Transcript Wireless Systems: Where are we heading?

Wireless Systems: Where are we
heading?
Outline
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Some definitions
Current situation
Near Future
4G: what we really want
What are the obstacles
Higher Layer Issues
Conclusions
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Definitions
• Definition of mobility:
– user mobility: users communicate anytime, anywhere, with
anyone
– device portability: devices can be connected anytime,
anywhere to the network
• Definition of wireless:
– Un-tethered, no physical wire attachment
• The need for mobility creates the need for integration of
wireless networks into existing fixed network
environments:
– local area networks: standardization of IEEE 802.11
– Internet: Mobile IP extension of the internet protocol IP
– wide area networks: e.g., internetworking of 3G and IP
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Current Situation
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Technological trends
Issues in Wireless Systems
Wireless vs Fixed
Wireless LANS
Wireless PANs
Cellular
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Technological Trends
• Advances in Technology
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more computing power in smaller devices
flat, lightweight displays with low power consumption
user interfaces suitable for small dimensions
higher bandwidths
multiple wireless interfaces: wireless LANs, wireless WANs,
home RF, wireless PANs
• New Electronic Computing Devices
– small, cheap, portable, replaceable and most important of all
USABLE!
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Sample Future Application: Vehicles
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transmission of news, road conditions, weather
personal communication using cellular
position identification via GPS
inter vehicle communications for accident prevention
vehicle and road inter communications for traffic
control, signaling, data gathering
• ambulances, police, etc.: early transmission of patient
data to the hospital, situation reporting
• entertainment: music, video
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An Integrated View
GSM, 3G, WLAN,
Bluetooth, ...
PDA, laptop, cellular phones,
GPS, sensors
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Constraints of Portable Devices
• Power consumption
– battery capacity -> limited computing power, low
quality/smaller displays, smaller disks, fewer options (I/O,
CD/DVD)
• Device vulnerability
– more rugged design required to withstand bumps, weather
conditions, etc.
– theft
• Limited Capabilities
– Small display size due to size and power
– compromise between comfort/usability and portability (e.g.,
keyboard size)
– integration of character/voice recognition, abstract symbols
– memory limited by size and power
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Wireless vs Fixed
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Higher loss-rates due to interference
– other EM signals, objects in path (multi-path, scattering)
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Limited availability of useful spectrum
– frequencies have to be coordinated
– lower transmission rates
• local area: 2 – 11 Mbit/s, -> 50 - 70Mbit/s
• wide area: 9.6 – 19.2 kbit/s -> 384 - 2000Kbit/s
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Higher delays, higher jitter
– connection setup time for cellular in the second range, several hundred
milliseconds for wireless LAN systems
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Lower security, simpler active attacking
– radio interface accessible for everyone
– base station can be simulated, thus attracting calls from mobile phones
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Always shared medium
– secure access mechanisms important
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Wireless LANs: Design Goals
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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
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Wireless LANs: Standards
• 802.11 (2M) -> 802.11b (11M) -> 802.11a (5070M)
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Wider spectrum -> Higher bitrates
Generally used with access points
Adhoc component not used, has flaws
Poor support for real-time communications
• HiperLAN
– European standard for high bit rate (~25M) local
transmission in 5GHz range over 50-300m
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Infrastructure vs Adhoc
infrastructure
network
AP
AP
AP: Access Point
wired network
AP
ad-hoc network
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IEEE 802.11 MAC
• Traffic services
– Asynchronous Data Service (mandatory)
– Time-Bounded Service (optional)
• Access methods: Distributed Foundation Wireless MAC
(DFWMAC)
– DFWMAC-DCF CSMA/CA (mandatory)
• collision avoidance via randomized „back-off“ mechanism
• minimum distance between consecutive packets
• ACK packet for acknowledgements (not for broadcasts)
– DFWMAC-DCF w/ RTS/CTS (optional)
• avoids hidden terminal problem
– DFWMAC- PCF (optional)
• access point polls terminals according to a list
