- 7What fundamental common engineering design
challenge do electric grids, packet networks, phone networks and
(non-rural) roads all face?
All of these engineered systems have a very high peak-to-average load,
whether that's a hot summer day, busy hours on packet networks, Mother's
Day or a natural disaster on (old) phone networks, and rush hour on
non-rural roads. None of these systems can be economically engineered
for peak load, but should avoid congestion collapse during those times.
- 9Suppose we replace today's broadcast AM,
FM and over-the-air TV technology with IP-based delivery, using the same
radio spectrum. What would be the major advantages and drawbacks or
challenges, based on what we discussed in class?
The advantage would be (possibly) more efficient delivery of content
since newer video or audio encodings could be used. Also, it would be
easy to multiplex many streams, in different audio and video quality, on
the same channel. The same channel could be used to deliver data, or
still images, not just audio and video. (As an aside, this was proposed
very recently by somebody from the National Association of Broadcasters
and there is a proposal for a satellite-based version for developing
countries.) The major drawback would be the difficulty of accommodating
all the legacy receivers on the same spectrum at the same time during the
transition. Also, radios, in particular, would likely be more
complicated than AM and FM radios.
- 6What's the difference between continuous
media and streaming media? Given an example for each.
Continous media describes all Internet-delivered media streams whose
rate (bandwidth) is, on average, the same as the encoding rate.
Streaming media is a sub-category where longer delivery or buffering delays
can be tolerated.
- 6We have seen three fundamental interaction
patterns. For each, name one application and one protocol (or indicate that
the protocol(s) are proprietary to the application.)
- Request-response: HTTP (e.g., web browsers)
- Continuous media: streaming video using HTTP or proprietary
protocols such as Flash video
- Events: instant messaging using XMPP, Skype, Facebook or maybe email
- 6You are asked to design a new Internet
(layer 3) protocol that includes a number of optional information
elements. What kind of serialization would you use and why?
Fixed length-elements and TLV for extensions, since that choice is
easiest to parse, including in hardware, and compact.
- 9Show how you would encode a simple
calendar entry (date, location, subject) in TLV, XML and JSON. (Rough
syntax is sufficient.)
For TLV, some hex or graphical representation, as in RFCs, is
  "2015-03-23"    "New York"   "exam"
"location": "New York",
- 6In the Internet protocol stack, what
layer(s) ensure reliability?
Transport layer such as TCP and, in some networks, the link layer (e.g.,
for Wi-Fi). In some cases, the application layer may also provide
reliability if an unreliable transport protocol such as UDP is used.
- 6Both virtual circuit and packet switching
use small data units (called packets, cells or frames, depending on the
technology). What is the difference between the two?
Packet switching does not set up any per-flow or per-session state in
routers/switches, [such as label mappings for MPLS]. Virtual circuits
do. Virtual circuits switch packets based on flow labels, while packets
are routed based on global IP addresses.
- 6A crosspoint switching network can be
either buffered (switching fixed-sized cells or frames, with memory at
each cross point) or unbuffered (as depicted in class). Why would you
add memory to a crosspoint switching network? What are the major
advantages and disadvantages of each approach?
Adding memory reduces blocking since intermediate nodes can buffer data
when the output link is busy. Thus, the number of switch points can
- 6A speaker produces an audio signal of 10
Watts, and the background noise is 1 mW. What is the SNR in dB?
The ratio is 10 / 0.001 = 10,000, which corresponds to 40 dB.
- 7A researcher has invented a new transport
protocol. Outline what all needs to change for the protocol to be
widely deployed and some of the obstacles. What are the typical steps
this type of technology traverses before it becomes a "product"?
- The solution has to be standardized, through some kind of
- One or more operating systems (e.g., Linux or Windows) will have to
implement it, likely in the OS kernel.
- Extensive testing under a variety of operating conditions.
- Applications will have to implement or use the protocol.
- Middleboxes such as firewalls will have to allow the protocol.
- 6What are the three largest components of
mobile Internet traffic? (There's no need to estimate percentages or
According to the Cisco data, video, web and mobile audio. [See slide 22
- 8A fiber can carry 10 Mb/s of packet data.
How much data can be carried across that fiber in a month? Is your
theoretical answer realistic? Justify your answer.
107 * 3600 seconds/hour * 24 hours/day * 30 days / 8 = 3.24 1012 or 3.24
TB. The number is unrealistic since no packet network can be loaded to 100%
of capacity. Alternatively, it also ignores packet header overhead and any
- 6"All other things equal, increasing the
power (measured in Watts) of a cellular radio signal increases the
achievable transmission rate." True or false? Justify your answer.
True. Increasing the power increases the signal-to-noise ratio and
thus, by Shannon, the achievable transmission rate. [The increased SNR
allows a higher-rate modulation or reduces the bit error rate, and thus
the effective transmission rate. The question is somewhat ambiguous
since it uses the imprecise term transmission rate rather than
information rate or net data transfer rate.]
- 6What is a delay-tolerant network? Name
two applications where such a network may be useful.
A delay-tolerant network continues to function even when the
transmission path has extremely long delay or no contiguous
connectivity. Examples include inter-planetary communications (e.g.,
from Earth to the outer reaches of the solar system), locations without
3G/4G cellular services or hard (or expensive)-to-reach places in
developing countries. Services that may use delay-tolerant networks
include message delivery (email), backups or content delivery (e.g.,
movies or educational content).