Urgent fast answer 1, 2, 6, 7, 8. All of the questions dont need to be answered.

COSC 650                             Exam 1                  Fall 2020               Name_______________________________

PLEASE INDICATE YOUR ANSWERS CLEARLY and only in the SPACES PROVIDED.

1. Do each calculation below. Please show the calculation steps to explain your answers. ms=milliseconds. A 2megabit/sec link has a 100ms one-way delay.  Use powers of 10 (instead of 2).
2. What is the delay bandwidth product in kilobits, if one-way delay is used?

• What is the total time(first bit sent to last bit received) in msto transfer 2MB (megabytes) of data?

• What is the throughputin megabits/secwhen transferring 2MB of data as in b) above?

2. a) In the OSI model,CIRCLE the name of the layer (only ONE) that deals with

• encoding schemes such as 4D-PAM5 for Ethernet: phy       link     net     trans     sesspres     app
• modulation of bits on a wireless network:              phy       link     net     trans     sesspres     app

b) Draw a protocol graphshowing the relation between Ethernet, IP, TCP, UDP in the TCP/IP model.

3. CIRCLE ONE of the choices to indicate your answer. In LTE, the

• layer immediately below the IP layer is: RRC       RLC                       PDCP                     none of these
• node that assigns IP addresses is: eNodeB             MME                     SGW                       none of these
• uplink uses:                   SC-FDMA             OFDMA CDMA                   none of these

4. a) The NRZI signal for 4 bits is: LtoHH HtoLL (L=Low, H=High).

What are the actual 4 bits corresponding to this signal? (use 0s and 1s to indicate) ____________________

b)For each of the following, circle ONE of True or False.

• 4B/5B encoding is used to prevent long strings of ones                                                       True        False   
• An STS-24SONET link transmits 810 bytes in 125µs                                                          True        False   
• Two-dimensional parity can detect all 3-bit errors                                                                True        False   
• The Internet checksum algorithm computes a value that is 16 bits in length                  True        False   

5. If the CRC polynomial is: x¹²+x⁸+x³+1,

a) How many bits are in the CRCthat is sent to detect errors in a message? _____

b) How many XOR gates are used when building the CRC hardware?    _____

c) The CRC can detect all burst errors of length 8 bits. Circle ONE of:     True        False

d) To find the CRC for a message M, add __ zeros to the end of M and divide by a string of length __ bits.

6. a) What is meant by saying that the type field in an Ethernet header is its demultiplexing key?

______________________________________________________________________________________

• For a captured Ethernet frame, Wireshark shows the source and destination MAC addresses and the type fieldin Ethernet header,and the data bytes carried in the frame. The total number of bytes in this frame according to Wireshark is 80 bytes. Show how to find the number of data bytes in the frame. __________________________________________________________________________________

7. Consider the sliding window algorithm.

a) If the sequence number field has 8 bits, how many sequence numbers are possible? _____

b) Suppose SWS=RWS=5. Smallest possible value for MaxSeqNum = ______

c) Suppose SWS=RWS=5 and MaxSeqNum=6. The sender sends frames with seq numbers 0, 1, 2, 3, 4. They are received in order and given to upper layer and the corresponding cumulative acks are sent.

c1) The value of LargestAcceptableFrame (LAF) after receiving all 5 frames above= _______

c2) The sender only gets the cumulative ack for frame 3. The sender then sends two new frameswith seq numbers 5, 0 and also retransmits the frame with seq number 4.

• If only frames with seq numbers 4, 0are received, the numbers of the cumulative acks sent= _______
• Thesecumulative acks are lost and the sender retransmits frames with seq numbers 4, 5, 0. If all these frames are received, which of these sequence numbers are accepted? ________
• Did the algorithm fail due to accepting a wrong frame? CIRCLE ONE of: Yes      No

8. a) In the figure below, X is an 802.11 station whose MAC address is MAC(X). X is connected to the 802.11 network via its AP whose MAC address is MAC(A). The AP connects the 802.11 network to the Internet via its Ethernet connection to the router R whose MAC address is MAC(R):   X——–AP——–R

R receives a frame sent to X from the Internet and forwards it to the AP. The AP sends the frame to X using the 802.11 wireless link. Fill in the MAC addresses A1, A2, A3 in the 802.11 framereceived by X.

A1=MAC_______           A2=MAC______ A3=MAC_______

b) X sends an RTS to the AP. Y is a hidden stationwith respect to X (but within range of the AP).

• When X sends its data frame, explain why Y does not send any frames that will collide with X’s frame.

______________________________________________________________________________________

• If X does not get an ack for its data frame, it assumes a collision occurred and uses binary exponential backoff. Explain how X chooses the time to wait before resending its data frame.

______________________________________________________________________________________

9.In the figure below, A, B, C, and D are hosts connected as shown by switches 1, 2, and 3.

Suppose that first 1) C connects to A, and then 2)D connects to A. In the table below, fill in the information in switch 1’s virtual circuit table after both connections are made (the lowest unused VCI on a link is always picked, starting with 0).

Incoming Interface	Incoming VCI	Outgoing Interface	Outgoing VCI

10. In the figure, W, X, Y, Z are LANs; B1, B2, B3 are bridges; W1, Y1, Z1 are hosts (tables are empty).

a) Suppose that now link (B2, W) fails. W1 sends packet 1 to Z1 and then Z1 sends packet 2 to W1. CIRCLE ALL bridge interfaces that see packet 2.

(B1, Z)                   (B1, X)              (B2, X)                        (B2, W)                  (B3, Y)                   (B3, Z)

b) Suppose that link (B2, W) is up again.Messages are of form (root, dist, sender).

• During the spanning tree algorithm, B2 first sends the message __________________ on all ports; after it gets the message (B1, 0, B1) on (B2, X), B2 sends  ______________________ on (B2, W).
• CIRCLEbelow ALL interfaces that are NOT in the spanning tree.

(B1, X)                   (B1, Z)                   (B2, X)                   (B2, W)                  (B3, W)             (B3, Y)        (B3, Z)                  

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