1. Communication Topics Jason Hill – jhill@cs.berkeley.edu

2. 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/consistency tradeoffs
Mask device mobility from services & applications
Provide application interface to low level network information (location)

3. 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

4. Quick look at existing wireless systems
GSM
Mobitex
CDPD

5. GSM Overview….
Mobility achieved through the use of the Home Location Register (HLR) and the Visitor Location Register (VLR).

6. Mobitex Architecture Used by Palm VII Packet Switched
Guaranteed message delivery (Message Queues)

7. 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”

8. Possible Architecture
Traditional Clients/Services IP
Infrastructure Based Services
Device Proxies
Delivery Q’s
Base Stations
Portable Devices

9. Device Properties Uniquely identifiable
Bootstrap to nearest Access Point
Not directly addressable
AP’s 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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?

16. 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

17. Is this a big naming problem?
Resolution and assignment of proximity based names
Querying of proximity based name space?

18. 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

20. 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