通信与电子信息工程专业英语Expanding_reading_Asynchronous_Transfer_Mode.docx

此文档收集于网络，如有侵权，请联系网站删除Asynchronous Transfer ModeAsynchronous Transfer Mode (ATM) is a switching technique for telecommunication networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from networks such as the Internet or Ethernet LANs that use variable sized packets or frames. ATM provides data link layer services that run over OSI Layer 1 physical links. ATM has functional similarity with both circuit switched networking and small packet switched networking. This makes it a good choice for a network that must handle both traditional high-throughput data traffic (e.g., file transfers), and real-time, low-latency content such as voice and video. ATM uses a connection-oriented model in which a virtual circuit must be established between two endpoints before the actual data exchange begins. ATM is a core protocol used over the SONET/SDH backbone of the Integrated Services Digital Network (ISDN).异步传输模式（ATM）是用于电信网络中的切换技术。它采用异步时分复用，并且它编码数据转换成小的，固定大小的细胞。这不同于网络，诸如使用可变大小的数据包或帧的因特网或以太网LAN。 ATM机提供了运行在OSI的第一层物理链路的数据链路层的服务。自动柜员机有功能的相似性与电路交换网络和小分组交换网络。这使得它对于必须同时处理传统的高吞吐量的数据流（如文件传输），以及实时，低延迟的内容，如语音和视频网络的好选择。 ATM使用其中一个虚电路必须在两个端点之间建立实际的数据交换开始之前，面向连接的模式。 ATM是使用过综合业务数字网（ISDN）的SONET / SDH骨干网核心协议。Consider a speech signal reduced to packets, and forced to share a link with bursty data traffic (traffic with some large data packets). No matter how small the speech packets could be made, they would always encounter full-size data packets, and under normal queuing conditions, might experience maximum queuing delays. That is why all packets, or cells, should have the same small size. In addition the fixed cell structure means that ATM can be readily switched by hardware without the inherent delays introduced by software switched and routed frames.考虑语音信号还原成数据包，并被迫（与一些大的数据包流量）分享与突发数据流量的链接。不管多么小的语音数据包可以做，他们总是遇到全尺寸的数据包，正常排队的条件下，可能会遇到的最大排队延迟。这就是为什么所有的数据包，或“细胞”应该具有相同的小尺寸。除了固定的细胞结构意味着ATM可以容易地通过硬件开关没有通过软件引入的固有延迟交换和路由帧。Thus, the designers of ATM utilized small data cells to reduce jitter (delay variance, in this case) in the multiplexing of data streams. Reduction of jitter (and also end-to-end round-trip delays) is particularly important when carrying voice traffic, because the conversion of digitized voice into an analog audio signal is an inherently real-time process, and to do a good job, the codec that does this needs an evenly spaced (in time) stream of data items. If the next data item is not available when it is needed, the codec has no choice but to produce silence or guess and if the data is late, it is useless, because the time period when it should have been converted to a signal has already passed.因此，ATM的设计者使用小的数据单元，以减少抖动（延迟变化，在这种情况下）中的数据流的多路复用。降低抖动（也最终到终端的往返时延）是承载语音流量时尤其重要，因为数字化的语音转换成模拟音频信号本身是一个实时的过程，做一个好工作，执行此需要的数据项的一个均匀分布的（在时间上）的流的编解码器。如果一个数据项不可用在需要的时候，该编解码器别无选择，只能产生沉默或猜测 - 如果数据已经晚了，这是没有用的，因为当它应该被转化成信号的时间段已经过去了。At the time of the design of ATM, 155 Mbit/s SDH (135 Mbit/s payload) was considered a fast optical network link, and many PDH links in the digital network were considerably slower, ranging from 1.544 to 45 Mbit/s in the USA, and 2 to 34 Mbit/s in Europe.在ATM的设计时，155 Mbit / s的SDH（135 Mbit / s的有效载荷）被认为是一个快速的光纤网络链路，数字网络在许多PDH链路都相当慢，从1.544到45 Mbit / s的中在美国和欧洲的2至34兆比特/秒。At this rate, a typical full-length 1500 byte (12000-bit) data packet would take 77.42 s to transmit. In a lower-speed link, such as a 1.544 Mbit/s T1 line, a 1500 byte packet would take up to 7.8 milliseconds.按照这个速度，一个典型的全长1500字节（12000位）的数据包将采取77.42微秒来传输。在低速链路，如1.544 Mbit / s的T1线路，1500字节的数据包会占用到7.8毫秒。