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Fundamentos basicos de redes Cap2
- 2. Una red típica de computadores
- 24. El módelo referencial OSI
- 26. El módelo OSI revisión de la Capa 1
- 28. El módelo OSI revisión de la Capa 2
- 30. El módelo OSI revisión de la Capa 3
- 32. El módelo OSI revisión de la Capa 4
- 34. El módelo OSI revisión de la Capa 5
- 36. El módelo OSI revisión de la Capa 6
- 38. El módelo OSI revisión de la Capa 7
Notes de l'éditeur
- The telephone system, known in the industry as the Public Switched Telephone Network (PSTN), is an example of a network. It allows people in virtually every corner of the world to communicate with anyone who has access to a telephone. Computer networks are not always independent of the telephone network. Telephone lines are often used for transmitting data as well as voice.
- A computer network can combine many different types of technology. Computers on the network share data and applications with other computers on the network. These computers also share common hardware devices such as printers and scanners. Specialized network devices such as routers and switches handle the flow of data traffic throughout the network to ensure that each device connected can communicate with other devices.
- The telephone system, known in the industry as the Public Switched Telephone Network (PSTN), is an example of a network. It allows people in virtually every corner of the world to communicate with anyone who has access to a telephone. Computer networks are not always independent of the telephone network. Telephone lines are often used for transmitting data as well as voice.
- The network should be simple. It should require no special skills of its users. The network should be manageable. It should be easy to monitor and adjust the performance of network elements . The network should be adaptable and scalable. Change and growth do not present barriers to network owners. The network should be reliable. When a user requests any connected tool or operation, it should be available. The network should be transparent. It should be just as fast and easy to access a resource that is near as one that comes from the network’s far end. All of these attributes are affected by the quality of the cabling and the cable installation. When planning and installing cable plants, always remember the ultimate goals of the network. A good way to help remember these points is to see that the first letter of each of these network objectives creates the acronym S M A R T.
- There are numerous benefits to networking computers and other devices. Users can share documents easily, backup data easily, share a common network connection, share hardware to accomplish tasks like printing documents, and group computers and devices together to more easily manage them.
- Computers that are networked together make securing data and resources easier. The term security can also be used when speaking of hardware or software problems that users can experience. When computers are networked together, it is much easier to backup the data that is on them. This provides the users a sense of security should any unexpected failure occur. The cost involved in linking computers which includes network interface cards (NICs), cabling or wireless media, hubs, and other connectivity devices, outweighs the cost of buying multiple printers and other shared devices. Networking also saves labor hours when users access and share data.
- The quality of the cabling and installation will determine whether the data or voice connection is established and the quality of that connection. If the wiring is not technically sound, the health of the network will suffer. This is why it is essential that good quality cabling is chosen, that it is installed by professionals, and that it is properly maintained.
- Fiber-optic cable is gaining in popularity. It uses pulses of light to transmit signals. Other networks communicate using radio, infrared, or microwave waves. Network cabling technicians must be familiar with each of these media, when each should be used, and how to connect them to the network.
- A server that must be moved from one location to another will still be able to provide the same services in the same manner from any location. If there is no central server, then each workstation must keep and update its own list of allowable users and resources. Peer-to-peer networks become unfeasible with much more than 10 or 15 computers. The need for high reliability usually makes servers expensive. When a server or the wiring to it fails, it can affect a multitude of users.
- Some networks are confined to a building or office while others encompass a city or are even global Localized networks, that is, those limited to a building or campus, can be for voice, data, or both.
- Telephones, for example, are physically connected to each other by telephone cable. A call from San Francisco, California to Sydney, Australia is first routed to a local telco (telephone central office). The call is then forwarded through a series of telcos until it reaches one that serves the telephone cables that have an international connection. Now the signals are sent through the cables that are laid on the floor of the Pacific Ocean. When the call reaches a telco in Australia, it is then routed through another series of telcos until it finally reaches the destination telephone in Sydney. Similarly, data is transferred through cables, whether to the computer of a coworker across the room or across the world.
