ATM信元結構 ATM是一種異步轉移模式。異步是指ATM統計復用的性質。轉移模式是指網路中所採用的復用、交換、傳輸技術，即信息從一地轉移到另一地所用的傳遞方式。在這種轉移模式中，信息被組織成信元（CELL），來自某用戶信息的各個信元不需要周期性地出現。因此，ATM就是一種在網路中以信元為單位進行統計復用和交換、傳輸的技術。 英文詳解 中文解釋
Table of Contents,OAM Cell Structure,LANE Data Frame,中文解釋,
Table of Contents
ATM Cell Structures
Formats of the ATM Cell Header
OAM Cell Structure
Generic Identifier Transport IE Used by Signaling
LANE Data Frame
ATM Cell StructuresThis appendix describes the various ATM cell types and their configurations and includes the following sections:
Formats of the ATM Cell Header
OAM Cell Structure
Generic Identifier Transport IE Used by signaling
LANE Data Frame
Formats of the ATM Cell HeaderThe ATM standards groups have defined two header formats. The User-Network Interface (UNI) header format is defined by the UNI specification, and the Network-Node Interface (NNI) header format is defined by the NNI specification.
The UNI specification defines communications between ATM endpoints (such as workstations and routers) and ATM switch routers in private ATM networks. The format of the UNI cell header is shown in Figure B-1.
Figure B-1: UNI Header Format
The UNI header consists of the following fields:
GFC---4 bits of generic flow control that are used to provide local functions, such as identifying multiple stations that share a single ATM interface. The GFC field is typically not used and is set to a default value.
VPI---8 bits of virtual path identifier that is used, in conjunction with the VCI, to identify the next destination of a cell as it passes through a series of ATM switch routers on its way to its destination.
VCI---16 bits of virtual channel identifier that is used, in conjunction with the VPI, to identify the next destination of a cell as it passes through a series of ATM switch routers on its way to its destination.
PT---3 bits of payload type. The first bit indicates whether the cell contains user data or control data. If the cell contains user data, the second bit indicates congestion, and the third bit indicates whether the cell is the last in a series of cells that represent a single AAL5 frame.
CLP---1 bit of congestion loss priority that indicates whether the cell should be discarded if it encounters extreme congestion as it moves through the network.
HEC---8 bits of header error control that are a checksum calculated only on the header itself.
The NNI specification defines communications between ATM switch routers. The format of the NNI header is shown in Figure B-2.
Figure B-2: NNI Header Format
The GFC field is not present in the format of the NNI header. Instead, the VPI field occupies the first 12 bits, which allows ATM switch routers to assign larger VPI values. With that exception, the format of the NNI header is identical to the format of the UNI header.
OAM Cell Structure
Operation, Administration, and Maintenance (OAM) performs standard loopback (end-to-end or segment) and fault detection and notification (alarm indication signal [AIS] and remote defect identification [RDI]) for each connection. It also maintains a group of timers for the OAM functions. When there is an OAM state change such as loopback failure, OAM software notifies the connection management software. You can enable or disable OAM operation for the following switch router components:
The entire switch router
A specific ATM interface
Each ATM connection
Figure B-3 shows the format of the OAM loopback cell.
Figure B-3: OAM Cell Structure
The OAM cell structure has the following features:
OAM cell type is coded as 0001.
OAM function type is coded as 0010.
350 bits that are specific to the OAM type are divided into the following elements:
Loopback indicator---A bit that is set to 1 before the cell is looped back. The loopback node then sets the bit to 0, indicating it has been looped back.
Correlation tag---Identifies (correlates) related OAM cells within the same connection.
Loopback location ID---An optional field that identifies the site that is to loopback the cell.
Source ID---An optional field that identifies the site generating the cell.
Generic Identifier Transport IE Used by Signaling The generic identifier transport information element (IE) is used by signaling to carry an identifier between two users.
Figure B-4 shows the format of the generic signaling IE.
Figure B-4: Generic Identifier Transport IE Used by signaling
The generic identifier transport IE used by signaling has the following fields:
Generic identifier transport information IE.
Ext.
Coding standard.
Flag.
Reserved.
IE instruction action Indication.
Length of generic indentifier transport IE.
Identifier related standard/application---Each application requiring a different set or structure of identifiers (coded in octet 6 and possibly in subsequent octet groups) should use a different value of octet 5.
Identifier type---This value is independent of the identifier related standard/application field, octet 5. The maximum length is 20 octets.
Identifier length---A binary number indicating the length in octets of the identifier code in the subsequent octets of the octet group.
