January 22, 2023

Computer Networks: Data communication Standards and Models

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Introduction

The subject of data communications involves the reliable transportation of data from one computer to another. It might be fair to assume that we can guarantee communication by only using the equipment and products of one manufacturer (known as a homogenous environment). In reality, it will more probably be the case that the network comprises products and services from a number of different supplier (a heterogeneous network).

There are many standards bodies active in the area of voice and data communications and there are many different standards to choose from when specifying LAN and WAN solutions. This article looks more deeply into this area to identify major bodies and the standards that they have produced.

Major Standard Organisations

The area of communication is one that has evolved through time rather than following a dictated plan. As network implementations became more complex, it was more and more essential to regulate the interfaces that existed between different manufacturers equipment.

Initial efforts to standardise equipment, interfaces and the protocols used between them came in part from private industry and in part from the pressures of public telecommunications providers. As a result of this drive, standards first appeared for the elements of public networks. Customer demand for open systems and the desire to breakaway from proprietary systems led to the increased involvement of the manufacturing companies.

The information below provides a high level summary of the main standards bodies whose work impacts on communication activities. It highlights the principal functions of members and some of the main areas in which standards work has been carried out.

• C.C.I.T.T. This international body is composed of Public Telecommunications Operators (PTO) or the national bodies responsible for issuing licenses. Under certain circumstances, private manufacturers may be licensed to attend the sessions of the C.C.I.T.T. This body does not produce standards that have mandatory clauses but delivers recommendations once every four years. The recommendations are coded by colour e.g. X.25 Red Book and X.25 Blue Book. Care should be taken to check which recommendation is complied with when specifying or purchasing equipment.

  Some examples of recommendations that the C.C.I.T.T. have produced include Signalling System No. 7 for Public Switched Networks, X.25 for Public Data networks, X400 for Electronic Messaging Systems and X500 for Directory Services.

• IEEE The Institution of Electrical and Electronic Engineers operates primarily out of the USA although its membership is drawn from a worldwide audience. Members include representatives of national standards bodies, PTOs, private equipment manufacturers and individuals who have attained minimum educational qualifications or appropriate industry experience.

  The standards areas undertaken by the IEEE focus primarily on the LAN environment with some attention given to MANs. Standards related to data communications have been produced by the 802 committee and include specifications on cable types, network topologies and cable access methods. Standards have also been produced relating to network management, security and the use of redundant paths when connecting two networks.

• ETSI The European Telecommunications Standards Institute was formed to handle the telecommunications responsibilities of the older CEPT agency. Its membership is drawn from representatives of national standards bodies, PTOs and private equipment manufacturers. This body which is directly answerable to the European Commission is responsible for all areas of pullic and private telecommunications. In reality, the task of producing standards for private telecommunications has been subcontracted to ECMA.

  The most significant standards produced by ETSI relate to the European implementation of the C.C.I.T.T. Q931 standard.

• ECMA Although the European Computer Manufacturer Association is a private trade body it has succeeded in winning the responsibility for producing standards related to private telecommunications. The membership of BCMA comes primarily from private industry although several PTOs do choose to send representatives to the meetings.
• ISO The International Standards Organisation is perhaps the most senior of all standards bodies. Its membership is drawn from the national standards bodies of each country or agents who have received permission to attend. The responsibilities of the ISO are quite wide ranging.

  The ISO has produced standards in several aspects of communications. On occasion this has been no more than a ratification of the work done by another body. Perhaps the most notable ISO standard related to communications is the 7 Layer Model and we shall examine this in more depth later.

• ANSI The American National Standards Institute is mentioned in this section as this body was responsible for producing the specifications relating to FDDI. As a national body, it cannot enforce its standards on any other nation although most FDDI implementations will follow the ANSI standard.

The OSI Seven Layer Model

The Open Systems Interconnection seven layer model was developed by the International Standards Organisation to facilitate a clearer understanding of data communications processes and allow buyers to move away from proprietary implementations.

In an effort to make the communications process more understandable, the model attempts to break communication into separate discrete tasks. This allows systems developers to avoid the duplication of effort, by relying upon the fact that certain responsibilities have already been discharged.

