ITNW 2313
LAN Hardware/Wiring & Installation

Lesson 2 - Exploring Networking Essentials

Instructor: Kenneth D. Stewart

Key Terms
networking LAN LAN Basics shared access NIC Bus network packet Ethernet IEEE 802.3 CSMA/CD bus topology star topology	UTP EMI STP 10Base-T (UTP) 10Base2 (ThinNet) 10Base5 (ThickNet) Fast Ethernet IEEE 802.3u 100Base-TX Token Ring IEEE 802.5 token passing	token ring topology MAU FDDI OSI Model ISO IEEE interoperability NOS network center seamless connectivity enterprise networks

The World of Computer Networking

In the last 15 years, LANs have gone from being an experimental technology to becoming a key business tool used by companies worldwide. A LAN is a high-speed communications system designed to link computers and other data processing devices together within a small geographic area such as a workgroup, department, or a single floor of a multistory building. Several LANs can also be interconnected within a building or campus of buildings to extend connectivity.

Some Background on LANs

LANs have become popular because they allow users to share vital computing resources electronically, including expensive hardware such as printers and CD-ROM drives, application programs, and, most importantly, the information the users need to do their jobs. Prior to the development of LAN technology, individual computers were isolated from each other and limited in their range of applications. By linking these individual computers over LANs, their usefulness and productivity have been increased enormously. But a LAN by its very nature is a local network, confined to a fairly small area such as a building or even a single floor of a building. To realize the full benefit of computer networking, it is critical to link the individual LANs into an enterprise-wide backbone network that connects all of the company's employees and computing resources, no matter how geographically dispersed they may be.

Today's LANs and LAN internetworks are powerful, flexible, and easy to use, but they incorporate many sophisticated technologies that must work together flawlessly. For a LAN to really benefit an organization it must be designed to meet the organization's changing communications requirements. Building a LAN is a process of choosing different pieces and matching them together. This course is designed to help students understand the fundamentals of how LANs operate, what the different technology choices are for building a LAN, and the ramifications of choosing one option over another. Also discussed is the concept of internetworking or connecting disparate and geographically dispersed LANs together to form an enterprise system, the different technologies and products available to do so, and the benefits and limitations of each.

To aid students unfamiliar with networking terminology, most od the networking terms in these lessons appear in the Networking Glossary.

The Basics of Local Area Networking

Today local area networking is a shared access technology. This means that all of the devices attached to the LAN share a single communications medium, usually a coaxial, twisted pair, or fiber optic cable. Figure 1 illustrates this concept: Several computers are connected to a single cable that serves as the communications medium for all of them. The physical connection to the network is made by putting a network interface card (NIC) inside the computer and connecting it to the network cable. Once the physical connection is in place, it is up to the network software to manage communications between stations on the network.

A Basic LAN Bus Network

When Station B sends a packet to another station on the LAN, it passes by all of the stations connected to that LAN. On the bus network illustrated here, the electrical signal representing the packet travels away from the sending station in both directions on the shared cable. All stations will see the packet, but only the station it is addressed to will pay attention to it.

In a shared media network, when one station wishes to send a message to another station it uses the software in the workstation to put the message in an "envelope." This envelope, called a packet, consists of message data surrounded by a header and trailer that carry special information used by the network software to the destination station. One of the pieces of information placed in the packet header is the address of the destination station.

The NIC then transmits the packet onto the LAN. The packet is transmitted as a stream of data bits represented by changes in electrical signals. As it travels along the shared cable, all of the stations attached to it see the packet. As it goes by the NIC in each station, the NIC checks the destination address in the packet header to determine if the packet is addressed to it. When the packet passes the station it is addressed to, the NIC at that station copies the packet and then takes the data out of the envelope and gives it to the computer.

The diagram shows one source station sending a single message packet to one destination station. If the message the source station wants to send is too big to fit into one packet, it will send the message in a series of packets. On a shared access LAN, however, many stations all share the same cable. Since each individual packet is small, it takes very little time to travel to the ends of the cable where the electrical signal dissipates. So after a packet carrying a message between one pair of stations passes along the cable, another station can transmit a packet to whatever station it needs to send a message. In this way, many devices can share the same LAN medium.

Ethernet

The most widely used LAN technology in use today is Ethernet. It strikes a good balance between speed, price, ease of installation, and supportability. Approximately 80 percent of all LAN connections installed use Ethernet.

The Ethernet standard is defined by the Institute of Electrical and Electronics Engineers (IEEE) in a specification commonly known as IEEE 802.3. The 802.3 specification covers rules for configuring Ethernet LANs, the types of media that can be used, and how the elements of the network should interact. The Ethernet protocol provides the services called for in the Physical and Data Link Layers of the OSI reference model.

