Hybrid Topologies

Hybrid Topologies

In a hybrid topology, two or more topologies are combined to form a complete network design. Networks are rarely designed using only one type of topology. For example, you may want to combine a star with a bus topology to benefit from the advantages of each.

Two types of hybrid topologies are commonly in use: star-bus topology and star-ring topology.

Star-Bus

In a star-bus topology, several star topology networks are linked to a bus connection. After a star configuration is full, you can add a second star and use a bus connection to connect the two star topologies.

In a star-bus topology, if a single computer fails, it will not affect the rest of the network. However, if the central component, or hub, that attaches all computers in a star fails, all computers attached to that component fail and are unable to communicate.

Mesh Topology

Mesh Topology

In a mesh topology, each computer is connected to every other computer by a separate cable. This configuration provides redundant paths through the network so that if one cable fails, another carries the traffic and the network continues to function. On a larger scale, multiple LANs can be connected to each other in a mesh topology by using leased telephone lines, ThickNet coaxial cable, or fiber-optic cable.

An advantage of a mesh topology is its back-up capabilities by providing multiple paths through the network. Because redundant paths require more cable than is needed in other topologies, a mesh topology can be expensive.

Ring Topology

Ring Topology

In a ring topology, computers are connected on a single circle of cable. Unlike the bus topology, there are no terminated ends. The signals travel around the loop in one direction and pass through each computer, which acts as a repeater to boost the signal and send it to the next computer. On a larger scale, multiple
LANs can be connected to each other in a ring topology by using ThickNet coaxial or fiber-optic cable.

The advantage of a ring topology is that each computer acts as a repeater, regenerating the signal and sending it on to the next computer, thereby preserving signal strength.

Token Passing

The method of transmitting data around the ring is called token passing. A token is a special series of bits that contains control information. Possession of the token allows a network device to transmit data to the network. Each network has only one token.

The sending computer removes the token from the ring and sends the requested data around the ring. Each computer passes along the data until the packet finds the computer that matches the address on the data. The receiving computer then returns a message to the sending computer indicating that the data has been received. After verification, the sending computer creates a new token and releases it to the network.

The advantage of a ring topology is that it can handle high-traffic environments better than bus networks. In addition, the impact of noise is reduced in the ring topology.

The disadvantage of a ring topology is that only one computer at a time can send data on a single token ring. Also, ring topologies are usually more expensive than bus technologies.

Star Topology

In a star topology, cable segments from each computer on the network are connected to a central component, or hub. A hub is a device that connects several computers together. In a star topology, signals are transmitted from the computer, through the hub, to all computers on the network. On a larger scale, multiple LANs can be connected to each other in a star topology.

An advantage of the star topology is that if one computer on the star topology fails, only the failed computer is unable to send or receive data. The remainder of the network functions normally.

The disadvantage of using this topology is that because each computer is connected to a hub, if the hub fails, the entire network fails. In addition, noise is created on the network in a star topology.

Bus Topology

Bus Topology

In a bus topology, all of the computers in a network are attached to a continuous cable, or segment, that connects them in a straight line. In this straight-line topology, a packet is transmitted to all network adapters on that segment.

Because of the way electrical signals are transmitted over this cable, the ends of the cable must be terminated by hardware devices called terminators, which act as the boundaries for the signal and define the segment. If there is a break anywhere in the cable or if an end is not terminated, the signal will travel back and forth across the network and all communication will stop.

The number of computers attached to a bus also affects network performance. The more computers there are on the bus, the greater the backup of computers waiting to put data on the bus, and consequently, the slower the network. In addition, because of the way computers communicate in a bus topology, there may be a lot of noise. Noise is the traffic generated on the network when computers attempt to communicate with each other simultaneously. An increase in the number of computers results in an increase in noise and a corresponding decrease in network efficiency.

Network Topologies

A network topology is the arrangement of computers, cables, and other components on a network. It is a map of the physical network. The type of topology you use affects the type and capabilities of the network's hardware, its management, and possibilities for future expansion.

Topology is both physical and logical;

•     Physical topology describes how the physical components on a network are connected.
•     Logical topology describes the way network data flows through the physical components.

There are five basic topologies:

•      Bus. Computers are connected to a common, shared cable
•      Star. Computers are connected to cable segments that branch out from a central location, or hub.
•     Ring. Computers are connected to a cable that forms a loop around a central location,
•       Mesh. Computers on the network are connected to every other computer by cable.
•      Hybrid. Two or more topologies are used together.

Scope of Networks

The scope of a network refers to its geographical size. A network can range in size from just a few computers in one office to thousands of computers linked together over great distances.

Network scope is determined by the size of the organization or the distance between users on the network. The scope determines how the network is designed and what physical components are used in its construction.

