The Absolute Basics of Local Area
Please choose from the following
A Local Area Network (commonly referred to as a LAN) refers to a communication network with the following characteristics:
A LAN is an increasingly popular way for organizations large and small to get additional productivity gains from the use of personal computers. Large businesses, government agencies and universities have long realized the need for "collaborative computing" or people using their computers as a group. Today, more and more small businesses, local governments and schools are using the power of LANs to increase productivity and efficiency. It is well beyond the scope of this document to discuss the "enterprise wide" local and wide-area networks typically found in large organizations. Rather, this document will concentrate on the designs (referred to as topologies), hardware and software typically found in smaller Workgroup LANs. The purpose here is to provide a base level of understanding of the terminology commonly used by networking professionals and how they interrelate.
The most basic elements of workgroup LANs are personal computers. They may either be used by individuals or connected as supporting hardware. LAN hardware and software is designed to connect all types of PCs including DOS and Windows PCs, Apple Macintoshes, UNIX workstations, desktop and portable systems. When any one of these computers are connected to a LAN they are referred to as a workstation whether they are PCs, Macs or true UNIX workstations. This is only the first example of confusing LAN terminology.
One of the great benefits of a LAN is the ability to share computing devices that are used only occasionally. Devices that are commonly shared on a LAN include printers, modems, scanners and the like. One can save considerable computing expense by sharing these types of devices over a LAN as opposed to dedicating individual devices to individual users. All computing devices on a network; workstations, printers, etc., are referred to as nodes.
The next group of hardware to be considered is the hardware that is required for the physical network itself. For any node to be connected to the LAN, interface hardware is required. This interface hardware does not typically come standard with the computing device and must be purchased separately as either an option from the original manufacturer or from the hundreds of manufacturers that specialize in networking hardware. When connecting a workstation, the interface hardware is referred to as an adapter. An adapter is typically connected to the workstation via an expansion slot. On a desktop system, these expansion slots are in the back of the system and are the same bus expansion slots that are used to add other hardware adapters that provide for or enhance the system's sound, video and multimedia capabilities. Most newer portable systems and notebook computers have slots dedicated to expansion devices that are referred to as PC Card slots. Different types of computers, portables or desktops, usually accept only adapters that are designed specifically for them. For example, a desktop PC will not accept the same type of adapter as a notebook PC or a desktop Mac. Networking adapters are also commonly referred to as Network Interface Cards or NICs. The LAN connection for a printer, however, is referred to as a print server and not a NIC.
Media Interfaces: RJ-45 (10BASE-T) at top right, BNC (10BASE2) at bottom right
Bus Interface: ISA connector is along bottom edge of adapter
Notes: This adapter supports both 10BASE-T with an RJ-45 and 10BASE2 with a BNC connector. Other adapters may have a single interface and therefore support only one Ethernet cable implementation while others can have three connectors (RJ-45, BNC and an AUI for 10BASE5) and thereby support all three implementations.
Media is a term that largely refers to the cable or wires connecting together the various computing devices that make up a LAN. There are many different media types in use today in LANs.
Twisted Pair is the most common type of LAN media. It consists of strands of copper wire twisted together to make up a single cable. Most typically eight wires make up a twisted pair cable. This twisted pair cable is then either Shielded (and referred to as Shielded Twisted Pair or STP) or unshielded and referred to as Unshielded Twisted Pair or UTP. Of the two, UTP is far more common and is similar to the type of wire that is used for telephones in the United States and is therefore often referred to as telephone cable.
The next most common type of LAN media is coaxial cable or coax. Two different versions of coax are used in LANs: Thin coaxial and thick coaxial. Of the two, thin coax is far more popular and is similar in appearance to the cable used in cable television systems in the UK.
Other media types include Fiber Optic cable, which allows for high speed and long distance data transmission and is principally found in only larger LANs. Another type of media is not a cable at all. This type of media is either infrared light (like a television remote control) or radio waves. Networks that have no physical media are referred to as wireless and are typically found only in niche applications where stringing cable is impossible or impractical.
A Hub is a generic term that is used for a device that acts as a central point for LAN cable. The most basic and popular types of hubs are devices that simply connect cables together and regenerate data thereby passing data from one device to another. These types of hubs are referred to as concentrators or repeaters. There are also hubs that provide additional functionality, including bridges which connect network segments and routers which connect different types of both local and wide area networks. The functionality of these types of hubs are highly specialized and closely related to the type of network they are used in. They are typically installed in very large, complex networks. As such they are beyond the scope of this document.
