Exam 1 - HW 1

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.

An image is 3840 x 2160 pixels with 3 bytes/pixel. Assume the image is uncompressed. How long does it take to transmit it over a 1-Mbps cable modem? (in seconds)

The image is 3840 × 2160 × 3 bytes or 8,294,400 pixels x 3 bytes x 8 bits/byte. This is 199,065,600 bits. At 1,000,000 bits/sec, it takes about 199.07 sec.

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?

0.000012 secs

Which of the following descriptions below correspond to a "services" view of the Internet?

A platform for building network applications. A place I go for information, entertainment, and to communicate with people.

"Wireless networks have surpassed wired networks in popularity, even though they typically provide less bandwidth. Select all the reasons listed below that are true and correct."

A potential problem is battery life, since most wireless devices tend to be mobile. Installing wires in buildings can be expensive. "Although wireless networks provide lower bandwidth than wired networks, their bandwidth has become large enough to support applications that people find meaningful." "Wireless networks allow people to move around, instead of tying them to a wall."

Match the function of a layer in the Internet protocol stack to its name in the pulldown menu. 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.

Application Transport Network Link Physical

"Layers one, two, and three of the OSI Model are implemented in the end systems but not in the routers in the network core."

False

"One disadvantage of using international standards is that if everyone uses the standard, everyone can talk to everyone."

False

ADSL bandwidth is shared, and Cable bandwidth is not shared.

False

Suppose 20 connections traverse the same link of rate 1 Gbps. Suppose that the client access links all have rate 10 Mbps. Then the maximum throughput for each connection is 50 Mbps.

False

The data link layer is responsible for sending data into packets.

False

The network layer encapsulates frames into segments.

False

The transport layer is responsible for sending data in frames.

False

Twisted-pair copper wire is no longer present in computer networks.

False

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.

"If the unit exchanged at the data link level is called a frame and the unit exchanged at the network level is called a packet, do frames encapsulate packets or do packets encapsulate frames?"

Frames encapsulate packets

Match the name of an Internet layer with unit of data that is exchanged among protocol entities at that layer, using the pulldown menu. Application Transport Network Link Physical

Message Segment Datagram Frame Bit

"A factor in the delay of a store-and-forward packet-switching system is how long it takes to store and forward a packet through a switch. If switching time is 20 µsec, is this likely to be a major factor in the response of a client-server system where the client is in New York and the server is in California? Assume the propagation speed in copper and fiber to be 2/3 the speed of light in vacuum." Assume the client and server are separated by 5000 km.

No

An image is 3840 x 2160 pixels with 3 bytes/pixel. Assume the image is uncompressed. How long does it take to transmit it over a 56-kbps modem channel? (in seconds)

The image is 3840 × 2160 × 3 bytes or 8,294,400 pixels x 3 bytes x 8 bits/byte. This is 199,065,600 bits. At 56,000 bits/sec, it takes about 3,554.74 sec.

"A client-server system uses a satellite network, with the satellite at a height of 40,000 km. What is the best-case delay in response to a request?"

The request has to go up and down, and the response has to go up and down. The total path length traversed is thus 160,000 km. The speed of light in air and vacuum is 300,000 km/sec, so the propagation delay alone is 160,000/300,000 sec or about 533 msec.

"A factor in the delay of a store-and-forward packet-switching system is how long it takes to store and forward a packet through a switch. If switching time is 20 µsec, what percentage of the trip time in the response of a client-server system where the client is in New York and the server is in California is due to switching delay? Assume the propagation speed in copper and fiber to be 2/3 the speed of light in vacuum and there are 50 switches and the client and server are separated by 5000 km."

The speed of propagation is 200,000 km/sec or 400 meters/sec. In 20 µsec, the signal travels 4 km. Thus, each switch adds the equivalent of 4 km of extra cable. If the client and server are separated by 5000 km, traversing even 50 switches adds only 200 km to the total path, which is only 4%. Thus, switching delay is not a major factor under these circumstances

"Another advantage of having international standards for network protocols is that widespread use of any standard will give it economies of scale, as with VLSI chips."

True

An image is 3840 x 2160 pixels with 3 bytes/pixel. Assume the image is uncompressed. How long does it take to transmit it over a gigabit Ethernet?

