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BOOKMARKS:   NETWORK STRUCTURE = MediumNetworking Model,  Network Topology             



Networking Model 


cable tolerances and specifications
cable glossary

patch cables
patch panels
crossover cables
half-duplex transmission
full-duplex transmission
broadband transmissions

baseband transmissions










frame relay cloud











The structure of the network depends on the age and associated equipment, the topology of both the network and the equipment associated with that topology, the size of the network and total number of segments.  The network structure comprises the medium, the networking model, and network topology.






The LAN provides a means of communication between network nodes.  Nodes can consist of workstations with network NICs, hubs, bridges, switches, routers and servers.  CAT 5 and fiber-optic cable usually constitute the physical media on a LAN.

The CAT 5,  horizontal,  is installed point to point between the workstation and the electronics that make up the network, such as a hub or a switch. CAT 5 terminates to an IDF that must be within 100 meters of the workstation.

The fiber-optic cable connects all the electronic devices throughout out the campus to an MDF.  The MDF houses the enterprise category network equipment such as switches capable of switching packets at a rate of 10 or more gigabits per second and routers with multiple ports connecting the enterprise together.




A WAN is a means of communicating between geographically separated locations.  The WAN consists of routers, channel service unit/data service units and leased lines.  



the cabling medium  is the foundation of all networks; allows the packets to travel at a rate of speed between the source and destination


Cable Tolerances and specifications   cable glossary


Cable Name


Maximum Length

Data Rate



RG58 U 
Coaxial cable

500 m per segment



Thicknet uses broadband signaling; one of the originals, becoming less popular. Solid copper core which accounts for the size.  The transceivers are fitted with connectors, or nonintrusive taps called vampire taps, which pierce the cable to allow attachment of devices to the network.  Taps can be attached at no closer than 2.5 meters along the segment.  A maximum of only four repeaters can be placed between between any two nodes on the network. Can have up to 100 nodes on any one segment, but are limited to only 3 Ethernet segments between any 2 end nodes.

When working with cable, it is important to consider its size. As the thickness, or diameter, of the cable increases, so does the difficulty in working with it. You must remember that cable must be pulled through existing conduits and troughs that are limited in size. Coaxial cable comes in a variety of sizes. The largest diameter was specified for use as Ethernet backbone cable because it had historically a greater transmission length and noise rejection characteristics. This type of coaxial cable is frequently referred to as thicknet. As its nickname suggests, this type of cable, because of its thickness, can be too rigid to install easily in some situations. The rule of thumb is: "the more difficult the network media is to install, the more expensive it is to install." Coaxial cable is more expensive to install than twisted-pair cable. Thicknet cable is almost never used anymore, except for special purpose installations


RG58 A/U

BNC connector Coaxial cable

185 m per segment for total of no more than 900 meters



Thinnet uses broadband signaling,   Stranded copper. see thin ethernet networks.  Can have a maximum of five segments but can populate only three of the segments.  Each of those three segments can have a maximum of 30 nodes, with the repeater counting as one node.  The shortest acceptable cable length between two nodes is .5 meters and each end of the cable must have a terminator that matches the ohm value of the cable.  T-connectors connect nodes to the Ethernet segment.  You can disconnect the node from the network, but if you disconnect the T-connector from the cable, you will break the path to the terminator and the entire network will stop functioning.

In the past, coaxial cable with an outside diameter of only .35 cm (sometimes referred to as thinnet) was used in Ethernet networks. It was especially useful for cable installations that required the cable to make many twists and turns. Since it was easier to install, it was also cheaper to install. This led some people to refer to it as cheapernet. However, because the outer copper or metallic braid in coaxial cable comprises half the electrical circuit, special care must be taken to ensure that it is properly grounded. This is done by ensuring that there is a solid electrical connection at both ends of the cable. Frequently, installers fail to do this. As a result, poor shield connection is one of the biggest sources of connection problems in the installation of coaxial cable. Connection problems result in electrical noise that interferes with signal transmittal on the networking media. It is for this reason that, despite its small diameter, thinnet is no longer commonly used in Ethernet networks.



100 m per segment


performs half-duplex transmissions through CAT3, CAT4 or CAT 5 UTP but uses only two of the four pairs.  It uses RJ-45 connectors, with pins 1 and 2 receiving data and pins 3 and 6 transmitting data.  Each pair is crossed over so that the transmitter at one end connects to the receiver at the other.  Uses CSMA/CD.  The maximum distance from the hub to a network node is 100 meters (328 feet).