The use of the OSI model when designing a protocol implementation hinges upon two critical factors. Firstly, the data (including control information) produced at any one level is only understood at the equivalent level in another communications stack. Secondly, the interfaces and rules governing how data is passed to the layer immediately above or below are clearly defined.

The majority of the layers have an overhead incurred designed to provide identification and control information. This overhead may include addressing information to identify the distant system and information to identify which protocols the data should be handed off to when it is received.

If the principles of identifying the distant address and the upper layer processes are adhered to, a new protocol can be designed for a single layer which will not effect any of the other layers (provided that it maintains the interface agreement).

It should be stressed that those programs which users commonly perceive as applications e.g. word processing do not have any place within the 7 Layer Model. They would actually utilise the facilities offered by the upper layers of the stack.

The OSI Seven layer Model is in reality just that. It is not necessary to design a practical implementation that strictly adheres to the model, with separate protocols for each layer.

The following explanation of the model identifies the roles of each of the layers. Protocols in common use may implement all or some of the suggested responsibilities at each layer. However, the less implemented the less robust the protocol. Some protocols leave out components to save on the large overheads that can occur in both processing and data transmission.

When an application wishes to communicate with another application, data is sent down the stack, collecting headers that contain the vital information to encode and route the data to its destination. The bottom four layers are commonly referred to as the network layers. These physically transport the data from one application to another, free from errors. The upper three layers are responsible for formatting of the information and providing multiple paths to the same application.

The process of adding the vital header component to the data is termed data encapsulation. Each layer treats all the information it received from the last layer as data and incorporates its own header to the front. This header may identify the peer entity at the receiving system, the process from which the data was received and the higher layer protocol, which the data should be handed up to. As the information climbs up the other side the headers are removed one by one and decoded.

The Physical Layer

The Physical Layer is responsible for defining the clectrical and mechanical interfaces between devices. It specifies the following:

• Data transfer rate or clocking speed.
• The type of cable used (Coax, twisted pair or fibre optics).
• The level of the electronic or light signal, represented by a one or a zero.

Typical examples of interfacing standards working at this level are RS232/V24 and X21/V11. Physical layer specifications for LAN environments are encapsulated with the IBBE 802.3, 802.4 and 802.5 protocols.

When connecting networks, at this level repeaters will faithfully reproduce the signal as received. Modern repeaters support the use of different cable types on each side and are used to convert one type of electrical signal into another. The one requirement that exists is that both sides of the repeater must use the same datalink protocol. Repeaters do not have the ability to filter data.

The Datalink Layer

The Datalink Layer is responsible for giving significance to the 1's and 0's of the physical layer as well as moving data from one end of a wire to the other. The network card will assemble a frame of data then add the addressing information to it. Further to this it then controls the placing of the data onto the wire.

Protocols that work at this level deliver data from card to card. They provide limited addressing schemes but are unable to group addresses into logical partitions (or networks). Dependent on the datalink protocol in operation, it may be possible to determine if any corruption has occurred and correct the errors.

WAN examples of this type of protocol include the following:

• High Level Datalink Control (HDLC), used by X25 networks.
• Frame Relay.
• ATM.

In the LAN arena a number of protocols exist. Chief amongst these are the 802 series of protocols which describe cable access methods, frame formats and hardware addressing. These protocols are:

• Ethernet (IEEE 802.3)
• Token Bus (IEEE 802.4)
• Token Ring (IBEE 802.5)

The IEEE 802.2 protocol operates at the upper portion of this layer and provides a consistent interface to the network protocols operating above. It identifies source and destination network layer protocols (also know as Service Access Points). The 802.2 standard also provides for error detection, error recovery and flow control.

Ethernet II is closely allied to the 802.3 standard in terms of its function and cable access methods. However, due to subtle differences in how the frame is formatted the two protocols are totally incompatible. Ethernet II retains a strong popularity due to the widespread existence of TCP/IP systems running over Ethernet.

Data can be filtered out at this level when two networks are connected together using bridges. Bridging is the term used for data switching device that operate at this level. Bridges can allow the administrator to use different access methods on each side provided that the network layer is the same.

These devices also have to understand the physical level. To this end a repeater component must be included.

