Final Flashcards
(34 cards)
Social Issues of computer networks
- Content ownership (DMCA takedowns)
- Privacy (Web tracking)
- Malicious software (Theft, phishing)
Types of networks classified by their scale (5), with examples
- PAN (Personal Area Network).
Ex: Bluetooth keyboard & mouse connected to computer) - LAN (Local Area Network)
Ex: Connect devices in a home or office building - MAN (Metropolitan Area Network)
Ex: Connect devices over a metropolitan area. Houses on a street connect to junction box that connects to antenna and internet. - WAN (Wide Area Network)
Ex: Connected devices over a country. ISP network, customers buy connectivity to use - The Internet (network of all networks)
Service vs Protocol
Service: Layer talks vertically to the other
Protocol: Layer talks to its peer horizontally
OSI 7 layers and their functions
- Application Layer
- Provides functions needed by users - Presentation Layer
- Translates, encrypts, and compresses data - Session Layer
- Manages task dialogues - Transport Layer
- Provides end-to-end message delivery and error recovery - Network Layer
- Sends packets from source host to destination host - Data Link Layer
- Organizes bits into frames, provides hop-to-hop delivery - Physical Layer
- Sends bits as signals from one hop to the next
Types of data deliveries
Node to node: Data link layer
Host to host: Network layer
Process to process: Transport layer
TCP model layers used in course (5)
- Application
- Transport
- Network
- Link
- Physical
Channelization definition and the 3 protocols
Def: multiple access method in which the available bandwidth of a link is shared in time, frequency, or code, between different stations.
- Frequency-Division Multiple Access (FDMA)
- Time-Division Multiple Access (TDMA)
- Code-Division Multiple Access (CDMA)
2 Basic rules of a Walsh table, & how to find the number of sequences
- W1=(+1)
- W2N = WN WN WN /WN
Number sequences need to be N = 2^k were k is an integer
Find the chips for a network with two stations and four stations
2: [+1+1] and [+1-1]
4: [+1+1+1+1], [+1-1+1-1], [+1+1-1-1] and [+1-1-1+1]
CDMA: What is the number of sequences if we have 90 stations in our network?
Needs to = 2^k
2^7 = 128 sequences
Random Access definition and the 4 protocols
Def: No station is superior to another station, and no station controls another in allowing it to send or not send. Each station uses a protocol to make a decision on whether or not to send
- ALOHA
- Carrier Sense Multiple Access
- Carrier Sense Multiple Access with Collision Detection
- Carrier Sense Multiple Access with Collision Avoidance
Explain the procedure for pure ALOHA protocol
Send Frame ->Wait Time-Out Time (2xTp)->Ack Received?->no-> K=K+1-> check if K>Kmax, if not->Choose random number R between 0 & 2^k-1 ->Wait Tb Time (R x Tp/Tfr)-> restart
What is the vulnerable time for pure ALOHA Protocol?
A pure ALOHA network transmits a 200-bit frame on a shared channel of 200kbps, what is the time requirement to make this frame collision-free?
Vulnerable time = 2 x Tfr
Tfr= 200bits/200kbps = 1ms
2 x 1ms = 2ms
What is the vulnerable time in CSMA?
Vulnerable time = propagation time
What are the 3 persistence methods and their flow diagrams
- 1-persistent. Highest chance of collision
Channel:
Idle = Station can transmit
Busy = Restart - Non-persistent. Reduces chance of collision, less efficient
Channel:
Idle = Station can transmit
Busy = Wait randomly - p-persistent. Reduces the chance of collision and improves efficiency.
Channel:
Idle = Probability Outcome?
- ≤ p = Station can transmit
- > p = Wait a slot
- Channel:- Idle = back to probability outcome
- Busy = Use back-off process as if collision occurred
A network using CSMA/CD has a bandwidth of 10 Mbps, if the maximum propagation time is 25.6 ms, what is the minimum size of the frame?
Tfr= 2 x Tp = 2 x 25.6 = 51.2 ms to detect collision
Size of frame = 10mbps x 51.2 ms = 512 bits or 64 bytes
What is controlled-access? Explain each of the 2 popular methods.
Def: Stations consult one another to find which station has the right to send. A station cannot send unless authorized by other stations.
- Polling.
- Poll each station - Token Passing. Connects all stations.
- Physical ring
- Dual ring
- Bus ring
- Star ring
How does a machine detect collisions in CSMA/CD protocol?
By measuring the energy level of the channel. If the measured energy level is almost twice the normal level then a collision is detected
What is the motivation for creating the CSMA/CA protocol?
Because detecting collisions (CSMA/CD) is very hard in wireless communications, since much of the energy is lost in transmission. The collision may add 5% to 10% additional energy.
What are the rules for IPv4 addresses? (4)
- There must be no leading zeros
- There can be no more than four numbers
- Each number needs to be less than or equal to 255
- A mixture of binary notation and dotted-decimal notation is not allowed
Explain the 2 ways of finding the first and last addresses in a block? How to find the number of addresses in the block?
- Set 32-n rightmost bits to 0 for first address, set 32-n bits to 1 for last address
- AND the address with the mask in binary form (28 means 28 1s and 4 0s) for first address, OR the address with the mask complement in binary form for the last address
Find # of addresses in a block:
- 2^(32-n)
- Complement the mask, interpret as decimal, add 1
What address changes from hop-to-hop, and what address usually remains the same?
Physical addresses change from hop-to-hop, Logical and port addresses usually remain the same
Explain NAT
Network Address Translation
- enables a user to have a large set of addresses internally and one address or a set of addresses externally.
- Fixes the issue of limited addresses
- NAT router replaces the source address in the packet with the global NAT address
Hubs vs Switches
Hubs
- Physical-layer repeaters
- Bits from one link go to all other links at the same rate without buffering
Switches
- Operates at the link layer
- Has multiple ports, each connected to a computer
- Forwards frames only to the destination computer since it knows the MAC address of each
