- A communications protocol developed by Apple Computer to allow networking between
Macintoshes. All Macintosh computers have a LocalTalk port, running AppleTalk over a 230K
bps serial line. AppleTalk also runs over Ethernet (EtherTalk) and Token Ring (TokenTalk)
- Attachment Unit Interface. A 15-pin shielded, twisted pair Ethernet cable used
(optionally) to connect between network devices and a MAU.
- Automatic determination and matching of transmission speed.
- American Wire Gauge. A system that specifies wire size. The gauge varies inversely with
the wire diameter size.
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- The main cable in a network.
- Bandwidth on Demand:
- Feature that allows a remote access device to initiate a second connection to a
particular site to increase the amount of data transferred to that site to increase the
desired threshold. The network manager configuring the remote access server will specify a
number of bits or a percentage of connection bandwidth threshold which will trigger the
secondary connection. Multilink PPP is an emerging standard to allow this feature to be
interoperable, but right now the only way to ensure correct operation is to use devices on
both end from the same vendor.
- Baseband LAN:
- A LAN that uses a single carrier frequency over a single channel. Ethernet, Token Ring
and Arcnet LANs use baseband transmission.
- Unit of signal frequency in signals per second. Not synonymous with bits per second
since signals can represent more than one bit. Baud equals bits per second only when the
signal represents a single bit.
- Binary, machine readable forms of programs that have been compiled or assembled. As
opposed to Source language forms of programs.
- Characteristic of having only two states, such as current on and current off. The binary
number system uses only ones and zeros.
- Specification for parallel printing which allows bidirectional communication on a
Centronics-type interface. Pioneered by Hewlett-Packard, mainly used for postscript
- The smallest unit of data processing information. A bit (or binary digit) assumes the
value of either 1 or 0.
- A standardized connector used with Thinnet and coaxial cable.
- A TCP/IP network protocol that lets network nodes request configuration information from
a BOOTP "server" node.
- Bits per second, units of transmission speed.
- A networking device that connects two LANs and forwards or filters data packets between
them, based on their destination addresses. Bridges operate at the data link level (or
MAC-layer) of the OSI reference model, and are transparent to protocols and to higher
level devices like routers.
- A data transmission technique allowing multiple high-speed signals to share the
bandwidth of a single cable via frequency division multiplexing.
- Broadband Network:
- A network that uses multiple carrier frequencies to transmit multiplexed signals on a
single cable. Several networks may coexist on a single cable without interfering with one
- A device that routes specific protocols, such as TCP/IP and IPX, and bridges other
protocols, thereby combining the functions of both routers and bridges.
- A LAN topology in which all the nodes are connected to a single cable. All nodes are
considered equal and receive all transmissions on the medium.
- A data unit of eight bits.
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- The data path between two nodes.
- (Challenge Handshake Authentication Protocol) Authentication scheme for PPP where the
password not only is required to begin connection but also is required during the
connection - failure to provide correct password during either login or challenge mode
will result in disconnect.
- Coaxial Cable:
- An electrical cable with a solid wire conductor at its center surrounded by insulating
materials and an outer metal screen conductor with an axis of curvature coinciding with
the inner conductor - hence "coaxial." Examples are standard Ethernet cable and
Thinwire Ethernet cable.
- The result of two network nodes transmitting on the same channel at the same time. The
transmitted data is not usable.
- Collision Detect:
- A signal indicating that one or more stations are contending with the local station's
transmission. The signal is sent by the Physical layer to the Data Link layer on an
Ethernet/IEEE 802.3 node.
- Communication Server:
- A dedicated, standalone system that manages communications activities for other
- The terminal used to configure network devices at boot (start-up) time.
- Noise passed between communications cables or device elements.
- Technique for examining incoming packets whereby an Ethernet switch looks only at the
first few bytes of a packet before forwarding or filtering it. This process is faster than
looking at the whole packet, but it also allows some bad packets to be forwarded.
- Carrier Sense Multiple Access with Collision Detection is the Ethernet media access
method. All network devices contend equally for access to transmit. If a device detects
another device's signal while it is transmitting, it aborts transmission and retries after
a brief pause.
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- Data Link:
- A logical connection between two nodes on the same circuit.
- Data Link Layer:
- Layer 2 of the seven-layer OSI reference model for communication between computers on
networks. This layer defines protocols for data packets and how they are transmitted to
and from each network device. It is a medium-independent, link-level communications
facility on top of the Physical layer, and is divided into two sublayers: medium-access
control (MAC) and logical-link control (LLC).
