Course Overview

Introduction and History

Data in Wireless Cellular Systems

Data in Wireless Local Area Networks

Internet Protocols

Routing and Ad-Hoc Networks

TCP over Wireless Link

Services and Service Discovery

System Support for Mobile Applications

Wireless Local Area Networks

“Traditional” LANs: WaveLan, Proxim, IEEE 802.11

More specific “personal” LANs, also called “Personal Area Networks”: Bluetooth, IEEE 802.15

High-speed wireless LANs (approaching ATM data rates): HiperLAN

WaveLAN

Commercial product, developed by Lucent Technologies (de-facto market leader), available since early 1990s

Development closely aligned with early IEEE 802.11 standard effort, first product generation differs in key aspects from final IEEE 802.11 standard

Will discuss WaveLAN version 1, newer version 2 follows finally approved 802.11 standard (plus enhancements to allow for data rates up to 10 Mbps)

Physical layer:

915 MHz (902-928 MHz) or 2.4 GHz (2.412-2.475 GHz) ISM band

Bands are divided into frequency channels of 26 MHz each

Modulation scheme: DSSS, raw data rate of 2 Mbps

WaveLAN: Protocol Stack

WaveLAN: Frame Formats

WaveLAN MAC Protocol

Basically CSMA/CA

Sense media before transmission, if free, transmit

Defer:

wait until end of current transmission, plus fixed delay (WaveLAN InterFrame Space, WIFS) of 60 msec

apply random backoff procedure: pick number between 0 and 31 (antenna slot number S)

each slot corresponds to 23 ms

wait S*23 ms, sense media again, if busy, double backoff range until we reach range 0-255

drop packet after 15 attempts

If station has more than one packet to send: wait WIFS plus backoff-period in range 0-15 (avoid monopolization)

WaveLAN: MAC Protocol (Example)

WaveLAN: Higher Protocols

WaveLAN: Beacon
(Higher Protocol Example)

IEEE 802.11

Standard for wireless local area networks, approved by IEEE in 1997

Scope: physical layer (PHY) and media access control sublayer (MAC) for wireless connectivity for fixed, portable, and moving stations with a local area

Supports data rates of 1 or 2 Mbps, using infrared or radio

Supports two basic architectures: independent basic support set (IBSS) and infrastructure networks

Most recent commercial products (including the new WaveLAN generation) are compatible with 802.11

802.11 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 (EES: Extended Service Set) based
on several BSS

802.11 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

IEEE Family of Standards

802.11 Layers and Functions

PLCP Physical Layer Convergence Protocol

clear channel assessment signal (carrier sense)

PMD Physical Medium Dependent

modulation, coding

PHY Management

channel selection, MIB

Station Management

coordination of all management functions

MAC

access mechanisms, fragmentation, encryption

MAC Management

synchronization, roaming, MIB, power management

802.11 Physical Layer

3 versions: 2 radio (typ. 2.4 GHz), 1 IR

data rates 1 or 2 Mbit/s

FHSS (Frequency Hopping Spread Spectrum)

spreading, despreading, signal strength, typ. 1 Mbit/s

min. 2.5 frequency hops/s (USA), two-level GFSK modulation

DSSS (Direct Sequence Spread Spectrum)

DBPSK modulation for 1 Mbit/s (Differential Binary Phase Shift Keying), DQPSK for 2 Mbit/s (Differential Quadrature PSK)

preamble and header of a frame is always transmitted with 1 Mbit/s, rest of transmission 1 or 2 Mbit/s

chipping sequence: +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1 (Barker code)

max. radiated power 1 W (USA), 100 mW (EU), min. 1mW

Infrared

850-950 nm, diffuse light, typ. 10 m range

carrier detection, energy detection, synchonization

IEEE 802.11 MAC Architecture

802.11 MAC Layer

Traffic services

Asynchronous Data Service (mandatory)

exchange of data packets based on “best-effort”

support of broadcast and multicast

Time-Bounded Service (optional)

implemented using PCF (Point Coordination Function)

