SONET标准详解：同步光网络基础

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"Sonet标准入门指南" Sonet（Synchronous Optical Network）是一种同步光网络标准，主要用于电信领域，提供高效、可靠的数据传输服务。这个标准最初由美国贝尔通信研究公司（Bellcore）制定，后来被国际电报电话咨询委员会（ITU-T）采纳，成为全球广泛使用的光纤通信体系之一。 **什么是SONET？** SONET是一种同步传输框架，旨在统一不同类型的数字通信系统，确保数据在长途传输中的稳定性和同步性。这个标准不仅规范了物理层的传输，还涵盖了网络的复用、分接和管理等高层功能。 **SONET的背景** 随着数字通信的发展，各种不同的数据传输速率和帧格式导致了兼容性和互操作性的问题。为了克服这些挑战，SONET应运而生，它定义了一种标准化的方法，将各种速度的信号（如T1，E1等）复用到单一的高带宽光信号中，同时提供了同步传输的能力。 **数字信号的同步** 在通信网络中，同步是确保不同设备之间数据正确传输的关键。SONET通过使用一个全局时钟来实现同步，使得多个独立的通信链路可以在同一时间框架内协调工作，提高了整体系统的效率。 **基本的SONET信号** 基础的SONET信号被称为STS-1（Synchronous Transport Signal - Level 1），传输速率为51.840 Mbps。STS-1信号可以进一步复用成更高层次的信号，如STS-3（STS-1的三个复用体，速率为155.520 Mbps），甚至更高级别的SONET层次。 **为何要同步？** 同步的优势在于减少了数据处理的复杂性，降低了信号失真的可能性，并允许在不增加额外开销的情况下高效地复用多个较低速率的信号。 **同步与异步的区别** 同步传输意味着所有信号共享一个时钟源，而异步传输则每个信号都有自己的时钟源。同步传输提供了更稳定的传输环境，尤其对于大规模的网络至关重要。 **同步层次** SONET定义了一个层次化的同步架构，包括STS-1、STS-3、STS-12、STS-48、STS-192等层次，允许灵活地组合和拆分不同级别的信号，以适应不同需求。 **帧格式结构** 每个SONET帧都是由固定数量的比特组成，具有明确的帧头和帧尾，用于错误检测和同步。例如，STS-1帧由9行和270列组成，总共包含2430个8位字节。 **STS-1作为构建块** STS-1是SONET的基本单元，其帧结构包含了同步开销（SOH）、段开销（SO）和净荷（payload）。这些开销部分用于网络管理和故障检测。通过理解这些基本概念，读者可以深入探索SONET的工作原理，包括其在光网络中的应用、多路复用技术、故障恢复机制以及与其他通信标准（如SDH）的关系。对于那些需要了解更多详细信息的人，建议参考贝尔通信和ITU-T的最新标准文档，以及附带的专业词汇表，以更好地掌握SONET通信的术语和细节。

www.tektronix.com/optical

Why Synchronize?

Synchronous versus Asynchronous

Traditionally, transmission systems have been asynchronous, with each

terminal in the network running on its own clock. In digital transmission,

“clocking” is one of the most important considerations. Clocking means

using a series of repetitive pulses to keep the bit rate of data constant

and to indicate where the ones and zeroes are located in a data stream.

Since these clocks are totally free-running and not synchronized, large

variations occur in the clock rate and thus the signal bit rate. For exam-

ple, a DS3 signal specified at 44.736 Mb/s + 20 ppm (parts per million)

can produce a variation of up to 1789 bps between one incoming DS3

and another.

Asynchronous multiplexing uses multiple stages. Signals such as asyn-

chronous DS1s are multiplexed, extra bits are added (bit-stuffing) to

account for the variations of each individual stream, and are combined

with other bits (framing bits) to form a DS2 stream. Bit-stuffing is used

again to multiplex up to DS3. DS3s are multiplexed up to higher rates

in the same manner. At the higher asynchronous rate, they cannot be

accessed without demultiplexing.

In a synchronous system, such as SONET, the average frequency of all

clocks in the system will be the same (synchronous) or nearly the same

(plesiochronous). Every clock can be traced back to a highly stable refer-

ence supply. Thus, the STS-1 rate remains at a nominal 51.84 Mb/s,

allowing many synchronous STS-1 signals to be stacked together when

multiplexed without any bit-stuffing. Thus, the STS-1s are easily

accessed at a higher STS-N rate.

