Local Area Network

Local area

network

Local area network scheme A local area network (LAN) is a computer network covering a small geographic area, like a home, office, or group of buildings. The defining characteristics of LANs, in contrast to Wide area networks (WANs), include their much higher data transfer rates, smaller geographic range, and lack of a need for leased telecommunication lines. Ethernet over unshielded twisted pair cabling, and Wi-Fi are the most common two technologies currently, but ARCNET, Token Ring and many others have been used in the past.

History In the days before personal computers, a site might have just one central computer, with users accessing this via computer terminals over simple low-speed cabling. Networks such as IBM's SNA (Systems Network Architecture) were aimed at linking terminals or other mainframes at remote sites over leased lines—hence these were wide area networks. The first LANs were created in the late 1970s and used to create high-speed links between several large central computers at one site. Of many competing systems created at this time, Ethernet and ARCNET were the most popular. The development and proliferation of CP/M and then DOS-based personal computers meant that a single site began to have dozens or even hundreds of computers. The initial attraction of networking these was generally to share disk space and laser printers, which were both very expensive at the time. There was much enthusiasm for the concept and for

several years, from about 1983 onward, computer industry pundits would regularly declare the coming year to be “the year of the LAN”. In reality, the concept was marred by proliferation of incompatible physical layer and network protocol implementations, and confusion over how best to share resources. Typically, each vendor would have its own type of network card, cabling, protocol, and network operating system. A solution appeared with the advent of Novell NetWare which provided even-handed support for the 40 or so competing card/cable types, and a much more sophisticated operating system than most of its competitors. Netware dominated[1] the personal computer LAN business from early after its introduction in 1983 until the mid 1990s when Microsoft introduced Windows NT Advanced Server and Windows for Workgroups. Of the competitors to NetWare, only Banyan Vines had comparable technical strengths, but Banyan never gained a secure base. Microsoft and 3Com worked together to create a simple network operating system which formed the base of 3Com's 3+Share, Microsoft's LAN Manager and IBM's LAN Server. None of these were particularly successful. In this same timeframe, Unix computer workstations from vendors such as Sun Microsystems, Hewlett-Packard, Silicon Graphics, Intergraph, NeXT and Apollo were using TCP/IP based networking. Although this market segment is now much reduced, the technologies developed in this area continue to be influential on the Internet and in both Linux and Apple Mac OS X networking—and the TCP/IP protocol has now almost completely replaced IPX, AppleTalk, NBF and other protocols used by the early PC LANs.

Technical aspects Although switched Ethernet is now the most common data link layer protocol and IP as a network layer protocol, many different options have been used, and some continue to be popular in niche areas. Smaller LANs generally consist of a one or more switches linked to each other - often with one connected to a router, cable modem, or DSL modem for Internet access. Larger LANs are characterized by their use of redundant links with switches using the spanning tree protocol to prevent loops, their ability to manage differing traffic types via quality of service, and to segregate traffic via VLANing. LANs may have connections with other LANs via leased lines, leased services, or by 'tunneling' across the Internet using VPN technologies.

Local area network - A local area network (LAN) is a group of computers and associated devices that share a common communications line or wireless link. Typically, connected devices share the resources of a single processor or server within a small geographic area (for example, within an office building). Usually, the server has applications and data storage that are shared in common by multiple computer users. A local area network may serve as few as two or three users (for example, in a home network) or as many as thousands of users (for example, in an FDDI network). Major local area network technologies are: Ethernet Token Ring FDDI Ethernet is by far the most commonly used LAN technology. A number of corporations use the Token Ring technology. FDDI is sometimes used as a backbone LAN interconnecting Ethernet or Token Ring LANs. Another LAN technology, ARCNET, once the most commonly installed LAN technology, is still used in the industrial automation industry. Typically, a suite of application programs can be kept on the LAN server. Users who need an application frequently can download it once and then run it from their local hard disk. Users can order printing and other services as needed through applications run on the LAN server. A user can share files with others at the LAN server; read and write access is maintained by a LAN administrator. A LAN server may also be used as a Web server if safeguards are taken to secure internal applications and data from outside access. In some situations, a wireless LAN may be preferable to a wired LAN because it is cheaper to install and maintain.

