RTS/CTS -unicast or broadcast + Random backoff time

Hi everybody,
Iam reading through the frame handshake in WiFi. Also went through some of the good articles in this forum.
I understand that to avoid collision, RST/CTS handshake is used in the wireless Air medium.
I have attached the RTS-CTS handshake and I would like to know if these frames are unicast or broadcast.
Since they are sent from a specific source say Wifi Client to a specifc destination say Wifi AP, i assume the RTS and CTS signals to be unicast but i dont understand then how the other wifi stations/clients will listen/get the duration (NAV)
Also regarding the backoff time interval, I would like to know how this time is decided by the waiting clients? Say client A and Client B are waiting for the medium to be free. Both get the NAV duration as client C is using the medium. Now after the NAV time is complete, the clients A and B will wait for a backoff time.
>how much is this random time?
>Can both set the same backoff time say 1? >Will the clients with min backoff time will get to check the medium first.
>What is the max time each client will get to use the medium? What if Cient C using the medium has lots of data to send. Will clients A and B have to wait till Client C finishes? OR is there a max time duration.
Pl advise.

Whenever a wireless client wants to send a data packet to the access point, it actually transmits a four-packet sequence called the RTS-CTS-DATA-ACK packet sequence. Each of the four 802.11 frames carries a NAV field that indicates the number of microseconds that the channel is reserved for by a wireless client. During the RTS/CTS handshake between the wireless client and access point, the wireless client sends a small RTS frame that includes a NAV interval large enough to complete the entire sequence. This includes the CTS frame, the data frame, and the subsequent acknowledgment frame from the access point.
When the wireless client transmits its RTS packet with the NAV set, the transmitted value is used to set the NAV timers on all other wireless clients associated to the access point. The access point replies to the RTS packet from the client with a CTS packet that contains a new NAV value updated to account for the time already elapsed during the packet sequence. After the CTS packet is sent, every wireless client that can receive from the access point has updated their NAV timer and defers all transmissions until their NAV timer reaches 0. This keeps the channel free for the wireless client to complete the process of transmitting a packet to the access point.

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CSMA/CA NAV/Duration field and random backoff timer - Process

Hi guys,
Gotta a quick one for you.
Pls see text below
As a condition to accessing the medium, the MAC Layer checks the value of its network allocation vector (NAV), which is a counter resident at each station that represents the amount of time that the previous frame needs to send its frame. The NAV must be zero before a station can attempt to send a frame. Prior to transmitting a frame, a station calculates the amount of time necessary to send the frame based on the frame's length and data rate. The station places a value representing this time in the duration field in the header of the frame. When stations receive the frame, they examine this duration field value and use it as the basis for setting their corresponding NAVs. This process reserves the medium for the sending station.
An important aspect of the DCF is a random back off timer that a station uses if it detects a busy medium. If the channel is in use, the station must wait a random period of time before attempting to access the medium again. This ensures that multiple stations wanting to send data don't transmit at the same time. The random delay causes stations to wait different periods of time and avoids all of them sensing the medium at exactly the same time, finding the channel idle, transmitting, and colliding with each other. The back off timer significantly reduces the number of collisions and corresponding retransmissions, especially when the number of active users increases.
So, if all clients in a WLAN see a frame come from station A, and station A has set the duration field, all other stations set their NAV and wait until it expires.
THEN, i would expect all stations to start to contend for the medium at exactly the same time.
BUT you have the random backoff timer.
The question is does the NIC add a random number to the NAV field, ie, once it sees station As duration field value (lets say 300us) it adds 47us to this and the NAV is 347us? -- or do all stations decrement the NAV field to zero and then start a random backoff timer?
Many thx for the help,
Kind regards,
Ken

