Lecture Note 3 - Secondary Memory

LECTURE NOTE 3 3. SECONDARY MEMORY Secondary memory addresses the limitations of main memory, which is always too small to store the vast amounts of data that users want. As technology advances, the demand for storage grows, leading to scenarios where substantial information, like a com...

LECTURE NOTE 3 3. SECONDARY MEMORY Secondary memory addresses the limitations of main memory, which is always too small to store the vast amounts of data that users want. As technology advances, the demand for storage grows, leading to scenarios where substantial information, like a comprehensive film archive, must be digitized and stored. This data can reach into the hundreds of terabytes, far exceeding the capacity of main memory. 3. SECONDARY MEMORY Figure 2-18. A five-level memory hierarchy. 3.1 Memory Hierarchies The hierarchy is structured as follows: 1.CPU Registers: These are the fastest and smallest storage, accessed at full CPU speed and typically hold around 128 bytes. 2.Cache Memory: Larger than registers, cache memory ranges from 32 KB to a few megabytes and is accessed slightly slower than registers. 3.Main Memory: This memory can hold tens of megabytes to several gigabytes, with access times in the range of tens of nanoseconds. 3.2 Memory Hierarchies 4. Magnetic Disks: These serve as the primary means for permanent storage, providing several gigabytes to tens of gigabytes of capacity, with access times measured in milliseconds. 5. Magnetic Tape and Optical Disks: Used primarily for archival storage, their access times can take seconds, and their capacity is only limited by budget. 3.3 Memory Hierarchies Access Time: Increases as you go from CPU registers to tape or optical disks. Storage Capacity: Increases downward, from small CPU registers to larger magnetic disks and tapes. Cost per Bit: Decreases down the hierarchy, with main memory costing dollars per megabyte, magnetic disk costing pennies per megabyte, and magnetic tape costing even less. 3.3 Magnetic Disks Figure 2-19. A portion of a disk track. Two sectors are illustrated. 3.3 Magnetic Disks What is a Magnetic Disk? Storage device with platters coated in a magnetizable material. Disk head writes/reads data by magnetizing areas on the surface. Originally large (up to 50 cm); now much smaller (3–12 cm). Track: Circular sequence of bits on the disk. Sector: Each track is divided into sectors (512 bytes of data each). Components: Preamble (for sync), data, ECC (error correction). Figure 2-19: Shows a section of a track with two sectors. 3.3 Magnetic Disks Disk Arm: Moves radially to access different tracks (concentric circles). Zone Bit Recording: Tracks are divided into zones; outer zones have more sectors per track, increasing capacity. Figure 2-21: Shows zones with increasing sectors outward. Manages read/write commands, arm movement, error correction, and remapping of bad sectors. Perpendicular Recording: Newer technique to increase data density by recording vertically. Winchester Disks: Sealed drives for dust protection, ensuring high surface quality 3.3 Magnetic Disks Figure 2-21. A disk with five zones. Each zone has many tracks. 3.4 Floppy disk Originally invented by IBM for mainframe maintenance. Quickly became popular for software distribution on personal computers. Early versions were physically flexible, hence the name "floppy." Organized into tracks and sectors like hard disks. Unlike hard disks, where heads float on a cushion of air, floppy disk heads touch the surface. This direct contact leads to more wear on the media and heads. 3.4 Floppy disk Floppy disks were widely used for about 20 years. Modern computers are now usually shipped without floppy drives, as newer storage media have replaced them. 3.5 RAID RAID Stands for Redundant Array of Independent Disks. RAID addresses slow disk performance relative to CPU by using multiple disks in parallel. It enhances disk performance and reliability. SLED vs. RAID: Unlike a Single Large Expensive Disk (SLED), RAID uses multiple disks and appears as one large disk to the OS. RAID levels are visually illustrated in Figure 2-23 3.5 RAID Figure 2-23. RAID levels 0 through 5. Backup and parity drives are shown shaded. 3.5 RAID RAID Level 0 (Striping) [Fig. 2-23(a)]: Data is split into strips and distributed across multiple drives. Performance: High for large requests; supports parallel I/O. Reliability: Low; lacks redundancy, so data is lost if any drive fails. RAID Level 1 (Mirroring) [Fig. 2-23(b)]: Data is duplicated across primary and backup disks. Performance: Improved read speed; write speed similar to a single disk. Reliability: High fault tolerance; data is recoverable if a drive fails. 3.5 RAID RAID Level 2 (Hamming Code Parity) [Fig. 2-23(c)]: Data split into bits across multiple drives with Hamming code for error correction. Performance: High throughput; requires synchronized drives. Usage: Suitable for systems with many drives but high overhead. RAID Level 3 (Single Parity Disk) [Fig. 2-23(d)]: Uses one parity disk for error correction; all drives are synchronized. Reliability: Can handle single drive failure. Limitation: Limited to handling one I/O request per time, similar to a single drive. 