In addition to the networking hardware previously discussed, software is required by the network to operate. This type of networking software is referred to as a Network Operating System or NOS. The various brands of NOSs divide into two types: those which provide for a Peer to Peer architecture and those that provide for a Client/Server architecture. Additionally, a small software program is required for the adapters installed in LAN nodes to operate with the NOS. These programs are referred to a device drivers or simply drivers. Drivers for popular adapters are often included with the NOS itself. Custom drivers which provide for better adapter performance or drivers for less popular adapters are included with the adapter itself and must be copied into the NOS when the adapter is installed.
Network Operating Systems and Required Drivers
|Developer||Product||Architecture||Type Driver Required|
|Artisoft||LANtastic||Peer to Peer||NDIS 2|
|DEC||Pathworks||Client Server||NDIS 2|
|Microsoft||Windows NT||Client/Server||NDIS 3|
|Microsoft||Windows 95||Peer to Peer||NDIS 3|
|Microsoft||Windows for Workgroups||Peer to Peer||NDIS 2 or NDIS 3|
|Novell||Netware||Client/Server||ODI or IPX|
|SCO||SCO UNIX||Peer to Peer||LLI|
A peer to peer network is an architecture in which all network nodes have essentially equal status. All workstations can directly access data stored in all other workstations. A user on a system configured to a peer to peer LAN would view the data stored on a peer system's hard drive as an additional hard drive accessible to that user's system. For LANs with a relatively small number of nodes, this is a fairly efficient and easily managed network architecture. As the number of workstations grows, it is easy to see how this architecture would become unwieldy. Imagine having 1,000 hard drives to choose from!
A client/server architecture is an architecture that can be scaled up to a nearly unlimited number of users. With this architecture, the personal computers attached to the LAN are designated as either clients or servers. From a hardware point of view, a server is nothing more than a workstation that is dedicated to serving the data needs of the actual workstations which are designated as clients. Any data required by the client that is not resident on the client's system is accessed from the server. If a client wishes to send data to another client (such as an e-mail), this data is routed through the server which then forwards it to the appropriate workstation. The server also acts as a central repository of data files which are accessible to all the workstations on the LAN. For this reason, the server is often referred to as a file server.
Given the high data storage and transfer requirements of the server, it is typically a very well enhanced system relative to the clients. It will typically have a very large hard drive or multiple hard drives for the network data that it stores. It will have a relatively large amount of system memory and will have very fast input and output devices. As client/server LANs grow, more servers can be dedicated to specific functions like e-mail or a particular database.
Given that the concept Local Area Networking has been established for well over twenty years and given that LANs serve the computing requirements of a wide range of organizations, it follows that different types of LANs have been standardized at various times to suit specific computing needs. The organizations that is responsible for establishing these standards is the Institute of Electrical and Electronic Engineers or the IEEE. This professional organization works with network product vendors and users to standardize product specifications so that hardware and software products from many different and competing vendors will work together on the LAN. Products that conform to these standardized specifications and therefore work together are considered to be Interoperable. Within the IEEE, various committees are set up to act as the standards setting bodies for different types of LANs. These LAN committees are designated by a number.
The two predominant types of standardized networking are Ethernet and Token Ring. Of the two, Ethernet networks are far more common than Token Ring networks. In general, Ethernet networks are designed for small and medium sized groups while Token Ring networks are best suited to larger groups. Amongst larger networks, it is not uncommon to have both types of networks which are connected via a router or similar hardware device. Given the focus of this document on smaller networks, Ethernet will be covered in more detail than Token Ring.
Ethernet was developed in the mid 1970's at the Xerox Palo Alto Research Center (PARC). This facility did pioneering research in what are today very popular elements of personal computing like Graphical User Interfaces and mice. Ethernet was standardized by the IEEE by the 802.3 committee.
Ethernet is predominately used as a baseband transmission network, where all the network nodes share access to the network media on an equal basis. Baseband transmission means that data sent over the media uses the entire bandwidth of the media as opposed to broadband transmission where data takes only a segment of the media by dividing the media into electronic channels. Each Ethernet node has the capability to send data at the Ethernet standard speed of ten million bits (megabits) per second (10Mbps). Since the nodes are sharing the media, the actual data speed tends to be significantly less than 10Mbps in much the same way that the speed of a car on a crowded freeway tends to be significantly less than the posted speed limit. Ethernet is based on a network access method called Carrier Sense Multiple Access with Collision Detection or CSMA/CD. On an Ethernet network, all nodes share the network much like an old party line telephone system. When nodes on an Ethernet network want to send data to another node, the node first senses (or listens to) the network to ascertain if there are other contending nodes already sending information. Any node can send information at any time provided that it first checks to see if the network is already in use. Once the data is sent, the node listens to the network to see if a collision occurred with data from another node that sent data at some time after the sending node last listened. If a collision occurs, the node simply re-sends the data until it successfully reaches its destination address. When there are a relatively small number of nodes on the network, collisions are fairly rare. As the number of nodes increases (as the number of cars on the freeway increase as in our previous example), the likelihood of collisions increases proportionately causing increased re-sends and generally decreasing network performance.
