Mesh Topoloy

In this topology, every node has a dedicated point-to-point connection to every other node on the network. A fully connected mesh network has n(n-1)/2 channels to link ‘n’ devices. Therefore, every device on the network must have ‘n-1′ input/output (I/O) ports.

In mesh network, each node is directly connected to all nodes on the network. This type of network involves the concept of routes. In this type of network, each node may send message to destination through multiple paths. It means that each node of mesh network has several possible paths to send (or to receive) message, but in Bus, Star, Ring and Tree topologies each node has only one path.

Advantages

Mesh topology has the following advantages:

• It has multiple links, so if one route is blocked then other routes can be used for data communication.
• Each connection can have its own data load, so the traffic problem is eliminated.
• It ensures the data privacy or security, because every message travels along a dedicated link.
• Troubleshooting of this topology is easy as compared to other networks.
• Its performance is not affected with heavy load of data transmission.

Disadvantages

Mesh topology has the following disadvantages:

• It becomes very expensive because a large number of cabling and 110 ports are required.
• It is difficult to install.

Tree Topology

In tree network, the nodes are connected to each other in such a way that forms a tree like structure. Typically to form a tree network, multiple star topologies are combined together. This type of network has combined features of bus and star topology.

On tree topology the hubs of each star topology are connected to the central h

ub that controls the entire network. However, some nodes can be directly connected to the central hub. The tree topology configuration is shown in figure below.

The central Hub in the tree network is an active hub. It contains a repeater (a hardware device), which re-generates the received bit patterns. The secondary hubs usually are passive hubs. The passive hub controls the nodes directly connected to it and exchange data to other devices connected to the other secondary hubs (or same hub) through the central hub. The secondary hub may also be active hub if another secondary hub is directly connected to it. The cable TV network is an example of tree topology, where main cable is divided into, branches and each branch is further divided into smaller branches and so on. The hub is used when a branch is created. 

Advantages

The tree topology has the same advantages as star topology but it has some additional advantages. These are.

• It allows more devices to be connected to the central Hub.

Disadvantages

The tree topology also has the same disadvantages as star topology built has some additional disadvantages such as:

• It because more expansive because more hubs are required to install the network.

Hybrid Topology

Hybrid network is the combination of different topologies such as star, Ring, Mesh, Bus etc. For example, if a department uses a Bus network, second department uses the ring network, third department uses the Mesh network and fourth department uses the star network. All the networks of different types (of four departments) can be connected together through a central hub (in the form of star network) as shown in the figure below.

The public switched telephone network (PSTN) is the world-wide telephone system and usually this network system uses the digital technology. In the past, it was used for voice communication only but now it is playing very important role for data communication in the computer network such as in the Internet. There are different types of telephone lines that are used for data communication in the network. These are discussed below.

1. Dial-Up Lines

It is a temporary connection that uses one or more analog telephone lines for communication. Modem is used at the sending end as well as at the receiving end. A telephone number is dialed from the sending end through modem and the modem at the receiving end answers the call. In this way, connection is established for data communication between computers or electronic devices. In dial up line connection, the cost of data communication is very low. The mobile users can also access the Internet through this connection.

2. Dedicated Lines

It is a permanent connection that is used to establish connection between two devices permanently. It is better than dial-up line connection because dedicated lines provide a constant connection. These types of connections may be digital or analog. The data transmission speed, of digital lines is very fast as compare to analog dedicated line. The data transmission speed is also measured in bits per second (bps). In dial-up and dedicated lines, it is up to 56 Kbps. The dedicated lines are mostly used for business purposes. The most important digital dedicated lines are described below.

1) ISDN Lines

ISDN stands for Integrated Services Digital Network. It is a set of standards used for digital transmission over telephone line. The ISDN uses the multiplexing technique to carry three or more data signals at once through the telephone line. It is because the data transmission speed of ISDN line is very fast. In ISDN line, both ends of connections require the ISDN modem and a special telephone set for voice communication. Its data transmission speed is up to 128 Kbps.

2) DSL

DSL stands for Digital Subscriber Line. It is another digital line. In DSL, both ends of connections require the network cards and DSL modems for data communication. The data transmission speed and other functions are similar as ISDN line. DSL transmits data on existing standard copper telephone wiring. Some DSLs provide a dial tone, which allows both voice an4 data communication.

3) ADSL

The ADSL (Asymmetric Digital Subscriber Line) is another digital connection. It is faster than DSL. ADSL is much easier to install and provides much faster data transfer rate. Its data transmission speed is from 128 Kbps up to 10 Mbps. This connection is ideal for Internet access.

4) Cable Television Line

The Cable Television (CATV) line is not a standard telephone line. It is a dedicated line used to access the Internet. Its data transmission speed is 128 Kbps to 3 Mbps.