3.5 RAID RAID Level 4 (Striping with Dedicated Parity) [Fig. 2- 23(e)]: Strips data with a dedicated parity disk. Advantage: Can recover data if a drive fails. Drawback: Small updates slow down performance due to parity disk bottleneck. RAID Level 5 (Distributed Parity) [Fig. 2-23(f)]: Distributes parity information across all disks, eliminating the bottleneck. Reliability: High fault tolerance; complex recovery if a drive fails. Best For: Systems needing high reliability and performance without a parity disk bottleneck. 3.6 CD-ROMS Origins of Optical Disks: Initially created for television recording, optical disks soon became popular for computer storage due to high capacity and low cost. LaserVision and Audio CDs: Early large-diameter optical disks were unsuccessful except in Japan. Philips and Sony later launched the Compact Disc (CD) in 1980, becoming the first mass-market digital storage medium. Red Book Standard: Published technical CD details allowed cross-compatibility among music publishers and electronics, ensuring CDs from various sources were compatible with different players. 3.6 CD-ROMS Pit-Land Structure: A low-power laser reads the pits (depressions) and lands (flat areas), as shown in Figure 2-24. The laser identifies transitions between pits and lands to read binary data, ensuring reliable encoding. CD-ROM Development: Recognizing CDs' data storage potential, Philips and Sony published the Yellow Book in 1984, setting standards for CD-ROMs and enhancing error correction for data reliability. 3.6 CD-ROMS Figure 2-24. Recording structure of a Compact Disc or CD-ROM. 3.6 CD-ROMS Data Organization: The CD-ROM format organizes data into 98-frame sectors (Fig. 2-25), with each sector containing a 16-byte preamble, 2048 data bytes, and a 288-byte error-correcting code. Mode 1: Has enhanced error correction. Mode 2: Merges data and ECC for applications where error correction is less crucial (e.g., audio). In 1986, Philips introduced the Green Book, enabling graphics and multimedia by interleaving audio, video, and data in the same sector. 3.6 CD-ROMS Figure 2-25. Logical data layout on a CD-ROM. CD-Recordables Emergence: Mid-1990s saw CD recorders becoming affordable and accessible. Usage: Primarily used for data backup, small-scale CD production, and as masters for duplication. Differences from CD-ROMs: Once written, data on CD-Rs cannot be erased. Structure: 120-mm polycarbonate blanks with grooves for laser guidance. CD-Recordables Reflective Layer: Initially gold (later replaced by aluminum) creates reflectivity. Dye Layer: Cyanine (green) and pthalocyanine (yellow- orange) dyes simulate pits and lands. Writing Mechanism: High-powered laser creates dark spots on dye, mimicking data pits. Compatibility: Can be read by standard CD-ROM drives and audio CD players. CD-Recordables Orange Book: Defines CD-R standards and introduces CD-ROM XA for incremental writing. Incremental Writing: Allows data to be written in tracks over time. VTOC Management: Each track has its own Volume Table of Contents (VTOC). Multisession CDs: Multiple tracks and sessions can coexist, though not compatible with standard audio CD players. CD-Recordables Buffer Underrun: Insufficient data flow causes burning failures, leading to “coasters.” Copy Protection: Various techniques like fake file sizes, incorrect ECCs, and nonstandard gaps make unauthorized copying difficult. Piracy Prevention: Protects against unauthorized replication but remains a challenge due to sophisticated countermeasures. CD-Rewritables (CD-RW) Purpose: CD-RW provides a rewritable CD-ROM alternative, meeting demand for reusable media. Material: Uses an alloy of silver, indium, antimony, and tellurium, which has two stable states (crystalline and amorphous) with different reflectivities. Laser Functionality: Three power levels High Power: Converts crystalline to amorphous state, creating low-reflectivity “pits.” Medium Power: Reverts alloy to crystalline state, restoring high-reflectivity “lands.” Low Power: Reads data without altering material state. CD-Rewritables (CD-RW) Cost: CD-RW discs are more expensive than CD-Rs, limiting their widespread replacement of CD-Rs. Accidental Deletion: CD-R is preferred for backups due to its write-once nature, reducing the risk of accidental data loss. DVD Background: Developed to meet demand for higher-capacity, high-quality optical storage by Hollywood, consumer electronics, and computer companies. DVD Evolution: Derived from CD technology but optimized for larger storage and multimedia. Core Improvements: Smaller pits (0.4 microns vs. 0.8 for CDs). Tighter spiral (0.74 microns vs. 1.6 for CDs). Red laser (0.65 microns vs. 0.78 for CDs). Capacity Increase: Up to 4.7 GB for single-layer DVDs (7x a CD’s capacity). DVD 1.Standard DVD Formats: Single-sided, single- layer: 4.7 GB 2.Single-sided, dual-layer: 8.5 GB 3.Double-sided, single-layer: 9.4 GB 4.Double-sided, dual-layer: 17 GB DVD Multimedia Capabilities: High-resolution video (720 x 480), supporting 133 minutes of content with MPEG-2 compression. Audio and Subtitles: Supports soundtracks in up to 8 languages and subtitles in 32 languages. Pan-and-Scan: Dynamic cropping of widescreen to fit 4:3 screens. Parental