Analysis of 802.11 MAC code in NS-2

0. MAC in ns-2

LAN is within Berkeley Architecture, WLAN cannot create with “newLan” command
Ethernet could be created as a LAN with common bandwidth and delay.
 

1. The general structure of MAC

• pktTx_ 
• pktRx_
• Macstate_ :
• index_  : mac address
  

• recv (packet, handler)
  • This is the entry from upper target (a callback handler is given as a parameter) to send a packet to MAC. After the MAC transmit this packet successfully, it will use this callback handler to inform the upper target that MAC is idle and  give another packet if there are packets buffered.
    
• SendUp
  • entry for receiving a packet. Sendup is function is directly called by the lower target of MAC, might be "netif" or "phy". And this function directly calls the upper_target. Because the uplink  to upper_target does not involve any phsical transmission delay, it does not need any timer and state change here. The question is that when the MAC is in MAC_RECV state? The answer is: The MAC here is supposed to be full-duplex and receive can be happened simultaneously. it does not care about collisions etc. This is a general MAC class
    
• SendDown
  • used to sending packet down. Called by recv().init a timer for tx, and the timer handler is defined to call resume().
    
• Handler* callback_;
  • when MAC is idle, the upper target has to be callback.
• Resume()
  • When tx timer out, reset MAC as idle state and callback.
    
• Discard
  • When a packet has to be drop, the drop_ (NsObject*) of bi-connector class has to be called to handle this, usually drop (p, why) is used. Why is a string of drop reason, in cmu-trace.h. three-character string is defined to describe those reasons in the trace file, such as "BSY", "CBK"....
    

2. The 802.11 MAC

• pktTx_ (inherited from Mac class): 
• pktRx_(inherited from Mac class):
  
• pktCtrl_
• pktRTS_
  

• recv(): the recv() function of MAC class has been overridden. Here, recv() means an incoming packet from both physical layer and upper layer.
  
• send(): this function is called by recv() when it get a packet supposed to sending down.  Similarly, a callback handler is given as a parameter. Immediately, the sendDATA(p) function is called to generate an appropriate mac_header for this packet, and set this to pktTx_. Then,  follew the CSMA/CA procedures to get this packet sent.
• sendDATA(). to compose a DATA MAC frame and set it as pktTx_
• recvDATA(). When heard a DATA MAC frame, hand it to upper layer.
  

void

Mac802_11::tx_resume()

{

 .....

else if(callback_) {

        Handler *h = callback_;

        callback_ = 0;

        h->handle((Event*) 0);

    }

    // change wrt Mike's code

    //SET_TX_STATE(MAC_IDLE);

   setTxState(MAC_IDLE);

}

void QueueHandler::handle(Event*)

{

        queue_.resume();

}


void Queue::resume()

{

        Packet* p = deque();

        if (p != 0) {

                target_->recv(p, &qh_);

        } else {

                if (unblock_on_resume_)

                        blocked_ = 0;

                else

                        blocked_ = 1;

        }

}

Backoff

The backoff implementation in mac-802_11.cc does not comply with the standard. Basically, this is due to the introduction of a defer timer. The defertimer is set in send() and after a packet transmission finishes. The deferred time value is usually (DIFS+rTime) where rTime is as same as the selection of backoff timeslots from CW (Contention Window). However, the defertimer is never paused or resumed.

So, after a packet transmission, according to the IEEE 802.11 standard, the node should do backoff immediately. here in ns-code, the defertime is first set. And after timer out, in check_pktTx() function. if the channel is not idle, a backoff timer is set. Therefore, here exists a double backoff problem. Also, according to IEEE 802.11 standard, a node should access channel directly if it sense the channel idle and still idle after at least DIFS time. Here, ns-code also deviates from the standard and add an additional rTime before sending DATA or RTS.
 
Determine the transmission time
 
txtime is calculated from the "size" variable in common header. Because all headers are present in the packet no matter what layer it is. Thus, the only way to know the packet size is to add or subtract some bytes in hdr_cmn->size(); In 802.11 MAC code, in recvDATA(), the 802.11 header size will be subtracted. And in SendDATA(), a 80211_hdr_length will be added.

Transmission Failure:

In both RetransmitRTS and RetransmitDATA functions, it is necessary to deal a packet drop :

if(ssrc_ >= macmib_.getShortRetryLimit()) {

		discard(pktRTS_, DROP_MAC_RETRY_COUNT_EXCEEDED); pktRTS_ = 0;

		/* tell the callback the send operation failed 

		   before discarding the packet */

		hdr_cmn *ch = HDR_CMN(pktTx_);

		if (ch->xmit_failure_) {

                        /*

                         *  Need to remove the MAC header so that 

                         *  re-cycled packets don't keep getting

                         *  bigger.

