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Cable Testing
Extra Info - Ethernet
In recent years, networking computers has taken on greater importance as organizations rely on a network for communication applications like electronic mail and for core business operations functions like database applications. This tutorial helps to explain Ethernet and Fast Ethernet, which are two of the most popular technologies used in networking.
LANs
Networks are collections of independent computers that communicate with one another over a shared network medium. Local area networks (LANs) are those networks usually confined to a geographic area, such as a single building or a college campus. LANs, however, are not necessarily simple in design, as they may link many hundreds of computers and be used by many thousands of users. The development of various standards for networking protocols and media has made possible the proliferation of LANs in organizations worldwide for business and educational applications.
WANs
Often a network is located in multiple physical locations. Wide area networking is the connecting of multiple LANs that are geographically separate. This is accomplished by connecting the different LANs using services including dedicated leased phone lines, dial-up phone lines both synchronous and asynchronous, satellite links, and data packet carrier services. Wide area networking can be as simple as providing modems and a remote access server to allow remote employees to dial in; or it can be as complex as linking hundreds of branch offices across the world using special routing protocols and filters to minimize the expense of sending data sent over vast distances.
Ethernet
Ethernet is the most popular physical layer LAN technology in use today. Other LAN types include Token Ring, Fast Ethernet, Fiber Distributed Data Interface (FDDI), Asynchronous Transfer Mode (ATM) and LocalTalk. Ethernet is popular because it strikes a good balance between speed, cost and ease of installation. These strong points, combined with wide acceptance in the computer marketplace and the ability to support virtually all popular network protocols, make Ethernet an ideal networking technology for most computer users today. The Ethernet standard is defined by the Institute for Electrical and Electronic Engineers (IEEE) as IEEE Standard 802.3. This standard defines rules for configuring an Ethernet as well as specifying how elements in an Ethernet network interact with one another. By adhering to the IEEE standard, network equipment and network protocols will interoperate efficiently.
Fast Ethernet
For Ethernet networks that need higher transmission speeds, the Fast Ethernet standard (IEEE 802.3u) has been established. This standard raises the Ethernet speed limit from 10 Megabits per second (Mbps) to 100 Mbps with only minimal changes to the existing cable structure.
There are three types of Fast Ethernet: 100BASE-TX for use with level 5 UTP cable, 100BASE-FX for use with fiber-optic cable, and 100BASE-T4 which utilizes an extra two wires for use with level 3 UTP cable. The 100BASE-TX standard has become the most popular due to its close compatibility with the 10BASE-T Ethernet standard. For the network manager, the incorporation of Fast Ethernet into an existing configuration presents a host of decisions. Each site in the network must determine the number of users that really need the higher throughput, decide on which segments of the backbone need to be reconfigured specifically for 100BASE-T and then choose the necessary hardware to connect the 100BASE-T segments with existing 10BASE-T segments.
Gigabit Ethernet is a future technology that promises a migration path beyond Fast Ethernet so that the next generation of networks will support even higher data transfer speeds.
Protocols
Network protocols are standards that allow computers to communicate. A protocol defines how computers should identify one another on a network, the form that the data should take in transit, and how this information should be processed once it reaches its final destination. Protocols also define procedures for handling lost or damaged transmissions or "packets." IPX (for Novell NetWare), TCP/IP (for UNIX, Windows NT, Windows 95 and other platforms), DECnet (for networking Digital Equipment Corp. computers), AppleTalk (for Macintosh computers), and NetBIOS/NetBEUI (for LAN Manager and Windows NT networks) are the main types of network protocols in use today.
Although each network protocol is different, they all are able to 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. This concept is known as "protocol independence," which means that devices that are compatible at the physical and data link layers allow the user to run many different protocols over the same medium.
Media
An important part of designing and installing an Ethernet is selecting the appropriate Ethernet medium for the environment at hand. There are four major types of media in use today: Thickwire for 10BASE5 networks, thin coax for 10BASE2 networks, unshielded twisted pair (UTP) for 10BASE-T networks and fiber optic for 10BASE-FL or Fiber-Optic Inter-repeater Link (FOIRL) networks. This wide variety of media reflects the evolution of Ethernet and also points to the technology?s flexibility. Thickwire was one of the first cabling systems used in Ethernet but was difficult to work with and expensive. This evolved to thin coax, which is easier to work with and less expensive.