When Ethernet was first standardized by the IEEE, the type of media specified was thick coaxial cable which allowed for a maximum cable length of 500 meters. Later, a second cable implementation of Ethernet was standardized based on thin coaxial cable that was less expensive and allowed segments of just under 200 meters. To differentiate between these two different cable implementations, the designations 10BASE5 and 10BASE2 were developed for thick and thin coaxial cable respectively. The "10" refers to the 10Mbps data transfer rate, "Base" refers to the fact that Ethernet is a baseband media access method and the "5" or the "2" refers to the maximum cable length between nodes.
With both 10BASE5 and 10BASE2, the nodes are arranged in a bus topology meaning that they are connected one after another with a terminator on the last node on the bus. While this is a fairly simple design, it does have a drawback in that if a node fails on the bus, it effectively terminates the bus making media access impossible for nodes further down. This is similar to older Christmas tree lights where if one light burned out, the entire strand failed. Trouble shooting a bus type topology to find the problem node is just as time consuming and frustrating as trouble shooting old Christmas tree lights. To address this issue as well as provide for even less expensive media, a third implementation was developed using twisted pair cable and allowing for nodes up to 100 meters from the hub. This implementation was designated as 10BASE-T. Unlike 10BASE5 and 10BASE2, the 10BASE-T physical network design is a star topology with each node connecting not to the next node but to a central hub. If a node fails, the hub partitions that node from the rest of the LAN leaving the remaining nodes unaffected.
As it stands today, the popularity of Ethernet cable implementations are in order: 10BASE-T, 10BASE2 and 10BASE5. These implementations are not mutually exclusive meaning that an Ethernet LAN can, as an example have a 10BASE5 segment from the file server to the hub, a 10BASE2 segment of older nodes and a newer segment using the 10BASE-T topology.
|IEEE Implementation||Interface (Connector) Type||Cable Type|
Token Ring is a network architecture that is based on a far more structured media access method. Not surprisingly, Token Ring was developed by IBM for larger LANs. The Token Ring IEEE designation is 802.5. With Token Ring, the network nodes are arranged in a ring pattern. LAN data, along with an electronic "token", circle around the ring. Unlike Ethernet, a Token Ring node cannot send data at any time: it must first capture the token which is constantly circling the ring. The node then attaches the data to be sent to the single token and sends it to its destination. In this manner, only one node can send at a time, eliminating the possibility of collisions. Historically, Token Ring hardware has been more expensive than Ethernet hardware and available from fewer vendors. Today Token Ring is found mostly in large corporate LANs.
In addition to Ethernet and Token Ring there are other network types with smaller installed bases. Arcnet is an established type of standardized networking that has declined dramatically in popularity relative to Ethernet over the years.
While Ethernet and Token Ring promise to be the predominant types of networking for the rest of the decade, new, higher performance networking technologies are on the horizon. Fast Ethernet is a 100Mbps version of Ethernet based on CSMA/CD and twisted pair cable. 100VGAnyLAN (another 100Mbps network type) is a sort of hybrid of Ethernet and Token Ring and uses twisted pair cable as well but with a "demand protocol" which structures media access. Asynchronous Transfer Mode or ATM is a high performance network type (25 and 155Mbps data transfer) that can be used for both local and wide area networking. ATM is generally held to be the network of the future, showing a lot of promise but very little implementation at present.
A Local Area Network is simply a way of using computer hardware and software to further increase the productivity of personal computers and those who use them. Two examples of typical workgroup LANs follow, as well as a glossary of all the boldface terms found in this document.
Network Type: Ethernet
Ethernet Implementation: 10BASE2
NOS Type: Peer to Peer
LAN Hardware: PC Ethernet adapters, print server and thin coaxial cable
Advantages: Easy and inexpensive to install.
Disadvantages: Performance tends to decrease and trouble shooting difficulties tend to increase as nodes are added.
Notes: Without a system designated as a file server, a peer to peer architecture is the only possibility. As the LAN grows a file server and the appropriate NOS could be loaded on the server and the workstations converting it to a client/server architecture. Similarly, a 10BASE-T segment can be added to the LAN at a later time. The omission of systems other than PCs from the example is not meant to suggest that only these types of systems can be connected to a 10BASE2 or peer to peer LAN. Macintosh systems, for example could be added.
Network Type: Ethernet
Ethernet Implementation: 10BASE-T
NOS Type: Client/Server
Workstations: PCs and Macs adapters, a print server and twisted pair network cable
LAN Hardware: PC and Mac Ethernet
File Server: PC
Advantages: Nodes can be added easily without compromising performance or increasing trouble shooting difficulties.