- Digital Equipment Corporation (DEC) proprietary network architecture, a system for
networking computers. It runs on point-to-point, X.25 and Ethernet networks.
- Dial on Demand:
- When a router detects the need to initiate a dial-up connection to a remote network, it
does so automatically according to pre-defined parameters set by the network manager.
- A security feature that ensures people do not log into modems that they shouldn't have
access to. When a connection is requested, the system checks the user name for validity,
then "dials back" the number associated with that user name.
- Distributed Processing:
- A system in which each computer or node in the network performs its own processing and
manages some of its data while the network facilitates communications between the nodes.
- Domain Name:
- A domain name is a text name appended to a host name to form a unique host name across
- The transfer of a file or information from one network node to another. Generally refers
to transferring a file from a "big" node, such as a computer, to a
"small" node, such as a terminal server or printer.
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- End Node:
- A node such as a PC that can only send and receive information for its own use. It
cannot route and forward information to another node.
- The most popular LAN technology in use today. The IEEE standard 802.3 defines the rules
for configuring an Ethernet network. It is a 10 Mbps, CSMA/CD baseband network that runs
over thin coax, thick coax, twisted pair or fiber optic cable.
- Apple Computer's protocol for Ethernet transmissions.
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- Fiberoptic Data Distribution Interface. A cable interface capable of transmitting data
at 100 Mbps. Originally specified for fiber lines, FDDI can also operate over twisted-pair
cable for short distances.
- Fiber-Optic Cable:
- A transmission medium composed of a central glass optical fiber cable surrounded by
cladding and an outer protective sheath. It transmits digital signals in the form of
modulated light from a laser or LED (light-emitting diode).
- File Server:
- A computer that stores data for network users and provides network access to that data.
- Process whereby an Ethernet switch or bridge reads the contents of a packet and then
finds that the packet does not need to be forwarded, drops it. a filtering rate is the
rate at which a device can receive packets and drop them without any loss of incoming
packets or delay in processing.
- Alterable programs in semipermanent storage, e.g., some type of read-only or flash
- Process whereby an Ethernet switch or bridge reads the contents of a packet and then
passes that packet on to the appropriate attached segment. A forwarding rate is the time
that it takes the device to execute all of the steps.
- Flash ROM:
- See ROM.
- Dividing data for transmission into groups of bits, and adding a header and a check
sequence to form a frame.
- File Transfer Protocol, a TCP/IP protocol for file transfer.
- Independent, simultaneous two-way transmission in both directions, as opposed to
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- A device for interconnecting two or more dissimilar networks. It can translate all
protocol levels from the Physical layer up through the Applications layer of the OSI
model, and can therefore interconnect entities that differ in all details.
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- Hardware Address:
- See Network Address.
- The initial part of a data packet or frame containing identifying information such as
the source of the data, its destination, and length.
- Ethernet defined SQE signal quality test function.
- Hertz (Hz):
- A frequency unit equal to one cycle per second.
- Generally a node on a network that can be used interactively, i.e., logged into, like a
- Host Table:
- A list of TCP/IP hosts on the network along with their IP addresses.
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- IEEE 802.3:
- The IEEE (Institute of Electrical and Electronic Engineers) standard that defines the
CSMA/CD media-access method and the physical and data link layer specifications of a local
area network. Among others, it includes 10BASE2, 10BASE5, 10BASE-FL and 10BASE-T Ethernet
- A series of interconnected local, regional, national and international networks, linked
using TCP/IP. Internet links many government, university and research sites. It provides
E-mail, remote login and file transfer services.
- IP Address:
- See Network Address.
- Internetwork Packet eXchange, a NetWare protocol similar to IP (Internet Protocol).
- (Integrated Services Digital Network): All digital service provided by telephone
companies. Provides 144K bps over a single phone line (divided in two 64K bps
"B" channels and one 16K bps "D" channel).
- ISO Layered Model:
- The International Standards Organization (ISO) sets standards for computers and
communications. Its Open Systems Interconnection (OSI) reference model specifies how
dissimilar computing devices such as Network Interface Cards (NICs), bridges and routers
exchange data over a network. The model consists of seven layers. From lowest to highest,
they are: Physical, Data Link, Network, Transport, Session, Presentation and Application.
Each layer performs services for the layer above it.
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- Network error caused by an interface card placing corrupted data on the network. Or, an
error condition due to an Ethernet node transmitting longer packets than allowed.
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- Kilobits per second.
- A popular file transfer and terminal emulation program.
