PREFACE - IEEE PROJECT 802 FRAMEWROK

IEEE 802 COMMITTE SPECIFIES THE FOLLOWING STANDARDS:

  • 802.1:High Level Interface
  • 802.2:Logical Link Control
  • 802.3:CSMA/CD Network
  • 802.4:Token-Bus Network
  • 802.5:Token-Ring Network
  • 802.6:DQDB Metropolitan Area Network
  • 802.7:Broadband Technical Advisory Group
  • 802.8:Fiber Optic Technical Advisory Group
  • 802.9:Integrated Voice and Data LAN Working Group
  • 802.10:LAN Security Working Group
  • 802.11:Wireless LAN
  • 802.12:Demand-Priority 100VG-Any Lan
    •

    INTRODUCTION - WIRELESS NETWORK FRAMEWORK AND FEATURE

    DIFFERENCE FROM WIRED NETWROK

  • The destination address is usually not the same as the destination loccation in the wireless network:
    One address often represent one fixed position in wired network; however, this matter may be wrong in wireless network. Because a computer assigned to a constant address will move to different location in wireless network randomly.
  • The media of wireless network will influence the design of the whole network:
    There are great differences in the physical layer between the wireless network and wired one. The following is the chanracteristic of wireless network:
  • The connection of nodes is limited because of involing the magnitude of signal.
  • nodes use the same channel.
  • The transfered signal is not protected and influenced in by the external noise easily.
  • The reliablity in wireless network is inferior to the one in wired net work.
  • With dynamic topology.
    • Because of the above reasons, the software and hardware network framework is different from traditional network. For example, because of the limitaion of the transfered signal range, we have to think of the hardware framework in wireless LAN which can be just designed in the resonable geometry distance.
  • Wireless network must be able to handle the mobile stantion:
    For wireless network, one important request is able to handle not only the portable stations but also the mobile stations. The portable stations can move from one posi tion to another, but theyare usually fixed in one position. However, the mobile stations is posible to move continuously in a short time and to access the data in the network.
  • The status of wireless network and other IEEE 802 network is different:
    In order to attain the transparency between networks, wireless LAN hopes to communicate with other networks, so the MAC layer in wireless LAN must have the ability of handleing mobile stations and keeping the transfered data correct. This is different fromthe ablility of MAC layer in LAN.

    • IEEE 802.11 WIRELESS LAN FEATUER

  • The transfer rate is 1 Mbps or 2 Mbps.
  • The frame is IEEE 802.11 CSMA/CA frame.
  • The media are radio.
  • The communication protocol is CSMA/CA providing priority transfer service.
  • transfer delay service:
    Providing the service of guaranteeing transfer dely. If there are two or beyond two workstation transfering frames simul taneously, collision occurs. CSMA/CA can avoid the most unnecessary colli sions, so it can provide the service of guaranteeing transfer delay.
  • occupying bandwidth unfairely:
    Not to guarantee to occupy the bandwidth fairly. The workstations compete the opportunity of transfered data to each other by CSMA/CA protocol not having the characteristic of transfering in turn. Thus, the bandwidth occupied by each workstation is different.
  • not adapted to transfering the multimedia information:
    It is not adapted to transfering the multimedia information. Though, network provides the service of guaranteeing transfer delay, the bandwidth of 2 Mbps is not sufficient to content the multimedia information requesting the real time service.


    • WLAN HARDWARE FRAMEWROK

    IEEE 802.11 SPECIFIES 2 KINDS OF WLAN FRAMEWORK

  • Infrastructure WLAN:
    The so-called infrastructure means an existent wired distrib ution system in which there are special nodes called access points. The function of the access points is to connect one or several WLAN to the existent wired distribution system to let the stations in WLAN be able to communicate with the stations in another WLAN and access the resources in wired distrubution system. This kind of WLAN communication range is often within a building such as store, hospital.
  • Ad Hoc WLAN:
    Ad Hoc WLAN let countless accounts be able to construct wireless communication network immediately. Because any two accounts can communicate with each other directly, this kind of WLAN framework is often applied to conference room. The framework specified by IEEE 802.11 allow ad hoc and infrastructure WLAN to use the same access protocol simultaneously. However, IEEE 802.11 WLAN hardware framework we discussing stress infras tructure WLAN.

