6.4 COMPUTER NETWORKS 321Network Architecture
Aprotocolis a formal description of message formats and the rules two or more machines
must follow to exchange those messages. Because TCP (transmission control protocol) and IP
(Internet protocol) are the two most fundamental protocols, the entire protocol suite that is used
by the Internet is often referred to as TCP/IP. X.25 is the CCITT (Consultative Committee on
International Telephony and Telegraphy) standard protocol employed by Telnet, and is most
popular in Europe. Ethernet utilizes CSMA/CD (carrier sense multiple access with collision
detection) protocol technology.
Computer network architecture refers to the convention used to define how the different
protocols of the system interact with each other to support the end users. The most common
network architecture model is the open-systems interconnections (OSI). Figure 6.4.1 shows
the ISO (International Standards Organization) seven-layer model for an OSI. Although not all
layers need be implemented, the more layers that are used, the more functionality and reliability
are built into the system. Starting from the bottom layer, the functions of the layers are as
follows.
- Physical—Defines the type of medium, the transmission method, and the transmission
rates available for the network; provides the means for transferring data across the
interconnection channel and controlling its use. - Data Link—Defines how the network medium is accessed, which protocols are used,
Layer 7: Application
Layer 7 protocolLayer 6 protocolLayer 5 protocolLayer 4 protocolLayer 3 protocolLayer 2 protocolLayer 1 protocolLayer 6: PresentationLayer 5: SessionLayer 4: TransportLayer 3: NetworkLayer 2: Data linkLayer 1: PhysicalInterconnection channelLayer 7: ApplicationProcess AVirtual communications
shown by dashed linesPhysical
communications
shown by
solid lines
Process BLayer 6: PresentationLayer 5: SessionLayer 4: TransportLayer 3: NetworkLayer 2: Data linkLayer 1: PhysicalFigure 6.4.1ISO 7-layer model for an OSI.