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- Local Area Network, a data communications system consisting of a group of interconnected
computers, sharing applications, data and peripherals. The geographical area is usually a
building or group of buildings.
- Local Area Transport, a Digital Equipment Corporation proprietary network communication
protocol. The protocol is based on the idea of a relatively small, known number of hosts
on a local network sending small network packets at regular intervals. LAT will not work
on a wide area network scale, as TCP/IP does.
- In networks, layers refer to software protocol levels comprising the architecture, with
each layer performing functions for the layers above it.
- Line Speed:
- Expressed in bps, the maximum rate at which data can reliably be transmitted over a line
using given hardware.
- Load Balancing:
- Shifting a user job from a more heavily loaded resource to a less loaded resource.
- Local Network Interconnect (LNI):
- A Port Multiplier, or concentrator supporting multiple active devices or communications
controllers, either used standalone or attached to standard Ethernet cable.
- Apple Computer's proprietary 230 Kbps baseband network protocol. It uses the CSMA/CD
access method over unshielded twisted pair wire.
- Logical Link:
- A temporary connection between source and destination nodes, or between two processes on
the same node.
- Line Printer Daemon, a process on Berkeley spooler implementations that provides LPR
- The LPR command is used to queue print jobs on Berkeley queuing systems.
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- Medium Attachment Unit, a device used to convert signals from one Ethernet medium to
- Megabits per second.
- Management Information Base, a database of network parameters used by SNMP and CMIP
(Common Management Information Protocol) to monitor and change network device settings. It
provides a logical naming of all information resources on the network that are pertinent
to the network's management.
- Modular Jack. A jack used for connecting voice cables to a faceplate, as for a
- Modified Modular Jack. These are the 6-pin connectors used to connect serial terminal
lines to terminal devices. MMJs can be distinguished from the similar RJ12 jacks by having
a side-locking tab, rather than a center-mounted one.
- A modulator-demodulator device for changing transmission signals from digital to analog
for transmission over phone lines. Used in pairs, one is required at each end of the line.
- Maintenance Operations Protocol, a DEC protocol used for remote communications between
hosts and servers.
- A multicast is a message that is sent out to multiple devices on the network by a host.
- A device that allows several users to share a single circuit. It funnels different data
streams into a single stream. At the other end of the communications link, another
multiplexer reverses the process by splitting the data stream back into the original
- Transmitting multiple signals simultaneously on a single channel.
- Multiport Repeater:
- A repeater, either standalone or connected to standard Ethernet cable, for
interconnecting up to eight Thinwire Ethernet segments.
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- Name Server:
- Software that runs on network hosts charged with translating (or resolving) text-style
names into numeric IP addresses.
- Network Control Program, a program run on VMS machines to configure local network
hardware and remote network devices.
- A Novell developed Network Operating System (NOS). Provides file and printer sharing
among networks of Personal Computers (PCs). Each NetWare network must have at least one
file server, and access to other resources is dependent on connecting to and logging into
the file server. The file server controls user logins and access to other network clients,
such as user PCs, print servers, modem/fax servers, disk/file servers, etc.
- Microsoft's networking protocols for it's LAN Manager and Windows NT products.
- An interconnected system of computers that can communicate with each other and share
files, data and resources.
- Network Address:
- Every node on a network has one or more addresses associated with it, including at least
one fixed hardware address such as "ae-34-2c-1d-69-f1" assigned by the device's
manufacturer. Most nodes also have protocol specific addresses assigned by a network
- Network Management:
- Administrative services for managing a network, including configuring and tuning,
maintaining network operation, monitoring network performance, and diagnosing network
- Network Interface Card, an adapter card that is inserted into a computer, and contains
the necessary software and electronics to enable the station to communicate over the
- Any intelligent device connected to the network. This includes terminal servers, host
computers, and any other devices (such as printers and terminals) that are directly
connected to the network. A node can be thought of as any device that has a "hardware
- Network Operating System, the software for a network that runs in a file server and
controls access to files and other resources from multiple users. It provides security and
administrative tools. Novell's NetWare, Banyan's VINES and IBM's LAN Server are NOS
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- Open System Interconnect (OSI):
- See "ISO."
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- A series of bits containing data and control information, including source and
destination node addresses, formatted for transmission from one node to another.
- (Password Authentication Protocol) Authentication scheme for PPP links. A password can
be specified for both devices on a remote link. Failure to authenticate will result in a
dropped connection prior to start of data transmission.
- Physical Address:
- An address identifying a single node.