The image is 3840 × 2160 × 3 bytes or 8,294,400 pixels x 3 bytes x 8 bits/byte. This is 199,065,600 bits. At 1,000,000,000 bits/sec it takes about 0.20 msec.

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.

H1 - Link H2 - Network H3 - Transport

Consider an application that transmits data at a steady rate (for example, the sender generates an N-bit unit of data every k time units, where k is small and fixed). Also, when such an application starts, it will continue running for a relatively long period of time. Suppose that a packet-switched network is used and the only traffic in this network comes from such applications as described above. Furthermore, assume that the sum of the application data rates is less than the capacities of each and every link. Is some form of congestion control needed?

Since the links have greater capacity than the total capacity of all the applications combined, congestion control is not needed. Because of this there will not be any congestion to control.

"A server sends packets to a client via satellite. The packets must traverse one or multiple satellites before they reach their destination. The satellites use store-and-forward packet switching, with a switching time of 100 µsec. If the packets travel a total distance of 29,700 km, how many satellites do the packets have to pass if 1% of the delay is caused by packet switching?"

((100 µs * n)/((29,700 km/300,000 km/s) + 100 µs * n)) = 0.01 where n is the number of satellites. (29,700 km/300,000 km/s) + 100 µs * n) = 100(100 µs * n) Yields 29,700 km/300,000 km/s = 99(100 µs * n) Yields 0.099 s = .0099 * n Yields n = 10

Consider the scenario shown below, with a single source client sending to a server over two links of capacities R1=100 Mbps and R3=10 Mbps. What is the maximum achievable end-end throughput (in Mbps, give an integer value) for the client-to-server pair, assuming that the client is trying to send at its maximum rate?

10 Mbps

Consider the scenario shown below, with 10 different servers (four shown) connected to 10 different clients over ten three-hop paths. The pairs share a common middle hop with a transmission capacity of R = 200 Mbps. Each link from a server has to the shared link has a transmission capacity of RS = 25 Mbps. Each link from the shared middle link to a client has a transmission capacity of RC = 50 Mbps. What is the maximum achievable end-end throughput (in Mbps, give an integer value) for each of ten client-to-server pairs, assuming that the middle link is fairly shared and all servers are trying to send at their maximum rate?

20 Mbps

Consider the scenario shown below, with 10 different servers (three shown) connected to 10 different clients over ten three-hop paths. The 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 = 90 Mbps. Each of the four links from the shared middle link to a client has a transmission capacity of RC = 50 Mbps. What is the end-end maximum throughput on one of the the-hop paths, assuming all servers are sending at the maximum rate possible to their clients?

30 Mbps

Suppose you would like to urgently deliver 40 terabytes data from Boston to Los Angeles. You have available a 100 Mbps dedicated link for data transfer. How long would it take in seconds to transfer the data?

40*10^12 * 8 = 320*10^12 bits, 320*10^12/100*10^6 = 3200000 seconds

Suppose you would like to urgently deliver 40 terabytes data from Boston to Los Angeles. You have available a 100 Mbps dedicated link for data transfer. Would you prefer to transmit the data via this link or instead use FedEx overnight delivery?

40*10^12 * 8 = 320*10^12 bits, 320*10^12/100*10^6 = 3200000 seconds = 37.037 days. Fedex can deliver it in one day.

"Suppose that two network endpoints have a round-trip time of 100 milliseconds, and that the sender transmits five packets every round trip. What will be the sender's transmission rate for this round-trip time, assuming 1500-byte packets? Give your answer in bytes per second."

5 × 1500 = 7,500 bytes per 100 milliseconds. So, the rate is 75,000 bytes per second.

Consider the network shown in the figure below, with three links, each with a transmission rate of 1 Mbps, and a propagation delay of 1 msec per link. Assume the length of a packet is 1000 bits. What is the end-end delay of a packet from when it first begins transmission on link 1, until is it received in full by the server at the end of link 3. Assume store-and forward packet transmission.

6 msec

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.

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 collection of billions of computing devices, and packet switches interconnected by links. A "network of networks".