Access methods

DCF CSMA/CA (mandatory)

collision avoidance via randomized „back-off“ mechanism

minimum distance between consecutive packets

ACK packet for acknowledgements (not for broadcasts)

DCF w/ RTS/CTS (optional)

Distributed Foundation Wireless MAC (DFWMAC)

avoids hidden terminal problem

PCF (optional)

access point polls terminals according to a list

802.11 MAC Layer

Priorities

defined through different inter frame spaces

no guaranteed, hard priorities

SIFS (Short Inter Frame Spacing)

highest priority, for ACK, CTS, polling response

PIFS (PCF IFS)

medium priority, for time-bounded service using PCF

DIFS (DCF, Distributed Coordination Function IFS)

lowest priority, for asynchronous data service

802.11 MAC: CSMA/CA

station ready to send starts sensing the medium (Carrier Sense based on CCA, Clear Channel Assessment)

if the medium is free for the duration of an Inter-Frame Space (IFS), the station can start sending (IFS depends on service type)

if the medium is busy, the station has to wait for a free IFS, then the station must additionally wait a random back-off time (collision avoidance, multiple of slot-time)

if another station occupies the medium during the back-off time of the station, the back-off timer stops (fairness)

802.11 MAC: Competing Stations

IEEE 802.11 DCF Protocol

Sense media before transmission

If media is free, transmit if media stays idle for a fixed amount of time (DCF Interframe Space, DIFS)

Defer:

wait until end of current transmission, plus DIFS

apply random backoff procedure: pick number between 0 and 7, check whether medium is idle during each backoff slot

if media is busy, suspend backoff process at beginning of current slot

after media was idle for selected number of slots, transmit immediately

if this transmission results in collision, backoff again, doubling the backoff

Upon receipt of packet:

receiver waits short interval (Short Interframe Space, SIFS)

transmits acknowledgement frame (ACK) back to sender

If sender receives no ACK within ACKTimeout interval, assume collision

802.11 MAC: CSMA/CA

Sending unicast packets

station has to wait for DIFS before sending data

receivers acknowledge at once (after waiting for SIFS) if the packet was received correctly (CRC)

automatic retransmission of data packets in case of transmission errors

802.11 MAC: RTS/CTS

Sending unicast packets

station can send RTS with reservation parameter after waiting for DIFS (reservation determines amount of time the data packet needs the medium)

acknowledgement via CTS after SIFS by receiver (if ready to receive)

sender can now send data at once, acknowledgement via ACK

other stations store medium reservations distributed via RTS and CTS

IEEE 802.11: DCF and PCF Coexistence

802.11 MAC:
Point Coordination Function

802.11 MAC Frame Format

Types

control frames, management frames, data frames

Sequence numbers

important against duplicated frames due to lost ACKs

Addresses

receiver, transmitter (physical), BSS identifier, sender (logical)

Miscellaneous

sending time, checksum, frame control, data

MAC Address Format

802.11 MAC Management

Synchronization

try to find a LAN, try to stay within a LAN

timer etc.

Power management

sleep-mode without missing a message

periodic sleep, frame buffering, traffic measurements

Association/Reassociation

integration into a LAN

roaming, i.e. change networks by changing access points

scanning, i.e. active search for a network

MIB - Management Information Base

managing, read, write

Synchronization using a Beacon (Infrastructure Network)

Synchronization using a Beacon
(Ad-hoc Network)

Power Management

Idea: switch the transceiver off if not needed

States of a station: sleep and awake

Timing Synchronization Function (TSF)

stations wake up at the same time

Infrastructure

Traffic Indication Map (TIM)

list of unicast receivers transmitted by AP

Delivery Traffic Indication Map (DTIM)

list of broadcast/multicast receivers transmitted by AP

Ad-hoc

Ad-hoc Traffic Indication Map (ATIM)

announcement of receivers by stations buffering frames

more complicated - no central AP

collision of ATIMs possible (scalability?)