Sorry chaps, but I think you've got this wrong...
The whole point of a NAV being transmitted by the TX & RX ends is to stop hidden node issues - with both ends transmitting the NAV, you are guaranteeing that every 802.11 device within range of either end of the transaction is quiet for the duration of the NAV.
Lets say we have a TX (A) and an RX (B), and another device (C). Lets say that (A) can hear (B), and that (B) can hear (A) and (C). (A) and (C) cannot hear each other.
(A) <<>> (B) <<>> (C)
Applying that to your scenario...
<>
it now counts down to 310, it checks the air, and sees the same packet still being transmitted
309, 308, 307 .............
gets down to 12us, checks the ait, NOW DOES NOT see a packet being transmitted.
you count down to 11, it check the air, you donot see a packet being transmitted.
<>
The part where it jumps down from 312 to 11 is wrong. Just because (C) can't hear anything, doesn't mean that (B) isn't receiving something from (A).
What actually happens with NAV's is;
[Lets say (A) is transmitting to (B)]
1. (A) transmits NAV, value = "NAV-A"
2. (B) receives packet, along with everything else in range of (A), on the same channel as (A)
3. (B) responds to (A) with "NAV-B". "NAV B" = "NAV-A" [minus] "Elapsed Time"
4. (A) receives packet, along with everything in range of (B), on the same channel as (B)
At this point, every device on that channel will be silent for the duration of the NAV, regardless of whather or not they hear anything during the NAV countdown duration.
When the NAV expires, the Random Backoff Timer begins, and the lowest/ quickest client to get to zero then gets to transmit, and the whole process starts again.
Let me know what you think, but I'm pretty sure that's how it actually works.
Rgds,
Richard

Random backoff timing.

Hi All,
Was going thro this post
http://forums.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Wireless%20-%20Mobility&topic=WLAN%20Radio%20Standards&topicID=.ee6e8c2&fromOutline=&CommCmd=MB%3Fcmd%3Ddisplay_location%26location%3D.2cc12631
And iam confused. Have some queries.
1. When does the random backoff timer start?
a) whenever the station wants to transmit after the completion of its NAV
b) Starts when Station wants to transmit irrespective of the NAV, later NAV gets added to backoff timer some other station is transmitting before completion of its backoff timer to 0
2. Who sets this Random number for the backoff time?
3. Can two devices on the same WLAN set the same random backoff time?
4. In the scenario that two stations finish their NAVs and start their ramdom backoff time. Is it possible that they both set the same time? if nope... how is this acheived? if yes then how r colisions avoided in such a scenario?
5. the above post spoke of intervals like 20ns etc
Can someone tel me the intervals for 802.11 a, b , b and n explicitely?
6. When calculating NAV, does it consider a whole frame or if there is frangments to that frame. does it consider only 1 fragment.
meaning... will the other stations have to wait for the whole frame to be transmitted or if there is fragmentation of that frame/data, after the 1st fragment, he other clients will get a chance to transmit data?
Pl help. any inputs r welcome.

When Both stations should have the same NAV values, and both will physically sense when the medium is idle. There is a high probability that both stations will attempt to transmit when the medium becomes idle, causing a collision. To avoid this situation, DCF uses a random backoff timer.
The random backoff algorithm randomly selects a value from 0 to the contention window (CW) value. The default CW values vary by vendor and are value-stored in the station NIC. The range of values for random backoff start at 0 slot times and increment up to the maximum value, which is a moving ceiling starting at CWmin and stopping at a maximum value known as CWmax. For the sake of this example, assume that the CWmin value begins at 7 and CWmax value is 255

802.11 RTS/CTS and hidden node problem

Guys,
A little confused here.
The hidden node problem is if two nodes within a cell can hear the AP but not each other. OK.
But, when we talk about 802.11b and 802.11g backwards compatibility causing reduced throughtput in terms of bandwidth, it seems that this is always blamed on 802.11g stations having to use RTS/CTS.
But,
Even in an 802.11g only cell, dont stations still have to use RTS/CTS mechanisms for the hidden node problem?
I'm confused.com!
Thx
Ken

