Computers in the real world Objectives Explain the need for secondary storage devices Understand the three main storage types – Optical – Magnetic – Solid state Learn the characteristics of secondary storage

Computers in the real world Secondary storage It is very common to save data externally from the main PC. There are many reasons we may have to do this - Transport data from one PC to another Backup or archive data Distribution of programs or data Submission of work to teachers or exam boards! Secondary storage devices allow permanent storage. They are volatile. Different devices will have different characteristics.

Computers in the real world Optical storage Optical storage devices will use lasers to store / read data from a disc. Optical medium tends to be used for multimedia data and to allow games / software to be installed. The image to the left shows what the surface of a CD looks like under a microscope. It has a series of holes and spaces. The white blobs are little holes in the surface! A laser will travel over the surface of the CD. If it hits a space it will reflect into a reader. If it hits a hole it will “diffuse” and err not hit the reader! This effectively allows us to store 1 or 0!

Computers in the real world Optical storage The video below shows how reading a CD works.

Computers in the real world CD-R / CD-RW CD –R stands for Compact Disc Writeable (yes I know it starts with a W!) CD – RW stands for Compact Disc Re-Writeable. Capacity – 700 Mb Speed – Up to 68.8 Mbit/s Portability – Highly portable but could be READ ONLY. Durability – CD’s are fairly durable but they are vulnerable to scratches and sunlight. Reliability – CD-RW have a limited number of times they can be written to. CD’s sometimes do not write correctly (known as a burn as it burns holes into the surface) which will destroy the disk, Main uses – Store music and software to be installed on a PC. Software tends is now coming on DVD more often than CD.

Computers in the real world DVD DVD stands for digital versatile disk (not video!) Capacity – 4.7 Gb Speed – Up to Mbit/s Portability – Highly portable but could be READ ONLY. Durability – DVD’s are fairly durable but they are vulnerable to scratches and sunlight. They are more vulnerable that CD’s Reliability – DVD-RW have a limited number of times they can be written to. DVD’s sometimes do not write correctly (known as a burn as it burns holes into the surface) which will destroy the disk. Main uses – DVD is used more video and software install. Xbox 260 games come on DVD.

Computers in the real world Bluray Bluray is the new HD video format. It is called bluray because the laser is blue (which is due to the wave length not making it pretty!) Capacity – 50 Gb Speed – Faster than DVD / CD! 72 Mb/s Portability – Highly portable but could be READ ONLY. Durability – Blurays are very vulnerable to scratches. Reliability – Blurays share the same problems with reliability as DVD and CD. There was a point where blurays were going to be shipped in protective cases like PSP disks! Main uses – Bluray is mainly used for HD movies but is also used for games on the PS3.

Computers in the real world Magnetic storage Magnetic storage uses special disks which are held inside a protective case. There could be more than one disk inside the case. Each disk is known as a platter. Data is saved using a small magnetic charge. A read/write read passes over the platter either setting a magnetic charge or removing it. When reading it will detect is a charge exists or not. The presence of a charge allows the reader to differentiate the difference between a 1 or a 0.

Computers in the real world Magnetic storage The video below shows how reading a HDD works

Computers in the real world External hard drives These come in many different shapes, sizes and capacities. Some are rugged (for durability) while some are designed for large backups. Capacity – can be up to and over 2TB Speed – Very fast compared to optical! Portability – Highly portable. May require an external power source. Durability – Some are highly durable and have special protective cases. Hard drives tend to suffer from vibrations or magnetic interference. The platters are very sensitive. Reliability – They will decay over time in the same way hard drives do. Main uses – Backup of large amounts of data. Tends not do be used to transport data as much as memory sticks but can still be used for this.

Computers in the real world Magnetic tape Looks a bit like the old video cassettes! It stores data magnetically onto a black tape. Capacity – Depends on number of tapes and type of tape. In the region of 20GB – 3 TB! Speed – Very slow. The tape is mechanical so has to be turned to the correct position which is slow. Portability – The tapes themselves tend to stored away from the main server. Durability – Susceptible to magnetic damage. Reliability – They will decay over time. Main uses – Backup of corporate data. Each cassette can store large amounts of data and multiple cassettes would be involved in a single backup.

Computers in the real world Solid state Solid state or flash memory is a special type of RAM which can retain its state once power has been disconnected. It is known as EEPROM or Electronically erasable programmable read only memory The image below shows what the inside of a memory stick looks like. Memory sticks use the USB interface to connect to the computer. Power is supplied to the device via USB and this is used to erase and set (program) data on the device. Once power is removed it acts as read only and the data is not changed.

Computers in the real world Memory sticks They can come in many shapes and sizes but the most common format is shown in the image below. Capacity – Can store between 1GB to well over 1 TB. The size of the device does NOT reflect the capacity. Speed – Very fast access time. Is faster than CD to read and write data to. Portability – Highly portable. Is the defacto standard for transferring files. Durability – Highly durable. These things can survive washing machines! They have NO moving parts which makes them very durable. Reliability – They have a limited number of times they can be written to. They are very reliable and data will not decay over time. Main uses – Transfer of files from one PC to another

Computers in the real world Solid state in devices Solid state memory is found in devices such as the Ipod nano, Iphone, mobiles in general, SD cards and other types of memory card. Internal memory in cameras also use solid state.

Computers in the real world Solid state in hard drives Solid state drive (SSD) acts like a normal hard drive but instead of using platters it uses flash memory to store data. Due to the lack of physical motion they can access data much faster than a standard hard drive. They are not in wide spread use yet due to their cost and the fact that each memory block has a limited number of times it can be written to. This means that if the drives are written to a lot then they will have a very limited life span.

Computers in the real world Activity Make notes on all the devices mentioned in this power point in your own revision guide. Then answer the questions found on this worksheet