GSCE Computer Science

GSCE Computer Science

1. UNITS

   • *BIT - Binary Digit 0 or 1. *NIBBLE - 4 Bits/Half a Byte. *BYTE - 8 Bits(Can Represent 1 Character in ASCII). *KILOBYTE(KB) - 1024 Bytes (Can Be 1,000 Bytes). *MEGABYTE(MB) - 1024 Kilobytes(Can Be 1,000,000  Bytes). *GIGABYTES(GB) - 1024 Megabytes(Can Be 1,000,000,000 Bytes). *TERABYTE(TB) - 1024 Gigabytes(Can Be 1,000,000,000,000). *PETABYTE(PB) - 1024 Terabytes(Can Be 1,000,000,000,000,000 Bytes).
   • Numbers
     • Converting binary and denary
       • CONVERT BINARY NUMBER TO DENARY USING  A TABLE 128 | 64 | 32 | 16 | 8 | 4 | 2 | 1 ----------------------------------------  0   |  0  |  1  |  1  | 0 | 1 | 1 | 0 Simply Add The Column Numbers Where There Is A 1. 32+16+4+2=54. THIS IS ONE BYTE.
       • CONVERTING DENARY NUMBER TO BINARY USING A TABLE. (using the table on slide 1) 54:  54 –  128  = can’t be done so record  0 54 –  64   = can’t be done so record  0 54 –  32   = 22 so record  1 22 –  16   = 6 so record  1 6 –  8  = can’t be done so record  0 6 –  4  = 2 so record  1 2 –  2  = 0 so record  1 0 –  1  = can’t be done so record  0
     • Adding binary numbers
       • We Use Column Addition And Carry The Extra Digit: 0 + 0 = 0 0 + 1 = 1 1 + 1 = 1 0 1 + 1 + 1 = 1 1 (1+1=2 which in binary we write as 10. 1+1+1=3 which in binary we write as 11).
   • ARITHMETIC OVERFLOW
     • When There Are Insufficient Bits To Store The Result From Addition Due To A Final Carry Bit
     • OVERFLOW ERROR
       • When We Add Two Binary Number Together There Is A Chance Of OVERFLOW. This Is Where We Attempt To Store a 9 Bit Number In Memory Allocated For 8 Bits. If We Have A Calculation That Returns 256 Or More We Will Get A 9 Bit Answer. This Is An Overflow Error. We Have To Ignore The Overflow To Give An 8 Bit Answer. This Can Lead To The Loss Of Data.

2. Protocols and layers

3. Wired and Wireless Networks, Protocols, Layers

   • Modes of connection - Wired and Wireless
     • Wired
       • Ethernet
         • •Ethernet is a standard that has been around for many years. •It has evolved as network topologies have evolved. •Here is an old-style bus network. •A shared backbone cable uses a protocol called CSMA/CD to listen for acommunication before transmitting and to detect when two computers aretransmitting at the same time.  
         • Ethernet and star network topologies:  •Here is a more modern star network.  •With twisted pair cables, duplex communication is possible – sending and receiving data at the same time.  •With this kind of network, the CSMA/CD protocol is not required because frames do not collide.   
     • Wireless
   • Wireless Encryption
     • Wireless networks are identified with a unique Service Set Identifier (SSID)
       • The SSID must be used by all devices that want to connect to the network and can be set to automatically broadcast to any device in range of a WAP.
       • Protecting your wireless network
         • > The SSID is set automatically, but you can change it if you wish  > It can also be hidden to make it harder to detect  > Additionally, it can be secured with a password
     • Encryption
       • Wireless networks broadcast data that must be encrypted in order to be secure --- this is done by scrambling the data to ciphertext using a master key created from the network's SSID and password
         • Data is decrypted by the receiver using the same master key -- the key is not transmitted
           •    Wireless encryption protocols include: WEP, WPA and WPA2. A handshake protocol is used to ensure the receiver has a valid master key before transmission begins.