Notes

Question 1

Data link layer

Answer 1

The layer 2 is responsible for moving a message from one computer or network device to the next computer. It controls the way messages are sent on the physical media.

Question 2

Data link protocol

Answer 2

A data link protocol determines who can transmit at what time, where a message begins and ends, and how a receiver recognizes and corrects a transmission error

Question 3

The data link layer performs two main functions, and therefore is often divided into two sublayers …

Answer 3

Logical link control [LLC] sublayer &

Media access control [MAC] sublayer

Question 4

Logical link control [LLC] sublayer

Answer 4

LLC is the data link layer’s connection to the network layer above it. At the sending computer, the LLC sublayer software is responsible for communicating with the network layer Protocol Data Unit (PDU) – usually an IP packet – and surrounding it with a data link layer PDU – often an ethernet frame. At the receiving computer, the LLC sublayer software removes the data link layer PDU and passes the message it contains to the network layer software.

Question 5

Media access control [MAC] sublayer

Answer 5

MAC controls the physical hardware. The MAC sublayer software at the sending computer controls how and when the physical layer converts bits into the physical symbols that are sent down the circuit.

At the receiving computer, the MAC sublayer software takes the data link layer PDU from the LLC sublayer, converts it into a stream of bits, and controls when the physical layer actually transmits the bits over the circuit. At the receiving computer, the MAC sublayer receives a stream of bits from the physical layer and translates it into a coherent PDU, ensures that no errors have occurred in transmission, and passes the data link layer PDU to the LLC sublayer.

Question 6

A data link protocol performs three functions:

Answer 6

1. Controls when computers transmit (media access control)
2. Detects and corrects transmission errors (error control)
3. Identifies the start and end of a message by using a PDU (message delineation)

Question 7

Media Access Control (MAC)

Answer 7

Refers to the need to control when computers transmit

Question 8

Why is Media Access Control unnecessary with point-to-point full-duplex configurations?

Answer 8

Because there are only two computers on the circuit and full duplex permits either computer to transmit at any time.

Question 9

When does Media Access Control becomes important?

Answer 9

It becomes important when several computers share the same communication circuit, such as point-to-point configuration with a half duplex configuration that requires computers to take turns, or a multipoint configuration in which several computers share the same circuit.

Question 10

Fundamental approaches to Media Access Control …

Answer 10

1. Contention
2. Controlled Access
   
   2. 1. Access Requests
   3. 2 Polling
      2. 2.1. Roll-call polling
      2. 2. Hub polling (also called token passing)

Question 11

Contention (Media Access Control) approach

Answer 11

With contention, computers wait until the circuit is free and then transmit whenever they have data to send. Contention is commonly used in Ethernet LANs

Question 12

Controlled Access (Media Access Control) approach

Answer 12

Most wireless LANs use controlled access. In this case, the wireless access point controls the circuit and determines which clients can transmit at what time.

Question 13

Controlled Access techniques:

Answer 13

1. Access Requests
2. Polling
   
   2. 1. Roll-call polling
   3. 2. Hub polling (also called token passing)

Question 14

Access Requests (Controlled Access technique)

Answer 14

steps:

1. Client computers that want to transmit send a request to transmit to the device that is controlling the circuit (e.g., the wireless access point).
2. The controlling device grants permission for one computer at a time to transmit.
3. When one computer has permission to transmit all other computers wait until that computer has finished, and then, if they have something to transmit, they use a contention technique to send an access request.

Question 15

Polling (Controlled Access technique)

Answer 15

Polling is the process of sending a signal to a client computer that gives it permission to transmit. With polling, the clients store all messages that need to be transmitted. Periodically the controlling device (e.g., a wireless access) polls the client to see if it has data to send. If the client has data to send, it does so. If the client has no data to send, it responds negatively, and the controller asks another client if it has data to send.

Question 16

Roll-call polling (Controlled Access technique - polling)

Answer 16

With roll-call polling, the controller works consecutively through a list of clients, first polling client 1, then client 2, and so on, until all are polled. Roll-call polling can be modified to select clients in priority so that some get polled more often than others. For example, one could increase the priority of client 1 by using a polling sequence such as 1, 2, 3, 1, 4, 5, 1, 5, 7, 1, 8, 9. The response might be an incoming message that was waiting to be sent, a negative response indicating nothing is to be sent, or the full “time-out-period” may expire because the client is temporarily out of service.

Question 17

Hub polling also called token passing (Controlled Access technique - polling)

Answer 17

One device starts the poll and passes it to the next computer on the multipoint circuit, which sends its message and passes the poll to the next. That computer then passes the poll to the next, and so on, until it reaches the first computer, which restarts the process again.

Question 18

Which media access control approach is best: controlled access or contention?

Answer 18

The key consideration is throughput – which approach will permit the most amount of user data to be transmitted through the network.

In general, contention approaches work better than controlled approaches for small networks that have low usage. In this case, each computer can transmit when necessary, without waiting for permission. Because usage is low, there is little chance of a collision. In contrast, computers in a controlled access environment must wait for permission, so even if no other computer needs to transmit, they must wait for the poll.

The reverse is true for large networks with high usage: controlled access works better. In high-volume networks, many computers want to transmit, and the probability of a collision using contention is high. Collisions are very costly in term of throughput because they waste circuit capacity during the collision and require both computers to retransmit later. Controlled access prevents collisions and makes more efficient use of the circuit, and although response time does increase, it does so more gradually.