14-08-2012, 04:00 PM
SONET (Synchronous Optical Network)
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INTRODUCTION
SONET (Synchronous Optical Network) is a standard for optical data transport. It defines optical signals and a synchronous frame structure for multiplexed digital traffic. It is a major contender as the physical transport layer for the next decade. SONET is an International standard which has been adopted in the United States, Europe and in Japan. It provides necessary bandwidth for today’s networks and can grow to accommodate the networks of the future. It provides advanced fault tolerance and survivability. The existing digital hierarchy in the United States suffered from a deficiency of overhead information to manage the network. SONET provides advanced OAM&P (Operations, Administration, Maintenance and Provisioning) features. SONET is a powerful, highly scalable technology. SONET is the North American standard for telecommunications transmission using fibrotic cables. SONET provides a set of protocols for the management and control of high bandwidth optical networking transmission. The SONET standard includes definitions for a multiplexing structure, optical parameters, service mappings, and network management (operations) support for existing and future services. The SONET technology section of this document makes up the body of this document. SONET is discussed from the viewpoint of the User Device into the SONET Multiplexer into a SONET ring and its operational aspect.
MOTIVATION
The existing digital multiplexing system is asynchronous at DS3 and lower levels. Asynchronous There is a growing trend towards using optical fibre media for data transport. Optical fibre has the following advantages over coaxial cable, twisted-pair and microwave networks.
• Optical fibre offers a much higher bandwidth
• Optical fibre can be carried over longer distances without repeaters (regenerators)
• Fibre is immune to interference
• Fibre is more secure. It is hard to tap and unwanted tapping can be detected
• Fibre is cheaper to maintain in the long run
• A small strand of fibre can offer enormous bandwidth. Fibre is also very light-weight
• Bandwidth upgrades can be made on the same fibre
• It has very low bit error rates (BER)
The existing North American Digital Hierarchy was created to carry digitized voice over twisted wire. Each level in the hierarchy is called a digital stream (DS). The lower level digital streams are multiplexed into the higher digital streams. The lowest level in the hierarchy is DS0 that carries a single voice channel.
Stream name Bit rate Structure Number of DSOs
DSO 64 Kbps 1 voice channel 1
DS1 1.544Mbps 24 x DSO 24
DS3 44.736Mbps 28 x DS1 672
Table1. Digital Stream Data Rates
The European and Japanese digital hierarchies are closely related to this structure. SONET is a recreation of the digital transmission hierarchy with a whole family of Optical Carriers (OC levels) running at speeds ranging from 51.84 Mbps in the US and 155.52 Mbps in Europe to about 9.9 Gbps.
multiplexing uses multiple stages. For example, when DS0 signals are multiplexed into a DS1 stream extra bits (bit-stuffing) are added to account for variations in the individual streams. When the DS1 stream is multiplexed into a DS-3 stream, bit-stuffing is used again. At this level, it is not possible to recover the DS0 without first demultiplexing the DS1 signal and the demultiplexing the DS0 out of the demultiplexed DS1 signal.
SYNCHRONOUS VS. ASYNCHRONOUS
SONET stands for Synchronous Optical Network. Synchronous used in this context refers to the multiplexing method used to combine channels on SONET. Synchronous multiplexing in SONET is achieved by ensuring that all the input channels into the multiplexer have clocks that are synchronized to a certain level of tolerance. SONET uses byte-interleaved multiplexing at all levels. The control information to separate the channels is inserted into the channels before they are multiplexed. Other than this there is no other overhead. For example, an STS-3 (Synchronous Transport Signal level) / OC-3 channel is combined by multiplexing three STS-1/OC-1 channels. The Line Rate of an STS-1 channel is 51.84 Mbps and the line rate of an STS-3 channel is 155.52 Mbps which exactly three times the input stream. Thus aggregating channels into higher levels add no additional overhead. In a set of synchronous signals, the digital transitions in the signals occur at exactly the same rate. There may, however, be a phase difference between the transitions of the two signals, and this would lie within specified limits. These phase differences may be due to propagation delays or jitter introduced into the transmission network. In a synchronous network, all the clocks are traceable to one primary reference clock (PRC) In the case of asynchronous signals, the transitions of the signals do not necessarily occur at the same nominal rate. Asynchronous, in this case, means that the difference between two clocks is much greater than a plesiochronous difference. For example, if two clocks are derived from free-running quartz oscillators, they could be described as asynchronous. If two digital signals are plesiochronous, their transitions occur at almost the same rate, with any variation being constrained within tight limits. For example, if two networks must interwork, their clocks may be derived from two different primary reference clocks (PRCs). Although these clocks are extremely accurate, there is a difference between one clock and the other. This is known as a plesiochronous difference.
SONET TOPOLOGIES
SONET technology enables a number of different network topologies to solve networking requirements, including survivability, cost, and bandwidth efficiencies. The following provides a description of 3 different SONET configurations, which are deployed in a variety of enterprise situations. The SONET configurations include:
1. Point-to-point configuration
2. Hubbed configuration
3. Linear Add/Drop configuration
4. Ring configuration
Point-to-Point Configuration
SONET point-to-point configurations (see figure 2) create a simple topology that terminates a SONET payload at each point of a fibre optic cable span. Point-to-point configurations are typically deployed in transport applications, which require a single SONET multiplexer in a single route. Point-to-point configurations can be enhanced to increase survivability by to deploying a protection path (second fibre span) over a different
path between two or more SONET multiplexers.
Hubbed Configuration
Hubbed configurations consolidate traffic from multiple sites onto a single
optical channel, which then can be forwarded to another site. This topology is often used in applications where the user wants to consolidate traffic from multiple satellite sites to a single site such as corporate headquarters, before extending it, in some cases to a central office. This topology helps to reduce the number of hops as well as the equipment required to create a multisite topology.
Linear Add/Drop Configuration
In the asynchronous digital signal hierarchy environment, every time a digital signal is accessed the entire signal needs to be multiplexed/ demultiplexed, costing time and money at each site along a given path. However, a Linear Add/Drop configuration enables direct access to VTS/STS channels at each intermediate site along a fibre optic path. Therefore the Linear Add/Drop configuration eliminates the need to process
(Multiplex/demultiplex) the entire optical signal for pass-through traffic.
Self-Healing Ring Configuration
In a Self-Healing Ring configuration, a mechanism referred to as Automatic Protection switching is employed. There are two types of protection ring topologies. The first is UPSR (unidirectional Path Switched Ring), the other is BLSR (Bi-directional Line Switched Ring). Each of these ring topologies is discussed later in this section of this document. The Self-Healing Ring configuration is the most commonly deployed SONET topology in mission critical government and enterprise backbones, due to its survivability
characteristics. Automatic Protection Switching is a mechanism provided within the SONET specification that is designed to provide duplicate finer span paths. In this configuration, a backup fiber span (protection ring) is enabled when and if there is a failure within the fibre span currently carrying traffic on a SONET network. It should be noted that during normal
operating conditions, both fibre spans are always active, and a SONET multiplexer selects which fibre span to receive traffic, based on an internal algorithm (e.g. based on which fibre module was installed in the multiplexer first). The SONET standard specifies that the protection ring should automatically become the fibre span (ring) the SONET multiplexer receives traffic from within 60 milliseconds (unnoticeable to the user) in the event of a failure on the other fibre span.