A queuing delay induced by several such data packets might exceed the figure of 7.8 ms several times over, in addition to any packet generation delay in the shorter speech packet. This was clearly unacceptable for speech traffic, which needs to have low jitter in the data stream being fed into the codec if it is to produce good-quality sound. A packet voice system can produce this in a number of ways:甲排队延迟引起的几个这样的数据包可能会超过780毫秒的数字数倍以上，此外，在更短的语音分组的任何分组产生延迟。这显然是不能接受的用于语音通信，这需要具有低抖动的数据流中被送入解码器，如果它是产生高质量的声音。一种分组语音系统可以以多种方式产生这样的：Have a playback buffer between the network and the codec, one large enough to tide the codec over almost all the jitter in the data. This allows smoothing out the jitter, but the delay introduced by passage through the buffer would require echo cancellers even in local networks; this was considered too expensive at the time. Also, it would have increased the delay across the channel, and conversation is difficult over high-delay channels.有在网络和编解码器，足够大，以潮编解码器之间的再现缓冲过的几乎所有数据中的抖动。这允许平滑的抖动，但通过缓冲器通过通道引入的延迟，需要回声消除器，即使在本地网络;这被认为是当时太昂贵了。此外，这将增加整个信道的延迟，并且会话是难以通过高延迟信道。Build a system which can inherently provide low jitter (and minimal overall delay) to traffic which needs it.The design of ATM aimed for a low-jitter network interface. However, to be able to provide short queueing delays, but also be able to carry large datagrams, it had to have cells. ATM broke up all packets, data, and voice streams into 48-byte chunks, adding a 5-byte routing header to each one so that they could be reassembled later. The choice of 48 bytes was political rather than technical. When the CCITT was standardizing ATM, parties from the United States wanted a 64-byte payload because this was felt to be a good compromise in larger payloads optimized for data transmission and shorter payloads optimized for real-time applications like voice; parties from Europe wanted 32-byte payloads because the small size (and therefore short transmission times) simplify voice applications with respect to echo cancellation. Most of the European parties eventually came around to the arguments made by the Americans, but France and a few others held out for a shorter cell length. With 32 bytes, France would have been able to implement an ATM-based voice network with calls from one end of France to the other requiring no echo cancellation. 48 bytes (plus 5 header bytes = 53) was chosen as a compromise between the two sides. 5-byte headers were chosen because it was thought that 10% of the payload was the maximum price to pay for routing information. ATM multiplexed these 53-byte cells instead of packets. Doing so reduced the worst-case jitter due to cell contention by a factor of almost 30, minimizing the need for echo cancellers.建立一个系统，该系统可本质上的交通，需要的ATM后援的设计旨在用于低抖动的网络接口，提供低抖动（和最小的总延迟）。然而，为了能够提供短的排队延迟，也能够进行大的数据报，它必须有细胞。 ATM分手的所有数据包，数据和语音流为48字节的块，加入了5个字节的路由报头到每个人，让他们可以在以后重新组装。 48字节的选择是政治而不是技术。当被CCITT标准化的ATM，来自美国各方需要一个64字节的有效载荷，因为这是被认为是处于进行数据传输和语音类的实时应用优化的有效载荷短优化更大的有效载荷一个很好的妥协;来自欧洲各方希望32字节的有效载荷，因为体积小（因此短的传输时间）简化语音应用方面回声消除。大多数欧洲各方最终到来的时候由美国人提出的论点，但法国和其他几个人坚守了一个较短的电池长。具有32字节，法国就已经能够实现一个基于ATM的话音网络和不需要回波消除来自法国的一端到另一呼叫。 48字节（加上5头字节=53）被选为双方之间的妥协。被选中5个字节的头，因为它被认为有效载荷的10为最高代价的路由信息。 ATM复用这53个字节的细胞，而不是包。这样做减少了最坏情况下的抖动是由于细胞的争用的近30倍，从而最小化需要回声消除器。ATM supports different types of services via ATM Adaptation Layers (AAL). Standardized AALs include AAL1, AAL2, and AAL5, and the rarely used AAL3 and AAL4. AAL1 is used for constant bit rate (CBR) services and circuit emulation. Synchronization is also maintained at AAL1. AAL2 through AAL4 are used for variable bit rate (VBR) services, and AAL5 for data. Which AAL is in use for a given cell is not encoded in the cell. Instead, it is negotiated by or configured at the endpoints on a per-virtual-connection basis.ATM支持通过ATM适配层（AAL）不同类型的服务。