- A variation on the PBX theme is the centrex (central office exchange service). The centrex is a PBX but with all switching done at a local telephone office instead of on the organization's premises. Typically, a telephone company owns and manages all the communications equipment necessary to implement the centrex, and then sells various services to the organization.
- A university or government that has several campuses or departments (each with its own LAN) within a metropolitan area can directly interconnect these LANs to create a MAN so that resources and data can be shared. Users do not connect directly to a MAN, rather they connect to a LAN, which in turn connects to the MAN. Similarly, users connect to a WAN by first connecting to a LAN.
- Since WANs connect users across great distances, they are also used by large corporations to communicate efficiently and quickly between offices within a country or around the world.
- Logical topologies - Describe the function of the network. They describe how the network gets voice and data from point to point. Common logical topologies include the ring and the bus. Physical topologies - Describe the actual physical layout of the network. Common physical topologies are the bus, ring, star, extended star, hierarchical, and mesh
- When pulling cable for LANs, a cable installer must know which physical topology is to be used or is in existence already. Physical topology is the layout of the networking cables, devices, and workstations.
- A bus requires termination at each end of the cable. Failure to terminate both ends of the cable results in signal bounce, which can disrupt or prevent communications on the network. One end of a bus should be grounded Troubleshooting a bus topology can be very difficult. If the cable breaks or fails, none of the devices located along will be able to communicate.
- Both the single ring and the dual ring topologies offer networks the benefits of providing predictable paths for error recovery. If there is ever a break in the network, automatic sensors can seal off the bad section and restore connectivity by using the other ring.
- The star topology is easy to design and install, and it is also scalable. To add more workstations, printers, or servers, simply plug in another cable into the central device like a hub or switch. Another advantage is that the entire network will not fail if one cable does. If a cable fails, only the device at the end of that cable is affected. The remainder of the network continues working. It is easy to diagnose problems since the fault can be quickly identified. The major disadvantage of the star topology, however, is its reliance on the central device (hub, switch, or router). If it fails, the entire network is affected The advantages of the extended star topology are the same as those for the star. It is ideal for medium to large networks. In fact, it is considered the backbone topology of choice for structured cabling systems by the Telecommunications Industry Association (TIA) and Electronic Industries Alliance (EIA) organizations.
- The disadvantage of a hierarchical topology is that if one cable fails, it can affect all the hosts that use it to access other parts of the network.
- Even though the mesh topology is an expensive solution, this topology is popular because many businesses want to be sure that if for some reason there is a problem with network services, there will be a redundant path that can be used to route the data in a different way.
- When analyzing or learning a complex subject, it often helps to break it down into separate parts. The Open System Interconnect (OSI) reference model does just that by breaking up the networking process into seven manageable layers. Each layer of the OSI model defines a specific function of the network. These functions are defined by the International Organization for Standardization (ISO) and are recognized worldwide
- The focus for network cabling installers and technicians is on Layer 1 of the OSI model, the physical layer, because it deals with the media.
- The OSI model is divided into seven different layers. The bottom layer is layer number one and deals with the actual transmission of signals throughout the network. As data is moved from the bottom of the model to the top, it is moved from hardware to software components until it reaches layer seven called the application layer. In order for two devices on the network to communicate, they both use the OSI model to ensure that data is sent and received in the same manner. Data being received moves through the layers from bottom to top and data being transmitted moves through from top to bottom. This method ensures common grounds for devices to communicate.
- 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 as well as radio waves, microwaves, laser beams and infrared light.
