Tutorial 1
1.
Two musicians located in different cities wish to have a jam session over a
communications network. Find the maximum possible distance between the
musicians if they are to interact in real-time, in the sense of experiencing the same
delay in hearing each other as if they were 10 meters apart. The speed of sound is
approximately 330 meters/second. Assume that the network transmits the sound at the
8
speed of light in cable, 2.3 x 10 meters/second.
Solution:
The first step is to find the delay for the sound when the musicians are 10 meters apart:
t10 = 10/330 = 30.30 milliseconds
The maximum distance is the time required for a real-time 'experience' times the cable speed:
8
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d = (2.3 x 10 ) x (30.30 x 10 ) = 6,969,000 meters = 6969 kilometers
2.
Describe the step-by-step procedure that is involved from the time you deposit a letter in a
mailbox to the time the letter is delivered to its destination. What role do names, addresses
and mail codes play? How might the letter be routed to its destination? To what extent can the
process be automated?
Solution:
The steps involved in mailing a letter are:
1. The letter is deposited in mailbox.
2. The letter is picked up by postal employee and placed in sack.
3. The letter is taken to a sorting station, where it is sorted according to destination, as
determined by the mail code and grouped with other letters with the same destination
mail code. (If there is no mail code, then it is determined by the largest geographical
unit, for example, country (if specified), otherwise state (if specified), otherwise city
(if specified).)
4. The letter is shipped to the post office that handles the mail for the specific mail code
(or country or city).
5. The letter is then sorted by street address.
6. The letter is picked up at the post office by the postal worker responsible for delivering
to the specified address.
7. The letter is delivered according to the number and street.
The name is not really used, unless the street address is missing or incorrect. The name is
at the destination to determine who the letter belongs to. (Unless of course the letter is
being sent to a small town, where most inhabitants are known to the postal worker.)
Data Communication & Networks
DKT 224/3
The mail delivery process can be automated by using optical recognition on the mail
code. The letter can then be sorted and routed to the destination postal station, and even
to the destination neighborhood, depending on the amount of geographical detail built
into the mail code.
2b. Repeat part (a) for an e-mail message. At this point, you may have to conjecture different
approaches about what goes on inside the computer network.
Solution:
The steps involved in e-mailing a message are:
1. The message is sent electronically by clicking 'Send'. (In Chapter 2 we see that the
Simple Mail Transfer Protocol (SMTP) is used to do this.)
2. The mail provider of the sender sends a request to a name server for the network
address of the mail provider of the recipient. The mail provider is determined by the
information following the @ symbol.
3. If the mail provider finds the network address of the recipient's mail provider, then it
sends the message to that address.
4. Upon receipt of the message the mail provider either informs the recipient
automatically that there is mail or waits until mail is requested by the recipient,
depending on how the mail service is set up. The recipient is determined by the
information before the @ symbol.
2c. Are the procedures in parts (a) and (b) connection-oriented or connectionless?
Solution:
The transfer service of letters in a postal system is connectionless. Users do not set up
connections prior to the transfer of each letter. (Note however that the postal system may
have regularly scheduled shipments of bundles of mail from city to city, because the
volume of mail between cities is predictable. These regular shipments can be viewed as
pre-arranged "connections.")
E-mail transfer is also a connectionless service in that the user does not set up end-to-end
connections to send a message. We will see in Chapter 2, however, that e-mail protocols
use connection-oriented procedures to exchange messages.
NOTE: For parts (a) and (b) other step-by-step procedures are possible.
3. Suppose that the letter in problem 2 is sent by fax. Is this mode of communications
connectionless or connection-oriented? Real-time or non-real time?
Solution:
In order to send the letter by fax, a telephone connection must first be established. Therefore
the mode of communications is connection-oriented. The transfer of information across the
network occurs in real-time.
4. Discuss the advantages and disadvantages of transmitting fax messages over the Internet
instead of the telephone network.
