ATM信元结构 ATM是一种异步转移模式。异步是指ATM统计复用的性质。转移模式是指网络中所采用的复用、交换、传输技术，即信息从一地转移到另一地所用的传递方式。在这种转移模式中，信息被组织成信元（CELL），来自某用户信息的各个信元不需要周期性地出现。因此，ATM就是一种在网络中以信元为单位进行统计复用和交换、传输的技术。 英文详解 中文解释

Table of Content(ATM Cell Structure)

OAM Cell Structure

LANE Data Frame

中文解释(通用流控制 虚拟路径标识符 虚拟通道标识符 有效载荷类型指示 信元丢失优先权 头错误校验)

Table of Content

ATM Cell Structure

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.

中文解释

通用流控制

通用流控制（GFC）字段是一个4位字段，最初加入支持的ATM网络连接共享的接入网络，如分布式队列双总线（DQDB）环。 这次全球金融危机领域的目的是给用户网络接口（UNI），4位在谈判各种ATM连接的细胞间的复用和流量控制。 然而，这次全球金融危机领域的使用和精确值没有被标准化，和外地总是设置为0000。

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虚拟路径标识符

虚拟路径标识符（VPI）的定义为这个特定的单元格的虚拟路径。 在交换虚电路（SVC）连接的连接设置过程中发现一个特定的虚拟通道连接VPIS和永久虚电路（PVC）连接手动配置。 在UNI，8位的VPI长度允许多达256个不同的虚拟路径。 存在的VPI 0默认情况下所有ATM设备上，如信号来创建和删除动态ATM连接的行政目的。

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虚拟通道标识符

虚拟通道标识符（VCI）定义为这个特定的细胞内指定的虚拟路径的虚拟通道。 正如与VPIS，VCIs也发现在交换虚电路（SVC）连接的连接设置过程和永久虚电路（PVC）连接手动配置。 VCI的长度为16位允许多达65,536个不同的虚拟通道，每个虚拟路径。 VCIs 0到15是由国际电信联盟（ITU）和VCIs from16 32个是由ATM论坛（每个虚拟的路径）保留保留。 这些保留VCIs用于信号，操作和维护，以及资源管理。

VPI和VCI值的组合标识为一个指定的ATM信元的虚电路。 的VPI / VCI组合提供了ATM的转发信息，ATM交换机的使用转发到其目的地的细胞。 的VPI / VCI组合并不如IP或IPX网络地址的网络层地址。

的VPI / VCI组合作为一个虚电路的本地标识符是类似的逻辑信道号在X.25的数据链路连接标识符（DLCI在帧中继）。 在任何特定的ATM终端或交换机的VPI / VCI唯一标识一个虚电路到下一个ATM终端或交换机。 不需要匹配的VPI / VCI对最终目的地的ATM端点使用的VCI / VPI。

为每个传输路径（即，每根电缆或连接到ATM交换机）是唯一的VPI / VCI组合。 然而，两个不同的虚电路，一个ATM交换机上的两个不同的端口可以有没有冲突的相同的VPI / VCI值。

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有效载荷类型指示

有效载荷类型指标（PTI）是一个3位的领域。 它的位使用如下：

第一个位表示如下ATM信元的类型。 第一个位设置为0表示用户数据设置为1位表示操作，行政及管理（OA&M）的数据。

第二位表示是否细胞经历了从源到目的地的旅程中的交通挤塞。 该位也被称为显式前向拥塞指示（EFCI）位。 第二位设置为0，由源，如果临时开关经验的挤塞情况，而路由的细胞，它设置为1位。 路径中的所有其他交换机后，它被设置为1，离开这个位值，在1。

目的地的ATM终端可以使用的EFCI位，实施流量控制机制，直到细胞与EFCI位设置为0，接收的油门上的传输速率。

第三位表示用户的ATM细胞的AAL5在一个块中的最后一个单元格。 对于非用户的ATM信元，第三位是用于OA&M功能。

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信元丢失优先权

信元丢失优先级（CLP）字段是1位的领域作为优先指标。 当它被设置为0，细胞是高度重视和临时开关必须尽一切努力向前细胞成功。 当电位设置为1，临时开关有时丢弃在拥塞情况下的细胞。 中电放弃资格（DE）在帧中继位位是非常相似。

一个ATM端点设置CLP位为1，创建一个细胞时表示低优先级的细胞。 ATM交换机可以设置为1的电，如果单元格超过虚拟通道连接的协商参数。 这是类似上述承诺信息速率（CIR），帧中继爆破。

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头错误校验

头错误校验（HEC）字段是一个8位的域，允许一个ATM交换机或ATM端点纠正单比特错误，或在第4个字节的ATM头多比特错误检测。 多比特误码细胞被静静的丢弃。 港灯只检查了ATM头，而不是在ATM有效载荷。 检查错误的有效载荷是上层协议的责任。