COMMUNICATIONS AND NETWORKS

COMMUNICATIONS AND NETWORKS

What is data communication?

This refers to one computer transferring data, instructions and information to another computer or some other computers.  The basic model for data communication consists of:

(a)    A sending device that initiates an instruction to transmit data, instruction or information e.g computer A which sends signals to another computer B.

(b)   A communications device that converts data, instructions or information from the sending device into signals that can be carried by a communications channel e.g Modem which converts digital signals into analog signals.

(c)    A communications channel or a path over which signals are sent e.g a standard telephone line along which the analog signals are sent.

(d)   A communications device that receives the signals from the communications channel and converts them into a form understood by the receiving device e.g Modem B, which converts the analog signals back into digital signals.

(e)     A receiving device i.e computer B that accepts the signals from computer A.

(f)    Communications software which consists of programs that manage transmission of data, instruction and information between computers.

Uses of data communications

▪      Voice mail

▪      Fax

▪      E-mail

▪      BBS

▪      Instant messaging

▪      Chart rooms

▪      Newsgroups

▪      Internet telephony

▪      Video conferencing

▪      Groupware

▪      Telecommunicating

▪      Global positioning system

COMMUNICATION DEVICES

These are devices that enable two or more computers to exchange items such as data, instructions and information with each other.  The primary function of a communication device (e.g a modem) is to convert or format signals so that they become suitable for the communications channel or a receiving device.

Common types of communications devices are;

1.      Dial-up modems

2.      ISDN and DSL modems

3.      Cable Modems

4.      Network Interface cards

5.      Multiplexer

1. DIAL-UP MODEMS.  A modem is a communications device that converts between analog and digital signals.  The word modem is derived from a combination of words Modulation and Demodulation

     Modulation is to convert digital signals into analog signals

     Demodulation is to convert analog signals into digital signals.

A modem connected to a sending computer converts the computers digital signals into analog signals (i.e modulation) which can travel over a communications channel e.g a telephone line.

Another modem at the receiving end converts the analog signals back into digital signals (i.e demodulation) that can be understood by the receiving computer.

 A modem can be external or internal.

External Modem is a standalone device that attaches to a serial port on a computer with a cable to a telephone outlet with a standard telephone cord.

Internal Modem is an expansion card that can be inserted into an expansion slot on a computers motherboard; and the modem then attaches to a telephone outlet with a standard telephone cord.

Most personal computers modems transmit data between 28.8kbps and 56kbps

Most modems today are also fax modems because they send computer prepared documents as faxes and also receive faxes.

2.  ISDN and DSL Modems

ISDN (Integrated Services Digital Network) is a set of standards for digital transmission of data over a standard telephone line.  With ISDN, the same telephone line that could normally carry only one signal can now carry three or more signals at once using a technique called Multiplexing.

ISDN requires that both ends of the connection have an ISDN modem.  ISDN lines also require a special ISDN telephone for voice communications.

Advantages of ISDN lines

-       Provides faster transfer rates that dial-up Modems.

-       Faster web page downloads and clearer video conferencing.

-       Produce very clear voice conversations.

DSL (Digital Subscribe Lines) provides high speed connections to the internet ove r a regular copper telephone line.  The user must have a special network card or DSL modem to connect to digital subscriber line.  It is much easier to install and can provide much faster data transfer rates than ISDN.

ADSL (Asymmetric Digital Subscriber line) is a type of DSL that supports faster transfer rates when receiving data (i.e downstream rate) than when sending data (i.e upstream).  ASDL is ideal for internet access because most users download more information from the internet than they upload.

3.  CABLE MODEMS.

A cable modem sends and receives data over the cable television (CATV) network.  Cable modems can transmit data at speeds (500kbps to 2mbps) much faster than dial-up modems or ISDN.  It can also be integrated with a set-to-box to provide faster viewing of multi-media websites.

4. NETWORK INTERFACE CARDS

A network interface card (NIC) or LAN adapter is an expansion card that enable a computer or device to connect to a network. The Ethernet card is the most common type of network card.  The transfer rate on Ethernet network can be 10mbps, 100mbps or 1,000mbps.

Fig:  Network interface card.

DEVICES USED TO INTERCONNECT NETWORKS

5. MULIPLEXER.  A Multiplexer is a communication device that combines two or more input signals from various devices into a single stream of data and then transmits over a single transmission medium.

N.B 

Transmission Media consists of materials or techniques capable of carrying one or more signals.  Transmission media can be physical or wireless.

(a)   Physical transmission media use wire, cable and other physical materials to send communication signals.

(b)   Wireless transmission media send communication signals through air or space using radio, microwave and infrared signals.

A multiplexer increases the efficiency of communications and reduces the need for and the cost of using separate transmission media.  Both the sending end and receiving ends need a multiplexer for data transmission to occur.  At the sending end, a multiplexer combines separate data transmission into a single data stream and then compress the data and sends it over a communications channel.  At the receiving end , the multiplexer separates the single stream of data into its original parts.

6.   A HUB. A hub is also called a concentrator or multi-station access unit.  It is a device that provides a central point for cables in a network.  It allows devices to be connected to a server.

7. A REPATER.  This is a device that accepts a signal from a transmission medium, amplifies it and retransmits it over a medium. As signals travel over a long distance, it undergoes a reduction in strength, an occurrence called Attenuation.

Repeaters are used in both copper wire cables carrying electrical signals and in fibre optics carrying light. Repeaters are also used extensively in broadcasting, where they are known as translators or booster.

8.  BRIDGE. A bridge is a device that connects two LANS using the same protocol, such as the Ethernet.  Bridges are uses to connect similar networks over a wide area communication links.

Fig: A bridge

Advantages of bridges

-       They are self configuring.

-       Primitive bridges are often inexpensive.

-       LANs interconnected are separate and physical constraints such as number of stations, repeaters and segment length don’t apply.

Disadvantages of bridges

-       Bridges are more expensive than repeaters.

-       Bridging of different MAC protocols introduces errors.

-       Do not limit the scope of broadcasts.

-       Because bridges do more than repeaters by viewing MAC address, the extra processing makes them slower than repeaters.

9. GATEWAY.  A gateway is a combination of hardware and software that connects networks that use different protocols.

10. ROUTER. A router is an intelligent communication device that sends 9routes) communication traffic to the appropriate network using the fastest available path.

11. SWITCH. A switch is used to store address of every device down each cable connected to it.  By delivering each message only to the connected device it was intended for, a network switch conserves network bandwidth and offers generally better performance than a hub.  It is more intelligent.