FILE
TRANSFER
·
Transferring files from one computer to
another is one of the most common tasks in a networking and internetworking
environment.
·
File transfer is accomplishFTed using a
protocol known as FTP.
File Transfer Protocol
(FTP)
·
FTP is a part of TCP/IP protocol suite.
·
It is a protocol that enables files to
be transferred between the computers.
·
FTP does not actually transfer a file
from one computer to another rather it copies it. Hense , the original files
exists and a copy of it is transferred.
·
FTP works on the client/server
principle. A client program enables the user to interact with a server in order
to access information and services on the server computer.
Features of FTP
1.
Data representation
·
FTP handles three types of data
representations- ASCII(7bit), EBCDIC(8 bit) and 8 binary data.
·
The ASCII file is the default format for
transferring text files.
·
Each character is encoded using 7-bit
ASCII. The sender transforms the file from its own representation into ASCII characters
and the receiver transform the ASCII character to its own representation.
·
The image file is the default format for
transferring binary files.
·
The file is sent as continous streams of
bits without any interpretation or encoding.
2.
File organization and data
structures
·
FTP supports both unstructured and
structured file.
·
An unstructured file contains string of
bytes and is end – marked by EOF (end of file).
·
The data structure that corresponds to
such a file is called file structure.
·
A structured file contains a list of
records and each record is delimited by EDR (end of record).
·
The data structure of such file is called record structure i.e. file is
divided into records.
·
Another structured file contains pages ,
with each page having a page number and a page header .
·
The pages can be stored and accessed
randomly or sequently the corresponding data structure is called page structure
i.e. file is divided into pages.
3.
Transmission modes
·
FTP can transfer a file by using one of
the following three modes:
Stream
mode
·
It is the default mode
·
File is transmitted as continous stream
of bytes to TCP.
·
TCP is responsible for copying data into
segments of appropriate size.
·
If data is simply a stream of bytes (file
structure), no end – of – file is needed. EOF in this case is the closing of
the data connection by the sender .
·
If data is divided into records( record
structure), each record has a 1-byte EOR (end – of - record), character and the
end of the file has a 1-byte EOF (end – of – five ) character.
Black
mode
·
Data is delivered from FTP to TCP in
blocks.
·
Each block is preceeded by three bytes
header.
·
The first bytes is called the block descriptor
.
·
The second the third byte define the
size of the block in bytes.
Compressed
mode
·
Data is usually compressed if the file
to be transmitted very big.
·
The comparison method normally used in
run – length and encoding.
·
In a text file , usually spaces (blanks
are removed).
·
In a binary file , null characters are
compressed.
4.
Error control
·
Since TCP is used for data transfer no
additional error recovery mechanism is required.
5.
Access control
·
File access protection is done using
login procedure with login name and password.
CLICK BELOW FOR FILE TRANSFER PROTOCOL PART 2
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