Open System LANs and their Global Interconnection. Electronics and Communications Reference Series

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Access to the network is guaranteed, and time-sensitive applications can be supported. However, these benefits come at a price. Component costs are usually higher, and the networks themselves are considered to be more complex and difficult to implement. Various PC vendors have been proponents of Token Ring networks.

After a physical connection has been established, network protocols define the standards that allow computers to communicate. A protocol establishes the rules and encoding specifications for sending data. This defines how computers identify one another on a network, the form that the data should take in transit, and how this information is processed once it reaches its final destination. Although each network protocol is different, they all share the same physical cabling.

This common method of accessing the physical network allows multiple protocols to peacefully coexist over the network media, and allows the builder of a network to use common hardware for a variety of protocols. The Open System Interconnection OSI model specifies how dissimilar computing devices such as Network Interface Cards NICs , bridges and routers exchange data over a network by offering a networking framework for implementing protocols in seven layers.

Beginning at the application layer, control is passed from one layer to the next. The following describes the seven layers as defined by the OSI model, shown in the order they occur whenever a user transmits information. This layer supports the application and end-user processes.

Within this layer, user privacy is considered and communication partners, service and constraints are all identified. File transfers, email, Telnet and FTP applications are all provided within this layer. Within this layer, information is translated back and forth between application and network formats.

This translation transforms the information into data the application layer and network recognize regardless of encryption and formatting. Within this layer, connections between applications are made, managed and terminated as needed to allow for data exchanges between applications at each end of a dialogue. Complete data transfer is ensured as information is transferred transparently between systems in this layer.

The transport layer also assures appropriate flow control and end-to-end error recovery. Using switching and routing technologies, this layer is responsible for creating virtual circuits to transmit information from node to node. Other functions include routing, forwarding, addressing, internet working, error and congestion control, and packet sequencing. Information in data packets are encoded and decoded into bits within this layer. Errors from the physical layer flow control and frame synchronization are corrected here utilizing transmission protocol knowledge and management.

This layer consists of two sub layers: the Media Access Control MAC layer, which controls the way networked computers gain access to data and transmit it, and the Logical Link Control LLC layer, which controls frame synchronization, flow control and error checking. This layer enables hardware to send and receive data over a carrier such as cabling, a card or other physical means.

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It conveys the bitstream through the network at the electrical and mechanical level. This order is then reversed as information is received, so that the physical layer is the first and application layer is the final layer that information passes through. In order to understand standard Ethernet code, one must understand what each digit means.

OSI Model Explained -- Open System Interconnection -- Real Time Example -- CCNA

Following is a guide:. An important part of designing and installing an Ethernet is selecting the appropriate Ethernet medium. Thickwire was one of the first cabling systems used in Ethernet, but it was expensive and difficult to use.

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This evolved to thin coax, which is easier to work with and less expensive. It is important to note that each type of Ethernet, Fast Ethernet, Gigabit Ethernet, 10 Gigabit Ethernet, has its own preferred media types. This is similar to telephone cable and comes in a variety of grades, with each higher grade offering better performance. Level 5 cable is the highest, most expensive grade, offering support for transmission rates of up to Mbps.

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Level 4 and level 3 cable are less expensive, but cannot support the same data throughput speeds; level 4 cable can support speeds of up to 20 Mbps; level 3 up to 16 Mbps. Fiber-optic cable is more expensive, but it is invaluable in situations where electronic emissions and environmental hazards are a concern.

Fiber-optic cable is often used in inter-building applications to insulate networking equipment from electrical damage caused by lightning. Because it does not conduct electricity, fiber-optic cable can also be useful in areas where heavy electromagnetic interference is present, such as on a factory floor. The Ethernet standard allows for fiber-optic cable segments up to two kilometers long, making fiber-optic Ethernet perfect for connecting nodes and buildings that are otherwise not reachable with copper media.