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MAC Operation
• 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, Point Coordination Function IFS)
• medium priority, for time-bounded service using PCF
– DIFS (DCF, Distributed Coordination Function IFS)
• lowest priority, for asynchronous data service
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Interframe Spacings
DIFS
DIFS
medium busy
PIFS
SIFS
contention
next frame
t
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Wireless PANs
• Bluetooth
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Bluetooth
• Low bitrate (1M), short distances (110m) in 2.4GHz ISM band
• Adhoc networking, cable and IrDA
replacement
• No mobility
• Next generation higher bit rate (10M),
longer distances (100m)
• Scatternets: multihop environment
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Usage Scenarios
• Cable replacement
• Adhoc PAN
Cable
Replacement
Personal Ad-hoc
Networks
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Technology
• Low-cost,
• Low-power,
• Small-sized,
• Short-range,
• Robust wireless technology
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Cellular Systems:
• The essential elements of a cellular
system are:
– Low power transmitter and small coverage areas
called cells
– Spectrum (frequency) re-use
– Handoff
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Cells (1/2)
• Space Division Multiplexing (SDM): base station
covers a certain transmission area (cell)
• Mobile stations communicate only via the base
station
• Advantages of cell structures:
– higher capacity due to frequency re-use -> higher
number of users
– less transmission power needed
– more robust, decentralized
– base station deals with interference, transmission
power, etc., locally
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Cells (2/2)
• Problems:
– fixed network needed for the base stations
– handoffs (changing from one cell to another)
necessary
– interference with other cells
• Cell sizes range from 100 m in dense urban
areas to, e.g., 35 km in rural areas
• Cells sizes drop for higher frequencies as
propagation loss increases
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Multiplexing Techniques
• Multiplexing techniques are used to allow
many users to share a common transmission
resource.
• In the cellular case the users are mobile and
the transmission resource is the radio
spectrum.
• Sharing a common resource requires an
access mechanism that will control the
associated multiplexing mechanism.
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Media Access Comparison Chart
SDMA
Approach
segment space into
cells/sectors
Terminals
only one te rmi nal can
be act ive in one
cell/one secto r
Signal
separation
cell structu re, d irected
antennas
Advantages very simple, increases
capac ity pe r km
Disadvantages
inflexible, antennas
typically fixed
Comment
only in co mbination
w ith TDM A, FDM A or
CDM A useful
TDM A
FDM A
CDM A
segment send ing
time into d isjoint
time-slots, de mand
driven o r fixed
patte rns
all termi nals are
active for short
periods of t ime on
the sa me frequency
synchronization in
the t ime do main
segment the
frequency band into
disjoint sub -bands
spread the spect rum
using o rthogona l codes
every te rmi nal has its
own frequency,
uninterrupted
all termi nals can be act ive
at the sa me place at the
same moment,
uninterrupted
code p lus spec ial
receivers
estab lished, fu lly
digital, flexible
simple, estab lished,
robust
guard space
needed (multipath
propagat ion),
synchronization
difficult
used in con junct ion
w ith FDM A/SDM A
in many mobile
networks
inflexible,
frequenc ies a re a
scarce resou rce
filtering in the
frequency do main
typically combined
w ith TDM Auand
SDM A (frequency
reuse )
flexible, less frequency
planning needed, soft
handove r
complex receivers, needs
more co mplicated po wer
cont rol for sende rs
gene rally integ rated w ith
TDM A/FDM A
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CDMA: Overview
• Each channel has a unique code
(not necessarily orthogonal)
• All channels use the same spectrum at the same time
• Advantages:
– bandwidth efficient
– no coordination and synchronization necessary
– good protection against interference and tapping
• Disadvantages:
– lower user data rates due to high gains required to
reduce interference
– more complex signal regeneration
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CDMA: Illustration
k1
k2
k3
k4
k5
k6
c
f
t
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CDMA C/Cs (1/2)
• A CDMA system can be either code limited or interference
limited.
• For an interference limited system, every user has a code,
but only uses it when active, this is referred to as a soft
capacity system. The more users active in the system, the
more codes that are used. However as more codes are used
the signal to interference (S/I) ratio will drop and the bit
error rate (BER) will go up for all users.
• CDMA requires tight power control as it suffers from farnear effect. In other words, a user close to the base station
transmitting with the same power as a user farther away will
drown the latter’s signal. All signals must have more or less
equal power at the receiver.