2 km


is composed of chemically formed glass and is slightly thicker than human hair.  Because it uses reflected photon (light) energy instead of electromagnetic charge to transmit its signal, it is resistant to electromagnetic interference. It does not radiate an electromagnetic field, so tapping into the strand is difficult because it will bring down everything that is on that cable run.  This makes it appropriate for deployment in high-security environments.  Signals can travel greater distances, with less degradation than over copper.  An "F" or "FX" suffix represents cable suitable for Ethernet.  Two types of cable are single mode and multimode.

  • Single-Mode Fiber-Optic Cable
    uses laser for signal transmission.  Signals transmitted by laser travel much farther than signals transmitted by diodes over multimode fiber.  Telephone companies and industries that provide communications over large geographical areas commonly deploy laser transmitters.

  • Multimode Fiber-Optic Cable
    uses diode transmitters and is used mainly in LAN and campus environments, up to a distance of 2 kilometers.  Multimode fiber uses two types of transmissions, most of which are diodes.  The most common multimode cable is 62.5/125 - the core alone is 62.5 microns, but is 125 microns with its cladding.

  • The fiber -optic cable connects all the electronic devices throughout the campus to an MDF.




100 m per segment




100VG -


100 m per segment

100Mbps Fast Ethernet

is an alternative to the Fast Ethernet standard and adheres to the IEEE 802.12 standard.  It is based on a demand-priority access scheme that provides higher performance than the CSMA/CD associated with Ethernet.   However, it does support both Ethernet and token ring frame formats.  It uses two or four pairs of CAT 3, CAT 4 or CAT 5 UTP.



4 pairs category
3,4 or 5 UTP

100 m per segment


a)    100BaseX consists of two basic types of cabling schemes for twisted-pair wire: 100BaseT4 and 100BaseTX.  b) Class 1 hubs accept 100BaseTx and 100BaseT4 signals and retransmit them. Class II hubs will retransmit only 100BaseTX signals.

  • 100BaseT4  This cabling scheme uses all four pairs of CAT 3, CAT 4 or CAT 5 UTP cable for Fast Ethernet transmissions.  The signal is transmitted and received across all four pairs, reducing the signal frequency and allowing you to use a lower category of twisted-pair cable.  Works in half-duplex operation and uses CSMA/CSD.

  • 100BaseTX This cabling scheme uses all four pairs of CAT 3, CAT 4 or CAT 5 UTP cable for Fast Ethernet transmissions.  The signal is transmitted and received across all four pairs, reducing the signal frequency and allowing you to use a lower category of twisted-pair cable.  It works in half-duplex operation and uses CSMA/CD.



2 pairs category 5 UTP or
cat 1 STP



100 m between the hub and the network node

Fast Ethernet



2 km

Fast Ethernet



Category 3


100 m, including patch panel and patch cables

see red

is rated  for signals of 16MHz or less.  It has four pairs of wires with a lay length of three to four twists per foot.  It supports 10Mbps Ethernet, 4Mbps and 15 Mbps token ring, and 100VGAnyLAN networks.  Individual cable lengths should not exceed 100 meters, including the patch panel and patch cables.


Category 5


90 m (100 m, including patch panels and patch

100Mbps Fast
Ethernet at 100MHz

has four pairs of wires with a lay length of eight twists per foot.Rated for signals of 100MHz or less and supports 100Mbps Fast Ethernet.  Individual cable runs should not exceed 100 meters including patch panel and patch cables.  All components, such as the patch panes, connectors, wall plated and patch cables in the network must be rated for CAT 5 compliance.

  • The CAT 5 cable terminates to an IDF that must be within 100 meters of the workstation.





Cable Glossary


coaxial cable

normally called coax, has a solid copper core surrounded by insulating material, a shield that doubles as a ground wire, a polymer casing for outer protection and 50-ohm terminators at each end of the cable.  The two main types of network are Thicknet and Thinnet because of the size of the coaxial cable.  Thicknet  cable  has a solid copper core.  Thinnet cable uses stranded copper.  Was deployed in the early days of networking.

The advantage of coax over UTP is the greater length of the segments between repeaters and greater total network length.  The disadvantage is that you can stop the entire network by disconnecting the cable at any point in the network.  Because of this, current technology is focusing on twisted-pair and fiber-optic rather than coax cable.


thin ethernet networks

consist of the following components: RG58 A/U cable, BNC-connectors, BNC T-connectors, BNC barrel connectors, BNC 50-ohm terminators, and Ethernet repeaters. The difference in construction between RG58 A and RG58 A/U makes it prohibitory to mix the two.

twisted pair cable

the most common cable type is UTP which has four individually twisted pairs of wires in a common sheath.  The lay lengths of the twists is different for each pair to reduce cross talk.  Lay length is the distance between the individual twists of wire.  UTP may have a plenum-insulating jacket if the building in which it is installed has an open air-return system.

