OSI (Open System Interconnection):

• Developed by ISO (International Organization for Standardization) in 1984 (idea to develop in 1977).
• Define communication between devices in 7 layers (steps).
• On one side, each layer taking service from its upper layer, does its own work and passes it to the lower layer. Just opposite happens on the receiving node.
• Upper three layers (Application, Presentation and Session) are application specific.
• Lower four layers (Transport, Network, Data-link and Physical) are transmission specific.

Application:

• Provide user interface to the application (network application such as web browser, outlook express, filezila etc.)
• Protocols: DNS, DHCP, FTP, SMTP, POP3, TFTP etc.

Presentation:

• Present data to the application layer (jpeg, mpeg, dat, docx, zip etc.)
• Responsible for translation of data (encrypt, decrypt, compress, uncompress, encode and decode etc.)

Session:

• Establish, maintain and terminate a connection (session).
• Dialogue control
• Protocols: NFS, RPC etc.

Transport:

• Segment and reassemble (on the other end) data.
• Flow control
  

  (Flow control determines the amount of data that can be transmitted in a given period of time. Flow control prevents the transmitting device from devastating the receiver.)

• Correctly sequence the segments
• Error Control (retransmit the segment with error or lost)
  

  (Error control detects errors in received frames and requests retransmission of frames. It checks whether the correct number of frames arrived and whether any of these are damaged.)

• Acknowledgement (if TCP)
• Address/Name resolution.
• Protocols: TCP, UDP.
  

Network:

• Logical addressing (provide a unique network address).
• Routing (select best path) to the destination network.
• Assemble / deassemble packets.
• Devices: Router, Layer 3 Switch.
• Protocols: IP, RIP, OSPF, ARP

Data-link:

• It deals with the communicating devices in the same network.
• Physical addressing (add source and destination mac-address) to provide unique identification to each node within a network.
• Framing (by putting header and trailer to the packet)
• And then convert frame into bits (1s and 0s) on one end and just opposite on the other end.
• Convert 1s and 0s received from physical layer into frame.
• Divided into two sub layers:
  MAC (Media Access Control):    
  Allow nodes in a network to communicate, by providing their physical address (mac-address).
  LLC (Logical Link Control):            
  Establish and manages logical link between communicating devices in a network.
• Protocols: Ethernet, PPP, HDLC, RARP

Physical:

• Responsible for the transmission of data over a transmission media.
• Define topology (connectivity of cable, devices, connector etc.)
• Signal conversion:
  a) Analog to digital and vice versa.
  b) bit-byte (1s and 0s) to electrical signal, radio wave, light signal and vice versa.
• Devices: NIC, Repeater, Hub, MAU, Modem etc.

 

Protocols used on each layer:

7 Application:HTTP, SMTP, SNMP, FTP, Telnet, SIP, SSH and Scp, NFS, RTSP, Feed, Webcal, XMPP, Whois, AppleTalk, Print Services

6 Presentation: XDR, ASN.1, SMB, AFP, NCP

5 Session: TLS, SSH, ISO 8327 / CCITT X.225, RPC, NetBIOS, ASP, Winsock, BSD sockets

4 Transport: TCP, UDP, RTP, SCTP, SPX, ATP

3 Network: IP, ICMP, IGMP, BGP, OSPF, RIP, IGRP, EIGRP, ARP, RARP, X.25

2 Data Link: Ethernet, Token ring, HDLC, Frame relay, ISDN, ATM, 802.11 WiFi, FDDI, PPP

1 Physical: Wire, radio, fiber optic, Carrier pigeon

TCP/IP Model (DOD Model):

Application:

• Allow application to gain access to the network.
• Protocols: HTTP, FTP, SMTP, DNS, TELNET

Transport:

• Data delivery is controlled by TCP/UDP.
• Port no. assignment.
• Protocols: TCP, UDP.

Session:

• Addressing, Packaging, Routing
• Protocols: IP, ICMP, ARP, IGMP

Network Access:

• Responsible for data transmission over the media.
• Protocols such as Ethernet, Token Ring, Frame Relay, ATM.