Networking Topology

Introduction

As the name suggests this lecture is a primer on computer networking. Our goal here is to demystify Networking Technology by painting a detailed picture of its key pieces and defining the terminology. While there are too many details they are all easy to understand. And surprisingly enough, knowing the terminology clarifies nine-tenths of the technology. Our goal in this chapter then, is to address network technology at its most fundamental layer - the wires, data, and connections. 

With this brief introduction we embark on our journey into computer networking.

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.

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 base-band 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.

Physical Topologies

The following four types of physical topologies are frequently used in computer networking:

  1. Star
  2. Bus
  3. Ring

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)

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.

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:

  • IBM Token Ring (although wired as a star)
  • Fiber Distributed Data Interface (FDDI)

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:

  • ARCnet, (Token bus)
  • Ethernet, (10Base2)

Communication Schemes

 

Communication schemes are those methods used by various types of local area networks for transferring data from one point to another. Another common term applied to this function is channel access method.

There are several schemes or methods used in networking today. The leaders include contention and token passing. Another method that has been used in the past is called polling.

 

1. Contention

 

The contention channel access method involves multiple devices sharing a common transmission media. An example would be Ethernet's modus operandi. In bus topology systems like this, all devices are attached to a common wire. Only one device may use the common wire at a time. Since several devices may need to use the wire at once, machines are said to be contending for the media. If the system is operating within tolerable limits, every machine will eventually get the opportunity to transfer data.

 


Figure 2-9: With contention systems, devices must listen for the opportunity to transmit data.

 

Ethernet systems use a channel access method known as CSMA/CD, short for Carrier Sense Multiple Access / Collision Detection. Though this seems a lot of words, the meaning is quite simple. Carrier Sense means that each device checks the LAN before it starts transmitting to see if some other device is using the media then. If another signal (containing a "carrier") was present, then the device attempting to send would wait until the LAN is clear. Then it transmits its data. The collision detection part means that each workstation listens to make sure that only one signal is present on the LAN. In the event there are two then obviously the data from one device has collided with that of another. Once a workstation detects a collision, it sends out a series of 1 bits (Jamming Signal) alerting the rest of the network. At that point everyone stops transmitting and each workstation waits a random amount of time before attempting to transmit again. The delay time is regulated by a random number generator on-board each Ethernet card.

 

LocalTalk LANs used by Macintosh PCs also use CSMA contention schemes, but these machines incorporate a technology called time- division multiplexing to allow avoidance of collisions. In fact, LocalTalk systems are said to be CSMA/CA systems, with CA standing for Collision Avoidance.

 

The major advantage of contention systems is that devices may transmit whenever they like just as long as the LAN is free. Consequently the overhead of devices waiting on the opportunity is generally low. Since any device can participate at almost any time, no attempt is made to prioritize LAN access in any way.

 

However, as traffic increases in a contention system, collisions can become excessive, severely impacting the overall performance of the network. The capacity of the LAN may be far underutilized in this event. The other major disadvantage is that contention systems do not follow an easily predictable pattern of performance degradation as traffic increases. The true loss in performance can only be guessed at statistically.

 

The Institute of Electrical and Electronic Engineers (IEEE) has created a standard for Ethernet-type systems that include specifications for implementation of contention in these types of LANs. The standard is called 802.3.

 

2. Token Passing Scheme

 

This technology is used for token ring systems. 

 


Figure 2-10: The token visits every device on the LAN giving each permission to transmit if ready.

 

In this channel access method, a small signal called a token regularly visits each device. The token gives permission for the device to transmit if it needs to. If a transfer of data is needed, the device receives a set amount of time to broadcast its data. When it is done, the machine then retransmits the token to another machine giving that recipient permission to transmit, and so the system continues. This mechanism ensures opportunity for all devices to gain access to the LAN. Because of its predictable behavior, token scheme LANs offer the advantage of priorities, where a certain group of devices may have enhanced access to the LAN if warranted.

 

Token passing systems may be implemented using either bus or ring topology. The IEEE standard governing token bus systems is called 802.4. The token ring specifications are called 802.5. Vendors count on standards such as these to help make sure that their products are interoperable with those of other vendors.

 

As traffic demand increases on a token LAN, the overall throughput of data rises as well as until a point is reached where the networks simply cannot accommodate anymore. The function in this case is somewhat like a waterwheel. The wheel itself receives water from a sluice. You may increase the capacity of the wheel, but the sluice can only hold so much water, so there is a finite limit to the throughput of the system.

 

Because the throughput characteristics of token LANs are so predictable, and because of the characteristics of traffic demand vs. throughput, these systems are ideal for heavy traffic situations. However, the complexity of such a LAN does come at some cost. Token systems require overhead to carry out their many functions including fault-tolerance. Plus, token ring systems are considerably more expensive than Ethernet systems. Factors weighing in deciding which system to choose should include traffic demand and budgetary restraints.