Information Technology Reference
In-Depth Information
packets in the receiver. This layer also can monitor what packets are
actually received during a transmission and detect packets that appear
lost; lost packets can be resent. Although several approaches may be
followed at Layer 4, TCP is widely used and provides reliable commu
nications for applications.
Layers 5, 6, and 7 provide additional services for the programs
that send and receive messages. Layers 5 and 6 offer specialized ser
vices, such as the ability to monitor large messages and to review spe
cific fields of messages. Many applications do not need such capabili
ties, and some networking topics do not mention them. Services, such
as FTP, email, and remote file access, run as applications in Layer 7.
As you explore this topic, focus first on understanding Layers 1
through 4, as these provide the basis for the higherlevel layers.
Thus, the abbreviation TCP/IP identifies a basic approach to
machine communications, highlighting two specific layers. TCP in
dicates a protocol at Layer 4, in which large messages are divided
into packets, and IP specifies a mechanism at Layer 3 by which
those packets are sent from a source to an intended destination.
When the packets get to their destination, TCP also indicates how
the material should be reassembled to get the original message.
Further, as part of this process, TCP checks that no data are lost,
asking for the retransmission of data as required.
Summary
When several people collaborate on a project, they may wish to
share files and programs. This sharing could occur in three basic ways.
Each person in the group could maintain her or his own copy of the
material, and they could email versions among group members.
Alternatively, the group could agree upon a common file format and
place the current version of their work on a common file server.
Group members then could use File Transfer Protocol (FTP) to down
load a copy of the current version from the common server to their
machines, edit their copy of the file, and upload a revised copy back to
the server when they are done. FTP may involve accounts and pass
words for individual users or “anonymous FTP” for general use. As a
third alternative, group members could work directly with the file on
the common server, using remote file access.
 
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