标准化的AAL包括AAL1，AAL2和AAL5，而很少使用AAL3和AAL4。 AAL1是用于恒定比特率（CBR）业务和电路仿真。同步也保持在AAL1。通过AAL4 AAL2被用于可变比特率（VBR）业务和AAL5的数据。其中的AAL是在使用一个给定的细胞中没有细胞编码。相反，它是通过协商或者在每个虚拟连接的基础端点配置。Following the initial design of ATM, networks have become much faster. A 1500 byte (12000-bit) full-size Ethernet packet takes only 1.2 s to transmit on a 10 Gbit/s optical network, reducing the need for small cells to reduce jitter due to contention. Some consider that this makes a case for replacing ATM with Ethernet in the network backbone. However, it should be noted that the increased link speeds by themselves do not alleviate jitter due to queuing. Additionally, the hardware for implementing the service adaptation for IP packets is expensive at very high speeds. Specifically, at speeds of OC-3 and above, the cost of segmentation and reassembly (SAR) hardware makes ATM less competitive for IP than Packet Over SONET (POS); because of its fixed 48-byte cell payload, ATM is not suitable as a data link layer directly underlying IP (without the need for SAR at the data link level) since the OSI layer on which IP operates must provide an MTU of at least 576 bytes. SAR performance limits mean that the fastest IP router ATM interfaces are STM16 - STM64 which actually compares, while as of 2004 POS can operate at OC-192 (STM64) with higher speeds expected in the future.继ATM机的初步设计，网络已经成为快得多。 1500字节（12000位），全尺寸以太网数据包只需1.2微秒传送一个10 Gbit / s的光纤网络上，减少了对小细胞减少抖动因争。有人认为，这使得案件，适用于以太网取代ATM的网络骨干。然而，应该指出的是，增加的链路速度本身并不减轻抖动由于排队。此外，为实施业务适配为IP数据包的硬件价格昂贵，在非常高的速度。具体而言，在速度的OC-3及以上的，分段和重组的成本（特别行政区）的硬件，使ATM的IP数据包比在SONET（POS机）的竞争力下降;由于其固定的48字节的信元有效载荷，ATM是不适合作为直接置于下面的IP（无需SAR在数据链路层）的数据链路层，因为IP操作，必须提供至少576的MTU在其上的OSI层字节。 SAR性能的限制意味着，最快的IP路由器，ATM接口STM16 - STM64实际上比较，而在2004年的POS机可以预计，在未来更高的速度在OC-192（STM64）操作。On slower or congested links (622Mbit/s and below), ATM does make sense, and for this reason most ADSL systems use ATM as an intermediate layer between the physical link layer and a Layer 2 protocol like PPP or Ethernet.在较慢或拥塞链路（为622Mbit/ s和下文），ATM确实是有意义的，因为这个原因最ADSL系统使用ATM作为物理链路层和第2层协议像PPP或以太网之间的中间层。At these lower speeds, ATM provides a useful ability to carry multiple logical circuits on a single physical or virtual medium, although other techniques exist, such as Multi-link PPP and Ethernet VLANs, which are optional in VDSL implementations. DSL can be used as an access method for an ATM network, allowing a DSL termination point in a telephone central office to connect to many internet service providers across a wide-area ATM network. In the United States, at least, this has allowed DSL providers to provide DSL access to the customers of many internet service providers. Since one DSL termination point can support multiple ISPs, the economic feasibility of DSL is substantially improved.在这些较低的速度，自动柜员机提供携带多个逻辑电路的单一实体或虚拟介质上的有用的能力，虽然其他技术存在，如多链路PPP和以太网的VLAN，这是可选的VDSL实现。 DSL可以被用作一个用于ATM网络的接入的方法，使得在电话中心局的是DSL终止点跨越广域ATM网络连接到多个互联网服务提供商。在美国，至少，这使得DSL提供商提供的DSL接入许多互联网服务供应商的客户。由于一个DSL终止点可以支持多个互联网服务供应商，DSL的经济可行性得到大大改善。Why virtual circuits?ATM operates as a channel-based transport layer, using Virtual circuits (VCs). This is encompassed in the concept of the Virtual Paths (VP) and Virtual Channels. Every ATM cell has an 8- or 12-bit Virtual Path Identifier (VPI) and 16-bit Virtual Channel Identifier (VCI) pair defined in its header. Together, these identify the virtual circuit used by the connection. The length of the VPI varies according to whether the cell is sent on the user-network interface (on the edge of the network), or if it is sent on the network-network interface (inside the network).