Understanding the network.pdf - Back to Home

Layer 7: Application
This layer provides the network service access to applications. All application
programs are included at this layer (including ones that do not require any
communication services), and each application must employ its own protocol to
communicate with the Lower-Layer Protocols (LLPs). Basic file and print services
also fall into the application layer.
Layer 7 services are the places where the application communicates with the user in
terms of actual (human) meaningful input and output data. The following standards
govern data between the user application and the network:
• ISO—X.500 Directory Services
X.400 Message handling (email) services
Virtual Terminal Protocol (VTP)
• TCP/IP—Telnet virtual terminal service
Simple Mail Transfer Protocol (SMTP)
Domain Name Service (DNS)
Berkeley Remote Commands
Sun's Network File System
CMU's Andrew File System
• AppleTalk—AppleShare print and file services
• IPX—NetWare Core Protocol
NetWare Shell (NetX)
Layer 6: Presentation
This layer addresses the problems of data representation as it appears to the user.
Data syntax, character sets, and data formatting also fall under Layer 6. Layer 6
also provides the means for the various Layer 7 services to exchange information in
an encoding scheme. Almost all systems use the ASCII encoding scheme to present
data so it can be transmitted in a computer-independent form. This way, computers
of various types can exchange information with one another. Overall, Layer 6