Low-speed synchronous virtual tributary (VT) signals are also simple to

interleave and transport at higher rates. At low speeds, DS1s are trans-

ported by synchronous VT-1.5 signals at a constant rate of 1.728 Mb/s.

Single-step multiplexing up to STS-1 requires no bit stuffing and VTs are

easily accessed.

Pointers accommodate differences in the reference source frequencies

and phase wander, and prevent frequency differences during synchro-

nization failures.

Synchronization Hierarchy

Digital switches and digital cross-connect systems are commonly

employed in the digital network synchronization hierarchy. The network

is organized with a master-slave relationship with clocks of the higher-

level nodes feeding timing signals to clocks of the lower-level nodes.

All nodes can be traced to a primary reference source, a Stratum 1

atomic clock with extremely high stability and accuracy. Less stable

clocks are adequate to support the lower nodes.

Synchronizing SONET

The internal clock of a SONET terminal may derive its timing signal from a

Building Integrated Timing Supply (BITS) used by switching systems and other

equipment. Thus, this terminal will serve as a master for other SONET nodes,

providing timing on its outgoing OC-N signal. Other SONET nodes will operate

in a slave mode called “loop timing” with their internal clocks timed by the

incoming OC-N signal. Current standards specify that a SONET network must

be able to derive its timing from a Stratum 3 or higher clock.

SONET Telecommunications Standard

Primer

Frame Format Structure

SONET uses a basic transmission rate of STS-1 – equivalent to 51.84

Mb/s. Higher-level signals are integer multiples of the base rate. For

example, STS-3 is three times the rate of STS-1 (3 x 51.84 = 155.52

Mb/s). An STS-12 rate would be 12 x 51.84 = 622.08 Mb/s.

STS-1 Building Block

The frame format of the STS-1 signal is shown in Figure 1. In general,

the frame can be divided into two main areas: Transport overhead and

the Synchronous Payload Envelope (SPE).

The synchronous payload envelope can also by divided into two parts:

STS path overhead and the payload. The payload is the revenue-produc-

ing traffic being transported and routed over the SONET network. Once

the payload is multiplexed into the synchronous payload envelope, it can

be transported and switched through SONET without having to be exam-

ined and possibly demultiplexed at intermediate nodes. Thus, SONET is

said to be service-independent or transparent.

Transport Overhead is composed of section overhead and line overhead.

The STS-1 path overhead is part of the synchronous payload envelope.

The STS-1 payload has the capacity to transport up to:

28 DS1s

1 DS3

21 2.048 Mb/s signals or combinations of the above.

STS-1 Frame Structure

STS-1 is a specific sequence of 810 bytes (6480 bits), which includes

various overhead bytes and an envelope capacity for transporting pay-

loads. It can be depicted as a 90 column by 9 row structure. With a

frame length of 125 µs (8000 frames per second), STS-1 has a bit rate

of 51.840 Mb/s. The order of transmission of bytes is row-by-row from

top to bottom, left to right (most significant bit first).

As shown in Figure 1, the first three columns of the STS-1 frame are

for the Transport Overhead. The three columns each contain nine bytes.

Of these, nine bytes are overhead for the Section layer (for example,

Section Overhead), and 18 bytes are overhead for the Line layer (for

example, Line Overhead). The remaining 87 columns constitute the

STS-1 Envelope Capacity (payload and path overhead).

As stated before, the basic signal of SONET is the Synchronous

Transport Signal level 1, or STS-1. The STS frame format is composed

of 9 rows of 90 columns of 8-bit bytes, or 810 bytes. The byte trans-

mission order is row-by-row, left to right. At a rate of 8000 frames

per second, that works out to a rate of 51.840 Mb/s, as the following

equation demonstrates:

9 x 90 bytes/frame x 8 bits/byte x 8000 frames/s =

51,840,000 bits/s = 51.840 Mb/s

This is known as the STS-1 signal rate – the electrical rate used

primarily for transport within a specific piece of hardware. The optical

equivalent of STS-1 is known as OC-1, and it’s used for transmission

across the fiber.

SONET Telecommunications Standard

Primer

www.tektronix.com/optical

Figure 1. STS-1 frame format.

B B B 87B

125

Transport

Overhead

Synchronous Payload Envelope

µs

B = an 8-bit byte

Figure 2. STS-1 frame elements.

90 Columns

Transport

Overhead

9 Rows

STS-1 Synchronous

Payload Envelope

87 Columns

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