What is BUS...?

Bus

*)     Bus is nothing but the cables, printed circuits, etc.


*)     A bus, in computing, is a set of physical connections which can be shared by multiple hardware components in order to communicate with one another. 


*)   The purpose of buses is to reduce the number of "pathways" needed for communication between the components, by carrying out all communications over a single data channel. This is why the metaphor of a "data highway" is sometimes used.

Characteristics of a bus

A bus is characterised by the amount of information that can be transmitted at once.

This amount, expressed in bits, corresponds to the number of physical lines over which data is sent simultaneously.

A 32-wire ribbon cable can transmit 32 bits in parallel.

The term "width" is used to refer to the number of bits that a bus can transmit at once. 

The bus speed is also defined by its frequency (expressed in Hertz), the number of data packets sent or received per second.

Each time that data is sent or received is called a cycle. 

This way, it is possible to find the maximum transfer speed of the bus, the amount of data which it can transport per unit of time, by multiplying its width by its frequency.

A bus with a width of 16 bits and a frequency of 133 MHz, therefore, has a transfer speed equal to: 

 

16 * 133.106 = 2128*106 bit/s,
or 2128*106/8 = 266*106 bytes/s
or 266*106 /1000 = 266*103 KB/s
or 259.7*103 /1000 = 266 MB/s

Bus sub assembly

• The address bus (sometimes called the memory bus) transports memory addresses which the processor wants to access in order to read or write data. It is a unidirectional bus.
• The data bus transfers instructions coming from or going to the processor. It is a bidirectional bus.
• The control bus (or command bus) transports orders and synchonisation signals coming from the control unit and travelling to all other hardware components. It is a bidirectional bus, as it also transmits response signals from the hardware.
  

 

what is port...?

*)    An interface on a computer to which you connect a device.

*) Internally there are several port for connecting                                 disk drive,display screen and key board.

*)  Externally personal computer have port for modem, printer and peripheral devices.

*) Almost all personal computers come with a serial RS-232C  port or RS-422  port for connecting a modem or mouse and a parallel port  for connecting a printer. 

*) The parallel port uses a 25-pin connector (type DB-25) and is used to connect printers.

Transmission Control Protocol (TCP)

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TCP Header

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Bit offset	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29	30	31

 ------------------------------------------------------------------------------------------------------

0

Source       port

Destination     port

 ------------------------------------------------------------------------------------------------------

     32               Sequence number 

 ------------------------------------------------------------------------------------------------------

     64                Acknowledgment number 

 ------------------------------------------------------------------------------------------------------

96   Data offset

Reserved

C W R

E C E

U R G

A C K

P S H

R S T

S Y N

F I N

window Size

 ------------------------------------------------------------------------------------------------------

128

Urgent pointer

 ------------------------------------------------------------------------------------------------------

  160             options(if data offset>5)

 

 ------------------------------------------------------------------------------------------------------

Tcp  is on of the core protocal for all protocal suite.

TCP operates at a higher level, concerned only with the two end systems.

TCP segment structure

• Source port (16 bits) – identifies the sending port
• Destination port (16 bits) – identifies the receiving port
• Sequence number (32 bits) – has a dual role

• If the SYN flag is set, then this is the initial sequence number. The sequence number of the actual first data byte (and the acknowledged number in the corresponding ACK) will then be this sequence number plus 1.
• If the SYN flag is clear, then this is the sequence number of the first data byte

• Acknowledgment number (32 bits) – if the ACK flag is set then the value of this field is the next sequence number that the receiver is expecting. This acknowledges receipt of all prior bytes (if any). The first ACK sent by each end acknowledges the other end's initial sequence number itself, but no data.
• Data offset (4 bits) – specifies the size of the TCP header in 32-bit words. The minimum size header is 5 words and the maximum is 15 words thus giving the minimum size of 20 bytes and maximum of 60 bytes, allowing for up to 40 bytes of options in the header. This field gets its name from the fact that it is also the offset from the start of the TCP segment to the actual data.
• Reserved (4 bits) – for future use and should be set to zero
• Flags (8 bits) (aka Control bits) – contains 8 1-bit flags

CWR (1 bit) – Congestion Window Reduced (CWR) flag is set by the sending host to indicate that it received a TCP segment with the ECE flag set and had responded in congestion control mechanism