Controls: Scene skipping to adjust content suitability. Regional Encoding: Intentional incompatibility between regions (e.g., U.S. vs. Europe) to protect theatrical releases. Hollywood Influence: Designed with movie rentals and sales as the primary focus, leading to features aimed at consumer viewing preferences. Blu-Ray What is Blu-Ray? Successor to DVD, named for its blue laser (shorter wavelength than DVD’s red laser). Improved Focus: Blue laser allows for smaller pits and lands, enabling higher data density. Storage Capacity: Single-sided Blu-Ray: 25 GB Double-sided Blu-Ray: 50 GB Blu-Ray Data Transfer Rate: ~4.5 MB/sec, higher than DVD but slower than modern magnetic disks (e.g., ATAPI-6 at 100 MB/sec). Expected Replacement: Likely to phase out CD-ROMs and DVDs over time due to higher capacity and performance. Adoption Timeline: Full transition expected to take several years as technology adoption spreads. 2.4 INPUT/OUTPUT As we mentioned at the start of this course, a computer system has three major components: the CPU, the memories (primary and secondary), and the I/O (Input/Output) equipment such as printers, scanners, and modems. So far, we have looked at the CPU and the memories. Now it is time to examine the I/O equipment and how it is connected to the rest of the system. Buses Definition of a Bus: A bus is a communication system that transfers data between components inside a computer or between computers. Components of a Motherboard: The motherboard contains critical components: CPU (Central Processing Unit): The brain of the computer, executing instructions. DIMM (Dual In-line Memory Module) Slots: Slots for RAM modules, allowing for memory expansion. Support Chips: Chips that manage various functions, such as memory control and interfacing. Bus System: A pathway etched onto the motherboard that facilitates communication among the CPU, memory, and I/O devices. Buses Figure 2-28 shows a typical motherboard layout, indicating the placement of the CPU, memory slots, support chips, and the bus system Buses Structure of I/O Devices: Components: Controller: Contains most of the electronics for managing the I/O device, usually found on an expansion card plugged into a motherboard slot. Device Itself: The physical unit, such as a hard drive, printer, or scanner. Function of Controllers: The controller translates commands from the CPU into actions for the I/O device. Direct Memory Access (DMA): Allows data transfer between I/O devices and memory without CPU intervention, streamlining data handling and improving Buses Interrupt Mechanism: After data transfer, the controller sends an interrupt signal to the CPU, which causes the CPU to halt its current operation and execute an interrupt handler routine to process the I/O completion. Figure Reference: Figure 2-29 depicts the logical structure of a computer with a single bus connecting the CPU, memory, and I/O devices, illustrating data flow and interaction. Buses Bus Utilization: The bus is a shared resource used by both the CPU for fetching instructions and by I/O controllers for data transfers. Bus Arbitration: A critical component called the bus arbiter determines which device has control of the bus at any time, ensuring orderly access. Priority Scheme: I/O devices typically receive priority to prevent data loss, particularly for devices like hard drives that cannot tolerate delays. When no I/O requests are present, the CPU can utilize the bus exclusively for its operations. Buses Cycle Stealing: I/O devices can request bus access, interrupting CPU processes. While this allows I/O operations to occur, it can slow overall performance as the CPU waits for access. Performance Challenges: The increasing speed of CPUs and I/O devices has led to bus contention issues, where the existing bus cannot handle the data load effectively, creating performance bottlenecks. Buses Legacy Buses: ISA Bus (Industry Standard Architecture): One of the first buses widely used in personal computers but is becoming obsolete due to its limitations in speed and bandwidth. EISA Bus (Extended ISA): An enhancement of the ISA bus that maintained backward compatibility while providing improved performance. Buses Modern Bus Technologies: PCI Bus (Peripheral Component Interconnect): Developed by Intel in the early 1990s, designed for high- speed data transfer and ease of use. Public Domain: Intel placed PCI patents in the public domain to encourage widespread adoption across the industry. Provides a high-speed connection between the CPU and high- bandwidth peripherals, reducing bus congestion. Figure 2-30 illustrates a typical PCI bus configuration, showing how the CPU interacts with a memory controller and various peripherals via the PCI bus. Buses Figure 2-30 illustrates a typical PCI bus configuration, showing how the CPU interacts with a memory controller and various peripherals via the PCI bus. Terminals Computer terminals consist of two parts: a keyboard and a monitor. In the mainframe world, these parts are often integrated into a single device and attached to the main computer by a serial line or over a telephone line. In the airline reservation, banking, and other mainframe-oriented industries, these devices are still