Power saving with wake-up patterns (Infrastructure Network)

Power saving with wake-up patterns
(Ad-hoc Network)

802.11 Roaming

No or bad connection? Then perform:

Scanning

scan the environment, i.e., listen into the medium for beacon signals or send probes into the medium and wait for an answer

Reassociation Request

station sends a request to one or several AP(s)

Reassociation Response

success: AP has answered, station can now participate

failure: continue scanning

AP accepts Reassociation Request

signal the new station to the distribution system

the distribution system updates its data base (i.e., location information)

typically, the distribution system now informs the old AP so it can release resources

Future Developments

IEEE 802.11a

compatible MAC, but now 5 GHz band

transmission rates up to 20 Mbit/s

close cooperation with BRAN (ETSI Broadband Radio Access Network)

IEEE 802.11b

higher data rates at 2.4 GHz

proprietary solutions already offer 10 Mbit/s

IEEE WPAN (Wireless Personal Area Networks)

market potential

compatibility

low cost/power, small form factor

technical/economic feasibility
è Bluetooth

Bluetooth: “Personal Area Networks”

open specification for wireless communication of data and voice

based on a low-cost short-range radio link, built into a 9 x 9 mm microchip (design goal: cost of US$ 5/device)

facilitates protected ad hoc connections for stationary and mobile communication environments

Bluetooth is a cooperation between computer and telecommunication industries (Ericsson, IBM, Toshiba, Intel, Nokia, …)

SIG started in February 1998 with above five members, has grown since (64 companies joined in January 1999 alone)

Bluetooth Vision

Bluetooth General Characteristics

operates in the 2.4 GHz Industrial-Scientific-Medical (ISM) band

nominal link range: 10 cm to 10 m, can be increased to 100 m (transmitting with more power)

uses Frequence Hop (FH) spread spectrum

supports up to 8 devices in a piconet (two or more Bluetooth units sharing a channel)

built-in security

non line-of-sight transmission through walls and briefcases (distinguishes it from IrDA)

omni-directional

supports both isochronous and asynchronous services; easy integration of TCP/IP for networking

Bluetooth Intended Uses

connect a wide range of computing and telecommunications devices without the need to buy, carry, or connect cables

delivers opportunities for rapid, ad hoc connections, and in the future, possibly for automatic, unconscious, connections between devices

power-efficient radio technology can be used in many of the same devices that use IR:

Phones and pagers

Modems

LAN access devices

Headsets

Notebook, desktop, and handheld computers

Bluetooth Radio

frequency hopping in 79 hops displaced by 1 MHz, starting at 2.402 GHz and stopping at 2.480 GHz

to function on a worldwide basis, Bluetooth requires a radio frequency that is license-free and open to any radio

2.45 GHz ISM band satisfies these requirements, although it must cope with interference from baby monitors, garage door openers, cordless phones and microwave ovens, which also use this frequency.

due to local regulations the bandwidth is reduced in Japan, France and Spain. This is handled by an internal software switch

the maximum frequency hopping rate is 1600 hops/s.

Bluetooth Frequency Hopping/Time Division Duplex Scheme

Bluetooth MAC protocol

Time Division Duplex (TDD) scheme for full-duplex transmissions

master device establishes connection, slave devices synchronize their clock with master clock for duration of connection

Synchronous Connection Oriented (SCO) type (used primarily for voice)

channel symmetric, only data packets retransmitted

Asynchronous Connectionless (ACL) type (used primarily for packet data)

master unit controls the link bandwidth and decides how much piconet bandwidth is given to each slave, and the symmetry of the traffic

slaves must be polled before they can transmit data.