• ECE (1 bit) – ECN-Echo indicates

• If the SYN flag is set, that the TCP peer is ECN capable.
• If the SYN flag is clear, that a packet with Congestion Experienced flag in IP header set is received during normal transmission

• (1 bit) – indicates that the Urgent pointer field is significant
• ACK (1 bit) – indicates that the Acknowledgment field is significant. All packets after the initial SYN packet sent by the client should have this flag set.
• PSH (1 bit) – Push function
• RST (1 bit) – Reset the connection
• SYN (1 bit) – Synchronize sequence numbers. Only the first packet sent from each end should have this flag set. Some other flags change meaning based on this flag, and some are only valid for when it is set, and others when it is clear.
• FIN (1 bit) – No more data from sender

Window (16 bits) – the size of the receive window, which specifies the number of bytes (beyond the sequence number in the acknowledgment field) that the receiver is currently willing to receive

 

• Checksum (16 bits) – The 16-bit checksum field is used for error-checking of the header and data
•  
• Urgent pointer (16 bits) – if the URG flag is set, then this 16-bit field is an offset from the sequence number indicating the last urgent data byte
•  
• Options (Variable 0-320 bits, divisible by 32) – The length of this field is determined by the data offset field. Options 0 and 1 are a single byte (8 bits) in length. The remaining options indicate the total length of the option (expressed in bytes) in the second byte. Some options may only be sent when SYN is set; they are indicated below as .
•  

• 0 (8 bits) - End of options list
•  
• 1 (8 bits) - No operation (NOP, Padding) This may be used to align option fields on 32-bit boundaries for better performance.
•  
• 2,4,SS (32 bits) - Maximum segment size 
• 
  
• 3,3,S (24 bits) - Window scale
•  
• 4,2 (16 bits) - Selective Acknowledgement supported. 
•  
• 5,N,BBBB,EEEE,... (variable bits, N is either 10, 18, 26, or 34)- Selective ACKnowlegement (SACK) These first two bytes are followed by a list of 1-4 blocks being selectively acknowledged, specified as 32-bit begin/end pointers.
•  
• 8,10,TTTT,EEEE (80 bits)- Timestamp and echo of previous timestamp 
• 
  
• 14,3,S (24 bits) - TCP Alternate Checksum Request. 
• 
  
• 15,N,... (variable bits) - TCP Alternate Checksum Data. 

 

Protocol operation

 

• LISTEN : In case of a server, waiting for a connection request from any remote client. 
•  
• SYN-SENT : waiting for the remote peer to send back a TCP segment with the SYN and ACK flags set. (usually set by TCP clients)
•  
• SYN-RECEIVED : waiting for the remote peer to send back an acknowledgment after having sent back a connection acknowledgment to the remote peer. (usually set by TCP servers)
•  
• ESTABLISHED : the port is ready to receive/send data from/to the remote peer.
•  
• FIN-WAIT-1
•  
• FIN-WAIT-2
•  
• CLOSE-WAIT
•  
• CLOSING
•  
• LAST-ACK
• TIME-WAIT : represents waiting for enough time to pass to be sure the remote peer received the acknowledgment of its connection termination request. According to RFC 793 a connection can stay in TIME-WAIT for a maximum of four minutes.
•  
• CLOSED

What is SPAM..?

* E- mail  spam  is  also  known as  Junk-Mail . It is a subset of spam that involves nearly identical message to the recipiants by email.

* A common synonym for spam is unsolicited bulk e-mail

Mail Bomb

*  e-mail bomb is a form of net abuse consisting of sending huge volumes of e-mail to an address in an attempt to overflow the mailbox or overwhelm the server where the email address is hosted in a denial of service attack.

There are two types are there,

1)  Mass mailing

2) list linking

MASS  MAILING :

Mass mailing consists of sending numerous duplicate mails to the same email address.

LIST LINKING :

List linking means signing a particular email address up to several email list subscriptions. The victim then has to unsubscribe from these unwanted services manually.

Hacking

 

HACKING :

     Hacking is unauthorized use of computer and network resources. 

HACKER: 

a hacker is a person who breaks into computers, usually by gaining access to administrative controls

Hacker attitudes :

 

 

 BLACK HAT:

 he breaks computer security without authorization or uses technology (usually a computer, phone system or network) for vandalism, credit card fraud, identity theft, piracy, or other types of illegal activity.

