I.INTRODUCTION
The existence of a faculty is based on two necessary parts: the professors and the students. During the years the last one have not been organized so, to be able to defend their rights with all the power necessary. In the 2001 a group of young enthusiastic students took the decision to organize the students from the Faculty of Automation, Computers and Electronics. So started OSACE, the student organization. One of the most important parts in a student organization and in fact in a very large group of people is the information. Having this in mind I decided to make a student network in the office of the Student Organization (OSACE) and also a web page containing the information necessary for the students. The web page also contains a forum for all the discussions needed. Now after a hard work all these exist. In the OSACE office there are 3 computers with access to Internet opened to all the students from Faculty of Automation, Computers and Electronics. One of them is server where it is also the web page with the forum. It also exists a separate group of discussion where now are enrolled almost 70% of our students.
II. COMPUTER NETWORKS
New business practices are driving changes in enterprise networks. Employees at corporate headquarters and in worldwide field offices, as well as telecommuters in home offices, need immediate access to data, regardless of whether the data is on centralized or departmental servers. Enterprises such as corporations, agencies, schools, and other organizations that tie together their data communication, computing, and file servers need:
• interconnected LANs that provide access to computers or file servers in other locations
• high bandwidth onto the LANs to satisfy the needs of the end users
• efficient WAN(s) to interconnect the LANs
To improve communication with partners, employees, and customers, enterprises are implementing new applications such as electronic commerce, videoconferencing, voice over IP, and distance learning. Businesses are merging their voice, video, and data networks into global enterprise networks as shown in . These enterprise networks are viewed as critical to the organization's business success.
Enterprise networks are designed and built to support current and future applications. To accommodate increasing requirements for bandwidth, scalability, and reliability, vendors and standards bodies introduce new protocols and technologies at a rapid rate. Network designers are challenged to develop state-of-the-art networks, even though what is considered state-of-the-art changes on a monthly, if not weekly basis.
Dividing and organizing the networking tasks into separate layers/functions allows new applications to be handled without problems. The OSI reference model organizes network functions into seven categories, called layers. Data flows from upper-level user applications to lower-level bits that are then transmitted through network media. The task of most wide area network managers is to configure the three lowest layers.
There are seven layers in the OSI reference model, each of which has separate distinct functions. This separation of networking functions is called layering. The Transmission Control Protocol/Internet Protocol (TCP/IP) models' functions fit into four layers. Regardless of the number of layers, however, the reasons for the division of network functions include the following:
• to divide the interrelated aspects of network operations into less complex elements
• to define standard interfaces for plug-and-play compatibility and multivendor integration
• to enable engineers to focus design and development efforts on a particular layer's functions
• to promote symmetry of the different internetwork modular functions for the purpose of interoperability
• to prevent changes in one area from significantly affecting other areas, so that each area can evolve more quickly
• to divide the complex operations of internetworking into discrete, more easily learned operational subsets
Each layer of the seven-layer OSI reference model serves a specific function. The functions are defined by the OSI and can be used by any network products vendor.
The layers are:
• Application -- This layer provides network services to user applications. For example, a word processing application is serviced by file transfer services at this layer.
• Presentation -- This layer provides data representation and code formatting. It ensures that the data arriving from the network can be used by the application, and it ensures that the information sent by the application can be transmitted on the network.
• Session -- This layer establishes, maintains, and manages sessions between applications.
• Transport -- This layer segments and reassembles data into a data stream. TCP is one of the transport layer protocols used with IP.
• Network -- This layer determines the best way to move data from one place to another. Routers operate at this layer. You will find the IP (Internet Protocol) addressing scheme at this layer.
• Data Link -- This layer prepares a datagram (or packet) for physical transmission across the medium. It handles error notification, network topology, and flow control. This layer uses Media Access Control (MAC) addresses.
• Physical -- This layer provides the electrical, mechanical, procedural, and functional means for activating and maintaining the physical link between systems. This layer uses physical media such as twisted-pair, coaxial, and fiber-optic cable.
The major characteristics of LANs are as follows:
• LANS are networks that operate within a building or floor of a building.
• LANs provide multiple connected desktop devices (usually PCs) with access to high-bandwidth media.
• By definition, the LAN connects computers and services to a common Layer 1 medium. There are several different
LAN devices:
• Bridges that connect LAN segments and help filter traffic
• Hubs that concentrate LAN connections and allow use of twisted-pair copper media
• Ethernet switches that offer full-duplex, dedicated bandwidth to segments or desktop traffic
• Routers that offer many services, including internetworking and broadcast control traffic
The following three LAN technologies account for virtually all deployed LANs :
• Ethernet -- The first of the major LAN technologies; it is deployed in the largest number of LANs.