One element of the 802.3 (or 802.3u) specification states that Ethernet networks run at a data rate of 10 million bits per second (10 Mbps) or 100 million bits per second (100 Mbps) in the case of Fast Ethernet. This means that when a station transmits a packet onto the Ethernet medium it travels along that medium at 10 Mbps.

Another important element defined by the 802.3 specification is the access method to be used by stations connected to an Ethernet LAN, called carrier sense multiple access with collision detection (CSMA/CD). In this method, each station contends for access to the shared medium. It is possible for two stations to try sending packets at the same time, which results in a collision on the LAN. In Ethernet networks, collisions are considered normal events and the CSMA/CD access method is designed to quickly restore the network to normal activity after a collision occurs.

Ethernet Media and Topologies

An important part of designing and installing a LAN is selecting the appropriate medium and topology for the environment. Ethernet networks can be configured in either a star or bus topology and installed using any of three different media.

Coaxial cable was the original LAN medium and it is used in what is called a bus topology In this configuration, the coaxial cable forms a single bus to which all stations are attached. This topology is rarely used in new LAN installations today because it is relatively difficult to accommodate adding new users or moving existing users from one location to another. It is also difficult to troubleshoot problems on a bus LAN unless it is very small.

A Basic Star Topology LAN

In a star topology all stations are wired to a central wiring concentrator called a hub. Similar to a bus topology, packets sent from one station to another are repeated to all ports on the hub. This allows all stations to see each packet sent on the network, but only the station a packet is addressed to pays attention to it.

The diagram illustrates a star topology LAN -- which is a more robust topology than the bus topology. In a star topology, each station is connected to a central wiring concentrator, or hub, by an individual length of twisted pair cable. The cable is connected to the station's NIC at one end and to a port on the hub at the other. The hubs are placed in wiring closets centrally located in a building.

Ethernet networks can be built using three different types of media: shielded and unshielded twisted pair, coaxial, and fiber optic cables. By far the most common is twisted pair because it is associated with the more popular star topology. It is inexpensive, and very easy to install, troubleshoot, and repair. Twisted pair cable comes both unshielded and shielded. Unshielded twisted pair (UTP) cable used for LANs is similar to telephone cable but has somewhat more stringent specifications regarding its susceptibility to outside electromagnetic interference (EMI) than common telephone wire. Shielded twisted pair (STP), as its name implies, comes with a shielding around the cable to provide more protection against EMI.

Of the two types of twisted pair cable, UTP is by far the most commonly used. The specification for running Ethernet on UTP is called 10Base-T. This stands for 10 Mbps, baseband signaling (the signaling method used by Ethernet networks), over twisted pair cable. Other Ethernet specifications include 10Base5, which uses a thick coaxial cable, and 10Base2, which uses a thin coaxial cable media. Today, 10Base5 is seldom installed in new Ethernet networks, and 10Base2 is used only in very small office networks or networks in high EMI areas. An additional standard, 10Base-FL, allows Ethernet to run on fiber optic link.

Fast Ethernet

An extension of the popular 10Base-T Ethernet standard, Fast Ethernet transports data at 100 Mbps. With rules defined by the IEEE 802.3u standard, Fast Ethernet leverages the familiar Ethernet technology and retains the CSMA/CD protocol of 10 Mbps Ethernet. Three types of Fast Ethernet are available: 100Base-TX, which runs over Category 5 UTP; 100Base-T4 which runs over existing Category 3 UTP; and 100Base-FX, which operates over multimode fiber optic cabling.

Token Ring LANs

Another major LAN technology in use today is Token Ring. Token Ring rules are defined in the IEEE 802.5 specification. Like Ethernet, the Token Ring protocol provides services at the Physical and Data Link Layers of the OSI model. Token Ring networks can be run at two different data rates, 4 Mbps or 16 Mbps.

The access method used on Token Ring networks is called token passing. Token passing is a deterministic access method in which collisions are prevented by assuring that only one station can transmit at any given time. This is accomplished by passing a special packet called a token from one station to another around a ring. A station can only send a packet when it gets the free token. When a station gets a free token and transmits a packet, it travels in one direction around the ring, passing all of the other stations along the way. As with Ethernet, the packet is usually addressed to a single station, and when it passes by that station the packet is copied. The packet continues to travel around the ring until it returns to the sending station, which removes it and sends a free token to the next station around the ring.

Token Ring Topology and Media

Token Ring networks use what is called a ring topology. However, it is actually implemented in what can best be described as a collapsed ring or star wired ring that looks like a physical star topology.

Basic Ring Topology LAN

The ring topology used in Token Ring networks is a collapsed ring that looks like a physical star. Each station is connected to a Token Ring wiring connector by a single twisted pair cable with two wire pairs. One pair serves as the "inbound" portion of the ring (also known as the receive pair) and the other pair serves as the "outbound" or transmit pair.