- Physical Layer:
- Layer 1, the bottom layer of the OSI model, is implemented by the physical channel. The
Physical layer insulates Layer 2, the Data Link layer, from medium-dependent physical
characteristics such as baseband, broadband or fiber-optic transmission. Layer 1 defines
the protocols that govern transmission media and signals.
- A circuit connecting two nodes only, or a configuration requiring a separate physical
connection between each pair of nodes.
- The physical connector on a device enabling the connection to be made.
- Port Multiplier:
- A concentrator providing connection to a network for multiple devices.
- A printer/display protocol developed by Adobe Corp. PostScript is an actual printing and
programming language to display text and graphics. Unlike line/ASCII printers, which print
character input verbatim, PostScript printers accept and interpret an entire PostScript
page before printing it.
- Point-to-Point Protocol. The successor to SLIP, PPP provides router-to-router and
host-to-network connections over both synchronous and asynchronous circuits.
- Print Server:
- A dedicated computer that manages printers and print requests from other nodes on the
- Programmable ROM, a read-only memory whose data content can be altered.
- Any standard method of communicating over a network.
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- Remote Access:
- Access to network resources not located on the same physical Ethernet. (Physical
Ethernet here refers to an entire site network topology.)
- Remote Control:
- Form of remote access where a device dialing in assumes control of another network node
- all keystrokes on the remote are translated into keystrokes on the network node. Used
primarily with IPX protocol.
- Remote Node:
- Form of remote access where the device dialing in acts as a peer on the target network.
Used with both IP and IPX protocols.
- A repeater is a network device that repeats signals from one cable onto one or more
other cables, while restoring signal timing and waveforms.
- A network topology in which the nodes are connected in a closed loop. Data is
transmitted from node to node around the loop, always in the same direction.
- Rlogin is an application that provides a terminal interface between UNIX hosts using the
TCP/IP network protocol. Unlike Telnet, Rlogin assumes the remote host is (or behaves
like) a UNIX machine.
- Read-Only Memory, a memory device that retains its information even when power to it is
removed. A ROM version of a network device does not need to download, since the ROM
contains the entire executable code and thus never needs to reload it. Frequently the ROM
is provided as "flash ROM", which can be reprogrammed by downloading if the user
- Device capable of filtering/forwarding packets based upon data link layer information.
Whereas a bridge or switch may only read MAC layer addresses to filter, routers are able
to read data such as IP addresses and route accordingly.
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- A computer that provides resources to be shared on the network, such as files (file
server) or terminals (terminal server).
- A connection to a network service.
- Serial Line Internet Protocol, a protocol for running TCP/IP over serial lines.
- Systems Network Architecture. IBM's layered protocols for mainframe communications.
- Simple Network Management Protocol, allows a TCP/IP host running an SNMP application to
query other nodes for network-related statistics and error conditions. The other hosts,
which provide SNMP agents, respond to these queries and allow a single host to gather
network statistics from many other network nodes.
- Source Code:
- Programs in an uncompiled or unassembled form.
- Spanning Tree:
- An algorithm used by bridges to create a logical topology that connects all network
segments, and ensures that only one path exists between any two stations.
- Store and Forward:
- Technique for examining incoming packets on an Ethernet switch or bridge whereby the
whole packet is read before forwarding or filtering takes place. Store and forward is a
slightly slower process than cut-through, but it does ensure that all bad or misaligned
packets are eliminated from the network by the switching device.
- Sequential Packet exchange. Novell's implementation of SPP (Sequential Packet Protocol).
- Ethernet-defined signal quality test function, frequently called "heartbeat."
- Multiport Ethernet device designed to increase network performance by allowing only
essential traffic on the attached individual Ethernet segments. Packets are filtered or
forwarded based upon their source and destination addresses.
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- A T-shaped device with two female and one male BNC connectors.
- Transmission Control Protocol (TCP) and Internet Protocol (IP) are the standard network
protocols in UNIX environments. They are almost always implemented and used together and
- Telnet is an application that provides a terminal interface between hosts using the
TCP/IP network protocol. It has been standardized so that "telnetting" to any
host should give one an interactive terminal session, regardless of the remote host type
or operating system. Note that this is very different from the LAT software, which allows
only local network access to LAT hosts only.
- Ethernet running on thin coax network cable.
- Ethernet running on Thickwire network cable.
- Ethernet running on unshielded twisted pair (UTP) cable. Note that 10BASE-T is a
point-to-point network media, with one end of the cable typically going to a repeater/hub
and the other to the network device.
- Terminal Server:
- A concentrator that facilitates communication between hosts and terminals.
