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Tuesday, January 3, 2017

Some Basic Theories on Open System Interconnection Reference



 by Aftab Ahmed

 The Open System Interconnection Reference Model or OSI Reference Model or OSI Model is basically a conceptual description for layered communications and computer network protocol design. It was created as part of the Open Systems Interconnection or OSI initiative. In its most fundamental form, it segments network architecture into seven distinct layers: the Application, Presentation, Session, Transport, Network, Data-Link, and Physical Layers. As a result it is frequently referred to as the OSI Seven Layer Model. A layer is a collection of theoretically similar functions that provide services to the layer directly above it and gets service from the layer beneath it. On each single layer an instance provides services to the instances at the layer above and requests service from the layer beneath. An example of this relates to a layer that provides error-free communications across a network which then provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that comprise the contents of the path.

Work on a layered model of network architecture commenced in the year 1977. Simultaneously the International Organization for Standardization or ISO started to develop its OSI framework architecture. OSI has two major parts: an abstract model of networking, called the Basic Reference Model or seven-layer model and a set of specific protocols. It is important to note that the standard documents that describe the OSI model can be freely downloaded from the ITU-T as the X.200-series of recommendations. A number of the protocol specifications can also be accessed as part of the ITU-T X series. The equivalent ISO and ISO/IEC standards for the OSI model are available from ISO, however not all of these are available free.

Every area of the OSI design evolved from experiences with the CYCLADES network, which also had a large influence on the design of the internet. The latest design has been meticulously documented in ISO 7498 and several of its addenda. In this latest model, a networking system is segmented into layers. Within each layer, one or more entities proves its functionality. Each entity will interact directly only with the layer just below it, and enables facilities for use by the layer above it. Protocols will allow an entity in one host to interact with a corresponding entity at the same layer in another host. Service definitions ideally describe the functionality provided to an (N)-layer by an (N-1) layer, where N is one of the seven layers of protocols operating in the local host.

Neither the OSI Reference Model nor OSI protocols designate any programming interfaces, other than a purposely abstract service specifications. Protocol specifications precisely define the interfaces between multiple computers, but the software interfaces within computers are implementation-specific.

Take for example, Microsoft Windows' Winsock, and Unix's Berkeley sockets and System V Transport Layer Interface, are interfaces between applications (Layer 5 and above) and the transport (Layer 4). NDIS and ODI are interfaces between the media in Layer 2 and the network protocol in Layer 3. Interface standards, with the exception of the Physical Layer to media, are estimated implementations of OSI Service Specifications.

Free to Air Satellite

FTA is in certain circumstances delivered by satellite television, but in many different locations of the world free-to-air television channels are actually broadcast unencrypted on UHF or VHF bands. Despite the fact that these channels are described as free, in some cases the viewer may have to pay for them. Some of these channels are paid directly by payment of a licence fee, for example as seen with the BBC or by voluntary donation for example PBS. Others indirectly pay by paying for consumer products and services where a portion of the cost is used for television advertising and sponsorship.
Free-to-air is typically used for international broadcasting, making it in some way a video equivalent of shortwave radio. Most authorized FTA retailers will list free to air channel guides and content available in North America for free to air use.

There are many competing systems presently in use. Many of the older channels used C-band satellite dishes of several feet in diameter to receive signals which were at first analogue microwaves, and then digital microwave using the 3.9-4.2 GHz band. Currently, in the 11.7-12.2 GHz Ku band permits the use of under one-meter dishes chiefly with the DVB-S standard. Hence FTA can now be used from apartment balconies.

The most prevalent North American sources for free-to-air DVB satellite television includes:


NASA TV Multi-channel(NASA HD, NASA Public, NASA Media, NASA Education) Broadcast on AMC 6 (72W)
Retro Television Network on AMC 9 (83W)
TVU Music channel and This TV on AMC 3 (87W)
ABC News Now on Galaxy 28 (89W)
Eternal Word Television Network on Galaxy 17 (91W)
My Family TV on Galaxy 3C (95W).
AMGTV and BYU Television on Galaxy 19 (97W)
English and foreign language broadcasters RT (TV network), MHz Worldview, Ebru TV, IRINN, Al Jazeera English and more up-linked by GlobeCast World TV on Galaxy 25 (97W)
Christian broadcasters The Word Network, Emmanuel TV, Daystar Television Network, JCTV, Trinity Broadcasting Network, The Church Channel, God's Learning Channel and more promoted by Glorystar and Spiritcast Satellite Systems TV on Galaxy 25 (97W)
Pentagon Channel on AMC 1 (103W)
Jewish Life Television, ResearchChannel, and University of Washington TV on Galaxy 18 (123W)
Montana Public Broadcasting Service on AMC 21 (125W)
Louisiana Connection Network on Galaxy 13 (127W)
White Springs Television on Galaxy 27 (129W)
Classic Arts Showcase on Galaxy 15 (133W)
Football, Basketball, Baseball, Soccer, and Hockey wildfeeds on various satellites

Many of these signals are carried by United States satellites. There is a small number of or no free Canadian DVB-S content available to users of medium-size dishes as much of the available Ku-band satellite bandwidth is used by pay-TV operators Shaw Direct and Bell TV, however the large style dish of over 3 feet/90cm does have a few extra choices. FTA signals may be dispersed across multiple satellites needing a motor or multiple LNBs to receive everything.

Free-to-air satellite TV is a feasible addition to any home video system, not only because of the specialized content it makes available but also for use in areas where terrestrial ATSC over-the-air reception is not at acceptable levels and additional channels are needed.



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Article Source: http://www.articlesphere.com/Article/Some-Basic-Theories-on-Open-System-Interconnection-Reference/252731