從這篇paper當中,隨著作者所要讓我們知道的事關於在無線網路(802.11 Wireless LAN)上,容量的分析跟protocol的改善.
From this paper, the author reveals something mainly related to wireless network (802.11 Wireless LAN ), analysis of the capacity and improvement for the protocol.
在無線網路中,MAC(Medium Access Control)protocol為無線網路傳輸通道上面一個重要的機制,根據作者所描述的,IEEE 802.11 protocol 有一個上限質,但是在現實的網路中,所呈現的容量效能往往會低於這個質很大,而這篇paper主要是講說如何提出一個良好的backoff algorithm然後去改善現時網路中的效能以至於可以很接近於理想值.
In the wireless network, MAC (Medium Access Control ) protocol is an important mechanism on the passageway for the wireless network, according to what the author describes, IEEE 802.11 protocol has a upper limit quality, but in the realistic network, the capacity efficiency appears much lower than this quality, and this paper mainly says how to put forward a good backoff algorithm and then improve the efficiency in the current network so that we makes it possible to approach the ideal value.
在WLAN的環境中,我們都知道WLAN的頻寬相對於Wired network來的小很多.所以我們必須對於傳送的機制有相當嚴格的規定.例如接收端在接收到正確的封包訊息之後,他會傳回一個可以讓傳輸端知道他已經接收到正確封包的訊息,這個訊息是ACK message,或者傳輸端在要傳送訊息給某各接收端的時候,他會先去聽看看傳輸的通道上面是不是有其他的傳輸正在進行,這項功能又稱為carrier sensing .
In the environment of WLAN, we all know WLAN bandwidth is relatively much smaller than Wired network. So we must have quite strict regulations to the conveyed mechanism . For example, after the receiving-end receives the correct packet message, it will send back a message to let him know he has already received the correct packet. The information is ACK message, or when transmission end is ready to send messages to a certain receiving end, it will go have a look at the transmission passageway beforehand to see if there’s any other on-going transmission. This function is also called carrier sensing.
在IEEE 802.11 MAC protocol當中,WLAN所用的protocol是DCF(Distributed Coordination Function),而DCF是CSMA/CA(Carrier Sense Multiple Access with Collision Avoidance)一種機制,而CSMA/CA又可分為很多種,譬如1-persistent CSMA/CA or p-persistent CSMA/CA等等.CSMA/CA主要是用於在無線網路當中,當傳輸端要傳送封包給接收端時,他必須要先知道他所用的通道上面是否有其他的傳輸正在進行,假若有的話,則它必須停止他的倒數器,且不會有任何的動作,一直到他發現這個通道上面並沒有任何傳輸在進行,則它會等待一個時間(DIFS)後,然後直到他自己的到數器數到”0”的時候,他才會將他所要傳送的封包丟到這個通道上面去.
Among IEEE 802.11 MAC protocol, protocol used in WLAN is DCF (Distributed Coordination Function ). DCF is CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance ) is a kind of mechanism, and CSMA/CA can be divided into many kinds, for example 1- persistent CSMA/CA or p-persistent CSMA/CA ,etc.. CSMA/CA is mainly used in the wireless network, when transmission end sends the packet to the receiving end, it must know beforehand if there’s any on-going transmission on the passageway used by itself., if it exists , then it must stop his reciprocal device without any movements, until he makes sure there is no any other on-going transmission. It will wait for a certain time (DIFS ), and not until the reciprocal device reaches 0 will he throw the packet intended to be transmitted to this pass way.
關於容量的分析部份,當傳送一個成功被接收的封包,其實除了傳送封包的時間外,還必須要加上propagation time跟ACK,DIFS,SIFS等…故實際上通到容量的大小並不會實際等於傳輸的速率,這就是為什麼WLAN Adapter雖然他的throughput為11Mbps,但是實際值卻只能到達5Mbps左右的原因.可是這些都是理想的考量情況下,因為假如在現實得無線網路情況下,不可能只有一個使用者而已,一定會有非常多的人會使用到無線網路,固所以我們還必須考量到很多複雜的問題,例如在傳輸的時候要避免跟其他人碰撞的演算法,或是已經碰撞之後我們所要去考慮到的事情,這些都是我們所要去研究的.
Regarding to the part of the capacity analysis, when transmitting a packet which receives successfully, in fact ,in addition to the time of delivering the packet , we have to add propagation time and ACK , DIFS , SIFS ,etc. As a result, the size of the capacity led to will not be equal to the speed rate of transmission practically. That’s why WLAN Adapter’s throughput is 11Mbps, but it can only reach the actual value around 5Mbps. Anyway all these occurs just under the ideal consideration. If in reality under the circumstance of the wireless network, it is impossible to exist just one user. There must be lots of persons in use of the wireless network, as a result we have to take other complicated problems into consideration. For example, in transmitting, we have to avoid performing the algorithm of colliding with the others, or something we have to consider when they have already collided. All these are what we have to research for this case.
