz
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
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
Ad-Hoc Mode
ap
ap
ap
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
ap
ap
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
RTS,
not CTS
Tx ok
CTS,
not RTS
Keep silent
CTS,
RTS
Keep silent
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 L
B responding with a CTS to A, whoever hears CTS shall remain silent for the
duration of L
A sends data (length L) to B
Further optimization as MACAW (MACA for Wireless)
Medium Access Control
Distributed Coordination Function
Distributed Control, Ethernet-like CSMA
CSMA/CA (collision avoidance)
•
Physical channel sensing
•
•
Sense channel, transmit entire frame, retry if necessary
Virtual channel sensing (MACAW)
•
Short frame (30B)
Contains data length
Add ACK frame
data length copied
from RTS
Network Allocation Vector
(quiet time)
Fragmentation for Throughput
fragment burst
Unreliable ISM bands
Error 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 Control
Point 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
RTS/CTS/ACK
Short
InterFrame
Spacing
Fragment burst
The 802.11 Data Frame Structure
Frame length plus
ack
(used for NAV)
Data
Control
Mgmt
RTS
CTS
ACK
Intercell
traffic
addresses
Fragment
sequence
Intercell
traffic
addresses
WEP
To AP
Sleep /
Awake
More
Frames
Ordered
frames
802.11 AP Services
Distribution Services
• Association
•
•
•
•
•
•
•
handover
routing
Integration
•
format conversion
•
conducted after
association
Deauthentication
Privacy
•
Distribution
•
•
station reports identity,
data rate, power
Disassociation
Reassociation
•
IntraCell Services
• Authentication
Wired-Equivalent
Privacy WEP RC4
Data Deliver
Association Process
-- Passive Scanning
Steps to Association:
Access
Point
A
Access
Point
B
Client sends probe
AP sends Probe Response
Client evaluates AP
response, selects best AP.
Client sends authentication
request to selected AP (A).
AP A confirms authentication
and registers client.
Client sends association
request to selected AP (A).
Initial connection to an Access Point
AP A confirms association
and registers client.
Re-association Process
Steps to Re-association:
Access
Point
A
Access
Point
B
Roaming from Access Point A
to Access Point B
Adapter listens for beacons
from APs.
Adapter evaluates AP
beacons, selects best AP.
Adapter sends association
request to selected AP (B).
AP B confirms association
and 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.
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