 WHITE HAT: 

A white hat hacker breaks security for non-malicious reasons, for instance testing their own security system. 

This type of hacker enjoys learning and working with computer systems, and consequently gains a deeper understanding of the subject.

Such people normally go on to use their hacking skills in legitimate ways.

 GREY HAT:

A grey hatted hacker is a hacker of ambiguous ethics and/or borderline legality, often frankly admitted.

 Script kiddie :

A script kiddie is a non-expert who breaks into computer systems by using pre-packaged automated tools written by others, usually with little understanding. These are the outcasts of the hacker community.

 Hacktivist

A hacktivist is a hacker who utilizes technology to announce a social, ideological, religious, or political message. 

In general, most hacktivism involves website defacement or denial-of-service attacks.

 Techniques used by computer criminals and security experts.

Vulnerability scanner :

A vulnerability scanner is a tool used to quickly check computers on a network for known weaknesses. 

Hackers also commonly use port scanner.

These check to see which ports on a specified computer are "open" or available to access the computer, and sometimes will detect what program or service is listening on that port, and its version number.

Password cracking:

 Password cracking is the process of recovering password from data that has been stored in or transmitted by a computer system.

A common approach is to repeatedly try guesses for the password.

Packet sniffer :

A packet analyzer (also known as a network analyzer, protocol analyzer or sniffer, or for particular types of networks an Ethernet sniffer or wireless sniffer) is computer software or computer hardware that can intercept and log traffic passing over a digital network or part of a network. As data stream flow across the network, the sniffer captures each packet and ,if needed, decodes and analyzes its content according to the appropriate RFC or other specifications.

 Spoofing attack:

 spoofing attack is a situation in which one person or program successfully masquerades as another by falsifying data and thereby gaining an illegitimate advantage.

Trojan horse :

A Trojan horse is a program which seems to be doing one thing, but is actually doing another. A trojan horse can be used to set up a back door in a computer system such that the intruder can gain access later.

Virus

 A virus is a self-replicating program that spreads by inserting copies of itself into other executable code or documents. Therefore, a computer virus behaves in a way similar to a biological virus  which spreads by inserting itself into living cells.

 

Worm 

Like a virus, a worm is also a self-replicating program. A worm differs from a virus in that it propagates through computer networks without user intervention

Windows Shortcuts

 

    *

      Shift + F10 right-clicks.
    *

      Win + L (XP Only): Locks keyboard. Similar to Lock Workstation.
    *

      Win + F or F3: Open Find dialog. (All Files) F3 may not work in some applications which use F3 for their own find dialogs.
    *

      Win + Control + F: Open Find dialog. (Computers)
    *

      Win + U: Open Utility Manager.
    *

      Win + F1: Open Windows help.
    *

      Win + Pause: Open System Properties dialog.
    *

      Win + Tab: Cycle through taskbar buttons. Enter clicks, AppsKey or Shift + F10 right-clicks.
    *

      Win + Shift + Tab: Cycle through taskbar buttons in reverse.
    *

      Alt + Tab: Display Cool Switch. More commonly known as the AltTab dialog.
    *

      Alt + Shift + Tab: Display Cool Switch; go in reverse.
    *

      Alt + Escape: Send active window to the bottom of the z-order.
    *

      Alt + Shift + Escape: Activate the window at the bottom of the z-order.
    *

      Alt + F4: Close active window; or, if all windows are closed, open shutdown dialog.
    *

      Shift while a CD is loading: Bypass AutoPlay.
    *
    
      Ctrl + Alt + Delete or Ctrl + Alt + NumpadDel (Both NumLock states): Invoke the Task Manager or NT Security dialog.
    *

      Ctrl + Shift + Escape (2000/XP ) or (Ctrl + Alt + NumpadDot) : Invoke the task manager. On earlier OSes, acts like Ctrl + Escape.
    *

