Computer Networks Chapter 1
Consider the network shown in the figure below, with three links, each with the specified transmission rate and link length. Assume the length of a packet is 8000 bits.What is the transmission delay at link 2? [Note: you can find more problems like this one here.]
8 x 10^(-5) secs
Consider the network shown in the figure below, with three links, each with the specified transmission rate and link length. Assume the length of a packet is 8000 bits. The speed of light propagation delay on each link is 3x10^8 m/secWhat is the propagation delay at (along) link 2?
.0033 secs
Which of the following physical layer technologies has the highest transmission rate and lowest bit error rate in practice?
Fiber optic cable
Choose one the following two definitions that makes the correct distinction between routing versus forwarding.
Forwarding is the local action of moving arriving packets from router's input link to appropriate router output link, while routing is the global action of determining the source-destination paths taken by packets.
Consider the figure below, showing a link-layer frame heading from a host to a router. There are three header fields shown. Match the name of a header with a header label shown in the figure. ANSWER LIST: Physical layer Transport layer Network Layer Link layer Application layer QUESTION LIST: Header H1 Header H2 Header H3
Header H1: Link layer Header H2: Network Layer Header H3: Transport layer
Which of the definitions below describe what is meant by the term "encapsulation"?
Taking data from the layer above, adding header fields appropriate for this layer, and then placing the data in the payload field of the "packet" for that layer.
When we say that the Internet is a "network of networks," we mean? Check all that apply (hint: check two or more).
The Internet is made up of a lot of different networks that are interconnected to each other. The Internet is made up of access networks at the edge, tier-1 networks at the core, and interconnected regional and content provider networks as well.
Which of the characteristics below are associated with the technique of packet switching? Select all correct answers. [Hint: more than one of the answers is correct].
This technique is used in the Internet. Data may be queued before being transmitted due to other user's data that's also queueing for transmission. Congestion loss and variable end-end delays are possible with this technique. Resources are used on demand, not reserved in advance.
Which of the characteristics below are associated with the technique of circuit switching? Select all correct answers. [Hint: more than one of the answers is correct].
This technique was the basis for the telephone call switching during the 20th century and into the beginning of this current century. Reserves resources needed for a call from source to destination. Frequency Division Multiplexing (FDM) and Time Division Multiplexing (TDM) are two approaches for implementing this technique.
Suppose a packet is L = 1200 bytes long (one byte = 8 bits), and link transmits at R = 100 Mbps (i.e., a link can transmit bits 100,000,000 bits per second). What is the transmission delay for this packet? [Note: you can find more problems like this one here.]
.000096 secs
Suppose a packet is L = 1500 bytes long (one byte = 8 bits), and link transmits at R = 1 Gbps (i.e., a link can transmit bits 1,000,000,000 bits per second). What is the transmission delay for this packet? [Note: you can find more problems like this one here.]
.000012 secs
Consider the scenario shown below, with four different servers connected to four different clients over four three-hop paths. The four pairs share a common middle hop with a transmission capacity of R = 300 Mbps. The four links from the servers to the shared link have a transmission capacity of RS = 50 Mbps. Each of the four links from the shared middle link to a client has a transmission capacity of RC = 90 Mbps. Assuming that the servers are all sending at their maximum rate possible, what are the link utilizations of the client links (with transmission capacity RC)? Enter your answer in a decimal form of 1.00 (if the utilization is 1) or 0.xx (if the utilization is less than 1, rounded to the closest xx). Your answer: The utilization of client link is: [A]
0.56
Consider the scenario shown below, with four different servers connected to four different clients over four three-hop paths. The four pairs share a common middle hop with a transmission capacity of R = 300 Mbps. The four links from the servers to the shared link have a transmission capacity of RS = 50 Mbps. Each of the four links from the shared middle link to a client has a transmission capacity of RC = 90 Mbps.Assuming that the servers are all sending at their maximum rate possible, what are the link utilizations of the shared link (with transmission capacity R)? Enter your answer in a decimal form of 1.00 (if the utilization is 1) or 0.xx (if the utilization is less than 1, rounded to the closest xx). Your answer: The utilization of shared link is: [A]
0.67
COMPUTING END-END DELAY (TRANSMISSION AND PROPAGATION DELAY) Consider the figure below, with three links, each with the specified transmission rate and link length. Assume the length of a packet is 8000 bits. The speed of light propagation delay on each link is 3x10^8 m/sec Round your answer to two decimals after leading zeros 1. What is the transmission delay of link 1? 2. What is the propogation delay of link 1? 3. What is the total delay of link 1? 4. What is the transmission delay of link 2? 5. What is the propogation delay of link 2? 6. What is the total delay of link 2? 7. What is the transmission delay of link 3? 8. What is the propogation delay of link 3? 9. What is the total delay of link 3? 10. What is the total delay?