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CDMA C/Cs (2/2)
• Rake receivers can be used to improve signal
reception. Time delayed versions (a chip or more
delayed) of the signal (multipath signals) can be
collected and used to make bit level decisions.
• Soft handoffs can be used. Mobiles can switch base
stations without switching carriers. Two base stations
receive the mobile signal and the mobile is receiving
from two base stations (one of the rake receivers is
used to listen to other signals).
• Burst transmission - reduces interference
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Spread Spectrum: Basis of CDMA
• Problem of radio transmission: frequency dependent
fading can wipe out narrow band signals for duration
of the interference
– Solution: spread the narrow band signal into a broad
band signal using a special code
• Side effects:
– coexistence of several signals without dynamic
coordination
– tap-proof
• Techniques: Direct Sequence, Frequency Hopping
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Operation of SS
P
P
i)
ii)
f
f
sender
P
iii)
P
iv)
f
power
interference
f
spread
signal
power
detection at
receiver
user signal
broadband interference
narrowband interference
signal
spread
interference
f
f
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SS and Fading
channel
quality
1
2
5
3
6
narrowband channels
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frequency
narrow band
signal
guard space
channel
quality
1
spread
spectrum
2
2
2
2
2
spread spectrum channels
frequency
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Cellular: 2G
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Digital wireless
Low bitrate voice and data services
Circuit switched
Multiple standards: GSM, IS 136, IS 95
Global roaming within similar systems
only
• Messaging services: SMS
• Web access: imode, wireless portals
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Cellular: 3G
• The next generation cellular, 3G, is envisioned to
enable communication at any time, in any place, with
any form, as such, it will:
– allow global roaming
– provide for wider bandwidths to accommodate different
types of applications
– support packet switching concepts
• The ITU named this vision: IMT-2000 (International
Mobile Telecommunications 2000) with the hope of
having it operational by the year 2000 in the
2000MHz range.
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IMT-2000 Vision
• Common spectrum worldwide (2.8 – 2.2 GHz band)
• Multiple environments, not only confined to cellular,
encompasses: cellular, cordless, satellite, LANs, wireless
local loop (WLL)
• Wide range of telecommunications services (data, voice,
multimedia, etc.)
• Flexible radio bearers for increased spectrum efficiency
• Data rates of: 9.6Kbps or higher for global (mega cell),
144Kbps or higher for vehicular (macro cell), 384Kbps or
higher for pedestrian (micro cell) and up to 2Mbps for
indoor environments (pico cell)
• Global seamless roaming
• Enhanced security and performance
• Full integration of wireless and wireline
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3G Technologies
• W-CDMA backward compatible with GSM
(called UMTS by the ETSI)
• The IS-95 standard (CDMAOne) is evolving
its own vision of 3G: CDMA2000
• The IS-136 standard is evolving its own
migration to 3G, Universal Wireless
Communications, UWC-136 or IS-136 HS
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3G Timeframe
• The Japanese are leading the pack with their WCDMA implementation. It is planned to be rolled out
in the year 2001 (pushed back from spring to late
fall).
• The Koreans plan to have CDMA2000 up an running
before the world cup in 2002.
• The Europeans are pushing hard to UMTS up soon
but the current push is for 2.5G, a middle of the road
to protect current infrastructure investments.
• In the US no major push yet, some service providers
are following in the footsteps of the Europeans by
pushing a 2.5G solution.
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IMT 2000 Services (1/2)
• All of 2G plus --->
• Higher Bit rates:
– 144Kbps or higher for vehicular (macro cell),
– 384Kbps or higher for pedestrian (micro cell) and
– up to 2Mbps for indoor environments (pico cell)
• Billing/charging/user profiles
– Sharing of usage/rate information between service
providers
– Standardized call detail recording
– Standardized user profiles
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IMT 2000 Services (2/2)
• Support of geographic position finding
services
• Support of multimedia services
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QoS
Asymmetric links
Fixed and variable rate
Bit rates of up to 2Mpbs
• Support of packet services
• Internet Access (wireless cellular IP - 3GPP)
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IMT 2000 Family Concept
• The IMT 2000 family concept defines some basic
interoperability capabilities between different IMT
2000 technologies to enable global roaming!
