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 decapsulation.

Answer 1

a)

logical link control (LLC)

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.

The Open System Interconnections (OSI) model is a 7 – 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

Functions of LLC Sublayer

• 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

As for LLC, this sublayer acts as a link between the upper layer (Network Layer) and the bottom layer (Physical Layer). It is responsible for identifying Network layer protocols. Put another way, when a frame coming from a remote host (source) reaches a computer in a local network (destination), as you know, the frame is first generated at the source machine starting from the Application Layer down to the Physical Layer, it is slipped off of the header and trailer of the Data-link Layer ( which you will see), then it is passed on to the Network layer. It is the one that sends the frame to its correct protocol at the Network Layer - for example, IP protocol.  

MAC (Media Access Control) sublayer

The MAC Layer is one of the two sublayers into which the data-link layer (layer 2) of the Open Systems Interconnection (OSI) reference model is subdivided based on the specifications of IEEE Project 802. The other sublayer is the logical link control (LLC) layer.

In addition to LLC (Logical Link Control), MAC sublayer has, equally, significant functions. It provides Data-link Layer addressing and puts Frames on the media to takes their journey across the network towards the destination.

How it works

Essentially, the MAC layer determines which computer on the network is allowed to use the media at any given moment. The MAC layer is thus responsible for implementing the media access control method for the particular network architecture, such as Ethernet or Token Ring. The MAC layer is also responsible for making sure that data is delivered without errors.

The MAC layer receives framed data from the LLC layer immediately above it, which is media independent and reframes the data, adding a source and destination physical address or MAC address to the frame for transmission on the medium.

Addressing in the Data-link Layer is providing the frame, generated at the sending machine, with a source address and destination address called MAC address so that the frame is destined to its correct receiving machine. More precisely, as you saw in the previous lesson, MAC address or hardware address is a unique identifier which consists of 48 bits in Hexadecimal. It looks like this 00:0a:9f:9d:6a:15. This address is burned on any network device, which means that it is unchangeable. NIC (Network Interface Card) card or Wireless card, for example, has a unique MAC address which identifies it. Routers, switches, hubs all have MAC addresses.

The primary function of this address is to enable devices to be located inside a local network (LAN). For example, when a frame coming from a remote network gets at the router of the receiving host, it needs to continue its path along the concerned host. To do that, the routers, in this case, depends on the MAC address to reach the concerned host (in this case the correct term is node rather than host).

b)

CSMA/CD stands for Carrier Sense Multiple Access/Collision Detection, with collision detection being an extension of the CSMA protocol. This creates a procedure that regulates how communication must take place in a network with a shared transmission medium. The extension also regulates how to proceed if collisions occur i.e. when two or more nodes try to send data packets via the transmission medium (bus) simultaneously and they interfere with one other.

To understand how CSMA/CD works, it makes sense to break down the individual components of the term:

• Carrier sense (CS): The carrier state detection makes sure that all network participants check whether the medium is currently free – only then does the protocol initiate data transmission
• Multiple access (MA): Several participants (computers connected to the network) share a transmission medium
• Collision detection (CD): The collision detection is an extension of the original protocol and regulates how to proceed in case data packets happen to collide

• How the CSMA/CD process works

  CSMA/CD is divided into several steps. The procedure is based on a normal group conversation: for good communication, it is important that the participants don’t all speak at once, which can be confusing. Instead, they should speak one after the other, so that each participant can fully understand what the others are contributing to the discussion. Without realizing, we actually behave like this ourselves in conversations: When someone else is talking, we stand back and listen.

  After the other participant has finished their contribution for the time being, we wait a short time and only start talking when the same participant or another participant in the conversation doesn’t start to say anything else. If we happen to start talking to someone else at the same time, we stop our attempt, wait a bit, and then try again.

  The CSMA/CD process is very similar. First, the station monitors the transmission medium. As long as this is occupied, the monitoring will continue. Only when the medium is free and for a certain time (in interframe spacing), will the station send a data packet. Meanwhile, the transmitter continues to monitor the transmission medium to see if it detects any data collisions. If no other participant tries to send its data via the medium by the end of transmission, and no collision occurs, the transmission has been a success.

