Lan Introduction

Introduction to Local Area Network (LAN) Protocols What Is a LAN? A LAN is a high-speed data network that is usually confined to a limited geographic area, such as a single building or a college campus. LANs can be small, linking as few as three computers, but can often link hundreds of computers used by thousands of people. The development of standard networking protocols and media has resulted in worldwide proliferation of LANs throughout business and educational organizations. It typically connects workstations, personal computers, printers, servers, and other devices. LANs offer computer users many advantages, including shared access to devices and applications, file exchange between connected users, and communication between users via electronic mail and other applications.

LAN Transmission Methods LAN data transmissions fall into three classifications: unicast, multicast, and broadcast. In each type of transmission, a single packet is sent to one or more nodes. In a unicast transmission, a single packet is sent from the source to a destination on a network. First, the source node addresses the packet by using the address of the destination node. The package is then sent onto the network, and finally, the network passes the packet to its destination. A multicast transmission consists of a single data packet that is copied and sent to a specific subset of nodes on the network. First, the source node addresses the packet by using a multicast address. The packet is then sent into the network, which makes copies of the packet and sends a copy to each node that is part of the multicast address. A broadcast transmission consists of a single data packet that is copied and sent to all nodes on the network. In these types of transmissions, the source node addresses the packet by using the broadcast address. The packet is then sent on to the network, which makes copies of the packet and sends a copy to every node on the network

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Types of LAN Technology • Ethernet Ethernet is the most popular physical layer LAN technology in use today. It defines the number of conductors that are required for a connection, the performance thresholds that can be expected, and provides the framework for data transmission. A standard Ethernet network can transmit data at a rate up to 10 Megabits per second (10 Mbps). Other LAN types include Token Ring, Fast Ethernet, Gigabit Ethernet, 10 Gigabit Ethernet, Fiber Distributed Data Interface (FDDI), Asynchronous Transfer Mode (ATM) and Local Talk. Ethernet is popular because it strikes a good balance between speed, cost and ease of installation. These benefits, 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 Institute for Electrical and Electronic Engineers developed an Ethernet standard known as IEEE Standard 802.3. This standard defines rules for configuring an Ethernet network and also specifies how the elements in an Ethernet network interact with one another. By adhering to the IEEE standard, network equipment and network protocols can communicate efficiently.

• Fast Ethernet The Fast Ethernet standard (IEEE 802.3u) has been established for Ethernet networks that need higher transmission speeds. This standard raises the Ethernet speed limit from 10 Mbps to 100 Mbps with only minimal changes to the existing cable structure. Fast Ethernet provides faster throughput for video, multimedia, graphics, Internet surfing and stronger error detection and correction. 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.

• Gigabit Ethernet

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Gigabit Ethernet was developed to meet the need for faster communication networks with applications such as multimedia and Voice over IP (VoIP). Also known as "gigabit-Ethernet-over-copper" or 1000Base-T, GigE is a version of Ethernet that runs at speeds 10 times faster than 100Base-T. It is defined in the IEEE 802.3 standard and is currently used as an enterprise backbone. Existing Ethernet LANs with 10 and 100 Mbps cards can feed into a Gigabit Ethernet backbone to interconnect high performance switches, routers and servers. From the data link layer of the OSI model upward, the look and implementation of Gigabit Ethernet is identical to that of Ethernet. The most important differences between Gigabit Ethernet and Fast Ethernet include the additional support of full duplex operation in the MAC layer and the data rates.

• 10 Gigabit Ethernet 10 Gigabit Ethernet is the fastest and most recent of the Ethernet standards. IEEE 802.3ae defines a version of Ethernet with a nominal rate of 10Gbits/s that makes it 10 times faster than Gigabit Ethernet. Unlike other Ethernet systems, 10 Gigabit Ethernet is based entirely on the use of optical fiber connections. This developing standard is moving away from a LAN design that broadcasts to all nodes, toward a system which includes some elements of wide area routing. As it is still very new, which of the standards will gain commercial acceptance has yet to be determined.

• Asynchronous Transfer Mode (ATM) ATM is a cell-based fast-packet communication technique that can support data-transfer rates from sub-T1 speeds to 10 Gbps. ATM achieves its high speeds in part by transmitting data in fixed-size cells and dispensing with error-correction protocols. It relies on the inherent integrity of digital lines to ensure data integrity. ATM can be integrated into an existing network as needed without having to update the entire network. Its fixed-length cell-relay operation is the signaling technology of the future and offers more predictable performance than variable length frames. Networks are extremely versatile and an ATM network can connect points in a building, or across the country, and still be treated as a single network.

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• Power over Ethernet (PoE) PoE is a solution in which an electrical current is run to networking hardware over the Ethernet Category 5 cable or higher. This solution does not require an extra AC power cord at the product location. This minimizes the amount of cable needed as well as eliminates the difficulties and cost of installing extra outlets.

