Topologies of Common Networks

As mentioned earlier, a network can have a logical topology different from its physical topology. The following are some common types of networks:

• Ethernet
• Token Ring.

1. Ethernet

An older, common wiring system for Ethernet (10Base2) and (10Base5) uses coaxial cable in a linear bus topology. In the most type of Ethernet, each node connects to the coax through a T-connector that taps into the signals on the coaxial cable. The nodes both transmit and receive through 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). 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. Therefore 10Base-T and 100Base-TX are physical stars,but logical buses.

2. Token Ring

In 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.

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 there is signal around the entire network, 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. Sometimes Token Ring referred as a star-wired-ring.

Logical Topologies

Logical topologies have the same names as physical topologies. But a physical topology describe the network, 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.

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

• Ring Logical topology
• Bus logical topology

1. Ring Logical Topology

Ring topology functions by passing data transmission 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.

2. Bus Logical Topology

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.

Considerations when choosing a topology:

1. Money: A linear bus network may be the least expensive way to install a network; Need not to purchase concentrators.
2. Length of cable needed: The linear bus network uses shorter lengths of cable.
3. Future growth: With a star topology, expanding a network is easily done by adding another concentrators.
4. Cable type: The most common cable in schools is unshielded twisted pair, which is most often used with star topologies.

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 weaknesses, several different types can be combined for maximum effectiveness.

1. Advantages of Hybrid topology
   • One company can combine the benefits of several different types of topologies.
   • Workgroup efficiency and traffic can be customized.
2. Disadvantages of Hybrid topology
   • Devices on one topology cannot be placed into another topology without some hardware changes.
3. Examples of Hybrid topology
   • A Company can place its accounting database users on a ring and its secretarial staff on a bus for ease of cabling

Tree Topology

A tree topology combines characteristics of linear bus and star topologies. It consists of groups of star-configured workstations connected to a linear bus backbone cable. Tree topologies allow for the expansion of an existing network and enable schools to configure a network to meet their needs.

1. Advantages of a Tree Topology
• Point-to-point wiring for individual segments.
• Supported by several hardware and software vendors.
2. Disadvantages of a Tree Topology
   • Overall length of each segment is limited by the type of cabling used.
   • If the backbone line breaks, the entire segment goes down.
   • More difficult to configure and wire than other topologies.
3. Rule (Ethernet)

A consideration in setting up a tree topology using Ethernet protocol is the 5-4-3 rule. One aspect of the Ethernet protocol requires that a signal sent out on the network cable reach every part of the network within a specified length of time. Each concentrator or repeater that a signal goes through adds a small amount of time. This leads to the rule that between any two nodes on the network there can only be a maximum of 5 segments, connected through 4 repeaters/concentrators. In addition, only 3 of the segments may be populated (trunk) segments if they are made of coaxial cable. A populated segment is one which has one or more nodes attached to it. The furthest two nodes on the network have 4 segments and 3 repeaters/concentrators between them.

This rule does not apply to other network protocols or Ethernet networks where all fiber optic cabling is used.

Ring Topology

The ring topology is a physical, closed loop consisting of point-to-point links. All devices are connected to one another in the shape of a closed loop, so that each device is connected directly to two other devices, one on either side of it. Each node on the ring acts as a repeater. It receives a transmission from the previous node and amplifies it before passing it on.

1. Advantages of Ring topology
   • Each repeater duplicates the data signals so that very little signal degradation occurs.
2. Disadvantages of Ring topology
   • 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 to be more expensive.
3. Examples of Ring topology
   • IBM Token Ring (although wired as a Star)
   • Fiber Distributed Data Interface (FDDI).

Star Topology

A star topology is designed with each node (file server, workstations and peripherals connected directly to a central network hub or concentrator.

Data on a star network passes through the hub or concentrator before continuing to its destination. The hub or concentrator manages and controls all functions of the network. It also acts as a repeater for the data flow. This configuration is common with twisted pair cable; however, it can also be used with coaxial cable or fiber optic cable.

• All peripheral nodes may thus communicate with all others by transmitting to and receiving from, the central node only.
• The failure of a transmission line, that is, channel linking any peripheral node to the central node will result in the isolation of that peripheral node from all others.
• If the star central node is passive, the originating node must be able to tolerate the reception of an echo of its own transmission, delayed by the two way transmission time, that is, to and from the central node, plus any delay generated in the central node.
• An active star network has an active central node that usually has the means to prevent echo-related problems.

The protocols used with star configurations are usually Ethernet or LocalTalk. Token Ring uses a similar topology called the star-wired ring.

1. Advantages of a Star Topology
   • Easy to install and wire.
   • No disruptions to the network when connecting or removing devices.
   • Easy to detect faults and to remove parts.
2. Disadvantages of a Star Topology
   • Requires more cable length than a linear topology.
   • If the hub or concentrator fails, nodes attached are disabled.
   • More expensive than linear bus topologies because of the cost of the concentrators.
3. Examples of Star Topology
   • ARCnet
   • 10Base-T, 100Base-TX
   • StarLAN.

Bus Topology

A linear bus topology consists of a main run of cable with a terminator at each end. All nodes (file server, workstations and peripherals) are connected to the linear cable. Ethernet and LocalTalk networks can use a linear bus topology. All devices are connected to a central cable called the bus or backbone.

1. Advantages of a Bus Topology
   
   • Easy to connect a computer or peripheral to a linear bus.
   • Requires less cable length than a star topology.

2. Disadvantages of a Bus Topology
   • Entire network shuts down if there is a break in the main cable.
   • Terminators are required at both ends of the backbone cable.
   • Different to identify the problem if the entire network shuts down.
   • Not meant to be used as a stand-alone solution in a large building.

3. Example of Bus Topology
   • ARCnet (Token Bus)
   • Ethernet (10Base2).

Physical Topologies

All physical topologies are variations of two fundamental methods of connecting devices. They are:

• Point-to-point
• Multipoint

1. 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.

2. 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 are different types of physical topologies are frequently used in computer networking:

• Bus
• Star
• Ring
• Tree
• Hybrid.

Network Topologies

Network Topology refers to the shape of network or the network's layout. How different nodes in a network are connected to each other and how they communicate is determined by the network's topology. Two network have the same topology if the connection configuration is the same, although the network may differ in physical interconnections, distances between nodes, transmissions rates and /or signal types.

There are two types of topology: physical and logical.

• The physical topology of a network refers to the configuration of cables, computers and other peripherals.
• Logical topology is the method used to pass the information between workstations.