in widespread use. In the personal computer world, the keyboard and monitor are independent devices. Either way, the technology of the two parts is the same. Keyboards Original IBM PC: Featured snap-action switches for tactile feedback and audible clicks. Cheaper Keyboards: Use simple mechanical contact for key activation. Advanced Methods: Some keyboards use magnets and coils to detect key presses through induced current. Keyboards Operation: Key Press: When a key is pressed, an interrupt is generated, triggering the keyboard interrupt handler in the operating system. Key Identification: The handler reads a hardware register in the keyboard controller to identify the key pressed (1-102). Key Release: A second interrupt occurs when the key is released, allowing for the recognition of key combinations. Software Handling: Multikey sequences (SHIFT, CTRL, ALT) are processed by software, enabling functions like CTRL-ALT-DEL for rebooting the system. CRT Monitors Structure: A CRT monitor consists of a box containing a Cathode Ray Tube (CRT) and power supplies. Functionality: The CRT features an electron gun that emits a beam of electrons towards a phosphorescent screen, producing images through a process called raster scanning. CRT Monitors Figure 2-31. (a) Cross section of a CRT. (b) CRT scanning pattern. CRT Monitors Raster Scanning Process Horizontal Sweep: The electron beam sweeps horizontally across the screen in about 50 µsec, tracing a line. Controlled by linearly increasing voltage applied to horizontal deflection plates. Vertical Sweep: After completing the horizontal sweep, a vertical retrace brings the beam back to the left side. Vertical motion is managed by a slower voltage increase on vertical deflection plates. Refresh Rate: The entire screen is repainted 30 to 60 times per second. Refer to Figure 2-31(b) for beam motion illustration. CRT Monitors Grid Function: Inside the CRT, a grid controls the acceleration of electrons based on the voltage applied. Positive voltage allows electrons to pass, causing the screen to glow; negative voltage repels them, preventing illumination. Visual Output: This binary control enables the conversion of electrical signals into bright and dark spots, forming the displayed image. Figure 2-31(a) for CRT structure and grid placement. Flat Panel Displays Transition from CRT to LCD: Cathode Ray Tube (CRT) monitors are too bulky for portable devices, leading to the adoption of Liquid Crystal Display (LCD) technology, which is lightweight and slim. Liquid Crystal Technology: Liquid crystals are organic molecules that flow like liquids but have a structured arrangement, enabling manipulation of light based on molecular alignment. Basic Structure: An LCD consists of two glass plates with a liquid crystal layer sandwiched in between, transparent electrodes, and light sources to illuminate the screen (Figure 2- 32a). Polarizing filters (polaroids) are attached to both sides to manage light polarization. Flat Panel Displays Figure 2-32. (a) The construction of an LCD screen. (b) The grooves on the rear and front plates are perpendicular to one another. Flat Panel Displays Twisted Structure: The TN (Twisted Nematic) display features horizontal grooves on the rear plate and vertical grooves on the front plate, causing the liquid crystal molecules to twist (Figure 2-32b). Flat Panel Displays Matrix Configurations: Passive Matrix Displays: Use perpendicular wires to create a grid (e.g., 640 vertical x 480 horizontal) and control voltage at selected pixels, painting the screen line by line, similar to CRT operation. Active-Matrix Displays: Incorporate a Thin Film Transistor (TFT) at each pixel for independent voltage control, resulting in better image quality. Most modern notebook and desktop displays utilize TFT technology. Colour Displays: Utilize similar principles as monochrome displays but incorporate optical filters to separate white light into red, green, and blue components, allowing for a full spectrum of colours through additive colour mixing. Video RAM Purpose of Video RAM: Video RAM (VRAM) is used in CRT and TFT displays to refresh the screen 60–100 times per second, storing bitmaps representing the pixel information for each screen image. Storage Requirements: For a resolution of 1600 × 1200 pixels, VRAM would require: 5.5 MB of storage: Each pixel represented by a 3-byte RGB value (one for each color component). Color Palette Option: To save memory, indexed color can be used with an 8-bit index pointing to a color palette of 256 colors, reducing memory needs by two-thirds but limiting simultaneous colors. Video RAM Bandwidth Challenges: Full-screen multimedia requires significant bandwidth, necessitating a data rate of 137.5 MB/sec for full-motion video (25 frames/sec), which exceeds the capabilities of earlier bus architectures like (E)ISA and PCI. Video RAM AGP Bus Introduction: To address bandwidth limitations, Intel introduced the Accelerated Graphics Port (AGP) with the Pentium II. Key features include: Data Transfer Rate: 32-bit transfers at 66 MHz, achieving a bandwidth of 252 MB/sec. Scalability: Subsequent AGP versions supported 2x, 4x, and 8x modes to further enhance bandwidth for interactive graphics