Twisted -pair cable  is grouped into five categories:

UTP Categories



Data Rate


Voice frequencies

Voice only


100 ohm UTP, 4MHz transmissions



100 ohm UTP, 16MHz transmissions



100 ohm UTP, 20MHz transmissions



100 ohm UTP, 100MHz transmissions


Enhanced CAT 5

100 ohm UTP, 200MHz transmissions


data rate

MBps or Mb/s - a unit of measurement for data transmission speed.

MHz - a unit measurement for line frequency.

NOTE:  Although the data rate (measured in Mbps) and frequency (measured in MHz) may look the same, they are two different transmission characteristics.  Frequency measures the signal frequency whereas data transmission speed measures how fast the signal can be sent down the line.

patch cables

in addition to the cabling plant, take into account the patch panels, patch cables and crossover cables when estimating the total length of the cable runs and segment lengths.  Patch cables can connect computers to UTP faceplates or connect a patch panel to a network device, such as a hub or switch.  Patch cables are constructed from stranded copper to allow flexibility.  Because the high-frequency characteristics of flexible patching cables are worse than those of solid conductor cables used for fixed installation, keep them as short as possible.  Patch cables should be of the same overall construction as the floor wiring they serve and should not be mixed, such as unshielded patch cable serving a shielded cable drop.  To make a patch cable, the pin-out (order that the colored strands of wire are in) is:

  1. Cut the UTP to desired length.

  2. Strip outer sheathing on cable and order the wires (pins) in this sequence: orange/white(1), orange(2), green/white (3), blue (4), blue/white(5), green (6), brown/white (7), and brown (8).

  3. Crimp RJ-45 connectors onto each end.

patch panels

allow flexibility within the structured cabling system and are used for either horizontal, service between desks, or vertical, service between floors.  Patch panels are grouped into four categories, the two most concerned are cable patching and fiber patching.  Cable patching is more common in a structured cabling environment.  Fiber patching performs linking by fiber-patching cables.


In an Ethernet, the carrier sense multiple access with collision detection (CSMA/CD) technique creates the opportunity to have collisions; CMSA/CD is a technique where two or more devices may want to transmit data at the same time.  Only one channel exists, so if they both transmit simultaneously, the data will collide on the single channel.


CSU  - channel service unit - a device that connects with the digital communication line and provides a termination for the digital signal.  Usually used with a DSU.

DSU -  data service unit - a device that is necessary in transmitting digital data over a hardware channel.  It converts signals from bridges, routers and multiplexers into digital signals.  Usually used with a CSU.

crossover cables

connect two network devices together, such as two hubs, computers without a hub, switches or a hub and a switch.  It is the same as a patch cable except that you switch (cross) two pairs of wires before crimping on the RJ-45 connectors.  To construct a crossover cable, follow the steps in making a patch cable, except switch pins 1 and 3 and pins 2 and 6 on one end of the cable.  pin-out (order that the colored strands of wire are in)   green/white (3),  green (6), orange/white(1), blue (4),   blue/white(5) ,orange(2),  brown/white (7), and brown (8) on ONE END of the cable.


half-duplex transmission

Duplexing refers to the send and receive criteria of the medium

allows data to flow in only one direction at a time.  UTP network topologies are rated for 100Mbps, which is half-duplex operation.

full-duplex transmission

one method of increasing network speed.  For full-duplex transmission, the signal must be a  point-to-point connection between two pieces of equipment and cannot be connected to a shared-access medium, such as a hub.  In a standard Ethernet and Fast Ethernet network, only two pairs of wires are utilized.  Such transmission is based on the CSMA/CD network arbitration scheme.  Full duplex uses all four pairs of wires in a CAT 5 cable.  Each piece of equipment uses different wire pairs to transmit and receive data.  This allows simultaneous communications between the two pieces of equipment, the design of which must specifically allow full-duplex operation.

broadband transmissions

analog transmissions that can handle a large number of frequencies.  Allow multiple, simultaneous transmissions on different frequencies.  Most common broadband network is cable television.  Broadband networks can span greater distances than baseband and can transmit in the gigabit range.

baseband transmissions

apply direct current to the cable to transmit digital signals.  The signal is either a high-voltage or low-voltage pulse that represents a binary 1 or 0.  Signals can travel limited distances over a baseband network before they become degraded beyond use.  In addition, baseband networks are susceptible to outside interference from electric fields generated by fluorescent lights or electric motors.  The higher the data rate, the more susceptible to interference the baseband network is.  Baseband networks have stringent guidelines for distance, cable types, shielding and other criteria to ensure a successful network implementation.  Two examples of baseband networks are Ethernet and token ring.


older technology, as networks grew, bridges connected segments, they helped control collisions.  Bridges have several types of ports that allow connections to older, unstructured network media to newer network media.