    • IEEE 802.11 STRESSES INFRASTRUCTURE WIRELESS LAN

    WLAN hardware framework difined by IEEE 802.11 consists of the following elements(refer to figure):
  • Wireless Medium(WM):
    Used by WLAN.
  • Station(STA):
    The device having the MAC layer and PHY layer of IEEE 802.11 can be called a station.
  • Station Services(SS):
    Providing the stations with the service to transfer data.
  • Basic Service Area(BSA):
    In the infrastructure WLAN, every geometric buliding block is called a Basic Service Area, BSA; the range of every building block depends on the environment and power of the station.
  • Basic Service Set(BSS):
    The set of all the stations in the BSA is BSS.
  • Distribute System(DS):
    Consisting of wired network, DS, can connect several BSAs.
  • Access Poin(AP):
    A device connecting BSS with DS have the ability of station and can let stations access DS as well. Often there is one AP in a BSA.
  • Extended Service Area(ESA):
    The area formed by several BSAs connected by DS is called an ESA.
  • Extended Service Set(ESS):
    Several BSSs connected by DS form an ESS.
  • Distribution System Services(DSS):
    The servicesprovided by DS let data be transfered in different BSSs.


    DESCRIPTION OF STANDARD - MAC COMMUNICATION PROTOCOL

    IEEE802.11 PROVIDES 2 KINDS OF ACCESS CHANNEL METHOD

    The so-called Coordination Function is one mechanism to decide when to be able to transfer the data for a certain station. DCF is the basic access method of IEEE 802.11 MAC, using the technique of CSMA/CA( carrier-sense multiple access/ collision avoidance) to let the stations transfer the assynchro data. We can apply the method to AD Hoc and Intrastructure WLAN. PCF provides the function that stations can transfer the time-bounded data by contention free method, so the frames collision won't occur. But PCF can be applied to the infrastructure WLAN. Figure describes the framework of IEEE 802.11 MAC communication protocol, and PCF operates by DCF.

    DCF

    BASED ON CSMA/CA

    DCF is the basic access method in IEEE 802.11. DCF mode letw different stations share the same media and solves the collision between stations using the tech nique of CSMA/CA. CSMA/CA judges that the signal magnitude is above one thres hold by carrier-sense technique. If the signal magnitude is under this thres hold, it appears that the bandwidth is not occupied and stations can use the bandwidth to transfer data. On the contrary, if the signal magnitude is above the threshold, it appears that the channel is busy and stations have to defer to transfer data. Stations can not transfer frames until the channel is free.

    INTER-FRAME SPACE(IFS) TIME INTERVALS

    In fack, IEEE 802.11 divides the frame by three different priority level; every frame with individual priority level has to wait for a fixed period before being transfered. The period is called Inter-Frame Space, IFS:
  • SIFS(short IFS):
    This is the shortest inter-frame space. It is for immediate respondent action. The waiting time for the following frames is SIFS level: CTS, ACK, etc.
  • PIFS(PCF IFS):
    This is the IFS with mid length. It is the waiting time for PCF to transfer time-bounded data.
  • DIFS(DCF IFS):
    This is the IFS with the longest lenth. It is the waiting time for DCF to transfer assynchronous data.

    • The higher priority level the frames have, the shorter IFS is. Thus, the frames with higher priority level have better chance to use media. In other words, when stations find that the channel becomes free from busy situation, they can't transfer frames immediately. They should wait for a proper period by the priority level of frame and during this period the channel keeps free; they just can transfer frames. With this method, it is very possible that the frames with the same priority level collide. Because the frames with the same priority level waiting for the same time(IFS) and the channel is free, then the stations will transfer the frames simultaneously and collision occurs. The solving method is that afer waiting for the IFS, the stations wait for a random delay time and then transfer the frames. We call this time as backoff time. Because the time slot of backoff window selected by every station is different probably. The rate of collision reduces. This is the property of backoff algorithm.