- Used on both ends of a standard Ethernet or Thinwire Ethernet segment, this special
connector provides the 50 ohm termination resistance needed for the cable.
- Trivial File Transfer Protocol. On computers that run the TCP/IP networking software,
TFTP is used to quickly send files across the network with fewer security features than
- Half-inch diameter coax cable.
- Thin coaxial cable similar to that used for television/video hookups.
- The amount of data transmitted between two points in a given amount of time, e.g., 10
- The character sequence or frame, passed in sequence from node to node, to indicate that
the node controlling it has the right to transmit for a given amount of time.
- Token Ring:
- Developed by IBM, this 4 or 16 Mbps network uses a ring topology and a token-passing
- The arrangement of the nodes and connecting hardware that comprises the network. Types
include ring, bus, star and tree.
- The actual device that interfaces between the network and the local node. The term
generally refers to any connector, such as a MAU, that actively converts signals between
the network and the local node.
- Transceiver Cable:
- Cable that attaches a device either to a standard or thin coax Ethernet segment.
- Twisted-Pair Cable:
- Inexpensive, multiple-conductor cable comprised of one or more pairs of 18 to 24 gauge
copper strands. The strands are twisted to improve protection against electromagnetic and
radio frequency interference. The cable, which may be either shielded or unshielded, is
used in low-speed communications, as telephone cable. It is used only in baseband networks
because of its narrow bandwidth.
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- A multitasking, multiuser computer operating system developed by AT&T. Several
versions exist, e.g., the Berkeley version.
- Unshielded twisted pair, one or more cable pairs surrounded by insulation. UTP is
commonly used as telephone wire.
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- Wide Area Network (WAN):
- A network using common carrier transmission services for transmission of data over a
large geographical area.
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- X.25 Gateway Access Protocol:
- Allows a node not directly connected to a public data network to access the facilities
of that network through an intermediary gateway node. X.25 is the protocol standard
governing packet-switched networks.
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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
one of the most popular technologies used in networking.
Networks are collections of independent computers that communicate with one another
over a shared 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.
Often a network is not located all in one physical location. 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 call in; or it can be as complex as linking
hundreds of branch offices across the world using special routing protocols and filters to
optimize the amount of data sent over vast distances to minimize expenses.
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, makes Ethernet an ideal networking technology for most computer
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
operate in the most efficient manner.
For Ethernet networks that need higher transmission speeds, a new 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
There are three types of Fast Ethernet standards; 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 to be made. The questions that must be answered
for each site are how many users really need the higher throughput, which segments of the
backbone need to be reconfigured specifically for 100BASE-T and what types of hardware
will be required 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
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 networks), TCP/IP (for UNIX and other
platforms), DECnet (for networking Digital Equipment Corp. computers), AppleTalk (for
Macintosh computers), and NetBEUI/ NetBIOS (for LAN Manager and WindowsNT 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
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 scheme is 10BASE-T, which makes use of inexpensive
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
throughput speeds; level 4 cable can support speeds of up to 20 Mbps, level 3 up to 16
Mbps. Level 2 and level 1 cables are not be 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 or inside a steel mill. 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.
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 or networking equipment like a repeater, switch or
a router. A bus topology consists of nodes connected 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.
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. While tranferring the packet onto the network, they
sometimes detect that another node has also started sending information. This event is
called a collision, and is a crucial element in the design and operation of networks. In
Ethernet, each node attempting to send stops when it detects a collision, and waits a
random time before attempting to resend. Network design rules take the minimum packet
size, and speed of transmission of the media into account, ensuring that collisions can be
detected by the sending node. Products that introduce delay (latency) into packet
transmission are limited in number. Lengths of cable are also restricted. In order to
transcend these limitations network managers can use devices that split the network into
collision domains. Back to Top
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 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 a transceiver. Thickwire (10BASE5) cables also use transceivers to allow
Repeaters are used to connect two or more Ethernet segments of any media type. As
segments exceed their maximum length, signal quality begins to deteriorate. Repeaters
provide the signal amplification required to allow a segment to be extended a greater
distance. A repeater takes any incoming signal and repeats it out all ports. Ethernet
repeaters are necessary in star topologies. A multiport twisted pair repeater (often
called a 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 repeater and the other is attached to
the computer. If the repeater 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 repeaters in any one collision domain is limited by the Ethernet rules. The
repeater rules are discussed in more detail later in this tutorial.
A very important fact to note about repeaters 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.
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 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 learning bridges, meaning that 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 houses a specific
address and force 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 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
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 repeaters which 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"
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.
