Download Data Modeling - Hiram College

Introduction
CPSC 363 Computer Networks
Ellen Walker
Hiram College
(Includes figures from Computer Networking by Kurose & Ross, © Addison Wesley
2002)
What is the Internet?
• INTERconnected
NETworks
• Hardware
– Hosts (end systems)
– Routers
– Communication Links
• Cables
• Wires
• Wireless
• Organization
– Standards (RFC’s)
– Protocols
“Cool” internet appliances
IP picture frame
http://www.ceiva.com/
Web-enabled toaster+weather forecaster
World’s smallest web server
http://www-ccs.cs.umass.edu/~shri/iPic.html
Message Transmission across the
Internet
• Source host creates the message to send
• Message is transmitted via the local network
to the local router (gateway)
• Message travels from router to router until the
destination’s gateway is reached
• Message is transmitted across the
destination’s LAN to the destination host.
• Destination host receives and acts upon the
message.
What is Provided?
• Communication services for distributed applications
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–
–
–
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Remote file transfer and login
WWW
Peer to Peer (e.g. Gnutella, Kazaa)
Multimedia applications
IM, chat
New applications all the time…
• Types of services
– Connection-oriented & reliable (TCP)
– Connectionless unreliable (UDP)
– No guarantee of how quickly the message is delivered
TCP vs UDP: An analogy
• TCP is like a conversation
– Each party makes sure the other party received
and understood each message.
– If you don’t understand, I repeat …
• UDP is like shouting from a rooftop
– I keep yelling (transmitting info) with no idea of
whether you can hear and understand me
What is a Protocol?
• Set of conventions surrounding a conversation, e.g.
–
–
–
–
–
–
–
–
–
Dial phone, hear ring
Hear ring, pick up phone, “Hello”
“Hello, may I speak to…”
“Just a minute…” [New person] “Hello”
“Hello”
[conversation here]
“Goodbye”
“Goodbye”, hang up phone
Hang up phone
• Conversation is aborted if appropriate response is
missing!
Examples of
Computer Network Protocols
• Application protocols
– HTTP (Hypertext transfer protocol)
– FTP (File transfer protocol)
– SMTP (Simple mail transfer protocol)
• Router and Network protocols
– TCP (Transmission Control Protocol)
– IP (Internet Protocol)
Human and Computer Protocols with
“Handshaking”
End Systems
• Servers
– Provide a “service” to others on the network
• Web server
• Email server
• Database Management System
• Clients
– Request “services” from others
• Web browser
• Email or DB client program
• Both
– Peer to Peer (P2P) hosts act as both client and server.
Network Core
• How is information passed through the
network?
• Goal: fast, accurate, avoid “idleness”
• Terms from telephony, information theory
– Circuit Switching vs. Packet Switching
– Multiplexing (Frequency vs. time)
– Bandwidth, Latency, Throughput
Circuit Switching
End-end resources
reserved for “call”
• link bandwidth, switch
capacity
• dedicated resources: no
sharing
• circuit-like (guaranteed)
performance
• call setup required
• Once setup, transmission
time depends only on
message length and
distance (not number of
switches)
Sharing Links
• Frequency Division Multiplexing (FDM)
– Each message travels in a unique Frequency
Band (like an FM radio station)
• Time Division Multiplexing (TDM)
– Time is divided into Frames, and Frames are
divided into Slots. Each message gets one slot.
Freq
Time
Disadvantages of Circuit Switching
• Time to set up the circuit
• Once the circuit is connected, bandwidth is
reserved -- even if no message is being sent
– Telephone “silence”
– Reading the message before responding…
• … but it worked pretty well for telephones for
years! (Mostly because people aren’t silent
much)
Packet Switching
• Divide messages into fixed-size packets
• Each packet uses full bandwidth
• Each packet travels its own path (message is
reassembled and reordered at the end)
• Switches (now called routers) collect and
pass packets along paths toward their final
destination
Store and Forward Transmission
• Router has several links
• When a packet is received:
– Router waits until the entire packet has been received before
doing anything (store-and-forward delay)
– Packet is stored in the output buffer until the outgoing link is
available (queuing delay)
– If the buffer is full, the packet is dropped (packet loss)
– Packet is sent on the outgoing link (to the next router)
How Long Does it Take?
• Assume we want to send a packet of L=1000 bits
from one host to another across the network
• The packet must pass through Q=4 links to get there.
• Each link can transmit R= 10,000 bps (bits per
second)
• From one router to the next will take (1000 bits) /
(10,000 bits / second) = 0.1 seconds
• After 0.1 seconds, the full packet is available to send
out. Assuming no delays, we can do this 4 times in
0.4 seconds, which is the total time to send the
message.