Identifier value---Value of an identifier according to the recommendation or the standard identifier in octet 5.
LANE Data Frame
The LAN emulation data frame for Ethernet is based on ISO 8802.3/CSMA-CD (IEEE 802.3) and is used to provide connectivity between ATM attached end systems and LAN attached stations.
Figure B-5 shows the format of the LANE data frame.
Figure B-5: LANE Data Frame Format for IEEE 802.3/Ethernet
The LANE data frame has the following fields:
LE header---Contains either the LAN emulation client identifier value, the sending client, or X'0000'.
Destination address.
Source address.
Type information---Logical link control (LLC) data frames whose total length, including the LLC field and data, but not including padding required to meet minimum data frame length, is less than 1536 (X"0600"). It must be encoded by placing the length value in the type/length field. LLC data frames longer than the maximum must be encoded by placing the value 0 in the type/length field.
Information---Encapsulated Ethernet data.
中文解釋
通用流控制
通用流控制（GFC）欄位是一個4位欄位，最初加入支持的ATM網路連線共享的接入網路，如分散式佇列雙匯流排（DQDB）環。 這次全球金融危機領域的目的是給用戶網路接口（UNI），4位在談判各種ATM連線的細胞間的復用和流量控制。 然而，這次全球金融危機領域的使用和精確值沒有被標準化，和外地總是設定為0000。
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虛擬路徑標識符
虛擬路徑標識符（VPI）的定義為這個特定的單元格的虛擬路徑。 在交換虛電路（SVC）連線的連線設定過程中發現一個特定的虛擬通道連線VPIS和永久虛電路（PVC）連線手動配置。 在UNI，8位的VPI長度允許多達256個不同的虛擬路徑。 存在的VPI 0默認情況下所有ATM設備上，如信號來創建和刪除動態ATM連線的行政目的。
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虛擬通道標識符
虛擬通道標識符（VCI）定義為這個特定的細胞內指定的虛擬路徑的虛擬通道。 正如與VPIS，VCIs也發現在交換虛電路（SVC）連線的連線設定過程和永久虛電路（PVC）連線手動配置。 VCI的長度為16位允許多達65,536個不同的虛擬通道，每個虛擬路徑。 VCIs 0到15是由國際電信聯盟（ITU）和VCIs from16 32個是由ATM論壇（每個虛擬的路徑）保留保留。 這些保留VCIs用於信號，操作和維護，以及資源管理。
為每個傳輸路徑（即，每根電纜或連線到ATM交換機）是唯一的VPI / VCI組合。 然而，兩個不同的虛電路，一個ATM交換機上的兩個不同的連線埠可以有沒有衝突的相同的VPI / VCI值。
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有效載荷類型指示
有效載荷類型指標（PTI）是一個3位的領域。 它的位使用如下：
第一個位表示如下ATM信元的類型。 第一個位設定為0表示用戶數據設定為1位表示操作，行政及管理（OA&M）的數據。
第二位表示是否細胞經歷了從源到目的地的旅程中的交通擠塞。 該位也被稱為顯式前向擁塞指示（EFCI）位。 第二位設定為0，由源，如果臨時開關經驗的擠塞情況，而路由的細胞，它設定為1位。 路徑中的所有其他交換機後，它被設定為1，離開這個位值，在1。
目的地的ATM終端可以使用的EFCI位，實施流量控制機制，直到細胞與EFCI位設定為0，接收的油門上的傳輸速率。
第三位表示用戶的ATM細胞的AAL5在一個塊中的最後一個單元格。 對於非用戶的ATM信元，第三位是用於OA&M功能。
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信元丟失優先權
信元丟失優先權（CLP）欄位是1位的領域作為優先指標。 當它被設定為0，細胞是高度重視和臨時開關必須盡一切努力向前細胞成功。 當電位設定為1，臨時開關有時丟棄在擁塞情況下的細胞。 中電放棄資格（DE）在幀中繼位位是非常相似。
一個ATM端點設定CLP位為1，創建一個細胞時表示低優先權的細胞。 ATM交換機可以設定為1的電，如果單元格超過虛擬通道連線的協商參數。 這是類似上述承諾信息速率（CIR），幀中繼爆破。
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頭錯誤校驗
頭錯誤校驗（HEC）欄位是一個8位的域，允許一個ATM交換機或ATM端點糾正單比特錯誤，或在第4個位元組的ATM頭多比特錯誤檢測。 多比特誤碼細胞被靜靜的丟棄。 港燈只檢查了ATM頭，而不是在ATM有效載荷。 檢查錯誤的有效載荷是上層協定的責任。