为什么虚拟电路？ATM作为一个基于通道的传输层，使用虚拟电路（VC）。这也包括在虚拟路径（VP）和虚通道的概念。每个ATM信元有一个8位或12位的虚拟路径标识符（VPI）及16位的虚拟信道标识符（VCI）对在它的报头中定义。总之，这些识别连接所使用的虚电路。VPI的长度根据该小区是否被发送的用户 - 网络接口上（在网络的边缘）而变化，或者如果它被发送到网络 - 网络接口上（网络内）。As these cells traverse an ATM network, switching takes place by changing the VPI/VCI values (label swapping). Although the VPI/VCI values are not necessarily consistent from one end of the connection to the other, the concept of a circuit is consistent (unlike IP, where any given packet could get to its destination by a different route than the others).因为这些细胞穿越ATM网络中，交换通过改变VPI / VCI值（标签交换）发生。虽然VPI / VCI值不一定从连接到其它的一端一致，电路的概念是一致的（不像IP，其中任何给定的数据包可以到达它的目的地由不同的路线比其他的）Another advantage of the use of virtual circuits comes with the ability to use them as a multiplexing layer, allowing different services (such as voice, Frame Relay, n* 64 channels, IP)using cells and virtual circuits for traffic engineering.采用虚电路的另一个优势来自与使用它们的复用层的能力，允许不同的业务（如语音，帧中继，N *64个信道，一个IP）使用电池和虚电路的流量工程。Another key ATM concept involves the traffic contract. When an ATM circuit is set up each switch on the circuit is informed of the traffic class of the connection.另一个关键ATM概念涉及到交通的合同。当ATM电路设置在电路上的每个开关被告知通信类连接的。ATM traffic contracts form part of the mechanism by which Quality of Service (QoS) is ensured. There are four basic types (and several variants) which each have a set of parameters describing the connection.ATM业务合同形成由“服务质量”（服务质量）保证的机制的一部分。有四种基本类型（和几个变种），其中各有一组描述连接参数CBR - Constant bit rate: a Peak Cell Rate (PCR) is specified, which is constant.VBR - Variable bit rate: an average cell rate is specified, which can peak at a certain level for a maximum interval before being problematic.ABR - Available bit rate: a minimum guaranteed rate is specified.UBR - Unspecified bit rate: traffic is allocated to all remaining transmission capacity.VBR has real-time and non-real-time variants, and serves for bursty traffic. Non-real-time is usually abbreviated to vbr-nrt.CBR - 恒定比特率：峰值信元速率（PCR）是指定的，这是不变的。 VBR - 可变比特率：指定的平均信元速率，可在一定程度是有问题的前高峰期的最大间隔。 ABR - 可用比特率：规定的最低保证率。 UBR - 未指定比特率：流量分配给所有剩余的传输容量。 VBR具有实时的和非实时的变体，并用于“突发”的流量。非实时的，通常缩写为VBR-NRT。Most traffic classes also introduce the concept of Cell Delay Variation Tolerance (CDVT), which defines the clumping of cells in time.大多数交通类也引进细胞时延抖动容限（CDVT），它定义在时间单元的“聚集”的概念。 To maintain traffic contracts, networks usually use shaping, a combination of queuing and marking of cells. Policing generally enforces traffic contracts.Traffic shaping usually takes place at the entry point to an ATM network and attempts to ensure that the cell flow will meet its traffic contract.To maintain network performance, networks may police virtual circuits against their traffic contracts. If a circuit is exceeding its traffic contract, the network can either drop the cells or mark the Cell Loss Priority (CLP) bit (to identify a cell as discardable farther down the line). Basic policing works on a cell by cell basis, but this is sub-optimal for encapsulated packet traffic (as discarding a single cell will invalidate the whole packet). As a result, schemes such as Partial Packet Discard (PPD) and Early Packet Discard (EPD) have been created that will discard a whole series of cells until the next frame starts. This reduces the number of useless cells in the network, saving bandwidth for full frames. EPD and PPD work with AAL5 connections as they use the frame end bit to detect the end of packets. ATM can build virtual circuits and virtual paths either statically or dynamically. Static circuits (permanent virtual circuits or PVCs) or paths (permanent virtual paths or PVPs) require that the provisioner must build the circuit as a series of segments, one for each pair of interfaces through which it passes.