OSI Model Flashcards
Addressing
Describing where data messages should go. At each layer, there are different mechanisms for identifying nodes and rules for how they can send and receive messages.
Encapsulation
Describing how data messages should be packaged for transmission. Encapsulation is like an envelope for a letter, with the distinction that each layer requires its own envelope. At each layer, the protocol adds fields in a header to whatever data (payload) it receives from an application or other protocol.
OSI model
Open Systems Interconnection (OSI) model
- Separates the function of hardware and software components to seven discrete layers. Each layer performs a different group of tasks required for network communication.
- The OSI model is not a standard or a specification; it serves as a functional guideline for designing network protocols, software, and appliances and for troubleshooting networks.
Network
A network is two or more computer systems that are linked by a transmission medium and share one or more protocols that enable them to exchange data.
OSI Model Layers
- Application
- Presentation
- Session
- Transport
- Network
- Data Link
- Physical
OSI Layer 1
Physical Layer
The physical layer (PHY) of the OSI model (layer 1) is responsible for the transmission and receipt of the signals that represent bits of data from one node to another node. Different types of transmission media can be classified as cabled or wireless:
- Cabled—A physical signal conductor is provided between two nodes. Examples include cable types such as copper or fiber optic
cable. Cabled media can also be described as bounded media. - Wireless—Uses free space between nodes, such as microwave radio. Wireless media can also be described as unbounded media.
OSI Layer 1 Devices
Devices that operate at the Physical layer include:
- Transceiver—The part of a network interface that sends and receives signals over the network media.
- Repeater—A device that amplifies an electronic signal to extend the maximum allowable distance for a media type.
- Hub—A multiport repeater, deployed as the central point of connection for nodes.
- Media converter—A device that converts one media signaling type to another.
- Modem—A device that performs some type of signal modulation and demodulation, such as sending digital data over an analog
line.
OSI Layer 2
Date Link Layer
- Transfers data between nodes on the same logical segment
- Segment: where all nodes can send traffic to each other regardless of whether they share access to same media
- Users hardware addresses
- Structures stream of information into unites called Frames
OSI Layer 2 Devices
Devices that operate at the data link layer include:
- Network adapter or network interface card (NICs)—An NIC joins an end system host to network media (cabling or wireless) and
enables it to communicate over the network by assembling and disassembling frames. - Bridge—A bridge is a type of intermediate system that joins physical network segments while minimizing the performance
reduction of having more nodes on the same network. A bridge has multiple ports, each of which functions as a network interface. - Switch—An advanced type of bridge with many ports. A switch creates links between large numbers of nodes more efficiently.
- Wireless access point (AP)—An AP allows nodes with wireless network cards to communicate and creates a bridge between
wireless networks and wired ones.
OSI Layer 3
Network Layer
- Responsible around a networks of networks (The internet)
- Uses Logical network and Host IDs
- Uses a variety of physical media and data link protocols
- Forwards information using destination network-layer addresses
Devices:
- Router
- Firewall
ACL
Access Control List
- Enforced by basic firewall at layer 3
- Network ACL is list of addresses and types of traffic that are permitted or blocked
OSI Layer 4
Transport Layer (End to end or Host to host Layer)
- Identifies each type of network application by assigning it a port number
- Sending host packages data from upper layers in Layer 4 PDUs, segments
- Each segment is tagged with port number
Devices:
- Multilayer Switch (Usually load balancers)
- Security devices like advanced firewalls and IDS (Intrusion detection systems)
OSI Layer 5
Layer 5 - Session
- Most protocols require exchange of multiple messages between client and server
- The session layer (layer 5) represents functions that administer the process of establishing a dialog, managing data transfer, and then ending (or tearing down) the session.
OSI Layer 6
Presentation Layer
- Transforms data between format required for network and format required for presentation
- Also can be conceived as supporting data compression and encrypted
OSI Layer 7
Application Layer
- Doesn’t encapsulate any other protocols.
- Provides interface for software programs on hosts that have established communications channels through lower level protocols
- Provides services that make networking useful
