Chapter 3 - data link layer

Question 1

What is information unit at data link? network layer?

Answer 1

Frame. packet.

Question 2

What makes channel channel? give examples.

Answer 2

Bits are delievered same order they exit sender. Coaxial cable, telephone line, and wireless channel.

Question 3

What limitations of channels requiring data link protocols?

Answer 3

A finite data rate, channels make errors and delay between bit leaving sender and arriving receiver.

Question 4

What we will study in this chapter?

Answer 4

Design issue, nature of errors and how they can be detected and corrected, study some protocols and present some others.

Question 5

What layer does data link layer provides services for? What layer does it utilize?

Answer 5

Network layer. Physical layer.

Question 6

What are two purposes of data link layer?

Answer 6

Regulate bits flow and deal with errors.

Question 7

What changes does data link layer do to packets?

Answer 7

It adds header and trailer to packet and calls it payload.

Question 8

What are three types of services data link layer provides?

Answer 8

Unacknowledged connectionless, acknowledged connectionless and acknowledged connection-oriented.

Question 9

What do acknowledged/unacknowledged and connectionless/connection-oriented mean in context of data link layer services context?

Answer 9

Acknowledgement refers to getting back to sender to acknowledge reception. Connection refers to session or circuet establishment before transfer which implys frames arrival in order.

Question 10

What example of unacknowledged connectionless service? When to use this service?

Answer 10

Ethernet. error rate is very low and with real-time traffic, such as voice.

Question 11

What example of acknowledged connectionless service? When to use this service?

Answer 11

With unriable channel. Wireless such as 802.11 (WiFi).

Question 12

What example of acknowledged connection-oriented service? When to use this service?

Answer 12

With unriable long channel. Satellite channel and long-distance telephone circuit.

Question 13

What are phases of onnection-oriented service?

Answer 13

Connection establishment when variables and counters are initialized. Transmittion. Connection freeup when counters and buffers are cleared.

Question 14

What is framing? What are four methods of framing?

Answer 14

Delimit start of new frames. Byte count, flag bytes with byte stuffing, flag bits with bit stuffing and physical layer coding violations.

Question 15

What is framing by byte count? What is its disadvantage?

Answer 15

It specifying bytes count in header. If count got error, receiver is out of syncronization.

Question 16

What is framing by flag byte? How middle frames starts are delimited? is this delimitation effected by escape process? What is byte stuff? why it’s used? What is one example of protocol build on flag byte frame method?

Answer 16

Frame start and end with special bytes. Two consecutive flag bytes. No. Escape byte (ESC) is stuffed before flag byte or escape byte at data. Point-to-point protocol (PPP)

Question 17

What is framing by bit stuffing? What is one protocol example with details of its flag and stuffing? What is example technology using bit falgging? What is advantage of bit flag?

Answer 17

Delemiting start and end with flag that is of any size. High level Data Link Control (HDLC). Its flag is 0x7e and is stuffed with one bit zero after fifth one. USB. It has minimum density of transitions.

Question 18

What is shortage of bit and byte flagging? What is percentage of this shortage?

Answer 18

Byte flagging can increase data length by 100% and bit flagging can increase data length by 12.5%.

Question 19

What is 4B/5B? Whay it is used?

Answer 19

Every four bits are mapped to five bits with no longer than three zeros in these five bits. Long run of zeros causes problem of syncronization because of long sileince.

Question 20

What is framing by physical layer coding violation? What is one example?

Answer 20

If physicl layer is using coding method that omit some sequence of bits, we could use these bits for framing. Example is using symbol from 4B/5B that is not used with 4B/5B and does not contain three consecutive zeros like 12.

Question 21

What is advantage of byte count and physical layer voilation over byte and bit flagging?

Answer 21

No need to escape.

Question 22

What is defentions of Ethernet, 802 and 802.11 protocols? What is difference between 802.11 and Ethernet in terms of wired and wireless?

Answer 22

Ethernet is protocols group for wired networks. 802 is protocols group for LAN, MAN and PAN. 802.11 is group of 802 protocols for MAC and PHY of WLAN. 802.11 is wireless and Ethernet is wired.

Question 23

What framing method is used for Ethernet and 802.11?

Answer 23

Frame begins with preample pattern followed by frame length.

Question 24

What is preample length at 802.11?

Answer 24

72 bits.

Question 25

What is error (malfunction) control of data link layer? What type service can use it?

Answer 25

Use of timers and sequence numbers. Connection-oriented acknowledge service.

Question 26

What are two types of flow control at data link layer? What are their defentions? At what layers they are used?

Answer 26

Feedback and rate based flow controls. Feedback flow control receiver sends permission to send data. Rate-based flow control limits rate of sender transmittion. Rate flow control is categorized with transport layer while feedback is categorized by link and higher lyaers.

Question 27

What does NIC stand for? What is its defention?

Answer 27

Network interface controller. Hardware to connect computer to network.

Question 28

What does it mean to say NIC run run at ‘wire speed?

Answer 28

It handlea frames as fast as

they can arrive.