A cable modem is used with the CATV. it provides a high speed Internet connections through the cable television network. A cable modem sends and receives digital data over the cable television network.

To access the Internet using the CATV network, the CATV Company installs a splitter inside your house. From the splitter, one part of the cable runs to your television and other part connects to the cable modem. A cable modem usually is an external device, in which one end of a cable connects to a CATV wall outlet while the other end plugs into a port (such as on an Ethernet card) in the system unit.

5) T-Carrier Lines

It is very fast digital line that can carry multiple signals over a single communication line whereas a standard dialup telephone line carries only one signal. T-carrier lines use multiplexing so that multiple signals share the line. T-carrier lines provide very fast data transfer rates. The T-carrier lines are very expensive and large companies can afford these lines. The most popular T-carrier lines are:

1. TI Line
2. T3 Line

i) T1 Line

The most popular T-carrier line is the Ti line (dedicated line). Its data transmission speed is 1.5 Mbps. Businesses often use Ti lines to connect to the Internet. Many ISPs use Ti, lines to connect to the Internet backbone. Another type of TI line is the fractional TI line. It is slower than TI line but it is less expensive. The home and business users use this line to connect to the Internet and share a connection to the Ti line with other users.

ii) T3 Line

Another most popular and faster T-carrier line is T3 line. Its data transmission speed is 44 Mbps. It is more expensive than Ti line. The main users of T3 line are telephone companies and ISPs. The Internet backbone itself also uses T3 lines.

6) Asynchronous Transfer Mode (ATM)

It is very, fast data transmission connection line that can carry data, voice, video, multimedia etc. Telephone networks, Internet and other network use ATM. In near future, ATM will become the Internet standard for data transmission instead of T3 lines. Its data transmission speed is from 155 Mbps to 600 Mbps.

Computer Networks: Communication Devices

An electronic device that can send and receive data on the network is called the communication device. The communication devices are used on both ends of the connections. Most of the communication devices have the ability to convert the digital signal of computer into analog signal so that the signals can be transmitted on the communication line to another end. The reverse is also true. The most important communication devices are described below.

Modem

Modem stands for Modulator and Demodulator. The modem (dialup modem) is an electronic device that can convert the digital signals into analog signals and analog signals into digital signals. It is used on both ends of the computers for data communication between, computers through telephone line. We knew that data is communicated through telephone line in the form of analog signals. So data in the form of digital signal from one computer must be converted into analog signals before to transmit it to another computer through telephone line. Similarly, to receive the data from another computer through telephone line in the form of analog signals must be converted to digital before to store into the computer. The modem can be external or internal.

The external modem is a separate device that is attached to the computer through RS-232 serial port. It can be easily moved from, one computer to another. It has also many functions but it is very costly.

An internal modem is an expansion card that is installed into the expansion slot on the computer’s motherboard. Most of the computers at home and offices use the internal modem because it is less expensive than external modem. The Fax machine also uses the internal card; it is because internal modem is also known as Fax Card.

Digital Modem

The modem that is used to send and to receive data to and from a digital telephone line is called digital modem. The examples of digital modems are ISDN and DSL. These modems send and receive the digital signals to and from the digital line. This modem does not convert the digital signals into analog because digital lines already use the digital signals. Therefore, in this case he definition of modem confuses the users, however manufacturers still use the term modem for devices used in digital line for data communication.

Network Interface Card

It is also referred to as LAN adapter or simply LAN card. It is used in Local Area Network to establish the communication between the devices (computers, printers and other devices) attached on the network. Usually, the personal computers use the network interface card (NIC) in Local Area Network (LAN). The Ethernet card is the most common type of network interface card. The data transmission speed of Ethernet card is from 10 Mbps to 1,000 Mbps. Some of the network interface cards also, have the combined features of Ethernet card and dialup modem card.

Bridge

It is an electronic device that connects two similar networks and controls the data flow between them. A bridge looks at the information in each packet header and forwards data (that is traveling) from one LRN to another.

Router

An electronic device that connects multiple computers together and transmits data to its correct destination using the available path on the network is called router. It stores the routing information of each node or computer on the network and to use these information to transfer data between nodes. Like a bridge, a router looks at each packet’s header to determine where the packet should go and then determines a route for, the packet to take and thus reach its destination; some routers also have built-in antivirus protection. Similarly, some routers also support wireless communications.

Gateway

A gate is an electronic device that can perform logical function. A gateway is a device or system (collection of hardware and software resources) that connects two networks and translates information from one to the other so that data can be transmitted between two dissimilar networks.

Packets from different networks have different kinds of information in their headers, and information can be of different formats. The gateway can take a packet from one type. of network, read the header, and then encapsulates the entire packet into a new one, adding a header that is understood by the second network.

Repeater

A device, which is used to join same type of networks to extend the length of the communication medium, is called repeater. It also amplifies the signals received from one network and sends the amplified signals to another network.