Telecommunications and Coordination Technologies
Prof. Arthur Goldberg
10 November 1997
Midterm 1 Answers.
Short answers
Some exams start with question 8 and, after the last question, continue with question 1.
1.Physical, (or layer 1). Physical properties of network communications, such as voltage and speed, are captured in the physical layer.
2.It is easy to solve because every host gets every packet. To route a message, the protocol simply asks every host to look at every message's destination address, and receive messages destined for itself. Saying “topology” misses the point.
3.Data layer. A solution to the medium access problem provides communication between machines on the LAN.
4.Multicast sends a message to a set of multiple destinations. Broadcast sends a message to all destinations on a network. (Each question is 1.5 points.)
5.One office. All phones with the same prefix are attached to a single end office so the phone company can connect the call there.
6.Both Nyquist’s and Shannon’s data transmission limits express upper speeds as a multiplicative factor of the link’s bandwidth, H. So if H is increased by a factor of 4 and the S/N stays the same, then the local loop could transmit 4 times more data. Any reference to Nyquist’s or Shannon’s limit or to S/N is a satisfactory explanation.
7.Yes. Each router independently routes each packet; therefore the router must be able to read the packet’s destination in the header. Note that packet switching is connectionless, not a virtual circuit.
8.About 2/3 times the speed of light, or 3 x 105 km/sec x 2/3 = 2 x 105 km/sec. Just saying ‘the speed of light’ loses 1 point. ‘How fast does it travel’ is not bandwidth.
9.Client, because it is the interface to the input source, the user. One point for the correct explanation.
10.The growth rate of physical bandwidth per decade is 100 (Tanenbaum) or square root of 100,000 (Forbes). Over the last few years, physical bandwidth speeds have tripled annually, according to Forbes. I allocated 1.5 points per question.
11.Chief Information Officer (CIO) or Chief Operating Officer (COO) or Chief Technical Officer (CTO).
12.It means that at most 1 in 108 bits received is wrong, on average. One could also say 1 error per 108 bits. It would be OK to leave out ‘average’ or ‘at most’.
13.A piece of data prepended to a message which contains information about the message, such as its length, destination address. An example of a header, such as email headers, gets 2 points. ‘Prepended’ not required.
14.A network number and a host number. ‘Subnet’ number is wrong.
Long answers
Some exams start with question 4 and, after the last question, continue with question 1.
1.A system that sends multiple data streams over a single communicating channel is said to multiplex the data over the channel. Computer communications involves many concurrent data streams traveling over many fewer cables. Demultiplexing takes the multiplexed signals off the cable and converts them into the original individual signals. For example, the phone system time division multiplexes 24 64kbps calls over a 1.544 Mbps T1 line. Other examples include time division multiplexing, frequency division multiplexing and wavelength division multiplexing. (Two points for each part or sentence.)
2.A machine on the Internet must be able to communicate using the internet protocol (IP) with other machines on the Internet. It must have the TCP/IP protocol software and a unique IP address. (I subtracted 2 points for missing one of these.) Note that some machines may be on the internet intermittently, e.g. those connected by dialin, and some may have limited. IP service, e.g.. those behind firewalls.
3.Baud measures the number of signals per second. Bps is bits per second--the total data rate of a system. If a signal has V levels, it encodes log 2(V) bits per signal. If it runs at B baud then its data rate is B log 2(V) bps. An example of signal levels earned full credit. Failure to describe the relationship loses 3 points.
4.If one person (computer) is talking (transmitting), then everyone else (other computers) keep quiet until they finish. 2. If multiple people (computers) start talking (transmitting) at the same time then they’ll all realize they’re interfering with each other and stop. 3. If one person (computer) is talking (sending) then they’ll talk (send) for a limited period of time to give another person (computer) a chance to talk (send). Any two ways is a complete answer. Failure to mention people loses 3 points.
5.Compare these systems.
System / Cost ($ per person per month) / Bandwidth (average Kbps per person) / Cost of bandwidth (dollars per person per month per Kb)ISDN / 100 / 128.0 / 0.78
T1 / 20 / 15.44 / 1.30
The ISDN provides a slightly lower cost of bandwidth when everyone is using the network. However, the T1 line would be more cost-effective because most of the time only a proportion of users would be transmitting so the typical bandwidth experienced by the T1 user would be a large fraction of the full T1 bandwidth. This stems from the bursty behavior of computer network users. No mention of usage loses 4 points. Just providing the alternatives loses 2 points. Just comparing the cost of bandwidth loses 5 points.
6. A space-time diagram shows communications. Time increases down, and the events at one program happen on a vertical line. Diagonal lines indicate messages between programs.
Client Server
> GET index.html
< [index.html page]
client parses HTML, and finds it uses ‘object’
> GET object
< [object]
No object loses 1 point. No picture loses 5 points.
Like a wire and a switch / X
The Internet (network layer) / X
TCP / X
UDP / X
Like the mail / X
Moves digital data / X / X
Complexity in the network / X
Complexity in the end points / X
Remote login / X
More reliable against router failure / X
POTS / X
100 years phone company experience / X
Sends packets / Either / X
Transmits data streams / X
Needs to be set up and taken down / X
Every message contains destination address / X
Virtual circuit / X
Uses time division multiplexing / X / Either
Acts like a tube / X
Data arrives in the order it was sent / X
Datagram service / X
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