Question

QUESTION : LAN Technology and Protocols

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

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

c) Discuss with the appropriate figure/s the concept of packet encapsulation and de-
capsulation.

Answer 1

1. LOGIC LINK CONTROL (LLC) LAYER:

The Open System Interconnections (OSI) model is a seven layered networking framework that conceptualizes how communications should be done between heterogeneous systems. The data link layer is the second lowest layer. It is divided into two sublayers

>>The logical link control (LLC) sublayer

>>The medium access control (MAC) sublayer

The following diagram depicts the position of the LLC and MAC sublayers:

The logical link control (LLC) is the upper sublayer of the data link layer of the Open System Interconnections (OSI) reference model for data transmission. It acts act an interface between the network layer and the medium access control (MAC) sublayer of the data link layer. The LLC sublayer is mainly used for its multiplexing property. It allows several network protocols to operate simultaneously within a multipoint network over the same network medium.

Basic funtions of the LLC Layer:

>>The primary function of LLC is to multiplex protocols over the MAC layer while transmitting and likewise to de-multiplex the protocols while receiving.

>>LLC provides hop-to-hop flow and error control.

>>It allows multipoint communication over computer network.

>>Frame Sequence Numbers are assigned by LLC.

>>In case of acknowledged services, it tracks acknowledgements

MEDIUM ACCESS CONTROL (MAC) LAYER:

The medium access control (MAC) is a sublayer of the data link layer of the open system interconnections (OSI) reference model for data transmission. It is responsible for flow control and multiplexing for transmission medium. It controls the transmission of data packets via remotely shared channels. It sends data over the network interface card.

Basic funtions of the LLC Layer:

>>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, or groups of destination stations.

>>It performs multiple access resolutions when more than one data frame is to be transmitted. It determines the channel access methods for transmission.

>>It also performs collision resolution and initiating retransmission in case of collisions.

>>It generates the frame check sequences and thus contributes to protection against transmission errors.

2.  CARRIER SENSE MULTIPLE ACCESS (CSMA):

This method was developed to decrease the chances of collisions when two or more stations start sending their signals over the datalink layer. Carrier Sense multiple access requires that each station first check the state of the medium before sending.

Vulnerable time = Propagation time (T_p)

The persistence methods can be applied to help the station take action when the channel is busy/idle.

CARRIER SENSE MULTIPLE ACCESS COLLISION DETECTION (CSMA/CD):

CSMA/CD (Carrier Sense Multiple Access/ Collision Detection) is a media-access control method that was widely used in Early Ethernet technology/LANs, When there used to be shared Bus Topology and each nodes (computers) were connected by coaxial cables. Now a days ethernet is full duplex and CSMA/CD is not used as topology is either star (connected via switch or router) or point to point ( direct connection) but they are still supported though.

Let us consider a scenario where there are ‘n’ stations on a link and all are waiting to transfer data through that channel. In this case all ‘n’ stations would want to access the link/channel to transfer their own data. Problem arises when more than one station transmits the data at the moment. In this case, there will be collisions in the data from different stations.

CSMA/CD is one such technique where different stations that follow this protocol agree on some terms and collision detection measures for effective transmission. This protocol decides which station will transmit when so that data reaches the destination without corruption.

In this method, a station monitors the medium after it sends a frame to see if the transmission was successful. If successful, the station is finished, if not, the frame is sent again.

In the diagram, A starts send the first bit of its frame at t1 and since C sees the channel idle at t2, starts sending its frame at t2. C detects A’s frame at t3 and aborts transmission. A detects C’s frame at t4 and aborts its transmission. Transmission time for C’s frame is therefore t3-t2 and for A’s frame is t4-t1.

So, the frame transmission time (Tfr) should be at least twice the maximum propagation time (Tp). This can be deduced when the two stations involved in collision are maximum distance apart.

Mechanism:

The entire process of CSMA/CD (Carrier Sense Multiple Access/ Collision Detection) can be explained as follows:

• Step 1: To check if the sender is ready for transmitting data packets.
• Step 2: To check if the transmission link is idle?

Sender has to keep on checking if the transmission link/medium is idle. For this it continously senses transmissions from other nodes. Sender sends dummy data on the link.If it does not receive any collision signal, this means the link is idle at the moment.If it senses that the carrier is free and there are no collisions, it sends the data. Otherwise it refrains from sending data.

• Step 3: To transmit the data & check for collisions.

Sender transmits its data on the link. CSMA/CD does not use ‘acknowledgement’ system. It checks for the successful and unsuccessful transmissions through collision signals. During transmission, if collision signal is received by the node, transmission is stopped. The station then transmits a jam signal onto the link and waits for random time interval before it resends the frame. After some random time, it again attempts to transfer the data and repeats above process.

• Step 4: If no collision was detected in propagation, the sender completes its frame transmission and resets the counters.

The entire process of collision detection can be explained in the form of a flowchart as follows:

3. PACKET ENCAPSULATION AND DECAPSULATION:

Packet encapsulation:

When data moves from upper layer to lower layer of TCP/IP protocol stack, during an outgoing transmission, each layer includes a bundle of relevant information called "header" along with the actual data. The data package containing the header and the data from the upper layer then becomes the data that is repackaged at the next lower level with lower layer's header. Header is the supplemental data placed at the beginning of a block of data when it is transmitted. This supplemental data is used at the receiving side to extract the data from the encapsulated data packet. This packing of data at each layer is known as data encapsulation.

Packet decapsulation:

The reverse process of encapsulation, that is, decapsulation occurs when data is received on the destination computer during an incoming transmission. As the data moves up from the lower layer to the upper layer of TCP/IP protocol stack (incoming transmission), each layer unpacks the corresponding header and uses the information contained in the header to deliver the packet to the exact network application waiting for the data.