LAN Technology Specifications Name

IEEE Data Standard Rate

Media Type

Maximum Distance

Ethernet

802.3

10 Mbps

10Base-T

100 meters

Fast Ethernet/ 802.3u 100Base-T

100 Mbps

100Base-TX 100Base-FX

100 meters 2000 meters

Gigabit Ethernet/ GigE

802.3z

1000 Mbps

1000Base-T 1000Base-SX 1000Base-LX

100 meters 275/550 meters 550/5000 meters

10 Gigabit Ethernet

IEEE 802.3ae

10 Gbps

10GBase-SR 10GBase-LX4 10GBase-LR/ER 10GBaseSW/LW/EW

300 meters 300m MMF/ 10km SMF 10km/40km 300m/10km/40km

• Token Ring Token Ring is another form of network configuration. It differs from Ethernet in a way that all messages are transferred in one direction along the ring at all times. Token Ring networks sequentially pass a “token” to each connected device. When the token arrives at a particular computer (or device), the recipient is allowed to transmit data onto the network. Since only one device may be transmitting at any given time, no data collisions occur. 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.

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Open Systems Interconnection Model OSI Model - History, Origin, Purpose The International Standards Organization (ISO) developed a theoretical model of how networks should behave and how they are put together. This model is called the Open Standards Interconnect (OSI) Model. The "ISO OSI Model" was developed because it appeared that IBM was going to patent the design of their SNA networks so that no one else could use IBM's design model for networking. The ISO OSI model is used throughout the network, internet and telecom industries today to describe various networking issues. The OSI model is also of use in a learning or training environment where a novice can use it as a point of reference to learn how various technologies interact, where they reside, what functions they perform and how each protocol communicates with other protocols. The ISO's Open Standards Interconnect document series defines a model for networking which specifies: • • • • •

How information should be handled when being transported over a network. How software should interact with the network. Layers at which specific networking functions are performed. Layer specific functions should be invisible to the layer above it and below it. The method of communication at the boundaries between layers.

OSI Model - A Layered Approach to Networking Host #1 DATA Applicatio n [DATA [[DATA Presentati on [[[DATA Session [[[[DAT A Transport [[[[[DAT Network A Data Link [[[[[[DA Physical TA

Host #2 Applicatio DATA n [DATA Presentati [[DATA on [[[DATA Session [[[[DATA Transport [[[[[DAT Network A Data Link [[[[[[DAT Physical A

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OSI MODEL - Basic Operation Layer 7: Application Network-capable Applications produce DATA.

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. Layer 6: Presentation (Syntax) Each layer in the OSI Model adds its own information to the front of the data it receives from the layer above it. This information in front of the data is called a header and contains information specific to the protocol operating at that layer. The process of adding the header is called encapsulation. Encapsulated data is transmitted in Protocol Data Units (PDUs). There are Presentation PDU's, Session PDU's, Transport PDU's etc. Thus, PDU's from an upper layer are encapsulated inside the PDU of the layer below. it.

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. Layer 5: Session PDU's are passed down through the stack of layers (called 'the stack' for short) optionally repeating the encapsulation process until they can be transmitted over the Physical layer. The physical layer is the wire connecting all the computers on the network.

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. Layer 4: Transport 6

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.

The OSI standards specify that a layer on host #1 speaks the same language as the same layer on host #2 or any other host on the network. Thus, all hosts can communicate via the Physical layer. This communication between layers is represented by the symbols . Layer 3: Network DATA passed upwards is unencapsulated before being passed farther up (represented by the colored brackets [[[[[[ ). 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, internetworking, error and congestion control, and packet sequencing. Layer 2: Data Link All information is passed down through all layers until it reaches the Physical layer (represented by the vertical red arrows).

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. Layer 1: Physical The Physical layer chops up the PDU's and transmits the PDU's over the physical connection (copper wire, fiber optic cable, radio link etc.). The Physical layer provides the real physical connectivity between hosts over which all communication occurs.

This layer enables hardware to send and receive data over a carrier such as cabling, a card or other physical means. It conveys the bitstream through the network at the electrical and mechanical level. Fast Ethernet, RS232, and ATM are all protocols with physical layer components.