1. 0.0008 2. 1.00E-5 3. 0.00081 4. 0.0008 5. 0.0017 6. 0.0025 7. 8.00E-6 8. 6.67E-6 9. 1.47E-5 10. 0.0033
COMPUTING THE ONE-HOP TRANSMISSION DELAY Consider the figure below, in which a single router is transmitting packets, each of length L bits, over a single link with transmission rate R Mbps to another router at the other end of the link. Suppose that the packet length isL= 16000 bits, and that the link transmission rate along the link to router on the right isR= 1000 Mbps. Round your answer to two decimals after leading zeros 1. What is the transmission delay? 2. What is the maximum number of packets per second that can be transmitted by this link?
1. 1.60E-5 2. 62500
QUANTITATIVE COMPARISON OF PACKET SWITCHING AND CIRCUIT SWITCHING This question requires a little bit of background in probability (but we'll try to help you though it in the solutions). Consider the two scenarios below: A circuit-switching scenario in which Ncs users, each requiring a bandwidth of 10 Mbps, must share a link of capacity 150 Mbps. A packet-switching scenario with Nps users sharing a 150 Mbps link, where each user again requires 10 Mbps when transmitting, but only needs to transmit 30 percent of the time. 1. When circuit switching is used, what is the maximum number of users that can be supported? 2. Suppose packet switching is used. If there are 29 packet-switching users, can this many users be supported under circuit-switching? Yes or No. 3. Suppose packet switching is used. What is the probability that a given (specific) user is transmitting, and the remaining users are not transmitting? 4. Suppose packet switching is used. What is the probability that one user (any one among the 29 users) is transmitting, and the remaining users are not transmitting 5. When one user is transmitting, what fraction of the link capacity will be used by this user? Write your answer as a decimal. 6. What is the probability that any 15 users (of the total 29 users) are transmitting and the remaining users are not transmitting? 7. What is the probability that more than 15 users are transmitting?
1. 15 2. No 3. 1.38E-5 4. 0.0004 5. 0.067 6. 0.0075 7. 0.0041
END TO END THROUGHPUT AND BOTTLENECK LINKS Consider the scenario shown below, with four different servers connected to four different clients over four three-hop paths. The four pairs share a common middle hop with a transmission capacity of R = 400 Mbps. The four links from the servers to the shared link have a transmission capacity of RS = 40 Mbps. Each of the four links from the shared middle link to a client has a transmission capacity of RC = 90 Mbps. 1. What is the maximum achievable end-end throughput (in Mbps) for each of four client-to-server pairs, assuming that the middle link is fairly shared (divides its transmission rate equally)? 2. Which link is the bottleneck link? Format as Rc, Rs, or R 3. Assuming that the servers are sending at the maximum rate possible, what are the link utilizations for the server links (RS)? Answer as a decimal 4. Assuming that the servers are sending at the maximum rate possible, what are the link utilizations for the client links (RC)? Answer as a decimal 5. Assuming that the servers are sending at the maximum rate possible, what is the link utilizations for the shared link (R)? Answer as a decimal
1. 40 2. Rs 3. 1 4. 0.44 5. 0.4
CAR - CARAVAN ANALOGY Consider the figure below, adapted from Figure 1.17 in the text, which draws the analogy between store-and-forward link transmission and propagation of bits in packet along a link, and cars in a caravan being serviced at a toll booth and then driving along a road to the next tollbooth. Suppose the caravan has 5 cars, and that the tollbooth services (that is, transmits) a car at a rate of one car per 5 seconds. Once receiving serving a car proceeds to the next tool both, which is 400 kilometers away at a rate of 10 kilometers per second. Also assume that whenever the first car of the caravan arrives at a tollbooth, it must wait at the entrance to the tollbooth until all of the other cars in its caravan have arrived, and lined up behind it before being serviced at the toll booth. (That is, the entire caravan must be stored at the tollbooth before the first car in the caravan can pay its toll and begin driving towards the next tollbooth). 