一些通信类问题，请帮忙回答一下。谢谢

askask

1.what packet switching provides assured delivery
a. X.25  b.Frame Relay   c.ATM   d. ISDN

2.which mthod can speed up the access memory disk virtual memory?
a. select a good replacement algorithm
b. make mapping from virtual address to real address fast
b. make memory accessing time shorten
d. all of above

3.In Unix, how many files(including directories) at maximum can be
contained in one directory
a. depends on the i-node space
b. depends on the file length
c. unlimited
d. 512




4.what limits the sensitivity of a receiver
a. thernal noise
b. nois figure of the receiver
c. phase noise in the receiver
d. all of the above

5.the use of SIM kard in GSM mobile phone
a. identification of the caller
b. authentification of the caller
c.
d.

谢谢