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802

With the IEEE Project 802 reworks the second layer of the OSI model to provide detailed specifications on most popular networks. In particular, to standardize and make them compatible with networks MAN LAN networks, the data link layer, as shown in the figure below, is divided into two sub-layers, called LLC (Logical Link Control) and MAC (Media Access Control).

1. The project
802.1
With the project, the IEEE 802.1 prepare general specifications of the entire project, introducing the idea that networks LAN and MAN , even though they use different transmission techniques, must have a common interface to the network level. Therefore defines the standard for connecting and managing local and metropolitan networks, including, in project 802.1D Spanning Tree Protocol.
The MAC sublayer then interface directly with the physical layer, and depends on the technology used to connect the network. It depends on the particular network architecture, as it communicates with the network card of the workstation and the transceiver and is involved in deciding which computer to use the means of transmission and how to send the information correctly. The MAC layer allows, through the card network to identify each host in a unique way.
The IEEE has decided to describe this tube in special forms, each of which reflects a given topology. The standard MAC sublayer and Layer 1 of the OSI model are in fact defined by the individual projects, in particular dall'802 .3, .4 dall'802, dall'802 .5 and so on.

2. The project
802.2 802.2
The project deals exclusively with the LLC sublayer , making it independent of the LAN technology used, so it includes its own protocol data unit and its interfaces and is in the process of encapsulation of the data.
This sublayer, common to all networks, it acts as an interface to the network layer above and to the MAC sublayer. It specifies how to use SAP, ie points of access to services; LLC adds to the package of control information, such as DSAP (Destination Service Access Point) and SSAP (Source Service Access Point), to allow is delivered to its destination. LLC supports both connectionless service that the connection-oriented protocol used by the highest level. The project 802.2 specifies a number of fields in the frame, which enable multiple higher layer protocols to share a single physical data link. Do not differ from an topology network to another, workstations thus refer to SAP to move the message from the LLC sublayer to the network layer of the OSI model.

3. The Standard IEEE 802.3


The Project refers to the IEEE 802.3 Ethernet already developed in 1975 by Xeros, and more particularly to a standard document, drawn up by Xeros in collaboration with Intel and Digital, which replaced the previous version of Ethernet, with a new one called Ethernet II or DIX, named after the three companies that collaborated in the new implementation. The first publication of the document by the IEEE was in 1983, and contained the physical and electrical specifications for a Ethernet network operating at 10 Mbps over coaxial cable. The project was extended several times n, and Ethernet was standardized for connecting networks with linear bus topology or star-bus, which use different types of cables, which Thicknet, Thinnet or UTP, base-band with variable bitrates, between 10 and 100 Mbps was also established as a means of accessing the transmission medium CSMA / CD.

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local networks

local networks are also called to LAN (Local Area Network). The definition given by these networks' IEEE is:
" a local area network is a communication system that allows a number of independent devices to communicate directly with each other, within an area geographical area of \u200b\u200bmoderate size and a physical channel along at a moderate speed communications .

This definition is actually a little antiquated, because it defines a local type of Peer to Peer ie a network where all machines have the same state, in other words, there is a machine that has control over the others. In fact a local network may also have a hierarchical or centralized control, in which case it is assumed that there is a machine with computing power than the other, which is given control of the communication. The second point of the definition given by the IEEE that specifies the geographical area should have a size not too large. A local network it should not exceed an area of \u200b\u200b5 km and therefore it is usually installed within the same building and buildings close enough to each other. With regard to the Communication
this must be done on a dedicated cable, then owned by the company, not public. The transmission medium allows the exchange of information at relatively high speed, including usually between 10 Mbps and 100 Mbps But now you're saying the use of fiber optic cables, which allow a higher transmission rate to 100 Mbps In addition, most local networks that are within walking distance, can be connected by a backbone (backbone) local, that is a high-speed network.
add that a LAN provides connectivity full-time local services.

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Project The project of a local network

In designing a local network keep in mind that there are two models of network, called a Peer to Peer that other Client-Server .
The model is characterized Peer to Peer a set of PCs, each of which can share their resources, such as Hard Disk, CD-ROM, printer, scanner and so on, with the other computer.
You can therefore work in "local" that is, use only the resources of your computer, or work in "Network", that use shared resources from other stations, though obviously it has access.
This type of network, has several limitations, first of all, the number of stations that can be connected is quite low, but also where the company is quite small and therefore does not require a large number of PCs, keep that if the PC is password protected and the employee who uses it is absent, you can not access shared resources on that PC. Moreover, a proper sharing of resources requires more training for users.
The client-server model provides instead that there is a car with a processing capacity, memory and mass superior to the others. This computer is also called the Master. In extended networks quite close to the main server can provide the machines with specialized functions.

Example:
Some computers may be used for print management, application software, etc. .., in which case they will take the name of Print Server, Application Server, and so on.

All other computers, namely the customers, as well as take advantage of their local resources, all server resources, provided of course that they have permission. In fact, the authorization is assigned to users who are connected, and not the client. Managing Client-Server makes it possible to adequately protect not only all the server resources, but also the local resources of each machine, inhibiting access to particular resources to specific users.


Example:
to protect the server or multiple computers, you can block access to floppy disk or operating system files in order to prevent tampering or infection, but you can also avoid the banal, the desktop is changed.


the different clients can access not only to the server simultaneously, but also to the same resources, working on files and shared printers. If the user data are centralized, that is stored in folders of the server, then you can make a centralized backup, otherwise each user is responsible for backing up their data, which must be run on any PC. The drawback of this system could be a possible server crash. However, the major software house offering to support the primary server network, a controller support, as it were a Server Backup , which comes into operation automatically in case of malfunction of the primary server.