1. Once a car enters service at the tollbooth, how long does it take until it leaves service? 2. How long does it take for the entire caravan to receive service at the tollbooth (that is the time from when the first car enters service until the last car leaves the tollbooth)? 3. Once the first car leaves the tollbooth, how long does it take until it arrives at the next tollbooth? 4. Once the last car leaves the tollbooth, how long does it take until it arrives at the next tollbooth? 5. Once the first car leaves the tollbooth, how long does it take until it enters service at the next tollbooth? 6. Are there ever two cars in service at the same time, one at the first toll booth and one at the second toll booth? Answer Yes or No 7. Are there ever zero cars in service at the same time, i.e., the caravan of cars has finished at the first toll both but not yet arrived at the second tollbooth? Answer Yes or No
1. 5 2. 25 3. 40 4. 40 5. 60 6. No 7. Yes
CIRCUIT SWITCHING Consider the circuit-switched network shown in the figure below, with circuit switches A, B, C, and D. Suppose there are 14 circuits between A and B, 10 circuits between B and C, 18 circuits between C and D, and 19 circuits between D and A. 1. What is the maximum number of connections that can be ongoing in the network at any one time? 2. Suppose that these maximum number of connections are all ongoing. What happens when another call connection request arrives to the network, will it be accepted? Answer Yes or No 3. Suppose that every connection requires 2 consecutive hops, and calls are connected clockwise. For example, a connection can go from A to C, from B to D, from C to A, and from D to B. With these constraints, what is the is the maximum number of connections that can be ongoing in the network at any one time? 4. Suppose that 11 connections are needed from A to C, and 12 connections are needed from B to D. Can we route these calls through the four links to accommodate all 23 connections? Answer Yes or No
1. 61 2. No 3. 29 4. Yes
Match the access network with the approximate speeds that a subscriber might experience. (Note: if you look these up, do so in the 8E textbook, slides,or video -- not in the 7E or earlier versions, since link access speeds are always increasing over the years). ANSWER LIST: Wired. Up to 10's to 100's of Mbps downstream per user. Wired. Up to 100's Gbps per link. Wireless. Up to 10's Mbps per device. Wired. Up to 10's of Mbps downstream per user. Wireless, up to 10's Kbps per device. Wireless. 10's to 100's of Mbps per device. Wired. Up to 1 Tbps per link. QUESTION LIST: 1. Ethernet 2. 802.11 WiFi 3. Cable access network 4. Digital Subscriber Line 5. 4G cellular LTE
1. Ethernet Wired. Up to 100's of Mbps downstream per link. 2. 802.11 WiFi Wireless. 10's to 100's of Mbps per device 3. Cable access network Wired. Up to 10's to 100's of Mbps downstream per user. 4. Digital Subscriber Line Wired. Up to 10's of Mbps downstream per user. 5. 4G cellular LTE Wireless. Up to 10's Mbps per device.
THE IP STACK AND PROTOCOL LAYERING In the scenario below, imagine that you're sending an http request to another machine somewhere on the network. 1. What layer in the IP stack best corresponds to the phrase: 'handles the delivery of segments from the application layer, may be reliable or unreliable' 2. What layer in the IP stack best corresponds to the phrase: 'moves datagrams from the source host to the destination host' 3. What layer in the IP stack best corresponds to the phrase: 'passes frames from one node to another across some medium' 4. What layer in the IP stack best corresponds to the phrase: 'handles messages from a variety of network applications' 5. What layer in the IP stack best corresponds to the phrase: 'bits live on the wire' 6. What layer corresponds to box 1? 7. What layer corresponds to box 2? 8. What layer corresponds to box 3? 9. What layer corresponds to box 4? 10. What layer corresponds to box 5? 11. What layer corresponds to box 6? 12. What layer corresponds to box 7? 13. What layer corresponds to box 8? 14. What layer corresponds to box 9? 15. What layer corresponds to box 10? 16. What layer corresponds to box 11? 17. What layer corresponds to box 12? 18. What layer corresponds to box 13? 19. What layer corresponds to box 14? 20. What layer corresponds to box 15?