When 802.11b clients are associated to an 802.11g access point, the access point will turn on a protection mechanism called Request to Send/Clear to Send (RTS/CTS). Originally a mechanism for addressing the "hidden node problem" , RTS/CTS adds a degree of determinism to the otherwise multiple access network. When RTS/CTS is invoked, clients must first request access to the medium from the access point with an RTS message. Until the access point replies to the client with a CTS message, the client will refrain from accessing the medium and transmitting its data packets. When received by clients other than the one that sent the original RTS, the CTS command is interpreted as a "do not send" command, causing them to refrain from accessing the medium. One can see that this mechanism will preclude 802.11b clients from transmitting simultaneously with an 802.11g client, thereby avoiding collisions that decrease throughput due to retries. One can see that this additional RTS/CTS process adds a significant amount of protocol overhead that also results in a decrease in network throughput.
In addition to RTS/CTS, the 802.11g standard adds one other significant requirement to allow for 802.11b compatibility. In the event that a collision occurs due to simultaneous transmissions (the likelihood of which is greatly reduced due to RTS/CTS), client devices "back off" the network for a random period of time before attempting to access the medium again. The client arrives at this random period of time by selecting from a number of slots, each of which has a fixed duration. For 802.11b, there are 31 slots, each of which are 20 microseconds long. For 802.11a, there are 15 slots, each of which are nine microseconds long. 802.11a generally provides shorter backoff times than does 802.11b, which provides for better performance than 802.11a, particularly as the number of clients in a cell increases. When operating in mixed mode (operating with 802.11b clients associated) the 802.11g network will adopt 802.11b backoff times. When operating without 802.11b clients associated, the 802.11g network will adopt the higher-performance 802.11a backoff times.

Same Channel on Different WLANs and RTS/CTS

Hi
I have 2 questions about physical layer of wireless lan.
1-) Many WLANs(SSID) use same channel for communication, that means they use same frequency, what prevents the collisions between different wlans but same channel?( I am asking about RF signal collision)
2-) Do rts and cts valid for just one wlan that computer which is generated, belongs to or do rts/cts synchronize all different wlans that use same frequency.
Thanks.

1.) Congestion Avoidance. Before the client transmits, it listens to for energy on the frequency it is transmitting on. If it hears energy, it backs off for a time. Then listens again.
http://en.wikipedia.org/wiki/Carrier_sense_multiple_access_with_collision_avoidance take a look at the diagram on the left
2.) RTS/CTS client sends the RTS, then the AP will send the CTS.
http://en.wikipedia.org/wiki/IEEE_802.11_RTS/CTS
HTH,
Steve

On RTS-CTS & DTR-DSR pairs, which and why?

Hello All
Is there somewhere in the Net a good article about when and why to use the flow control RTS-CTS and/or DTR-DSR.
I am dealing with duplex communication between two devices via RS232 and I would like to know when and why to control those lines.
I also found three types of Null-Modem cables and all it is connected with the control of those lines.
Where I can read about it?
Thanks in advance
Pawel

Most RS-232 communication is full duplex, meaning you can transmit and receive at the same time. RS-232 UARTs have separate transmit and receive machines and FIFOs. The processing speed and large available memory of today's electronics (PCs as well as other devices) drastically reduce the need for hardware handshaking. I've been writing RS-232 applications for almost 20 years. I haven't used hardware handshaking for over 15 years, and that application only required it because one piece of equipment was already 30 years old.
Hardware handshaking adds complexity to the software as well as the hardware. If you're only going a short distance, off the shelf cables include all RS-232 lines. But for longer runs, it will be easier and cheaper to run 3 or 4 conductor cabl
e without hardware handshaking.
If you need handshaking, consider software handshaking if your devices support XON/XOFF handshaking.
If you do a Google search for RS-232 handshaking, you'll find plenty of links. Some of these will give you an introduction.
http://www.airborn.com.au/rs232.html
http://www.bb-europe.com/tech_articles/faq_rs232_connections_work.asp
http://www.qkon.com/uploads/27_Practical%20RS232%20Design%20Considerations.pdf

Monitoring DTR RTS CTS

Hello,
I'using Labview 5.1.1
I just want to monitor DTR RTS CTS states of my COM port.
I think it's basic, I know Google is my friend but i've spend too much
time trying tips that do not work.
What is the easiest way to perform the task according to you ?
thank you.
Rudy

Are you using VISA or the older serial functions? With VISA, you can do it with a VISA property node (Modem Line Settings). In your old version of LabVIEW the property node may have been called something (attribute nodes?). The other serial driver has a function called serial line ctrl.vi and it's in vi.lib>platform>_sersup.llb.

