OSI

Open Systems Interconnection

Used to define how data communication occurs between devices.

The model is divided into 7 layers, each layer providing services to the layers above and below.

Physical Layer

Defines the physical and electrical medium for data transfer.

Components :  cables, jacks, patch panels, punch blocks, hubs, and MAUs

Concepts : analog versus digital/encoding - bit synchronization - baseband versus broadband - multiplexing, and serial data transfer

Unit of measurement: Bits

Baseband

Refers the fact that devices on the network use digital signaling over a single frequency.

Broadband

Systems use analog signaling over a range of frequencies enabling multiple channels over the same physical medium. 

Data Link Layer

Establishes, maintains, and decides how transfer is accomplished over the physical layer and ensures error-free transmission over the physical layer.

Physical addresses (the hexadecimal address that is burned into the ROM of the NIC, known as the MAC address uniquely identify each hardware device) work at the Data Link Layer.

Components : network interface cards and bridges.

Unit of measurement : frames

MAC

Media Access Control Address

Unique identifiers assigned to network adapters by the manufacturer.

Six octets in length written in hexadecimal.

Layer 2 switches

Use the MAC address of each host computer ’s network adapter when deciding where to direct data frames.

Ports on the switch are mapped to the specific MAC address of the device attached.

VLAN

Layer 2 switching can also allow for a virtual LAN (VLAN) to be implemented.

A VLAN is implemented to segment and organize the network, to reduce collisions, boost performance.

IEEE 802.1Q is the standard that supports VLANs.

A tag is added to the data frame to identify the VLAN

Network layer

Controls the operations of routing and switching information to different networks.Translates logical addresses or names to physical addresses.

Internet Protocol (IP) is a Network Layer protocol.

Devices that work at the network layer are routers and IP switches.

Components : IP addresses, subnets

Unit of measurement : Packets

Layer 3 Switches

Determines paths for data using logical addressing (IP addresses) instead of physical addressing (MAC addresses for a layer 2 switch)

Layer 3 switches forward packets, whereas layer 2 switches forward frames.

Transport layer

Ensures messages are delivered error-free, in sequence and with no losses or duplications.

Protocols that work at this layer :

• segment messages
• ensure correct reassembly at the receiving end
• perform message acknowledgement and message traffic control

Contains both connection-oriented and connectionless protocols.

Unit of measurement :  segments or messages

Connection Oriented Communications

Require both devices involved in the communication establish an end-to-end logical connection before data can be sent.

These communications are considered reliable network services. Packets not received by the destination device can be resent by the sender.

Connectionless Communications

TCP and UDP

Transmission Control Protocol (TCP)

Provides a connection-based, reliable, byte-stream service to programs.

Used for :

• logging on
• file and print sharing
• replication of information between domain controllers transfer of browse lists
• other common functions

TCP can only be used for one-to-one communications.

User Datagram Protocol (UDP)

Provides a connectionless, unreliable transport service.

Used for one-to-many communications, using broadcast or multicast IP datagrams.

Ports

Layer 4 protocol that a computer uses for data transmission.

Ports act as logical communications endpoint for specific program on computers for delivery of data sent.

There are a total of 65,536 ports, numbering between 0 and 65,535.

Ports are defined by the Internet Assigned Numbers Authority or IANA and divided into categories.

Session Layer

Manages session establishment, maintenance and termination between network devices.

Example: when you log on and log off.

This layer controls the name and address database for the OS.

NetBIOS (Network Basic Input Output System) is a protocol that works at this layer.

Presentation layer

This layer translates the data format from sender to receiver in the various OSes that may be used.

Concepts :

• character code conversion
• data compression
• data encryption

Redirectors work on this layer, such as mapped network drives that enable a computer to access file shares on a remote computer.

Application layer

Serves as a the window for users and application processes to access network services.

This layer is where message creation begins.

End-user protocols such as FTP, SMTP, Telnet, and RAS work at this layer.

This layer is not the application itself, but the protocols that are initiated by this layer.

Tableau OSI

Comparaison TCP / OSI

Tableau TCP

Couches OSI avec protocoles et devices