1. Transport Layer 2. Network Layer 3. Link Layer 4. Application Layer 5. Physical Layer 6. Application Layer 7. Transport Layer 8. Network Layer 9. Link Layer 10. Physical Layer 11. Physical Layer 12. Link Layer 13. Physical Layer 14. Link Layer 15. Network Layer 16. Physical Layer 17. Link Layer 18. Network Layer 19. Transport Layer 20. Application Layer
QUEUING DELAY Consider the queuing delay in a router buffer, where the packet experiences a delay as it waits to be transmitted onto the link. The length of the queuing delay of a specific packet will depend on the number of earlier-arriving packets that are queued and waiting for transmission onto the link. If the queue is empty and no other packet is currently being transmitted, then our packet's queuing delay will be zero. On the other hand, if the traffic is heavy and many other packets are also waiting to be transmitted, the queuing delay will be long. Assume a constant transmission rate of R = 800000 bps, a constant packet-length L = 4100 bits, and a is the average rate of packets/second. Traffic intensity I = La/R, and the queuing delay is calculated as I(L/R)(1 - I) for I < 1. 1. In practice, does the queuing delay tend to vary a lot? Answer with Yes or No 2. Assuming that a = 21, what is the queuing delay? Give your answer in milliseconds (ms) 3. Assuming that a = 63, what is the queuing delay? Give your answer in milliseconds (ms) 4. Assuming the router's buffer is infinite, the queuing delay is 1.1205 ms, and 1158 packets arrive. How many packets will be in the buffer 1 second later? 5. If the buffer has a maximum size of 923 packets, how many of the 1158 packets would be dropped upon arrival from the previous question?
1. Yes 2. 0.4921 3. 1.1205 4. 266 5. 235
Consider the scenario shown below, with four different servers connected to four different clients over four three-hop paths. The four pairs share a common middle hop with a transmission capacity of R = 300 Mbps. The four links from the servers to the shared link have a transmission capacity of RS = 50 Mbps. Each of the four links from the shared middle link to a client has a transmission capacity of RC = 90 Mbps.Assuming that the servers are all sending at their maximum rate possible, what are the link utilizations for the server links (with transmission capacity RS)? Enter your answer in a decimal form of 1.00 (if the utilization is 1) or 0.xx (if the utilization is less than 1, rounded to the closest xx). Your answer: The utilization of the server links is: [A]
1.00
What is the maximum throughput achievable between sender and receiver in the scenario shown below?
1.5 Mbps
Consider the scenario shown below, with four different servers connected to four different clients over four three-hop paths. The four pairs share a common middle hop with a transmission capacity of R = 300 Mbps. The four links from the servers to the shared link have a transmission capacity of RS = 50 Mbps. Each of the four links from the shared middle link to a client has a transmission capacity of RC = 90 Mbps.What is the maximum achievable end-end throughput (an integer value, in Mbps) for each of four client-to-server pairs, assuming that the middle link is fairly shared (divides its transmission rate equally) and all servers are trying to send at their maximum rate? Your answer: [A] Mbps
50
Consider the circuit-switched network shown in the figure below, with four circuit switches A, B, C, and D. Suppose there are 20 circuits between A and B, 19 circuits between B and C, 15 circuits between C and D, and 16 circuits between D and A. What is the maximum number of connections that can be ongoing in the network at any one time? [Note: you can find more questions like this one here.
70
Which of the following descriptions below correspond to a "nuts-and-bolts" view of the Internet? Select one or more of the answers below that are correct. [Hint: more than one of answers below are correct].
A collection of billions of computing devices, and packet switches interconnected by links. A collection of hardware and software components executing protocols that define the format and the order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event. A "network of networks".
Which of the following descriptions below correspond to a "services" view of the Internet? Select one or more of the answers below below that are correct below that are correct. [Hint: more than one of answers below are correct].
A place I go for information, entertainment, and to communicate with people. A platform for building network applications
Match the name of an Internet layer with unit of data that is exchanged among protocol entities at that layer, using the pulldown menu. ANSWER LIST: Message Frame Bit Datagram Segment QUESTION LIST: Application layer Transport layer Network layer Link layer Physical layer
Application layer: Message Transport layer: Segment Network layer: Datagram Link layer: Frame Physical layer: Bit
Consider a scenario in which 5 users are being multiplexed over a channel of 10 Mbps. Under the various scenarios below, match the scenario to whether circuit switching or packet switching is better. ANSWER LIST: Circuit switching Packet switching Neither works well in this overload scenario QUESTION LIST: Each user generates traffic at an average rate of 2.1 Mbps, generating traffic at a rate of 15 Mbps when transmitting Each user generates traffic at an average rate of 2 Mbps, generating traffic at a rate of 2 Mbps when transmitting Each user generates traffic at an average rate of 0.21 Mbps, generating traffic at a rate of 15 Mbps when transmitting
Each user generates traffic at an average rate of 2.1 Mbps, generating traffic at a rate of 15 Mbps when transmitting: Neither works well in this overload scenario Each user generates traffic at an average rate of 2 Mbps, generating traffic at a rate of 2 Mbps when transmitting.: Circuit switching Each user generates traffic at an average rate of 0.21 Mbps, generating traffic at a rate of 15 Mbps when transmitting.: Packet switching
Match the networking event with the time frame when the event occurred. ANSWER LIST: 2010 - 2020 Early 1960's 2000-2010 1990's 1970's Early 1980's Late 1960's Late 1980's QUESTION LIST: Early studies of packet switching by Baran, Davies, Kleinrock. First ARPAnet node operational. Internetting: DARPA researchers connect three networks together. The Internet Protocol (IP) is standardized in RFC 791. Congestion control is added to the TCP protocol. The WWW starts up (note: the WWW design started at the end of previous decade). Software-defined networking begins. The number wireless Internet-connected devices surpasses the number of connected wired devices.
