October 8, 2003
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October 8, 2003






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October 8, 2003 Presentation Transcript

  • 1. Outline for This Lecture
    • Reality Check of Wireless Network Usage
    • Overview of Wireless and Mobile Networking
  • 2. Comparisons of 3G and 802.11
    • Coverage
      • 3G: large coverage
      • 802.11: small
    • Throughput
      • 802.11: up to 11/54 Mbps
      • 3G: up to 2 Mbps
    • Cell size and density
      • 802.11: several hundred feet
      • 3G: up to several kilometers
    • Applications supported:
      • 802.11: mainly data, but may support VoIP
      • 3G: data plus voice in 1XEVDV
  • 3. Measurements on 802.11 WLAN
    • Mobile host is prevalent, but mobile flows are not
    • Network usage is highly dependent on applications
    • Highly nonstationary traffic pattern
      • Days and evenings
      • Workdays and weekends
  • 4. Migration to 3G (Src: Wireless Week Research)
    • Carrier Network Technology Estimated Deployment
    • ATT Wireless GSM/GPRS Overlay conclude year-end 2002
    • upgrade to E-GPRS or EDGE mid-2002
    • W-CDMA-based tech. Late 2002
    • Cingular GPRS Overlay on GSM Year-end 2001
    • Wireless EDGE Overlay on TDMA 2002 & 2003
    • EDGE overlay on GPRS/GSM 2002 & 2003
    • next step not determined
    • Sprint PCS CDMA 1X (release 0) to early 2002
    • CDMA 1X (release A) to early 2003
    • CDMA 1XEV-DO early 2003
    • CDMA 1XEV-DV 2003~2005
    • Verizon CDMA 1X year-end 2001
    • Wireless CDMA 1XEV following 1X rollout
    • next step not determined
  • 5. Overview
    • Fundamental issues and impact
      • wireless
      • mobility
    • For each layer in the protocol stack
      • A subset of design requirements
      • Design challenges/constraints
      • Possible design options
  • 6. Wireless Channel Characteristics
    • Radio propagation
      • Multipath, fade, attenuation, interference & capture
      • Received power is inversely proportional to the distance: distance-power gradient
        • Free space: factor 2
        • Inbuilding corridors or large open indoor areas: <2
        • Metal buildings: factor 6
        • Recommended simulation factors: 2~3 for residential areas, offices and manufacturing floors; 4 for urban radio communications
  • 7. Wireless Channel
    • Wireless transmission is error prone
    • Wireless error and contention are location dependent
    • Wireless channel capacity is also location dependent
  • 8. Mobility
    • Why mobility?
      • 30~40% of the US workforce is mobile (Yankee group)
      • Hundreds of millions of users are already using portable computing devices and more than 60% of them are prepared to pay for wireless access to the backbone information
  • 9. Mobility
    • Four types of activities for a typical office work during a workday:
      • Communication (fax, email)
      • Data manipulation (word processing, directory services, document access & retrieval)
      • Information access (database access and update, internet access and search)
      • Sharing of information (groupware, shared file space)
    • Question: how does mobility affect each of the above activities?
  • 10. Mobility
    • Possible scenarios of mobility
      • Scenario 1: user logs out from computer 1, moves to computer 2 and logs in
        • Should the user see the same workspace?
      • Scenario 2: different devices for different network
      • Scenario 3: user docks a laptop, works in a networked mode for a while, then disconnects and works in the standalone mode for a while, and then docks back
        • In stand-alone mode
          • What kind of activities can the user do?
          • What cannot be done?
          • Can we provide an illusion of connectivity in this case?
          • Can we automatically re-integrate the work (s)he has done while disconnected when (s)he finally reconnects to the network server?
  • 11. Impact of Mobility
    • Scenario 4: a user has a notebook with a wireless connection, connects to a remote host via network 1, shuts down connections, connects to the remote host via network 2, continues to work
      • Is the disconnection between network migration necessary?
      • When can we make the disconnection transparent to users? When we cannot?
      • What are the key issues to ensure seamless network migration?
      • Is it really important or users do not care about the automatic process? For what applications? What to change for the applications?
  • 12. Protocol Stack
    • Draw the entire protocol stack
      • For each component/layer
        • Some requirements
        • Issues to address
        • Possible design options
  • 13. Physical/MAC Layer
    • Requirements:
      • Continuous access to the channel to transmit a frame without error
      • Fair access to the channel: how is fairness quantified?