Tutorial #1
Pn. Shahadah Ahmad
Data Communication & Networks
DKT 224/3
Solution: The comparison of fax transmission over the Internet and over the telephone network
reflect the differences between e-mail and telephone calls. The transmission of a fax message as
an attachment to e-mail means that the transfer of the message is not in real time and that delivery
is not necessarily confirmed. The transmission of a fax message over the telephone network, on
the other hand, is real time and is confirmed with fairly high certainty. On the other hand, the cost
of transmission of a fax message over the Internet is very inexpensive and is not distancesensitive. The transmission of fax messages over telephone networks may involve long distance
charges.
5. Give two features that the data link layer and transport layer have in common. Give two
features in which they differ. Hint: Compare what can go wrong to the PDUs that are handled by
these layers.
Solution: Features they have in common: • Both layers can provide recovery from transmission
errors. • Both layers can provide flow control. • Both layers can support multiplexing.
Features in which they differ: • The transport layer is end to end and involves the interaction of
peer processes across the network. The data link layer involves the interaction of peer-to-peer
processes that are connected directly. In general, the time that elapses in traversing a data link is
much smaller than the time traversing a network, where packets can become trapped in temporary
routing loops. Consequently, transport layer protocols must be able to deal with out-of-sequence
PDUs and a much larger backlog of PDUs than data link layers. • The data link layer is concerned
with framing and the transport layer is not. • The data link layer may be concerned with medium
access control, the transport layer does not have this concern.
6. Suppose an application layer entity wants to send an L-byte message to its peer process, using
an existing TCP connection. The TCP segment consists of the message plus 20 bytes of header.
The segment is encapsulated into an IP packet that has an additional 20 bytes of header. The IP
packet in turn goes inside an Ethernet frame that has 18 bytes of header and trailer. What
percentage of the transmitted bits in the physical layer correspond to message information, if L =
100 bytes, 500 bytes, 1000 bytes?
Solution:
TCP/IP over Ethernet allows data frames with a payload size up to 1460 bytes. Therefore, L =
100, 500 and 1000 bytes are within this limit.
The message overhead includes:
• TCP: 20 bytes of header
• IP: 20 bytes of header
• Ethernet: total 18 bytes of header and trailer.
Therefore
L = 100 bytes, 100/158 = 63% efficiency.
L = 500 bytes, 500/558 = 90% efficiency.
L = 1000 bytes, 1000/1058 = 95% efficiency.
Tutorial #1
Pn. Shahadah Ahmad
Data Communication &Network
DKT224/3
7. What are the responsibility of the transport link layer in the internet model.
The transport layer oversees the process-to-process delivery of the entire message.
It is responsible for
a. dividing the message into manageable segments
b. reassembling it at the destination
c. flow and error control
8. . What is the difference between a port address, a logical address, and a physical
address?
The physical address is the local address of a node; it is used by the data link layer
to deliver data from one node to another within the same network. The logical
address defines the sender and receiver at the network layer and is used to deliver
messages across multiple networks. The port address (service-point) identifies the
application process on the station.
9. How do the layers of the internet model correlate to the layers of the OSI model?
The application, presentation, and session layers of the OSI model are represented
by the application layer in the Internet model. The lowest four layers of OSI
correspond to the Internet model layers.
10. Match the following to one or more layers of the OSI model:
a. Route determination: network layer
b. Flow control: data link and transport layers
c. Interface to transmission media: physical layer
d. Access for the end user: application layer
11. In figure 2 , computer A sends a message to computer D via LAN1,routerR1 and
LAN2. Show the contents of the packets and frames at the network and data link
layer for each hop interface.
See Figure 2
Exercise1
Pn. Shahadah Ahmad
Data Communication &Network
DKT224/3
12. Suppose a computer sends a frame to another computer on a bus topology LAN.
The physical destination address of the frame is corrupted during the
transmission. What happen to the frame? How can the sender be informed
about the situation.
If the corrupted destination address does not match any station address in the network,
the packet is lost. If the corrupted destination address matches one of the stations,
the frame is delivered to the wrong station. In this case, however, the error
detection mechanism, available in most data link protocols, will find the error and
discard the frame. In both cases, the source will somehow be informed using one
of the data link control mechanisms .
Exercise1
Pn. Shahadah Ahmad