Today, the most popular wiring schemes are 10BASE-T and 100BASE-TX which both use unshielded twisted pair (UTP) cable. 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 100 Mbps. Level 4 and level 3 cable are less expensive, but cannot support the same data through put speeds; level 4 cable can support speeds of up to 20 Mbps, level 3 up to 16 Mbps. The 100BASE-T4 standard allows for support of 100 Mbps Ethernet over level 3 cable, but at the expense of adding another pair of wires (4 pair instead of the 2 pair used for 10BASE-T); for most users, this is an awkward scheme and therefore 100BASE-T4 has seen little popularity. Level 2 and level 1 cables are not used in the design of 10BASE-T networks.
For specialized applications, fiber-optic, or 10BASE-FL, Ethernet segments are popular. Fiber-optic cable is more expensive, but it is invaluable for situations where electronic emissions and environmental hazards are a concern. Fiber-optic cable is often used in interbuilding 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 large amounts of electro-magnetic interference is present, such as on a factory floor. The Ethernet standard allows for fiber-optic cable segments up to 2 kilometers long, making fiber optic Ethernet perfect for connecting nodes and buildings that are otherwise not reachable with copper media.
Topologies
Ethernet media are used in two general configurations or topologies; bus and star. These two topologies define how ?nodes? are connected to one another. 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 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. Examples of bus topology include 10BASE2 and 10BASE5. 10BASE-T Ethernet and Fast Ethernet use a star topology. Generally a computer is located at one end of the segment, and the other end is terminated in a central location with a hub. 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. The primary advantage of this type of network is reliability, for if one of these ?point-to-point? segments has a break, it will only affect the two nodes on that link. Other computer users on the network continue to operate as if that segment were nonexistent.
Collisions
Ethernet is a shared media, so there are rules for sending packets to avoid conflicts and protect data integrity. Nodes on an Ethernet network send packets when they determine the network is not in use. It is possible that two nodes at different locations could try to send data at the same time. When both PCs are transferring a packet to the network at the same time, a collision will result. 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, which results in a lot of contention for network bandwidth. This can slow the performance of the network from the users point of view. Segmenting the network, where a network is divided into different pieces joined together logically with a bridge or switch, is one way of reducing an overcrowded network.
Ethernet Products
The standards and technology that have just been covered are translated into specific products that network managers use to build Ethernet networks. The following text discusses the key products needed to build a Ethernet LAN.
Transceivers
Transceivers 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, allowing them to be connected directly to Ethernet without requiring an external transceiver. Many Ethernet compatible devices provide an AUI connector to allow the user to connect to any media type via an external transceiver. The AUI connector consists of a 15 pin D-shell type connector, female on the computer side, male on the transceiver side. Thickwire (10BASE5) cables also use transceivers to allow connections.
For Fast Ethernet networks, a new interface called the MII (Media Independent Interface) was developed to offer a flexible way to support 100 Mbps connections. The MII is a popular way to connect 100BASE-FX links to copper-based Fast Ethernet devices.
Network Interface Cards
Network interface cards, commonly referred to as NICs, are used to connect a PC to a network. The NIC provides a physical connection between the networking cable and the computer?s internal bus. Different computers have different bus architectures, PCI bus master slots are most commonly found on 486/Pentium PCs and ISA expansion slots are commonly found on 386 and older personal computers. Network interface cards come in three basic varieties; 8 bit, 16 bit, and 32 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.
Many NIC adapters comply with Plug-n-Play (PnP) specifications. On PnP systems, the NICs are automatically configured without user intervention, while on non-PnP systems, configuration is done manually through a setup program and/or manually set DIP switches.
Cards are available to support almost all networking standards, including the latest Fast Ethernet environment. Fast Ethernet NICs are often 10/100 capable, and will automatically set to the appropriate speed. Full duplex networking is another option, where a dedicated connection to a switch allows a NIC to operate at twice the speed.
