Data Communication and Computer Networks

Data Communication and Computer Networks

Data Communications has an ancient history, as people have always had an interest in communication with each other. Different methods have been used and associated with each method are various advantages and disadvantages. A major problem with communications is ensuring that the receiver gets the message sent by the transmitter.

In every form of communication there are common elements:

1.      Transmitter (Sender, Source)

2.      Receiver (Destination)

3.      Message to be Communicated

4.      Medium (How message is carried)

Medium	Problem (Noise)
Post	Strike, Loss
Telegraph	Broken Wires
Broken Wires	Electrical
Computer Cable	Electrical

Anything That interferes with the message is Technically Called Noise.

The connection between two Exchanges is called a Trunk and trunk switches in each exchange route calls. The connection between a subscriber and the local exchange is called The Local Loop.

In the telegraph network text was transmitted using codes, beginning with Morse, and then Baudot codes. These were predecessors of the modern ASCII code, which is frequently used in computers to represent text.

The concept of a start-stop code system was developed to tell a receiver that a character was being transmitted. Seven Pulses were transmitted on the line. Five were used to represent the character. A start pulse indicated that a character was to be transmitted and a stop pulse that the character was finished.

This idea is the basis of the RS232 serial Interface. The serial interface is the communications interface used between computes and devices such as modems, printers and computer terminals.

RS232 is the name od the standard that defines the interface (e.g. how many wires are used, what each wire is used for and soon).

This type of transmission is asynchronous. Characters are transmitted independently of each other as opposed to synchronous transmission where blocks of characters are transmitted and precise timing is critical.

Computer Networks:

Definition: A Computer network is an interconnected collection of autonomous computers. The goals of a computer network include:

·         Resource Sharing: Programs (O.S., application), data, equipment (printers, disks) are available to all users of the users of the network regardless of location.

·         High Reliability: By replicating files on different machines and having spare CPUs, users are more immune from hardware/software failure.

·         Less Cost: Small machines have about 1/10 the power of a mainframe but 1/1000 the cost. By using such machines with file server machines, a local area network LAN can be cheaply installed. It is easy to increase the capacity by adding new machines.

·         Communication medium: users have access to email and the internet.

Wide Area Networks (WANs)

When the computers in a network are separated by long distances (from a few kms to global distances) the network is called a WAN.

LANs can carry data at varying speeds depending on the communications medium used. If they are based on the telephone network this speed may range from 56 kbps to less than 10 Mbps.

However, if they are based on optical fibre, much higher speeds are possible.

WANs are widely used in banking. The financial markets are also heavily dependent on wide area computer networks.

They provide for remote database access, which is the basis for airlines reservations and home banking. They also provide electronic mail or e-mail. the point to point subnet, is mainly found in Wide Area Network (WANs).

Local Area Networks (LANs)

When the computers in a network are located close together close together (Usually less than 1 or 2 kms), the network is called a LAN.

LANs are used in many offices, schools, colleges and hospitals. They are suitable for networking within a building or campus area. Because of the short distances involved they are typically (though not necessarily) faster than WANs in that it is cheap to use high quality communications media such as coaxial cable and optical fibre. They can carry data at very high speeds e.g. from 10Mbps to 100Mbps being very common speeds, while speeds of upto 1000mbps are possible.

LANs usually use Broadcast subnets.

Metropolitan Area Network (MAN)

The metropolitan area network is designed to cover an entire city. It can be a single network such as cable TV or a number of LANs connected together within a city to form a MAN. Privately laid cables or public leased lines may be used to form such network. For instance, a business organisation may choose MAN to inter connect all its branch offices within the city.

Data Transmission

There is a minimum limit to the amount of data that can be transmitted using any transmission medium (Shannon’s Law). Each medium has its own maximum.

