CPE 448/548 Exam #1 (100 pts) February 14, Name Class: 448

Transcription

1 Name Class: 448 1) (14 pts) A message M = is transmitted from node A to node B using the CRC code. The CRC generator polynomial is G(x) = x 3 + x ( bit sequence 1101) a) What is the transmitted code word perform the polynomial long division to find this result b) If the transmitted word is corrupted during transmission by a bit error pattern of (XOR the bit error pattern and the transmitted code word to obtain the received code word). What is the received code word for the transmitted word determined in part a? c) Assume node B receives the following code word: By using the CRC, does node B detect any bit errors introduced by the link? Use generator polynomial from part a. Page 1 of 8

2 2) (12 pts) Answer the following short answer questions. a) What are the differences between datagram packet switching (DPS) and virtual circuit packet switching (VCPS)? b) Explain the exposed node problem in a wireless network? c) In the following 2 dimensional parity problem, 4 bit words are used. 1) Fill in the missing (shaded part) values if even parity is used. 2) What percentage of the bits transmitted are data bits? Page 2 of 8

3 3) (24 pts) A hypothetical network has an end to end length of 7500 meters with a propagation speed of 2.0x10 8 m/s. The bandwidth of the link is 10 Mbps. The maximum frame size for transmission on this network is 1000 bits. a) What is the transmission time for a maximum sized frame on the network? b) What is the one way (from one end to the other) propagation delay for the network? c) Assume CSMA/CD (carrier sense multiple access with collision detection) is used on this network. What is the smallest size frame (in bits) that can be used such that a transmitting node is guaranteed of detecting a collision? d) For this network, is the maximum frame size sufficient for CSMA/CD? Explain e) Which delay (from parts a and b) dominates(larger impact on delay) for this link? f) If the bandwidth of the link increases to 100 Mbps, which delay dominates for this link? Page 3 of 8

4 4) (20 pts) Consider the following virtual circuit network and the table showing the next Virtual Circuit Identifier () to use for each port. The outgoing and incoming s can be the same for a given interface/port (i.e. port 3 on a switch can have a of 5 for incoming packets and a of 5 for outgoing packets). An interface is the same as port, and only the interfaces of interest for each switch are shown (i.e. interfaces 1 and 2 on switch 1). Note: the network does not show all of the ports available on all switches, and it does not show all of the other nodes in the network. Lastly, each port has its own set of virtual circuit identifiers (i.e each port on a switch has its own s 0, 1, 2, etc. ) A S1 S2 S The next to use for interfaces on the switches Switch Incoming Next to Use S1 1 4 S1 2 2 S2 2 5 S2 3 6 S3 0 4 S3 1 1 Z Host A starts a connection to Host Z by sending a setup message. A short while later Host Z starts a connection with Host A by sending a setup message. Use the table above to complete the switch tables below to show the new entries created during these virtual circuit setups. Assume that all previous connections remain active during the setups. Use a next of 4 for Host Z and a next of 8 for Host A Setup creating entry A to Z Z to A Setup creating entry A to Z Z to A Setup creating entry A to Z Z to A Virtual Circuit Table for Switch 1 (S1) Incoming Incoming Outgoing Virtual Circuit Table for Switch 2 (S2) Incoming Incoming Outgoing Virtual Circuit Table for Switch 3 (S3) Incoming Incoming Outgoing Outgoing Outgoing Outgoing Page 4 of 8

5 5) (20 pts) Consider the 1Gpbs ethernet shown A 10 µ S S1 10 µ S S2 10 µ S B The propagation delay between any two hosts (A, B or a switch) is 10 µ s. The data to transmit from node A to node B consists of 40,000 bits. Each switch is a store and forward switch that starts retransmission of a packet 25µ s after receiving the last bit of a frame. a) What is the latency for transmitting the data as a single frame from A to B (time from the first bit of the frame transmitted by node A until last bit of the frame is received at node B)? b) What is the effective data throughput rate in bits per second(bps) for the circuit analyzed in part a? (answer is less than 1Gbps) Continued on Next Page Page 5 of 8

6 5 cont) c) If the data is split into 4 frames so that each frame to be transmitted consists of 10,000 bits of data, what is the latency for transmitting all of the data (4 frames) from A to B? Node A will transmit the frames one right after the other. d) What is the effective data throughput rate in bits per second(bps) for the circuit analyzed in part c? (Answer is higher than that of part b) Page 6 of 8

7 6) (10 pts) Spanning Trees a) The spanning tree algorithm is performed on the following network. Put an X on the links that are not used after the algorithm has established the spanning tree. LAN A LAN B LAN C B2 B1 B3 LAN D LAN F LAN E B5 B4 LAN G b) Draw one of the possible spanning trees for the network shown on the left: Page 7 of 8

8 Extra Credit) (5 pts) A Go-Back-N ARQ protocol is being implemented with a sending window size of 3 frames (SWS = 3). Frames are sequenced using numbers 0, 1, 2, 3, etc. Acknowledgments are sent for each frame that is received in order. If a higher sequence numbered frame is received out of order, it is discarded and a NAK (negative acknowledgement) is sent back with the number of the frame the receiver is expecting. The sender must then resend ALL frames starting with the frame referenced in the NAK. For example, frames 0, 1, 2 and 3 are sent, and frame 0 is received (ack 0 sent back) then frame 2 is received. Frame 2 is dropped and a NAK1 is sent back. The sender must then resend frames 1,2 and 3. Complete the timeline for this protocol given the following information: Sender needs to send 6 frames with sequence numbers 0 through 5 When allowed to send (SWS = 3), the sender transmits one frame every ¼ of a RTT Receiver sends ACKs and NACKs as indicated above The timeout period is 2 Round Trip Times (2 RTT) During transmission, Frame 2 is lost A frame is received ½ of a RTT after transmission starts (propagation delay) (Frame 0 and ACK 0 are shown) Bandwidth is infinite, so transmit time of frames is instantaneous Sender Receiver 0 F 0 0 A 0 1 RTT 1 RTT 2 RTT 2 RTT 3 RTT 3 RTT Page 8 of 8