CS 640 Fall 1996 Midterm Exam 1. Provide brief answers (3 points each) How do packet switching and circuit switching differ with respect to buffers at switches (1) and bandwidth reservation on links (2). Give a disadvantage of all packet switching methods (3) and a disadvantage of circuit switching (4). State two key differences between datagrams and virtual circuits (5), (6) and the key feature they have in common (7). Does a connection-oriented service interface have to be reliable (8)? 2. Two parts; 5 points for each part; no partial credit Assume a generator polynomial G(x) = x**5 + x**2 + x + 1 and a frame 101101011010. Compute the CRC for this G(x) and frame (9). Give the shift register circuit for G(x) (10). 3. Assume the following communication between two TCP entities. Specify the values omitted for the indicated TCP segments. (2 points each) Last Data Byte Sent = 399 Last Window Field = 220 Last Ack Sent = 50 Seq # = 50 Ack # = 400 -----------> Arrive Window = 250 DataLen=120bytes Seq# = 400 Arrive <----------- Ack# = (11) Window = 300 DataLen=550bytes Seq# = (12 Ack# = (13) -----------> DataLen=30bytes 4. Fill in the blanks for the seq numbers in the following connection (1 point each) TCP A, starts with sequence number 1000 TCP B, starts with sequence number 5000 Assume Window field in all segments set to 64K Time 0: TCP A: CLOSED STATE; TCP B: CLOSED STATE Time 1: TCP A: Send S1 = Time 2: TCP B: Recv S1; Send S2 = Time 3: TCP A: Recv S2; Send S3 = Time 4: TCP B: Recv S3 Time 5: TCP A: Send S4 = DataLen=1500bytes Time 6: TCP B: Recv S4 ; Send S5= DataLen=700bytes 5. Fill in the blanks (2 points each question) a. What four values uniquely specify a TCP connection? (23) b. What field in the TCP header requires information from the IP header? (24) c. Assuming a RTT of 100 ms for a transport connection, a maximum packet length of 500 bytes, and a maximum receive window size (credit) of 1000 packets, what is the maximum throughput in bytes per second that could be achieved on this connection. (25) d. To achieve 8mpbs TCP throughput on a connection with RTT 200ms, what is the minimum window scale factor required? (26) e. Which TCP flag is used for graceful close? (27) and which is used to abort a connection (28) f. The three components of end-end latency are (29). g. The principal benefit of NAKs in data link protocols is (30). h. The DLE is used by (31) data link protocols. i. The 3-way handshake is used to (32). j. SABM and UA are used by (33) to (34). 6. The following are to be answered with True (T) or False (F) (1 point each) The following are used by LAPB (X.25 Level 2) (35) cumulative acks (36) credits (37) 3-way handshake (38) bit stuffing (39) NAKs (40) sliding window (41) retransmission 802.5 Questions (42) Automatic retransmission occurs when a frame returns to the sender with an incorrect FCS.. (43) Tokens are distinguished from data frames by a single bit. (44) A station that receives a frame removes it from the ring if the FCS is not correct. (45) The token is put back on ring at the earlier of a) when the sending station receives the header of the frame it sent or b) when the sending station finishes transmitting the frame. (46) Any station is capable of being the active monitor. (47) The MAC protocol provides a fair service to all stations. 802.3 Questions (48) The backoff procedure on collisions guarantees that no station will be starved. (49) If the maximum network size (in the standard) were increased, the minimum frame size would decrease. (50) If the bandwidth (in the standard) were decreased, the minimum frame size would decrease. (51) Ethernet and 802.3 differ in the way that they compute backoff times. (52) Every station that senses a collision, sends a jam signal. (53) If an ack is not received within a specified time, 802.3 retransmits the frame. (54) A station never has to wait more than k slot times to transmit, where k is derived from the number of stations attached to the bus. (55) Ethernet and 802.3 differ by one field in the header. FDDI Questions (56) The receiving station never removes the frame from the ring. (57) More than one frame can be on the ring at the same time. 802.2 Questions (58) The protocol provides either a connection-oriented or a connectionless service interface. (59) A field in the header specifies the upper (network) layer protocol. (60) Sliding windows are used to provide a connectionless service interface. (61) Data link protocols usually have variable size receive windows. (62) Byte-oriented data link protocols are preferable to bit-oriented data link protocols because they depend on a fixed character set representation. (63) When a LAPB data link frame arrives with incorrect FCS, its N(R) field is still processed if the frame sequence number, N(S), is that of the next expected frame.