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OSI Networking Model Layer 7

 

7 - APPLICATION - Network process to application (User Interface)

 

Function: Communication between programs. This is the layer that user programs talk to. This is where most Industrial Automation applications are run. Serial encapsulation of older protocols, ModbusTCP, Ethernet / IP and others operate at this level.

 

Data unit: Messages & Packets

 

Services (examples): gateways, Web browser, Network File System (NFS), Telnet and Remote Login (rlogin), X.400, FTAM, Database software, Print Server Software.

 

Protocols (examples): DNS, FTP, TFTP, BOOTP, SNMP, RLOGIN, SMTP, MIME, NFS, FINGER, TELNET, NCP, APPC, AFP, SMB.

 

Components: Gateway.

 

Topics & Methods Explained


Network services

Supports file, print, message, application, and database services. Does not include user software packages such as Word, but includes the technologies that allow user software to take advantage of network services.

Service advertisement

Active:
Each server sends out periodic messages (including service addresses) to announce its availability. Clients collect advertisements and build tables of currently available services.

 

Passive:
Servers register their service and address with a directory. When clients want to identify which services are available, they ask the directory to locate the service and provide its address.

Service use

OS call interception:
The local operating system is completely unaware of the network services. A special piece of software intercepts a service request before it reaches the local OS and sends out a request for network services.

 

Remote operation:
The local operating system is aware of the network and is responsible for submitting requests for network services. However, the server is unaware of the client. All requests for services appear the same to the server's OS.

 

Collaborative:
Both the service requestor and the service provider recognize the existence of the other and work together to coordinate service use. Typically required to perform Peer-to-Peer collaborative computing.

 

 

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