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Communication Network Port (CNP) PDF Print E-mail
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Saturday, 04 June 2011 22:56

 

Communication networks elaborate the Fundamental Model of Communications. The model shown in Figure 1 describes point-to-point communications well, wherein the link between transmitter and receiver is straightforward, and they have the channel to themselves. One modern example of this communications mode is the modem that connects a personal computer with an information server via a telephone line. The key aspect, some would say flaw, of this model is that the channel is dedicated: Only one communications link through the channel is allowed for all time. Regardless whether we have a wire line or wireless channel, communication bandwidth is precious, and if it could be shared without significant degradation in communications performance (measured by signal-to-noise ratio for analog signal transmission and by bit-error probability for digital transmission) so much the better.

 

The idea of a network first emerged with perhaps the oldest form of organized communication: the postal service. Most communication networks, even modern ones, share many of its aspects.

·         A user writes a letter, serving in the communications context as the message source.

·         This message is sent to the network by delivery to one of the network's public entry points. Entry points in the postal case are mailboxes, post offices, or your friendly mailman or mailwoman picking up the letter.

·         The communications network delivers the message in the most efficient (timely) way possible, trying not to corrupt the message while doing so.

·         The message arrives at one of the network's exit points, and is delivered to the recipient (what we have termed the message sink).

 

What is most interesting about the network system is the ambivalence of the message source and sink about how the communications link is made. What they do care about is message integrity and communications efficiency. Furthermore, today's networks use heterogeneous links. Communication paths that form the Internet use wireline, optical fiber, and satellite communication links.

The first electrical communications network was the telegraph. Here the network consisted of telegraph operators who transmitted the message efficiently using Morse code and routed the message so that it took the shortest possible path to its destination while taking into account internal network failures (downed lines, drunken operators). From today's perspective, the fact that this nineteenth century system handled digital communications is astounding. Morse code, which assigned a sequence of dots and dashes to each letter of the alphabet, served as the source coding algorithm. The signal set consisted of a short and a long pulse. Rather than a matched filter, the receiver was the operator's ear, and he wrote the message (translating from received bits to symbols).

NOTE: 

Because of the need for a comma between dot-dash sequences to define letter (symbol) boundaries, the average number of bits/symbol, as described in Subtleties of Coding, exceeded the Source Coding Theorem’ supper bound.

Internally, communication networks do have point-to-point communication links between network nodes well described by the Fundamental Model of Communications. However, many messages share the communications channel between nodes using what we call time-domain multiplexing: Rather than the continuous communications mode implied in the Model as presented, message sequences are sent, sharing in time the channel's capacity. At a grander viewpoint, the network must route messages—decide what nodes and links to use—based on destination information—the address—that is usually separate from the message information. Routing in networks is necessarily dynamic: The complete route taken by messages is formed as the network handles the message, with nodes relaying the message having some notion of the best possible path at the time of transmission. Note that no omnipotent router views the network as a whole and pre-determines every message's route. Certainly in the case of the postal system dynamic routing occurs, and can consider issues like inoperative and overly busy links. In the telephone system, routing takes place when you place the call; the route is fixed once the phone starts ringing. Modern communication networks strive to achieve the most efficient (timely) and most reliable information delivery system possible.

Different Types of Communication Networks

Modern society is dependent on communication networks to transmit information among a wide number of people. Humans use these networks to exchange information and ideas through a system of symbols, writing, signals or speech. Networks employ many tools for distributing this information, including television sets, radios and computers. Society uses many different networks for communication purposes.

Local Area Network

o    A local area network is a network of computers in a localized area, such as in an office or a school. All the computers are connected to each other through the LAN via a hub or a switch. A lot of computers, however, drag down the speed of a LAN.

Wide Area Network

o    A wide-area network covers a large geographical area and usually consists of multiple computer networks. The Internet is a WAN which relies on a large global network of service providers who use routers, switches, modems and servers to provide connectivity to people and organizations around the world. It is a network of interconnected computers that carries data, media and web pages.