Setting up a Local Area Network Using Red Hat Linux to connect two or more computers

Level: Introductory Darrick Addison ([email protected]), Senior Software Engineer/Consultant, ASC Technologies 01 Feb 2001 This article describes how to build a Local Area Network (LAN) consisting of two or more computers running the Red Hat Linux operating system. The article begins with the basics: an overview of the TCP/IP (Transmission Control Protocol/Internet protocol) suite, and an explanation of assigning IP addresses in a LAN. Then the article covers the LAN hardware and configuration using a tool called LinuxConf in the Red Hat Linux operating system environment. Lastly, the article walks you through the critical steps of testing and troubleshooting your LAN. Linux is increasingly popular in the computer networking/telecommunications industry. Acquiring the Linux operating system is a relatively simple and inexpensive task since virtually all of the source code can be downloaded from several different FTP or HTTP sites on the Internet. In addition, the most recent version of Red Hat Linux can be purchased from computer retail stores for between $25 and $50, depending on whether you purchase the standard or full version. The retail brand is indeed a worthwhile investment (vs. the free FTP or HTTP versions) since valuable technical support is included directly from the Red Hat Linux engineers for at least a year. This can be very helpful if, for instance, you can not resolve an installation/configuration problem after consulting the Red Hat Linux manuals. This article describes how to put together a Local Area Network (LAN) consisting of two or more computers using the Red Hat Linux 6.2 operating system. A LAN is a communications network that interconnects a variety of devices and provides a means for exchanging information among those devices. The size and scope of a LAN is usually small, covering a single building or group of buildings. In a LAN, modems and phone lines are not required, and the computers should be close enough to run a network cable between them. For each computer that will participate in the LAN, you'll need a network interface card (NIC) to which the network cable will be attached. You will also need to assign a unique hostname and IP address to each computer in the LAN (described later in this article), but this requires a basic understanding of TCP/IP (Transmission Control Protocol/Internet Protocol). Introduction to TCP/IP TCP/IP is the suite of protocols used by the Internet and most LANs throughout the world. In TCP/IP, every host (computer or other communications device) that is connected to the network has a unique IP address. An IP address is composed of four octets (numbers in the range of 0 to 255) separated by decimal points. The IP address is used to uniquely identify a host or computer on the LAN. For example, a computer with the hostname Morpheus could have an IP address of 192.168.7.127. You should avoid giving two or more computers the same IP address by using the range of IP addresses that are reserved for private, local area networks; this range of IP addresses usually begins with the octets 192.168. LAN network address The first three octets of an IP address should be the same for all computers in the LAN. For example, if a total of 128 hosts exist in a single LAN, the IP addresses could be assigned starting with 192.168.1.x, where x represents a number in the range of 1 to 128. You could create consecutive LANs within the same company in a similar manner consisting of up to another 128 computers. Of course, you are not limited to 128 computers, as there are other ranges of IP addresses that allow you to build even larger networks. There are different classes of networks that determine the size and total possible unique IP addresses of any given LAN. For example, a class A LAN can have over 16 million unique IP addresses. A class B LAN can have over 65,000 unique IP addresses. The size of your LAN depends on which reserved address range you use and the subnet mask (explained later in the article) associated with that range (see Table 1.). Table 1. Address ranges and LAN sizes Address range

Subnet mask Provides

Addresses per LAN

10.0.0.0 - 10.255.255.255.255 255.0.0.0

1 class A LAN

16,777,216

172.16.0.0 - 172.31.255.255

16 class B LANs

65,536

255.255.0.0

Local-Area Network (Lan) The term "local-area network" (or LAN) describes a group of computers connected by usually no more than 1,000 feet of cable that allows these computers to share data. LANs can be created using different types of cables, configurations, and protocols, and still remain within the requirements of a network system specification. Networks consist of four basic components: •

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Network Operating System (NOS). The NOS consists of several computer programs that run in the networked computers. Some of these computers share files, printers, and other peripherals over a network and are called servers. Other computers that use these shared resources are called clients. Network Peripherals. Printers and scanners with their own network connections are examples of network peripherals. These components use specialized processors to operate networking server software without having to be connected to a computer. Network Interface Card (NIC). NICs are also called LAN adapters. NICs have several functions inside a computer. They convert the low-power signals from inside a computer into stronger signals that can propagate across a network cable. The NIC also prepares data for transmission by including important messaging or packaging information that is used by other computers on the LAN. The NIC also controls access to the network cable that is shared by other computers on the network. Network Cabling. Network data can be sent as 1) electrical pulses over different types of copper cable, 2) pulses of light over fiber optic cable, 3) through the air as radio or light waves, or 4) data can be sent via a combination of these.