IDF  and  MDF

  • Intermediate Distribution Facility  Modern LANS follow a structured cabling scheme that allows for dynamic change.  These LANs use mainly CAT 5 UTP cable that is terminated in a intermediate distribution facility (IDF), with multiple IDFs connecting to a MDF.

  • The MDF  houses the enterprise-category network equipment such as switches capable of 10 or more gigabits per second, and routers with multiple ports connecting the enterprise together.

  • The connection from the IDF to the MDF (main distribution facility) is usually a fiber-optic cable connected to a layer 3 routing switch that distributes packets for the entire network.





CLIENT/SERVER-BASED  the data, print queues, data backup and security are stored on a central server.  The workstations connect to the server and receive a security login prompt.  The username and password determine the level of access that the workstations have to the system, according to their user rights and profile restrictions.  The advantages of a server-based network are ease of management and central control of critical company data.


PEER-TO-PEER  decentralizes all data access and security control allowing each machine on the network to serve as both server and workstation.  Each workstation must locally enforce access control through local security tools.  Generally they have less security than server-based networks making them acceptable for a small office or department that does not require the checks and balances that a server-based network offers.  However, most shared applications require a server-based network, thus requiring small offices and departments to deploy more server-based networks than peer-to-peer.  Peer-to-peer can share files and printers like a client/server network, but they lack the functionality and security of a server.





Size determines topology  always design the the electronics to maximize the speed of the media and to segment the large network into smaller workgroups.  Larger networks will use several different types of topologies to accomplish different goals.

  • The size of the network determines the best topology to use.  Small business networks usually terminate in a centrally located closet where a shared media hub acts as the backbone.

  • Medium business networks might have one or several IDFs connected to an MDF.  In addition, one or several WAN connections will terminate in the MDF, which will not always require a layer 3 switch.

  • Large business networks use the same scenario as medium business networks but add enterprise-level equipment to the scheme.  Enterprise-level equipment has many ports and can transfer gigabytes of information.



  • The router connects the LAN to the WAN, acting as a gateway between the LAN and WAN topologies. The LAN port on the router has to match the LAN topology and signaling characteristics, such as Ethernet.

  • The WAN port is generally a serial connection to a CSU/DSU.  The CSU/DSU connects to a T1 or FT1 that the local telephone company normally provides.  The connection can be a dedicated point-to-point circuit between two locations, or it can terminate into a frame relay cloud.  The frame relay cloud is a shared packet switch network, all major telephone companies offer this service.  Frame relay is distance insensitive and is the most cost-effective method of connecting multiple locations over a large geographic area.

  VLAN   RFC1390  802.1Q specification establishes a standard method for inserting VLAN membership information into Ethernet frames   A VLAN acts like an ordinary LAN, but connected devices don't have to be physically connected to the same segment. While clients and servers may be located anywhere on a network, they are grouped together by VLAN technology, and broadcasts are sent to devices within the VLAN. A VLAN is an administratively configured LAN or broadcast domain.
  • VLANs configured by using Media Access Control addresses can recognize when a station has been moved to another port on a switch. VLAN management software can then automatically reconfigure that station into its appropriate VLAN without the need to change the station's MAC or IP address.
  •  Instead of going to the wiring closet to move a cable to a different LAN, network administrators can accomplish this task remotely by configuring a port on an 802.1Q-compliant switch to belong to a different VLAN. The ability to move endstations to different broadcast domains by setting membership profiles for each port on centrally managed switches is one of the main advantages of 802.1Q VLANs.
  • The 802.1Q specification establishes a standard method for inserting virtual LAN (VLAN) membership information into Ethernet frames.

    In a LAN, datalink-layer broadcast and multicast traffic is delivered to all endstations, but this traffic cannot go beyond the LAN boundary. In the past, shared cabling or hubs were the boundaries for LANs.


ROUTERS  connect similar and heterogeneous network segments into inter-networks, allowing each interconnected network to retain its sub-network address and broadcast characteristics while allowing the networks to communicate through the router connections to other networks.


GATEWAYS  a network device that acts as a translator between two systems that do not communicate using the same protocols.  Gateways can provide some security functions, such as acting as routers or firewalls.  Most protocol gateways operate at the Applicationlayer of the OSI model.  A gateway (not a default IP gateway) connects two separate systems that do not communicate using the same protocol.

