Important Short Questions and Answers: Computer Networks - Data Link Layer

1. What are the responsibilities of data link layer?

Specific responsibilities of data link layer include the following.

a) Framing

b) Physical addressing

c) Flow control

d) Error control

e) Access control

2. State the difference between Fast Ethernet and Gigabit Ethernet.

3. What is the purpose of Network Interface Card?

A NIC is a computer circuit board or card that is installed in a computer so that it can be connected to a network. NIC provides a dedicated, full time connection to the network.

4. What are Virtual LANs?

A VLAN acts like an ordinary LAN, but connected devices don’t have to be physically connected to the same segment. VLANs are considered likely to be used with campus environment networks. Among many companies likely to provide products with VLAN support are Cisco, Bay Networks and 3Com.

5. What are the functions of MAC?

MAC sublayer resolves the contention for the shared media. It contains synchronization, flag, flow and error control specifications necessary to move information from one place to another, as well as the physical address of the next station to receive and route a packet.

6. What are the functions of LLC?

The IEEE project 802 models take the structure of an HDLC frame and divide it into 2 sets of functions. One set contains the end user portion of the HDLC frame – the logical address, control information and data. These functions are handled by the IEEE 802.2 logical link control (LLC) Protocol.

7. What is Ethernet?

Ethernet is a multiple access network, meaning that a set of nodes send and receive frames over a shared link.

8. Define Bluetooth.

Bluetooth is a wireless LAN technology designed to connect devices of different functions such as telephones, notebooks, computers, cameras, printers, coffee maker and so on.

9. Why Ethernet is said to be 1-persistent protocol?

The Ethernet is said to be a 1-persistent protocol, because an adapter with a frame to send transmits with probability one, whenever a busy line goes idle.

All the nodes can distinguish between idle and a busy link and “collision detect” means that a node listens as it transmits and can therefore detect when a frame it is transmitting as interfered (collided) with a frame transmitted by another node.

11. Define flow control.

Flow control refers to a set of procedures used to restrict the amount of data. The sender can send before waiting for acknowledgment.

12. What is a buffer?

Each receiving device has a block of memory called a buffer, reserved for storing incoming data until they are processed.

13. Mention the categories of flow control.

There are 2 methods have been developed to control flow of data across communication links.

·    Stop and wait- send one from at a time.

·    Sliding window- send several frames at a time.

14. What is the function of stop and wait flow control?

In this method, the sender sends one frame and waits for an acknowledgement before sending.

15. Mention the advantage and disadvantage of stop and wait f low control.

Advantage: simplicity and Disadvantage: inefficiency.

16. Define ARQ.

Error control in the data link layer is based on Automatic repeat request (ARQ), which means retransmission of data in 3 cases.

1. Damaged frame

2. Lost frame

3. Lost acknowledgment.

17. Mention the function of go-back N-ARQ.

It is the popular mechanism for continuous transmission error control. In the method, if our frame is lost or damaged, all frames sent since the last frame acknowledged are retransmitted.

18. What is selective reject ARQ?

In selective reject ARQ only the specific damaged or lost frame is retransmitted. If a frame is corrupted in transit, a NAK is returned and the frame is resent out of sequence.

19. Define HDLC.

It is bit-oriented data link protocols designed to support both half-duplex and full duplex communication over point to point and midpoint links.

20. List the types of stations is HDLC.

HDLC differentiates between 3 types of stations.

a) Primary

b) Secondary

c)        Combined

21. What are the different communication modes in HDLC?

HDLC supports 3 modes of communication between stations.

a) Normal response mode (NRM)

b) Asynchronous response mode (ARM)

c) Asynchronous balanced mode (ABM)

22. Mention the types of frames in HDLC.

There are 3 types of HDLC frames.

a) Information frames (I-frames)

b) Supervisory frames (S-frames)

c) Unnumbered frames (U-frames)

23. Give the usage of I, S, U frames.

I frames – used to transport user data and control information relating to user data.

S frames – used only to transport control information, primarily data link layer and error controls.

U frames – reserved for systems management.

24. Write the types of frame fields contained in HDLC.

Each frame in HDLC may contain up to 6 fields.

a) Beginning flag field

b) An address field

c) A control field

d) An information field

e) A frame check sequence (FCS) field

f)  An ending flag field.

25. What is meant by bit stuffing?

Bit stuffing is the process of adding one extra 0 whenever there are 5 consecutive in the data so that the receiver doesn’t mistake the data for a flag.

26. Define LAN.

A Local Area Network (LAN) is a data communication system that allows a number of independent devices to communicate directly with each other in a limited geographic area.

27. Mention the various architecture in a LAN.

LAN is dominated by 4 architectures.

a) Ethernet

b) Token bus

c) Token ring

d) Fiber distributed data interface (FDDI)

28. Define a standard 802.3

IEEE 802.3 supports a LAN standard originally developed by Xerox and later extended by a joint venture between digital equipment corporations. Intel Corporation and Xerox. This was called ‘Ethernet’.

29. List the most command kinds of Base band 802.3 LAN.

a) 10 Base 5

b) 10 Base 2

c) 10 Base T

d) 1 Base 5

e) 100 Base T

30. Mention the different kinds of Ethernet networks.

a) Switched Ethernet

b) Fast Ethernet

c) Gigabit Ethernet

31. Describe the three HDLC station types?

The three HDLC station types are:

Primary station: The primary station has the complete control of the link. The Primary stationsends commands to the secondary station.

Secondary station: The secondary station sends responses.

Combined station: The combined station is one which acts either as a primary or a Secondary,depending upon the nature and direction of the transmission. Combined station sends both commands and responses.