The ACL link also supports broadcast messages from the master to all slaves in the piconet

Bluetooth MAC Protocol

Error correction:

1/3 rate forward error correction code (FEC)

for SCO only

2/3 rate forward error correction code FEC

Automatic repeat request (ARQ) scheme for data

data transmitted in one slot is directly acknowledged by the recipient in the next slot.

Authentication and Privacy

one-way, two-way, or no authentication possible

use stream cipher based on secret keys (0, 40, 64 bits)

key management left to higher layer software

if stronger protection (longer key is needed), use better encryption at network and/or application level

Bluetooth Data Rates

Bluetooth PicoNet Example

Bluetooth Power States

Bluetooth Scatternets

Bluetooth Scatternets

Multiple overlapping piconets (sets of communicating devices) with own hopping sequence, max of one master and 8 slaves

Collisions do occur when two piconets use same frequency at the same time

as more piconets overlap, performance degrades

degradation gradual: 10 overlapping piconets reduce aggregate bandwidth by 10%

Single device can participate in multiple piconets, though only one at a time

need to re-adjust clock to re-sync with master when entering a piconet

inform master when device leaves piconet, will suppress data being sent/device being polled

Bluetooth Scatternet

HIPERLAN

ETSI standard

European standard, cf. GSM, DECT, ...

Enhancement of local Networks and interworking with fixed networks

integration of time-sensitive services from the early beginning

HIPERLAN family

one standard cannot satisfy all requirements

range, bandwidth, QoS support

commercial constraints

HIPERLAN 1 standardized since 1996

Original HIPERLAN Protocol Family

HIPERLAN 1 Characteristics

Data transmission

point-to-point, point-to-multipoint, connectionless

23.5 Mbit/s, 1 W power, 2383 byte max. packet size

Services

asynchronous and time-bounded services with hierarchical priorities

compatible with ISO MAC

Topology

infrastructure or ad-hoc networks

transmission range can be larger then coverage of a single node („forwarding“ integrated in mobile terminals)

Further mechanisms

power saving, encryption, checksums

HIPERLAN 1 Services and Protocols

CAC service

definition of communication services over a shared medium

specification of access priorities

abstraction of media characteristics

MAC protocol

MAC service, compatible with ISO MAC and ISO MAC bridges

uses HIPERLAN CAC

CAC protocol

provides a CAC service, uses the PHY layer, specifies hierarchical access mechanisms for one or several channels

Physical protocol

send and receive mechanisms, synchronization, FEC, modulation, signal strength

HIPERLAN 1 Physical Layer

Scope

modulation, demodulation, bit and frame synchronization

forward error correction mechanisms

measurements of signal strength

channel sensing

Channels

3 mandatory and 2 optional channels (with their carrier frequencies)

mandatory

channel 0: 5.1764680 GHz

channel 1: 5.1999974 GHz

channel 2: 5.2235268 GHz

optional (not allowed in all countries)

channel 3: 5.2470562 GHz

channel 4: 5.2705856 GHz

HIPERLAN 1 Physical Layer Frames

Maintaining a high data-rate (23.5 Mbit/s) is power consuming - problematic for mobile terminals

packet header with low bit-rate comprising receiver information

only receiver(s) address by a packet continue receiving

Frame structure

LBR (Low Bit-Rate) header with 1.4 Mbit/s

450 bit synchronization

minimum 1, maximum 47 frames with 496 bit each

for higher velocities of the mobile terminal (> 1.4 m/s) the maximum number of frames has to be reduced

Modulation

GMSK for high bit-rate, FSK for LBR header

HIPERLAN 1 CAC Sublayer

Channel Access Control (CAC)

assure that terminal does not access forbidden channels

priority scheme, access with EY-NPMA

Priorities

5 priority levels for QoS support

QoS is mapped onto a priority level with the help of the packet lifetime (set by an application)

if packet lifetime = 0 it makes no sense to forward the packet to the receiver any longer

standard start value 500ms, maximum 16000ms

if a terminal cannot send the packet due to its current priority, waiting time is permanently subtracted from lifetime

based on packet lifetime, waiting time in a sender and number of hops to the receiver, the packet is assigned to one out of five priorities

the priority of waiting packets, therefore, rises automatically

HIPERLAN 1 EY-NPMA

EY-NPMA (Elimination Yield Non-preemptive Priority Multiple Access)

