ITNW 2313 - Networking Hardware
( LAN Hardware/Wiring & Installation )
Prof. Michael P. Harris, CCNA, CCAI

Lesson #3:  Exploring Network Topologies

Topology

The word topology is commonly used to discuss the properties of various types of networks.  Topology is the branch of mathematics that examines the characteristics of geometric shapes.  Networks have shapes, and the shape a network takes has much to do with the way it functions.

Two different definitions of topology are as follows:  physical and logical.  You encounter the distinction between physical and logical frequently in networking. The distinction is necessary because the way something looks can be quite different from the way it functions.

A motion picture may look like a natural moving scene, but it actually consists of many snapshots flashed on the screen so quickly that your eye interprets the individual pictures as moving.  The logical, moving picture is much different from the physical reality.

So it is with computer networks.  Your eye may tell you one thing as you examine the cabling, but the network may operate very differently at the invisible level of electrical signals in the wire.

NOTE:  The following rule of thumb distinguishes physical from logical:

  • If you can see it and touch it, it is physical.
  • If you cannot see it or touch it, it is logical.

The physical topology  of a network is the layout or actual appearance of the cabling scheme used on a network.

The logical topology  of a network describes how the data flows through the physical topology.  You soon discover that similar-looking networks can have quite different logical topologies.

A network's logical topology is closely related to the mechanism used to manage the way stations access the network.  Because a given cable in a baseband network services only one station at a time,  it is necessary to have procedures in place that manage network access so that all stations have access without conflicting with each other.  These procedures are called access control methods  and will be discussed in the next lesson.

Physical Topologies

All physical topologies are variations of two fundamental methods of connecting devices:  point-to-point and multipoint.  After examining these two fundamental topologies, several variations will be examined.



Point-to-Point Topology

Point-to-point (PTP) topology connects two nodes directly together.

The following examples are pure point-to-point links:

  • Two computers communicating via modems
  • A mainframe terminal communicating with a front-end processor
  • A workstation communicating along a parallel cable to a printer

In a point-to-point link,  two devices monopolize a communication medium.  Because the medium is not shared,  a mechanism is not needed to identify the computers.  Therefore, a simple, two-device point-to-point network has no need for addressing.

Point-to-point links can be simplex, half-duplex, or full-duplex.  When devices must engage in bi-directional communication on a half-duplex link,  some turnaround mechanisms must be in place to switch the roles of the sending and receiving devices.

Multipoint Topology

Multipoint topologies link three or more devices together through a single communication medium.  Multipoint topologies work much like a party-line telephone service where several subscribers are connected to the same telephone line.

Because multipoint topologies share a common channel, each device needs a way to identify itself and the device to which it wants to send information.  The method used to identify senders and receivers is called addressing.

The following four types of physical topologies are frequently used in computer networking:
  1. Star
  2. Bus
  3. Ring
  4. Hybrid

NOTE:  Most of the preceding topologies are discussed in greater detail in later lessons.



[Top] Star Topology

The star topology is a popular method of connecting the cabling in a computer network. In a star, each device connects to a central point via a point-to-point link. Depending on the logical architecture used, several names are used for the central point including the following:

  • Hub
  • Multipoint Repeater
  • Concentrator
  • Multi-Access Unit (MAU)

The central hubs also can be classified in the following manner:
  • Passive hub - A passive hub is a simple signal splitter. Its main function is to connect the arms of the star while maintaining the proper electrical characteristics.

    • A passive hub routes all traffic to all nodes. This means that a tremendous load can be created when much communication takes place between computers. Every computer has the additional burden of reading the address of each piece of information it receives to determine if the information is intended for that computer. Information containing other addresses are discarded.

  • Active hub - An active hub performs the same function as a passive hub, but contains electronic circuits that regenerate and retransmit the information. Thus, active hubs can be used to extend the size of a network.

  • Intelligent hub - Intelligent hubs perform the same functions as passive and active hubs; however, they can make informed path selections and perform some network management. Intelligent hubs route traffic only to the branch of the star on which the receiving node is located. If redundant paths exist, an intelligent hub can route information around normally used paths when cable problems occur.