    2 PROBLEMS IN WIRELESS LAN

    In WLAN, there are two problems:
  • It is difficult to detect collison:
    solution --
    Before transfering frames, the sender transfers the control frame "Request to Send, RTS" ; and when the receiver receives this control frame, it sends another control frame "Clear to Send, CTS" immediately. Only when sender receives CTS sended frome receiver properly (it indicates that RTS doesn't collide.) , sender can transfer frames. When other stations receive CTS which is transfer to receiver, they will stop transfering frames temporarily. Thus, the probabili ty of collision reduces.Figure shows the illustration of RTS/CTS.

  • In physical layer, it is easy to judge whether the channel is busy in error:
    solution --
    IEEE 802.11 solves this problem by Virtual Carrier Sense, which makes use of Net Allocation Vector, NAV, which records the time for other station to transfer frames, and stations can judge whether the channel is busy according to the information. Mentioned above, when one station transfers frames by RTS/CTS, data or ACK, if other stations receive these frames, they will stop transfering frames temporarily. In fact, the correct wording is that RTS, CTS and data frame all include a field record ing the duration for the sender to transfer frames. When other stations receive the RTS, CTS and data frame, they will register the duration field to their own NAV. The duration of NAV may be accumulated; the value of duration not becoming zero indicates that the station can't transfer frames because the channel is busy now ( the period for other station to transfer frames does not end yet). The NAV seems been equipped with the function of carrier sense, being able to tell stations whether now channel is busy. Thus we call it Virt ual Carrier Sense. Figures shows the illustratuion of NAV.


    • PCF

    SUPERFRAME:CONSISTING OF PCF and DCF

    The service offered by DCF is a contention service, that is used for asynchronous traffic. This service does not guarantee any boundaries for access delay or available bandwidth. In order to also offer contention free srevice for time bounded traffic or contention free asynchronous traffic the point coordinated mode PCF may also be used on top of DCF.
  • superframe:
    In point coordinated mode both services are available, the two coordination modes share the bandwidth available in a superframe structure(see figure): After the PCF-paret in the superframe, the PCF passes control to the DCF and regains control to the DCF and regains control of the bandwidth once the DCF-part is over.
  • polling:
    In PCF mode, the central coordination station polls stations, that are on its polling list and allows them undisturbed, contention free access to the medium. To get on the polling list, either once or repeatedly, the stations have to apply during DCF period at the point coordinator.

    • CSMA/CA + RTS/CTS FLOW FIGURE


    CSMA/CA + RTS/CTS ALGORITHMS

    while(1)
  {cs0location
    case1(qanting to send RTS)
    d0(we can write this as fcn1)
      {cs1location
        if(channel is free)
        then
          {
            time(start);
            while(1)
              {
                time(stop);
                if(receiving RTS)
                then goto cs2location;
                else if(receiving CTS)
                then goto cs3location;
                if(time(stop)>=time(start)+DIFS)
                then if(checking channel is free)
                     then
                       {
                         time(start2);
                         while(1)
                           {
                             time(stop2);
                             if(receiving RTS)
                             then goto cs2location;
                             else if(receiving CTS)
                             then goto cs3location;
                             if(time(stop2)>=time(start2)+random(10)*SIFS)
                             then if(checking channel is free)
                                  then
                                    {
                                      time(start3);
                                      sending RTS;
                                      while(1)
                                        { 
                                          time(stop3);
                                          if(receiving CTS)
                                          then break;
                                          else if(time(stop3)>time(star+SIFS)
                                          then goto cs1location;
                                        }
                                      if(channel free>=SIFS)
                                      then
                                        {
                                          if(wanting to send #packet n>=5)
                                          then send i=5 packet
                                          else send i=n packet
                                          if(receiving ACK during timer<=i packet+SIFS)
                                          then goto cs1location;
                                        } 
                                      else goto cs1location;
                                    }            
                                  else goto cs1location;
                           }       
                       } 
                     else goto cs1location;
              }                          
          }                         
        else goto cs1location;
      }

