M.Sc. Computer Science
Computer Systems
Additional Questions on Week 9 Materials – Part I
Question #1: Review the car-caravan analogy. Assume a propagation speed of 100 km/hour.
a) Suppose the caravan travels 150 km, beginning in front of one tollbooth, passing through a
second tollbooth, and finishing just after a third tollbooth. What is the end-to-end delay?
b) Repeat (a), now assuming that there are eight cars in the caravan instead of ten.
Tollbooths are 75 km apart, and the cars propagate at 100km/hr. A tollbooth services a car at a rate of
one car every 12 seconds.
a) There are ten cars. It takes 120 seconds, or 2 minutes, for the first tollbooth to service the 10 cars.
Each of these cars has a propagation delay of 45 minutes (travel 75 km) before arriving at the second
tollbooth. Thus, all the cars are lined up before the second tollbooth after 47 minutes. The whole
process repeats itself for traveling between the second and third tollbooths. It also takes 2 minutes
for the third tollbooth to service the 10 cars. Thus the total delay is 96 minutes.
b) Delay between tollbooths is 8*12 seconds plus 45 minutes, i.e., 46 minutes and 36 seconds. The total
delay is twice this amount plus 8*12 seconds, i.e., 94 minutes and 48 seconds.
Question #2: How long does it take a packet of length 1,000 bytes to propagate over a link of distance
2,500 km, propagation speed 2.5 x 108 m/s, and transmission rate 2 Mbps? More generally, how long
does it take a packet of length L to propagate over a link of distance d, propagation speed s, and
transmission rate R bps? Does this delay depend on packet length? Does this delay depend on
transmission rate?
10msec; d/s; No; No
Question #3: Why are standards important for protocols?
Standards are important for protocols so that people can create networking systems and products that
interoperate.
Question #4: SMS, iMessage and WhatsApp are all smartphone real-time messaging systems. After
doing some research on the Internet, for each of these systems, write one paragraph about the protocols
they use. Then write a paragraph explaining how they differ?
SMS (Short Message Service) is a technology that allows the sending and receiving of text messages
between mobile phones over cellular networks. One SMS message can contain data of 140 bytes and it
supports languages internationally. The maximum size of a message can be 160 7-bit characters, 140 8-
bit characters, or 70 16-bit characters. SMS is realized through the Mobile Application Part (MAP) of
the SS#7 protocol, and the Short Message protocol is defined by 3GPP TS 23.040 and 3GPP TS
23.041. In addition, MMS (Multimedia Messaging Service) extends the capability of original text
messages, and support sending photos, longer text messages, and other content.
iMessage is an instant messenger service developed by Apple. iMessage supports texts, photos, audios
or videos that we send to iOS devices and Macs over cellular data network or WiFi. Apple’s iMessage
is based on a proprietary, binary protocol APNs ( Notification Service).
WhatsApp Messenger is an instant messenger service that supports many mobile platforms such as
iOS, Android, Mobile Phone, and Blackberry. WhatsApp users can send each other unlimited images,
texts, audios, or videos over cellular data network or WiFi. WhatsApp uses the XMPP protocol
(Extensible Messaging and Presence Protocol).
iMessage and WhatsApp are different than SMS because they use data plan to send messages and they
work on TCP/IP networks, but SMS use the text messaging plan we purchase from our wireless carrier.
Moreover, iMessage and WhatsApp support sending photos, videos, files, etc., while the original SMS
can only send text message. Finally, iMessage and WhatsApp can work via WiFi, but SMS cannot.
Question #5: Suppose Host A wants to send a large file to Host B. The path from Host A to Host B has
three links, of rates R1 = 500 kbps, R2 = 2 Mbps, and R3 = 1 Mbps
a) Assuming no other traffic in the network, what is the throughput for the file transfer?
b) Suppose the file is 4 million bytes. Dividing the file size by the throughput, roughly how long
will it take to transfer the file to Host B?
c) Repeat (a) and (b), but now with R2 reduced to 100 kbps.
a) 500 kbps b) 64 seconds c) 100kbps; 320 seconds
Question #6: In this problem, we consider sending real-time voice from Host A to Host B over a
packet-switched network (VoIP). Host A converts analog voice to a digital 64 kbps bit stream on the fly.
Host A then groups the bits into 56-byte packets. There is one link between Hosts A and B; its
transmission rate is 2 Mbps and its propagation delay is 10 msec. As soon as Host A gathers a packet, it
sends it to Host B. As soon as Host B receives an entire packet, it converts the packet’s bits 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)?
Consider the first bit in a packet. Before this bit can be transmitted, all of the bits in the packet must be
generated. This requires
56 ⋅8
64×10
3 sec=7msec.
The time required to transmit the packet is
56 ⋅8
2×10
6 sec=224 μsec.
Propagation delay = 10 msec.
The delay until decoding is
7msec +224 μsec + 10msec = 17.224msec
A similar analysis shows that all bits experience a delay of 17.224 msec.