Is SIFS, DIFS, AIFS or nothing used before beacon frame is sent?
I got the frame duration in us by adding 192 us preample and data_bits/1Mbps together but I don't know if I should add DIFS SIFS etc to get total airtime that is required to send one beacon.
I am not totally clear about what you are trying to ask.There are so many parameters regarding airtime for any 802.11 frame.
You mentioned 192us that [Long preamble] is time taken to send PHY layer header of 802.11 Beacon frame.
And Beacon frame is kept in Highest Priority queue in AP and uses the same CSMA/CA method to gain access to wireless medium.I am not sure about which IFS time AP uses before sending beacon.I will get back to you on this.
Update:
--Please ignore my comment.I was not allowed to edit :P --
After discussing with Experienced WLAN Developer,this is the best information i got.
There is no special facilities are given for Beacon.This means, to send Beacon AP has to content the Wireless medium as usual.But beacon is given higher priority than other TX packets ,present in Queue.Thats why you will notice there will be delay [ from actual TBTT ] in Beacon Timestamps [As medium was busy so AP could not able to send Beacon in TBTT]. But STAs will be synchronized according to TimeStamp in Beacon.
Before transmitting a Beacon APs shall use DIFS.
Beacons follow the same channel access procedure that data frames (See IEEE 802.11-2016, Section 11.1.3.2), this is, the AP shall perform a backoff and wait for the channel to be idle during DIFS before sending the Beacon.
Beacons shall be scheduled at the nominal beacon interval, that's why they shall be prioritised (as mentioned by #Bamdeb), but again, they must follow the regular IEEE 802.11 channel access procedure and wait for DIFS.
Related
Let's say I have a car with different sensors: several cameras, LIDAR and so on, the data from this sensors are going to be send to some host over 5G network (omnetpp + inet + simu5g). For video it is like 5000 packets 1400 bytes each, for lidar 7500 packets 1240 bytes and so on. Each flow is encoded in UDP packets.
So in omnetpp module in handleMessage method I have two sentTo calls, each is scheduled "as soon as possible", i.e., with no delay - that corresponds to the idea of multiple parallel streaming. How does omnetpp handle situations, when it needs to send two different packets at the same time from the same module to the same module (some client, which receives sensor data streams)? Does it create some inner buffer on the sender or receiver side, therefore allowing really only one packet sending per handleMessage call or is it wrong? I want to optimize data transmission and play with packet sizes and maybe with sending intervals, so I want to know, how omnetpp handles multiple streaming at the same time, because if it actually buffers, maybe than it makes sense to form a single package from multiple streams, each such package will consist of a certain amount of data from each stream.
There is some confusion here that needs to be clarified first:
OMNeT++ is a discrete event simulator framework. An OMNeT++ model contains modules that communicate with each other, using OMNeT++ API calls like sendTo() and handleMessage(). Any call of the sendTo() method just queues the provided message into the future event queue (an internal, time ordered queue). So if you send more than one packet in a single handleMessage() method, they will be queued in that order. The packets will be delivered one by one to the requested destination modules when the requested simulation time is reached. So you can send as many packets as you wish and those packets will be delivered one by one to the destination's handleMessage() method. But beware! Even if the different packets will be delivered one by one sequentially in the program's logic, they can still be delivered simultaneously considering the simulation time. There are two time concepts here: real-time that describes the execution order of the code and simulation-time which describes the time passes from the point of the simulated system. That's why, while OMNeT++ is a single threaded application that runs each events sequentially it still can simulate infinite number of parallel running systems.
BUT:
You are not modeling directly with OMNeT++ modules, but rather using INET Framework which is a model directly created to simulate internet protocols and networks. INET's core entity is a node which is something that has network interface(s) (and queues belonging to them). Transmission between nodes are properly modeled and only a single packet can travel on an ethernet line at a time. Other packets must queue in the network interface queue and wait for an opportunity to be delivered from there.
This is actually the core of the problem for Time Sensitive Networks: given a lot of pre-defined data streams in a network, how the various packets interfere and affect each other and how they change the delay and jitter statistics of various streams at the destination, Plus, how you can configure the source and network gate scheduling to achieve some desired upper bounds on those statistics.
The INET master branch (to be released as INET 4.4) contains a lot TSN code, so I highly recommend to try to use it if you want to model in vehicle networks.