  If, however, a collision is detected, the participant immediately interrupts the transmission and instead sends an interference signal (JAM signal) so that all other stations can also detect the collision. Now the participant waits for a random amount of time (backoff) before trying the transmission again. The backoff must be random so that the next collision doesn’t occur straight away. Since both stations select a random value, the probability that both of them will start a transport attempt at the same time is quite low.

  The transmission attempts are counted. If the following attempts also fail and the maximum number of attempts (16) is reached, the station reports the error to the next higher network layer and then terminates the transmission permanently. Since it is highly unlikely that a network participant will reach the maximum number of attempts during a normal process, it can be assumed that a system error has occurred.

c)

ENCAPSULATION:

The Application layer is where the user interface exists, here the user interacts with the application he or she is using, then this data is passed to the Presentation layer and then to the Session layer. These three layer add some extra information to the original data that came from the user and then passes it to the Transport layer. Here the data is broken into smaller pieces (one piece at a time transmitted) and the TCP header is a added. At this point, the data at the Transport layer is called a segment.

Each segment is sequenced so the data stream can be put back together on the receiving side exactly as transmitted. Each segment is then handed to the Network layer for network addressing (logical addressing) and routing through the internet network. At the Network layer, we call the data (which includes at this point the transport header and the upper layer information) a packet.

The Network layer add its IP header and then sends it off to the Datalink layer. Here we call the data (which includes the Network layer header, Transport layer header and upper layer information) a frame. The Datalink layer is responsible for taking packets from the Network layer and placing them on the network medium (cable). The Datalink layer encapsulates each packet in a frame which contains the hardware address (MAC) of the source and destination computer (host) and the LLC information which identifies to which protocol in the prevoius layer (Network layer) the packet should be passed when it arrives to its destination. Also, at the end, you will notice the FCS field which is the Frame Check Sequence. This is used for error checking and is also added at the end by the Datalink layer.

If the destination computer is on a remote network, then the frame is sent to the router or gateway to be routed to the desination. To put this frame on the network, it must be put into a digital signal. Since a frame is really a logical group of 1's and 0's, the Physical layer is responsible for encapsulating these digits into a digital signal which is read by devices on the same local network.

There are also a few 1's and 0's put at the begining of the frame, only so the receiving end can synchronize with the digital signal it will be receiving.

DECAPSULATION:

The receiving computer will firstly synchronize with the digital signal by reading the few extra 1's and 0's as mentioned above. Once the synchonization is complete and it receives the whole frame and passes it to the layer above it which is the Datalink layer.

The Datalink layer will do a Cyclic Redundancy Check (CRC) on the frame. This is a computation which the comupter does and if the result it gets matches the value in the FCS field, then it assumes that the frame has been received without any errors. Once that's out of the way, the Datalink layer will strip off any information or header which was put on by the remote system's Datalink layer and pass the rest (now we are moving from the Datalink layer to the Network layer, so we call the data a packet) to the above layer which is the Network layer.

At the Network layer the IP address is checked and if it matches (with the machine's own IP address) then the Network layer header, or IP header if you like, is stripped off from the packet and the rest is passed to the above layer which is the Transport layer. Here the rest of the data is now called a segment.

The segment is processed at the Transport layer, which rebuilds the data stream (at this level on the sender's computer it was actually split into pieces so they can be transferred) and acknowledges to the transmitting computer that it received each piece. It is obvious that since we are sending an ACK back to the sender from this layer that we are using TCP and not UDP. Please refer to the Protocols section for more clarification. After all that, it then happily hands the data stream to the upper-layer application.

You will find that when analysing the way data travels from one computer to another most people never analyse in detail any layers above the Transport layer. This is because the whole process of getting data from one computer to another involves usually layers 1 to 4 (Physical to Transport) or layer 5 (Session) at the most, depending on the type of data