- Encoding is another function of Layer 1. Encoding is the conversion of the information into bits (0s and 1s). It is these bits that are then transmitted on the cable. When the source host sends data, the physical layer converts the data (for example, an e-mail message and its addressing information) into bits and then transmits those bits over the medium. When the destination host receives these bits, Layer 1 converts the bits back into the original format (the e-mail message in this example). As a signal travels on a wire, it grows weaker. This is referred to as attenuation. To keep the signal from becoming unrecognizable to the receiving host, a repeater is placed on the wire. A repeater is a networking device that takes in the weakened signal, cleans it up, and regenerates it before sending it on its way. Repeaters are generally used near the outer edges of networks where attenuation is most likely to occur. Like repeaters, active hubs also regenerate signals. The difference between the two is that hubs have many more ports than repeaters. Hence hubs are often called multi-port repeaters.
- Data Link - This layer prepares packets for physical transmission across the network medium. It handles error notification, network topology, and flow control. This layer uses MAC addresses.
- Network - This layer determines the best way to move data from one place to another across a network. This layer uses IP addresses.
- Transport - This layer segments and reassembles data into a data stream, and is responsible for reliable delivery of data. .
- Session - This layer establishes, maintains, and manages sessions between applications.
- Presentation - This layer provides data code formatting, compression, and encryption. It ensures that the data that arrives 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.
- Presentation - This layer provides data code formatting, compression, and encryption. It ensures that the data that arrives 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.
- One of the most common wiring errors by cable installers is laying cables near other wires, particularly power cables, or sources of power. Power cables emit background noise, which can interfere with the signals on network cables. Another common error is improperly terminating wire to jacks and plugs. This can lead to the wires emitting signals that interfere with the signals on other wires, a condition called crosstalk. When errors are caused by crosstalk or other interference, it means that data is lost and must be retransmitted. Finally, wires can be damaged as they are pulled into place. Pulling cables too tightly, nicking them, or bending them can cause problems that may not be apparent immediately, but can cause the electrical properties of the wire to change slowly over time.
- Less traffic can also mean a decrease in collisions. Collisions are when data packets collide on the media. Most networks are of a type called Ethernet. In an Ethernet network, a complete data frame is transferred with one bit allowed on the wire at a time. Only when that frame transmission is complete can a new frame begin. If the bits from more than one data frame are sent at the same time, the bits collide and the contents are destroyed. The frames have to be re-sent, tying up the network and possibly causing other collisions. The number of collisions may become so great that the network uses most of its resources to detect and recover from collisions. This results in excessive network congestion and significant slow down of the network. To solve this problem, bridges and switches are used to create several collision domains rather than just a single large one
- Switches are also useful because several ports can be grouped together into a virtual local area network (VLAN). VLANs can be used to secure certain parts of the network or to manage departments within a company. For instance, a company may group all accounting PCs and relevant servers on the same VLAN so that they can communicate with each other and not allow any other user access to the information.
- In addition to addressing, another function of the network layer is to help determine the best path that data will take through the LAN or a WAN. This is achieved by using a device called a router.
- Path determination is the process that the router uses to select the next hop, that is, the path to the next connected router that will move the data toward its destination. This process is known as routing.
- Since data packets may be sent by different paths and arrive at the destination at different times, sequence numbers ensure that the data file will be reassembled so that it appears as the same file sent. When the data file is segmented, each segment receives a sequence number. When the data segments reach the destination, they are sorted in order according to the sequence numbers so that the original data file can be reassembled.
- The session layer sets up, maintains, and then terminates sessions between hosts on the network. This includes starting, stopping, and re-synchronizing two computers as they communicate, a process called dialogue control. Another primary role of the session layer is that it provides services to the presentation layer.
- The presentation layer also performs data compression and encryption functions. Compression is when frequently repeated words or combinations of characters can be indicated by a single character, thus reducing the size of the file. When the destination host receives the compressed file, it uses a key to decompress the file to its original size. Encryption protects data from being read by unauthorized viewers. Encryption is crucial for sensitive data, such as financial transactions, personal information, or company trade secrets, that are being transmitted to a computer on the same network or across the Internet.