Small Office/Home Office (SOHO) Networking

SOHO 1

Small Office/Home Office (SOHO) Networking

1. The representation of numeric data.

1.1Number systems

(a)Denary System

Numbers are made up of ten digits: 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9

The value represented by each digit depends on the digit itself as well as its position relative to the other digits.

The value represented by the number is the sum of all the values represented by the digits, taking into account of their positions.

e.g.486(10) = 4 x 102 + 8 x 101 + 6 x 100

(b)Binary System

Numbers are made up of ____ digits: ______.

e.g.10110(2) =

(c)Hexadecimal System

Numbers are made up of ____ digits:

0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A(10), B(11), C(12), D(13), E(14), F(15)

e.g.A2C(16) =

Note: i)The number of digits used in a number system is called its base.

ii)Although human beings find the denary system convenient, this need not be the case for machines. The coding system for computers is the binary system because computers are made up of two-state devices.

iii)Binary numbers can be easily converted into numbers in the hexadecimal system, which is easier to deal with.

1.2Conversion between number systems

1. Conversion of any number systems into denary.

Method: Multiply each digit with its place value and add up all the products.

e.g.Convert the numbers 111010(2) and 1AB(16) into denary.

1. Conversion from denary into other systems (say base b)

Method:Step 1Divide the denary number by b.

Step 2Keep dividing the resulting quotient by b until it is smaller than b.

Step 3Combine the final quotient with the remainders obtained in step 2 in the correct order.

e.g.Convert 37 into binary.e.g.Convert 1997 into hexadecimal

1. Conversion from binary into hexadecimal

Method:Step 1Grouping the original binary digit in groups of four starting from the rightmost digit.

Step 2Replacing each four-digit binary number by its hexadecimal equivalent.

e.g.Convert 11011000(2) and 101010(2) into hexadecimal.

1. Conversion from hexadecimal into binary

Method:Convert each hexadecimal digit into its four-digit binary equivalent and combine all resulting binary digits in order.

e.g.Convert CD(16) and 3A2B3C(16) into binary.

1.3Addition and subtraction of numbers in any base

1. Addition

Two numbers in any base are added in the same way as denary numbers.

When the sum of two individual digits exceeds the base, a carry to the next position is necessary.

e.g.Calculate 10110(2) + 101111(2) and 486(16) + A9C(16).

1. Subtraction

Numbers in any base are subtracted in the same way as denary numbers.

When a larger digit is subtracted from a smaller digit, a borrow from the next position is necessary.

e.g.Calculate 100011(2) – 11110(2) and 8A2(16) – 4FE(16).

1. Data representation in the computer

2.1Representation of characters

8 bits (1 byte) are used to represent a character. (8 bits have ____ combinations)

______is the most common representation for characters.

(ASCII stands for American Standard Code for Information Interchange)

Originally ____ bits are used in ASCII. (Extended ASCII uses 8 bits)

ASCII coding system includes alphabets, numerals, punctuations and control characters.

A partial Character List for ASCII

Write down the ASCII code for the following characters

Write down the characters represented by the following ASCII code

2.2Representation of Chinese Characters

Chinese characters are NOT made up of ______.

Each Character should be represented by a ______code.

The number of Chinese characters is several ______.

We use ______bits (2 bytes) to represent Chinese characters.

The most commonly used coding system for traditional Chinese characters is ______code and that for simplified Chinese characters is ______code.

______is introduced to standardize all coding systems and to solve the problem of combining all kinds of characters in one document.

Partial Big-5 Code Character List

2.3Representation of Physical Address (MAC address)

An address system is required to uniquely identify computers and interfaces to allow for local delivery of data. A physical address (MAC address) uses 48 bits in length and expressed as 12 hexadecimal digits. The first six hexadecimal digits, which are administered by the IEEE, identify the manufacturer or vendor. This is known as the Organizational Unique Identifier (OUI). The remaining six hexadecimal digits represent the interface serial number or another value administered by the manufacturer. MAC addresses are sometimes referred to as burned-in MAC addresses because they are burned into ROM.

Examples of MAC addresses

2.4Representation of Network Address (IP address)

Addresses assigned to computers on the Internet are 32-bit binary numbers (IPv4). To make it easier to work with these addresses, the 32-bit binary number is broken into a series of decimal numbers. First the binary number is split into four groups of eight binary digits. Then each group of eight bits, or octet, is converted into its decimal equivalent. When written, the complete binary number is represented as four groups of decimal digits separated by periods. This is called dotted decimal notation and provides a compact and easy way to refer to 32-bit addresses.