    • Routers, bridges, & switches  are examples of hub devices that can route transmissions intelligently. Intelligent hubs also can incorporate diagnostic features that make it easier to troubleshoot network problems.
Benefits of Stars

Most modern cabling systems are designed in a star physical topology. The benefits of the star topology are many, including the following:

  • Each device is isolated on its own cable. This makes it easy to isolate individual devices from the network by disconnecting them from the wiring hub.

  • All data goes through the central point, which can be equipped with diagnostic devices that make it easy to trouble shoot and manage the network.

  • Hierarchical organization allows isolation of traffic on the channel. This is beneficial when several, but not all, computers place a heavy load on the network. Traffic from those heavily used computers can be separated from the rest or dispersed throughout for a more even flow of traffic.

Disadvantages of Star topology

Star topology has the following disadvantages:

  • Because point-to-point wiring is utilized for each node, more cable is required.
  • Hub failures can disable large segments of the network.
Examples of Star topology

The following types of networks are examples of star topologies:

  • ARCnet
  • 10Base-T, 100Base-TX
  • StarLAN

Token Ring also is wired in a physical star. However, as you will learn later in this lesson, the physical wiring and the logical characteristics of Token Ring are quite different.

[Top] Ring Topology

The ring topology is a physical, closed loop consisting of point-to-point links. In the diagram, you can see how each node on the ring acts as a repeater. It receives a transmission from the previous node and amplifies it before passing it on.

Benefits of Ring topology

Ring topology has the following advantage:

  • Each repeater duplicates the data signals so that very little signal degradation occurs.
Disadvantages of Ring topology

Ring topology has the following disadvantages:

  • A break in the ring can disable the entire network. Many ring designs incorporate extra cabling that can be switched in if a primary cable fails.

  • Because each node must have the capability of functioning as a repeater, the networking devices tend to be more expensive.

Examples of Ring topology

The following are examples of ring topologies:

[Top] Bus Topology

In a bus topology, all devices attach to the same transmission medium. The medium has a physical beginning and end. All buses are implemented using electrical cable, usually coax, and the ends of the cable must be terminated with a terminating resistor that matches the impedance of the cable. The terminating resistor prevents data reflections from coming across as data corruption. The bus is considered a multipoint system because all devices tap into the same backbone cable.

An important characteristic to remember in bus topologies is that all data signals are broadcast throughout the bus structure. In the following diagram, if node B sends a signal to node C, the signal propagates for the length of the cable and is seen by nodes A and D as well. It is necessary to have an addressing mechanism so that each node understands which messages it is to receive and which to ignore.

Benefits of Bus topology

Bus topology has the following advantage:

  • Cabling costs are minimized because of the common trunk.
Disadvantages of Bus topology

Disadvantages of bus topology are as follows:

  • Difficult to trouble shoot because no central distribution points exist.
  • Cable breaks can disable the entire segment because they remove the required termination from each of the two cable fragments.
Examples of Bus topology

The following networks are examples of bus topology:

[Top] Hybrid Topology

The hybrid topology scheme combines multiple topologies into one large topology. The hybrid network is common in large wide-area networks. Because each topology has its own strengths and weaknesses, several different types can be combined for maximum effectiveness.

Benefits of Hybrid topology

Hybrid topology has the following advantages:

  • One company can combine the benefits of several different types of topologies.
  • Workgroup efficiency and traffic can be customized.
Disadvantages of Hybrid topology

The disadvantages of hybrid topology include the following:

  • Devices on one topology cannot be placed into another topology without some hardware changes.
Examples of Hybrid topology

An example of a hybrid topology is as follows:

  • A company can place its accounting database users on a ring for better throughput, and its secretarial staff on a bus for ease of cabling.
STUDY NOTE: Throughput  is the amount of data transferred per unit of time. It is viewed in terms of bits, bytes, or packets per second.