• Different Radio Access Networks (RANs):
– CDMA2000
– W-CDMA
– UWC-136
• Different Core Network standards
– IS 41
– GSM
– ISDN
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Challenge of the Family Concept
• With IMT 2000 Standard Interfaces and Capabilities:
– Any Family RAN could interface with any Family Core Network
for some minimum set of features.
• More advanced features are possible in limited regions
where the Family RAN and the Family Core Network are
optimally matched
• The Core Network functionality should be kept independent
of the Radio technology.
• By maintaining independence, each can evolve separately
based on needs
• User Identity Modules (UIM) Plug-In modules could be used
in locally rented handsets for Global Roaming with at least
the minimum feature set. (similar to GSM SIMs)
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UIM Roaming
• UIM cards should allow a subscriber to obtain:
– Any IMT 2000 service/capability basic feature set on
– Any IMT 2000 Network family member (W-CDMA,
CDMA2000 and UWC-136)
• UIM Card: will be a superset of the current GSM SIM
– Contains all necessary information about the user’s
service subscriptions
– Supports user identity separate from handset identity:
• Allows a user to use different handsets, with all usage
billed to the single user
• Allows a user to rent a handset in a foreign
country/network and obtain instant service
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To realize the IMT 2000 Vision
• Physical interfaces are being standardized:
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UIM to handset interface
Radio/Air interfaces
RAN to Core Network
Network to Network Interfaces (NNI) between
Core Networks
• Radio independent functions are being
standardized:
– UIM to handset
– Handset to Core Network
– NNI
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The next vision: 4G
• Higher bit rates (what else???):
– 2Mbps outdoor, high speed
– 20Mbps indoor, low speed
• Full integration with IPv6, IP QoS and MoIP
• High capacity: 5 to 10 increase
• Multimode terminals: seamless switching
between different systems
• Cheaper infrastructure cost
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How to realize 4G
• Higher spectrum is required to accommodate higher
bit rates (e.g., 2-4Mbps requires ~ 20MHz)
– Problems with propagation loss, attenuation
– Higher RF circuit losses
– Both of these require higher output power, e.g., 2Mbps
at 5GHz requires 2400 times more power than 8Kbps at
2GHz
• Adaptive phased arrays are needed to achieve higher
gains to counteract the losses listed above
– With better antennas we get higher capacity systems as
co-channel interference is reduced
– These antennas are expensive but generally constitute
the cheapest component of the system
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Issues to be considered
• Few studies exist that characterize the behaviour of
the channel at these higher frequencies
• The increased gains claimed by phased antennas are
based on theoretical studies and remain to be verified
in live scenarios
• New space time channel codes need to be defined
that work optimally in this higher frequency range
• Equalization and decoding algorithms need to studied
for space time coded systems
• To achieve better performance 3G uses specialized
circuits, 4G should use instead general purpose DSP,
and implement soft radios
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Higher Layer Issues
• Network Layer
• Transport Layer
• Mobility Support
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Network Layer
• What do cellular networks and wireless LANs provide?
– Wireless connectivity
– Mobility at the data link layer
• What is Dynamic Host Configuration Protocol (DHCP)?
– It provides local IP addresses for mobile hosts
– Is not secure
– Does not maintain network connectivity when moving around
• What the above do not provide:
– Transparent connectivity at the network layer
– Mobility with local access, i.e, mobility at the data link layer
The difference between mobility and nomadicity!
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Mobile IP
• Mobile IP provides network layer
mobility
• Provides seamless roaming
• ‘‘Extends’’ the home network over the
entire Internet
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Motivation for MoIP
• IP Routing
– based on IP destination address, network prefix (e.g.
129.13.42) determines physical subnet
– change of physical subnet implies change of IP address to
have a topologically correct address (standard IP) or needs
special entries in the routing tables
• Specific routes to end-systems?
– requires changing all routing table entries to forward packets
to the right destination
– does not scale with the number of mobile hosts and frequent
changes in the location, security problems
• Changing the IP-address?
– adjust the host IP address depending on the current location
– almost impossible to find a mobile system, DNS updates slow
– TCP connections break, security problems
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Scope of MoIP
• Mobile IP solves the following problems:
– if a node moves without changing its IP address it will be
unable to receive its packets,
– if a node changes its IP address it will have to terminate and
restart its ongoing connections everytime it moves to a new
network area (new network prefix).