Disadvantages: Tends to be more expensive and difficult to initially set up.
Notes: This example is not meant to imply that only 10BASE-T implementations can support Client/Server architectures. Bus implementations support Client/Server as well. It is also not to be taken that a 10BASE-T LAN must have both PCs and Macs.
Adapter - A device that connects to the expansion slot of a system and allows additional system functionality i.e. hardware connection to LAN media.
ATM-Asynchronous Transfer Mode - A packet based, broadband technology for transmitting voice, video, and data over LANs or WANs. Transmission is specified for either 25Mbps or 155Mbps.
Baseband - A transmission method where the entire line is used as a single channel for transmission.
Bridges - An internetworking device that provides a communication pathway between two or more network segments or subnetworks.
Bus Topology - A network architecture using thin coaxial cable as a single cable trunk that connects one workstation to the next in a daisy-chain configuration..
Coax - A common term of Coaxial Cable
Coaxial Cable - A multilayered type of cable commonly used in Ethernet 10BASE2 and 10BASE5 LANs.
Concentrators - A device that provides a central point of connection of terminal, computer, or communication devices.
CSMA/CD- Carrier Sense Multiple Access with Collision Detection - The network access control method used in Ethernet.
Enterprise Wide - A term used to describe an organization's information infrastructure encompassing both local and wide area networks.
Ethernet - Networking system originally developed by DEC, Intel and Xerox. Ethernet has 10Mbps throughput and uses a carrier-sensing access method in which workstations share a network cable, but only one can use the cable at a time.
Fast Ethernet - IEEE standard for High speed 100Mbps Ethernet.
Fiber Optic - A type of cable typically used for high speed, enterprise wide LANs that consists of fiber optic strands.
Hub - A generic term for the central point of connection for the wires from workstations and nodes.
IEEE - Institute of Electrical and Electronic Engineers, the standards body that creates the Networking specifications.
LAN - A common abbreviation of Local Area Network
Local Area Network - A system of hardware and software connected by a common media that allows for sharing and or transferring data between computing devices
Media - That which acts to connect the various hardware components that make up a LAN. Media largely refers to the cable or wires in a typical wired LAN, but can also be a radio frequency or a segment of the color spectrum in the case of wireless LANs.
Network Interface Card - A specific type of adapter that allows for hardware connection to LAN media.
NIC - A common abbreviation for Network Interface Card.
Node - Any computing device (i.e. PC, printer or scanner) connected to a LAN.
NOS-Network Operating System - The software required by the network to control the transmission of data.
Peer-to-Peer - A network architecture where all machines have equal status. All stations can access data stored in all other workstations.
Print Server - A specific type of LAN hardware that allows for connection of a printer to LAN media.
Repeaters - A device that regenerates network signals so that they can travel further, extending the cable length.
Routers - A device used to interconnect networks over local or wide areas and provide traffic control and filtering functions.
Shielded Twisted Pair - A type of LAN cabling containing eight wires all of which are shielded by an insulating cover. Commonly used in Token Ring LANs.
Star Topology - A network architecture using twisted pair cabling to provide workstations individual connections back to a hub providing a single point of connection.
STP - A common abbreviation for Shielded Twisted Pair cable.
Telephone Cable - A type of cable commonly used in Ethernet 10Base-T and Token Ring LANs. See Unshielded Twisted Pair.
Thick Coaxial - A type of coaxial cable used in Ethernet 10BASE5 LANs that is approximately twice the diameter of Thin Coaxial cable.
Thin Coaxial - A type of coaxial cable used in Ethernet 10BASE2 LANs that is approximately half the diameter of Thick Coaxial cable. Similar in looks to the cable used in cable television systems.
Token Ring - The IEEE standard for a token-passing ring network.
Topology - The physical design of a LAN.
Twisted Pair - A type of LAN cabling containing either four or eight wires. Commonly used in Ethernet 10BASE-T and Token Ring LANs.
Unshielded Twisted Pair - A type of LAN cabling containing eight wires all of which are covered with a thin insulating cover. Often referred to as telephone wire. Commonly used in Ethernet 10BASE-T and Token Ring LANs
UTP - A common abbreviation for Unshielded Twisted Pair cable.
Workgroup LAN - A term used to describe a relatively small network that connects PC users in a single group or department.
Workstation - Any computer (i.e. PC, Macintosh or UNIX system) connected to a LAN.
10Base2 - Ethernet network based on thin coaxial cable.
10Base5 - Ethernet network based on thick coaxial cable.
10Base-T - Ethernet network based on twisted pair cable.
802.3 - IEEE specification for 10Mbps Ethernet.
802.5 - IEEE specification for tokenring.
100VGAnyLAN - IEEE standard for a competing high speed 100Mbps transmission, not based on CSMA/CD.