• Token-Ring -- From IBM, it followed Ethernet and is now widely used in a large number of IBM networks.
• FDDI -- Also uses tokens, and is now a popular campus LAN.
On a LAN, the physical layer provides access to the network media. The most common Layer 1 media used in networking today: coaxial, fiber-optic, and twisted-pair cable. The data link layer provides support for communication over several types of data links, such as Ethernet/IEEE 802.3. Addressing schemes such as Media Access Control (MAC) at Layer 2, and Internet Protocol (IP) at Layer 3, provide a very structured method for finding and delivering data to computers or to other hosts on a LAN.
The Ethernet and IEEE 802.3 standards define a bus topology LAN that operates at a baseband signaling rate of 10 Mbps. The three defined wiring standards:
• 10BASE2 (thin Ethernet) -- allows coaxial cable network segments up to 185 meters long
• 10BASE5 (thick Ethernet) -- allows coaxial cable network segments up to 500 meters long
• 10BASE-T -- allows Unshielded Twisted Pair (UTP) cable runs up to 100 meters long
The 10BASE5 and 10BASE2 standards provide access for several stations to the same LAN segment. Stations are attached to the segment by a cable that runs from an attachment unit interface (AUI) in the station to a transceiver that is directly attached to the Ethernet coaxial cable.
Because 10BASE-T provides access for a single station only, stations that are attached to an Ethernet LAN by 10BASE-T are almost always connected to a hub or a LAN switch. In this arrangement the hub or LAN switch is the same as an Ethernet segment.
The Ethernet and 802.3 data links prepare data for transport across the physical link that joins two devices. The Macintosh on the left and the Intel-based PC in the middle show MAC addresses used by the data link layer. The router on the right also uses MAC addresses for each of the LAN side interfaces. The Ethernet/802.3 interface on the router uses the Cisco IOS interface type abbreviation "E" followed by an interface number.
Broadcasting is a powerful tool that can send a single frame to many stations at the same time. Broadcasting uses a data link destination address of all 1s (FFFF.FFFF.FFFF in hexadecimal). When improperly used, broadcasting can seriously affect the performance of stations by unnecessarily interrupting them. Broadcasts should, therefore, be used only when the MAC address of the destination is unknown, or when the destination is all stations
On an Ethernet LAN only one transmission is allowed at any given time. An Ethernet LAN is referred to as a Carrier Sense Multiple Access with Collision Detection (CSMA/CD) network. This means that one node's transmission traverses the entire network and is received and examined by every node. When the signal reaches the end of a segment, terminators absorb it to prevent it from going back onto the segment.
When a station wishes to transmit a signal, it checks the network to determine whether another station is currently transmitting. If the network is not being used, the station proceeds with the transmission. While sending a signal the station monitors the network to ensure that no other station is transmitting at that time. It is possible that two stations could both determine that the network is available and starts transmitting at approximately the same time. If this should occur they would cause a collision, as is illustrated in the upper part of the graphic.
When a transmitting node recognizes a collision, it transmits a jam signal that causes the collision to last long enough for all other nodes to recognize it. All transmitting nodes then stop sending frames for a randomly selected period of time before attempting to retransmit. If subsequent attempts also result in collisions, the node backs off for a period and then tries to transmit again. The node will attempt to transmit fifteen times before it gives up. The clocks indicate various backoff timers. If the two timers are sufficiently different, one station succeeds in transmitting on the next try.
An essential component of any network system is the process that enables information to locate specific nodes on a network. Various addressing schemes are used for this purpose, depending on the protocol family being used. For example, AppleTalk addressing is different from TCP/IP addressing, which in turn is different from IPX addressing. Two important types of addresses are data link layer addresses and network layer addresses. Data link layer addresses, also called physical addresses, hardware addresses, or MAC addresses, are typically unique for each network connection. In fact for most LANs, data link layer addresses are permanently encoded on the NIC (network interface card). Because a typical computer system has one physical network connection, it has only a single data link layer address. Routers and other systems that are connected to multiple physical networks can have multiple data link layer addresses. As their name implies, data link layer addresses exist at Layer 2 of the OSI reference model.
Network layer addresses (also called logical addresses or IP addresses for the Internet Protocol suite) exist at Layer 3 of the OSI reference model. Unlike data link layer addresses, which usually exist within a flat address space, network layer addresses are usually hierarchical. In other words, they are like postal addresses that describe a person's location by indicating a country, state, ZIP Code, city, street, house address, and name. One example of a flat address is a U.S. Social Security number. Each person has a unique Social Security number. People can move around the country and obtain new logical addresses depending on their city, street, or ZIP Code, but their Social Security numbers remain unchanged.