32. What is piggy backing?

Piggy backing means combining data to sent and acknowledgement of the frame received in one single frame. Piggy backing can save bandwidth because the overhead from a data frame and an ACK frame can be combined in to just one frame

33. Name the four types of S-frames?

The four types of S-frames are

Receive ready (RR).T0 e value of the code sub field is 00

Receive not ready (RNR). The value of the code sub field is 10

Reject (REJ). The value of the code sub field is 01

Selective reject (SREJ). The value of the code sub field is 11

34. Name the five categories of U-frames?

The five categories of U-frames are Mode setting, unnumbered exchange, Disconnection, Initialization mode &Miscellaneous mode

Glossary:

Access point (AP): A central base station in a BSS.

Acknowledgment (ACK): A response sent by the receiver to indicate the successful receipt ofdata.

ALOHA: The original random multiple access method in which a station can send a frame anytime it has one to send.

Asynchronous Transfer Mode (ATM): A wide area protocol featuring high data rates andequal-sized packets (cells); ATM is suitable for transferring text, audio, and video data.

ATM LAN: A LAN using ATM technology.

ATM layer: A layer in ATM that provides routing, traffic management, switching, andmultiplexing services.

Automatic Repeat Request (ARQ):An error-control method in which correction is made byretransmission of data.

Autonegotiation:A Fast Ethernet feature that allows two devices to negotiate the mode or datarate.

Backward Explicit Congestion Notification (BECN):A bit in the Frame Relay packet thatnotifies the sender of congestion.

Basic Service Set (BSS): The building block of a wireless LAN as defined by the IEEE 802.11standard.

Bluetooth: A wireless LAN technology designed to connect devices of different functions suchas telephones and notebooks in a small area such as a room.

Bursty Data: Data with varying instantaneous transmission rates.

Data Link Control The responsibilities of the data link layer: flow control and error control. 

Data Link Layer The second layer in the Internet model. It is responsible for node-to-nodedelivery.

Encryption Converting a message into an unintelligible form that is unreadable unlessdecrypted.

Ethernet: A local area network using the CSMA/CD access method.

Extended Service Set (ESS): A wireless LAN service composed of two or more BSSs with APsas defined by the IEEE 802.11 standard.

Fast Ethernet: Ethernet with a data rate of 100 Mbps.

Forward Explicit Congestion Notification (FECN): A bit in the Frame Relay packet thatnotifies the destination of congestion.

Forwarding: Placing the packet in its route to its destination.

Frame Relay: A packet-switching specification defined for the first two layers of the Internetmodel. There is no network layer. Error checking is done on end-to-end basis instead of on each link.

Gigabit Ethernet: Ethernet with a 1000 Mbps data rate.

Go-Back-N ARQ: An error-control method in which the frame in error and all following framesmust be retransmitted.

High-level Data Link Control (HDLC): A bit-oriented data link protocol defined by the ISO.

Institute of Electrical and Electronics Engineers (IEEE): A group consisting of professionalengineers which has specialized societies whose committees prepare standards in members' areas of specialty.

Interframe space (IFS): In wireless LANs, a time interval between two frames to control accessto the channel.

Logical link control (LLC): The upper sublayer of the data link layer as defined by IEEEProject 802.2.

Medium access control (MAC) sublayer: The lower sublayer in the data link layer defined bythe IEEE 802 project. It defines the access method and access control in different local area network protocols.

Network Address Translation (NAT): A technology that allows a private network to use a setof private addresses for internal communication and a set of global Internet addresses for external communication.

Network Allocation Vector (NAV): InCSMA/CA,the amount of time that must pass before astation can check for an idle line.

Normal Response Mode (NRM): In HDLC, a communication mode in which the secondarystation must have permission from the primary station before transmission can proceed. 

Password Authentication Protocol (PAP): A simple two-step authentication protocol used inPPP.

Piconet: A Bluetooth network.

Piggybacking: The inclusion of acknowledgment on a data frame.

Poll: In the primary/secondary access method, a procedure in which the primary station asks asecondary station if it has any data to transmit.

P-Persistent: A CSMA persistence strategy in which a station sends with probabilitypif it findsthe line idle.

Preamble: The 7-byte field of an IEEE 802.3 frame consisting of alternating 1s and 0s that alertand synchronize the receiver.

Project 802: The project undertaken by the IEEE in an attempt to solve LAN incompatibility.Routing Table: A table containing information a router needs to route packets. The informationmay include the network address, the cost, the address of the next hop, and so on.

Scatternet:A combination of piconets.

Select: In the poll/select access method, a procedure in which the primary station asks asecondary station if it is ready to receive data.

Selective-Repeat ARQ: An error-control method in which only the frame in error is resent.

Sliding Window: A protocol that allows several data units to be in transition before receiving anacknowledgment.

Sliding Window ARQ: An error-control protocol using sliding window concept.

Slotted ALOHA: The modified ALOHA access method in which time is divided into slots andeach station is forced to start sending data only at the beginning of the slot.

Standard Ethernet: The conventional Ethernet operating at 10 Mbps.

Stop-and-Wait Protocol: A protocol, in which the sender sends one frame, stops until itreceives confirmation from the receiver, and then sends the next frame.

Switched Ethernet: An Ethernet in which a switch, replacing the hub, can direct a transmissionto its destination.

Throughput: The number of bits that can pass through a point in one second. 

Unicast Address: An address belonging to one destination.

Virtual Circuit (VC): A logical circuit made between the sending and receiving computer. 

Virtual Circuit Switching: A switching technique used in switched WANs.

Virtual Link: An OSPF connection between two routers that is created when the physical link isbroken. The link between them uses a longer path that probably goes through several routers. 

Virtual Local Area Network (VLAN): A technology that divides a physical LAN into virtualworkgroups through software methods.

X.25: An ITU-T standard that defines the interface between a data terminal device and a packet-switching network