Transcript Computer Systems

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
Network
Memory
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
millisecond
microsecond
nanosecond
picosecond
sec
ms
s
ns
ps
1
1/1,000
1/1,000,000
1/1,000,000,000
1/1,000,000,000,000
Size
Unit
Abbr
bit
b
byte
B
kilobyte kB
megabyte MB
gigabyte GB
terabyte TB
Bytes
1/8
1
1,024
1,048,576
1,073,741,824
1,099,511,627,776
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
Cache
RAM
Disk
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
millisecond
microsecond
nanosecond
picosecond
sec
ms
s
ns
ps
1
1/1,000
1/1,000,000
1/1,000,000,000
1/1,000,000,000,000
Size
Unit
Abbr
bit
b
byte
B
kilobyte kB
megabyte MB
gigabyte GB
terabyte TB
Bytes
1/8
1
1,024
1,048,576
1,073,741,824
1,099,511,627,776
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 Work
Rotation
Sector
Seek
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
www
sam
rac2
rac3
rac4
wam
kim
ttclass
ann
joe
www
glue
teal
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
www
rac2
rac3
rac4
sam
kim
ann
joe
www
teal
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
www
sam
rac2
rac3
rac4
wam
kim
ttclass
ann
joe
www
glue
teal
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)