Information Systems

Session 2

LBSC 690

Information Technology

Agenda

• Questions

• Convergence: computing and communications

• What is the Internet made of?– Computer processors– The storage hierarchy– Networks– Software

The Development of Computing:An Example of Industrial Policy

• Hardware: all developed for the government– Mechanical: essentially a big adding machine– Analog: designed for calculus, limited accuracy– Digital: early machines filled a room– Microchips: designed for missile guidance

• Software: initial applications were military – Numeric: computing gun angles– Symbolic: Code-breaking

History of Telephone Networks• Connecting central offices

– Initial connections passed analog signals on wires– Digital signals allow several signals to share a wire– Microwave avoids the cost of laying the wire– Satellites avoid the cost of the microwave towers– Fiber optic cables have nearly unlimited “bandwidth”

• “The last mile”– Step/crossbar: Analog connection, mechanical control– ESS: Analog connection, digital control– DSL/Cellular: Digital connection, digital control

Commercial Developments

• 1960’s Mainframes Teletypes– IBM

• 1970’s Minicomputers Modems– DEC

• 1980’s Personal computers LANs– Apple, Microsoft

• 1990’s Internet Internet– AOL, Search Engines

• 2000’s Convergence with television?

Hardware Processing Cycle

• Input comes from somewhere– Keyboard, mouse, microphone, etc.

• The system does something with it– Processor, memory, software, network

• Output goes somewhere– Monitor, speaker, robot controls, etc.

The Big Picture

Processor Memory

Network

Computer Hardware

• Central Processing Unit (CPU)– Intel Pentium, Motorola Power PC, …

• Internal communications “Bus”– PCI, ISA, …

• Storage devices– Cache, RAM, hard drive, floppy disk, …

• External communications– Modem, LAN, …

Thinking About Speed

• Speed can be expressed two ways:– How long does it take do something once?

• Memory speed is often measured as access time

– How many times can you do it in one second?• Processor speed is measured in instructions per second

• Convenient units are typically used– “10 microseconds” rather than “0.00001 seconds”

• When comparing speeds, convert units first!

Converting Units

Speed

Unit Abbr Fraction of a sec

second sec 1

millisecond ms 1/1,000

microsecond s 1/1,000,000

nanosecond ns 1/1,000,000,000

picosecond ps1/1,000,000,000,000

Size

Unit Abbr Bytes

bit b 1/8

byte B 1

kilobyte kB 1,024

megabyte MB 1,048,576

gigabyte GB 1,073,741,824

terabyte TB1,099,511,627,776

Two Aspects of Speed

• Time to get from one place to another– California is 1/80 of a second (at speed of light)– Routing delays can slow this down

• The amount of information sent each second– “Fat pipes” are “faster” in this sense– Typically measured in bits per second

• The total transfer time is what counts– Each bit’s travel time + time from first bit to last– For large files, the second factor dominates

Speed vs. Cost

• The problem:– Fast memory devices are expensive

• So large memory devices are slow!

– But fast access to large memories is needed

• The solution:– Keep what you need often in small (fast) places

• Keep the rest in large (slow) places

– Copy quickly between the two

The Storage Hierarchy

CPU

RAM

Disk

Cache

Internet

Types of Memory• Random-Access Read-Only Memory (ROM)

– Bytes can be read in any order– No power needed to retain the contents– Used to store the Basic Input/Output System (BIOS)

• Random Access Read-Write Memory (RAM, disk)– Static RAM (SRAM) is very fast, but expensive– Dynamic RAM (DRAM) is cheap, but too slow for P III– Video RAM (VRAM) is shared by CPU and display chip

• Sequential-Access Read-Write Memory (tape)– Bytes must be read in the order they were written

CPU “Memory”

• The CPU is the fastest part of a computer– 500 MHz Pentium III = 250 MIPS

• One operation every 4 ns

• Cache memory is fast enough to keep up– 32 kB of 4 ns level 1 (L1) cache on chip, 32 kB– 512 kB of slower L2 cache also on chip

• RAM is larger, but even slower– 100 Mhz now common, typically at least 64 MB

Converting Units

Speed

Unit Abbr Fraction of a sec

second sec 1

millisecond ms 1/1,000

microsecond s 1/1,000,000

nanosecond ns 1/1,000,000,000

picosecond ps1/1,000,000,000,000

Size

Unit Abbr Bytes

bit b 1/8

byte B 1

kilobyte kB 1,024

megabyte MB 1,048,576

gigabyte GB 1,073,741,824

terabyte TB1,099,511,627,776

Types of Disks• Fixed magnetic disks

– May be partitioned into multiple volumes• In Windows, referred to as C:, D:, E:, …• In Unix, referred to as /software, /homes, /mail, …

• Removable magnetic disks– 3.5 inch floppy disks, zip drives, ...