                         */

			// change wrt Mike's code

                        //ch->size() -= ETHER_HDR_LEN11;

			ch->size() -= phymib_.getHdrLen11();

                        ch->xmit_reason_ = XMIT_REASON_RTS;

                        ch->xmit_failure_(pktTx_->copy(),

                                          ch->xmit_failure_data_);  //callback by upperlayer
                }
		//printf("(%d)....discarding RTS:%x\n",index_,pktRTS_);
		discard(pktTx_, DROP_MAC_RETRY_COUNT_EXCEEDED); pktTx_ = 0;
		ssrc_ = 0;
		rst_cw();
	} 

So, it is a way to let upper layer , such as DSR routing to know there is a route-error and need to send a route-error message.

Frame Formats in ns-2

The 802.11 header in ns-2 is not conform to the 802.11 standard. It is a different one. Thus, this overhead cannot be regarded as authentic as that of
the experiment. And the throughput measurements also seems different.

Other IEEE 802.11 features

RTS/CTS is an important part of code. There are functions like sendRTS ( is to form a RTS ctrl packrt) , retransmitRTS...... Basically it has to compare with RTSThreshold first, if the size is small than the threshold, there is no" virtual carrier sense" scheme used.

[ns] Dropping Packets with RTR CBK

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Hi,

It means that the MAC layer couldn't successfully deliver the packet. You can 
find the reasons for this at the bottom of this page: 
http://masimum.inf.um.es/nsrt-howto/html/node23.html

Regards,
fran

P.S: answers to frequent questions like this can be easily found with 
http://www.isi.edu/nsnam/htdig/search.html

On Tuesday 19 September 2006 11:48, michelenuti at alice.it wrote:
> Hi ns users,
> does anybody know what does it mean when a TCP packet is dropped and in the
> trace file is indicated as RTR CBK?
>
> Thanks a lot,
> Michele Nuti

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Re: segmentation fault (core dumped) using ZRP in ns-2.27

영어로는 Preposition라고 합니다.
전치사는 명사(명사구 포함), 대명사 앞에 놓이며, 뒤에 따르는 명사 또는 대명사와 함께 형용사구 또는 부사구를 만든다.

1) 시간을 나타낼때
       
      ① at - 시각 
        Can you come at nine? (9시에 올 수 있어요?)
         
      ② on - 날짜, 요일, 특정한 날
        I play tennis on Sunday. (나는 일요일에 테니스를 한다) 
         
      ③ in - 주, 월, 계절, 년, 세기 등 
        Are there many holidays in October?(10월에는 많은 휴일이 있습니까?)
     
      ④ before <∼전에>  after <∼후에>  till, until <∼까지>
        I played tennis after school. (나는 방과후에 테니스를 했다) 
      
      
    (2) 장소를 나타내는 전치사 
     ① at <∼에(서)>라는 뜻으로 비교적 좁은 장소에 쓰인다.
     **in <∼에(서)>라는 뜻으로 비교적 넓은 장소에 쓰인다. 
      I met him at the station. (나는 정거장에서 그를 만났다)
        Let's play soccer in the park. (공원에서 축구하자)
         
      ② on (접촉), over (바로 위), above (보다 위쪽에) <∼위에>under <(접촉해서)∼아래에>
        There is a bridge over the river. (강 위에는 다리가 있다)
         
    (3) for, by
      ① for <∼을 향하여(목적)>, <∼동안(기간)>, <∼에게는>, <∼을 향하여>
        I bought a book for you. (나는 당신을 위해 책을 한 권 샀다)
         
      ② by <∼로(수단, 교통 수단)>, <∼까지>, <∼옆에>
        I go to school by bus. (나는 버스를 타고 학교에 갔다)
         
  4. 구
    <전치사+명사??대명사>의 형태로 부사처럼 동사를 수식하는 <부사구>와 형용사처럼 명사를 수식하는 <형용사구>가 있다. 
      The book on the desk is mine. 형용사구 (책상 위에 있는 책)
      There is a book on the desk. 부사구 (책상 위에 있다)
         
 

 
  
  
   
    1. 전치사의 성격
     
      전치사는 명사·대명사의 목적격 앞에서, 혹은 동명사 앞에서 쓰여 형용사구·부사구를 만든다. 
 