applications. Impact on Graphics Performance: The AGP bus allows for smoother graphics rendering and improved performance in applications requiring high data rates without overwhelming the main PCI bus. Mice Evolution of Computer Users: Early computers (ENIAC era) were operated solely by the experts who built them. By the 1950s, skilled programmers were the primary users. Today, people with minimal technical expertise widely use computers for everyday tasks. Why a Mouse? Early interfaces relied on text-based command lines, challenging for non- experts. Point-and-click interfaces (like Windows and Macintosh) were developed for easier navigation. A mouse serves as a practical tool for users to point, click, and select items on the screen without needing advanced computer knowledge Mice 1. Mechanical Mouse First design featured two perpendicular rubber wheels that tracked movement. Wheels were replaced by a rolling ball, shown in Fig. 2- 33, which rotated as the mouse moved. Ball-driven motion sensors track x- and y-axis movement. Mice Figure 2-33. A mouse being used to point to menu items Mice 2. Optical Mouse Uses LED and photodetector; no moving parts like wheels or balls. Moves on a special grid-lined pad, sensing the number of grid lines crossed. More durable than mechanical mice due to fewer moving parts. Mice 3. Optomechanical Mouse Combines elements of both mechanical and optical designs. Uses a rolling ball connected to 90-degree-aligned shafts with light pulse encoders. As the ball moves, the encoders detect light pulses, tracking precise movements. Mice Data Transmission: Each movement triggers the mouse to send 3-byte data packets to the computer. Byte 1: x-axis movement. Byte 2: y-axis movement. Byte 3: Button state (clicked/not clicked). Units of movement (e.g., 0.01 inch) are sometimes called “mickeys.” Printer Matrix Printers Technology: Uses a print head with 7-24 needles that strike to form dots, arranged in matrices (e.g., 5x7) to build characters. Quality and Speed: More needles provide better print quality by creating overlapping dots, but it reduces speed. Use Cases: Best for printing on large forms, receipts, and multipart continuous forms with carbon paper. Pros & Cons: Economical and reliable but noisy, slow, and limited in graphics. Illustrate: Figure 2-34(a) and (b) - Letter "A" printed on 5x7 and overlapping dot matrices. Printer Figure 2-34(a) and (b) - Letter "A" printed on 5x7 and overlapping dot matrices. Printer Inkjet Printers Technology: Piezoelectric: Uses crystals to control droplet size based on voltage (e.g., Epson). Thermal: Heats ink to form vapor bubbles that push ink droplets out (e.g., Canon, HP). Resolution: Generally high, from 1200 to 4800 dpi, allowing near-photographic quality on coated paper. Pros & Cons: Affordable and good quality but relatively slow and uses costly ink cartridges. Printer Laser Printers Technology: Process: A rotating drum is charged and exposed to a laser, creating light and dark spots where toner is applied. Halftoning: Converts gray scales by creating halftone cells of black/white pixels, giving the illusion of grayscale. Resolution and Speed: High speed, often 600 dpi or more; excellent for sharp images and text. Use Cases: Ideal for high-quality, fast printing, and combining multiple functions (e.g., printing, copying, and sometimes faxing). Pros & Cons: High quality and speed but initially more expensive than inkjet. Printer Figure 2-35 - Basic technology diagram of a laser printer drum and halftone patterns as shown in Figure 2-36. Colour Printers Color Representation: Transmitted Light: Monitors use RGB (Red, Green, Blue) for colors. Reflected Light: Printers use CMYK (Cyan, Magenta, Yellow, Black) because they absorb and reflect light. Gamut and Calibration: Each device has a limited color range (gamut). Color matching from screen to print requires calibration due to RGB (monitor) vs. CMYK (printer) differences. Colour Printers CMYK Color Printer Types 1.Inkjet Printers: 1. Uses CMYK cartridges. 2.Ink Types: 1.Dye-based: Bright colors but fade in sunlight. 2.Pigment-based: Longer-lasting but may clog nozzles. 3.Best For: Graphics, photos with special paper. 2.Solid Ink Printers: 1. Uses melted wax ink, creating vibrant colors. 2. Long startup time due to ink melting. 3.Best For: High-volume color prints with stable colors. Colour Printers Advanced Color Printers 1.Color Laser Printers: 1. Works like a monochrome laser printer but with CMYK toners. 2. Requires large memory for high-resolution images. 3. Best For: Fast, high-quality color prints; stable over time. 2.Wax Printers: 1. Uses heated wax ribbons, producing vibrant colors. 2. Best For: High-quality photos but less common now. 3.Dye Sublimation Printers: 1. Vaporizes dye for near-continuous colors without halftoning. 