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              Transmission media  In data communication terminology, a transmission medium is a physical path between the transmitter and the receiver i.e it is the channel through which data is sent from one place to another. Transmission Media is broadly classified into the following types: 1. Guided Media: It is also referred to as Wired or Bounded transmission media. Signals being transmitted are directed and confined in a narrow pathway by using physical links. Features: High Speed Secure Used for comparatively shorter distances There are 3 major types of Guided Media: (i) Twisted Pair Cable – It consists of 2 separately insulated conductor wires wound about each other. Generally, several such pairs are bundled together in a protective sheath. They are the most widely used Transmission Media. Twisted Pair is of two types: 1. Shielded twisted pair  2. Unshielded twisted pair. (ii) Coaxial Cable – It has an outer plastic covering containing 2 parallel

            Device management The main function of the device manager are :- 1. Monitor the status of all devices , including storage drives , printer and other peripheral. 2. Enforce pre-set policies on which process gets which device for how long. 3. Deal with the allocation of devices to processes. 4. Deal with the de- allocation of devices to processes ,both at a temporary basis (e.g. when the processes is interrupted) and on a permanent basis(e.g. when the processes is completed). Example of devices. 1. Keyboard 2. Mouse  3. Printer  4. Monitor 5. Scanner Etc. There are three main type of devices:- 1. dedicated devices 2. Shared devices 3. Virtual devices Dedicated device :- These are devices that are assigned to one process at a time, and the processes only release once it is completed. This make sense for devices like plotter and tape drives. The problem with this is that it means only one user using it at a time, and it migh

           Principal of I/o software I/O software is often organized in the following layers − User Level Libraries  − This provides simple interface to the user program to perform input and output. For example,  stdio  is a library provided by C and C++ programming languages. Kernel Level Modules  − This provides device driver to interact with the device controller and device independent I/O modules used by the device drivers. Hardware  − This layer includes actual hardware and hardware controller which interact with the device drivers and makes hardware alive. A key concept in the design of I/O software is that it should be device independent where it should be possible to write programs that can access any I/O device without having to specify the device in advance. For example, a program that reads a file as input should be able to read a file on a floppy disk, on a hard disk, or on a CD-ROM, without having to modify the program for each different device. Device

Principal of I/o hardware I/O Devices I/O devices can be roughly divided into two categories: block devices and character devices. Block Devices It stores information in fixed-size blocks, each one with its own address. Common block sizes range from 512 to 65,536 bytes. Examples are hard disks, Blu-ray discs, and USB sticks. Character Device It delivers or accepts a stream of characters, without regard to any block structure. Not addressable and does not have any seek operation. Examples are printers, network interfaces, mice, and most other devices that are not disk-like can be seen as character devices. Doesn't Really Fit Some devices don't fit into this division: For instance, clocks aren't block addressable, nor do they accept character streams. All they do is cause interrupts... at timed intervals. Memory-mapped screens do not fit this division either

Protection mechanism Authentication Authentication refers to identifying each user of the system and associating the executing programs with those users. It is the responsibility of the Operating System to create a protection system which ensures that a user who is running a particular program is authentic. Operating Systems generally identifies/authenticates users using following three ways − Username / Password  − User need to enter a registered username and password with Operating system to login into the system. User card/key  − User need to punch card in card slot, or enter key generated by key generator in option provided by operating system to login into the system. User attribute - fingerprint/ eye retina pattern/ signature  − User need to pass his/her attribute via designated input device used by operating system to login into the system. One Time passwords One-time passwords provide additional security along with normal authentication. In One-Time Pass

            Security issues  - The os security issues is critical issue. - Each processes determines whether it has control of a system resource exclusively or whether it is isolated from the other processes or whether it shares resource common to set of processes. - the os then configures when a resource is isolated from one process and a resource is shared with a defined set of processes. - The os should also have the flexibility to change this configuration when needed to full fill the requirement all the processes. - For example , a process has 32 memory blocks at an instance and the os configures the system accordingly. - The os should provide protection mechanism and implement a system administrator(s) - defined security policy. - An application software programmer can find a hole in the protection mechanism and an unauthorized access. Important security issues :- 1. Protection mechanism 2. Flexibility to change. 3. Control resource sharing 

 File system implementation :- Numerous on-disk and in-memory configurations and structures are being used for implementing a file system. These structures differ based on the operating system and the file system but applying some general principles. Here they are portrayed below: A boot control block usually contains the information required by the system for booting an operating system from that volume. When the disks do not contain any operating system, this block can be treated as empty. This is typically the first chunk of a volume. In UFS, this is termed as the boot block; in NTFS, it is the partition boot sector. A volume control block holds volume or the partition details, such as the number of blocks in the partition, size of the blocks or chunks, free-block count along with free-block pointers. In UFS, it is termed as superblock; in NTFS, it is stored in the master file table. A directory structure per file system is required for organizing the files. In UFS, it held