Bhodhitree 2. physical layer –theory Q4
Max data rate
0 / 2 points
0 / 2 attempts
A transmission line has a bandwidth of 1 Mhz. When 10 dBm power is transmitted, the receiver receives -60 dBm. The noise level at the receiver is -100 dBm. What is the maximum theoretical rate (in Mbps) that can be achieved on this link? Format X.x (E.g 5.1)
Note dBm is nothing but power ratio in decibels (dB) of the observed power referenced to one milliwatt (mW). So it boil downs to 10log_10(P in mw). Eg. 0dBm = 10 log_10(1mw)
Q5 Packet error rate
0 / 1 points
0 / 1 attempts
What is the Packet Error Rate (PER) in percentage at a bit error rate of 10^-7 if packet size is 10k bits. 1%
0.1%
0.01%
0.001%
physical layer –encoding 6 que
Q1 Clock Synchronization
0 / 1 points
0 / 1 attempts
Which of the following encodings do not provide proper clock synchronization? synchronization?
NRZ
NRZI
Manchester
4B/5B
Q2 Sequence
0 / 1 points
0 / 1 attempts
Which of the following encodings have problem with long sequence of 1's NRZ
NRZI
Manchester
4B/5B
Q3
Manchester Encoding
0 / 2 points
0 / 2 attempts
Manchester encoding can be thought of as XB/YB encoding. What are X,Y? Format X,Y (E.g. 1,3 )
Q4
Clock Skew
0 / 4 points
0 / 2 attempts
Assume a bit duration of 1ms. Suppose the sender clock is running 5% slower than the receiver clock and the receiver samples the bit in what it believes to be the middle of the bit. If NRZ encoding is used, at what bit position starting from 1, can the receiver possibly make an error. This means the sender sends first bit from 0 to 1.05ms, second bit from 1.05 to 2.1ms and so on. The receiver samples first bit at 0.5, second bit at 1.5 and so on.
Express answer as a bit position (starting from 1). Format X (e.g. 7)
Q5
4B/5B
0 / 1 points
0 / 1 attempts
When employing 4B/5B encoding, what would the below bit sequence translate to?
001110001111
Format: bit sequence without spaces (E.g. 001100011011010)
Q6 Encoding Design
0 / 0 points
0 / 1 attempts
Can you design an encoding scheme with an even stronger requirement of no more than 2 consecutive 0's in the encoded bit stream?
What would be the efficiency of such a scheme?
Data Link Layer –Framing 4Que
Q1 Frame Errors
0 / 1 points
0 / 1 attempts
Can errors in one frame affect the next frame? yes
no
Q2 Byte Stuffing
0 / 1 points
0 / 1 attempts
Suppose the last two bytes of the BiSYNC protocol are DLE ETX, what sequence of bytes precede CRC? DLE DLE DLE ETX ETX
DLE DLE ETX
DLE DLE ETX ETX
DLE DLE DLE ETX
Q3 Bit Stuffing
0 / 1 points
0 / 1 attempts
If the following bit pattern is received as part of HDLC data, what does the HDLC decoder output after removing stuffed bits?
1101111101001 Express the answer as a sequence of bits with no spaces (e.g. 1100110011) 110111111001
Q4 Escape characters
0 / 1 points
0 / 1 attempts
Can this sequence 'A DLE DLE ETX' appear in the data portion of the transmission when using the Bisync protocol, where A is the ASCII character 'A'. yes
no
DATA LINK LAYER –error control 4que
Q1
Hamming distance
0 / 1 points
0 / 1 attempts
If a code has a hamming distance of 7, how many errors can it detect? Format X (e.g. 2) 6
If a code has a hamming distance of 7, how many errors can it correct? Format X (e.g. 1)3
Q2
Code
0 / 3 points
0 / 2 attempts
If a code contains the following code words:
c1 = 0101010, c2 = 1111111, c3 = 1010101, c4 = 0000000 and c5 = 1100001
What is the hamming distance of this code?3
Q3
Error detection vs correction:
Mr. Error Prone wants to decide on whether to use error correction or detection on a link with packet error rate of 10^-5. Suppose the packet size is 923 bits and he is considering BCH ( Bose-Chaudhuri-Hocquenghem) code for error correction, where 1023 code bits are sent for 923 data bits and CRC for error detection ,where 32 bits are added to 923 data bits.