Public Switched Network

o    The public switched network is essentially the telephones' version of the Internet. It is a network of public circuit-switched telephones. The network today is largely digital and includes services for both cellular and landline phones.

Telecommunication

o    Telecommunication is the transmission of signals over a large distance, usually by electromagnetic waves. It is used for television, radio and telephones. Computer data transmission is also a form of telecommunication.

Wireless Networks

o    Wireless networks provided information transmission and network connectivity to devices without cables or wires. Some examples of a wireless network include broadcast radio, which sends data over long geographical distances and is available to anyone with a radio who knows the frequency. Wi-Fi is a wireless network for computers, which can access the network remotely. Bluetooth, which connects with a nearby mobile phone, is a shorter-range version of a wireless network, which supports transmission of voice and data but only at a distance of a few feet from the communication device with which it works.

Satellite Networks

o    Satellite networks come in a number of different varieties. Phone companies use satellites for data and voice transmission to mobile phones on the ground. Some satellite networks provide navigation information, military surveillance or weather data. Still others provide television programming, radio broadcasts and even broadband Internet service.

 

ADVANTAGES OF NETWORK

Speed. Sharing and transferring files within Networks are very rapid. Thus saving time, while maintaining the integrity of the file.

Cost. Individually licensed copies of many popular software programs can be costly. Networkable versions are available at considerable savings. Shared programs, on a network allows for easier upgrading of the program on one single file server, instead of upgrading individual workstations. 

Security. Sensitive files and programs on a network are passwords protected (established for specific directories to restrict access to authorized users) or designated as "copy inhibit," so that you do not have to worry about illegal copying of programs.

Centralized Software Management.  Software can be loaded on one computer (the file server) eliminating that need to spend time and energy installing updates and tracking files on independent computers throughout the building. 

Resource Sharing. Resources such as, printers, fax machines and modems can be shared. 

Electronic Mail. E-mail aids in personal and professional communication. Electronic mail on a LAN can enable staff to communicate within the building having tot to leave their desk. 

Flexible Access. Access their files from computers throughout the firm. 

            Workgroup Computing. Workgroup software (such as Microsoft BackOffice)     allows many users to work on a document or project concurrently.

DISADVANTAGES OF  NETWORK  

·         Server faults stop applications being available

·         Network faults can cause loss of data.

·         Network fault could lead to loss of resources

·         User work dependent upon network

·         System open to hackers

·         Decisions tend to become centralized

·         Could become inefficient

·         Could degrade in performance

  • Resources could be located too far from users
  • Network management can become difficult

 

OSI MODELS

The standard model for networking protocols and distributed applications is the International Standard Organization's Open System Interconnect (ISO/OSI) model. It defines seven network layers.

Layer 1 - Physical

  •  This layer defines the cable or physical medium itself, e.g. unshielded twisted pairs (UTP).  All media of transmission are functionally equivalent in this layer and  the main difference is in convenience and cost of installation and maintenance. 

Layer 2 - Data Link

  • Data Link layer defines the format of data on the network ( a network data frame,  packet  and destination address). The Maximum Transmission Unit (MTU) is defined by the largest packet that can be sent through a data link layer. 

Layer 3 - Network

  •  This layer defines the protocols that are responsible for data delivery at the required destination, and requires.

Layer 4 - Transport

  • This layer subdivides user-buffer into network-buffer sized datagrams and enforces desired transmission control. Two transport protocols, Transmission Control Protocol (TCP) and User Datagram Protocol (UDP), sits at the transport layer. Reliability and speed are the primary difference between these two protocols. 

Layer 5 - Session

  • This leyer defines the format of the data sent over the connections. 

Layer 6 - Presentation

  • This layer converts local representation of data to its canonical form and vice versa. The canonical uses a standard byte ordering and structure packing convention, independent of the host.

Layer 7 - Application

  • Provides network services to the end-users. e.g Mail.

 

 

 

Last Updated on Friday, 07 October 2011 21:45
 
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