• Mobile IP is a routing protocol with a very specific purpose.
• Mobile IP is a network layer solution to node mobility in the
Internet.
• Mobile IP is not a complete solution to mobility, changes to
the transport protocols need to be made for a better
solution (i.e., the transport layers are unaware of the mobile
node’s point of attachment and it might be useful if, e.g.,
TCP knew that a wireless link was being used!).
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Requirements of MoIP
• Transparency
– mobile end-systems keep their IP address
– continuation of communication after interruption of link possible
– point of connection to the fixed network can be changed
• Compatibility
– support of the same layer 2 protocols as IP
– no changes to current end-systems and routers required
– mobile end-systems can communicate with fixed systems
• Security
– authentication of all registration messages
• Efficiency and scalability
– only little additional messages to the mobile system required
(connection typically via a low bandwidth radio link)
– world-wide support of a large number of mobile systems
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Problems with MoIP
• Security
– authentication with FA problematic, for the FA typically belongs to
another organization
– no protocol for key management and key distribution has been
standardized in the Internet
– patent and export restrictions
• Firewalls
– typically mobile IP cannot be used together with firewalls, special
set-ups are needed (such as reverse tunneling)
• QoS
– many new reservations in case of RSVP
– tunneling makes it hard to give a flow of packets a special treatment
needed for the QoS
• Security, firewalls, QoS etc. are topics of current research
and discussions!
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Transport Layer
• Transport protocols typically designed for
– Fixed end-systems
– Fixed, wired networks
• TCP congestion control
– packet loss in fixed networks typically due to (temporary)
overload situations
– routers have to discard packets as soon as the buffers are full
– TCP recognizes congestion only indirectly via missing (I.e.,
timed out) acknowledgements
– Immediate retransmissions unwise, they would only contribute
to the congestion and make it even worse
– slow-start algorithm is used as a reactive action to reduce the
network load
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Influences of Mobility and Wireless
• TCP assumes congestion if packets are dropped
– typically wrong in wireless networks, here we often have
packet loss due to transmission errors
– furthermore, mobility itself can cause packet loss, if e.g. a
mobile node roams from one access point (e.g. foreign agent
in Mobile IP) to another while there are still packets in transit
to the old access point and forwarding from old to new access
point is not possible for some reason
• The performance of unmodified (i.e., as is) TCP degrades
severely
– note that TCP cannot be changed fundamentally due to the
large base of installation in the fixed network, TCP for mobility
has to remain compatible
– the basic TCP mechanisms keep the whole Internet together
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Modified TCP
Approach
Indirect TCP
Snooping TCP
M-TCP
Fast retransmit/
fast recovery
Transmission/
time-out freezing
Selective
retransmission
Transaction
oriented TCP
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Issues with Proposed Solutions
• Not one of these is a good solution
• Each offers a solution to a part of the
problem but not the whole
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Mobility Support
• File Systems
• Databases
• WWW
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File Systems
• Goal
– efficient and transparent access to shared files within a mobile
environment while maintaining data consistency
• Problems
– limited resources of mobile computers (memory, CPU, ...)
– low bandwidth, variable bandwidth, temporary disconnection
– high heterogeneity of hardware and software components (no
standard PC architecture)
– wireless network resources and mobile computer are not very
reliable
– standard file systems (e.g., NFS, network file system) are very
inefficient, almost unusable
• Solutions
– replication of data (copying, cloning, caching)
– data collection in advance (hoarding, pre-fetching)
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Databases
• Request processing
– power conserving, location dependent, cost efficient
• example: find the fastest way to a hospital
• Replication management
– similar to file systems
• Location management
– tracking of mobile users to provide replicated or location
dependent data in time at the right place (minimize access
delays)
• example: with the help of the HLR (Home Location Register) in
GSM a mobile user can find a local towing service
• Transaction processing
– “mobile” transactions cannot necessarily rely on the same
models as transactions over fixed networks (ACID: atomicity,
consistency, isolation, durability)
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WWW 1/3
• Protocol (HTTP, Hypertext Transfer Protocol) and language
(HTML, Hypertext Markup Language) of the Web have not
been designed for mobile applications and mobile devices,
thus creating many problems!