There are two general types of network scope;

•  Local Area Networks
• Wide Area Networks

Local Area Network

A local area network (LAN) connects computers that are located near eachother.

For example, two computers connected together in an office or two buildings connected together by a high-speed wire can be considered a LAN. A corporate network that includes several adjacent buildings can also be considered a LAN.

Wide Area Network

A wide area network (WAN) connects a number of computers located at a greater distance from one another.

For example, two or more computers connecting opposite sides of the world is considered a WAN. A WAN can be made up of a number of interconnected LANs. For example, the Internet is really a WAN.

Basic Connectivity Components

• Network Adapters   
• Network Cables
• Wireless Communication Devices

The basic connectivity components of a network include the cables, network adapters, and wireless devices that connect the computers in the network.

These components enable data to be sent to each computer on the network, thereby permitting the computers to communicate with each other.

Common connectivity components of a network are:

•   Network adapters.
•   Network cables.
•   Wireless communication devices.

Network Adapters

Network adapters constitute the physical interface between the computer and the network cable. Network adapters, also known as network interface cards, are installed into an expansion slot in each computer and server on the network. After the network adapter is installed, the network cable is attached to the adapter's port to physically connect the computer to the network.

As the data passes through the cable to the network adapter, it is formatted into packets. A packet is a logical grouping of information that includes a header, which contains location information and user data. The header contains address fields that include information about the data's origin and destination. The network adapter reads the destination address to determine if the packet is to be delivered to this computer. If it is, the network adapter then passes the packet on to the operating system for processing. If not, the network adapter discards the packet

Each network adapter has a unique address that is incorporated into chips on the card. This address is called the physical, or media access control (MAC), address.

The network adapter performs the following functions:

•     Receives data from the computer's operating system and converts it into electrical signals that are transmitted onto the cable

•     Receives electrical signals from the cable and translates them into data that the computer's operating system can understand

•     Determines whether data received from the cable is intended for the computer

•     Controls the flow of data between the computer and the cabling system

To ensure compatibility between the computer and the network, the network adapter must meet the following criteria:

•  Fit in the computer's expansion slot
•  Use the correct type of cable connector for the cabling
•   Be supported by the computer's operating system

Network Cables

You connect computers together in a network by using cables to carry signals between computers. A cable that connects two computers or network components is called a segment. Cables differ in their capabilities and are categorized according to their ability to transmit data at varying speeds, with different error rates. The three major categories of cables that connect most networks are:

• Twisted-pair
•  Coaxial
• Fiber-optic

Twisted-Pair Cable

Twisted-pair cable (lObaseT) consists of two insulated strands of copper wire twisted around each other. There are two types of twisted-pair cable: unshielded twisted pair (UTP) and shielded twisted pair (STP). These are the most common cables used in networks and can carry signals for 100 meters (about 328 feet).

•   UTP cable is the most popular type of twisted-pair cable and is the most popular LAN cable.
•    STP cable uses a woven copper-braid Jacket that is more protective and of a higher quality than the jacket used by UTP. STP also uses a foil wraparound each of the wire pairs. This gives STP excellent shielding that protects the transmitted data from outside interference, which in turn allows STP to support higher transmission rates over longer distances than UTP.

Twisted-pair cabling uses Registered Jack 45 (RJ-45) connectors to connect to a computer. These are similar to Registered Jack 11 (RJ-11) connectors.

Coaxial Cable

Coaxial cable consists of a copper wire core surrounded by insulation, a braided metal shielding, and an outer cover. The core of a coaxial cable carries the electronic signals that make up the data. This wire core can be either solid or stranded. There are two types of coaxial cable: ThinNet coaxial cable (10Base2) and ThickNet coaxial cable (10Base5). Coaxial cabling is a good choice when transmitting data over long distances and for reliably supporting higher data rates when using less sophisticated equipment.

Coaxial cable must be terminated at each end.

•     ThinNet coaxial cable can carry a signal for approximately 185 meters

(about 607 feet).

•     ThickNet coaxial cable can carry a signal for 500 meters (about 1,640 feet).

Both ThinNet and ThickNet cable use a connection component, known as a BNC connector, to make the connections between the cable and the computers.

Fiber-Optic Cable

Fiber-optic cable uses optical fibers to carry digital data signals in the form of modulated pulses of light. Because fiber-optic cable carries no electrical impulses, the signal cannot be tapped and its data cannot be stolen. Fiber-optic cable is good for very high-speed, high-capacity data transmission because the signal is transmitted very quickly and with very little interference.

A disadvantage of fiber-optic cable is that it breaks easily if you are not careful during installation. It is more difficult to cut than other cables and requires special equipment to cut it.