Download Chapter 9 - University of Scranton: Computing Sciences Dept.

Chapter 9
How Do Users Share Computer Files?
What is a File Server
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A (central) computer which stores files which
can be accessed by network users.
File Servers (Continued)
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Example: The CS dept has a file server in its
network where each user can store files that can be
accessed from other computers. It is easy to
connect to from my department’s network and
possible (but complicated) to access from
elsewhere.
My files are available to me and to no one else.
If I wish to share my files I have to send copies to
other individuals via e-mail or put them on a web site
where others can download them.
How are files accessed on a network?
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Download the file from the server and make
changes to that file. This leaves the original
on the server unchanged unless the user
uploads the change (This is what I do to
update my website).
The user modifies the file on the server
without downloading it. (I sometimes do this
to fix something quick.)
FTP (File Transfer Protocol)
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What you do to copy files to your Web site.
It can also be used to copy files from your
Web site.
It can also be used independent of the Web
to copy files from one computer to another as
long as you have permission to use both
resources.
Remote File Access
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This technique is used when a program on
your computer is acting on the file on the
server and not a copy of it.
Updating the (only) copy of the file
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Write-Through – immediate update, each change
is automatically reflected in the document
Write-on-close – the changes are reflected in the
document when the user is finished.
Write-delayed – batch updates, for example the
file could be updated every two minutes.
How are files shared?
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FTP
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We’ve covered this a bunch of times
Anonymous FTP
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It’s just like regular FTP except that you don’t need a
username and password to access the account.
There is a standard username usually anonymous or
possibly guest. The password is your e-mail address.
So if you were to make something anonymously
available to me I would use anonymous as my
username and [email protected] as my
password.
How are files shared (continued)
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Remote File Access
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The problem occurs if several people have the right
to access the file simultaneously, i.e. if you and I
were editing the same paper at the same time or if
you were reading it while I was editing it.
Cache cohesion – the copy you’re reading is not the
copy I’m editing so you’re out of sync
Client-initiated update – the server version updates
when I finish editing but you aren’t told
Server-inititated update – when I finish your copy is
automatically updated.
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Why can some computers access some
files but not others?
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Necessities for file access
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Your computer must be able to physically connect to the
server.
You must have permission to access the server and the
directory and the file in the way that you need to access the
file.
Your computer (software) must be able to interpret the data
in the file.
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Encryption
Common file format
When someone wants a file from a server, how
does the server know what method to use?
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TCP/IP (Transmission Control Protocol/Internet
Protocol
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Data is sent in packets.
Each packet consists of the data being sent plus a
header for that packet.
One of the items in the header is a 16 bit port address
(which is essentially a number from 0 to 65535).
The ports are associated with particular applications.
For example ports 20 and 21 are associated with FTP,
port 80 with HTTP.
What makes TCP/IP work?
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Logical levels are used to subdivide the work
to be done.
The ISO (international standards
organization) has specified OSI (open
systems interconnection reference model)
with seven levels:
What Makes TCP/IP work (cont)?
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OSI Layers
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Layer 1: Physical Layer – listens to the cables to see if stuff
is being sent and sends bits when the line is clear
Layer 2: Data Link Layer – moves groups of bits called
frames from machine to machine
Layer 3: Network Layer – routes packets from source to
destination
Layer 4: Transport Layer – divides messages into packets
Layer 5: Session Layer – higher level capabilities
Layer 6: Presentation Layer – higher level capabilities
Layer 7: Application Layer – higher level capabilities
Summary
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Files can be shared in three different ways:
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Each member of a group can keep their own
personal copy of the data and they can e-mail
copies to each other and be personally
responsible for keeping the copies in sync.
The members could agree on a common file
format and keep copies of the file on a server and
use FTP to transfer between their machines and
the server. They would work on the files on their
own machines
Summary (Continued)
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Files can be shared in three different ways
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The members could work on the file on the server
itself using remote file access.
The application that the group is using must
insure that either only one person can work on a
particular file at one time or that if several people
work on a file simultaneously, that the correct
copy is maintained.
Summary (Continued)
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If different software (o/s) is used by different
members of a team to view data, it may look
different on different platforms.
TCP/IP is probably the most common method
of transmitting data through a network. Port
numbers tell the receiving computer which
application to use. These port numbers are
standardized by ICANN.
Terminology
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Anonymous FTP
Cache Coherence
Client
Client-initiated update
Concurrent update
problem
File Server
File Transfer Protocol
(FTP)
Frame
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Location Independence
Location Transparency
Lock Port Address
Packet
Remote File Access
Server
Server-initiated Update
Transmission Control
Protocol/Internet
Protocol (TCP/IP)