Consider an application that transmits data at a steady rate (for example, the sender generates an N-bit unit of data every k time units, where k is small and fixed). Also, when such an application starts, it will continue running for a relatively long period of time. Would a packet-switched network or a circuit-switched network be more appropriate for this application?

Circuit-switched

Which of the characteristics below are associated with the technique of packet switching?

Congestion loss and variable end-end delays are possible with this technique. This technique is used in the Internet. Resources are used on demand, not reserved in advance. Data may be queued before being transmitted due to other user's data that's also queueing for transmission.

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. 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

Neither Circuit Packet

Match the description of each component of packet delay to its name in the pull-down 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 needed for bits to physically propagate through the transmission medium from end one of a link to the other.

Processing Delay Queueing Delay Transmission Delay Propagation Delay

An image is 3840 x 2160 pixels with 3 bytes/pixel. Assume the image is uncompressed. How long does it take to transmit it over a 10-Mbps Ethernet? (in seconds)

The image is 3840 × 2160 × 3 bytes or 8,294,400 pixels x 3 bytes x 8 bits/byte. This is 199,065,600 bits. At 10,000,000 bits/sec, it takes about 19.91 sec.

An image is 3840 x 2160 pixels with 3 bytes/pixel. Assume the image is uncompressed. How long does it take to transmit it over a 100-Mbps Ethernet? (in seconds)

The image is 3840 × 2160 × 3 bytes or 8,294,400 pixels x 3 bytes x 8 bits/byte. This is 199,065,600 bits. At 100,000,000 bits/sec, it takes about 1.99 sec.

"Another disadvantage international standards for network protocols is that once a standard has been widely adopted, it is difficult to change, even if new and better techniques or methods are discovered. Also, by the time it has been accepted, it may be obsolete."

True

"One advantage of having international standards for network protocols is that if everyone uses the standard, everyone can talk to everyone."

True

A disadvantage international standards for network protocols is that the political compromises necessary to achieve standardization frequently lead to poor standards.

True

Category 6 copper cable can be used for 10 Gbps Ethernet.

True

Negotiation has to do with getting both sides to agree on some parameters or values to be used during the communication.

True

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")?

Two people introducing themselves to each other. 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. One person asking, and getting, the time to/from another person.

Consider a packet of length L which begins at end system A and travels over three links to a destination end system. These three links are connected by two packet switches. Let di, si, and Ri denote the length, propagation speed, and the transmission rate of link i, for i = 1, 2, 3. The packet switch delays each packet by dproc. Assuming no queuing delays, in terms of di, si, Ri,(i = 1,2,3), and L, what is the total end-to-end delay for the packet? Suppose now the packet is 1,500 bytes, the propagation speed on all three links is 2.5 *108 m/s, the transmission rates of all three links are 2 Mbps, the packet switch processing delay is 0 msec, the length of the first link is 5,000 km, the length of the second link is 4,000 km, and the length of the last link is 1,000 km. Also suppose R1 = R2 = R3 = R and dproc = 0. Further suppose the packet switch does not store-and-forward packets but instead immediately transmits each bit it receives before waiting for the entire packet to arrive. For these values, what is the end-to-end delay in msec?

dend-to-end = L/R1 + L/R2 + L/R3 + d1/s1 + d2/s2 + d3/s3 + dproc + dproc = 6 + 20 + 16 + 4 = 46 msec

Consider a packet of length L which begins at end system A and travels over three links to a destination end system. These three links are connected by two packet switches. Let di, si, and Ri denote the length, propagation speed, and the transmission rate of link i, for i = 1, 2, 3. The packet switch delays each packet by dproc. Assuming no queuing delays, in terms of di, si, Ri,(i = 1,2,3), and L, what is the total end-to-end delay for the packet? Suppose now the packet is 1,500 bytes, the propagation speed on all three links is 2.5 *108 m/s, the transmission rates of all three links are 2 Mbps, the packet switch processing delay is 3 msec, the length of the first link is 5,000 km, the length of the second link is 4,000 km, and the length of the last link is 1,000 km. For these values, what is the end-to-end delay in msec?

dend-to-end = L/R1 + L/R2 + L/R3 + d1/s1 + d2/s2 + d3/s3 + dproc + dproc = 6 + 6 + 6 + 20 + 16 + 4 + 3 + 3 = 64 msec