*IPv4 can only represent ______different addresses. It is not enough for current applications. A new addressing system ______is introduced. It uses ______to represent and it can provide ______different addresses.

Examples of IPv4 addresses

Classification of IPv4

 Reserved IP addresses

Certain host addresses are reserved and cannot be assigned to devices on a network. These reserved host addresses include the following:

Network address – Used to identify the network itself. An IP address that has binary 0s in all host bit positions is reserved for the network address.

Broadcast address – Used for broadcasting packets to all the devices on a network. An IP address that has binary 1s in all host bit positions is reserved for the broadcast address.

Examples of Network address and Broadcast address

Public and private IP addresses

Public IP addresses are unique. They are managed by IANA to ensure no duplications. With the rapid growth of Internet, public IP addresses are not enough. One of the solutions is to use Ipv6. Another solution is to use private IP addresses within the private network so that the hosts in a local network can share a public IP address.

There are three blocks of private IP address:

1. Boolean or Binary logic in computer

Boolean logic is based on digital circuitry that accepts one or two incoming voltages. Based on the input voltages, output voltage is generated. For computers the voltage difference is represented as an ON or OFF state. These two states are associated with a binary 1 or 0.

A)AND operation

AND operation accepts two inputs (say x, y). If the x is ON(1) and y is ON(1), the output (say f) is ON(1). Otherwise, it is OFF (0)

Truth table of AND

B)OR operation

OR operation accepts two inputs (say x, y). If the x is ON(1) or y is ON(1), the output (say f) is ON(1). Otherwise, it is OFF (0)

Truth table of OR

C)NOT operation

NOT operation accepts one input(say x). If the x is ON(1), the output (say f) is OFF(0). Otherwise, it is ON (1)

Truth table of OR

4.IP addresses and Subnet Masks

When IP addresses are assigned to computers, some of the bits on the left side of the 32-bit IP number represent a network. The number of bits designated depends on the address class. The bits left over in the 32-bit IP address identify a particular computer on the network. A computer is referred to as a host. The IP address of a computer consists of a network and a host part.

To inform a computer how the 32-bit IP address has been split, a second 32-bit number called a subnetwork mask is used. This mask is a guide that determines how the IP address is interpreted. It indicates how many of the bits are used to identify the network of the computer. The subnetwork mask sequentially fills in the 1s from the left side of the mask. A subnet mask will always be all 1s until the network address is identified and then it will be all 0s to the end of the mask. The bits in the subnet mask that are 0 identify the computer or host.

An example of a Class A IP address and its subnet mask

A Boolean AND operation is used to extract the network part of an IP address. So the network address of the IP address 00111100 01011100 11110011 00111101 with subnet mask 11111111 00000000 00000000 00000000 is

00111100 01011100 11110011 00111101

AND11111111 00000000 00000000 00000000

Network address reflects which network it belongs. Host can only communicate locally with another host in the same network.

Subnet masks for network of different classes

Can the following hosts communicate with each other locally? Why?

i)24.37.52.3/8 and 24.0.0.2/8ii) 137.6.8.4/16 and 137.9.3.3/16

5.Network basics

A network is a number of computers interconnected in some way.

5.1Why do we need networks?

(a)Resource sharing

(b)Exchange information

(c)Easier software management

(d)Flexibility of working location

5.2Types of networks

(a)Local Area Network (LAN)

LAN connects computers located within a limited area.

(b)Metropolitan Area Network (MAN)

MAN connects computers located within the same city.

(c)Wide Area Network (MAN)

WAN connects computers located in different countries.

5.3Network Devices

(a)Cable

(i)Twisted-pair wires

-cheapest transmission medium

-may easily be influenced by electronic

noise

-For short distance communication

(ii)Coaxial cable

-cheap transmission medium

-reduces electronic noise

(iii)Optical fibre

-an expensive transmission medium

-transmit light, therefore no electronic noise

-for long distance communication

(b)NIC (Network Interface Card)

NIC is an interface card of computer to send and receive messages from the LAN. Each NIC is identified by a unique code called a Media Access Control (MAC) address.

(c)Repeater

A repeater is a network device used to regenerate a signal. Repeaters regenerate signals that are distorted by transmission loss due to attenuation.

(d)Hub

Hub is a device to interconnect computers to form a LAN. As a hub cannot manage data flow, collision may arise frequently. Active hub acts as a multi-port repeater.

(e)Bridge

Bridges provide connections between LANs. They also check data to determine if it should cross the bridge.

(f)Switch

A switch functions as a multi-port bridge. It can transfer data only to the connection that needs it. It can reduce collision.