为了维持交通的合同，网络通常使用“塑造”，排队的组合，细胞的标记。 “警务”通常是强制交通contracts.Traffic通常成形发生在入口点到一个ATM网络，并尝试以确保电池流将满足其业务contract.To维持网络性能，网络可能警察对他们的业务合同的虚电路。如果电路超过其流量的合同，该网络既可以丢弃电池或标记信元丢失优先级（CLP）位（识别为可放弃越往下行的单元格）。基层警务工作在细胞通过细胞的基础上，但这是次优的封装数据包的流量（如废弃的单个单元格将整个数据包无效）。其结果是，计划，如部分分组丢弃（PPD）和分组抛弃（环保署）已创建将放弃一系列的细胞，直到下一帧开始。这减少了无用的细胞的数目在网络中，节省带宽为全帧。 EPD和PPD工作与AAL5连接，因为它们使用的帧结束比特，以检测数据包的末尾。 ATM可静态或动态建立虚电路和虚拟路径。静态电路（永久虚拟电路或虚电路）或路径（永久虚拟路径或PVP）规定该置备必须建立在电路为一系列段，每一个对通过它传递的接口。PVPs and PVCs, though conceptually simple, require significant effort in large networks. They also do not support the re-routing of service in the event of a failure. Dynamically built PVPs (soft PVPs or SPVPs) and PVCs (soft PVCs or SPVCs), in contrast, are built by specifying the characteristics of the circuit (the service contract) and the two endpoints.PVP和PVC，虽然概念上很简单，需要在大型网络显著努力。它们也并不支持重路由服务中的故障的事件。动态创建的PVP（软PVP或SPVPs）和虚电路（软PVC或SPVCs），与此相反，通过指定的线路的特性（该服务“合同”）和两个端点建立。Finally, ATM networks build and tear down switched virtual circuits (SVCs) on demand when requested by an end piece of equipment. One application for SVCs is to carry individual telephone calls when a network of telephone switches are inter-connected by ATM. SVCs were also used in attempts to replace local area networks with ATM.最后，ATM网络建设和拆除交换虚电路（SVC）需求时，由最终的设备要求。对SVC的一个应用是进行单个电话时电话交换机网络是相互连接的自动柜员机。储存虚拟化控制器还用于在尝试与ATM取代局域网。Most ATM networks supporting SPVPs, SPVCs, and SVCs use the Private Network Node Interface or Private Network-to-Network Interface (PNNI) protocol. PNNI uses the same shortest-path-first algorithm used by OSPF and IS-IS to route IP packets to share topology information between switches and select a route through a network. PNNI also includes a very powerful summarization mechanism to allow construction of very large networks, as well as a call admission control (CAC) algorithm that determines whether sufficient bandwidth is available on a proposed route through a network to satisfy the service requirements of a VC or VP.大多数ATM网络支持SPVPs，SPVCs和svcs使用专用网络节点接口或专用网络到网络接口（PNNI）协议。 PNNI使用使用OSPF协议相同的最短路径优先算法和IS-IS路由IP数据包的交换机之间共享的拓扑信息，并选择通过网络的路由。 PNNI还包括一个非常强大的聚合机制，以允许施工非常大的网络中，以及一个呼叫接纳控制（CAC）的算法，用于确定足够的带宽是否可用提议的路线上通过网络来满足一个VC或服务要求副总裁。A network must establish a connection before two parties can send cells to each other. In ATM this is called a virtual circuit (VC). It can be a permanent virtual circuit (PVC), which is created administratively on the endpoints, or a switched virtual circuit (SVC), which is created as needed by the communicating parties. SVC creation is managed by signaling, in which the requesting party indicates the address of the receiving party, the type of service requested, and whatever traffic parameters may be applicable to the selected service. Call admission is then performed by the network to confirm that the requested resources are available and that a route exists for the connection.网络必须建立双方之前的连接可以发送单元彼此。在ATM中这被称为虚拟电路（VC）。它可以是一个永久虚电路（PVC），其被管理员创建的端点上，或一个交换虚拟电路（SVC），其根据需要，通过通信方创建。 SVC的创建是通过信令，其中，所述请求方指示接收方，请求的服务的类型的地址进行管理，任何流量参数可以是适用于所选择的服务。 “呼叫准入”由网络然后进行以确认所请求的资源可用以及是否存在路由的连接。Asynchronous Transfer Mode (ATM) is a high-performance, cell-oriented switching and multiplexi