Data is transmitted as signals. The number of signals per second is the frequency of the signal. This is measured in Hertz (Hz). One cycle per second is 1 Hz. One Million cycles per second is 1MHz

Bandwidth

The maximum range of frequencies that can be transmitted is the bandwidth of the medium. This determines the maximum amount of data that can be transmitted. A coaxial cable might have a bandwidth of 10 to 100MHz. An optic fibre has a bandwidth of upto 108MHz.

The higher the bandwidth the more data that can be transmitted per second, hence the popularity of optic fibres. Infact, computers cannot yet utilise the full capacity of optic fibres. i.e., they cannot transmit at the speed which the fibre is capable of carring.

Transmission Media:

·         Copper Wires	·         Fibre Optic	·         Wireless
*Twisted Pair (tp)		*Microwave
*Coaxial Cable (coax)		*Infrared, O Radio

Twisted Pairs (tp)

They are used by telephones for the local loop (connection between your home phone and the local telephone exchange). They carry electrical signals. A tp consists of two insulated copper wires (1mm diameter) twisted to reduce electrical interference.

 Capacity: dependent on the distances involved but can be upto several Mbps over a few Kms. For example, ISDN (Integrated Services Digital Network) lines offer speeds from 64Kbps to over 1Mbps and have been available to home users for internet access, for several years. More recently (2003), DSL (Digital Subscriber Line) and in particular ADSL (Asymmetric DSL) lines are available to home users with speeds of 1.5 to 6Mbps. ISDN and ADSL both use digital transmission and so must use a digital line unlike the standard analog telephone line where a modem is used. You must install an ISDN card or an ADSL card into your PC to use an ISDN or ADSL line.

TPs may be shielded (stp) or Unshielded (utp) with the shielded having extra insulation. However, it is the rate of twisting (number of twists per inch) that is the most important characteristic. They are also classified into category-5 (CAT-5) and category (CAT-6).

CAT-5 can carry 10 or 100 Mbps (10/100Mbps) over short distances e.g. upto 100 metres approx. The communications standard used in this case is called 10/100-Base-T.

This is the type of cable that is often used in building to connect PCs to a LAN. Usually, the CAT-5 cable connects to a device know as a hub which is less than 100 metres from each PC. There may be a hub for each floor/laboratory in a building.

CAT-6 cable operates at 100/1000mbps (Gigabit Ethernet) and is typically used to interconnect hubs. It is more expensive than CAT-5 cable.

Coaxial Bale (coax)

Carry electrical signals. It consists of a copper core surrounded by 3 outer layers of insulation. It has a high bandwidth and good noise immunity.

The original Ethernet standard was based on 10Mbps coaxial cable. Ethernet is the most popular LAN standard and was developed at Rank Xerox (who also developed the mouse, laser print and graphical user interface (GUI) software. Ethernet LANs can be based on tp, coax or optic fibre.

Optic Fibre

Uses light to carry data and has a huge bandwidth. Very thin glass fibres used. To date capacity 1000Mbps over 1 Km is feasible.

It is used in WANs, LANs for interconnecting hubs and also for linking telephone exchanges. Excellent noise immunity as it does not suffer from electrical interference and is therefore suitable for harsh environments such as factory floor.

Line of Sight: infrared and Microwave

Physical cables have a major problem if you have to cross private or public property where it may be difficult or very expensive to get permission, in addition to the costs of laying the cable. Using line of sight transmitters avoids this problem.

Microwave can be used over long distances e.g. a 100m tower can transmit data for distance over 100km. cheaper than digging a trench. Relatively high speeds of 10 Mbps upwards are possible.

Satellite: operate in same fashion as microwaves where the satellite operates as a ‘Big microwave repeater in the Sky’!! satellite communication has a high bandwidth giving up 50 Mbps speed and a given satellite may be able to have many “channels” at this speed.

Wireless: Radio LANs or wireless (wi-fi) LANs are becoming common in offices, universities, hotels, restaurants and airports. A wireless LAN enables users to connect to the internet from a laptop computer with a wireless network card. In UCD, commerce students use such laptops with wireless cards to connect to the college network, for course work and email.