Information Technology Reference
In-Depth Information
Ethernet, this involves listening to a cable to determine when the ca
ble is not being used by others and sending materials bit by bit.
Matters of transmission media and speeds are handled at this layer.
Layer 2, the Data Link Layer, handles groups of bits, called
frames
. When a message is sent, the material is broken down into
frames and is monitored by the Data Link Layer. In this layer, ma
terials go from one machine to the next, and some checking is done
to be sure that the data sent match the data received. When frames
(groups of bits) are received in jumbled form, the receiving machine
asks the sender to retransmit the material. Because of this work, we
can have some confidence regarding individual transmissions of
data from one machine to the next.
Layer 3, the Network Layer, involves routing a message from a
source (such as a client) to a destination (such as a server). Although
the Data Link Layer handles data transmission from machine to ma
chine within a single segment, broader communications may require
messages to move along several segments to reach their desired des
tination. For example, in the case of the World Wide Web, a mes
sage may have to move from machine to machine repeatedly be
tween countries and across continents. The Network Layer handles
how the message is routed to get to its destination. Messages within
this layer are sometimes called
packets
. Computers can move the
packets through the network and towards their destinations with
the help of Internet Protocol, or IP. Thus, the IP part of TCP/IP
specifies a way of sending packets of information from a source to a
destination along a network. (Another common mechanism for the
Network Layer is called a
connectionoriented protocol
, and is used
by telephone companies and other public networks.) As has already
been suggested several times and as may be inferred by its name, IP
is the communication mechanism of choice for much of the Internet.
In considering IP, know that each packet is routed separately,
according to a variety of factors, including the congestion of vari
ous Ethernet segments. This means that if you have requested a
large amount of data through your browser and if that data were
split into several packets, then the various packets or parts of the
data might have each followed different routes from the Web server
to your browser!
Layer 4, the Transport Layer, divides large messages into smaller
packets that can be sent over the IP layer, and then recombines those