• Typical transfer sizes
– HTTP request: 100-350 byte
– Responses avg. <10 Kbyte, header 160 byte, GIF 4.1Kbyte,
JPEG 12.8 Kbyte, HTML 5.6 kbyte
– And many large files
• The Web is no file system
– Web pages are not simple files to download
– static and dynamic content, interaction with servers via forms,
content transformation, push technologies etc.
– many hyperlinks, automatic loading and reloading, redirecting
– a single click might have big consequences!
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WWW 2/3
• Characteristics
– stateless, client/server, request/response
– needs a connection oriented protocol (TCP), one connection
per request (some enhancements in HTTP 1.1)
– primitive caching and security
• Problems
– designed for large bandwidth (compared to wireless access)
and low delay
– large and redundant protocol headers (readable for humans,
stateless, therefore large headers in ASCII)
– uncompressed content transfer
– using TCP
– DNS lookup by client causes additional traffic and delays
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WWW 3/3
• Caching
– quite often disabled by information providers to be able to
create user profiles, usage statistics etc.
– dynamic objects cannot be cached
• numerous counters, time, date, personalization, ...
– mobility quite often inhibits caches
– security problems
• caches cannot work with authentication mechanisms that are
contracts between client and server and not the cache
– today: many user customized pages, dynamically generated
on request via CGI, ASP, ...
• POSTing (i.e., sending to a server)
– can typically not be buffered, very problematic if currently
disconnected
• Many unsolved problems!
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HTML and Mobility
• HTML
– designed for computers with “high” performance, color highresolution display, mouse, hard disk
– typically, web pages optimized for design, not for communication
• Mobile devices
– often only small, low-resolution displays, very limited input
interfaces (small touch-pads, soft-keyboards)
• Additional “features”
– animated GIF, Frames, ActiveX Controls, Shockwave, movie clips,
– many web pages assume true color, multimedia support, highresolution and many plug-ins
• Web pages ignore the heterogeneity of end-systems!
– e.g., without additional mechanisms, large high-resolution pictures
would be transferred to a mobile phone with a low-resolution display
causing high costs
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WWW and Mobility
• Application gateways, enhanced servers
– simple clients, pre-calculations in the fixed network
– Compression, transcoding, filtering, content extraction
– automatic adaptation to network characteristics
• Examples
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–
–
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picture scaling, color reduction, transformation of document format
Present only parts of the image: detail studies, clipping, zooming
headline extraction, automatic abstract generation
HDML (handheld device markup language): simple language
similar to HTML requiring a special browser
– HDTP (handheld device transport protocol for HDML
• Problems
– proprietary approaches, require special enhancements for browsers
– heterogeneous devices make approaches more complicated
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What is happening 1/2
• HTTP/1.1
– client/server use the same connection for several
request/response transactions
– multiple requests at beginning of session, several responses
in same order
– enhanced caching of responses (useful if equivalent
responses!)
– semantic transparency not always achievable: disconnected,
performance, availability -> most up-to-date version...
– several more tags and options for controlling caching
(public/private, max-age, no-cache, etc.)
– encoding/compression mechanism, integrity check, security
of proxies, authentication, authorization...
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What is Happening 2/2
• Enhanced browsers
– Pre-fetching, caching, off-line use
• e.g. Internet Explorer
• Client Proxy
– Pre-fetching, caching, off-line use
• e.g., Caubweb, TeleWeb, Weblicator, WebWhacker, WebEx
• Client and network proxy
– combination of benefits plus simplified protocols
• e.g., MobiScape, WebExpress
• Special network subsystem
– adaptive content transformation for bad connections, prefetching, caching
• e.g., Mowgli
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Conclusions
• The problems with 3G are mostly infrastructure cost
related
• The problems facing 4G are much more fundamental
– It is absolutely imperative that we start to think about what
the future will be like so that we can direct our energies to
solving these problems
• Wireless systems will become pervasive and will
exist in a multitude of flavors (sensors, satellites,
LANs, PANs, cellular, access, etc,).
– We need to be able to provide a seamless integration of all
these systems
• Still need work at higher layers for true
nomadicity, not just wireless and mobility
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