2. Best For: High-quality photos on special paper (often used in snapshot photo printers) Telecommunication Equipment Most computers nowadays are connected to a computer network, often the Internet. Achieving this access requires special equipment. In this section we will see how this equipment works. Modems Definition: A modem is a device that modulates and demodulates data for transmission over phone lines. Purpose: Connects computers over telephone lines, allowing communication between home computers, workplaces, ISPs, and banking systems. Challenge: Raw digital signals (0 and 1) don’t transmit well over voice-grade telephone lines; they require modulation. Digital Signal Transmission Issue: Digital signals (0 as 0V, 1 as 3-5V) distort over phone lines. Solution: Modulation, using a carrier sine wave (1000-2000 Hz), which carries data with minimal distortion. Modems Modulation is the process of converting digital data into analog signals for transmission over communication channels. It modifies certain aspects of a carrier wave (like amplitude, frequency, or phase) to encode information. Demodulation is the reverse process, where the received analog signals are converted back into digital data, retrieving the original information. Modems Types of Modulation Amplitude Modulation (AM): Changes the signal’s voltage for 0 and 1. Frequency Modulation (FM): Maintains constant voltage but changes frequency for 0 and 1 (also called Frequency Shift Keying). Phase Modulation (PM): Uses phase shifts (180° or multiple degrees) to represent data transitions between 0 and 1. Modems Advanced Modulation Techniques Dibit Phase Encoding: Transmits 2 bits per time interval by using different phase shifts (e.g., 45°, 135°, 225°, 315°). Baud Rate vs. Bit Rate: Baud rate refers to signal changes per second. Bit rate can exceed baud rate if multiple bits are sent per baud. Modems Data Transmission in Modems Serial Transmission: 8-bit characters sent serially over single-channel telephone lines. Start and Stop Bits: Each character is preceded by a start bit and followed by a stop bit (10 bits total per character). Example: 9600 baud implies one signal change every 104 µsec. Modems Types of Data Transmission Modes Full-Duplex: Simultaneous two-way transmission using different frequencies. Half-Duplex: Data travels one direction at a time. Simplex: Data travels only one direction. Modems Modern Modem Capabilities Data Rates: Common rates range from 28,800 bps to 57,600 bps with low baud rates. Advanced Techniques: Combine amplitude, frequency, and phase modulation for higher bit rates. Digital Subscriber Line (DSL) DSL: Digital Subscriber Line (DSL) is a high-speed Internet access technology provided over traditional telephone lines. ADSL: ADSL (Asymmetric DSL) is the most common DSL type, offering higher download speeds than upload speeds. Context: While modems provided only 56 kbps, cable and satellite offered much faster speeds, pushing telcos to develop a competitive alternative. Broadband Concept: DSL services offer broader bandwidth than traditional dial-up and are often marketed as "broadband" for high-speed access. Digital Subscriber Line (DSL) The Limitations of Traditional Phone Lines Voice Optimization: Traditional phone lines were optimized for voice, with bandwidth capped at around 3000 Hz due to filters at the telco's office. Potential Bandwidth: Removing the filter allows for bandwidth up to 1.1 MHz, enabling higher data rates. Digital Subscriber Line (DSL) ADSL Technology (Referencing Fig. 2-38) Channel Division: The 1.1 MHz bandwidth is divided into 256 channels, each at 4312.5 Hz. Channel Allocation: Channel 0: Reserved for POTS (Plain Old Telephone Service). Channels 1–5: Left unused to avoid interference. Remaining Channels: Most allocated for downstream, with a smaller portion for upstream (e.g., 80-90% downstream). ADSL Modulation: Data is sent via amplitude and phase modulation, achieving up to 15 bits per baud. Digital Subscriber Line (DSL) Figure 2-38. Operation of ADSL ADSL Speed and Bandwidth Downstream & Upstream: Split prioritizes downstream bandwidth for typical internet use (e.g., downloading content). Speed Ranges: Up to 13.44 Mbps theoretically, but practically 4-8 Mbps is achievable over quality lines. Digital Subscriber Line (DSL) Downstream: This is data sent from the service provider (e.g., headend in a cable network) to the user. It's the data you receive, such as loading a webpage, streaming a video, or downloading files. Since most online activities involve receiving more data than sending, downstream speeds are often prioritized and typically higher. Upstream: This is data sent from the user to the service provider, like uploading a file, sending an email, or sending requests to a server. In most consumer services like Internet over cable or DSL, upstream speeds are lower than downstream, as average users tend to consume more content than they create. Digital Subscriber Line (DSL) ADSL Setup (Referencing Fig. 2-39) Network Interface Device (NID): Installed on customer premises, marking the end of telco property. Splitter: Separates voice (0-4000 Hz) for POTS from data signals for the ADSL modem. ADSL Modem: Acts as 250 parallel modems across different frequencies, connecting to the computer via Ethernet or USB  Digital Subscriber Line (DSL) Figure 2-39. A typical ADSL equipment configuration Connection to the Internet DSLAM (Digital Subscriber Line Access Multiplexer): Located on the telco side, this device separates voice from data, recovers digital signals, and sends data Internet over Cable Internet over Cable Overview: Many cable TV