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• 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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• Advances in Technology
– 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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• 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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GSM, 3G, WLAN,
Bluetooth, ...
PDA, laptop, cellular phones,
GPS, sensors
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• 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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• Higher loss-rates due to interference
– other EM signals, objects in path (multi-path, scattering)
• 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
• Higher delays, higher jitter
– connection setup time for cellular in the second range, several hundred milliseconds for wireless LAN systems
• Lower security, simpler active attacking
– radio interface accessible for everyone
– base station can be simulated, thus attracting calls from mobile phones
• Always shared medium
– secure access mechanisms important
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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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• 802.11 (2M) -> 802.11b (11M) -> 802.11a (50-
70M)
– 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 network
AP
AP wired network
AP: Access Point
AP ad-hoc network
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• 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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• 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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DIFS medium busy
DIFS
PIFS
SIFS contention next frame t
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Cable
Replacement
Personal Ad-hoc
Networks
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• Low-cost,
• Low-power,
• Small-sized,
• Short-range,
• Robust wireless technology
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– Low power transmitter and small coverage areas called cells
– Spectrum (frequency) re-use
– Handoff
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• 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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• 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 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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Approach
Termi nals
SDMA seg ment space into cells/secto rs only one te rminal can be act ive in one cell/one secto r
Signal separation cell structu re, directed antennas
Advantages ve ry simple, increases capac ity pe r km
TDMA seg ment send ing time into d isjoint time-slots, de mand driven o r fixed patte rns all terminals a re active fo r sho rt periods of t ime on the sa me frequency synch ronizat ion in the t ime do main estab lished, fu lly digital, flexible
Disadvantages
Commen t inflexible, antennas typ ically fixed only in combinat ion wi th TDM A, FDMA or
CDMA usefu l
FDMA seg ment the frequency band into disjoint sub -bands eve ry terminal has its own frequency, uninte rrupted filtering in the frequency do main all terminals can be act ive at the sa me place at the same moment, uninte rrupted code p lus spec ial receive rs simple, estab lished, robust gua rd space needed (multipath propagat ion), synch ronizat ion difficult used in con junct ion wi th FDMA/SDMA in many mobile net works inflexible, frequenc ies a re a scarce resou rce typ ically combined wi th TDM Auand
SDMA (frequency reuse )
CDMA spread the spect rum using o rthogona l codes flexible, less f requency plann ing needed, soft handove r complex receive rs, needs more complicated po wer cont rol for sende rs gene rally integ rated wi th
TDM A/FDMA
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• 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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k
1 k
2 k
3 k
4 k
5 k
6 c f t
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• 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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• 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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• 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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P P i) ii) f sender
P P iii) f iv) f f power interference spread signal power f detection at receiver user signal broadband interference narrowband interference signal spread interference f
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channel quality
1 channel quality narrow band signal
2
3
4 guard space
5 6 frequency
1
2
2
2
2
2 frequency spread spectrum narrowband channels spread spectrum channels
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• 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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• 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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• 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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• The Japanese are leading the pack with their W-
CDMA 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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• 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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• Support of geographic position finding services
• Support of multimedia services
– 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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• 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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• 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 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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• Physical interfaces are being standardized:
– 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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• 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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• 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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• 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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• 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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• 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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• 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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• 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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• 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 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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• 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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Approach
Indirect TCP
Snooping TCP
M-TCP
Fast retransmit/ fast recovery
Transmission/ time-out freezing
Selective retransmission
Transaction oriented TCP
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good
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• 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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• 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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• 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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• 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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• 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
– 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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• Application gateways, enhanced servers
– simple clients, pre-calculations in the fixed network
– Compression, transcoding, filtering, content extraction
– automatic adaptation to network characteristics
• Examples
– 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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• 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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• 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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• 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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