Communication Topics

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Communication Topics

• Jason Hill jhill_at_cs.berkeley.edu
• http//www.cs.berkeley.edu/jhill/cs294-8/MiniRetr
  eat.ppt

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High Level Communication Goals

• Support low power network devices
• Give intermittently connected devices the
  illusion and appearance of continuous IP
  connectivity
• Make scalability independent of end device
• Allow application to manage availability/consisten
  cy tradeoffs
• Mask device mobility from services applications
• Provide application interface to low level
  network information (location)

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How?

• Push as much of the complexity as possible into
  the infrastructure
• Assume compute cycles and network bandwidth are
  free once you get to the wired world

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Quick look at existing wireless systems

• GSM
• Mobitex
• CDPD

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GSM Overview.

• Mobility achieved through the use of the Home
  Location Register (HLR) and the Visitor Location
  Register (VLR).

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Mobitex Architecture

• Used by Palm VII
• Packet Switched
• Guaranteed message delivery (Message Queues)
• http//www.ericsson.se/wireless

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CDPD Wireless IP

• Uses Analog Cellular System
• 19.2 kbps data w/ 128 bit encryption
• Sends small data packets over unused voice
  channels
• Mobile data intermediate systems route packets
• Fixed address

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Possible Architecture
Traditional Clients/Services
IP
Infrastructure Based Services
Device Proxies
Delivery Qs
Base Stations
Portable Devices
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Device Properties

• Uniquely identifiable
• Bootstrap to nearest Access Point
• Not directly addressable
• APs can provide them temporary names (proximity
  based naming)
• Must phone home to inform proxy of temp. name
• Special client of proxy push/pull data to/from
  proxy

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Access Points/Base Station Properties

• Permanently Connected
• Globally Addressable via IP
• Provides route to local devices
• Queues Data for recently seen devices
• Informs device proxies of local devices that it
  is in communication with

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Proxy Service Properties

• Easy to find (immobile)
• Always available
• Has approximation of device state
• What is the consistency policy?
• Locatable by both devices and services
• Handles all requests to and from mobile devices
  provides scalability
• Can learn temporary route to portable devices

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Optimizations.

• Ninja style code migration allows layers to
  collapse as an optimization
• Higher power devices can take on additional
  functionality
• Complex base stations can perform protocol
  translations for clients
• Device ? communication can avoid going to proxies

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Optimization for data flowing from the device

• Forward by any base station that hears clear
  transmission minimize retransmissions at
  expense of wired bandwidth
• Duplicates removed by proxy
• If all 5 clients try to transmit, 4 would get
  through

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Data flowing to the device

• Base stations uses wired network to orchestrate
  communication
• Base stations learn which other base stations
  they conflict with for a given client
• Primary base station transmits to client but
  neighboring base stations also queue data in case
  client moves into their area

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Last link communication options

• One option is use RF Monolithics Low Power/Cost
• Device to AP protocol should not determine lowest
  power consumption of devices (i.e. Require device
  to be active minimum amount of time.)
• Are there any standard protocols we could use?

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Issues

• What is the internal structure of a device proxy?
• How inter-related are the designs for the
  proxies, mobility support, and last link
  communication protocols?
• Could a device proxy be built on top of
  CDMA/Mobitex providing continuous IP
  connectivity?
• Naming Name Discovery
• Both proxy names and temp. names

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Is this a big naming problem?

• Resolution and assignment of proximity based
  names
• Querying of proximity based name space?

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Strengths

• Complex Device drivers are pushed into the
  infrastructure where they are easy to write,
  debug, upgrade
• Devices can be composed by the infrastructure
  (Speaker Camera Display ? Video Conference
  Device)
• Allow small devices to reliably service many
  clients (device proxies/surrogates)
• Heterogeneity can be masked by software adapters

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Infrastructure Based Communication Protocols

• Support mobility in a transparent way
• Provide tiny devices the illusion of IP
  connectivity
• Make brief periods of disconnection invisible to
  the real world
• Buffer and Cache information for clients
• Attempt to optimize delivery overhead