OPEN SYSTEMS INTERCONNECT Reference Model

Prepared by: Amos Satterlee 16 May 1999
Reviewed by: Joe Orr MCSE, MCT 24 May 1999

LAYER

SERVICE

HARDWARE

PROTOCOL/SERVICES

Application Layer
  • Provides access by applications to the network
  • Controls access by applications to the network
  • Informs applications of network errors
  • Gateway: translates between protocols and data formats
  • FTP, TELNET etc.
  • NFS, SMB

Presentation Layer
  • Translates data between application format and network format
  • Compression
  • Encryption
  • Gateway: translates between protocols and data formats
  • In NT, the Network Redirector

Session Layer
  • Establishes session – connection between two nodes
  • Requests connection
    Sends periodic messages to maintain connection
  • Tears down connection
  • Gateway: translates between protocols and data formats
  • Protocol to determine which nodes are allowed to send data
  • Name resolution services
  • Security services
  • Other services required to establish a session

Transport Layer
  • Assembles packets into proper sequence; or
  • Breaks data into appropriate-sized packets
  • Checks for errors
  • Acknowledgement sent on receipt of data
  • Retransmission request sent for packets not received correctly
  • Gateway: translates between protocols and data formats
  • TCP
  • UDP
  • SPX
  • NetBEUI

Network Layer
  • Adds headers to frames to make packets
  • Defines logical address (such as IP or IPX address)
  • Translates between hardware and logical address
  • Router: filters and moves packets to network segment based on logical address
  • IP
  • IPX (NWLink)

 

Data Link Layer

subdivided by IEEE 802.1

described by IEEE 802.2
  • Organizes data into frames
  • Defines hardware address (MAC address, physical address) set in NIC
  • Performs error control using checksum (cyclic redundancy check, CRC)
  • Flow control to ensure receiving device ready
  • Bridge: filters and moves data between network segments based on hardware address
  • Switch: sends packets to nodes based on hardware address
  • NDIS (Network Device Interface Specification) by Microsoft and IBM
  • ODI (Open Datalink Interface) by Apple and Novell

Physical Layer
  • Deals with network media
  • Defines media, connectors, and topologies
  • Translates bits into signals and vice versa
  • Defines signaling methods
  • Deals with synchronization
  • Repeater: connects network segments to extend total length
  • Hub: connects nodes to a central point in a star topology
  • Switch: connects nodes to central point
  • Connectors
  • Network Interface Cards
  • Transceiver: communicates between NIC and media
  • Cables
  • SLIP
  • PPP

 

Reference:  MCSE: The Core Exams in a Nutshell by Michael Moncur; O’Reilly & Associates, Inc.  1998.