Wednesday, March 21, 2007


I would like to discuss about 2 network reference models which are called Open Systems Interconnect (OSI) and Transfer Control Protocol/ Internet Protocol (TCP/ IP). Basically, these models implement procedures to follow when transmitting and receiving data. In networking and communications, this formal specification is called as protocol. Protocols define the format, timing, sequence and error checking used by the network.

OSI is a standard legislated model defined by International Standards Organization (ISO) which is a worldwide federation of national standard bodies. OSI implement seven layer models for networking which consist of Application Layer, Presentation Layer, Session Layer, Transport Layer, Network Layer, Data Link Layer, and Physical Layer. These layers are known as a protocol stack.
  1. Application Layer is the highest level of the model. It defines the manner in which applications interact with the network, including database management, e-mail, and terminal-emulation program. It provides a set of interfaces for applications to obtain access to networked services as well as access to the kinds of network services that support applications directly, e.g. Hyper Text Transfer Protocol (HTTP).
  2. Presentation Layer is reserved for certain kinds of data manipulation or consistent data types being encapsulated or encrypted for transmission. It defines the way in which data is formatted, presented, converted, and encoded.
  3. Session Layer deals with creating a session, transmission of data, and then tears down of the created session. Session Layer will be active if Presentation Layer has data needing to be passed on to a different location. It coordinates communications and maintains the session for as long as it is needed, performing security, logging, and administrative functions.
  4. Transport Layer defines protocols for structuring messages and supervises the validity of the transmission by performing some error checking (checksums). This layer also responsible for resizing the data into allowed size, so that it would fit into the streamed packet.
  5. Networking Layer deals with defining the address of data (i.e. routing). It defines protocols for data routing to ensure that the information arrives at the correct destination node.
  6. Data Link Layer validates the integrity of the flow of data from one node to another by synchronizing blocks of data and controlling the flow of data. It is often done by packaging bits into cells with a predictable beginning and end and special calculations performed on the data known as checksums.
  7. Physical Layer defines the mechanism for communicating with the transmission medium and interface hardware. It moves bits across a shared media between two points.

The advantages of OSI model are:
  • It is standard legalized by International Standards Organization (ISO).
  • All OSI layers providing error checking and handling.
  • Provides connection-oriented and connectionless model.
  • OSI protocols are well hidden and can be replaced easily as the technology changes.
  • Emphasis on providing reliable data transfer service.

The disadvantages of OSI model are:
  • OSI is complex and costly
  • Not so widespread as TCP/ IP

Furthermore, TCP/ IP model are quite similar; it is just that TCP/ IP model implement 4 layers instead of 7 layers. These layers are Application Layer, Transport Layer, Network Layer, and Network Access Layer.
  1. Application Layer groups the functions of OSI Application, Presentation, and Session Layer. In accessing services, end-to-end connection is established and socket is set up at each end of the connection. This socket is created at a particular port number, depending on the application in use.
  2. Transport Layer is divided into 2 protocols which are Transmission Control Protocol (TCP) for reliable data transfer and User Datagram Protocol (UDP) for unreliable data transfer.
  3. Network Layer has the Internet Protocol (IP) as its primary protocol to facilitate and manage the routing process.
  4. Network Access Layer is a combination of OSI Data Link Layer and OSI Physical Layer. However, there is no exact description and agreement about TCP/ IP in layered model; it is just generally agreed that it applies 4 layers model architecture.

The advantages of TCP/ IP model are:
  • Widespread use in networking.
  • Simpler than and not as costly as OSI.
  • Robust compared to alternatives such as OSI.
  • Available on virtually every hardware and operating system platform (often free).
  • The protocol suite on which Internet depends.
  • Provides error checking and handling.
  • Provides both connection-oriented and connectionless model.
  • Enable both reliable and unreliable data transfer service.

The disadvantages of TCP/ IP are:
  • The protocols are unwell hidden.
  • The protocols are hardly possible to be used to describe other models.
  • Not all TCP/ IP layers provide error checking; only Transport layer does.
  • It hasn’t been standardized by ISO

The similarity between OSI model and TCP/ IP model are:
  • Both are used for network data transmission.
  • Both employ all connection and connectionless models at transport layer.
  • The routing principals of both models are the same

The differences between OSI model and TCP/ IP model are:
  • OSI is standard legislated by official recognized body (ISO), while TCP/ IP are standard adopted due to widespread use.
  • The OSI reference model was devised before the protocols were invented, while in TCP/ IP model, the protocols came first.
  • The protocols in the OSI model are better hidden than in the TCP/ IP model and can be relatively easily as the technology changes.
  • TCP/ IP are more widespread used than OSI.
  • TCP/ IP are simpler than OSI.
  • TCP/ IP are not as costly as OSI.
  • OSI is more commonly used as teaching aids, while TCP/ IP is more commonly used for internetworking because TCP/ IP protocols suite on which the Internet depends.
  • OSI uses Horizontal approach, while TCP/ IP use Vertical approach.
  • OSI network layer provides both connectionless and connection-oriented services, while the TCP/ IP internet layer is exclusively connectionless.
  • Implementation of OSI model emphasis on providing a reliable data transfer service, while the TCP/ IP model treats reliability as an end-to-end problem.
  • Each layer of the OSI model detects and handles errors, while TCP/ IP concentrate errors handling and detection on its Transport Layer.
  • OSI use checksums, while TCP/ IP use acknowledgements and timeouts besides using checksums.
  • OSI Transport Layer checks source-to-destination reliability, while TCP/ IP transport layer provides end-to-end verification.

Bibliography: Lecturer’s Reading Materials, Networking Concept Textbook, Dictionary of Networking(.pdf)

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