OSI model

OSI model(Open Systems Interconnection) is a way to explain network comunications. This model defines 7 different layers.

7- Application:

Provides services/protocols to applications.

  • Protocols: FTP, DNS, Telnet, HTTP, SNMP, NFS

6- Presentation:

Provides codign and coversion funcion to the application layer suach as compression, decompression, encryption and decryption.

  • Protocols: Binary, ASCII, GIF, Encryption.

5- Session:

Controls sessions. Sets up, manages and tears down sessions between presentation layer entities. It has three communication modes: SIMPLEX, HALF-DUPLEX, FULL-DUPLEX

  • Protocols: SMB, RPC, SQL, NetBIOS, SAP

4- Transport:

Provides flow control to prevent loss of data. Supports reliable (connection-oriented, TCP) and unreliable(connectionless-oriented, UDP) data transport.

  • Protocols: TCP, UDP, SPX, SSL, TLS, SCTP

Flow control methods:

  1. Buffering
  2. Windowing
  3. Congestion prevention

3- Network:

End to end delivery logical addressing fragmentation for MTU Routing.

  • Protocols: IP, ICMP, IPsec, IGMP, IPX, AppleTalk

2- Data Link:

Provides media access(MAC) error detection and assembles frames from bits.

Switches or Bridges functions and features:

Filters the network using MAC addresses.

Provides low data transfer latency using Application-Specific Integrated Circuits (ASICs)

Builds the CAM Table(Content Addressable Memory) to associate destination MAC address with local interface.

Break down collision domains.

Hubs functions:

Won’t break broadcast or collision domain.

Acts like a wire, all nodes connected to a HUB will see each other’s transmitted data.

  • Protocols: ARP, PPP, PPTP, L2TP, Ethernet, Frame Relay

1- Physical:

Media interface. Transmition method. Sends and receive bits. Provides specification voltage, wire speed and calbe pin-outs.

  • Protocols: RS-232, DSL, POTS, BLUETOOTH, SONET/SDH, T1, E1


Media layers: Physical,Data-Link,Network

Host Layers: Transport,Session,Presentation,Application

In layers 1 to 4 data is moving. Layers 5 to 7 data is being created.

There are 5 different encapsulation methods depending of layers:

Bit or Data-stream in physical layer.

Frame in Data-Link layer.

Packet or Datagram in Network layer.

Segment in Transport layer.

Data in Session layer. The concept of each layer appending its header to the data received and passing it to the next layer.

Interesting Concepts:

ARP: Address Resolution Protocol is a computer networking protocol for determining a network host’s link layer or hardware address when only its internet laer (IP) or network layer address is know.

MTU:Maximum transmission unit of a layer of a comunicatinos protocol is the size in bytes of the largest protocol data unit that the layer can pass onwards. MTU parameters appear in association with a comunications interface.

TCP Header. See below: