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CS 134
Midterm
116
Computer Science
Undergraduate 1
03/15/2009

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Term
Declaration (3 types)
Definition
Tell Java which names you plan to use and what types of value or object can be associated with each name
1. Instance variable declaration
2. Local variable declarations
3 Formal parameter declarations
Term
Ex of Instance Variable declaration
Definition
private Netconnection toServer;
Term
Ex of Local Variable declaration
Definition
String latestLine;
Term
Ex of parameter declaration
Definition
the JButton which in
public void buttonClicked(JButton which) {
Term
Statements (4 types)
Definition
Tell Java to perfrom a particular action, commands
1. mutator method invocations
2. assignments
3. Compound statements
4. if statements
Term
Ex of mutator method invocations
Definition
contentPane.add(aButton); (change objects states)
Term
Ex of Assignments
Definition
toServer = new NetConnection(...);
Term
compound statements
Definition
any list of statements
Term
Ex of if statement
Definition
if(condition) {
statement
}else if{
statement
Term
Expressions (5 types)
Definition
Describes variables or objects. They appear on the right hand side of assignments or as actual parameters in invocations and constructions
1. constructions
2. accessor method invocations
3. literals
4. variable names
5. formulas
Term
Ex of constructions
Definition
new JButton("Click Here");
Term
Ex of accessor method invocations
Definition

hostName.getText(); toServer.in.nextLine();

Accessor methods access information about a the state of an object 

Term
literals
Definition
"cortland.cs.williams.edu"
300
Term
Ex of variable names
Definition
toSever
hostName
Term
formula
Definition
"USER" + id.getText()
WINDOW_WIDTH/2
Term
Digital
Definition
Having digits, Expressed in digits, using or giving a reading in digits
Term
!
Definition
not
Term
&&
Definition
and
Term
||
Definition
or
Term
Start bit
Definition
one unit signal that is not part of message but signals when it begins
Term
Ethernet time variables (R,D,L,T,c,W,Q,M
Definition
R = the rate at which bits are transmitted (measured in bits per second. This is what the Ethernet paper calls C)
D = number of data bits in each entry
L = the maximum allowed length of a network (measured in meters)
c = the speed at which signals propagate on network cables (measured in meters/second --- close to the speed of light)
W = the expected number of slots of contention before a successful transmission
Q = the number of active stations attached to the network
M = the minimum allowable packet size (measured in bits)
Term
boolean
Definition
variable to measure true/false
Term
Width of bit on fiber
Definition
W = c*T = c/R (T=1/R)
Term
Transmission Delay
Definition
D/R (amount it takes to get the entire message on the wire)
Term
Propagation Delay
Definition
L/C (amount it takes for a bit of that message to travel down the wire)
Term
Total Delay
Definition
D/R + L/C
Term
!=
Definition
does not equal
Term
>=
Definition
greater than or equal to
Term
==
Definition
if and only if
Term
.=
Definition
use with strings
Term
Get a substring for tom in addr1 which is tom@cortland.edu
Definition
addr1.substring(0,4);
Term
what indexOf will spit out if addr1 which is tom@cortland.edu addr1.indexOf( d, 12);
Definition
14
Term
Common structure of while loops
Definition
1. Initialize (buttonDisplayed =1)
2. Condition
3. Does the work of the loop
4. Increment step - makes sure condition becomes false so loop doesn't run forever
Term
Worst Case amount of time you have to wait to detect a collision
Definition
2L/C
Term
Collections
Definition
any collection is a thing (or empty) that is that thing plus everything else in the collection next to it
Term
number of bits in ASCII code
Definition
8 (128 possible symbols)
Term
Bits in Unicode
Definition
16 (65536 = 2^16 possible symbols - used for all the world's alphabet
Term
prefix free code
Definition
no codeword used in the scheme appears as a prefix of any other codeword. Guaranteed to produce coded messages that are uniquely decodable
Term
Huffman Codes
Definition
Replace the two least used symbols with a new symbol:
• Represent the new symbol as pair composed of
symbols it replaces
• Assume new symbol occurs as often as the symbols it
replaces
• REPEAT UNTIL THERE IS ONLY ONE SYMBOL LEFT!
Term
On Off Keying
Definition
Sender and Sendee agree to a certain amount of time that a signal for that amount of time is one bit of information
Term
bits in a byte
Definition
8
Term
Manchester Encoding
Definition
_|- means 1; -|_ means 0
Term
class header
Definition
informs the computer that the text that follows describes a new class (ex:public class TouchyButton extends GUIManager
Term
constuctor
Definition
specifies the actions to be performed when this program is first started ex:public TouchyButton(){
Term
method definition
Definition
named sequence of program instruction Ex: public void buttonClicked() {
Term
Ethernet
Definition
branching broadcast communication system for carrying digital data among locally distributed computing stations
Term
Time division multiplexing
Definition
refers to taking turns when two computers try to use a network at the same time
Term
self-synchronizing transmission system
Definition
based on the contents of the message sender and receiver can figure out the message without precise time measurements
Term
carrier sense multiple access with collision detection (CSMA/CD)
Definition
Transmissions initiated by a station defer to any which may already be in progress. Once started, if interference with other packets is detected, a
transmission is aborted and rescheduled by its source station. After a certain period of interference-free transmission,a packet is heard by all stations and will run to completion without interference. Ethernet controllers in colliding stations each generate random retransmission intervals to avoid repeated collisions. The mean of a packet's retransmission intervals is adjusted as a function of collision history to keep Ether utilization near
the optimum with changing network load.
Term
Ethernet addressing
Definition
Each packet has a source and destination, both of which are identified in the packet's header. A packet placed on the Ether eventually propagates to all stations. Any station can copy a packet from the Ether into its local memory, but normally only an active destination station matching its address in the packet's header will do so as the packet passes.
Term
broadcast packet
Definition
packet with address 0 that will go to everyone
Term
Acquisition Probability
Definition
A = (1-(l/Q))^(Q-l) where Q is the # of computers with a message to transmit. Comes from the equation N/Q*(1-(1/Q))^(N-1) with assumption N=Q where N = The total number of computers
that are trying to send
Term
waiting time
Definition
w = (1 - A)/A where A is probability exactly one computer attempts a transmission in a given slot
Term
Efficiency
Definition
E = (P/C)/((P/C) + (W*T)) where W is the average number of times it takes to succeed in sending and T is the delay that one collision causes
Term
Ethernet packet format
Definition
Preamble: 8 bytes Destination address: 6 bytes Source Address: 6 bytes Length 2 bytes Data: Min 46 Checksum: 4bytes
Term
baseband transmission
Definition
data in transmission is based only on whether a signal is or is not being sent regardless of any underlying signal frequency; ethernet uses this
Term
broadband transmission
Definition
different frequencies are used to encode different signals
Term
carrier sense
Definition
computers do not transmit when the computer is aware of a signal on the cable
Term
collision detection
Definition
while a computer is transmitting it continues to listen to the message. If it changes to sound like a different signal than the one it was supposed to transmit it will detect a collision and stop transmitting
Term
1 persistent
Definition
after carrier sense detects there is a something being transmitted it will continue to sense the cable until it detects the entire packet has been sent. Then it begins to send its own packet.
Term
collision resolution
Definition
once a computer senses a collusion it sends a jamming signal then stops transmitting until system is idle then it picks a "random" wait time and will repeat if necessary until the messages get through
Term
Message switching
Definition
high delay good line sharing
Term
circuit switching
Definition
low delay reserved lines waste capacity
Term
packet switching
Definition
medium delay and line sharing better error check
Term
token ring
Definition
optimal fiber network that connects computers in a circle which takes turns in order
Term
How IP works
Definition
Computers take IP and puts it in correct protocol which when it arrives gets translated back into IP
Term
POPConnection class
Definition
takes message from server and then adds it to cache and fetches it recursively
Term
router if statement
Definition
if(the packet is for this comp) {deliver to the correct application} else if(the destination is on one of my networks) { send packet to destination } else { send packet to a good router}
Term
IP address prefix
Definition
the first three bytes are for network address the next three you make up
Term
router
Definition
computer that is part of two or more distinct networks and is configured to relay messages between networks to which it is connected
Term
Frame format for wireless network
Definition
Control/16/Duration/16/DestinationAdd1
/48/SourceAdd1/48/DestinationAdd2/48
/SequenceControl/48/SourceAdd2/48/
Data/2312/Checksum/32/ Wireless frame also has a preamble but it goes in a few different places...longer than ethernet preamble
Term
Control and duration field wireless packets
Definition
help the process wireless networks use to approximate carrier sense
Term
Control field in wireless packets
Definition
used to distinguish special RTS and CTS packets from data packets
Term
Duration field in wireless packet
Definition
used to hold transition duration info essential to the process of virtual carrier sense
Term
purpose of Internet Protocl
Definition
serves as an intermediate language used to simplify the construction of software required to translate between other protocols
Term
translation after/for receiving packet
Definition
In hardware, there will be wireless and Ethernet transmission/wireless then there will be software that translate both forms to IP and then can untranslate IP into the other format
Term
How IP can simplify the task of constructing client programs
Definition
our program will send and receive all of its messages in IP to the software that translates it into either Ethernet or wireless
Term
active program
Definition
program associated with the top-most window on your screen
Term
How information is displayed on your computer
Definition
input comes in from the keyboard or network messages and the operating system decides what application program the information should be sent to
then the application program sends what they what displayed to the operating systems display software which will display it on the screen
Term
Where all incoming messages are translated to IP
Definition
operating system before being sent out to application programs
Term
virtual network
Definition
the illusion that all the networks connected by routers form a single network
Term
physical networks
Definition
independent networks that make up the virtual network
Term
IP packet
Definition
Goes in data field of Ethernet or wireless packet
IP version/4/Hdr len/4/Service class/8/
Packet Length/16/Packet Number/16/ FragmentNumber/16/TTL/8/Protocol/8/
Error Check/16/From Addr/32/To Addr/32/
DATA (up to 65516 bytes)
Term
fragmentation
Definition
packet # and fragment # are used to enable the receiving comp to identify and reassemble the fragments of a single message
Term
Encapsilation
Definition
putting one packet inside the data field of another packet of a different format
Term
physical network standards
