Course Overview

	Introduction
	Data in Wireless Cellular Systems
	Data in Wireless Local Area Networks
	Internet Protocols
	TCP over Wireless Link
	Ad-Hoc Networks, Sensor Networks
	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

Characteristics of Wireless LANs

	Advantages
		very flexible within the reception area
		Ad-hoc networks without previous planning possible
		(almost) no wiring difficulties (e.g. historic buildings, firewalls)
		more robust against disasters like, e.g., earthquakes, fire - or users pulling a plug...
	Disadvantages
		typically very low bandwidth compared to wired networks (1-10 Mbit/s)
		many proprietary solutions, especially for higher bit-rates, standards take their time (e.g. IEEE 802.11)
		products have to follow many national restrictions if working wireless, it takes a vary long time to establish global solutions like, e.g., IMT-2000

Design Goals for Wireless LANs

	global, seamless operation
	low power for battery use
	no special permissions or licenses needed to use the LAN
	robust transmission technology
	simplified spontaneous cooperation at meetings
	easy to use for everyone, simple management
	protection of investment in wired networks
	security (no one should be able to read my data), privacy (no one should be able to collect user profiles), safety (low radiation)
	transparency concerning applications and higher layer protocols, but also location awareness if necessary

Infrared vs. Radio Transmission

	Infrared
		uses IR diodes, diffuse light, multiple reflections (walls, furniture etc.)
	Advantages
		simple, cheap, available in many mobile devices
		no licenses needed
		simple shielding possible
	Disadvantages
		interference by sunlight, heat sources etc.
		many things shield or absorb IR light
		low bandwidth
	Example
		IrDA (Infrared Data Association) interface available everywhere
	Radio
		typically using the license free ISM band at 2.4 GHz
	Advantages
		experience from wireless WAN and mobile phones can be used
		coverage of larger areas possible (radio can penetrate walls, furniture etc.)
	Disadvantages
		very limited license free frequency bands
		shielding more difficult, interference with other electrical devices
	Example
		WaveLAN, HIPERLAN, Bluetooth

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:
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)

IEEE TSF: Does not scale!

Alternative Solution: CSMNS

Comparison TSF and CSMNS

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

WLAN: IEEE 802.11b

	Data rate
		1, 2, 5.5, 11 Mbit/s, depending on SNR
		User data rate max. approx. 6 Mbit/s
	Transmission range
		300m outdoor, 30m indoor
		Max. data rate ~10m indoor
	Frequency
		Free 2.4 GHz ISM-band
	Security
		Limited, WEP insecure, SSID
	Cost
		$100 adapter, $250 base station, dropping
	Availability
		Many products, many vendors
	Connection set-up time
		Connectionless/always on
	Quality of Service
		Typ. Best effort, no guarantees (unless polling is used, limited support in products)
	Manageability
		Limited (no automated key distribution, sym. Encryption)
	Special Advantages/Disadvantages
		Advantage: many installed systems, lot of experience, available worldwide, free ISM-band, many vendors, integrated in laptops, simple system
		Disadvantage: heavy interference on ISM-band, no service guarantees, slow relative speed only

Channel Selection (Non-Overlapping)

WLAN: IEEE 802.11a

	Data rate
		6, 9, 12, 18, 24, 36, 48, 54 Mbit/s, depending on SNR
		User throughput (1500 byte packets): 5.3 (6), 18 (24), 24 (36), 32 (54)
		6, 12, 24 Mbit/s mandatory
	Transmission range
		100m outdoor, 10m indoor
			E.g., 54 Mbit/s up to 5 m, 48 up to 12 m, 36 up to 25 m, 24 up to 30m, 18 up to 40 m, 12 up to 60 m
	Frequency
		Free 5.15-5.25, 5.25-5.35, 5.725-5.825 GHz ISM-band
	Security
		Limited, WEP insecure, SSID
	Cost
		$280 adapter, $500 base station
	Availability
		Some products, some vendors
	Connection set-up time
		Connectionless/always on
	Quality of Service
		Typ. best effort, no guarantees (same as all 802.11 products)
	Manageability
		Limited (no automated key distribution, sym. Encryption)
	Special Advantages/Disadvantages
		Advantage: fits into 802.x standards, free ISM-band, available, simple system, uses less crowded 5 GHz band
		Disadvantage: stronger shading due to higher frequency, no QoS

WLAN: IEEE 802.11 – Future Developments

	802.11d: Regulatory Domain Update
	802.11e: MAC Enhancements – QoS
		Enhance the current 802.11 MAC to expand support for applications with Quality of Service requirements, and in the capabilities and efficiency of the protocol.
	802.11f: Inter-Access Point Protocol
		Establish an Inter-Access Point Protocol for data exchange via the distribution system.
	802.11g: Data Rates > 20 Mbit/s at 2.4 GHz; 54 Mbit/s, OFDM –802.11h: Spectrum Managed 802.11a (DCS, TPC)
	802.11i: Enhanced Security Mechanisms
		Enhance the current 802.11 MAC to provide improvements in security.
	Study Groups
		5 GHz (harmonization ETSI/IEEE)
		Radio Resource Measurements
		High Throughput

