ISO First Standards for Computer Networking – An Overview History
One of the first standards for computer communications was proposed and developed by the International Standards Organization (ISO) in the early 1980s. This network architecture model, the open systems interconnection (OSI) reference model shown in Fig. 1, describes a network through seven layers. On any of these layers, one or more protocols can implement the functions specified for the layer. Some protocol specifications based on this model, the “X dot” series X.25, X.400, etc., were specified by the International Telecommunications Union (ITU).
Even though most of today’s protocols do not follow this reference model, it captures very well the concepts of service, protocol, and interface. OSI design is general and quite complex, but it is able to present the functionality of a network in a way it can be used as a guide for designing new networks.
Types of Layer:
There are seven layers in the OSI model, starting with the physical layer handling the raw data transmission over a physical medium. The most common transmission media are twisted pair (copper wires), coaxial cable, and fiber optics.
1. Data Link Layer:
The Data Link Layer, usually implemented in the network adaptors, is above the physical layer and is concerned with the organization of data into frames and the reliable transportation of these frames over a direct link.
The specific problems of multi-access links such as channel allocation and collision detection are handled by the data link sub-layer called Medium Access Control (MAC).
2. Sliding Window Protocols:
Reliable frame delivery, frame ordering, and frame retransmission are provided in the layer by Sliding Window Protocols. This is a set of protocols for full-duplex data frame transmission, in which the sender and the receiver both keep windows of frame acknowledgements and send frames only if a certain number of already sent frames were acknowledged by the receiver. The data link layer also includes some error detection and correction functions such as parity bit code and cyclic redundancy code (CRC).
3. Network Layer:
The next higher layer is called Network Layer, and it addresses the problem of finding a route through the network from the source to the destination. This layer also addresses the problems of interconnecting different networks.
Protocols in this layer are used to glue together heterogeneous networks into one scalable internetwork or internet. Mind the difference between Internet and internet, where the former is the global, widely used internetwork and the latter is the general term for a logical network consisting of a collection of physical networks.
4. Transport Layer:
The Transport Layer takes care of the efficient and reliable delivery of data from the source to the destination node. Protocols in this layer are sometimes called end-to-end protocols. Modified versions of the sliding window protocols ensure reliable delivery, synchronization, and flow control. The fifth layer, the Session Layer, is the one that manages traffic direction and synchronization. The last two layers, the presentation and application layers, are mainly application-oriented service layers. The Presentation Layer is responsible for data representation and data coding, while the Application Layer offers a variety of services for particular applications, such as e-mail or file transfer.
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