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284 Part Two Information Technology Infrastructure

TCP/IP and Connectivity
In a typical telecommunications network, diverse hardware and software
components need to work together to transmit information. Different
components in a network communicate with each other only by adhering to
a common set of rules called protocols. A protocol is a set of rules and proce-
dures governing transmission of information between two points in a network.
In the past, many diverse proprietary and incompatible protocols often forced
business firms to purchase computing and communications equipment from
a single vendor. But today, corporate networks are increasingly using a single,
common, worldwide standard called Transmission Control Protocol/Internet
Protocol (TCP/IP). TCP/IP was developed during the early 1970s to support
U.S. Department of Defense Advanced Research Projects Agency (DARPA)
efforts to help scientists transmit data among different types of computers over
long distances.
TCP/IP uses a suite of protocols, the main ones being TCP and IP. TCP refers
to the Transmission Control Protocol, which handles the movement of data
between computers. TCP establishes a connection between the computers,
sequences the transfer of packets, and acknowledges the packets sent. IP refers
to the Internet Protocol (IP), which is responsible for the delivery of packets and
includes the disassembling and reassembling of packets during transmission.
Figure 7.4 illustrates the four-layered Department of Defense reference model
for TCP/IP, and the layers are described as follows:
1. Application layer. The Application layer enables client application programs to
access the other layers and defines the protocols that applications use to
exchange data. One of these application protocols is the Hypertext Transfer
Protocol (HTTP), which is used to transfer Web page files.
2. Transport layer. The Transport layer is responsible for providing the Application
layer with communication and packet services. This layer includes TCP and
other protocols.
3. Internet layer. The Internet layer is responsible for addressing, routing, and
packaging data packets called IP datagrams. The Internet Protocol is one of the
protocols used in this layer.

FIGURE 7.4 THE TRANSMISSION CONTROL PROTOCOL/INTERNET
PROTOCOL (TCP/IP) REFERENCE MODEL

This figure illustrates the four layers of the TCP/IP reference model for communications.

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