3 phases: priority resolution, contention resolution, transmission

finding the highest priority

every priority corresponds to a time-slot to send in the first phase, the higher the priority the earlier the time-slot to send

higher priorities can not be preempted

if an earlier time-slot for a higher priority remains empty, stations with the next lower priority might send

after this first phase the highest current priority has been determined

HIPERLAN 1 EY-NPMA

Several terminals can now have the same priority and wish to send

contention phase

Elimination Burst: all remaining terminals send a burst to eliminate contenders (11111010100010011100000110010110, high bit- rate)

Elimination Survival Verification: contenders now sense the channel, if the channel is free they can continue, otherwise they have been eliminated

Yield Listening: contenders again listen in slots with a nonzero probability, if the terminal senses its slot idle it is free to transmit at the end of the contention phase

the important part is now to set the parameters for burst duration and channel sensing (slot-based, exponentially distributed)

data transmission

the winner can now send its data (however, a small chance of collision remains)

if the channel was idle for a longer time (min. for a duration of 1700 bit) a terminal can send at once without using EY-NPMA

synchronization using the last data transmission

HIPERLAN 1 MAC Layer

Compatible to ISO MAC

Supports time-bounded services via a priority scheme

Packet forwarding

support of directed (point-to-point) forwarding and broadcast forwarding (if no path information is available)

support of QoS while forwarding

Encryption mechanisms

mechanisms integrated, but without key management

Power conservation mechanisms

mobile terminals can agree upon awake patterns (e.g., periodic wake-ups to receive data)

additionally, some nodes in the networks must be able to buffer data for sleeping terminals and to forward them at the right time (so called stores)

Information Bases

Route Information Base (RIB) - how to reach a destination

[destination, next hop, distance]

Neighbor Information Base (NIB) - status of direct neighbors

[neighbor, status]

Hello Information Base (HIB) - status of destination (via next hop)

[destination, status, next hop]

Alias Information Base (AIB) - address of nodes outside the net

[original MSAP address, alias MSAP address]

Source Multipoint Relay Information Base (SMRIB) - current MP status

[local multipoint forwarder, multipoint relay set]

Topology Information Base (TIB) - current HIPERLAN topology

[destination, forwarder, sequence]

Duplicate Detection Information Base (DDIB) - remove duplicates

[source, sequence]

Ad-hoc Networks using HIPERLAN 1


	Introduction and History
	Data in Wireless Cellular Systems
	Data in Wireless Local Area Networks
	Internet Protocols
	Routing and Ad-Hoc Networks
	TCP over Wireless Link
	Services and Service Discovery
	System Support for Mobile Applications


	“Traditional” LANs: WaveLan, Proxim, IEEE 802.11
	More specific “personal” LANs, also called “Personal Area Networks”: Bluetooth, IEEE 802.15
	High-speed wireless LANs (approaching ATM data rates): HiperLAN


	Commercial product, developed by Lucent Technologies (de-facto market leader), available since early 1990s
	Development closely aligned with early IEEE 802.11 standard effort, first product generation differs in key aspects from final IEEE 802.11 standard
	Will discuss WaveLAN version 1, newer version 2 follows finally approved 802.11 standard (plus enhancements to allow for data rates up to 10 Mbps)
	Physical layer:
		915 MHz (902-928 MHz) or 2.4 GHz (2.412-2.475 GHz) ISM band
		Bands are divided into frequency channels of 26 MHz each
		Modulation scheme: DSSS, raw data rate of 2 Mbps


