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Tuesday, December 29, 2009

Day 30: Layered Model Applications

Don't forget to distinguish between the Open Systems Interconnection (OSI) model and the Transmission Control Protocol/Internet Protocol (TCP/IP) model - stacks made up of layers.

The  TCP/IP model has four layers:
  • Application layer (http, data)
  • Transport layer (TCP, UDP, segments)
  • Internet layer (IP, ICMP, ARP, RARP, packets)
  • Network acces layer (frames, bits)
The OSI model has seven layers:
  • Application (ftp, http, dns, dhcp, snmp, telnet, smtp, data)
  • Presentation (mime, ssl, shells, ascii, .txt, also known as the translator, data)
  • Session (SQL, API, RPC, NetBIOS, data)
  • Transport (TCP, UDP, segments)
  • Network (IP, NAT, packets)
  • Data link (MAC, error correction, FDDI, HDLC, Frame Relay, frames)
  • Physical (bits)
If you compare both models, its important to remember that the Network access layer of TCP/IP is the same as the first two OSI-layers (data link and physical)!
The OSI upper layers deal with the data's format, organization and communication. The lower layers implement protocols to transport and route data across a network.
Don't forget that a layered model has several benefits: helps design of protocols, interoperability of vendors, changing one technology without affecting other layers, common terminology to teach and learn.

The data link layer has two sublayers to provide physical media indepedence: the upper logical link control (LLC) layer  and the lower media access control (MAC) layer. The Ethernet protocol operates at the data link and physical layer of the OSI model!
LLC: what to do with a packet after it is receive.
MAC: how data is placed and transported over the physical wire.

The layers communicate with each other using service access points (SAP) and protocol data units (PDU).
Encapsulation wraps data with the necessary protocol information before network transmission.

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