Question

a) Explain with appropriate diagrams the Logic Link Control (LLC) Layer and Medium Access Control (MAC) layer. [5 Marks]

b) Explain the scenario and the mechanism of Carrier Sense Multiple Access Collision Detection (CSMA/CD) in a network setting. [5 Marks]

c) Discuss with the appropriate figure/s the concept of packet encapsulation and decapsulation. [5 Marks]

Answer 1

APPLICATION LAYER PRESENTATION LAYER SESSION LAYER TRANSPORT LAYER NETWORK LAYER DATA LINK LAYER LLC Sublayer MAC Sublayer PHYSICAL LAYER Physical Link

a)Logical Link Control (LLC) is the upper sublayer of the Open System Interconnections (OSI) data transmission reference model's data link layer. This functions as an interface between the network layer and the data connexion layer sublayer of the Medium Access Control ( MAC). LLC's primary purpose is to multiplex protocols when transmitting over the MAC layer and also to de-multiplex the protocols when receiving.Hop-to-hop flow and error control are provided by LLC. Multipoint communication over the computer network is allowed. it assigns Frame Sequence Numbers.In case of acknowledged services, it tracks acknowledgements.

Medium Access Control ( MAC) is a sublayer of the data link layer of the data transmission reference model for open device interconnections (OSI). It is responsible for the regulation of flow and multiplexing of the medium of transmission. It controls the transmission, through remotely shared channels, of data packets. It sends data over the interface card of the network. It provides an abstraction of the physical layer to the LLC and upper layers of the OSI network. It is responsible for encapsulating frames so that they are suitable for transmission via the physical medium. It resolves the addressing of source station as well as the destination station,) frame is to be transmitted. It decides the methods of channel access for transmission.

b) CSMA / CD stands for Carrier Sense  Multiple Access / Collision Detectionwith an extension of the CSMA protocol being collision detection. This creates a procedure which regulates how communication in a network with a shared transmission medium must take place. The extension also governs how to proceed if collisions occur, that is, when two or more nodes attempt to concurrently send data packets through the transmission medium (bus) and interact with each other. CSMA / CD is split into several stages. The protocol is based on a standard group conversation: it is crucial that the participants don't all talk at once for effective communication, which can be confusing. Instead, one after the other should talk, so that each individual can fully appreciate what the others are adding to the debate. In conversations, without understanding, we simply act like this ourselves: when someone else is talking, we stand back and listen. We wait a short time after the other participant has completed their contribution for the moment and only begin talking when the same participant or another participant in the conversation does not begin to say anything else. When, at the same time, we start talking to someone else, we stop trying, wait a while, and then try again.

The process for CSMA / CD is very similar. First, the station monitors the means of transmission. The monitoring will continue as long as this is dealt with. The station will send a data packet only when the medium is free and for a certain period (in interframe spacing). Meanwhile, to see if it detects any data collisions, the transmitter continues to track the transmission medium. If no other participant attempts to send his or her data via the medium by the end of the transmission and no collision occurs, the transmission will take place. If, however, a collision is detected, the participant interrupts the transmission immediately and sends an interference signal (JAM signal) instead, so that the collision can also be detected by all other stations. Now, before trying the transmission again, the participant waits for a random amount of time (backoff). In order for the next collision to not happen immediately, the backoff must be random. Since a random value is selected by both stations, the likelihood that both of them will begin a transport attempt at the same time is quite low. The attempts at transmission are counted. The station reports the error to the next higher network layer if the following attempts also fail and the maximum number of attempts (16) is reached and then terminates the transmission permanently. As a network participant is highly unlikely to reach the maximum number of attempts during a normal process, it can be assumed that a system error has occurred.

c)Data goes through the data communication layers. Two terms related to passing data through each layer are Encapsulation and Decapsulation. The main difference between encapsulation and decapsulation is that the data moves from the upper layer to the lower layer in encapsulation, and each layer contains a bundle of information known as a header along with the actual data, while the data moves from the lower layer to the upper layers in decapsulation, and each layer unpacks the corresponding headers to obtain the actual data.

Encapsulation is the process of adding information as it is sent through each model layer to the application layer data. A new Protocol Data Unit (PDU) is created each time the data goes through a layer. A header with information about TCP / UDP in the transport layer has been added to the data sent from the application layer. Now the information is known as a segment. When that segment reaches the Internet layer, a header with IP addresses is added to the segment. It is called a packet now. A header with MAC addresses is added when the packet reaches the network access layer. It is known as a frame now. A corresponding Protocol Data Unit (PDU) is likewise created in each layer. In each layer, adding this information is known as Encapsulation. The frame is sent to the network when the encapsulation process is finished.

The frame goes out of the host computer into the network, as explained in the encapsulation process. Then it reaches the host 's destination. In the destination host, the frame is decapsulated up to the application layer in reverse order. Data, TCP / UDP headers, headers with IP addresses and headers with MAC addresses are included in the frame that reaches the network access layer. It is a packet and has data, TCP / UDP headers and IP address headers when it is sent to the network layer. Then the transport layer is reached by the packet. It is now segmented and contains the TCP / UDP header and data. The segment finally reaches the layer of the application. In the application layer, the data sent from the source computer can be seen by the host. This cycle is referred to as Decapsulation.

Following figure shows the encapsulation and de-encapsulation in OSI model. Sending computer Receiving computer Encapsulation & De-encapsulation in OSI Model Application Application Presentation Presentation Data Session Session Encapsulation Transport Segment header Data Transport Deencapsulation Network Packet header Segment header Data Network Data Link > Frame header Packet header Segment header Data Frame trailer Data Link Physical 010110011010101010110001 Bits Physical

Following figure shows the encapsulation and de-encapsulation in TCP/IP model. Sending computer Encapsulation & De-encapsulation in TCP/IP Model Receiving computer Application Data Application Encapsulation Transport Segment header Data Transport De-encapsulation Network Packet header Segment header Data Network Data Link Frame header Packet header Segment header Data Frame trailer Data Link Physical 010110011010101010110001Bits Physical