• Read-only optical disks– CD-ROM, DVD-ROM

• Writable optical disks– CD-Recordable (CD-R), CD-Rewritable (RW)

How Disks WorkRotation

Seek

Sector

Memory Disk

• Hard disk is larger than RAM but much slower– 10 ms access time and 8 GB is now typical

• 1 hundred times larger than RAM

• 1 million times slower than RAM!

• The initial access is the slow part– Subsequent bytes sent at 17 MB/sec (60 ns/byte)

• As “virtual memory,” makes RAM seem larger– But things slow down beyond physical RAM

System Network• Network file system: use disks on nearby machines

– OK for small files, not good for disk-intensive operations

• Wide area networks are far slower than disks– Ethernet runs at 10 or 100 Mb/sec– 12 ms each way to California using fiber optics

• 250 ms each way using a satellite

– Sharing the circuit adds more delay

• Two solutions– Use local disk to store some network data

• Called “temporary Internet files” in Internet Explorer

– Transfer files using FTP and then work locally

Digital Signals: The Last Mile

• Analog modems– No faster than 56 kb/sec (typically slower)– Each byte (B) takes 10 bits (not 8!)

• Integrated Services Digital Network (ISDN)– Up to 128 kb/s (two 64kb/s channels)

• Digital Subscriber Line (DSL)– 640 kb/sec (down), slower upload, dedicated line

• Cable modems– 10 Mb/sec (down), slower upload, shared line

System User

• People are usually the slowest component – Fast typists produce 10 characters per second– Human perception time is about 100 ms

• Batch processing lets machine run fast– Type everything in, then process it all at once– Very common when computing was expensive

• Networks are sometimes slower than people – Use a “type-ahead” buffer for congested periods

Tape Backup

• Tapes store and access data sequentially– Very fast transfer– But not “random access”

• Used as backup storage for fixed disks– Disks rotate, and mechanical devices will fail– Weekly incremental backup is a good idea

• With a complete (“level zero”) monthly backup

– Best thought of as a batch process• CD-RW provides an affordable alternative

Networks

sam

kim

joe

annttclass

teal

wwwglue

rac4rac3www rac2

wam

Some Network Vocabulary

• Hosts– The computers that share the network

• Communications links– Essentially modems with dedicated phone lines

• Routing– Find a path from one host to another

Local Area Networks

• Within a campus or an office complex– Short-distance lines are fast and cheap– Fast communications makes routing simple

• Ethernet is a common LAN technology– All computers are connected to the same cable

• Ordinary “twisted pair” can carry 10 Mb/sec

– Every host broadcasts everything to all others• Collisions limit throughput to about 50% utilization

Local Area Networks

sam

kim

joe

ann

teal

www

rac4rac3www rac2

Wide Area Networks

• Regional, national, or global scale– Expensive communications must be used well

• Limiting to two hosts to allows 100% utilization

– Routing is complex with point-to-point circuits• Which path is shortest? Which is least busy? …

• Internet routers exchange routing tables– Which routes seem fast, which seem slow?

A Simple Wide Area Network

sam

kim

joe

annttclass

teal

wwwglue

rac4rac3www rac2

wam

Packet Switching

• Break long messages into short packets– Keeps one user from hogging a line

• Route each packet separately– Number them for easy reconstruction

• Request retransmission for lost packets– Unless the first packet is lost

Domain Names

• IP addresses are 32 bit numbers– Part of every packet, designed for easy routing

• People prefer to use names for things– Need to covert “domain names” to numbers

• Each name server knows one level of names– WAM’s name server knows rac1, rac2, …– UMD’s name server knows wam, glue, ttclass, …– .edu name server knows umd, umbc, stanford, …– “Top level” name server knows .edu, .com, .mil, …

Types of Software

• Application programs (e.g., Powerpoint)– What you normally think of as a “program”

• Compilers and interpreters (e.g., Java)– Programs used to write other programs

• Operating system (e.g., Windows 95)– Manages display, CPU, memory, disk, tape, …

• Embedded program (e.g., a disk controller)– Permanent software inside some device

Trends

• Moore’s Law and related laws– processing capacity doubles every 18 months– disk capacity, networking speed, pixels on displays all

increasing– miniturization (television camera and transmitter on an

artificial insect) – More information in more places: Ubiquitous information

- wireless

• Personalized services but loss of privacy– “de-massification” of mass media

Summary

• Speed, cost, and size (or distance): – You can easily get any 2, but not all 3– Computers use cache as a compromise strategy

• The Internet includes LAN’s and WAN’s– Each contributes unique characteristics

• Hardware and software work synergistically– Our focus will be on software and the Internet– But understand hardware capabilities and limitations

Obtaining Recordings of Class

• RealVideo is available over the Internet– Click the appropriate icon on the syllabus page

• Videotapes go from here to CLIS Library– Available for viewing there (not for check out)– You can hand carry it to IDSC to get a copy

• Allow 24 hours (for IDSC to schedule tape machines)