	Basically CSMA/CA
	Sense media before transmission, if free, transmit
	Defer:
		wait until end of current transmission, plus fixed delay (WaveLAN InterFrame Space, WIFS) of 60 msec
		apply random backoff procedure: pick number between 0 and 31 (antenna slot number S)
		each slot corresponds to 23 ms
		wait S*23 ms, sense media again, if busy, double backoff range until we reach range 0-255
		drop packet after 15 attempts
	If station has more than one packet to send: wait WIFS plus backoff-period in range 0-15 (avoid monopolization)


	Standard for wireless local area networks, approved by IEEE in 1997
	Scope: physical layer (PHY) and media access control sublayer (MAC) for wireless connectivity for fixed, portable, and moving stations with a local area
	Supports data rates of 1 or 2 Mbps, using infrared or radio
	Supports two basic architectures: independent basic support set (IBSS) and infrastructure networks
	Most recent commercial products (including the new WaveLAN generation) are compatible with 802.11


	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 (EES: Extended Service Set) based on several BSS


	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


	PLCP Physical Layer Convergence Protocol
		clear channel assessment signal (carrier sense)
	PMD Physical Medium Dependent
		modulation, coding
	PHY Management
		channel selection, MIB
	Station Management
		coordination of all management functions
	MAC
		access mechanisms, fragmentation, encryption
	MAC Management
		synchronization, roaming, MIB, power management


	3 versions: 2 radio (typ. 2.4 GHz), 1 IR
		data rates 1 or 2 Mbit/s
	FHSS (Frequency Hopping Spread Spectrum)
		spreading, despreading, signal strength, typ. 1 Mbit/s
		min. 2.5 frequency hops/s (USA), two-level GFSK modulation
	DSSS (Direct Sequence Spread Spectrum)
		DBPSK modulation for 1 Mbit/s (Differential Binary Phase Shift Keying), DQPSK for 2 Mbit/s (Differential Quadrature PSK)
		preamble and header of a frame is always transmitted with 1 Mbit/s, rest of transmission 1 or 2 Mbit/s
		chipping sequence: +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1 (Barker code)
		max. radiated power 1 W (USA), 100 mW (EU), min. 1mW
	Infrared
		850-950 nm, diffuse light, typ. 10 m range
		carrier detection, energy detection, synchonization


Traffic services
	Asynchronous Data Service (mandatory)
		exchange of data packets based on “best-effort”
		support of broadcast and multicast
	Time-Bounded Service (optional)
		implemented using PCF (Point Coordination Function)
Access methods
	DCF CSMA/CA (mandatory)
		collision avoidance via randomized „back-off“ mechanism
		minimum distance between consecutive packets
		ACK packet for acknowledgements (not for broadcasts)
	DCF w/ RTS/CTS (optional)
		Distributed Foundation Wireless MAC (DFWMAC)
		avoids hidden terminal problem
	PCF (optional)
		access point polls terminals according to a list


Priorities
	defined through different inter frame spaces
	no guaranteed, hard priorities
	SIFS (Short Inter Frame Spacing)
		highest priority, for ACK, CTS, polling response
	PIFS (PCF IFS)
		medium priority, for time-bounded service using PCF
	DIFS (DCF, Distributed Coordination Function IFS)
		lowest priority, for asynchronous data service


		station ready to send starts sensing the medium (Carrier Sense based on CCA, Clear Channel Assessment)
		if the medium is free for the duration of an Inter-Frame Space (IFS), the station can start sending (IFS depends on service type)
		if the medium is busy, the station has to wait for a free IFS, then the station must additionally wait a random back-off time (collision avoidance, multiple of slot-time)
		if another station occupies the medium during the back-off time of the station, the back-off timer stops (fairness)