      • Low power consumption
      • Increase channel throughput within the given frequency band
    • Constraints:
      • Interference, fade, multi-path, and signal attenuation cause the channel to be error prone
      • Channel contention and error are location dependent
      • Channel capacity is fluctuating
      • Transmission range is limited (but also enables channel reuse)
      • Shared channel (hidden/exposed station problem)
  • 14. Physical/MAC Layer
    • Possible options:
      • Physical layer:
        • Narrow band vs wide band: direct sequence, frequency hopping, OFDM
        • Antenna technology: smart antenna, directional antenna, MIMO
        • Adaptive modulation
      • MAC layer
        • Multiple access protocols (CSMA/CA, MACAW, etc.)
        • Frame reservation protocols (TDMA, DQRUMA, etc.)
  • 15. Link Layer
    • Requirements:
      • Error sensitive application
        • A reliable link abstraction on top of error-prone physical channels
      • Delay sensitive application
        • A bounded delay link abstraction on top of error-prone channels
    • Constraints:
      • Errors in the channel
      • Spatial congestion
      • Link capacity is changing due to modulation techniques
  • 16. Link Layer
    • Possible options at the link layer
      • Windowing to provide error and flow control
      • Combating error:
        • Proactive: error correction via e.g. FEC
        • Reactive: error detection+retransmission, ARQ
        • Channel-state prediction+channel swapping
      • A few possible definitions of fairness: long term vs short term, deterministic vs probabilistic, temporal vs throughput
        • All users are treated equal
        • Users in error prone or congested location suffer
  • 17. Network Layer
    • Requirements:
      • Maintain connectivity while user roams
      • Allow IP to integrate transparently with roaming hosts
        • Address translation to map location-independent addressing to location dependent addressing
        • Packet forwarding
        • Location directory
      • Provide connection to packet flow as opposed to datagram (connection oriented networks)
      • Support multicast, anycast
      • Ability to switch interfaces on the fly to migrate between failure-prone networks
      • Ability to provide quality of service: what is QoS in this environment?
  • 18. Network Layer
    • Constraint:
      • Unaware hosts running IP
      • Route management for mobile hosts needs to be dynamic
      • A backbone may not exist (ad-hoc network)
  • 19. Network Layer
    • Possible options:
      • Mobile IP and its variants
        • Two-tier addressing (location independent addressing <-> location dependent addressing)
        • A smart forwarding agent which encapsulates packets from unware host to forward them to MH
        • Location directory for managing location updates)
      • Connection-oriented mobility support
        • Multicast
        • Finding the first branch point and rerouting packets
      • Ad hoc routing
        • Shortest path, source routing, multipath routing
  • 20. Transport Layer
    • Requirements:
      • Congestion control and rate adaptation
        • Doing the right thing in the presence of different packet losses
      • Handling different losses (mobility-induced disconnection, channel, reroute)
      • Improve transient performance
    • Constraints:
      • Typically unware of mobility, yet is affected by mobility
      • Packet may be lost due to congestion, channel error, handoffs, change of interfaces, rerouting failures
      • Link-layer and transport layer retransmit interactions
  • 21. Transport Layer
    • Options:
      • Provide indirection
      • Make transport layer at the end hosts ware of mobility
      • Provide smarts in intermediate nodes (e.g. BS) to make lower-layer transport aware
      • Provide error-free link layers
  • 22. Operating Systems
    • Requirements:
      • Provide the same environment to the user whether mobile (partially connected) or on the backbone network: same files, same context, ability to run same programs, access the same databases, servers & services, retain the same ID
      • Provide an abstraction of the environment for the aware application to adapt intelligently
    • Constraints:
      • Scheduling limited CPU resources & limited energy
      • Limited disk, memory
      • Partial connectivity
  • 23. File Systems
    • Requirements:
      • Access the same file as if connected
      • Retain the same consistency semantics for shared files as if connected
      • Availability and reliability as if connected
      • ACID (atomic/recoverability, consistent, isolated/serializable, durable) properties for transactions
    • Constraints:
      • Disconnection and/or partial connection
      • Low bandwidth connection
      • Variable bandwidth and latency connection,
      • Connection cost
  • 24. File systems
    • Four major aspects of disconnected or partially connected operations:
      • Hoarding: what to pre-fetch
      • Consistency: what to keep consistent when connectivity is partial
      • Emulation: how to operate when disconnected
      • Conflict resolution: how to resolve conflicts
    • Many choices within each aspect
  • 25. Applications/Services
    • A few questions for application designs:
      • How much to know about mobility (dynamic state)?
      • How much to control the activity of OS?
      • How to structure the interaction btw. App and systems
      • How to write location-aware applications?
      • What kind of filtering, data retrieval, and control support to be provided at the backbone?