    case2(receiving RTS) 
    do
      {cs2location
        if(this RTS is for myself)
        then
          {
            if(channel free time>=SIFS)
            then
              {
                time(start4);
                sending CTS;
                while(1)
                  {
                    time(stop4);
                    if(receiving data)
                    then break;
                    else if(time(stop4)>time(start)+CTS+SIFS)
                    then goto cs0location;
                  }
                if(channel free>=SIFS)
                then
                  {
                    sending ACK;
                    goto cs0location;
                  }
                else goto cs0location;
              }
            else goto cs1location;
          }
        else 
          {
            tmp_begin=time(begin1);
            tmp_NAV=NAV1;
            while(1)
              {
                if(receiving RTS for others and time()=tmp_begin+tmp_NAV)
                    then
                      {
                        tmp_begin=new_begin;
                        tmp_NAV=new_NAV;
                      }      
                  }  
                  if(time(now)>=tmp_begin+tmp_NAV)
                  then goto cs0location;
               }
               
          }            
      }    


    case3(receiving CTS)
    do
      {cs3location
        if(this CTS is for myself)
        then goto cs1location;
        else waiting for the duration in CTS;
      }
  }

    SUMMARY

    The 802.11 draft standards consist of three main elements, the physical layer specification, the medium access control specification, and the power saving functionality. Two working modes ,Ad-Hoc and Infrastructure mode, are defined. In infrastructure mode either a contention service is ofered by using the distributed coordination function(DCF), or a contention free service with possible support for limited delay guarantees by using the point coordination function(PCF).Figure shows the elements in 802.11 and the services offered as well as their relation towards each other.
  • superframe:
    Both coordiantion modes coexist simultaneously within a superframe structure.
  • IFS:
    Different interframe spaces(IFS) are integrated as the supporting mechanism to separatethe different packet classes. They regulate the minimum time between packets.
  • CSMA/CA and RTS/CTS:
    As the basic access mechanism a CSMA/CA algorithm with rotaing backoff is applied,that may optionally be extended with the RTS/CTS mechanism to increase robustness against hidden terminals.


    • SIMULATING 802.11

    For our simulations we used PTOLEMY, an object oriented simulation tool. Oure packet sources read the packet sizes from a trace file of an Ethernet, the packet interarrival times are poison distributed. The trace contains the arrival times and corresponding packetsizes of an ethernet. The average packet size of the tracefile is 432 byte, before MAC header and physical header are added. If not stated otherwise no hidden terminals were simulated. For 802.11 we decide using frequency hopping spread spectrum as the physical layer (SIFS=28Ms, DIFS=128Ms, backoff slottime=50Ms, physical preamble=122bit) at the optional 2MBit/s transmission rate.

    AD-HOC NETWORKS

    Our first simulations intend to show the performance, that can be expected from both protocols with respect to the number of stations, that are active in a BSA. Figure shows the network throughput with increasing load for different numbers of stations in an ad-hoc network. One can see, that the overall network throughput decreases with increasing number of sending stations. This worsens the fact, that the throuphput available has to be shared among more stations, thus reducing even more the achievable throughput per station.

    Parallel to the decreasing throughput one can see quite obviously an increase in the mean access delay, as shown in figure.


    INFRASTRUCTURE NETWORK

    Only IEEE 802.11 offers a special access scheme for a point coordinated infrastructure mode. In order to evaluate the benefit of such a scheme we simulated a BSA with 8 stations and compared the resulting throughput if either only distributed mode or only point coordinated mode is applied (no time-bounded packet sources, only the contention free asynchronous service is used). For this simulation we have used a different trace file than the one described before, whose average packet size is larger (682 byte compared to 432 byte) which results in a higher efficiency of the access scheme. Figure shows, that due to assured collision avoidance and reduced access overhead higher throughput is reached with PCF, even through the improve ments are not very impressive (87% to 90% ). However, we have left out the additional overhead of managing the access tables in PCF, managing the application to contention free service and other functionality needed to adaptive operate in point coordination mode, since this has not yet been specified in the draft standard. This would cause performance degradation to a yet unknown degree.