OSI Reference Model Flashcards
LAYER 1: PHYSICAL
Deals with the physical characteristics of the transmission medium (hardware).
It defines the specifications for communication between the physical link and the end systems.
Deals with such characteristics as:
. Voltage levels,
. Timing of voltage changes,
. Physical data rates,
. Maximum transmission distances,
. Physical connectors.
* Examples :- EIA/TIA-232, RJ45.
LAYER 2: DATA LINK
Provides access to the networking media and physical layer
Deals with transmission across the media
Location of the intended destination on a network.
Can provide reliable transit of data across a physical link by using the Media Access Control (MAC) addresses.
Uses the MAC address in order for multiple stations to share the same medium and still uniquely identify each other.
Concerned with network topology, network access, error notification, ordered delivery of frames, and flow control.
- Examples :- Ethernet, Frame Relay, FDDI.
LAYER 3: NETWORK
Defines end-to-end delivery of packets.
Defines logical addressing
Defines how routing works and how routes are learned so that the packets can be delivered.
How to fragment a packet into smaller packets to accommodate different media.
Routers operate at Layer 3.
LAYER 4: TRANSPORT
Regulates information flow to ensure end-to-end connectivity between host applications reliably and accurately.
Segments data from the sending host’s system and reassembles the data into a data stream on the receiving host’s system.
Layer 4 protocols include TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).
LAYER 5: SESSION
The session layer defines how to start, control and end conversations (called sessions) between applications.
It uses a dialogue control for management of multiple bi-directional messages.
It synchronizes dialogue between two hosts’ presentation layers and manages their data exchange.
It offers provisions for efficient data transfer.
LAYER 6: PRESENTATION
Ensures that the information that the application layer of one system sends out is readable by the application layer of another system.
It translates between multiple data formats by using a common format.
Provides encryption and compression of data.
Layer 7 APPLICATION
The OSI layer that is closest to the user.
It provides network services to the user’s applications.
It does NOT provide services to any other OSI layer, only to applications
It checks the availability of intended communication partners
It synchronizes and establishes agreement on procedures for error recovery and control of data integrity.
OSI Reference Model
Physical layer, Bit, Repeater, Hub or Modem
Data Link layer, Frame, Bridge or Switch
Network layer, Packet, Router
Connection and Connectionless Transport
Connection-oriented TCP is used for transport when there is a need for delivery assurances:
>Applications can assume reliable delivery
Connectionless UDP is used when the application performs any needed recovery:
. In request/response applications
. By repeating the request after a timeout
. This can cause a duplicate operation if the response was delayed
Connectionless UDP is also used for:
. Broadcast
. Real-time e.g. VoIP
The importance of Networking Standards
The use of standards is fundamental to Open Systems
Independence from vendor proprietary approaches
Open procurement
Interoperability
Standards should be international in scope
It is important to track new standards
Knowing when it is “safe” to use an emerging standard
Organisations that produce Networking Standards
International Standardisation Organisation (ISO)
European Telecommunications Standards Institute (ETSI)
The TCP/IP Internet Engineering task force (IETF)
– Publishes ‘Request for Comments’ (RFC)
Institute of Electrical and Electronics Engineers (IEEE)
American National Standards Institute (ANSI)