Network Layer

The transport layer is responsible for delivering the data from the source system to the destination system. The administrator implements addressing schemes ensuring that each computer on the system can be identified through use of a unique address. Having placed the devices into logical groups, part of the address identifies the network and a further part identifies the individual device within this network.

As these addresses are resident in computer memory, it might be argued that the network layer is responsible for getting information from processor to processor. At this level data is assembled into packets with control information contained in the header. This control information may include the address of the peer level on the distant system, the process from which the data was received and the process for which the data is destined. The packet is then placed within the datalink frame and transmitted to the relevant device.

Devices may have the same host addresses provided they belong to different networks. The gateway can examine the network part of the address and therefore route packets without identifying the individual device. This function is carried out by a router. The router must be aware of the functionality of all three lower layers of the model but actual routing takes place at the network layer.

This means that each device has two addresses, a datalink address and a network address. To ensure that each device can be clearly identified, specific protocols within the network layer are used purely to link the two together.

The following are typical LAN protocols that operate in this area:

• Internetwork Packet eXchange (IPX) from Novell
• Internet Protocol (IP), used extensively on UNIX networks as well as the
• Internet.
• X25, well established as a WAN protocol at this level.

This layer is not responsible for getting the data to a particular application, only to a particular network endpoint. Nor is it necessarily responsible for guaranteeing its safe delivery.

The Transport Layer

The transport layer is responsible for the establishment of connections, maintaining the connection quality and clearing the connection in an orderly fashion when the conversation is over. If it is accepted that the network layer is responsible for processor to processor communication. Depending on the protocol, it may provide safety mechanism and guarantee integrity of the data from end-to-end. Some of the tasks that may be carried out at this layer include:

• Identify the individual process within the system
• Ensure information arrives in the correct order
• Identify any corruption within individual packets
• Acknowledge receipt of each packet
• Implement data recovery mechanisms

The following protocols are currently in use for this layer:

• Transmission Control Protocol (TCP)
• Sequenced Packet eXchange (SPX)

For speed benefits, some implementations relinquish the use of acknowledgements and will allocate responsibility for error correction to upper layer protocols.

Examples of these types of protocols include:

• User Datagram Protocol
• Internetwork Packet eXchange

The Session Layer

The Session Layer is responsible for establishing and regulating multiple connections to the same process from other processes. Prior to this layer, all of the model's effort has been concentrated on getting the data to the right process. When two processes require simultaneous connections to a single process, each needs to be individually identified. One of the key features associated with this layer is that of security.

If the Transport Layer is responsible for establishing and maintaining the connection between processes then the Session Layer regulates the dialogues or conversations that take place.

One protocol which is used at this layer is known as the Remote Procedure Call (RPC).

The Presentation Layer

Prior to the Presentation Layer, the data stream has been treated purely as ones and zeros as no previous layer was responsible for interpreting this information. The Presentation Layer assembles the data into bytes and characters. The protocols used at this layer define the rules for this conversion process. These rules can include the quantity of bits that characters are constructed from, the direction in which the bit stream should be read and whether or not any encryption mechanism has been employed.

The example below clearly demonstrates the different results that would be achieved by dividing the bit stream into 7 or 8 bits respectively.

1000100, 1101010, 1101001, 0010101, 0010111
10001001, 10101011, 01001001, 01010010, 11101101

Systems using floating-point numbers must construct the data into three components:

Mantissa, Exponent and Sign bit.

Individual systems may have different numbers of bits to represent each component.

A number of standards exist at this layer for the representation of characters and include:

American Standard Code for Information Interchange (ASCII).

Extended Binary Coded Decimal Interchange Code (EBCDIC).

A common floating point number standard is eXternal Data Representation (XDR).

The Application Layer

The Application Layer provides the final interface to the user. It contains control information which relates to the type of information and the language in which the characters are assembled. It indicates whether the data is text based or graphical. The Application Layer could perhaps be best described as the services available to the users end programs. Services such as file transfer, messaging, print services and directory services would occur at this level.

The DoD model subdivides the communications process into four primary components. It uses a similar principle to the OSI model such as the use of control information that contains addressing, source and destination applications.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2023 Mr Singh

Reference: https://discover.hubpages.com/technology/Computer-Networks-Data-communication-Standards-and-Models

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