Wireless Access. Practice 15 November 14, 2018

Transcription

1 Practice 15 November 14, 2018 Wireless Access Graduate course in Telecommunications School of Engineering University of Rome La Sapienza Rome, Italy

2 Token ring queueing time analysis with OMNeT++

3 Goal Verify the following relation for the time in queue in a token ring network: W = R + Y 1 ρ = λe X ρ = λe X ρ ( )v ( )v ( ) + 1 ρ 2( 1 ρ) + m 1 2( 1 ρ) = ( )v ( ) + m ρ 2( 1 ρ) by using token ring for OMNeT++, available at: _files.zip and considering the traffic scenarios and settings presented in the next slide 3

4 Traffic scenarios and settings Traffic scenarios: Low traffic: Packet interarrival time at each node set to 100 times the average packet duration High traffic: Packet interarrival time at each node set to 20 times the average packet duration Settings: Parameter Packet size in bytes: Value Transmission bit rate 4 Mb/s Randomly distributed according to a uniform distribution in the interval [1000, 4000]; Number of stations {5,10,15,20,50} Propagation delay 10-6 s 4

5 A network of m nodes Token ring in OMNeT++ Each node is a compound module organized as follows: Generates packets Generates MAC frames Generates and handles token Keeps packets in queue Receives packets and evaluates statistics 5

6 Token ring in OMNeT++ The MAC module keeps track of the following variables (in.vec files): Queueing time (s): the time spent by each packet in queue, measured in seconds Queue length (packets): the length of the queue as a function of time, measured in packets Queue length (bytes): the length of the queue as a function of time, measured in bytes Note that the.vec file for a given run will have a value of the above variables for each node (so, 5 values for a network of 5 nodes, and so on) We will be interested in the average of the mean value of one of the above variables (guess which one ) 6

7 MAC Frame generation: Token ring in OMNeT++ PL Generator PL=8*uniform(1000, 4000) bits HL PL MAC HL=8*21=168 bits Token generation: TL MAC TL=8*3=24 bits The token is generated by node 0 7

8 Token ring in OMNeT++ Token handling: When a node receives the token it keeps it for a maximum time of THT seconds The node (at MAC layer) keeps on transmitting packets in the transmit queue until either of the following two conditions applies: a) The packet queue is empty b) The transmission time of the next packet in queue would not fit within the time THT When either a) or b) is verified the token is released Note that a node that has no packets to send forwards the token immediately to the next node in the ring Queue handling: when the transmit queue is full packets coming from the Generator are dropped 8

9 Evaluation of time in queue based on theory We will proceed by following three steps: 1. Determine the expression of the parameters determining W 2. Determine the value of the parameters 3. Evaluate W by substituting in its formula the values of the parameters 9

10 Evaluation of time in queue in OMNeT++ We will proceed by following three steps: 1. Adapt the OMNeT++ model in order to meet the assumptions made in deriving the theoretical formula for W 2. Carry out simulations for several values of m (at least in one traffic scenario) 3. Compare the values of the average time in queue recorded by the simulator with the theoretical value of W 10