        The building on the hill is our school. <명사 수식 : 형용사구> 
        (언덕 위에 있는 건물은 우리 학교다)
         
        There is a book on the table. <동사 수식 : 부사구>
        (탁자 위에 책이 한 권 있다)
         
    2. 전치사와 부사, 전치사와 접속사의 역할을 동시에 하는 것
 
      ⑴ in
     
        We live in Seoul. (우리는 서울에서 산다) 전치사 (∼에) 
        Come in, please. (안으로 들어오십시오) 부사 (안으로)
         
      ⑵ On
        He goes to church on Sunday. 전치사 (∼에) (그는 일요일에 교회에 간다)
        I'll keep a diary from now on. 부사 (계속해서)
        (나는 지금부터 계속해서 일기를 쓸 것이다.)
         
       
      ⑶ before
     
        Do you usually study before supper? 전치사
        (너는 보통 저녁 먹기 전에 공부하느냐?)
         
        Do you have many things to do before you go to bed? 접속사 
        (너는 잠자리에 들기 전에 할 일이 많이 있니?)
 
      ⑷ after
        I play tennis after school. 전치사 (나는 방과후에 테니스를 했다)
        I arrived after he left. 접속사 (나는 그가 떠난 후에 도착했다)
       Let's wait here until two o'clock. 전치사 (2시까지 여기서 기다립시다)
        I'll wait here until he comes. 접속사 (그가 올 때까지 여기서 기다릴 것이다)
         
         
    3. 중요한 전치사
         
      ⑴ during  <∼동안>특정한 기간을 나타내는 명사와 함께 쓴다. 
        He kept sleeping during the lesson. (그는 수업 시간 동안 내내 잤다)
         
      ⑵ from A to B, from A till B  <A부터 B까지>
        It takes thirty minutes from my house to my school.(나의 집에서 학교까지 30분이 걸린다)
        He works hard from morning till night. (아침부터 밤까지 열심히 공부한다)
         
      ⑶ in front of (=before) <∼앞에>
        There was a car in front of the house. (집 앞에 차 한 대가 있었다)
         
      ⑷ like  <∼처럼>
        I want to be a great scientist like Newton.(나는 Newton 같은 위대한 과학자가 되고 싶다)
      <주의> like가 동사로 쓰이면 「좋아하다」의 뜻이다.
        I like to climb the mountain every Sunday.(나는 일요일마다 등산하기를 좋아한다)
         
      ⑸ among  <∼중에>
        Jane is the most beautiful girl among three.(Jane이 셋 중에서 가장 아름다운 소녀이다)
         
      ⑹ between A and B  <A와 B사이에> 
        Many men tried to reach the North Pole between 1898 and 1905.
        (많은 사람들이 1898년과 1905년 사이에 북극에 도달하려고 노력했다)
         
      ⑺ without  <∼없이, ∼않고> 
        He ran all the way from his school to his home without stopping. 
        (그는 학교에서 집까지 쉬지 않고 내내 달렸다)
         
      ⑻ behind  <∼뒤에> 
        The house stands behind the church. (그 집은 교회 뒤에 있다)
         
      ⑼ out of  <∼에서>
        They made the rings out of gold. (그들은 반지를 금으로 만들었다)
        He ran out of the room. (그는 방에서 뛰어나갔다)
         
      ⑽ along  <∼을 따라>
        We walked along the street. (우리는 길을 따라 걸었다)
         
      ⑾ on  <(표면에 접촉하여) 위에>   beneath  <(표면에 접촉하여) 아래에>
        There is a picture on the wall. (벽에 그림이 한 점 있다)
        The ice broke beneathe our feet. (얼음이 우리 발 아래서 부서졌다)
         
      ⑿ over  <∼(바로) 위에>     under  <∼(바로) 아래에>
        The dog jumped over the fence. 
        There is a sleeping dog under the table. 
         
      ⒀ above  <∼보다 높은 위쪽에[으로]>    below  <∼보다 낮은 아래쪽에[으로]>
        The moon rose above the mountain. (저쪽 산 너머로 달이 솟았다)
        the sun set below the horizon. (지평선 아래로 해가 졌다)
         
      ⒁ up  <∼의 위쪽으로>     down  <∼의 아래쪽으로>   모두 방향성이 있다.
        He ran up[down] the stairs. (계단 위로[아래로] 뛰어 갔다)
         
      ⒂ around  <∼의 주위에, 둘레에> 정지 상태
      **round  <∼의 주위로> 운동 상태
      **about  <∼의 부근에, 여기저기에> 막연한 주위
         
        The earth move round[around] the sun. 
        They were walking about the street. 

1. 준비

korean-amitasia-44eb.awl

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 - 필요 프로그램

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    : trojankiller : 파일 첨부 해놓았습니다.

팝업 윈도우가 뜨면 꼭 취소를 누르셔야합니다. 확인을 누를시 autorun.inf 파일이 자동실행되어 바이러스가 전염됩니다.

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