If you are not interested in the in-vehicle communication, bit rather want to stream some data over 5G, then TSN is not your interest, but you should NOT start to multiplex/demultiplex data streams at application level. The communication layers below your UDP application will fragment/defragment and queue the packets exactly how it is done in the real world. You will not gain anything by doing mux/demux at application layer.
In Ethernet networks, the MAC layer is the first layer to detect the destination address of the received message.
my questions: is that means that the transceiver shall take a copy of each message on the bus and forward it to the MAC layer who will decide to accept that message or discard it? If so, this means that the MAC layer must have a very large buffers to save all that intended and non intended message. am I correct ?
The MAC layer does not typically have much buffering. It may not even be able to store a full packet. Packets instead stream through the MAC.
Packets enter and exit the MAC one flit at a time. It may take hundreds of cycles for a full packet to pass into a MAC depending on the size of the packet and the width of the interface. For example, a MAC with an 8-byte interface (8-byte flit size) will take 1000 cycles to receive an 8kB packet.
The MAC may only have 800 bytes of buffering. In that case, the packet will start coming out the other end after 100 cycles when only 10% of the packet has entered. In fact, many MACs have a latency well below 100 cycles.
Packets which are rejected on the basis of destination address stream in one side but nothing comes out the other side. The frames are simply forgotten/dropped as they arrive.
I am using MAX144 ADC and in the Datasheet there is no information given about the control register to read the ADC values. I am using STM32L452RE micro-controller and using SPI to get data from ADC. Datasheet of the ADC is:
https://datasheets.maximintegrated.com/en/ds/MAX144-MAX145.pdf
anyone who encountered the same problem please guide.
my idea is to create a buffer of 2 bytes for SPI RX and store values in it. but i don't know what control register address should be assigned to it.
The conversion data is not stored internally in a register set. When you pull CS low the state of SCLK will determine wither it holds the conversion product(after a high to low transition to start it) or start streaming it on the falling edge of the second clock pulse.
This is all noted on page 9 of the data sheet. Pages 10 & 11 detail how to interface them to standard SPI.
CANopen is point to point communication while EtherCAT is bus based. Point to point means there will be node address. But this is redundant in EtherCAT. So I was wondering how this node address bytes are handled in the CANopen over EtherCAT. I tried searching for information but couldn't find anything specific on this.
Also, I assume both cyclic and acyclic data of the CANopen device is sent only cyclically over the EtherCAT because it is Master triggered cyclic transmission protocol. This basically means I cannot send asynchronous, event-triggered information at the trigger of the event, on EtherCAT (which is counter-intuitive for CAN's priority based because all of them get the same priority). Please correct me if I am wrong about this. Also please tell me how can I make a higher priority byte reach quicker than the lower priority one (assuming both occurred at the same time and assume there is bandwidth to send both at the start of new frame).
CANopen provides Process Data Object (PDO) and Service Data Object (SDO). PDO is sent cyclically over the EtherCAT and SDO is sent acyclically. Therefore, if you use the SDO, you send asynchronous, event-triggered information at the trigger of the event.
Additionally, CANopen is usually used in the servo control and most of servo controller support the PDO and the SDO.
I am using latest version of veins. I have been playing it with for a while and understand the basics now. I followed tictoc tutorial for omentpp, but I still couldn't figure out how to solve the following probelm:
I want Vehicles and RSU to send messages to each other. I want these messages to be sent in all the four catagories. When a message is received I want to measure the time it took to travel from source to destination.
By default, veins, can send data, and based on this post, I know that I have to change someparts in TraCIDemo11p, but I couldn't figure out what. It would be great if someone could provide an answer.
To answer my own question. I modified BaseWaveAppLayer.cc to accomplish my goal(though it is not right way to do it. The right way would be to extend this class and make your changes in that class. But since I just wanted to make changes quickly I chose this quicker way). I modified the method for sending beacons. Since beacons will be scheduled to be sent based on the time that the user can specify in .ini file. Now every time a beacon is scheduled to be sent, I randomly generate a priority from the range [0-4) and assign it to the packet. In this way I get to send beacons with different priorities over the network.
Also as I had a requirement of sending each packet in a different rate. To achieve this I implemented the random generation function in such a way that certain numbers of the range gets generated more than others. It's sorta biased. So as an example, in .ini file I would specify that priorities 0-2 should be sent at rate of 0.2 while priority 4 should be sent at rate of 0.4(it can interpreted as the sending rate for each priority). The random generation function would then generate 4 twice more than any other number, while numbers 0,1,2 would get generated the same number of times.