Contents   Index

Summary

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
	Fixed LAN/WAN	Dial-up
		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 operation	delayed writes & optimistic replication	cache reliance
Access	continuous	continuous or on demand	none
Bandwidth	high bandwidth constant	low bandwidth varying	none
Latency	low latency	high latency	none
Link	hard-wired e.g., LAN	serial link e.g., phone line or wire-less	none
Network (**)	Ethernet	PSTN or GSM	none
Guaranteed consistency	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).

Footnotes

... 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.

  Contents   Index