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计算机网络代写|CSCI-GA.2262 Midterm

计算机网络代写|CSCI-GA.2262 Midterm



1. General Q&As – (8 points)

Indicate whether each of the following statement is true or false (T/F) and explain
your answers if/as needed:

1a. (2 points) The internal data rates of LANs are typically much greater than
those of WANs.

1b. (2 points) Name servers are server programs that hold information about a
portion of the domain name tree structure and the associated resource records

1c. (2 points) Synchronous time division multiplexing is only possible when the
achievable data rate (sometimes called bandwidth) of the medium exceeds the
data rate of digital signals to be transmitted.

1d. (2 points) A set of devices and LANs connected by layer 2 (i.e., link layer)
switches is considered to have a “flat” address space where the term “flat” means
that all users share a common MAC broadcast address.

2. Computer Networks and the Internet – (7 points)

As an example of Voice over IP (VoIP), a real-time voice is being sent from Host
A to Host B over a single link packet-switched network. Host A converts analog
voice to a digital 128 kbps bit stream on the fly and groups the bits into 56-byte
packets. The single link between Host A and Host B has a transmission rate of 5
Mbps and a propagation delay of 12 msec. Packets gathered by Host A are
immediately sent to Host B and as soon as Host B receives an entire packet, it
converts it to an analog signal.

How much time elapses from the time a bit is created from the original analog
signal at Host A until the bit is decoded as part of the analog signal at Host B?

3. Computer Networks and the Internet – (10 points)

Explain how phone calls made from a PC to an ordinary phone (e.g., via Skype)
are typically supported. (Note: It is assumed here that the voice call would pass
through the Internet and a telephone carrier.)

4. Application Layer – SMTP (8 points)

The following questions pertain to electronic mail applications in the Internet and
the related SMTP protocol;.

4a. (2 points) Excluding the connection establishment and termination, what is the
minimum number of network round trips to send a small email message using

4b. (2 points) Suppose you need to send one message to three different users:,, and Is there
any difference between sending one separate message per user and sending only
one message with multiple (i.e., three) recipients? Please explain.

4c. (2 points) Because SMTP uses two different port numbers (i.e., UDP ports
161 and 162), a single system can easily run both a manager and an agent. What
would happen if the same port number were used for both?

4d. (2 points) Electronic mail systems differ in the manner they handle multiple
recipients. In some systems, the originating user agent or mail sender makes all
the necessary copies and these are sent out independently. An alternative
approach is to determine the route for each destination first. Then a single
message is sent out on the common portion of the route and copies are only made
when the routes diverge; this process is referred to as “mail-bagging”. Discuss the
relative advantages and disadvantages of the two methods.

5. Application Layer – HTTP and P2P – (12 points)

The following questions pertain to the Internet Directory Service and the World
Wide Web and the related DNS and HTTP protocols.

5a. (6 points) Prior to persistent connections, one separate TCP connection was
used to fetch each URL. Analyze the advantages of persistent connections over
the old HTTP paradigm of one connection per data transfer.