providers offer Internet access via their existing cable infrastructure. Key Distinction from ADSL: Cable Internet uses a shared medium (a single cable for multiple users) vs. the private line of ADSL. Internet over Cable Cable Network Structure Network Components: Main Office and Headends: The main office is connected to multiple headends throughout the service area. Shared Cable System: Each headend is connected to homes and offices via cables that hundreds of users share. Bandwidth: Typical cable bandwidth is 750 MHz. Internet over Cable Shared Bandwidth: Cable Internet performance can vary by time of day (better at off-peak times). Impact of Neighborhood Density: Fewer users per cable line (e.g., in less populated neighborhoods) can lead to better performance. In a cable network, a headend is a facility or hub where signals (for television, Internet, and other services) are received, processed, and sent out to subscribers. Internet over Cable Channel Allocation (Referencing Fig. 2-40) Frequency Allocation: TV Channels: Use the 54–550 MHz range. Internet Traffic: Downstream traffic occupies frequencies above 550 MHz, while upstream uses 5–42 MHz. One-Way Amplification Solution: Separate upstream and downstream amplifiers help maintain distinct traffic flows. Digital Subscriber Line (DSL) Figure 2-40. Frequency allocation in a typical cable TV system used for Internet access. Internet over Cable Cable Modem Operation Two Interfaces: A cable modem connects to both the computer (typically via Ethernet) and the cable network. "Always-On" Connection: Cable modems are typically always connected to the network when powered on. Internet over Cable Modem Initialization Process 1.Downstream Channel Scanning: The modem identifies the headend’s broadcast channel to receive system parameters. 2.Channel Assignment: The headend assigns specific upstream and downstream channels to the modem. 3.Ranging (Distance Calibration): The modem calculates its distance from the headend to adjust timing for upstream minislots. Internet over Cable Data Transmission (Referencing Fig. 2-41) Upstream (Minislot Mechanism): Modems use minislots for upstream data, with dynamic allocation managed by the headend. Contention Management: In cases of contention, modems wait and retry, doubling the delay on repeated failures. Downstream (Fixed Packet Size): Downstream packets are 204 bytes, optimized for digital TV compatibility using MPEG-2. Internet over Cable Figure 2-41. Typical details of the upstream and downstream channels in North America. QAM-64 (Quadrature Amplitude Modulation) allows 6 bits/Hz but only works at high frequencies. QPSK (Quadrature Phase Shift Keying) works at low frequencies but allows only 2 bits/Hz Internet over Cable Security and Final Setup Encryption: Both directions of traffic are encrypted to protect user privacy on the shared medium. Login and IP Assignment: After establishing a secure connection, the modem requests an IP address and synchronizes time with the ISP. Digital Cameras Digital cameras as computer peripherals that capture images electronically. Mechanism: Uses a lens to form an image on a CCD (Charge-Coupled Device) array instead of film. Key Differences: Digital sensor replaces film, and an analog-to-digital converter reads light intensity to generate digital data (Fig. 2-42). Digital Cameras Figure 2-42. A digital camera Digital Cameras CCD Array: Light-sensitive sensors that convert light into electrical charges, with values ranging from 0–255 or 0–4095. Color Filtering: A Bayer filter on the CCDs separates light into red, green, and blue components. Pixel Claim: Each pixel consists of four CCDs (two for green, one for red, one for blue). Interpolation: The camera's software interpolates to create a full-color image. Digital Cameras Image Capture Process Shutter Button Functions: 1. Focus: Adjusts lens based on image detail. 2. Exposure: Determines light intensity for optimal capture. 3. White Balance: Adjusts color based on light source spectrum. Storage: Image data saved in camera's RAM; high-end SLRs require ~1 GB for rapid frame capture Digital Cameras Post-Capture Processing in Camera White Balance Correction: Adjusts colors to reflect true tones. Noise Reduction: Reduces image noise, compensates for defective CCDs. Sharpening: Enhances edges to make details more defined. Compression: JPEG format compresses images via spatial Fourier transform, reducing file size with some detail loss. Digital Cameras Image Storage and Transfer Storage Media: Saved on flash memory or microdrives. Image Transfer: Connects to computers via USB or FireWire for transfer. Editing Software: Further editing in tools like Adobe Photoshop for cropping, color adjustments, and more. Digital Cameras Embedded Computing Power in Digital Cameras Embedded System: Modern digital cameras have powerful processors for: Managing lens and flash communication. Real-time image display on LCD. Controlling buttons and settings, often rivaling the capabilities of desktop computers. ASCII (American Standard Code for Information Interchange) Definition: ASCII is a character encoding standard for text. Bit Length: 7 bits per character. Character Count: 128 unique characters (including