0 / 2 points
0 / 2 attempts
What is the transmission overhead of the error correction in %? Transmission overhead is the percentage of 'additional' bits transmitted per data bit. Format X.x (e.g. 15.3) 10.8
Submit
0 / 4 points
0 / 2 attempts
What is the transmission overhead of the error detection in %? Transmission overhead is the percentage of 'additional' bits transmitted per data bit. Format X.x (e.g. 15.3) Hint
Need to determine the average number of retransmission required
Q4 Puzzle
0 / 0 points
0 / 1 attempts
A bit challenging but fun nonetheless...
An absent minded professor's glasses are inside one of three rooms. He wants to send five of his students in groups to the three rooms to check for his glasses. One of his students may lie, while others always tell the truth about whether they found the glasses in the room or not.
How should the professor divide the students into groups so that he can determine where his glasses are?
Also, how can he determine in which room his glasses are?
Could the professor determine where his glasses are if there were only four students, and only one of them might lie?
Map this problem to error correcting codes and solve. Hint
Codeword (student's answers) represent rooms
DATA LINK LAYER –error detection 5 que
Q1
Single bit parity
0 / 3 points
0 / 1 attempts
Smarty Pants wants to enhance single bit party scheme and decided to use two redundant bits. One bit represents the parity of all odd-numbered bits and second bit the parity of all even-numbered bits. Example: 0111 is encoded as 011110 (assuming 'even-parity').
Whats the hamming distance of this code? Note message bits can be any number, redundant bits are 2. Answer Format X (e.g. 5)
Has he done better than single bit parity in terms of hamming distance? Do not answer this in the box, just for your thinking.
Q2 Two dimensional parity
0 / 0 points
0 / 1 attempts
What is the general set of circumstances under which a 4-bit error cannot be detected by a two dimensional parity?
Submit Hint
Rectangle
Q3 Internet Checksum
0 / 0 points
0 / 1 attempts
The IP protocol implements Internet checksum over just the IP header. As the packet passes through the routers, one field called TimeToLive (TTL) (8-bits long) in the IP header is decremented at each router. So, each router needs to decrement this field and then update the checksum in the header. Is there a way to update the checksum without having to recalculate the checksum over the entire header?
Submit Hint
16-bit word in checksum calculation is made up of two bytes. Consider two cases, where TTL field falls in low byte order (least significant byte) and another where it falls in higher byte order (most significant byte).
Q4
CRC
0 / 2 points
0 / 1 attempts
For a generator polynomial of x^3+x^2+1, what is the CRC value if the message is 10011010? Express answer as a bit sequence with no spaces (Eg. 011)
Q5
CRC Burst Errors
0 / 0 points
0 / 1 attempts
Show that any bursts of length <= k can be detected if C(x) includes a constant term (x^0 term).
DLL-reliable data transfer 3que
Q1 Packet sequence
0 / 3 points
0 / 2 attempts
Tracing revealed the following sequence of packets sent by a sender employing stop and wait protocol. '0' and '1' refer to the sequence number in the packet.
000110000101110
Which of the following is most likely to produce the pattern. Bad forward channel (data path)
Bad reverse channel (ack path)
Bad timeout setting
Bug in error detection algorithm (reproducible bug)
Q2
Stop and wait
0 / 0 points
0 / 1 attempts
In stop and wait protocol, suppose both sender and receiver retransmit the previous packet on receipt of a duplicate ACK or data frame.
Draw a timeline to show the sequence of events which will happen if somehow the first data frame is duplicated.
Q3 Stop and wait data rate: Suppose the stop and wait protocol is employed over a link A to B, where the A to B link bandwidth is 1Mbps with a propagation delay of 20ms and B to A link bandwidth is 10Mbps with a propagation delay of 10ms (reverse link different from forward link). Suppose that the data packet size is 1000 byte and ACK size is 100 byte.
0 / 3 points
0 / 2 attempts
What is the throughput achieved by the protocol? Ignore other delays. Express answer in kbps. Format X (e.g. 512)
Submit
0 / 2 points
0 / 1 attempts
If you are designing a new RDT protocol, what should your target throughput be i.e what is the maximum throughput that can (potentially) be achieved over this link? Express the answer in kbps. Format X (Eg. 250).
DATA LINK LAYER –sliding window protocol 7que
Q1
Sending Window: Assume a link with data rate of 1Mbps and one way latency of 40ms. Assume a packet size of 1000 bytes.