  • INTERNET GATEWAY - Three types of Internet gateways: standard router, firewall and proxy server.  All TCP/IP networks that are connected to an external network must have a gateway specified at the host for communication beyond the local subnet to take place.

  1. Standard router  The most common gateway for access to the Internet.  Initially, it offered basic connectivity to external networks but eventually had to implement TCP/IP port filtering for network security.

  2. Firewall gateway  Offers a greater level of security from external intrusion and provides greater protection for trusted networks.

  3. Proxy server  Acts as an intermediate between trusted and nontrusted networks.  It receives all internal requests for Internet access and passes those requests on to the Internet.

  • SMTP GATEWAY  (Simple Mail Transfer Protocol)  the main email transport.  The SMTP gateway uses DNS addressing to deliver the mail to the correct location throughout the Internet.  SMTP gateway forwards the mail to the mail server.  The server transmits mail to a client through POP3 by using the DNS to direct the mail, whereas SMTP transfers mail to mail servers on the Internet.  POP3 transfers mail to the Internet client and SMTP is the protocol that the client uses to attach to the Internet mail server.

  • SNA GATEWAY (System Network Architecture Gateway) translates LAN protocols (TCP/IP, IPX, NetBEUI)  to IBM's SNA protocol used on their midrange and mainframe computers.


TOPOLOGIES  network topologies are named like their configuration.  A star topology has a central location with "legs" going out from the center like points on a star; a mesh network has multiple paths to many points, like a screen mesh;  a ring looks like a ring.

  • STAR  is so named because all the devices connect individually to a central hub at the local wiring closet (IDF or MDF).  The simplest type of star topology has workstations connected to a hub and the cable acts as a point-to-point connection between the workstations and the hub.  If a break occurs in the cable, the workstation is the only network node affected.  Because of its simple straightforward design, the star topology lets you make equipment changes at one location. The star topology  is the basis for structured cabling systems and can be combined with a bus, mesh, ring or hierarchical star topology to form a LAN or a WAN.  It is usually configured so that lines terminate within a multi-access unit (MAU), hub or switch.  It is also well suited to carrying out polling mechanisms for multiple access from the central location to outlying nodes.

  • MESH  networks are characterized by multiple paths to the same point, connecting one set of routers to other routers.  The two types of mesh topologies are the simple mesh and the fully interconnected mesh.  Many large companies employ mesh topologies for their WANs.  The largest mesh network is the Internet.

  • RING  the most common type of ring topology is the token ring network.  An electronic token is passed around the network.  When a station receives the token, it then has the authorization to transmit and receive data.  The network nodes are connected to an MAU device that has a collapsed ring inside it.  It also has special circuitry to detect when a device is connected to a port and allows the device into the ring or shuts the device out of the ring.  The early token ring networks were 4Mbps, but current networks operate at 16Mbps.  The IEEE 802.5 standard defines the token ring topology.


SEGMENT  a segment is a separate broadcast domain.  The most common segmentation device is a switch.  A switch creates segments that are logically separated from other segments, yet are physically connected to the same network.  The two types of segments are basic segments and complex segments.


BACKBONE  in a hierarchical network structure, a high-speed backbone connects the servers while the workstations connect to shared media in a star configuration.  A backbone structure is beneficial when several servers in a campus environment are remotely located and they all need access to the high-speed backbone.  If all the servers are in a central location, you should attach them directly to a high speed switch.  FIDDI (fiber distributed data interface) and ATM (asynchronous transfer mode) are the usual choices for high-speed backbones.


BUS  a bus topology is a single backbone cable that interconnects all the workstations on a LAN.  The bus structure provides simultaneous access to a central bus by several nodes and is the most commonly used networking structure in office LANs.  Tree topologies are also forms of bus structures.  This was the main type of network in the early days of Ethernet.  In today's network environment, you might implement a bus topology to connect network equipment that would connect workstations in a star topology.  Its main drawback is that if a break occurs in the cable, it takes the whole backbone segment down and all workstations will lose connection with the network.



a collection of sources for this information
Microsoft Certified System Engineer, TCP/IP, Certification Insider Press, Ed Tittel, Kurt Hudson, and J Michael Stewart, 1998
Comptia Certified Network Technician, Network +, Certification Insider Press, Scott Reeves and Kalinda Reeves,  1999
Cisco Router Handbook, McGraw Hill, George C Sackett, 1999
Mastering Windows NT Server 4,  Sybex Network Press, Mark Minasi, Christa Anderson, Elizabeth Creegan,  1996
Cisco Certified Network Associate (CCNA) Basics 1.0, Cisco Systems, 2000 (CD)
and the list goes on...

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