control characters). Purpose: Used to represent text in computers, telecommunications, and other devices. ASCII (American Standard Code for Information Interchange) Range: Codes 0 to 1F (in hexadecimal).Purpose: Primarily used for data transmission control. Examples: SOH (Start of Header): Marks beginning of a message header. STX (Start of Text): Marks the start of the actual message text. ETX (End of Text): Marks the end of the message text. EOT (End of Transmission): Marks the end of the message. Current Use: Mostly obsolete for modern data transmission. ASCII (American Standard Code for Information Interchange) Figure 2-43. The ASCII character set. ASCII (American Standard Code for Information Interchange) Range: Includes uppercase and lowercase letters, digits, punctuation, and some math symbols. Basic Usage: Widely used for representing readable characters in text files and programming. Examples: Uppercase Letters: A–Z (codes 41 to 5A in hexadecimal). Lowercase Letters: a–z (codes 61 to 7A in hexadecimal). Digits: 0–9 (codes 30 to 39 in hexadecimal). Symbols: Common punctuation and mathematical UNICODE The Need for UNICODE Limitations of ASCII: Designed for U.S. English, inadequate for languages with accents, diacritical marks, and non-Latin alphabets. Examples: French: "syste`me" German: "fu¨r", β Chinese: Entirely different script, no alphabet. Initial Solutions: Latin-1 (IS 646): Extended ASCII with 128 additional characters (8- bit). IS 8859: Introduced "code pages" for different languages, but struggled with mixing languages and lacked support for CJK (Chinese, Japanese, Korean). UNICODE Creation: Formed by a consortium of companies to create a universal standard (IS 10646). 16-bit Encoding: Assigns a unique code point to every character (no multibyte characters). 65,536 Code Points: Covers major world languages and symbols. ASCII Compatibility: Latin-1 is mapped to code points 0-255 for smooth transition. UNICODE Code Point Allocation UNICODE Structure: Code points are divided into blocks of 16, organized by script. Examples: Latin (336), Greek (144), Cyrillic (256) Arabic, Hebrew, Devanagari, and other regional scripts. Extended Scripts: CJK ideographs (20,992) and Korean Hangul (11,156). Other Symbols: Math, currency, geometric shapes, and special characters (e.g., Braille). UNICODE Diacritical Marks and Combining Characters Diacritical Marks: Each mark has its own code point; software combines with characters to form new symbols. Example: Combining "e" with an accent mark to form "é". Benefits: Simplifies software but requires careful handling in text rendering. UNICODE UNICODE Challenges Sorting Issues: Latin Scripts: Simple sorting by comparing code points. CJK Ideographs: No dictionary order; requires external tables for sorting. Dynamic Language Growth: New Words: English adds words without new code points; Japanese may require additional characters. Personal/Place Names: Demand for code points for 20,000+ new Chinese names. UNICODE Cultural Sensitivity and Controversies Han Ideographs: Only 20,992 code points for Chinese and Japanese characters, whereas a Japanese dictionary has 50,000 kanji. Unicode Decisions: Some critics argue that the process reflects cultural biases, especially with symbol allocation. Example: Similar characters may have identical code points, leading to possible misinterpretation. Summary Computer systems are composed of three key components: processors, memories, and I/O devices. The processor’s role is to execute the fetch-decode- execute cycle, fetching instructions from memory, decoding them, and then executing them. This cycle is typically described as an algorithm and may even be carried out by a low-level software interpreter. To enhance performance, many modern computers utilize pipelines or superscalar designs, where multiple functional units operate in parallel. Summary Parallel computing has become increasingly common, involving systems with multiple processors. There are different types of parallel systems: array processors, which perform the same operation on multiple data sets simultaneously; multiprocessors, where multiple CPUs share a common memory; and multicomputers, where multiple computers each have their own memory and communicate via message passing. Summary Memory in a computer is categorized as primary or secondary. Primary memory is used to store the program being executed and has a very fast access time, typically in the range of tens of nanoseconds, with caches further enhancing this speed. Secondary memory, on the other hand, has slower access times, often in the milliseconds range, and is dependent on the location of the data. Common types of secondary memory include tapes, magnetic disks (floppy disks, IDE, SCSI disks, and RAIDs), and optical disks (CD-ROMs, CDs, and DVDs). Summary I/O devices are essential for transferring data into and out of the computer and are connected to the processor and memory via buses. Examples of I/O devices include terminals, mice, printers, and modems. While many I/O devices use ASCII for character encoding, UNICODE is gaining popularity as the global standard for character representation as the computer industry continues to expand internationally.