Early studies of packet switching by Baran, Davies, Kleinrock.: Early 1960's First ARPAnet node operational.: Late 1960's Internetting: DARPA researchers connect three networks together.: 1970's The Internet Protocol (IP) is standardized in RFC 791.: Early 1980's Congestion control is added to the TCP protocol.: Late 1980's The WWW starts up (note: the WWW design started at the end of previous decade).: 1990's Software-defined networking begins.: 2000-2010 The number wireless Internet-connected devices surpasses the number of connected wired devices.: 2010 - 2020
Match the function of a layer in the Internet protocol stack to its its name in the pulldown menu. ANSWER LIST: Physical layer Link layer Application Layer Network layer Transport layer QUESTION LIST: Protocols that are part of a distributed network application. Transfer of data between one process and another process (typically on different hosts). Delivery of datagrams from a source host to a destination host (typically). Transfer of data between neighboring network devices. Transfer of a bit into and out of a transmission media.
Protocols that are part of a distributed network application.: Application Layer Transfer of data between one process and another process (typically on different hosts).: Transport layer Delivery of datagrams from a source host to a destination host (typically).: Network layer Transfer of data between neighboring network devices.: Link layer Transfer of a bit into and out of a transmission media.: Physical layer
Match the description of a security defense with its name. ANSWER LIST: Encryption Access control Authentication Firewall Digital signatures QUESTION LIST: Specialized "middleboxes" filtering or blocking traffic, inspecting packet contents inspections Provides confidentiality by encoding contents Used to detect tampering/changing of message contents, and to identify the originator of a message. Limiting use of resources or capabilities to given users. Proving you are who you say you are.
Specialized "middleboxes" filtering or blocking traffic, inspecting packet contents inspections: Firewall Provides confidentiality by encoding contents: Encryption Used to detect tampering/changing of message contents, and to identify the originator of a message.: Digital Signatures Limiting use of resources or capabilities to given users.: Access Control Proving you are who you say you are.: Authentication
Match the description of each component of packet delay to its name in the pull down list. ANSWER LIST: Transmission delay Propagation delay Processing delay Queueing delay QUESTION LIST: Time needed to perform an integrity check, lookup packet information in a local table and move the packet from an input link to an output link in a router. Time spent waiting in packet buffers for link transmission. Time spent transmitting packets bits into the link. Time need for bits to physically propagate through the transmission medium from end one of a link to the other.
Time needed to perform an integrity check, lookup packet information in a local table and move the packet from an input link to an output link in a router.: Processing delay Time spent waiting in packet buffers for link transmission.: Queueing delay Time spent transmitting packets bits into the link.: Transmission delay Time need for bits to physically propagate through the transmission medium from end one of a link to the other.: Propagation delay
Which of the following human scenarios involve a protocol (recall: "Protocols define the format, order of messages sent and received among network entities, and actions taken on message transmission, receipt")? Select one or more answers below that are correct. Hint: more than one of answers below are correct.
Two people introducing themselves to each other. One person asking, and getting, the time to/from another person. A student raising her/his hand to ask a really insightful question, followed by the teaching acknowledging the student, listening carefully to the question, and responding with a clear, insightful answer. And then thanking the student for the question, since teachers love to get questions.
Perform a traceroute from your computer (on whatever network you happen to be on) to gaia.cs.umass.edu. Use traceroute (on Mac terminal) or tracert (on Windows command line) or tracepath (on a Linux command line). Enter the missing part of the name of the router just before the host gaia.cs.umass.edu is reached: ??.cs.umass.eduNote: Routing may change, so the answer here may not be correct anymore. Also, if you are a Verizon user, there are known problems using traceroute with Verizon - if traceroute shows you two hops only to gaia.cs.umass.edu or any destination, skip this question.
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