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   802.1Q- -

802.1Q

An IEEE standard for providing VLAN identification and quality of service (QoS) levels. Four bytes are added to an Ethernet frame, increasing the maximum frame size from 1518 to 1522 bytes. Three bits are used to allow eight priority levels (QoS) and 12 bits are used to identify up to 4096 VLANs. 802.1q is the IEEE specification for implementation of VLANs (Virtual Local Area Networks) in Layer 2 LAN switches, with emphasis on Ethernet. Similar to 802.1P, prioritization of traffic is accomplished through an additional four bytes of data in the frame header. Most data fields in this addition to the header are specific to VLAN operation. Also included is a field which provides the same 3-bit priority flag specified in 802.1P&＃039;s priority-mapping scheme. In addition to conventional data traffic, 802.1Q supports voice and video transmission through Ethernet switches. In short, the 802.1Q specification provides a 32-bit header for VLAN frame tagging. Each 802.1Q tag sits in an Ethernet frame between the source address field and the media access control (MAC) client type/length field. An important feature of 802.1Q is the ability to share multiple subnets across a high-speed link. This capability not only reduced the number of lower speed links needed for physical separation, but it also allowed for asymmetrical traffic management so that different speed links could be managed more easily. With IEEE 802.1p and 802.1Q, we saw the introduction of some important concepts that have been carried forward for further QoS (Quality of Service) development. These 802.1 features also can be mapped into higher layer protocols like IP and ATM.

An IEEE standard for providing VLAN identification and quality of service (QoS) levels. Four bytes are added to an Ethernet frame, increasing the maximum frame size from 1518 to 1522 bytes. Three bits are used to allow eight priority levels (QoS) and 12 bits are used to identify up to 4096 VLANs. 802.1q is the IEEE specification for implementation of VLANs (Virtual Local Area Networks) in Layer 2 LAN switches, with emphasis on Ethernet. Similar to 802.1P, prioritization of traffic is accomplished through an additional four bytes of data in the frame header. Most data fields in this addition to the header are specific to VLAN operation. Also included is a field which provides the same 3-bit priority flag specified in 802.1P&＃039;s priority-mapping scheme. In addition to conventional data traffic, 802.1Q supports voice and video transmission through Ethernet switches. In short, the 802.1Q specification provides a 32-bit header for VLAN frame tagging. Each 802.1Q tag sits in an Ethernet frame between the source address field and the media access control (MAC) client type/length field. An important feature of 802.1Q is the ability to share multiple subnets across a high-speed link. This capability not only reduced the number of lower speed links needed for physical separation, but it also allowed for asymmetrical traffic management so that different speed links could be managed more easily. With IEEE 802.1p and 802.1Q, we saw the introduction of some important concepts that have been carried forward for further QoS (Quality of Service) development. These 802.1 features also can be mapped into higher layer protocols like IP and ATM.

IEEE關于提供VLAN（虛擬局域網(wǎng)）識別和服務質(zhì)量（QoS）等級的標準。在以太網(wǎng)一個幀當中加入4個字節(jié)，將一幀的最大長度從1518字節(jié)提高到1522字節(jié)。4個字節(jié)中的3比特提供8種優(yōu)先等級，12比特用于最多識別4096種虛擬以太網(wǎng)。在第二層局域網(wǎng)交換，尤其是以太網(wǎng)中，802.1q成為IEEE虛擬以太網(wǎng)實施的技術規(guī)范。與802.1P相類似，通信業(yè)務優(yōu)先權的確定是通過在幀標題中額外加入一個4字節(jié)數(shù)據(jù)來完成的。在幀標題中額外加入的大多數(shù)數(shù)據(jù)段僅限于虛擬以太網(wǎng)操作。幀標題中還包括一個數(shù)據(jù)段，提供802.1P優(yōu)先權設定方案中給出的相同的3比特優(yōu)先標記。除了一般的數(shù)據(jù)通信業(yè)務，802.1Q還通過以太網(wǎng)交換支持話音和視頻傳輸。簡而言之，802.1Q技術規(guī)范為虛擬以太網(wǎng)幀標簽提供一個32位標題。每一個802.1Q標簽位于以太網(wǎng)幀源地址字段和介質(zhì)訪問控制（MAC）客戶類型/長度字段之間。802.1Q的一個重要特點是能夠通過一個高速鏈路共享多重子網(wǎng)。這一功能不僅減少了物理分離所需的低速鏈路數(shù)量，而且還允許進行非對稱通信業(yè)務管理，這樣能夠更容易對不同速度的鏈路進行管理。通過IEEE 802.1p和802.1Q，我們可以看到一些已經(jīng)為服務質(zhì)量（QoS）進一步開發(fā)所采納的重要理念。這些802.1特性還可以被引入更高一層的協(xié)議，如IP和ATM。
   

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