[Top] Logical Topologies

You have just examined five types of physical topologies. Now you will examine two types of logical topologies. Logical topologies have the same names as physical topologies, but keep in mind that the physical topology describes the network you can see, whereas the logical topology describes the network from the viewpoint of the data traveling on the network. Networks can have different physical and logical topologies (most do!).

The following two logical topologies are discussed in the following sections:

  • Ring logical topology
  • Bus logical topology

[Top] Ring Logical Topologies

Ring topologies function by passing data transmissions from one node to the next. This operation is clearest when the physical topology is also a ring. Any time data are passed from node-to-node, the network has a ring logical topology.

Another way to identify a ring is to determine whether each node has separate receive and transmit circuits. If that is the case, the node is functioning as a repeater and is probably connected in a logical ring network.

[Top] Bus Logical Topologies

In a bus topology, each data transmission passes by each node on the network. Essentially, each transmission is broadcast throughout the network, and the nodes use addresses to determine whether they should pay attention. Any time all transmissions are available to all nodes on the network, the network has a bus logical topology.

If the nodes on a network use the same circuits to transmit and receive, the logical network is a bus.


Topologies of Common Networks:

As mentioned earlier, a network can have a logical topology different from its physical topology. In this class, you work with the following four common types of networks:
  1. Ethernet
  2. Token Ring
  3. ARCnet
  4. FDDI

[Top] Ethernet

An older, common wiring system for Ethernet (10Base2) and (10Base5) uses coaxial cable in a linear bus topology. In the most common type of Ethernet, each node connects to the coax through a T-connector (BNC) that taps into the signals on the coaxial cable. The nodes both transmit and receive through the same connector. Therefore, 10Base2 Ethernet is a logical as well as a physical bus.

A newer variation of Ethernet, 10Base-T and 100Base-TX, are cabled using wiring hubs (concentrators), as shown below. Each station is connected to the hub via an individual UTP twisted pair cable. Within the hub, however, the individual signals are combined into a bus. Therefor 10Base-T and 100Base-TX are physical stars, but logical buses.

[Top] Token Ring

If you examine the wiring of a Token Ring, it meets all the specifications of a star. Token Ring uses central wiring hubs, and each node is wired to the hub with an individual run of cable.

If you look inside the hubs and wires, however, you can see why this is called a ring network. The figure above shows the path that a transmission follows through the network. Starting at the hub, the signal travels through a pair of wires to the receive circuit on the node's network interface. The receive circuit passes the signal to the transmit circuit, which repeats the signal on a separate pair of wires and sends the signal back to the hub.

If you follow the signal around the entire network, you can see that it completes a circuitous path, proving that Token Ring has a ring logical topology.

Token Ring is wired in a physical star to obtain the advantages of a central wiring hub. All stations can be connected and disconnected at a central point, and the wiring hub can be equipped with hub management and diagnostic systems. We therefor sometimes refer to Token Ring as a star-wired-ring. Note the dual shielded twisted pair cable.

[Top] ARCnet

ARCnet is wired in a star using coaxial cable. The diagram above shows an extremely simple ARCnet using a single, passive hub.

Recall from the earlier discussion about passive hubs, that they simply split the signal and pass it on to all the nodes connected to the hub. This is an indication that ARCnet is a logical bus: all nodes see all signals on the network.

That ARCnet is a logical bus also can be determined by examining the network cards. They both receive and transmit through the same coax connector.

ARCnet is wired as a physical star, and has some of the advantages of a star network. However, ARCnet functions as a logical bus . Because of its signaling characteristics, ARCnet is also called a token-passing-bus.

[Top] FDDI

Fiber Distributed Data Interface (FDDI) can be configured as both a physical and a logical ring. FDDI is commonly used to connect widely dispersed areas with a high-speed fiber network. When doing so, a physical ring is the most cost-effective cabling plan. The diagram above shows an FDDI network covering a large metropolitan area. FDDI uses the same signaling mechanism as Token Ring; each node repeats the incoming signal and transmits it to the next node in the ring.



This page is maintained by:   Prof. Michael P. Harris, CCNA, CCAI

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 Last modified:  17-Jun-2008
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