Unit Math
• If you forget the formulas for computing times
and distances, remember you can always do
the “unit math” for example:
Bits / (Bits / Second) =
Bits * (Seconds / Bits) =
(Bits * Seconds) / Bits =
(Seconds * Bits) / Bits =
Seconds * (Bits / Bits) =
Seconds
Scientific Notation Math
• Another useful “math trick” for these types of
calculations is “exponent math”. Start by converting
to Scientific Notation.
• Scientific notation represents a number as a value
between 1 and 10 (or -1 and -10) multiplied by 10 to
an appropriate power
– Powers greater than 0 are 10, 100, 1000…
– Powers less than 0 are 0.1, 0.01, 0.001, …
– Examples:
• 20,000 = 2.0* 104 (2.0e4 in Java notation)
• 0.1 = 1 * 10-1 (1.0e-1 in java notation)
Exponent Math
• To multiply, multiply the coefficient and add the
exponent
– 2.0e4 * 1.6e2 = (2.0*1.6)e (4+2) = 3.2e6
– Check: 20,000 * 160 = 3,200,000
• To divide, divide the coefficient and subtract the
exponent
– 4e3 / 2e4 = (4/2)e(3-4) = 2e-1
– Check: 4,000 / 20, 000 = 0.2
• If your coefficient is too large or small, adjust the
exponent
– 1.2e3 / 4e4 = (1.2/4)e(3-4) = 0.3e-1 = 3e-2
(Since the coefficient got larger, the exponent must get
smaller)
Another Example
• Link bandwidth is 2400 bits / second
• Message is 120,000 bits long
• Message must pass through 3 links
• With no queueing delays or propagation
delays, how long will the message take?
Message Switching vs.
Packet Switching
• Message switching: 1 packet per message
– Store-and-forward delay is per message
• Packet switching: many packets per message
– Packets will always be of a fixed size
– Store-and-forward delay is per packet
– Packet 1 can go from router A to router B while
packet 2 is going from host to router A
– This pipelining adds parallelism to the network.
Packet Switching Example
• Link bandwidth is 2400 bits / second
• Message is 120,000 bits long, divided into 4
packets of 30,000 bits each
• Message must pass through 3 links
• With no queuing delays or propagation
delays, how long will the message take?
Packet Switching Transmission
Delay Formula
•
•
•
•
Message length: L bits
Packet size: P bits
Number of links: N links
Transmission rate: R bits / second
• Delay = (P/R) * (N+(L/P)-1)
• P/R is the time to pass 1 packet
• N+(L/P)-1 is the number of “links”: N steps to get the
first packet to the end, then (P/L)-1 more steps until
the last packet comes out.
• Note: take the ceiling of L/P to get an integer value.
Considering Delay and Loss
• Processing Delay (typically microseconds)
– Time to examine header and determine where to store the
packet
• Queuing Delay (varies based on traffic)
– Time waiting for outbound link to be available
• Transmission Delay (packet length / transmission rate)
– Time to push all bits of the packet onto the link
• Propagation Delay (distance / speed)
– Time for one bit to travel the length of the link
• Packet Loss
– When the queue is completely full, packets are dropped.
Tollbooth Analogy
• Road has tollbooths every 100km.
• Messages are convoys; cars are packets
• Transmission delay = time for one car to pass
one toll barrier
• Propagation delay = time for one car to travel
100km to the next toll barrier
R and S in Tollbooth Analogy
• R = transmission rate of the router (bits / second)
– Corresponds to number of cars that can be served by a toll
barrier in a fixed amount of time (say, cars / hour).
– Increase by adding more toll collectors at the barrier
• S = propagation speed of the link (e.g. speed of
electrons in copper; speed of light)
– Corresponds to car speed
– Increase (legally) by raising the speed limit
Some Tradeoffs in Network Design
• We have little control over propagation delay
– Can’t move Europe (or Mars) closer
– Can’t increase the speed of light!
• Transmission delay is easier to control
– Faster hardware, or more hardware in parallel
increases transmission rate
• Make sure transmission rate is at least as fast
as average incoming bit rate (traffic intensity)
– Otherwise, queues are guaranteed to fill and stay
full
Javascript Applets
• These can be found at the book’s website
– Start at our site
(http://cs.hiram.edu/~walkerel/cs363)
– Or go direct to the textbook site
(http://www.awl.com/kurose-ross-3)
– All 3 chapter 1 applets are very nice
Finding Real Delays
• Application called “traceroute”
– Sends a special message to the destination
– Each host along the way replies back with timing
information
• To run in linux
traceroute <address> , e.g.
traceroute www.yahoo.com
How Are Packets Directed?