Major Types of LAN Technology Asynchronous Transfer Mode (ATM) Asynchronous Transfer Mode (ATM) is a connection-based type of networking that can be used in LANs and wide-area networks (WANs). ATM is what is responsible for highspeed data switching, often at gigabit rates. However, most computers have a difficult time being able to absorb data at gigabit speeds. ATM networks, therefore, handle traffic from many other computers and are more expensive to build and maintain. ATM networks use optical fibers for connections because optical fibers transfer data at a higher rate than does copper (ATMs can operate at speeds between 1.544 Mbps to 1.2 Gbps). The lowest levels of an ATM network use fixed-size data packets called cells. The one-size cells allows ATM switch hardware to process them quickly.

Because ATM networks carry voice and data, many telephone system networks use ATM technology. Bridge A bridge is a computer that connects two or more networks and forwards data packets among them. A bridge can also link the faster portion of a LAN to slower links, such as networks of leased telephone lines. Bridges help extend the network and help isolate network traffic. Bridges send packets (small blocks of data sent across packet-switching networks) and frames (packages of data) between different types of media, but they can only forward information that is addressed to devices on another network system. Local bridges connect fast cable segments on one LAN (this usually requires one "local" bridge). Remote bridges connect fast local cables to slower, longer-distance cables in order to connect physically separated networks (this usually requires two "remote" bridges). Broadcasting In LAN context, a broadcast is a packet-delivery system that delivers a copy of a particular packet to all hosts that are attached to it, much the way a television signal is broadcast to all the television stations tuned to it. Broadcasting is a technique used by the most popular network connection technology, Ethernet. With a broadcast-network system, a single node on the network will transmit information to all other nodes at the same time. However, not all of the nodes will receive the message; only those nodes that have been addressed in the message will be able to receive it. This is similar to the television signal analogy where, for example, only those televisions able to receive an HBO or pay-per-view signal (because a signal blocking filter has been removed) can receive the broadcast. In the LAN world, once the signal has been received, the receiving stations will send an acknowledgement to the sending station. Carrier-Sense Multiple Access (CSMA) Carrier-Sense Multiple Access (CSMA) is a method used in Ethernet networks to transmit data only during breaks in the network traffic. This method is typically used by network interface cards that share a common cable. CSMA with collision detection, also known as CSMA/CD, is a technique used if two transmit stations begin transmission at the same time, they can detect a collision, stop, and retry after a predefined period of time. Ethernet networks use a binary exponential backoff policy, which is simply an algorithm that helps avoid massive data collisions in the unlikely event many transmitting stations try to transmit simultaneously. Ethernet

In the late 1970s IBM and Digital Equipment Corporation developed methods for several large computers to operate over local networks. Later, Xerox Corporation developed a method that allowed many computers to be connected over a LAN. This method later developed into a set of standards (minimum performance requirements) and protocols (rules for how different elements interact) called Ethernet, which today is the most popular type of LAN. Today's Ethernet LANs allow for mixing and matching of various hardware and software, network cables, and configurations, and still remain within the boundaries of the Ethernet specification, also known as IEEE 802.3. Today the Ethernet can achieve data-transfer rates in excess of one Gigabit (one Gigabit = 1,000 Megabits). In thin Ethernet networks, the cable configuration uses a T-shaped coaxial connector at each network interface card (NIC). The more common Ethernet configuration uses unshielded twisted-pair wire available in three configurations: 10Base-T (10 megabits per second, or Mbps, signal speed), 100Base-T (100 Mbps, also known as fast Ethernet), and 1000Base-T (1000 Mbps, also known as Gigabit Ethernet). Unshielded twisted pair wire uses a small plastic connector known as an RJ-45 connector at each end of the wire, which plugs into the back of a computer, wiring hub, or NIC. Fiber Distributed Data Interface (FDDI) Fiber Distributed Data Interface (FDDI) is a type of high-speed LAN that uses optical fiber to carry signals that contain data encoded in pulses of light. Another similar technology called Copper Distributed Data Interface (CDDI) works similar to FDDI, but uses copper cables to carry signals. Unlike other copper-based networks, optical fiber is immune from electrical interference, and because optical fiber uses light, it can transmit much higher data rates than copper cables carrying electrical signals (per unit time). FDDI is known as a "ring" because the network configuration starts and ends at the same source. The most unusual fact about FDDI is its ability to detect and diagnose problems; in fact, because the network hardware can automatically accommodate a network failure, it is sometimes called a selfhealing network. IBM Token-Ring