Hubs/Repeaters
Hubs/repeaters are used to connect together two or more Ethernet segments of any media type. As segments exceed their maximum length, signal quality begins to deteriorate. Hubs provide the signal amplification required to allow a segment to be extended a greater distance. A hub takes any incoming signal and repeats it out all ports. Ethernet hubs are necessary in star topologies. A multiport, 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. A network of repeaters is termed a "shared Ethernet", meaning that all members of the network are contending for transmission of data onto a single network (collision domain). This means that individual members of a shared network will all only get a percentage of the available network bandwidth.
Bridges
The function of a bridge is to connect separate networks together. Bridges can connect different networks types (such as Ethernet and Fast Ethernet) or networks of the same type. Bridges map the Ethernet addresses of the nodes residing on each network segment and then allow only the necessary traffic to pass through the bridge. When a packet is received by the bridge, the bridge determines the destination and source segments. If the segments are the same, the packet is dropped ("filtered"); if the segments are different, then the packet is "forwarded" to the right segment. Additionally, bridges prevent all bad or misaligned packets from spreading by not forwarding them. Bridges are called "store-and-forward" devices because they look at the whole Ethernet packet before making their filtering or forwarding decisions. Filtering of packets, and the regeneration of forwarded packets enables bridging technology to split a network into separate collision domains. This allows for greater distances and more repeaters to be used in the total network design.
Most bridges are self learning task bridges, meaning they determine the user Ethernet addresses on the segment by building a table as packets are passed through the network. This address self-learning capability dramatically raises the possibility of creating network loops in networks that have many bridges. As each device learns the network configuration, a loop presents conflicting information on which segment a specific address is located and forces the device to forward all traffic. The Spanning Tree Algorithm is a software standard (found in the IEEE 802.1d specification) for describing how switches and bridges can communicate to avoid network loops.
Ethernet Switches
Ethernet switches are an expansion of the concepts in Ethernet bridging. If it makes sense to link two networks through a bridge, why not develop a device that can link four, six, 10 or more networks together? That?s exactly what a LAN switch does. LAN switches come in two basic architectures, cut-through and store-and-forward. Cut-through switches have, in the past, held a speed advantage because when a packet comes into the switch, it only examines the destination address before forwarding it on to its destination segment. A store-and-forward switch, on the other hand, accepts and analyzes the entire packet before forwarding it to its destination. It takes more time to examine the entire packet, but it allows the switch to catch certain packet errors and keep them from propagating through the network. Today, the speed of store-and-forward switches has caught up with cut-through switches to the point where the difference between the two is minimal. Also, there are a large number of hybrid switches available that mix both cut-through and store-and-forward architectures.
Both cut-through and store-and-forward switches separate a network into collision domains, allowing network design rules to be extended. Each of the segments attached to an Ethernet switch has a full 10 Mbps of bandwidth shared by fewer users which results in better performance (as opposed to hubs that only allow sharing of bandwidth from a single Ethernet).
Newer switches today offer high-speed links, either FDDI, Fast Ethernet or ATM, that can be used to link the switches together or to give added bandwidth to particularly important servers that get a lot of traffic. A network composed of a number of switches linked together via uplinks is termed a "collapsed backbone" network.
Routers
Routers work in a manner similar to switches and bridges in that they filter out network traffic. Rather than doing so by packet addresses they filter by specific protocol. Routers were born out of the necessity for dividing networks logically instead of physically. An IP router can divide a network into various subnets so that only traffic destined for particular IP addresses can pass between segments. The price paid for this type of intelligent forwarding and filtering is usually calculated in terms of speed of the network. Such filtering takes more time than that exercised in a switch or bridge which only looks at the Ethernet address but in more complex networks network efficiency is improved.
Servers
When there is a demand for particular files or device access among network users, a means must be found to allow such resources to be shared. Servers are networked devices that allow their files, devices or other resources to be shared by network users. File servers are computers designed to give users access to files stored on their hard drives. Print servers are devices that attach a printer to the network and allow all network users access to the printer. Terminal servers allow terminals to attach directly to a network and access any host available.
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