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. Back to Top
Network Design Criteria
Ethernets and Fast Ethernets have design rules that must be followed in order to
function correctly. The maximum number of nodes, the number of repeaters and maximum
segment distances are defined by the electrical and mechanical design properties of each
type of Ethernet and Fast Ethernet media.
A network using repeaters, for instance, has restrictions having to do with the timing
constraints of Ethernet. Although electrical signals on the Ethernet media travel near the
speed of light, it still takes a finite time for the signal to travel from one end of a
large Ethernet to another. The Ethernet standard assumes it will take roughly 50
microseconds for a signal to reach its destination.
If the design of the network violates the rules for the placing of the number of
repeaters, then this timing guideline will not be met and the sending station, having not
received an acknowledgment of its sent packet, will resend that packet. This can lead to
lost packets and slow network performance and create trouble for applications.
Ethernet is subject to the "5-4-3" rule of repeater placement: the network
can only have five segments connected; it can only use four repeaters; and of the five
segments, only three can have users attached to them; the other two must be inter-repeater
links. Fast Ethernet has modified repeater rules, since the minimum packet size takes less
time to transmit than regular Ethernet. The length of the network links and the standard
allows a fewer number of repeaters. In Fast Ethernet networks, there are two classes of
repeaters. Class I repeaters have a latency of .7 microseconds or less and are limited to
one repeater per network. Class II repeaters have a latency of .46 microseconds or less
and are limited to two repeaters per network. The following are the distance (diameter)
characteristics for these types of Fast Ethernet repeater combinations:
|One Class I repeater
|One Class II repeater
|Two Class II repeaters
When conditions require more distance or an increase in the number of nodes/repeaters,
a bridge, router, or switch can be used to connect multiple networks together. These
devices essentially "join" two separate networks, allowing the network design
criteria to be restarted. With switches, network designers can build large networks that
function well. Each network connected via one of these devices is referred to as a
separate collision domain in the overall network. The reduction in costs of bridges and
switches has reduced the impact of repeater rules on network design. Back
When Ethernets Become Too Slow
As more users are added to a shared network or as applications requiring more data are
added, performance deteriorates. This is so because all users on a shared network are
competitors for the Ethernet bus. On a moderately loaded 10 Mbps Ethernet network being
shared by 30-50 users, that network will usually only be able to sustain something in the
neighborhood of 2.5 Mbps after packet overhead, interpacket gaps and collisions are
accounted for. Increasing amounts of users (and therefore packet transmissions) create
increasing potential for collisions. Collisions occur when two or more nodes attempt to
send information at the same time - when they realize that a collision has occurred, each
node backs off for a random time before attempting another transmission. With shared
Ethernet, the likelihood of collisions increases as more nodes are added to the shared
collision domain of the shared Ethernet.
One of the first steps to alleviating problems is to segment the traffic by using a
bridge or switch. Simple to install, a switch can replace a hub and have dramatic impact
on network performance. For example, an eight port switch can support eight Ethernets,
each running at a full 10 Mbps. Another option is to dedicate one or more of these
switched ports to a high traffic device such as a file server.
Multimedia and video applications demand as much as 1.5 Mbps of continuous bandwidth -
as we have seen above a single such user would be hardpressed to get this amount of
bandwidth alone as their share of an average 10 Mbps network. If you add in the fact that
video will look disjointed or "clunky" if the data rate is not sustained, then
the pressure will be on the network manager to provide greater throughput to support this
When added to the network, Ethernet switches provide a number of enhancements over
shared networks. The foremost enhancement is the ability to divide networks into smaller
and faster segments. Ethernet switches examine each packet, determine where that packet is
destined and then forward that packet to only those ports to which the packet needs to go.
Modern switches are able to do all these tasks at "wirespeed", that is without
Aside from deciding when to forward the packet or when to filter the packet, Ethernet
switches also completely regenerate the Ethernet packet. This regeneration and retiming of
the packet allows each port on a switch to be treated as a complete Ethernet segment,
capable of supporting the full length of the cabling along with all of the repeater
Additionally, bad packets are identified by Ethernet switches and immediately dropped
from any future transmission. This "cleansing" activity keeps problems isolated
to a single segment and keeps them from disrupting other network activity. This aspect of
switching cannot be underemphasized in a network environment where hardware failures are
to be anticipated.
Full duplex is another method to increase bandwidth to dedicated workstations or
servers. To use full duplex, special network interface cards are used in the server or
workstation, and the switch must support full duplex operation. Full duplex doubles the
bandwidth on that link, providing 20 Mbps for Ethernet and 200 Mbps for Fast Ethernet.
