Synchronous optical networking

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The Synchronous optical network, commonly known as SONET, is a standard for communicating digital information using lasers or light emitting diodes (LEDs) over optical fiber as defined by GR-253-CORE from Telcordia. It was developed to replace the PDH system for transporting large amounts of telephone and data traffic and to allow for interoperability between equipment from different vendors. The more recent Synchronous Digital Hierarchy (SDH) standard developed by ITU (G.707 and its extension G.708) is built on experience in the development of SONET. Both SDH and SONET are widely used today; SONET in the U.S. and Canada, SDH in the rest of the world. SDH is growing in popularity and is currently the main concern with SONET now being considered as the variation.

SONET differs from PDH in that the exact rates that are used to transport the data are tightly synchronized across the entire network, made possible by atomic clocks. This Telecom Synchronization system allows entire inter-country networks to operate synchronously, greatly reducing the amount of buffering required between each element in the network.

Both SONET and SDH can be used to encapsulate earlier digital transmission standards, such as the PDH standard, or used directly to support either ATM or so-called Packet over SONET networking. As such, it is inaccurate to think of SONET as a communications protocol in and of itself, but rather as a generic and all-purpose transport container for moving both voice and data.

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Structure of SONET/SDH signals

The basic unit of transmission for SONET is a signal that operates at 51.840 Mbit/s, designated STS-1 (Synchronous Transport Signal one). This differs from SDH's basic unit, the STM-1 (Synchronous Transport Module-level 1), which operates at 155.52 Mbit/s.

The two major components of the STS-1 frame are the transport overhead and the synchronous payload envelope (SPE). The transport overhead (27 bytes) comprises the section overhead and line overhead. These bytes are used for signalling and measuring transmission error rates. The SPE is comprised of two components: the payload overhead (9 bytes, used for end to end signalling and error measurement) and the payload of 774 bytes. The STS-1 payload is designed to carry a full DS-3 frame. When the DS-3 enters a SONET network, path overhead is added, and that SONET network element is said to be path terminating. Where multiple DS-3 paths are multiplexed, the SONET NE is said to be line terminating. Note that wherever the line or path is terminated, the section is terminated also. SONET Regenerators (see below) terminate the Section but not the path or line.

The entire STS-1 frame is 810 bytes. The STS-1 frame is transmitted in exactly 125 microseconds on a fiber-optic circuit designated OC-1 (optical carrier one). In practice, the terms STS-1 and OC-1 are sometimes used interchangeably, though the OC-N format refers to the signal in its optical form. It is therefore incorrect to say that an OC-3 contains 3 OC-1s: an OC-3 can be said to contain 3 STS-1s.

Three OC-1 (STS-1) signals are multiplexed by time-division multiplexing to form the next level of the SONET hierarchy, the OC-3 (STS-3), running at 155.52 Mbit/s. The multiplexing is performed by interleaving the bytes of the three STS-1 frames to form the STS-3 frame, containing 2430 bytes and transmitted in 125 microseconds.

Higher speed circuits are formed by successively aggregating multiples of slower circuits, their speed always being immediately apparent from their designation. For example, four OC-3 or STM-1 circuits can be aggregated to form a 622.08 Mbit/s circuit designated as OC-12 or STM-4.

The highest rate that is commonly deployed is the OC-192 or STM-64 circuit, which operates at rate of just under 10 Gbit/s. Speeds beyond 10 Gbit/s are technically viable and are under evaluation. Where fiber exhaust is a concern, multiple SONET signals can be transported over multiple wavelengths over a single fiber pair by means of Dense Wave Division Multiplexing (DWDM). Such circuits are the basis for all modern transatlantic cable systems and other long-haul circuits.

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SONET/SDH and relationship to 10 Gigabit Ethernet

Another fast growing circuit type amongst data networking equipment is 10 Gigabit Ethernet (10GbE). This is similar in rate to OC-192/STM-64, and, in its wide area variant, encapsulates its data using a light-weight SDH/SONET frame so as to be compatible at low level with equipment designed to carry those signals.

However, 10 Gigabit Ethernet does not explicitly provide any interoperability at the bitstream level with other SDH/SONET systems. This differs from WDM System Transponders, including both Coarse- and Dense-WDM systems (CWDM, DWDM) that currently support OC-192 SONET Signals, which can normally support thin-SONET framed 10 Gigabit Ethernet.

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SONET/SDH data rates

SONET/SDH Designations and bandwidths
SONET Optical Carrier Level SONET Frame Format SDH level and Frame Format Payload bandwidth<ref>Payload bandwidth is the actual data carrying capacity.</ref> (kbit/s)Line Rate (kbit/s)
OC-1 STS-1 STM-0 48 960 51 840
OC-3 STS-3 STM-1 150 336 155 520
OC-12 STS-12 STM-4 601 344 622 080
OC-24 STS-24 STM-8 1 202 688 1 244 160
OC-48 STS-48 STM-16 2 405 376 2 488 320
OC-192 STS-192 STM-64 9 621 504 9 953 280
OC-768 STS-768 STM-256 38 486 016 39 813 120
OC-1536 STS-1536 STM-512 76 972 032 79 626 120
OC-3072 STS-3072 STM-1024 153 944 064 159 252 240

<references/>

Note that the typical data rate progression starts at OC-3 and increases by multiples of 4. As such, while OC-24 and OC-1536, along with other rates such as OC-9, OC-18, OC-36, and OC-96 may be defined in some standards documents, they are not available on a wide-range of equipment.

