EDUSAT SESSION FOR COMPUTER NETWORKSI CS64

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EDUSAT SESSION FOR COMPUTER NETWORKS-I
(CS64) Date 31.05.2006 Session
IX TopicExercises Faculty Anita Kanavalli
MSRIT
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Exercises
1 Describe B-ISDN model (model QP
8marks) Answer Contains three planes User
plane-concerned with transfer of data, flow
control and error recovery Control plane-deals
with the signaling required to setup manage and
release connections Management plane-deals with
management of n/w resources and coordination of
other planes User plane has three layers
Physical-2 sublayers ATM layer-sequenced transfer
adds 5 byte header
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Exercises
1 Describe B-ISDN model (model QP
8marks) Answer AAL layer-has different
types ATM provides Qos parameters
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Exercises
2 Take any two Qos parameters under the ATM
standard and describe. (August 2005 6
marks) Answer There are 6 the first three not
negotiated but the next three may be
negotiated Cell error ratio-CER-ratio of number
of cells delivered with one or more bit error to
the total no of cells. CER depends on PM Cell
misinsertion rate-CMR-average no of cells/sec
that are delivered by mistake to wrong connection
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Exercises
2 Take any two Qos parameters under the ATM
standard and describe. (August 2005 6
marks) Answer. Severely errored cell block
ratio-SECBR-it is the ratio of errored to the
total no of cells determined by the error
mechanism The next three are Cell loss
ratio-CLR-ratio of the lost cells to the total no
of cells transmitted can be specified or not.
Depends on buffer allocation strategy Cell
transfer delay-CTD-is the time that elapses from
the instant when a cell enters the network at the
source UNI to the
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Exercises
2 Take any two Qos parameters under the ATM
standard and describe. (August 2005 6
marks) Answer. instant when it exits at the
destination UNI.CTD includes propagation delays,
processing delays and queuing delays in mux and
switches. Cell delay variation-CDV- measures the
variability of the total delay encountered by
cells in a connection.
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Exercises
3 Draw the diagram of the ATM cell header format
and give the brief description of the different
fields (August 2005 8 marks) Answer
GFC(4bits)
VPI(4bits)
VPI(4bits)
VCI(4bits)
VCI(8bits)
5 byte Header
VCI(4bits)
PT(3bits)
CLP(1)
HEC(8bits)
Payload 48bytes
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Exercises
3 Draw the diagram of the ATM cell header format
and give the brief description of the different
fields (August 2005 8 marks) Answer. GFC-provide
flow control has significance for UNI currently
not specified VPI-8 bit long 256 virtual path in
a given UNI link. VCI-16 bit long allows 65536
virtual channels per virtual path PT-eight type
payload can be specified (refer table in
book) CLP-two levels of priority-CLP0 higher
priority used during congestion
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Exercises
3 Draw the diagram of the ATM cell header format
and give the brief description of the different
fields (August 2005 8 marks) Answer. HEC-8 bit
checksum uses CRC recomputed at every switch
since VPI/VCI changes at every hop (since they
are local identifiers)
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Exercises

• 4 Describe the function of ATM adaptation layer
  (August 2005 6 marks)
• Answer
• ATM Adaptation Layer (AAL) adapts upper layers
  (IP or native ATM applications) to ATM layer
  below
• AAL present only in end systems, not in switches
• AAL layer segment (header/trailer fields, data)
  fragmented across multiple ATM cells
• analogy TCP segment in many IP packets

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Exercises

• 4 Describe the function of ATM adaptation layer
  (August 2005 6 marks)
• Answer..
• Different versions of AAL layers, depending on
  ATM service class
• AAL1 for CBR (Constant Bit Rate) services, e.g.
  circuit emulation
• AAL2 for VBR (Variable Bit Rate) services, e.g.,
  MPEG video
• AAL3/4for the bursty data e.g, multiple users
• AAL5 for data (eg, IP datagrams)

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Exercises

• 4 Describe the function of ATM adaptation layer
  (August 2005 6 marks)
• Answer.
• AAL is divided into two sublayers
• Segmentation and reassembly-SAR
• Convergence sublayer-CS
• CS subdivided into Common part-CPCS
• Service specific part-SSCS