Implementing Fast Ethernet to increase performance is the next logical step. The higher
traffic devices can be connected to switches or each other via 100 Mbps Fast Ethernet,
providing tremendous amounts of bandwidth. Many switches are designed with this in mind,
and have Fast Ethernet uplinks for connection to a file server or other switches.
Eventually, Fast Ethernet can be deployed to the users' desktops, by equipping all
computers with Fast Ethernet network interface cards and using Fast Ethernet switches and
repeaters. With an understanding of the underlying technologies and products in use in
Ethernet networks, we can now progress to a discussion of some of the most popular real
world applications. Back to Top
For many years, the Simple Network Management Protocol (SNMP) has been the most popular
tool for managing networks. SNMP relies on agents in each device on the network which
collect data based on industry standard Management Information Bases (MIBs). An SNMP
management station can poll these agents to collect this information and then display it
so the network manager can track the events occurring on the network.
In 1991, the Remote Monitoring (RMON) protocol was created to augment SNMP in networks
that are segmented by switches or that have many remote links. RMON uses intelligent
agents or probes to provide filtered data and information only when it is required by the
SNMP management station. It reduces the polling that had previously hampered the use of
SNMP on larger networks and extends the range of information that can be sent back to the
By allowing the network manager to set thresholds, RMON enables probes to measure
network performance. When the threshold for acceptable network behavior is exceeded, the
RMON probe alerts the SNMP management station to the problem. The RMON protocol reports
statistics at OSI layer two (the data link layer) although some new extensions now allow
the reporting of OSI layer three (the network layer) information. Approved in 1996, RMON
II enables RMON probes to completely provide information at OSI layer three.
As networks have become more complicated, new tools for troubleshooting them have
developed. A network analyzer is a device designed to monitor, capture and analyze network
packet traffic on a specified network or network segment. Analyzers allow a network
manager to examine actual packet traffic between nodes, which is necessary to solve
complex network problems. In the past, such products were limited in use to actual
protocol developers - ease of use features such as automatic protocol decoding and alarm
warnings for critical conditions have made these products more desirable for every network
manager. Back to Top
Remote Access Servers
While Ethernet is local to a geographic area, like a building, remote users, such as
traveling sales people, are increasingly requesting access to network-based resources.
Remote LAN access or remote access is quickly becoming a popular way to provide this
connectivity. Remote access solutions use telephone services to link a remote user or
office with an office network. For demanding applications, where speed and full-time
access is crucial, a leased-line solution should be considered. This involves purchasing a
router and a special leased line service which essentially sets up a dedicated telephone
line with a set amount of bandwidth - ranging from 56 Kbps to many megabits per second.
This solution is limited to the two connected offices and can be very expensive.
Dial-up remote access solutions such as ISDN or asynchronous dial up introduce more
flexibility into a remote access solution. Dial-up remote access offers both the remote
office and the remote user the economy and flexibility of "pay as you go"
telephone services. ISDN is a special telephone service that offers three channels, two 64
Kbps "B" channels for user data and a "D" channel for setting up the
connection. With ISDN, the B channels can be combined for double the bandwidth or
separated and used for different applications or users.
With asynchronous remote access, regular telephone lines are combined with modems and
remote access servers to allow users and networks to dial anywhere in the world and have
data access. Remote access servers provide connection points for both dial-in and dial-out
applications on the network to which they are attached. These hybrid devices are capable
of routing nd filtering protocols and offer other services such as modem pooling and
terminal/printer services. For the remote PC user, there is the flexibility of connecting
from any available telephone jack, including those in a hotel or on an aircraft.
Remote Access Applications
Remote access technology is optimized for a number of remote applications. Remote node
and remote control applications are when a remote user on a PC or workstation dials into a
network and is able to function as if he or she were directly attached to the network. A
remote access server provides dial-in services and support for PPP to allow full
functionality of the remote user as a network peer (remote node) or to allow the remote
user to take over a local node (remote control).
LAN-to-LAN is when an entire remote network is supported over a dial-up connection.
Remote access servers on each end act as routers to automatically generate a connection
when remote resources are requested. The dial-up connection is maintained according to
parameters established by the network manager for timeouts, allowed protocols and for
connection duration Internet access applications involve the use of a remote access server
as a router to "firewall" the local network from security problems present on
the Internet. Filters are configured by the network manager to ensure that only authorized
traffic is allowed to pass between the local network and the Internet. These applications
are actually a hybrid form of LAN-to-LAN connections.
