CTI-120 Lesson 2, Defining Networks with the OSI
Model
Open Systems Interconnection(OSI) - ANS-Reference model used to define how data
communication occurs on computer networks. This model is divided into layers, each of which
provides services to the layers above and below
Layer 1 - Physical Layer - ANS-This is the physical and electrical medium for data transfer.
Includes cables, jacks, patch panels, punch blocks, hubs, and MAUs
Layer 2 - Data Link Layer(DLL) - ANS-This layer establishes, maintains, and decides how
transfer is accomplished over the Physical Layer.
Unit of measurement on this layer is "frames"
Layer 3 - Network Layer - ANS-This layer is dedicated to routing and switching information to
different networks, LANs, or internetworks. Devices that exist on this network layer are network
layers and IP switches.
Unit of measurement used on this layer is "packets"
Layer 4 - Transport Layer - ANS-This layer ensures error-free transmission between hosts
through local addressing. Manages the transmission of messages between layers 1 through 3.
When you think of ports, think of the transPORT layer.
Unit of measurement used on this layer is "segments" or "messages"
Layer 5 - Session Layer - ANS-This layer governs the establishment, termination, and
synchronization of sessions within the OS over the network and between hosts - for example,
when you log on and off.
This layer Controls the name and address database for the OS and NOS
Layer 6 - Presentation Layer - ANS-This layer translates the data format from sender to receiver
in the various OSes that may be used.
Concepts include code conversion, data compression, and file encryption
Layer 7 - Application Layer - ANS-This layer is where message creation - and, therefore packet
creation - begins. Database access is on this level. End-user protocols such as FTP, SMTP,
Telnet, and RAS work on this layer.
This layer is not the application itself, but the protocols that are initiated by this layer
Communication Subnetwork - ANS-The guts of the OSI model transmissions, consisting of
layers 1 through 3
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