Table 1-1: OSI/RM layers
Layer
|
Layer Number
|
Description
|
Application
|
7
|
The interface to the user in an OSI environment. Supports file transfer, network management and other services. The first layer used when a packet is being created in a system.
|
Presentation
|
6
|
Provides useful transformations on data to support a standardized application interface and general communications services. For example, it converts text from American Standard Code for Information Interchange (ASCII) format into Extended Binary Coded Decimal Interchange Code (EBCDIC).
|
Session
|
5
|
Establishes, manages and terminates connections (sessions) among cooperating applications. Also adds traffic flow information.
|
Transport
|
4
|
Provides reliable, transparent transport between end points (the source and destination hosts). Also supports end-to-end error recovery and flow control.
|
Network
|
3
|
Organizes data into datagrams (in other words, packets). Provides reliable addressing services among hosts and networks. Ensures that datagrams are forwarded and routed to their destinations.
|
Data link
|
2
|
Provides reliable data transfer across the physical link. Frames are transmitted with the necessary synchronization, error control and flow control. In short, the data link layer prepares the information so it can be placed on the transmission medium, such as a copper wire.
In the IEEE 802 series of LAN standards (a group of popular network standards you will study in this course), the data link layer is divided into two sublayers: the Logical Link Control (LLC) layer and the Media Access Control (MAC) layer. The LLC is responsible for error and flow control, and the MAC layer is responsible for placing data on the transmission medium.
|
Physical
|
1
|
Associated with transmission of unstructured bit streams over a physical link. Responsible for the mechanical, electrical and procedural characteristics that establish, maintain and deactivate the physical link.
|
Figure 1-13: Headers added at each level of OSI/RM
Table 1-2: OSI/RM protocol examples
Layer(s)
|
Corresponding Protocol
|
Protocol Description
|
Application (upper layer)
Presentation Session |
Simple Mail Transfer Protocol (SMTP)
|
Used to send e-mail messages
|
Post Office Protocol 3 (POP3)
|
Used to allow clients to log on to a remote server and obtain e-mail messages
| |
Internet Message Access Protocol (IMAP)
|
Used to allow clients to access and manage e-mail messages without first downloading them
| |
Hypertext Transfer Protocol (HTTP)
|
Used to interconnect Web pages
| |
Server Message Block (SMB)
|
Allows files to be shared on a Microsoft network
| |
NetWare Core Protocol (NCP)
|
Allows files and printers to be shared on a NetWare network
| |
Network file system (NFS)
|
Allows files and printers to be shared on a UNIX network
| |
Transport
|
Transmission Control Protocol (TCP)
|
Part of the TCP/IP protocol suite; provides reliable delivery and manages sessions
|
Sequenced Package Exchange (SPX)
|
Part of the IPX/SPX protocol suite; similar to TCP in that it manages communication sessions
| |
NWLink
|
Microsoft's implementation of IPX/SPX; operates at both the transport and the network layers
| |
NetBEUI
|
Allows different applications on different computers using NetBIOS to communicate with one another; a nonroutable protocol that operates at both the transport and the network layers
| |
Network
|
Internet Protocol (IP)
|
Part of the TCP/IP protocol suite; responsible for addressing hosts and routing packets in any network running TCP/IP
|
Internetwork Package Exchange (IPX)
|
Provides addressing services for the Novell IPX/SPX suite
| |
NWLink
|
Microsoft's implementation of IPX/SPX; operates at both the transport and the network layers
| |
NetBEUI
|
Allows different applications on different computers using NetBIOS to communicate with one another; a nonroutable protocol that operates at both the transport and the network layers
| |
Data link
|
Ethernet
|
A LAN protocol that was created by Xerox, Digital Equipment Corp. and Intel; the most popular LAN technology
|
Token Ring
|
A LAN protocol that was created by IBM to provide more reliability than Ethernet; not as commonly implemented
|
