e-business Concepts  «Prev 

History of reference model

Internetworking Models and Layering Protocols

People started thinking about frameworks when the monolithic, single-tier architecture of the mainframe started to die out in the mid-1980s. The rise of desktop computing and client-server architecture brought about a shift in thinking about technology, information and business processes. It was becoming apparent that separating these domains was resulting in dysfunctional systems that could not inter-operate.

OSI (7-Layer) model

The (OSI model) Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a way of sub-dividing a communications system into smaller parts called layers. A layer is a collection of similar functions that provide services to the layer above it and receives services from the layer below it. On each layer, an instance provides services to the instances at the layer above and requests service from the layer below. For example, a layer that provides error-free communications across a network provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that make up the contents of the path. Two instances at one layer are connected by a horizontal connection on that layer.

Advanced Digital Architectures

TCP/IP Models

The most widely known reference model is that of the Open Systems Interconnections (OSI), referred to commonly as the 7-layer model. It is most often compared directly to the transmission control protocol/Internet protocol (TCP/IP) model and we shall follow the same pattern in this module. For any model the concept of a service provider and receiver is used. Each standard divides the reference model into a number of layers, each of which acts as a service receiver or provider, depending upon the logical state it is in. The OSI reference model was produced by an ISO working group[1] and is occasionally referred to as the ISO model. It is the generic model for all networked systems although other protocol standards use a different number of layers.

In the OSI 7-layer model the following layers are present: physical layer, data link layer, network layer, transport layer, session layer, presentation layer, and the application layer. It is common practice to use the numbers associated with these layers rather than their names. Therefore the physical layer is known as layer 1, the data link layer is known as layer 2, the Network Layer is known as layer 3, and so on.
These interact with their corresponding layers across the network on a peer-to-peer basis[2] so that each believes it has direct contact with its own peer layer. This is shown in Figure 1 for a single connection. The fact that each layer conceptually responds to its neighboring layer at the same level allows network connections to operate without the upper layers.
Figure 8.1: The OSI 7-layer model as applied to two networked hosts, A and B.

Physical Layer

The physical layer (layer 1) defines the physical and electrical characteristics of a network interconnection. These are implemented in the wiring, the connectors, and the network interface cards of the hosts. The data link layer (layer 2) determines how the physical medium is accessed. HDLC, SLIP, and PPP protocols operate in part at this level. For the Ethernet MAC addresses are located at level 2 and this allows a LAN to become aware of its constituent members by their unique hardware addresses. The network layer (layer 3) allows systems using the same protocol to establish, maintain, and terminate connections.
The IP protocol operates at the network layer, as do local hardware components such as routers. The transport layer (layer 4) ensures data reliability and integrity (TCP is usually considered to operate at this layer); retransmission requests and packet duplication problems are also resolved. The session layer (layer 5) has responsibility for ensuring sessions are completed without interference when necessary. This is important to e-commerce where the transaction cannot be disturbed by load sharing during its processing. The presentation layer (layer 6) deals with decryption, protocol conversion, and graphics expansion prior to presenting the information to the application layer. Layer 7, the application layer, is the part the user sees and uses. At this layer are applications such as FTP and other packages that require network access.
The TCP/IP structure is similar and due to its earlier commercial support (and support from the DoD in the USA) is the de facto applied standard. (It should be noted that the Department of Defense did eventually formally move to the OSI model.) The structure is shown in Figure 8.2.

Figure 8.2: The reference model for TCP/IP. Note the lack of the session and presentation layers.

[1] ISO standards are developed by a small group of experts representing participating countries in the ISO Technical Committee (TC) or Sub-Committee (SC). These experts represent their expertise on the subject matter and not their country as they discuss the finer points of terminology used, interpretation and technical aspects of the standard. They are the members of the Working Group (WG).

[2] Peer-to-peer interaction describes an approach to interaction and collaboration between participants in a shared project or activity that is characterized by networkbased organizational structures, a shared common resource base, and an assumption that all participants have the potential to make constructive contributions.