0 / 2 points
0 / 2 attempts
What should the sending window size be (in packets) over this link? Format X (e.g. 7)
Submit
0 / 2 points
0 / 2 attempts
For SWS as obtained in previous question, if RWS was set to 1, what would the minimum number of bits needed to represent the sequence number? Format X (e.g. 8)
Q2 Alternating bit protocol
0 / 1 points
0 / 1 attempts
Is the alternating bit protocol the same as Go-Back-N protocol with a sender and r eceiver window size of 1? yes
no
Q3
Sliding Window ACK
0 / 0 points
0 / 1 attempts
In the sliding window protocol, can the sender receive an ACK for a packet that falls outside its current window? If so, specify the scenario under which this occurs as well.
Q4
Minimum sequence number
0 / 0 points
0 / 1 attempts
Derive a general rule for the minimum sequence number space in terms of SWS and RWS.
Q5
Sliding Window Timeline-1
0 / 0 points
0 / 1 attempts
Draw a timeline of the sliding window protocol with SWS=RWS=3. Assume frames 4,5,6 are lost (first copies only). Use a time-out of 2*RTT and that the sender re-transmits the lost frames on timeout. Draw the timeline till the frames 4,5,6 are successfully received at the receiver.
Q6
Sliding Window Timeline-2
0 / 0 points
0 / 1 attempts
Same problem as before except that the sender retransmits frames on receiving first duplicate ACK. Draw a timeline of the sliding window protocol with SWS=RWS=3. Assume frames 4,5,6 are lost (first copies only). Use a time-out of 2*RTT. Draw the timeline till the frames 4,5,6 are successfully received at the receiver.
Q7
Sliding Window Selective Ack
0 / 0 points
0 / 1 attempts
Draw a timeline of the sliding window protocol that employs selective acknowledgment with SWS=RWS=4. Assume frames 4,5 are lost. Use a time-out of 2*RTT. Show the timeline till frame 11 is sent.
DAT LINK LAYER –media access control overview 3 que
Q1 In which of the following MAC protocols is there a delay (latency) associated with accessing the channel to transmit a packet? Assume the packet is at the head of the queue. FDM TDM CDMA Polling
Q2 Overhearing
0 / 2 points
0 / 2 attempts
In which of the following techniques can a node prepared to receive from its sender receive (whether it wants to or not) other sender's transmissions? Assume a simple receiver with no special capabilities. FDM
TDM
Random Access
Polling
Q3
Performance
0 / 0 points
0 / 1 attempts
Among the protocols you studied so far, which comes close to the ideal MAC?
DATA LINK LAYer –Aloha 4que
Q1 G
0 / 1 points
0 / 1 attempts
A value of G greater than 1 means the system is Balanced
Under-loaded
Over-loaded Slotted Aloha
Q2
0 / 1 points
0 / 1 attempts
In slotted Aloha, what is the average wait time for a new frame before its transmission? Assume an empty queue and a slot time of T. 0
T/2
T
2T
Q3Pure
Aloha
0 / 3 points
0 / 2 attempts
For a given frame, what is the expected number of transmissions in a Pure Aloha system? e
e^G
e^(2G)
2G Hint
Whats the probability of success for a given frame?
Throughput of slotted aloha
Q4
0 / 3 points
0 / 1 attempts
For the same assumptions as made in the a nalysis of Slotted Aloha involving poisson arrivals, if for a specific scenario 20% of the slots were found to be idle. What is the achievable throughput for that scenario? Format X.xx (E.g. 0.85)
Submit Hint
Determine value of G first.
DATA LINK LAYER –Ethernet technology 5 que
Q1
Hub vs switch
0 / 0 points
0 / 1 attempts
A switch can connect 10Mbps and a 100Mbps link, while a hub cannot. Why?
q2
Ethernet adaptor
0 / 1 points
0 / 1 attempts
How many Ethernet adaptors can potentially be manufactured? Express it as a power of 2. Format 2^X (e.g. 2^10).