In Token Ring LANs, each station is connected to a Token Ring wiring concentrator, called a multistation access unit (MAU), using an individual run of twisted pair cable. Like Ethernet hubs, MAUs are located in wiring closets.

FDDI

Fiber Distributed Data Interface, commonly known as FDDI, provides data transport at 100 Mbps, a much higher data rate than Ethernet or Token Ring. Originally, FDDI networks required fiber optic cable, but today they can be run on UTP as well. Fiber is still preferred in many FDDI networks because it can be used over much greater distances than UTP cable. Like Token Ring, FDDI uses a token passing media access method. It is also usually configured in a collapsed ring, or physical star, topology. FDDI is used primarily as a backbone, a segment of network that links several individual workgroup or department LANs together in a single building. It is also used to link several building LANs together in a campus environment.

Standards and Protocols

LANs are complex systems that implement many different services in order to provide communication between all of the types of devices that can be connected to them. A communications model called the Open Systems Interconnect (OSI) reference model was developed by the International Standards Organization (ISO) to define all of the services a LAN should provide. This model defines seven layers, each of which provides a subset of all of the LAN services. This layered approach allows small groups of related services to be implemented in a modular fashion that makes designing network software much more flexible. A network software module that implements services at the Network and Transport Layers of the model can be paired up with different Physical and Data Link Layer modules depending on the requirements of the user's application.

But the OSI model doesn't say how these services should actually be implemented in LAN equipment. The "how to" part has been defined in a number of different protocols that have been developed by international standards bodies, individual LAN equipment vendors, and ad hoc groups of interested parties. These protocols typically define how to implement a group of services in one or two layers of the OSI model. For example, Ethernet and Token Ring are both protocols that define different ways to provide the services called for in the Physical and Data Link Layers of the OSI model. They have both been approved by the Institute of Electrical and Electronics Engineers (IEEE), an international communications standards body.

The International Standards Organization (ISO), the primary standard-setting body in the data communications industry, developed the framework for LAN standards called the Open Systems Interconnect reference model. This reference model represents a standard approach to communicate information throughout a network so that a variety of independently developed computer and communications devices can interoperate.

Because they are approved and published by the IEEE, both the Ethernet and Token Ring protocols are said to be industry standards. Any company can acquire the specifications and design Ethernet or Token Ring NICs. Users can purchase an Ethernet NIC, for example, from any vendor and be assured that it will operate in a network with Ethernet NICs from other vendors. This degree of interoperability is highly desirable. However, there are many more protocols for providing services at the higher layers of the OSI model and very few of them have been approved by an international standards bodies. In fact, most upper layer LAN protocols are incorporated into proprietary network operating systems, such as Novell's NetWare, IBM's LAN Server, and Microsoft's LAN Manager. A user has to buy only that vendor's products in order to be assured that they will interoperate on a LAN.

Network Operating Systems

Ethernet and Token Ring technologies are just one part of a complete LAN. They provide the services specified in the Physical and Data Link Layers of the OSI model, but several other services must be added on top of the connectivity of Ethernet or Token Ring. Network operating systems (NOSs) are most often used to provide the additional communications services.

A NOS defines client and server systems. Clients are individual user workstations attached to the network where application programs are run and data is generated. Servers are shared network resources that provide hard disk space for users to store files, printer services, and a number of other network services. The network operating system provides a set of protocols in software that run on both servers and client systems and allow them to communicate with each other, share files, printers, and other network resources.

Networking Today

LAN technology is evolving. In the early 1980s LANs were strictly local area networks, linking small groups of computers in company departments. As workgroup LANs proliferated over the past 10 years, users began connecting them to form internetworks, first with bridges and later with routers. Today's networks typically comprise a combination of workgroup and campus hubs, bridges, and routers. Switches are also beginning to become more prevalent.

The next few years will see networks evolve to include more sophisticated LAN switches and switching hubs. They will be designed using several different types of components, both old and new. Ethernet and Token Ring LANs will be built with stackable workgroup hubs, which, in turn, will be interconnected by larger modular hubs that may incorporate LAN switching functionality. Large networks will include another layer of consolidation with network center hubs linking workgroup hubs and switches. Routers will continue to be used as gateways to the wide area network linking other buildings and remote sites.

For networks to deliver the performance today's users require, their many components must work together to deliver seamless connectivity between all of the users and computing systems throughout the enterprise. Flexibility to grow, power to support applications, and seamless connectivity are what users expect in the products they choose to build LANs and enterprise networks.

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This page is maintained by:   Kenneth D. Stewart

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	viking.delmar.edu Last modified 08 Jan 2002		kstewart@delmar.edu Copyright © 2002