Network topology is the geometric arrangement of nodes and cable links in a LAN. Two general configurations are used, bus and star. These two topologies define how nodes are connected to one another in a communication network. A node is an active device connected to the network, such as a computer or a printer. A node can also be a piece of networking equipment such as a hub, switch or a router.

A bus topology consists of nodes linked together in a series with each node connected to a long cable or bus. Many nodes can tap into the bus and begin communication with all other nodes on that cable segment. A break anywhere in the cable will usually cause the entire segment to be inoperable until the break is repaired. Generally a computer is located at one end of the segment, and the other end is terminated in central location with a hub or a switch.


Because UTP is often run in conjunction with telephone cabling, this central location can be a telephone closet or other area where it is convenient to connect the UTP segment to a backbone. Other computer users on the network continue to operate as if that segment were non-existent. Ethernet is a shared medium, so there are rules for sending packets of data to avoid conflicts and to protect data integrity. Nodes determine when the network is available for sending packets. It is possible that two or more nodes at different locations will attempt to send data at the same time.

When this happens, a packet collision occurs. Minimizing collisions is a crucial element in the design and operation of networks.

Increased collisions are often the result of too many users on the network. Segmenting the network is one way of reducing an overcrowded network, i.

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Ethernet allows each device to send messages at any time without having to wait for network permission; thus, there is a high possibility that devices may try to send messages at the same time. After detecting a collision, each device that was transmitting a packet delays a random amount of time before re-transmitting the packet. If another collision occurs, the device waits twice as long before trying to re-transmit. The standards and technology just discussed will help define the specific products that network managers use to build Ethernet networks.

The following presents the key products needed to build an Ethernet LAN. They are used to connect nodes to the various Ethernet media. Most computers and network interface cards contain a built-in 10BASE-T or 10BASE2 transceiver which allows them to be connected directly to Ethernet without the need for an external transceiver.

Many Ethernet devices provide an attachment unit interface AUI connector to allow the user to connect to any type of medium via an external transceiver. The AUI connector consists of a pin D-shell type connector, female on the computer side, male on the transceiver side.

Different computers have different bus architectures. NICs come in three basic varieties: 8-bit, bit, and bit. The larger the number of bits that can be transferred to the NIC, the faster the NIC can transfer data to the network cable. Most NICs are designed for a particular type of network, protocol, and medium, though some can serve multiple networks. Many NIC adapters comply with plug-and-play specifications.

Cards are available to support almost all networking standards. Full duplex networking is another option where a dedicated connection to a switch allows a NIC to operate at twice the speed. In larger designs, signal quality begins to deteriorate as segments exceed their maximum length. Hubs provide the signal amplification required to allow a segment to be extended a greater distance. A hub repeats any incoming signal to all ports. A multi-port twisted pair hub allows several point-to-point segments to be joined into one network. One end of the point-to-point link is attached to the hub and the other is attached to the computer.

If the hub is attached to a backbone, then all computers at the end of the twisted pair segments can communicate with all the hosts on the backbone. The number and type of hubs in any one-collision domain is limited by the Ethernet rules. These repeater rules are discussed in more detail later. A very important fact to note about hubs is that they only allow users to share Ethernet.

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Therefore, if a collision or electrical interference occurs on one segment, repeaters make it appear on all others as well. This means that individual members of a shared network will only get a percentage of the available network bandwidth. Basically, the number and type of hubs in any one collision domain for 10Mbps Ethernet is limited by the following rules:. Open Menu. Close Menu. Local Area Networks LANs A network is any collection of independent computers that exchange information with each other over a shared communication medium.

These networks themselves are run by computers. A telecommunications network is an arrangement of computing and telecommunications resources for communication of information between distant locations. A telecommunications network includes the following components:. Terminals for accessing the network. Computers that process information and are interconnected by the network. Telecommunications links that form a channel through which information is transmitted from a sending device to a receiving device. Telecommunications equipment that facilitates the transmission of information.

Telecommunications software that controls message transmission over the network.

Functions of the OSI Layers

Scope of Telecommunications Networks. Two principal types of telecommunications networks can be distinguished from the point of view of their geographical scope.