q3 Ethernet Address
0 / 1 points
0 / 1 attempts
If the first bit in an Ethernet MAC address is a zero, the address is what type of address? unicast
broadcast
multicast
q4 Collision domains
0 / 2 points
0 / 2 attempts
In the figure shown, how many collision domains are there? Format X (e.g. 8)
Q5
Competing nodes
0 / 2 points
0 / 1 attempts
See Fig. Identify the collision domains. In the largest collision domain, how many nodes are competing for the channel? Format X (e.g. 9)
DATA LINK LAYER –CSMA family protocol 3 que
Q1 Idle Channel
0 / 1 points
0 / 1 attempts
In which of the following protocols can a channel be idle at the end of a transmission even when there are nodes with traffic to send? Slotted Aloha
1-persistent
non-persistent
p-persistent
q2 Performance
0 / 1 points
0 / 1 attempts
Which of the following protocols does better in terms of channel utilization at high loads? Slotted Aloha
1-persistent
non-persistent
0.5 persistent
q3 Slotted system
0 / 1 points
0 / 1 attempts
Which among the following is a slotted system? Pure Aloha
1-persistent CSMA
non-persistent CSMA
p-persistent CSMA
DAT LINK LAYER –Ethernet MAC CSMA/CD 5 que
Q1
Frame Size
0 / 2 points
0 / 1 attempts
Suppose you want to employ CSMA/CD (not the Ethernet standard as such) on a bus topology of length 2km and rate 10Mbps. What minimum size frame in bytes would you use? Format X (E.g. 57)
Assume speed of light is 2* 10^8m/s
Q2 Padding
0 / 1 points
0 / 1 attempts
If an IP packet of size 50 bytes is delivered to the link-layer employing Ethernet, how many bytes of padding is needed? Format X (E.g. 10)
Q3 Minimum Frame Size
0 / 1 points
0 / 1 attempts
The minimum frame size needed for 10 Mbps Ethernet is 512 bits. For 100 Mbps Ethernet, what should this size (in bits) be if we assume same network diameter? Format X (e.g 1000)
Submit
0 / 1 points
0 / 1 attempts
The minimum frame size needed for 10 Mbps Ethernet is 512 bits. For 100 Mbps Ethernet, if the size is L for same network diameter, is this packet size desirable or not? Justify. Desirable
Non-desirable
Submit
0 / 1 points
0 / 1 attempts
Same problem as before. If the packet size is un-desirable, what can you do to address the problem. Not applicable -- Packet size is desirable
Increase network diameter
Decrease network diameter
Give up on 100 Mbps Ethernet
Q4
Ethernet Capture Effect:
Two stations A, B employing Ethernet standard are competing for access. Suppose, both stations have many frames to send (labelled A1, A2, ... for A and B1, B2, .... for B). Suppose A and B simultaneously transmit their respective first frames and collide. After their first collision, A chooses k=0, while B chooses k=1. This ensures that transmission of A1 is successful while B1 is yet to meet with success. At the end of A1's transmission, A attempts to send A2 and B attempts to send B1.
0 / 3 points
0 / 2 attempts
What is the probability that A2's transmission succeeds over B1's? Format 0.xxx (e.g. 0.125) 0.625
Submit
0 / 3 points
0 / 2 attempts
Suppose A2's transmission succeeds again compared to B1's. Now A is attempting A3, while B is still attempting B1. What is the probability that A3's transmission succeeds over B1's? Format 0.xxx (E.g. 0.563) 0.813
Q5
Ethernet Transmissions: Suppose two nodes A and B both transmit packets at the same time and collide. As soon as they collide, they send the jamming signal and enter exponential backoff. Assume the time at which both And B finish transmitting the last bit of the jamming signal is set to 0, and the one way propagation delay between the nodes is 250 bit times. Also suppose that A chose a value of k=0 and B chose a value of k=1 (waits 512 bit times).
0 / 2 points
0 / 2 attempts
When does node A begin transmission? Express the answer in terms of bit times. Format X (e.g. 284)
Submit
0 / 1 points
0 / 1 attempts
When does node A's first bit reaches B relative to time 0? Express answer in bit times. Format X (e.g. 543)
Submit
0 / 2 points
0 / 2 attempts
When does node B attempt to transmit its first bit?
DDL- Token ring 4Que
Q1Number of Stations
0 / 2 points
0 / 2 attempts
Suppose the total wire length of a Token Ring operating at 4Mbps is 200m. What is the minimum number of stations that needs to be attached to it so that the token can circulate properly? Assume each host provides just 1 bit of buffering. Assume speed of light as 2* 10^8m/s. Format X (e.g. 90)
Submit Hint
Determine how many bits the wire can hold
Q2
Throughput: Determine the maximum throughput a 'single' backlogged user can obtain on a 16Mbps Token Ring. Assume a packet size of 500 bytes, a ring latency of 20us. Ignore transmission time of the token.
