Module 4: Switching Concepts 
  
4.1 Introduction to Ethernet/802.3 LANs 
   
4.1.1 Ethernet/802.3 LAN development  
略

4.1.2 Factors that impact network performance  
略

4.1.3 Elements of Ethernet/802.3 networks  
略

4.1.4 Half-duplex networks  
略

4.1.5 Network congestion  
略

4.1.6 Network latency  
略

4.1.7 Ethernet 10BASE-T transmission time  
略

4.1.8 The benefits of using repeaters  
略

4.1.9 Full-duplex transmitting  
 略
4.2 Introduction to LAN Switching 
   
4.2.1 LAN segmentation  
略

4.2.2 LAN segmentation with bridges  
略

4.2.3 LAN segmentation with routers  
略

4.2.4 LAN segmentation with switches  
略

4.2.5 Basic operations of a switch 

The following are the two basic operations that switches perform:

下面是交換器二個基本操作功能：

• Switch data frames - The process of receiving a frame on a switch interface, selecting the correct forwarding switch port(s), and forwarding the frame.
• Maintain switch operations - Switches build and maintain forwarding tables. Switches also construct and maintain a loop-free topology across the LAN.
• 交換資料訊框-程序是接收訊框在交換器介面，選擇正確傳輸的交換埠Port，然後傳送訊框。
• 維護交換器操作-交換器建立與維護傳送表格，在區域網路裡，交換器同時建立與維護無迴圈的拓樸。

   Figures through show the basic operations of a switch.

  圖二 到圖六 顯示交換器的基本操作。

4.2.6 Ethernet switch latency 
 略

4.2.7 Layer 2 and Layer 3 switching 

The difference between Layer 2 and Layer 3 switching is the type of information inside the frame that is used to determine the correct output interface. Layer 2 switching is based on MAC address information. Layer 3 switching is based on network layer addresses, or IP addresses. The features and functionality of Layer 3 switches and routers have numerous similarities. The only major difference between the packet switching operation of a router and a Layer 3 switch is the physical implementation. In general-purpose routers, packet switching takes place in software, using microprocessor-based engines, whereas a Layer 3 switch performs packet forwarding using application specific integrated circuit (ASIC) hardware.

第二層交換與第三層路由的不同點是訊框裡用不同資訊來決定往正確介面輸出。第二層交換技術是以MAC的 位址資訊來決定，第三層路由技術是以網路層的位址資訊來決定，或是IP位址。第三層交換路由器的特色與功能與路由器有許多相似，唯一主要不同是路由器以軟體操作路由封包而第三層交換器是以硬體操作為主，一般用途的路由器 封包交換操作以軟體，使用微處理機當基礎動力，然而第三層交換器執行封包傳送是以特別設計的積體電路(ASIC) 硬體來處理。

4.2.8 Symmetric and asymmetric switching  
略

4.2.9 Memory buffering  
略

4.2.10 Two switching methods 
略
 
4.3 Switch Operation 
   
4.3.1 Functions of Ethernet switches  
略

4.3.2 Frame transmission modes 

This page will describe the three main frame transmission modes:

• Cut-through - A switch that performs cut-through switching only reads the destination address when receiving the frame. The switch begins to forward the frame before the entire frame arrives. This mode decreases the latency of the transmission, but has poor error detection. There are two forms of cut-through switching:
  1. Fast-forward switching - This type of switching offers the lowest level of latency by immediately forwarding a packet after receiving the destination address. Latency is measured from the first bit received to the first bit transmitted, or first in first out (FIFO). This mode has poor LAN switching error detection.
  2. Fragment-free switching - This type of switching filters out collision fragments, with are the majority of packet errors, before forwarding begins. Usually, collision fragments are smaller than 64 bytes. Fragment-free switching waits until the received packet has been determined not to be a collision fragment before forwarding the packet. Latency is also measured as FIFO.
• Store-and-forward - The entire frame is received before any forwarding takes place. The destination and source addresses are read and filters are applied before the frame is forwarded. Latency occurs while the frame is being received. Latency is greater with larger frames because the entire frame must be received before the switching process begins. The switch has time available to check for errors, which allows more error detection.
• Adaptive cut-through - This transmission mode is a hybrid mode that is a combination of cut-through and store-and-forward. In this mode, the switch uses cut-through until it detects a given number of errors. Once the error threshold is reached, the switch changes to store-and-forward mode.