	Sense media before transmission
	If media is free, transmit if media stays idle for a fixed amount of time (DCF Interframe Space, DIFS)
	Defer:
		wait until end of current transmission, plus DIFS
		apply random backoff procedure: pick number between 0 and 7, check whether medium is idle during each backoff slot
		if media is busy, suspend backoff process at beginning of current slot
		after media was idle for selected number of slots, transmit immediately
		if this transmission results in collision, backoff again, doubling the backoff
	Upon receipt of packet:
		receiver waits short interval (Short Interframe Space, SIFS)
		transmits acknowledgement frame (ACK) back to sender
	If sender receives no ACK within ACKTimeout interval, assume collision


	Sending unicast packets
		station has to wait for DIFS before sending data
		receivers acknowledge at once (after waiting for SIFS) if the packet was received correctly (CRC)
		automatic retransmission of data packets in case of transmission errors


	Sending unicast packets
		station can send RTS with reservation parameter after waiting for DIFS (reservation determines amount of time the data packet needs the medium)
		acknowledgement via CTS after SIFS by receiver (if ready to receive)
		sender can now send data at once, acknowledgement via ACK
		other stations store medium reservations distributed via RTS and CTS


	Types
		control frames, management frames, data frames
	Sequence numbers
		important against duplicated frames due to lost ACKs
	Addresses
		receiver, transmitter (physical), BSS identifier, sender (logical)
	Miscellaneous
		sending time, checksum, frame control, data


	Synchronization
		try to find a LAN, try to stay within a LAN
		timer etc.
	Power management
		sleep-mode without missing a message
		periodic sleep, frame buffering, traffic measurements
	Association/Reassociation
		integration into a LAN
		roaming, i.e. change networks by changing access points
		scanning, i.e. active search for a network
	MIB - Management Information Base
		managing, read, write


Idea: switch the transceiver off if not needed
States of a station: sleep and awake
Timing Synchronization Function (TSF)
	stations wake up at the same time
Infrastructure
	Traffic Indication Map (TIM)
		list of unicast receivers transmitted by AP
	Delivery Traffic Indication Map (DTIM)
		list of broadcast/multicast receivers transmitted by AP
Ad-hoc
	Ad-hoc Traffic Indication Map (ATIM)
		announcement of receivers by stations buffering frames
		more complicated - no central AP
		collision of ATIMs possible (scalability?)


	No or bad connection? Then perform:
	Scanning
		scan the environment, i.e., listen into the medium for beacon signals or send probes into the medium and wait for an answer
	Reassociation Request
		station sends a request to one or several AP(s)
	Reassociation Response
		success: AP has answered, station can now participate
		failure: continue scanning
	AP accepts Reassociation Request
		signal the new station to the distribution system
		the distribution system updates its data base (i.e., location information)
		typically, the distribution system now informs the old AP so it can release resources


	IEEE 802.11a
		compatible MAC, but now 5 GHz band
		transmission rates up to 20 Mbit/s
		close cooperation with BRAN (ETSI Broadband Radio Access Network)
	IEEE 802.11b
		higher data rates at 2.4 GHz
		proprietary solutions already offer 10 Mbit/s
	IEEE WPAN (Wireless Personal Area Networks)
		market potential
		compatibility
		low cost/power, small form factor
		technical/economic feasibility è Bluetooth


	open specification for wireless communication of data and voice
	based on a low-cost short-range radio link, built into a 9 x 9 mm microchip (design goal: cost of US$ 5/device)
	facilitates protected ad hoc connections for stationary and mobile communication environments
	Bluetooth is a cooperation between computer and telecommunication industries (Ericsson, IBM, Toshiba, Intel, Nokia, …)
	SIG started in February 1998 with above five members, has grown since (64 companies joined in January 1999 alone)


	operates in the 2.4 GHz Industrial-Scientific-Medical (ISM) band
		nominal link range: 10 cm to 10 m, can be increased to 100 m (transmitting with more power)
	uses Frequence Hop (FH) spread spectrum
	supports up to 8 devices in a piconet (two or more Bluetooth units sharing a channel)
	built-in security
	non line-of-sight transmission through walls and briefcases (distinguishes it from IrDA)
	omni-directional
	supports both isochronous and asynchronous services; easy integration of TCP/IP for networking


