[C#]簡易的Backoff window實現類別

在無線網路的領域中，若是節點間要進行傳輸，會試圖嚐試發送RTS訊號，當接收端收到且允許傳送時，接收端會發送CTS訊號，傳送端就會知道可以進行傳送的動作。但若是傳送端發送了RTS後過段時間沒收到CTS訊號，代表訊號被碰撞掉了，這時會挑選個backoff值，決定要多久後再重試傳輸。這邊的backoff值會隨著碰撞而變大backoff值的區間，也就是無線網路領域所謂的Backoff window機制。

這樣的機制很簡單，但卻能分散碰撞，並將重試的時間調在適當的區間。這樣的機制一樣可以拿來套用在我們一般程式的開發，就像有時後訂Delay time時不知道該設多少，訂多訂少都會有不好的影響，那倒不如導入這樣的一個機制讓它自我調適。

這邊筆者憑著印象刻了一個簡單的BackOff類別。

程式碼如下：

using System; using System.Collections.Generic; using System.Linq; using System.Text; using System.Collections.ObjectModel;

namespace ConsoleApplication12 { public class BackOff { #region Var private int _level; private Random _random; #endregion

#region Private Property private ReadOnlyCollection m_BackOffWindows { get; set; }

private Random m_Random { get { if (_random == null) _random = new Random(Guid.NewGuid().GetHashCode()); return _random; } } #endregion

#region Public Property /// /// Gets or sets the level. /// /// The level. public int Level { get { return _level; } set { if (value m_BackOffWindows.Count) throw new Exception(“Level value incorrect!”);

_level = value; } } #endregion

#region Constructor /// /// Initializes a new instance of the class. /// /// The back off windows. public BackOff(params int[] backOffWindows) { CheckBackOffWindows(backOffWindows);

this.m_BackOffWindows = new ReadOnlyCollection(backOffWindows.ToList()); ResetLevel(); } #endregion

#region Private Method /// /// Checks the back off windows. /// /// The back off windows. private void CheckBackOffWindows(params int[] backOffWindows) { if (backOffWindows == null) throw new ArgumentNullException(“backOffWindows”);

if (backOffWindows.Length == 0) throw new ArgumentException(“backOffWindows must contains elements!”);

if (backOffWindows.First() backOffWindow) throw new ArgumentOutOfRangeException(“backOffWindows must order by increase!”);

if (minValue == backOffWindow) throw new ArgumentOutOfRangeException(“Can’t contains duplicated elements!”);

minValue = backOffWindow; } } #endregion

#region Public Method /// /// Increases the level. /// /// public Boolean IncreaseLevel() { if (Level >= m_BackOffWindows.Count) return false;

Level += 1; return true; }

/// /// Decreases the level. /// /// public Boolean DecreaseLevel() { if (Level /// Gets the next value. /// /// public int NextValue() { var minValue = (Level == 1) ? 0 : m_BackOffWindows[Level - 2]; var maxValue = m_BackOffWindows[Level - 1];

return m_Random.Next(minValue, maxValue); } #endregion } }

實際使用時只要建立物件實體，並帶入backoff window的區間值，透過NextValue取得backoff值就可以了，若是碰撞則呼叫IncreaseLevel去將區間上調。

using System; using System.Collections.Generic; using System.Linq; using System.Text;

namespace ConsoleApplication12 { class Program { static void Main(string[] args) { BackOff backoff = new BackOff(16,32,64);

Console.WriteLine(“Level1…”); Console.WriteLine(backoff.NextValue().ToString()); Console.WriteLine(backoff.NextValue().ToString()); Console.WriteLine(backoff.NextValue().ToString()); Console.WriteLine(new string(’=’, 50));

backoff.IncreaseLevel(); Console.WriteLine(“Level2…”); Console.WriteLine(backoff.NextValue().ToString()); Console.WriteLine(backoff.NextValue().ToString()); Console.WriteLine(backoff.NextValue().ToString()); Console.WriteLine(new string(’=’, 50));

backoff.IncreaseLevel(); Console.WriteLine(“Level3…”); Console.WriteLine(backoff.NextValue().ToString()); Console.WriteLine(backoff.NextValue().ToString()); Console.WriteLine(backoff.NextValue().ToString()); Console.WriteLine(new string(’=’, 50)); } } }