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11 Cards in this Set
- Front
- Back
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What are the five layers in the internet protocol stack? What do they do? |
Application, transport, network, link, physical. App - HTTP, FTP, applications running on end devices, Transport - UDP, TCP, Network
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What is NAT? Describe a scenario where you would use NAT. |
Network Address Translation. Translates private addresses to public addresses. |
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Suppose a computer is moved from one department to another. Does it's physical address need to change? Does the IP address need to change? Why/why not? |
It's physical address does not change - it will always reference the same machine. It's IP address will change because //////////TODO/////// |
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What info is used by a process running on one host to identify a process running on another host? |
IP address of destination host, and socket number of destination nost |
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Does FTP run over TCP or UDP? Why/why not? |
TCP. Needs acknowledgement. |
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Which one is efficient: Slotted Aloha, or pure Aloha? Why? |
Slotted, because it uses time intervals, and has a lesser probability of collisions. |
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Which on is preferred for data networks? Circuit switching or packet switching? Why? |
Packet switching, because it dynamically sets bandwidth that can be used, whereas circuit switching sets the bandwidth to be used, regardless of whether it gets used or not. |
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Alice sends an email to Bob. Describe the protocol used and the steps that follow. |
Alice uses her computer to send a message to bob@someone.com. Then alice's computer sends the message to her mail server. The mail server then establishes a TCP connection with bob's mail server. Then the mail server sends the message. Then bob's mail server puts the message in bob's mailbox. |
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Suppose the bandwitdth on a point to point link is 1Mbps and the RTT is 19ms. Assume that the size of each packet is 1000 bits. Also assume that the size of ACK packet is negligible. Answer each of the following for both go-back-n and selective repeat schemes. Assume that the sender always has data for transmission. A. Assuming that the link is error-free, what should be the minimum window size to achieve at least 75% utilization. B. What is the minimum number of bits needed to represent the sequence number corresponding to the above window size to ensure the correct operation of the scheme? |
A. 0.75 = N*(L/R/(19ms + L/R)). N = 15. WIndow size of 15. B. Go-back-n is always window size + 1. Selective repeat is always window size*2. Convert that number to the number of appropriate bits. |
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Answer the following assuming that a node under 10Mbps Ethernet just detected a fourth collision. Note that the slot size under Ethernet is 512 bits. A. What is the max backoff time it waits before contending for the channel for retransmission? What is the probability? B. What is the probability that the wait time is greater than 150 micro (u) seconds. |
A. Do this: 2^n. Where n = 4 (fourth collision). So 2^4 = 16, but since 0 is included, we subtract 1 to get 15. Then since it's a 10Mbps link, we'll move the decimal from 512 bits one place, then multiply that by 15, which equals 768 micro seconds. The probability is 1/16. B. The probability that the wait time is greateer than 150, is anything over or equal to N = 3 (51.2*3 > 150), so the probability is 13/16. |
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Suppose we want to transmit the data bits 100110 and protect them from errors using the generator 1001. A. What are the bits (data bits and CRC bits) that would be transmitted? B. Suppose the 1st and fourth bits are corrupted druing transmission. Will the reciever detect that there was some error? Justify. C. What if the 1st and 2nd bits are corrupted during transmission? Justify |
A. Do long division of 100110000/1001. The bold part is always the size of the generator minus 1. The CRC is the remainder of the division. The data bits is the original bits. B. Corrupt the 1st and 4th bits by switching their value, then do the division of that number and 1001. If the remainder is 0, then we would not detect the error. C. Same process as B. |