	connect a wide range of computing and telecommunications devices without the need to buy, carry, or connect cables
	delivers opportunities for rapid, ad hoc connections, and in the future, possibly for automatic, unconscious, connections between devices
	power-efficient radio technology can be used in many of the same devices that use IR:
		Phones and pagers
		Modems
		LAN access devices
		Headsets
		Notebook, desktop, and handheld computers


	frequency hopping in 79 hops displaced by 1 MHz, starting at 2.402 GHz and stopping at 2.480 GHz
		to function on a worldwide basis, Bluetooth requires a radio frequency that is license-free and open to any radio
		2.45 GHz ISM band satisfies these requirements, although it must cope with interference from baby monitors, garage door openers, cordless phones and microwave ovens, which also use this frequency.
	due to local regulations the bandwidth is reduced in Japan, France and Spain. This is handled by an internal software switch
	the maximum frequency hopping rate is 1600 hops/s.


	Time Division Duplex (TDD) scheme for full-duplex transmissions
		master device establishes connection, slave devices synchronize their clock with master clock for duration of connection
	Synchronous Connection Oriented (SCO) type (used primarily for voice)
		channel symmetric, only data packets retransmitted
	Asynchronous Connectionless (ACL) type (used primarily for packet data)
		master unit controls the link bandwidth and decides how much piconet bandwidth is given to each slave, and the symmetry of the traffic
		slaves must be polled before they can transmit data.
		The ACL link also supports broadcast messages from the master to all slaves in the piconet


Error correction:
	1/3 rate forward error correction code (FEC)
		for SCO only
	2/3 rate forward error correction code FEC
	Automatic repeat request (ARQ) scheme for data
		data transmitted in one slot is directly acknowledged by the recipient in the next slot.
Authentication and Privacy
	one-way, two-way, or no authentication possible
	use stream cipher based on secret keys (0, 40, 64 bits)
	key management left to higher layer software
	if stronger protection (longer key is needed), use better encryption at network and/or application level


	Multiple overlapping piconets (sets of communicating devices) with own hopping sequence, max of one master and 8 slaves
	Collisions do occur when two piconets use same frequency at the same time
		as more piconets overlap, performance degrades
		degradation gradual: 10 overlapping piconets reduce aggregate bandwidth by 10%
	Single device can participate in multiple piconets, though only one at a time
		need to re-adjust clock to re-sync with master when entering a piconet
		inform master when device leaves piconet, will suppress data being sent/device being polled


ETSI standard
	European standard, cf. GSM, DECT, ...
	Enhancement of local Networks and interworking with fixed networks
	integration of time-sensitive services from the early beginning
HIPERLAN family
	one standard cannot satisfy all requirements
		range, bandwidth, QoS support
		commercial constraints
	HIPERLAN 1 standardized since 1996


	Data transmission
		point-to-point, point-to-multipoint, connectionless
		23.5 Mbit/s, 1 W power, 2383 byte max. packet size
	Services
		asynchronous and time-bounded services with hierarchical priorities
		compatible with ISO MAC
	Topology
		infrastructure or ad-hoc networks
		transmission range can be larger then coverage of a single node („forwarding“ integrated in mobile terminals)
	Further mechanisms
		power saving, encryption, checksums


	CAC service
		definition of communication services over a shared medium
		specification of access priorities
		abstraction of media characteristics
	MAC protocol
		MAC service, compatible with ISO MAC and ISO MAC bridges
		uses HIPERLAN CAC
	CAC protocol
		provides a CAC service, uses the PHY layer, specifies hierarchical access mechanisms for one or several channels
	Physical protocol
		send and receive mechanisms, synchronization, FEC, modulation, signal strength