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 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

	History
		1994: Ericsson (Mattison/Haartsen), “MC-link” project
		Renaming of the project: Bluetooth according to Harald “Blåtand” Gormsen [son of Gorm], King of Denmark in the 10th century
		1998: foundation of Bluetooth SIG, www.bluetooth.org
		1999: erection of a rune stone at Ericsson/Lund ;-)
		2001: first consumer products for mass market, spec. version 1.1 released
	Special Interest Group
		Original founding members: Ericsson, Intel, IBM, Nokia, Toshiba
		Added promoters: 3Com, Agere (was: Lucent), Microsoft, Motorola
		> 2500 members
		Common specification and certification of products

History and hi-tech…

…and the real rune stone

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

Piconet

	Collection of devices connected in an ad hoc fashion
	One unit acts as master and the others as slaves for the lifetime of the piconet
	Master determines hopping pattern, slaves have to synchronize
	Each piconet has a unique hopping pattern
	Participation in a piconet = synchronization to hopping sequence
	Each piconet has one master and up to 7 simultaneous slaves (> 200 could be parked)

Forming a Piconet

	All devices in a piconet hop together
		Master gives slaves its clock and device ID
			Hopping pattern: determined by device ID (48 bit, unique worldwide)
			Phase in hopping pattern determined by clock
	Addressing
		Active Member Address (AMA, 3 bit)
		Parked Member Address (PMA, 8 bit)

Bluetooth PicoNet Example

Baseband States of a Bluetooth Device

Bluetooth Power States

Bluetooth Scatternets

Scatternet

	Linking of multiple co-located piconets through the sharing of common master or slave devices
		Devices can be slave in one piconet and master of another
	Communication between piconets
		Devices jumping back and forth between the piconets

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

Security

SDP – Service Discovery Protocol

	Inquiry/response protocol for discovering services
		Searching for and browsing services in radio proximity
		Adapted to the highly dynamic environment
		Can be complemented by others like SLP, Jini, Salutation, …
		Defines discovery only, not the usage of services
		Caching of discovered services
		Gradual discovery
	Service record format
		Information about services provided by attributes
		Attributes are composed of an 16 bit ID (name) and a value
		values may be derived from 128 bit Universally Unique Identifiers (UUID)

WPAN: IEEE 802.15-1 – Bluetooth

	Data rate
		Synchronous, connection-oriented: 64 kbit/s
		Asynchronous, connectionless
			433.9 kbit/s symmetric
			723.2 / 57.6 kbit/s asymmetric
	Transmission range
		POS (Personal Operating Space) up to 10 m
		with special transceivers up to 100 m
	Frequency
		Free 2.4 GHz ISM-band
	Security
		Challenge/response (SAFER+), hopping sequence
	Cost
		$50 adapter, drop to $5 if integrated
	Availability
		Integrated into some products, several vendors
	Connection set-up time
		Depends on power-mode
		Max. 2.56s, avg. 0.64s
	Quality of Service
		Guarantees, ARQ/FEC
	Manageability
		Public/private keys needed, key management not specified, simple system integration
	Special Advantages/Disadvantages
		Advantage: already integrated into several products, available worldwide, free ISM-band, several vendors, simple system, simple ad-hoc networking, peer to peer, scatternets
		Disadvantage: interference on ISM-band, limited range, max. 8 devices/network&master, high set-up latency

WPAN: IEEE 802.15 – Future Developments 1

	802.15-2: Coexistance
		Coexistence of Wireless Personal Area Networks (802.15) and Wireless Local Area Networks (802.11), quantify the mutual interference
	802.15-3: High-Rate
		Standard for high-rate (20Mbit/s or greater) WPANs, while still low-power/low-cost
		Data Rates: 11, 22, 33, 44, 55 Mbit/s
		Quality of Service isochronous protocol
		Ad hoc peer-to-peer networking
		Security
		Low power consumption
		Low cost
		Designed to meet the demanding requirements of portable consumer imaging and multimedia applications

WPAN: IEEE 802.15 – Future Developments 2

	802.15-4: Low-Rate, Very Low-Power
		Low data rate solution with multi-month to multi-year battery life and very low complexity
		Potential applications are sensors, interactive toys, smart badges, remote controls, and home automation
		Data rates of 20-250 kbit/s, latency down to 15 ms
		Master-Slave or Peer-to-Peer operation
		Support for critical latency devices, such as joysticks
		CSMA/CA channel access (data centric), slotted (beacon) or unslotted
		Automatic network establishment by the PAN coordinator
		Dynamic device addressing, flexible addressing format
		Fully handshaked protocol for transfer reliability
		Power management to ensure low power consumption
		16 channels in the 2.4 GHz ISM band, 10 channels in the 915 MHz US ISM band and one channel in the European 868 MHz band