2. The Open Systems Interconnection Specifications

Question 1

1. Host 1 sent a SYN packet to Host 2. What will Host 2 send in response?
   a. ACK
   b. NAK
   c. SYN/ACK
   d. SYN/NAK
   e. SYN

Answer 1

c. A connection-oriented session is set up using what is called a three-way handshake. The transmitting host sends a SYN packet, the receiving host sends a SYN/ACK, and the transmitting host replies with the last ACK packet. The session is now set up.

Question 2

2. TCP and UDP reside at which layer of the OSI model?
   a. 1
   b. 2
   c. 3
   d. 4

Answer 2

d. TCP and UDP are transport layer protocols. The transport layer is layer 4 of the OSI model.

Question 3

3. Which layer of the OSI model provides an entry point for programs to access the network infrastructure?
   a. Application
   b. Transport
   c. Network
   d. Physical

Answer 3

a. The top layer of the OSI model gives applications access to the services that allow network access.

Question 4

4. You are connected to a server on the Internet and you click a link on the server and receive a time-out message. Which layer could be the source of this message?
   a. Application
   b. Transport
   c. Network
   d. Physical

Answer 4

a. If the remote server is busy or does not respond to your web browser request, this is an Application layer problem.

Question 5

5. Which layer of the OSI model is responsible for code and character-set conversion as well as recognising data formats?
   a. Application
   b. Presentation
   c. Session
   d. Network

Answer 5

b. The Presentation layer makes data “presentable” for the Application layer.

Question 6

6. At which layers of the OSI model do bridges, hubs and routers primarily operate, respectively?
   a. Physical, physical, data link
   b. Data link, data link, network
   c. Data link, physical, network
   d. Physical, data link, network

Answer 6

c. Bridges, like switches, are data link layer devices. Hubs, like repeaters, are physical layer devices. Routers are network layer devices.

Question 7

7. Which layer of the OSI model is responsible for converting data into signals appropriate for the transmission medium?
   a. Application
   b. Network
   c. Data link
   d. Physical

Answer 7

d. The physical layer’s job is to convert data into impulses that are designed for the wired or wireless medium being used on the attached segment.

Question 8

8. A receiving host has failed to receive all the segments that it should acknowledge. What can the host do to improve the reliability of this communication session?
   a. Send a different resource port number.
   b. Restart the virtual circuit.
   c. Decrease the sequence number.
   d. Decrease the window size.

Answer 8

d. A receiving host can control the transmitter by using flow control (TCP uses windowing by default). By decreasing the window size, the receiving host can slow down the transmitting host so the receiving host does not overflow its buffers.

Question 9

9. Which layer 1 devices can be used to enlarge the area covered by a single LAN segment? (choose two)
   a. Firewall
   b. NIC
   c. Hub
   d. Repeater
   e. RJ-45 transceiver

Answer 9

c, d. Not that you really want to increase a single collision domain, but a hub (multiport repeater) will provide this functionality for you.

Question 10

10. Segmentation of a data stream happens at which layer of the OSI model?
    a. Physical
    b. Data Link
    c. Network
    d. Transport

Answer 10

d. The transport layer receives large data streams from the upper layers and breaks these up into smaller pieces called segments.

Question 11

11. When data is encapsulated, which is the correct order?
    a. Data, frame, packet, segment, bits
    b. Segment, data, packet, frame, bits
    c. Data, segment, packet, frame, bits
    d. Data, segment, frame, packet, bits

Answer 11

c. The encapsulation order is data, segment, packet, frame, bits

Question 12

12. What are two purposes for segmentation with a bridge? (choose two)
    a. To add more broadcast domains
    b. To create more collision domains
    c. To add more bandwidth for users
    d. To allow more broadcasts for users

Answer 12

b, c. Bridges and switches break up collision domains, which allows more bandwidth for users.

Question 13

13. Acknowledgements, sequencing and flow control are characteristic of which OSI layer?
    a. Layer 2
    b. Layer 3
    c. Layer 4
    d. Layer 7

Answer 13

c. A reliable Transport layer connection uses acknowledgements to make sure all data is received reliably. A reliable connection is defined by the use of acknowledgements, sequencing, and flow control, which is characteristic of the of the transport layer (layer 4)

Question 14

14. Which of the following is true regarding sequencing and acknowledgements? (choose all that apply)
    a. The segments delivered are acknowledged back to the sender upon their reception.
    b. If a segment is not received, the virtual circuit must be restarted from the beginning at a slower transmit interval.
    c. Any segments not acknowledged are retransmitted.
    d. Segments are sequenced back into their proper order upon arrival at their destination.
    e. All segments are retransmitted on time slot intervals.

Answer 14

a, c, d. When sequencing and acknowledgements are used, the segments delivered are acknowledged back to the sender upon their reception. At this point, any segments not acknowledged are retransmitted, and segments are sequenced back into their proper order upon arrival at their destination.

Question 15

15. What is the purpose of flow control?
    a. To ensure that data is retransmitted if an acknowledgement is not received.
    b. To reassemble segments in the correct order at the destination device.
    c. To provide a means for the receiver to govern the amount of data sent by the sender.
    d. To regulate the size of each segment.

Answer 15

c. Flow control allows the receiving device to control the pace of the transmitting device so the receiving device’s buffer does not overflow.

Question 16

16. At which layer of the OSI model would you find IP?
    a. Transport
    b. Network
    c. Data Link
    d. Physical

Answer 16

b. IP is a network layer protocol. TCP is an example of a transport layer protocol, ethernet is an example of a data link layer protocol, and T1 can be considered a physical layer protocol.

Question 17

17. Of the following, which is the highest layer of the OSI model?
    a. Transport
    b. Session
    c. Network
    d. Presentation

Answer 17

d. The presentation layer is the 6th layer of the model. Only the application layer is higher, but it is not listed.Session is layer 5, Transport layer 4, and Network layer 3.

Question 18

18. Routers perform routing at which OSI layer?
    a. Physical
    b. Data link
    c. Network
    d. Transport
    e. Application

Answer 18

c. A router is specified at the network layer and a router routes packets. Routers can also be called layer 3 switches.

Question 19

19. Which of the following mnemonic devices can you use to remember the first letter of the name of each layer of the OSI model in the proper order?
    a. All people seem to need processed data.
    b. Always should people never threaten dog police.
    c. Please do not throw sausage pizza away.
    d. All day people should try new profession.

Answer 19

c. The phrase “please do not throw sausage pizza away” contains the first letters of the layers in order, from layer 1 through layer 7, “all people seem to need data processing” works from top down. The other options have all the right letters, just not in the right order.

Question 20

20. Which IEEE standard specifies the protocol for CSMA/CD?
    a. 802.2
    b. 802.3
    c. 802.5
    d. 802.11

Answer 20

b. The 802.3 standard, commonly associated with ethernet, specifies the media-access method used by ethernet, which is known as Carrier Sense Multiple Access with Collision Detection (CSMA/CD)