這一頁將敘述三種主要訊框傳輸模式。

• 穿透式分封交換(Cut-through )---交換器執行穿透式分封交換(Cut-through )當收到訊框只要讀到目的地位址，交換器會在還未完整收到訊框便開始傳送訊框，這種模式降低傳輸的延遲，但是錯誤偵測比較差，穿透式分封交換有二種作法:

  1.快速傳輸交換(Fast-forward switching)-這一種型式的交換提供最低等級的延遲， 只要一 接收到目的地位址就馬上傳送封包，延遲的計算是從第一個bit接收到第一個bit被傳送出去，或者是第一個進第一個出，這種模式有較差的區域網路錯誤偵測。

  2. 無片段交換 (Fragment-free switching) - 這種型式的交換會過濾掉碰撞片段，在開始傳輸之前有大多數的錯誤封包，通常碰撞片段都小於64 bytes，無碰撞片段交換會等到已收到封包已被認為是無碰撞片段才會開始傳送封包，延遲的計算也是從第一個bit接收到第一個bit被傳送出去。

• 儲存後推送封包交換(Store-and-forward)---接收完整的訊框後才開始傳送，目的地與來源位址都被讀到且已經過濾訊框才能傳送，當訊框 接收完延遲已發生，大的訊框就有大的延遲因為完整的訊框必須被接收完整交換的程序才會開始，交換器有時間去檢查錯誤，且允許更多錯誤偵測。
• 可調式穿透式分封交換-這種傳輸模式是一種混合模式它是穿透式分封交換(Cut-through )與儲存後推送封包協定(Store-and-forward)的組合， 在這種模式交換器先使用穿透式分封交換直到它偵測一些錯誤，當錯誤達到一定的門檻，交換器就會更換成儲存後推送封包協定模式。如圖二

4.3.3 How switches and bridges learn addresses 

The switching table is stored using Content Addressable Memory (CAM). CAM is used in switch applications to perform the following functions:

• To take out and process the address information from incoming data packets
• To compare the destination address with a table of addresses stored within it

內容尋址性記憶體(CAM)用來儲存交換表格，CAM 是用來做交換應用去執行下列功能:

• 從進來的資料封包從裡面拿出位址資訊並處理它。
• 去比較目的地位址與儲存在位址表格的位址。

The CAM stores host MAC addresses and associated port numbers. The CAM compares the received destination MAC address against the CAM table contents. If the comparison yields a match, the port is provided, and the switch forwards the packet to the correct port and address.

內容尋址性記憶體(CAM)儲存主機MAC位址與它相關的埠(port)號碼，CAM比較接收到訊框的目的地MAC位址對照CAM表格內容，假使比對成功，交換器開始傳輸封包到正確埠(port)與位址。如圖二

4.3.4 How switches and bridges filter frames 

Today, switches are also able to filter according to the network-layer protocol. This blurs the demarcation between switches and routers. A router operates on the network layer using a routing protocol to direct traffic around the network. A switch that implements advanced filtering techniques is usually called a brouter. Brouters filter by looking at network layer information but they do not use a routing protocol.

今天,交換器能夠過濾依據網路層的通信協定，這是個介於交換器與路由器之間的模糊功能，路由器在網路層操作使用路由通信協定四處指導交通在網路上，交換器應用進階的過濾技術通常叫做橋接器與路由器合成語(Brouter)，Brouters 它們過濾會參考網路層的訊息但不會使用路由通信協定。

4.3.5 Why segment LANs?  
略

4.3.6 Microsegmentation implementation  
略

4.3.7 Switches and collision domains  
略

4.3.8 Switches and broadcast domains  
略

4.3.9 Communication between switches and workstations  
略