computer science paper 1

computer science paper 1

1. PRIMARY STORAGE

   • volatile- temporary memory loses contents when power is turned off
   • CPU has easy and quick access to
   • CPU registers, cache, ROM, RAM
   • fast read/write speeds

2. SECONDARY STORAGE

   • non-volatile
   • data stored when not in use
     • operating systems, applications, user files, etc
   • HDDs, SSDs, CDs, SD cards
     • HDDs: stack of magnetised metal disks. data stored magnetically, in small areas (sectors) within circular tracks. Read/write heads on moving arm access sectors on disk. Portable HDDs used for transporting and backing up large amounts of data.
       • longlasting, reliable (despite moving parts), damaged by large impacts
       • HDD ADVANTAGES- cheaper, higher capacity, longer read/write life, SSDs can only be written a certain no. of times before begin to deteriorate
     • SSDs: no moving parts, use flash memory (non-volatile memory, stores data in electrical circuits by trapping electrons), uses for internal storage (same as HDDs). Much faster read/write speeds than HDDs, much quicker time for booting up and opening programs and files.portable SSDs used for same as portable HDDs
       • More flash storage: USB pen drives, memory cards (SD cards), much slower and shorter read/write life than SSDs, used to expand storage capacity - capacity high relative to size.
       • SDD ADVANTAGES- faster, don't need defragmenting, more shock- proof, silent
     • optical disks- CDs, DVDs, Blu-Ray
       • CDs- 700MB
         • DVDs- 4.7GB
           • Blu-Rays- 25GB
       • 3 forms- read only, write-once, rewriteable
       • Use declining due to internet- streaming + downloading, they're cheap, portable, not damaged by water/shocks, easily scratched
       • data stored as microscopic indentations on shiny surface, data read by shining laser beam on surface and detecting changes in position of reflected beam
     • Magnetic tapes are used for archiving
       • greater storage capacity than HDDs, very cheap, store huge amounts of data, used by large organisations, require tape-drive for read/writing, read/written sequentially- read/written from beginning to end or until stopped- very slow when finding specific data, but fast once in right place
   • slow read/write speeds
   • optical disc memory card magnetic tape HDD SDD SLOWEST FASTEST
     • magnetic tape optical disc HDD memory card SSD CHEAPEST PRICIEST
       • optical disc memory card SSD HDD Magnetic tape LOWEST CAPACITY HIGHEST CAPACITY

3. BACKUP

   • Full backup
     • all files and folders backed up entirely
     • ADVANTAGES- fast, easy recovery as complete data readily available, files + folders backed up to one back up set, easy veresion control
     • DISADVANTAGES- more storage space needed, additional bandwidth required, time consuming
   • incremental backup
     • backs up all files changed since last back up , whether backup full or incremental
     • ADVANTAGES- fast back up windows as there's less data, less storage space needed, allow retention of several versions of same file
     • DISADVANTAGES - slower recovery - all incrememnts must be recovered, initial backup needed, longer to restore specific file as need to search more than one backup set, if one backup fail then recovery will be incomplete

4. PACKET SWITCHING

   • 1. sending device splits data into packets to be sent across network, each packet has header - contains control info, includes packet's destination address, source address and packet number to show order of data. Each packet has payload - part of actual data wanted. Each packet has a checksum number - form of validation used to check payload data hasn't been corrupted during transit, sending and receiving devices both calculate checksum value on payload data, if values match data has been received correctly.
     • 2. Each router reads packet header and decides which way to send the packet next according to IP rules
       • 3. the way the data is sent changes depending on network traffic so the packets can take different routes, if router receives too many at once it may prioritise some over others.
         • 4. as packets take different routes they can arrive in wrong order. Receiving device uses packet numbers to reassemble them in right order
           • 5. sometimes packets go missing in transit so receiving device checks periodically that all packets have been received, if it hasn't received them in a certain time it sends a timeout message back to sending device.
             • 6. if all data is received and the checksums match, a receipt is sent to the sending device.