• Virtual Circuit
– All packets take the same path from source to
destination
– Packets contain virtual circuit number (VC#) changes at each hop
– VC# tells router where to send the packet next
– Each router has table of incoming interface# and
VC#, and outgoing interface# and VC#
– Special packets are used to establish and break
virtual circuits
How are Packets Directed?
• Datagram
– Each packet has a header with an address
– Address is hierarchically structured
– Router looks at appropriate part of the address to
select next destination
•
•
•
•
•
Paris -> plane to USA [USA]
USA customs -> truck to Akron, OH post office [442]
Akron -> truck to Hiram, OH post office [44234]
Hiram post office -> Service Center [Hiram College]
Service center -> my mailbox [Ellen Walker, CS Dept]
Virtual Circuit vs. Datagram
• Virtual Circuit
– All packets take the same path, arrive in order
– Circuit, once established, subject to delays, etc.
– Routers must keep connection state information
• Datagram
– Packets can take different paths, must be
reassembled
– Paths can vary to account for outages; delays
– No connection state information maintained in
routers
Network Taxonomy
Internet Service Providers (ISPs)
• Tier 1 (“backbone”)
– AT&T, Sprint, UUNet, etc.
– National & international connections
• Tier 2
– Provide regional connections, e.g. Oarnet (Ohio)
– Purchase services from Tier 1 providers
• Tier 3 and local
– “Last mile” connections
– Purchase services from Tier 2
– Sell services directly to individual & small business
customers
Message Path through Internet
local
ISP
Tier 3
ISP
Tier-2 ISP
local
ISP
local
ISP
local
ISP
Tier-2 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP
local
ISP
NAP
Tier 1 ISP
Tier-2 ISP
local
ISP
Tier-2 ISP
local
ISP
Residential “gateway” connections
• Dialup modem
– Standard analog telephone line
– <= 56Kbps
• Digital subscriber line (DSL)
– Standard phone lines, restricted distance to modem
– 3 Frequency channels (downstream, upstream, voice)
– 384K–1.5Mbps downstream, 128K-256Kbps upstream
• Hybrid Fiber Coaxial Cable (HFC)
– Fiber optic cable into neighborhood terminal, connected to
coax cable to each house
– 2 channels (downstream & upstream)
– Shared medium (all messages to all connected modems)
Residential Access: Cable
Local Area Networks
• Ethernet (most common wired technology)
– 10 Mbps , 100 Mbps, 1Gbps even now 10Gps
– Twisted pair copper wire or coax cable
• Wireless LAN
– Base station (access point) connected to wired LAN
– IEEE 802.11b is 11Mbps (802.11g is faster)
– Typically good for 10s of meters
• WAP (Europe, US) and I-mode (Japan)
– Extend cell-phone network to Internet
– Eg. Cingular GPRS, still fairly expensive
Building Links: Guided Media
• Twisted pair copper wire (phone, CAT-5)
– 2 wires twisted around each other to help limit interference
– CAT-5 cable has more twists, better insulation
• Coaxial Cable
– Cable wrapped by insulation surrounded by another
conductor
• Fiber optics
– Thin flexible “glass pipe”
– Require optical rather than electrical transmitters,
receivers, switches, amplifiers, etc.
Unguided Media
• Terrestrial radio link
– Bands allocated for wireless transmission
– FCC regulated in US
• Satellite radio channels
– Geostationary satellites used for phone network &
internet backbone
Dealing with Complexity
• The path a message takes is complex; dealing with
hosts, switches, packets, media, etc.
• Therefore we use an abstract model to divide the
transmission into layers
• The sender at each layer uses the lower layers (as a
black box) to send information directly to the recipient
at that layer.
• Each layer considers information from layers above
to be “data bits”
Example: Airplane Routing
ticket (purchase)
ticket (complain)
baggage (check)
baggage (claim)
gates (load)
gates (unload)
runway takeoff
runway landing
airplane routing
airplane routing
airplane routing
Internet Protocol Stack
• Application (web, email)
• Transport (TCP, UDP, message transport)
• Network (IP, datagram routing between
endhosts)
• Link (Ethernet, datagram transfer between
neighboring switches)
• Physical (bits on the “wire”)
Internet Hardware
• End host (all 5 layers)
• Intermediate switches (generally 3,
sometimes 2 layers only)
Enveloping a Message
• Each layer takes data from the above layer
– Breaks it apart if necessary (e.g. packetizing)
– Adds an “envelope” = header and/or footer bits
– Passes it to the next layer below, which will treat
envelope as part of the data