這篇paper裡面主要利用到的概念去算出較好的機制來提升IEEE 802.11原本所遇到的一些困難.
In this paper we mainly take advantage of the concept of Contention Window to figure out the much better mechanism to help facilitate some original problems arisen from IEEE 802.11.
1 :
作者他先在固定傳輸端數目（Ｍ）的情況下,去找出Capacity跟Packet size(slots q)之間的關係,可由下圖來解釋
The author find out the relationship between capacity and packet size(slots q) under the circumstance of fixing the number of transmission end.(M)
因為假使要傳送的封包太小,則會發現到Capacity會受到的影響很大,但如果Packet size逐漸變大時,則Capacity會逐漸趨向於某各定值(隨著傳輸者數量不同而不同).
If the packet in intent to be transmitted appears too small, we will find out a fact that ACK,DIFS,SIFD will have a great influence on Capacity. But if packet size is getting larger, capacity will tend to a certain value( it varies as the different quantity of transmitter.)
2 :
這個圖主要是說明,假若我們固定了傳輸者數量(M)跟Packet size(q)時所得到的傳輸機率(p)跟實際的傳輸時間之間的關係,由上圖可以發現的到,在某傳輸數量時,它的某一個傳輸機率( )一定會得到最低的傳輸時間,而作者他所要求的就是這個因為假若固定了傳輸數量的話,則我們又求到了這個的值,則相對的我們就可以將低傳輸的時間,那整個無線網路的效能就會被大大的提升了.
This figure mainly explain if we fix the quantity of transmitters(M) and Packet size (q), what the relationship is between the transmission probability (p) and the actual transmission time( ) From the above figure, we can find out in a certain transmission quantity, its certain transmission probability will get the shortest transmission time. However , what the author requests is this value . Because if we fix the transmission quantity, we will get the value of. , we can thus lower the transmission time relatively. Then the efficiency of the wireless network will be highly promoted.
3 :
由figure6跟figure7可以更明顯的清楚發現,假若我們可以求得 ,則相對的就可以藉由作者推倒出來的公式而求到最佳的window size,如果我們可以求到,則可以發現到,不論傳輸端的數量為何,它們的Capacity均可以達到最大的值.而這也是作者他們所盡力要去達到的目標.
From figure6 and figure 7, we can see more obviously. If we can get , then we can get the best window size relatively through the formula proposed by the author. If we can get optimal window size, then we can find out despite the quantity of the transmission end, their capacity will averagely reach the largest value. And this is what the authors struggle for.
4 :
這是作者他們自己提出如何計算 的方法,
the following is the formula proposed by the quthors how to compute (for M=100 and q=0.99)
àp lower than correspond to the cases in which the average virtual time is mainly determined by the .
àp greater than correspond to an average virtual time caused above all by collisions .
5 :
由上面這個圖就可以明顯的知道作者他們所提出的方法相對於原本的IEEE 802.11有明顯的改善(M為已知)
From the figure above, we can know obviously the method proposed by the author has apparent improvement compared with that of IEEE 802.11. (M is known)
6 :
可是現實網路當中,傳輸者數量到底為多少其實並不固定,故所以像第5所推得出來的數值,其實並不適合用於現實生活當中的無線網路上面,因為假如真的把傳輸端的人數給限定住的話,則就會發生以下的問題
But Network in reality, the quantity of the transmitters actually is not fixed, as a result just like the value computed from the number 5 figure, it is in fact not proper to be used in the wireless network in reality. Cause if we really fix the number of the transmission end, the following problems may occur,
很明顯的可以看的出來,若真實的傳輸者數量為50個的話,其實還是會比IEEE 802.11來的好很多,但是如果傳輸者數量很少的話(10),則會發現到效能將會比IEEE 802.11差很多很多.
We can see obviously, if the quantity of the real transmitters is 50, actually it will still be much better than IEEE 802.11. But if the quantity of the transmitter appears few(10), you can find the efficiency will be much worse than that of IEEE 802.11.
7 :
因為現實生活中傳輸者數量並無法限定住,故必須要去考慮一個隨著傳輸者數量變化而變化的演算法
Because in reality the quantity of transmitters can not be limited, we have to think of a mathematical calculation which will vary as the quantity of transmitters.
作者提出美各傳輸端(Station)均會利用上面的公式來更新所有傳輸者數量的資料,藉以求得較正確的傳輸者數目.
Every station proposed by the author will use the above formula to update the data of the quantity of all the transmitters so that we can get the much more correct number of the transmitter quantity.
而由上圖即可知道,經過了每個station都會更新傳輸者數量的資料後,則效能會比較接近於理想值.
From the figure above, we can know after every station updates the data of the quantity of all the transmitters, the efficiency will get closer to the optimal value.
文章定位：