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Local Area Network Topologies Local Area Networks (LANs) use one of the following physical layout designs. These designs are referred to as 'topologies'. Topology

(Logical Ethernet)

Bus Hub and (Star)

Types

Spoke (Physical Ethernet)

Hybrid (Bus & Star)

Ethernet

Point To Point / Daisy Serial Chaining Point to Multipoint

Frame Relay

Ring

FDDI, Ring

Token

BUS TOPOLOGY Networks employing a bus topology use a common physical connection for communication. That means the physical media is shared between stations. When one station transmits on the bus, all devices hear the transmission. If more than one device transmits at the same

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time, the two transmissions will collide with each other and both transmissions will destroy each other. When two or more of these devices attempts to access the network bus at the same time, some method must be used to prevent a collision (CSMA/CD). Historically, bus networks used coaxial cable as their medium of transmission. Token Bus, Ethernet (Thinnet and Thicknet) are common examples of bus topologies. Although some installations of Ethernet using coaxial cable still exist, all modern installations now use a hub and spoke or star topology.

HUB AND SPOKE (STAR) TOPOLOGY Note that this is not called a hub and spoke design because there is a network hub in the drawing. This drawing is to show how a star or hub and spoke network resembles the hub and spokes of a wheel. The Hub and Spoke topology refers to a network topology where there is a central connection point to which multiple devices are connected. A network hub device is not the only device usable in this configuration. A switch may also be used and in some cases, a router. Ethernet utilizing twisted pair still considered a bus architecture from a logical standpoint; however, physically, an Ethernet network can be physically wired as a hub and spoke model.

RING TOPOLOGY Ring topologies are similar to bus topologies, except they transmit in one direction only from station to station. Typically, a ring architecture will use separate physical ports and wires for transmit and receive. Token Ring is one example of a network technology that uses a ring topology.

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POINT TO POINT (Daisy Chaining) TOPOLOGY Point to Point topologies are simplest and most straightforward. You must picture them as a chain of devices and another name for this type of connectivity is called daisy chaining. Most computers can 'daisy chain' a series of serial devices from one of its serial ports. Networks of routers are often configured as point-topoint topologies.

POINT TO MULTIPOINT TOPOLOGY This is not quite the same as a hub and spoke configuration. In a hub and spoke topology, all transmissions from all devices pass through the hub--the hub broadcasts all communication from any single device to all other devices connected to it. In a multipoint topology the hub can send to one or more systems based on an address. Frame Relay is the most common technology to implement this scheme, and it is typically used as a WAN technology. All the remote connection points are connected to a single Frame Relay switch or router port, and communication between sites is managed by that central point. In hub and spoke, all spokes or only one spoke hears a given transmission. In point to multipoint, any number of remote stations can be accessed.

Logical Network Topologies • Peer-to-Peer A peer-to-peer network is composed of two or more self-sufficient computers. Each computer handles all functions, logging in, storage, providing a user interface etc. The computers on a peer-to-peer network can communicate, but do not need the resources or services

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available from the other computers on the network. Peer-to-peer is the opposite of the client-server logical network model. A Microsoft Windows Workgroup is one example of a peer-to-peer network. UNIX servers running as stand-alone systems are also a peerto-peer network. Logins, services and files are local to the computer. You can only access resources on other peer computers if you have logins on the peer computers.

• Client - Server The simplest client-server network is composed of a server and one or more clients. The server provides a service that the client computer needs. Clients connect to the server across the network in order to access the service. A server can be a piece of software running on a computer, or it can be the computer itself. One of the simplest examples of client-server is a File Transfer Protocol (FTP) session. File Transfer Protocol (FTP) is a protocol and service that allows your computer to get or put files to a second computer using a network connection. A computer running FTP software opens a session to an FTP server to download or upload a file. The FTP server is providing file storage services over the network. Because it is providing file storage services, it is said to be a 'file server'. A client software application is required to access the FTP service running on the file server. Most computer networks today control logins on all machines from a centralized logon server. When you sit down to a computer and type in your username and password, your username and password are sent by the computer to the logon server. UNIX servers use NIS, NIS+ or LDAP to provide these login services. Microsoft Windows computers use Active Directory and Windows Logon and/or an LDAP client. Users on a client-server network will usually only need one login to access resources on the network.

• Distributed Services Computer networks using distributed services provide those services to client computers, but not from a centralized server. The services are running on more than one computer and some or all of the functions provided by the service are provided by more than one server.

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The simplest example of a distributed service is Domain Name Service (DNS) which performs the function of turning humanunderstandable names into computer numbers called IP addresses. Whenever you browse a web page, your computer uses DNS. Your computer sends a DNS request to your local DNS server. That local server will then go to a remote server on the Internet called a "DNS Root Server" to begin the lookup process. This Root Server will then direct your local DNS server to the owner of the domain name the website is a part of. Thus, there are at least three DNS servers involved in the process of finding and providing the IP address of the website you intended to browse. Your local DNS server provides the query functions and asks other servers for information. The Root DNS server tells your local DNS server where to find an answer. The DNS server that 'owns' the domain of the website you are trying to browse tells your local DNS server the correct IP address. Your computer stores that IP address in its own local DNS cache. Thus, DNS is a distributed service that runs everywhere, but no one computer can do the job by itself.

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