Modem service is the ability of the remote access server to provide access for network
users to a bank of modems for both dial-in and dial-out applications. Software run on
networked hosts allows them to connect to modems attached to a remote access server,
providing cost-effective communications from the central site and preserving the
investment in modems and communications hardware.
The key to controlling costs is the ability of the remote access server to route the
desired protocols and to implement policy-based decisions on how the dialup connections
between sites are managed. In a LAN-to-LAN application, IP and IPX protocol traffic on the
network is monitored by a server and when a connection to resources on a remote network is
required, the server automatically dials up and connects to that network. Once the network
connection is established, the server will monitor the link according to criteria defined
by the network manager and manage the link to those specifications. These parameters
include: the amount of time the link is to remain connected if no data is being passed;
whether the link is to remain connected if only certain types of traffic are present (i.e.
disconnect if only the keep alive or broadcast messages are being transmitted); whether or
not to allow a particular protocol or packet type to travel the link between the two
networks. Additional convenience features are automatic redialing in case of a busy
answering modem or an unplanned disconnect, and time-of-day limits for dial-in/dial-out
Printer servers allow printers to be shared by other nodes on the network. Supporting
either parallel or serial interfaces (sometimes both), a printer server accepts print jobs
from any node on the network using the supported protocols and manages the printing of
those jobs on the appropriate printer.
The earliest printer servers were external devices, which supported printing via
parallel or serial ports on the device. Typically, only one or sometimes two protocols
were supported. The latest generation of printer servers supports multiple protocols, has
multiple parallel and serial connection options and in some cases, are small enough to fit
directly on the parallel port of the printer itself. Some printers have printer servers
that are internal to the printers themselves, this type of design has an integral
communication benefit between the printer and the printer server, but lacks flexibility if
a printer has physical problems.
Printer servers as a rule do not contain a large amount of memory. Rather than store
each print job in memory, they simply store the information about the host and the
protocol involved in a queue. When the desired printer becomes available, then they allow
the host to transmit the data to the appropriate printer port on the server. The printer
server can then simply queue and print each job in the order in which print requests are
received, regardless of protocol used or the size of the job.
The original role of terminal servers was to enable terminals to transmit data to and
receive data from host computers across local area networks, without requiring each
terminal to have its own direct connection. And while the terminal server's existence is
still usually justified by convenience and cost considerations, its inherent intelligence
provides many more advantages. Among these is enhanced remote monitoring and control.
Terminal servers that support protocols like SNMP make networks easier to manage.
Devices that are attached to a network through a terminal server can be shared between
terminals and hosts at both the local site and throughout the network. A single terminal
may be connected to several hosts at the same time (in multiple concurrent sessions), and
can switch between them. Terminal servers can also be used to link devices that have only
serial outputs over a network. A network connection between serial ports on different
servers is opened, allowing data to move between the two devices.
With the advent of multiprotocol terminal servers, the problem of a user needing two
terminals to reach hosts that used different communications protocols was alleviated. As
long as the terminal server supports the protocol used by the host, the terminal attached
to that server can access that host as if it were using the terminals own native protocol.
Economically, it also makes sense to have a single connection to the network instead of
individual interface cards and transceivers for each terminal.
Digital systems using the LAT protocol and Unix systems using TCP/IP have no natural
means to communicate with each other, in spite of how common it is to have VAX and Sun
workstations in the same facility. Given its natural translation ability, a multi-protocol
terminal server can perform conversions between the protocols it knows, like LAT and
TCP/IP, at least for those which are set up to work with terminals. While terminal server
bandwidth isn't adequate for large file transfers, it can easily handle host-to-host
inquiry/response applications, electronic mailbox checking, etc. And it is far more
economical than the alternatives of acquiring expensive host software special-purpose
converters. Terminal and printer servers give their users great flexibility in configuring
and managing their networks.
Whether it is moving printers and other peripherals from one network to another,
expanding the dimensions of interoperability, or preparing for growth. And you can do it
all without major rewiring. The demand for dial up remote access applications is causing
terminal and server functionality to evolve. The requirement for support of PPP and SLIP
connections has created the need for a "communication" server which does not
offer the routing capabilities of a true remote access server, but still offers
sophisticated dial up modem support.
We hope this introduction to local area networks has been helpful and informative.
Unfortunately we cannot explain everything there is to know about planning, installing,
administering and troubleshooting a LAN in a few, or even a hundred, pages. Many books and
magazines exist that explain all aspects of computer networks, from LANs to WANs, from
network applications, to running cable. Check your local bookstore, software retailer or
newsstand. Back to Top