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I will summarize this chapter by means of three tables that "say it all".

Table 2.3 shows the difference in hardware for the different types of computers; stationary ones (servers & workstations) and mobile ones (laptops & palmtops). From the table it is obvious that processing power and storage capacity (and to some extent also quality of user interface and reliability) is traded for portability!

Table 2.3: Characteristics of computer hardware
Characteristic Hardware
Stationary Mobile
Server Workstation Laptop Palmtop
Processing power Maximum High Medium Low
Storage capacity Maximum High (*) Medium Low
Portability None Limited Slightly limited Full
User interface - Full Slightly limited Limited
Reliability High Medium Limited Limited

(adapted from [34])

(*): The stationary workstation itself can be diskless, but then it has access to storage on or through a file server.

Table 2.4 shows the difference in quality of the different network technologies; fixed vs. dial-up wired or wire-less. Fixed is preferable to dial-up, dial-up wired (e.g., using telephone lines) is preferable to dial-up wireless (e.g., using GSM). In both cases the only gain is availability (i.e., mobility).2.4

Table 2.4: Characteristics of network technology
  Network technology
Wire Cellular
Operation Connected Weakly connected Weakly connected
Bandwidth High Medium Low
Reliability High Medium Low
Initial cost High Low Low
Latency Low Medium High
Cost to use Low Medium High
Topology Fixed, continuous Fixed, varying Dynamic
Available at Office outlet Phone outlet "Anywhere"

(adapted from [34])

Table 2.5 sums up the characteristics of the different modes of operation (or communication states, if you prefer) encountered by mobile computers. If I have not expressed myself clearly, this is what I have been trying to communicate. I would like to emphasize that consistency is weakened (delayed writes, optimistic replication, and cache reliance) in order to increase availability!

Table 2.5: Modes of operation (states) and their characteristics
  Connected Weakly Connected Disconnected
Mobility none medium or high (*) unlimited
Position fixed roaming
Method normal
delayed writes &
optimistic replication
Access continuous continuous
or on demand
Bandwidth high bandwidth
low bandwidth
Latency low latency high latency none
Link hard-wired
e.g., LAN
serial link
e.g., phone line
or wire-less
Network (**) Ethernet PSTN or GSM none
normal weakened none

(*): In theory, high when using wireless; but in practice? When using a phone line medium; you can find a phone outlet in almost every house but you cannot move around during the connection.

(**): Only the network types considered in this project is listed, other possibilities exist, e.g., ATM, WaveLAN, Infrared, etc.

As a last remark I would like to point out once again that mobile computers normally are used by a single person (at a time).


... mobility).2.4
Also initial cost, but I am talking about servicing mobile clients from an existing distributed system--and hopefully I do not have to worry about financing it.

next up previous contents index
Next: Replica Control Up: Mobile Computing Previous: Mobility   Contents   Index