Token-Ring network interface cards use a complex network cable-access control methodology. While Ethernet NICs contend for cable access, Token-Ring NICs require permission to transmit into a cable that forms a completed loop. The active Token-Ring NICs "negotiate" (using their serial numbers) to determine which card is the master NIC.

The master NIC transmits a special message called a "free token." When a data-loaded NIC receives a free token, it converts the free token into a message and forwards it to the next station on the loop for relay. After the addressed NIC receives the message, and the message returns to the originating NIC, that master NIC generates another free token and the process repeats. A Token-Ring network has two physically separate wiring hubs that are connected by fiber optic cable that can be thousands of feet apart. However, the devices attached to the Token-Ring network (connected by unshielded twisted pair, or UTP, wires) must be no farther than 100 feet from the wiring hub. The wiring configuration more resembles a star-shape, whereas the actual "ring" in the Token-Ring network is contained within the wiring hubs. Multicasting Broadcasting involves the delivery of a packet to all computers on a specific network. Unicasting involves sending a packet to a single device. Multicasting involves the delivery of a packet to a specific subset of hosts. Unlike broadcasting, multicasting allows each system the choice of participating in a particular multicast. Large numbers of addresses are reserved in a hardware device that will later be used in the multicast. When a collection of devices wants to communicate, one specific multicast address is selected for use. All connected devices will receive a copy of any packet sent to the multicast address, once the NICs have been configured to recognize the selected multicast address. A type of multicasting that was popular for a short time in the late 1990s was called Push. Push (or "server-push") is the delivery of information on a network (in this case, the World Wide Web) that is initiated by the information server rather than by the information user or client, as it usually is. An early Web service that specialized in "pushing" information rather than having it "pulled" as the result of requests for Web pages was Pointcast, a site that provided late-breaking news and other information customized to a previously defined user profile. Marimba was another similar site (and product) that pushed information to the user on a predefined schedule. The information pushed from a server to a user actually arrives as the result of a programmed request from the client in a computer. Any information pusher on the Web requires the download of a client program. This program captures a user profile and then occasionally initiates requests for information on the user's behalf from the server.

LAN Topologies Topology is simply the physical layout and configuration of the devices on a network, or the "network architecture." There are four fundamental LAN topologies.

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Star Topology. The star topology is the oldest type of network architecture. It is similar to a telephone system in which calls from one person to another are handled through a central switching station. With star LANs, all messages are routed through a central hub. Adding nodes to a star topology is simple and easy: just add a NIC and cable to connect to the central hub. This configuration allows certain nodes to be given higher priority over others. The central hub simply searches for signals from these higher priority nodes before acknowledging the others. Clustered Star Topology. A clustered star topology is simply several star topologies linked together. In this configuration, if one star malfunctions or fails, the entire network does not fail. Bus Topology. In this simple configuration, a bus (single master cable carrying all traffic) serves as the focal point for the LAN, where the nodes connect along the length of the bus. Devices on the network need to be sufficiently spaced apart to avoid electrical interference. The problem with such a configuration is that one malfunctioning node on the bus can shut down the LAN. Mesh Topology. A mesh topology is exactly what it sounds like. It is a network without a defined network architecture or topology. Such topologies tend to be extremely complex and have all types of built-in redundancies to minimize network downtime or failures.