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Exercises
5. Explain how the notion of layering and
internetworking make the rapid growth of
applications such as the World Wide Web
possible? Answer Internetworking allows many
component networks each with different underlying
technology and operation to work together and
form one large network. This provides the
ubiquitous connectivity for applications like
WWW. The layering concept hides the specific
underlying network technology from the upper
layers and
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Exercises
5 Explain how the notion of layering and
internetworking make the rapid growth of
applications such as the World Wide Web
possible? Answer.. provides a common networking
platform. Using the communication service
provided by the layers below, new applications
can be introduced independently and at a rapid
rate.
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Exercises
6 Can a connection-oriented, reliable message
transfer service be provided across a
connectionless packet network? Explain. Answer Ye
s. To provide connection-oriented service, the
transport layer can establish a logical
connection across the connectionless packet
network by setting up state information (for
example, packet sequence number) at the end
systems. During the connection setup, the message
is broken into separate packets, and each packet
is assigned a sequence number. Using the sequence
numbers, the end-system transport-layer entities
can acknowledge received packets, determine and
retransmit lost packets, delete duplicate
packets,
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Exercises
6 Can a connection-oriented, reliable message
transfer service be provided across a
connectionless packet network? Explain. Answer..
and rearrange out-of-order packets. In so doing,
the connectionless packet network is implementing
reliable packet transfer. Once all packets have
arrived at the receiving end, they are
reassembled into the original message. For
example, TCP provides a connection-oriented
reliable transfer service over IP, a
connectionless packet transfer service.
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Exercises
7Suppose 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?
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Exercises
7 Answer 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.
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Exercises
7 Answer. Therefore L 100 bytes, 100/158
63 efficiency. L 500 bytes, 500/558 90
efficiency. L 1000 bytes, 1000/1058 95
efficiency.
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Exercises
8Suppose that the TCP entity receives a 1.5
megabyte file from the application layer and that
the IP layer is willing to carry blocks of
maximum size 1500 bytes. Calculate the amount of
overhead incurred from segmenting the file into
packet-sized units. Answer 1500 - 20 -20 1460
bytes 1.5 Mbyte / 1460 byte 1027.4, therefore
1028 blocks are needed to transfer the
file. Overhead ((1028 x 1500 - 1.5M)/1.5M) x
100 2.8
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Exercises
9Suppose we need a communication service to
transmit real-time voice over the Internet. What
features of TCP and what features of UDP are
appropriate? Answer TCP is desirable in that it
provides a connection for the transfer of a
stream of information, which characterizes a
digital voice stream. However, to provide
reliable service TCP uses acknowledgments and
retransmissions that result in packet delay that
can not be tolerated by real-time traffic.
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Exercises
9Suppose we need a communication service to
transmit real-time voice over the Internet. What
features of TCP and what features of UDP are
appropriate? Answer UDP provides connectionless
service and delivers packets quickly. In case of
packet loss, UDP does not provide retransmission,
but some degree of packet loss can be tolerated
by voice.
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Exercises
10 Use IEEE 802.3 and IEEE 802.11 to discuss
three differences between wired and wireless
LANs. Answer Error rate Unlike wired LANs,
wireless LANs have high error rate due to
interference and noise. Wireless LANs needs to
implement ARQ and error correction to increase
the reliability of the communication
channel. Station mobility Unlike wired LANs
where stations connected to the LANs are static,
in wireless LANs, the stations can be mobile and
portable. Wireless LAN protocols may have to
implement dynamic traffic routing and service
handoff when the station moves from one service
area to another.
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Exercises
10 Use IEEE 802.3 and IEEE 802.11 to discuss
three differences between wired and wireless
LANs. Answer Collision detection Collision
detection is not possible in wireless LANs due to
physical limitations. So the sender must wait for
explicit acknowledgment (e.g. RTS/CTS) from the
receiver to know whether or not a collision has
occurred. The wireless LAN protocol implements a
collision avoidance algorithm rather than the
collision detection in wired LAN, and the delay
in the contention period is longer the round-trip
delay of 2tprop of wired LAN because of waiting
for the receivers acknowledgment.
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Exercises
10 Use IEEE 802.3 and IEEE 802.11 to discuss
three differences between wired and wireless
LANs. Answer Other differences Security In
wired LAN, the transmission medium is physically
secured. In wireless LAN, any device within the
geographic transmission area can intercept the
transmissions. To provide data security, wireless
LAN need to implement encryption at the expense
of higher cost and reduced performance.
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Exercises
10 Use IEEE 802.3 and IEEE 802.11 to discuss
three differences between wired and wireless
LANs. Answer Other differences Power
consumption Portable and mobile devices are
usually battery powered, and thus have limited
power capacity. The wireless LAN protocol must be
designed to be power efficient. All these issues
are addressed in the IEEE 802.11 wireless LAN
protocol.