Definition
802.3 Ethernaet, 802.11 wireless
Term
802.xx addresses
Definition
groups of 4 binary digits are replaced by a single hexadecimal equivalent
Term
how 802.xx address are assigned
Definition
first 24 bits of an address notates a manufacturer next 24 bits are chosen so they do not overlap with any other next 24 bits that the manufacturer has made
Term
FF:FF:FF:FF:FF:FF
Definition
Ethernet Broadcast address
Term
convert to dotted decimal notation
Definition
take an IP address and break the sequence of binary digits that form the address into four 8 bit subsequences then translate them to decimal notation and put periods in between
Term
IP address meaning
Definition
first 16 bits (6 digits) are associated with a physical network operated by my Internet service provider
Term
How classes open up new IP address
Definition
Three classes for size of network...machines on smaller networks will have no network prefix while machines on bigger networks will have network prefix
Term
domain name servers
Definition
maintains a database listing the names and IP addresses of many machines and the addresses of servers that can be used to look up other names
Term
domain name cashe
Definition
collection of saved IP addresses that correspond to domain name servers
Term
DHCP
Definition
Dynamic Host Configuration Protocol. DHCP server must be installed in each physical network. Tells computers their IP address, the IP address of a good router and IP address of at least one DNS
Term
How DHCP works
Definition
when machine is first connected to the internet they will send a broadcast packet out to every machine in the network. All the machines will ignore it except for the DHCP server which will send back the information
Term
How DHCP assigns address
Definition
if it is important to limit access to authorized users than a list of hardware addresses and IP addresses can be set up so that a when the DHCP server recieves a hardware address they just look at the list. If you don't have to limit access the server can just assign any number not in use. To ensure this number is not in use it put it in a list for a time limit called lease. Once the lease is up, the server sends a message to the machine saying that it must renew its IP address. When the machine leaves the network, once the lease is up the number will be usable once again
Term
How to fill out Ethernet source address
Definition
Comes from machine
Term
How to fill out ethernet destination address
Definition
ARP - will place IP address in a broadcast packet...if machine is that IP address than they will send back a packet with their Ethernet address
Term
ARP cashe
Definition
list of recently used Ethernet address and their corresponding IP addresses
Term
routing/forwarding table
Definition
every entry in the table contains network prefix and the address of the best router to use as the first step to get to that network
Term
Dijkstra's algorithm
Definition
1. Mark starting point as KNOWN with length 0 2. Identify eacn neighbor of start as ADJACENT 3. Set first step of each neighbor to itself 4. Set route length of each neighbor to first step distance 5. While you don't know how to reach all the cities {6. Select adj city with shortest route 7. Identify adj city with shortest route as KNOWN 8. Mark neighbors of new KNOWN city that were DISTANT as ADJACENT 9. update path lengths and record first steps to ADJACENT neighbors of new KNOWN city}
Term
link state update
Definition
periodic messages from every other machine in the network about hwo long it takes for info to travel to all destinations connected to the sender that would provide the necessary complete description of the network
Term
OSPF
Definition
Important Internet routing protocol with the basis of Dijkstra's algorithm
Term
TCP
Definition
Transport Control Protocol- a general purpose transport protocol
Term
Services TCP provides
Definition
Connection orientation, point to point communication, complete reliability, Full duplex communication, stream interface, reliable connection startup and graceful connection close
Term
end to end protocol
Definition
protocol that provides a connection from one application on one computer to another application on another computer
Term
How TCP works
Definition
Each TCP packet is encapsulated in a IP packet which sends it to another computer and the TCP packet gets delivered to the application
Term
retransmission
Definition
when TCP receives data it sends an acknowledgment back to the sender. Whenever it sends data TCP starts a timer. If the timer expires before an acknowledgment arrives the sender retransmits the data
Term
Adaptive retransmission
Definition
TCP monitors current delay on each connection and adapts the retransmission timer to accommodates changing transmissions
Term
window
Definition
TCP uses this to control flow of data. When a connection is established each end of the connection allocates a buffer to hold incoming data. acks also send back the remaining buffer size...if buffer size equals 0 than the receiver will receive a zero window and must stop sending data
Term
SYN segment
Definition
describes messages in in three way handshake used to create a connection
Term
FIN segment
Definition
describes messages in three way handshake used to close a connection
Term
3-way handshake
Definition
used to establish or end connections where host 1 will send FIN + ACK to host 2 who will send FIN+ACK back to host 1 who will send an ACK to host 2 if creating a connection a random 32 bit sequence is generated
Term
Network Address Translation
Definition
a router like a home router takes from IP addresses and replaces them with its own IP address than it puts those values in a chart with inside/outside IP address and Port number
Term
3d matching problem, partition problem
Definition
subset sum problems
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