0 / 2 points
0 / 2 attempts
Consider immediate token release. Express your answer in Mbps with one decimal point. E.g. X.Y (26.3)
Submit
0 / 2 points
0 / 2 attempts
Consider delayed token release. Express your answer in Mbps with one decimal point. E.g. X.Y (26.3)
Q3
Channel Efficiency
0 / 3 points
0 / 2 attempts
Determine the efficiency of the channel (in %) when 'many' backlogged users are transmitting on a 100Mbps Token Ring with a ring latency of 100us. Assume a packet size of 500 bytes and delayed token release. Ignore transmission time of the token. The number of backlogged users is not necessary for an approximate calculation. Format X (e.g. 56)
Q4Monitor Bit
0 / 1 points
0 / 1 attempts
If in a frame, the monitor bit in the header is set to 1. This means This frame was generated by the monitor
This frame was seen by the monitor
This frame is part of monitor election
This frame has to be drained by the monitor
DDL –Switching 4Que
Q1Time of Transfer: Suppose Host A is connected to another Host B via a switch. The data rate of the link connecting the hosts to the switch is Rbps. Assume a packet length of L bits. In the calculation below, ignore processing, propagation and queuing delay. A ---- S ---- B
0 / 1 points
0 / 1 attempts
If circuit switching was used to forward the packet, how much time would it take to send the packet from A to B after connection establishment? Assume a dedicated wire is allocated to the connection (not TDM). L/R
R/L
2L/R
3L/R
Submit
0 / 1 points
0 / 1 attempts
If using packet switching, how much time would it take to send the packet from A to B? L/R
2L/R
3L/R
4L/R
Q2
Payload Size
0 / 4 points
0 / 2 attempts
Suppose that 100kBytes of user data has to be sent from host A to host B passing through 4 switches employing packet switching. Suppose the data rate on each of the link is 1Mbps and that each packet carries a header of 40bytes. What value of payload size (payload in the packet excluding header) minimizes the total delay in delivering the user data? Total delay: first bit was sent from A to when last bit was received at B. Ignore propagation/processing delay. Express the answer in bytes. Format X (e.g. 200)
Q3
Packet vs Circuit Switching:
In this question, we will compare circuit with packet switching. Suppose N (B for Bytes) of user data has to be sent from host A to host B passing through 3 switches employing packet switching. Suppose the data rate on each of the link is 1Mbps and propagation delay i s 1ms. For circuit switching, assume that a 1000B packet has to be sent from source to destination and back from destination to source before the user data can be sent. During this connection set-up time, assume that each switch introduces 1ms of processing delay (packet fully removed from link before processing). After connection set up, assume a dedicated wire is allocated to the connection. There is no processing delay at the switches during the data transfer phase. For packet switching, assume a payload size of 1000B and a header size of 20B. For forwarding the packet, each switch incurs 1ms processing delay. Also assume that the file size is a multiple of 1000B.
0 / 5 points
0 / 2 attempts
For what value of N is the total delay in delivering the user data (first bit from sender to last bit received at receiver) less for circuit switching compared to packet switching. Express N in Kilobytes in the answer. Format X (E.g. 200)
Submit
0 / 3 points
0 / 2 attempts
For what value of N (minimum), the total number of bytes sent (including payload, headers, connection-setup packets, bytes sent by sender/receiver ) in the network "less" for circuit switching compared to packet switching? Express N in Kilobytes in the answer. Format X. E.g. 200 (to indicate 200KB)
Q4Sharing
0 / 2 points
0 / 2 attempts
Suppose, you have a 1 Mbps link being shared by 3 users at a switch. Each user is active only 10% of the time and when active needs 400kbps .
If packet switching was being used, for what fraction of the time (expressed in percentage) can the queue at the switch grow. Express your answer in X.yy format. Eg. 0.25 (to indicate 0.25%).
Submit Hint
Determine under what circumstances will incoming rate exceed outgoing rate
DDL –Packet Switching 5Que
Q1
Forwarding Tables
0 / 1 points
0 / 1 attempts
Host A is connected to Host B via 6 switches. How many forwarding tables are consulted to forward packets from A to B? Format X (e.g. 3)
Q2 Q3 Q4 Q5
