通信与电子信息工程专业英语Unit4 Expanding reading Asynchronous Transfer Mode.docx

1、通信与电子信息工程专业英语Unit4 Expanding reading Asynchronous Transfer ModeAsynchronous 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 net

2、works 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

3、 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

4、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

5、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

6、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 inheren

7、t 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 (del

8、ay 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 goo

9、d 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

10、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

11、 (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 /

12、 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T

13、1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 str

14、eam 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

15、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

16、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)

17、 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

18、 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 t

19、ransmission 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 ar

20、ound 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

21、 = 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 contenti

22、on 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

23、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 diffe

24、rent 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

25、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

26、。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 cel

27、ls 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 servic

28、e 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 lin

29、k 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 c

30、an 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S

31、ONET（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 syste

32、ms 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 physic

33、al 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