RTS/CTS Mechanism

Guys,
Could you please explain briefly about RTS/CTS mechanism.
when an environment use RTS/CTS mechanism?
when CTS to self frames occurs?
How CSMA/CA mechanism involved in RTS /CTS mechanism?
Thanks,
Suresh

What happen when the rts packet destroy or collide with another packet.
When the RTS doesn't reach the radio the client will not get a CTS and will retry the RTS frame again.
What is difference b/w DCF & EDCA.
DCF doesn't support QoS and is considered equal access tocall. QoS is supported in HCF. EDCA work with HCF and QoS. It is the mechanism that supports cW sizes.
What is the time duration for “cts-to-self”.
Depends on really on how long the client needs the medium for. If it send large frames @ 1500 bytes at 1 PHY, it will need more time of course.
What is the max frame size of the rts & cts & cts-to-self.
Not sure, but they are small. These are control frames and have no payload.
In which 802.11 standard does rts and cts are implemented.
RTS/CTS is used when the protection mechanism is used for example b clients on a g/n network. Its also used in the new 11ac as well.
Does 802.11b supports rts and cts. If not then which CSMA/CA is used
I can't recall if b does I want to say yes. CSMA/CA is more about the overall mechanics not RTS/CTS.

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RTS/CTS Problem ?

Hi..
I have to communicate between RS485 (NI card) to RS485 of the application board "with handshaking RTS/CTS"
I use serial VISA of labview and set Flow Control = RTS/CTS. and I monitor the status of RTS and CTS by property node to LED.
I espect that when I run my program first RTS should be green but it always red (no signal when I measure from Oscilloscope).
When I connect with the board, CTS is green and I can receive some wrong data from the board !!
I don't understand firstly, why the status of RTS is always red ? Is this behavior is correct ?
Thanks for any kind help.

If you are using RTS/CTS flow control, you should not be looking at the RTS state. It will always return -1. Depending on how you handle the -1 case, I could believe your LED would be either green or red (I know you said it is red) but it will never change color.
The driver itself will properly set the state of RTS, but it's just not something we allow you to monitor, because you are not directly in control of it. If you measured the RTS signal from the pin with an oscilloscope, you should see it change when you write data with VISA Write.
CTS, on the other hand, should always be returning a valid value. If CTS is set, your device is telling you that you have data to read. And if you get garbage, perhaps your instrument is not properly set up in RT
S/CTS mode?
Dan Mondrik
National Instruments

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Turning off wireless RTS/CTS feature

Is it feasible to turn on/off wireless card RTS/CTS feature for temporary from program code?
Which API to be used ?

nope. not from java. unless you use JNI and write code to interface with the network card software.

Protecting against Virtual Jamming (RTS/CTS) attacks ?

I am new to Wireless, and was wondering how Cisco products guard againts "Virtual Jamming" attacks where a station keeps sending RTS/CTS packets, and causes the NAV of all other stations to be reset.
I believe that the "Rouge AP" detection / prevention mechanism infact uses this very same method to block out rouge access points.
So what prevents a rouge station from doing the same ?
I am also not sure if this problem is eliminated in 802.11n due to its full-duplex like behaviour ??
Thanks

802.11 uses CSMA/CA.
There are 2 ways a radio will sense the medium.
Physical Carrier Sense - is a mech that allows the radio to sense if there is transmissions on the channel
Virtual Carrier Sense - is the use of rts-cts and cts-to-self to reserve the network with NAV timers.
yes, you can do DoS attacks with the correct software to 'jam' the MAC later and not allowing ANY radios to talk at ALL.
Rogue detector with the WLC does not operate in this way. It simply spoofs the rogues access point BSSID and sends deauth frames telling surrounding clients not to attach.

RS232 - Timeout / RTS-CTS

Is it possible to define a Timeout when using the "ComRdByte" Function,
so that CVI will not break down, when no byte was send.
I´ve also problems with the RTS/CTS-Signal. If i use them , it is not
possible to read from the port ! There is nothing happening on these
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You can either set a timeout for read/write operations with the SetComTime function or set up a callback function to be called whenever there is a certain number of bytes at the port with the InstallComCallback function. I am a fan of using the InstallComCallback method and if you want to know exactly how to use it look at the example in the C:\MeasurementStudio\cvi\samples\rs232 directory.
Only use the handshaking modes that are supported by the device you are connecting to. Meaning either use hardware only, software only, hardware and software, or none. All are very common. In your case it doesn't appear that hardware is working for you, so to turn it off use the SetCTSMode function to turn hardware handshaking off.
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nts
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RTS/CTS -unicast or broadcast + Random backoff time

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