      Print screen: Copy screenshot of current screen to clipboard.
    *

      Alt + Print screen: Copy screenshot of current active window to clipboard.
    *

      Ctrl + Alt + Down Arrow: Invert screen. Untested on OS's other than XP.
    *

      Ctrl + Alt + Up Arrow: Undo inversion.
    *

      Win + B : Move focus to systray icons.
    *

      Ctrl + C or Ctrl + Insert: Copy.
    *

      Ctrl + X or Shift + Delete: Cut.
    *

      Ctrl + V or Shift + Insert: Paste/Move.
    *

      Ctrl + N: New... File, Tab, Entry, etc.
    *

      Ctrl + S: Save.
    *

      Ctrl + O: Open...
    *

      Ctrl + P: Print.
    *

      Ctrl + Z: Undo.
    *

      Ctrl + A: Select all.
    *

      Ctrl + F: Find...
    *

      Ctrl+W : to close the current window
    *

      Ctrl + F4: Close tab or child window.
    *

      F1: Open help.
    *

      F11: Toggle full screen mode.
    *

      Alt or F10: Activate menu bar.
    *

      Alt + Space: Display system menu. Same as clicking the icon on the titlebar.
    *

      Escape: Remove focus from current control/menu, or close dialog box.
    *

      Tab: Forward one item.
    *

      Shift + Tab: Backward one item.
    *

      Ctrl + Tab: Cycle through tabs/child windows.
    *

      Ctrl + Shift + Tab: Cycle backwards through tabs/child windows.
    *

      Enter: If a button's selected, click it, otherwise, click default button.
    *

      Space: Toggle items such as radio buttons or checkboxes.
    *

      Alt + (Letter): Activate item corresponding to (Letter). (Letter) is the underlined letter on the item's name.
    *

      Ctrl + Left: Move cursor to the beginning of previous word.
    *

      Ctrl + Right: Move cursor to the beginning of next word.
    *

      Ctrl + Up: Move cursor to beginning of previous paragraph. This and all subsequent Up/Down hotkeys in this section have only been known to work in    Rich Edit controls.
    *

      Ctrl + Down: Move cursor to beginning of next paragraph.
    *

      Shift + Left: Highlight one character to the left.
    *

      Shift + Right: Highlight one character to the right.
    *

      Shift + Up: Highlight from current cursor position, to one line up.
    *

      Shift + Down: Highlight from current cursor position, to one line down.
    *

      Ctrl + Shift + Left: Highlight to beginning of previous word.
    *

      Ctrl + Shift + Right: Highlight to beginning of next word.
    *

      Ctrl + Shift + Up: Highlight to beginning of previous paragraph.
    *

      Ctrl + Shift + Down: Highlight to beginning of next paragraph.
    *

      Home: Move cursor to top of a scrollable control.
    *

      End: Move cursor to bottom of a scrollable control.
    *

      Arrow Keys: Navigate.
    *

      Shift + Arrow Keys: Select multiple items.
    *

      Ctrl + Arrow Keys: Change focus without changing selection. "Focus" is the object that will run on Enter. Space toggles selection of the focused item.
    *

      (Letter): Select first found item that begins with (Letter).
    *

      BackSpace: Go up one level to the parent directory.
    *

      Alt + Left: Go back one folder.
    *

      Alt + Right: Go forward one folder.
    *

      Enter: Activate (Double-click) selected item(s).
    *

      Alt + Enter: View properties for selected item.
    *

      F2: Rename selected item(s).
    *

      Ctrl + NumpadPlus: In a Details view, resizes all columns to fit the longest item in each one.
    *

      Delete: Delete selected item(s).
    *

      Shift + Delete: Delete selected item(s); bypass Recycle Bin.
    *

      Ctrl while dragging item(s): Copy.
    *

      Ctrl + Shift while dragging item(s): Create shortcut(s).
    *

      In tree pane, if any:
    *

      Left: Collapse the current selection if expanded, or select the parent folder.
    *

      Right: Expand the current selection if collapsed, or select the first subfolder.
    *

      Numpad Asterisk: Expand currently selected directory and all subdirectories. No undo.
    *

      Numpad Plus: Expand currently selected directory.
    *

      Numpad Minus: Collapse currently selected directory.


    *

      Switch FilterKeys on and off. Right SHIFT for eight seconds
    *

      Switch High Contrast on and off. Left ALT +left SHIFT +PRINT SCREEN
    *

      Switch MouseKeys on and off. Left ALT +left SHIFT +NUM LOCK
    *

      Switch StickyKeys on and off. SHIFT five times
    *

      Switch ToggleKeys on and off. NUM LOCK for five seconds