QualNet Exercises: Chapter 5. Link Layer and Local Area Networks*

Objective

Overview

Procedure

1. Medium Access Control (MAC)
   Under the IEEE 802 series of standards, the data link layer of the OSI Reference Model is subdivided into two sublayers: Logical Link Control (LLC) and the Medium Access Control (MAC). The MAC sublayer is responsible for checking the channel and transmitting data if the channel is idle, checking for occurrence of a collision, and taking a series of predefined steps if a collision is detected. The MAC layer is an interface between user data and the physical placement and retrieval of data on the network.
   
   This exercise focuses on one major function performed by the MAC sublayer -- transmission medium access management. The activities associated with this functionality are:
   • Defer transmission if the medium is busy.
   • Delay transmission for a specified interframe gap period.
   • Present a serial bit stream to the physical layer for transmission.
   • Halt transmission when a collision is detected.
   • Transmit a jam signal to ensure that the news of a collision propagates throughout the network.
   • Reschedule retransmission after a collision until successful, or until a specified retry limit is reached.
   
   
2. Carrier sense
   CSMA/CD can be described as a listen-before-speaking multiple access method. Thus, the first function associated with transmission media access management is to find out whether any data is already being transmitted on the network and, if so, to defer transmission. During the listening process, each station attempts to sense the carrier signal on the bus, hence the name carrier sense (CS) is used for this access method.
   
   Under Manchester encoding, a transition occurs in the middle of each bit period. The binary bit 1 is represented by a high-to-low transition, while the binary bit 0 is represented by a low-to-high voltage transition. Thus, an examination of the voltage on the medium of a base-band network enables a station to determine whether a carrier is present. If a carrier signal is found, the station with data to transmit will continue to monitor the channel.
   
   
3. Waiting time
   If the medium is busy, the station waits until it goes idle; otherwise it transmits immediately. If two or more stations simultaneously begin transmitting on an idle cable, they will collide. All colliding stations then terminate their transmission, wait for a random time, and repeat the whole process all over again.
   
   
4. Collision detection
   A transmitting station keeps listening to the channel to check for collisions while it is sending. Since IEEE 802.3 Manchester encoded signals have a 1 volt average DC voltage level, a collision results at an average DC level of 2 volts. Thus, a tranceiver or network interface card can detect collision by monitoring the voltage level of the Manchester line signal.
   
   
5. Jam sequence
   If a collision is detected during transmission, the transmitting station will cease transmitting of data and initiate transmitting of a jam pattern. The jam pattern consists of 32 ~ 48 bits. These bits can have any value other than the CRC value that corresponds to the partial frame transmitted before the jam. The transmission of the jam pattern ensures that the collision lasts long enough to be detected by all stations on the network.
   
   
6. Collision backoff and retransmission
   Once a collision is detected, the transmitting station waits for a random number of time slots before attempting to retransmit. The term "time slot" represents the time required to transmit 512 bits on the network. The actual number of time slots the station waits is selected by a randomization process, formaly known as "truncated binary exponential backoff". Under this randomization process, a randomly selected integer r defines the number of time slots the station must wait before listening again to determine whether the channel is clear. The number of time slots to delay before the n-th retransmission attempt is chosen as a uniformly distributed random integer r in the range between 0 and (2^k)-1 , where k=min(n,10). After the backoff, the station begins to retransmit the frame when it senses the channel to be free, while listening for another collision. After a user-defined maximum number of attempts, the MAC entity assumes that some problem exists, gives up sending, and reports failure to LLC.

1. Mac802_3Layer() : This function handles various messages received such as MSG_MAC_StartTransmission (which indicates that some other node has started to send a frame) and MSG_MAC_TransmissionFinished (which indicates that a frame has come up to this node), etc.
   
   
2. Mac802_3SendJamSequence(): This function creats and broadcasts a jam sequence to inform all nodes of the collision.
   
   
3. Mac802_3HandleBackOffSituation(): This function checks the collision counter. If excessive number of collisions occured then it rejects the frame. Otherwise, it schedules a message for future retransmission.

1. Download the compressed tar file here, including the skeleton file (mac_802_3.cpp) to complete and configuration files (mac_exercise.config and mac_exercise.app) to run.
   
   
2. Backup the original source file mac_802_3.cpp in QualNet directory $QUALNET_HOME/libraries/developer/src by renaming it to mac_802_3.original.cpp.
   
   
3. Copy the downloaded file mac_802_3.cpp into the directory $QUALNET_HOME/libraries/developer/src and all other files into the directory $QUALNET_HOME/bin.
   
   
4. Study the files mac_802_3.original.cpp, mac_802_3.h (in the same directory), and mac.cpp (in $QUALNET_HOME/main), if needed.
   
   
5. Fill in the missing codes for above three functions to complete the skeleton file mac_802_3.cpp, basesd on the provided inline pseudo codes and comments.
   
   
6. Re-compile QualNet and run simulation using the configuration file mac_exercise.config.

Questions

1. What happens after a sending station detects a collision ?
   
   
2. Why does a sending station continue to transmit for some time even after it detects a collision?
   
   
3. What is the purpose of the minimum frame size? How is it computed?
   
   
4. What is the length of jam sequence in mac_802_3.cpp?
   
   
5. Submit the generated statistics file (mac_exercise.stat) and do the following:
   • Specify the name(s) of MAC layer statistics related to collision;
   • Point out the instance(s) where frame loss happens due to collisions, if any;
   • Point out the instance(s) where all frames are transmitted successfully in their first tries, if any.

References and Further Readings

1. Sections 5.3.2 and 5.5.2 of the textbook.
2. Section 4.5.5 of the QualNet 5.0 Programmer's Guide.

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