As of 2006, OC-3072 is still a work in progress. It has not yet been manufactured.

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SONET Physical Layer

The "SONET Physical Layer" actually comprises a large number of layers within it, only one of which is the optical/transmission layer (which includes bitrates, jitter specifications, optical signal specifications and so on). The SONET and SDH Standards have within them a host of features for isolating and identifying signal defects and their origins.

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SONET/SDH system management protocols

SONET equipment is often managed with the TL1 protocol. TL1 is a traditional telecom language for managing and reconfiguring SONET network elements. TL1 (or whatever command language a SONET Network Element utilizes) must be carried by other management protocols, including SNMP, CORBA, and XML.

SONET Network Management is a large, difficult, and arcane subject, but there are some features that are fairly universal. First of all, most SONET NEs have a limited number of management interfaces defined. These are:

  • Electrical Interface. The electrical interface (often 50 Ω) sends SONET TL1 commands from a local management network physically housed in the Central Office where the SONET NE is located. This is for "local management" of that NE and, possibly, remote management of other SONET NEs.
  • Craft Interface. Local "craftspersons" can access a SONET NE and issue commands through a dumb terminal or terminal emulation program running on a laptop.
  • SDH has dedicated Data Communication Channels DCCs for management traffic. According to ITU-T G. 7712, there are three modes used for management:
  • IP-only stack, using PPP as data-link
  • OSI-only stack, using LAP-D as data-link
  • Dual (IP+OSI) stack using PPP or LAP-D with tunneling functions to communicate between stacks.

An interesting fact about modern SONET NEs is that, to handle all of the possible management channels and signals, most NEs actually contain a router for routing the network commands and underlying (data) protocols.

The main functions of SONET Network Management include:

  • SONET Network and NE Provisioning. In order to allocate bandwidth throughout a SONET Network, each SONET NE must be configured. Although this can be done locally, through a craft interface, it is normally done through a Network Management System (sitting at a higher layer) that in turn operates through the SONET/SDH Network Management Network.
  • Software Upgrade. SONET NE Software Upgrade is in modern NEs done mostly through the SONET/SDH Management network.
  • Performance Management. SONET NEs have a very large set of standards for Performance Management. The PM criteria allow for monitoring not only the health of individual NEs, but for the isolation and identification of most network defects or outages. Higher-layer Network monitoring and management software allows for the proper filtering and troubleshooting of network-wide PM so that defects and outages can be quickly identified and responded to.
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SONET Equipment

With recent advances in SONET and SDH chipsets, the traditional categories of SONET NEs are breaking down. Nevertheless, as SONET Network architectures have remained relatively constant, even newer SONET Equipment (including "Multiservice Provisioning Platforms") can be examined in light of the architectures they will support. Thus, there is value in viewing new (as well as traditional) SONET Equipment in terms of the older categories.

  • SONET Regenerator. Traditional SONET Regenerators terminate the SONET Section overhead, but not the line or path. SONET Regens extended long haul routes in a way similar to most regenerator, by converting an optical signal that has already traveled a long distance into electrical format and then retransmitting a regenerated high-power signal.
Since the late 1990s, SONET regenerators have been largely replaced by Optical Amplifiers. Also, some of the functionality of SONET Regens has been absorbed by the Transponders of Wavelength Division Multiplexing systems.
  • SONET Add-Drop Multiplexer. SONET ADMs are the most common type of SONET Equipment. Traditional SONET ADMs were designed to support one of the SONET Network Architectures, though new generation SONET systems can often support several architectures, sometimes simultaneously. SONET ADMs traditionally have a "high speed side" (where the full line rate signal is supported), and a "low speed side", which can consist of electrical as well as optical interfaces. The low speed side takes in low speed signals which are multiplexed by the SONET NE and sent out from the high speed side, or vice versa.
  • SONET Digital Cross Connect System. Recent SONET Digital Cross Connect systems (DCSs or DXCs) support numerous high-speed signals, and allow for cross connection of DS1s, DS3s and even STS-3s/12c and so on, from any input to any output. Advanced SONET DCSs can support numerous subtending rings simulataneously.
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SONET Network Architectures

Currently, SONET (and SDH) have a limited number of architectures defined. These architectures allow for efficient bandwidth usage as well as protection (i.e. the ability to transmit traffic even when part of the network has failed), and are key in understanding the almost worldwide usage of SONET and SDH for moving digital traffic. The three main architectures are:

  • Linear APS (Automatic Protection Switching): This involves 4 fibers: 2 working fibers in each direction, and two protection fibers.
  • UPSR (Unidirectional Path Switched Ring): In a UPSR, two redundant (path-level) copies of protected traffic are sent in either direction around a ring. A selector at the egress node determines the higher-quality copy and decides to use the best copy, thus coping if deterioration in one copy occurs due to a broken fiber or other failure. UPSRs tend to sit nearer to the edge of a SONET network and, as such, are sometimes called "collector rings".
  • BLSR (Bidirectional Line Switched Ring): BLSR comes in two varieties, a 2-fiber BLSR and 4-fiber BLSR. BLSRs switch at the line layer. Unlike UPSR, BLSR does not send redundant copies from ingress to egress. Rather, the ring nodes adjacent to the failure reroute the traffic "the long way" around the ring. BLSRs trade cost and complexity for bandwdith efficiency as well as the ability to support "extra traffic", which can be pre-empted when a protection switching event occurs. BLSRs can operate within a metropolitan region or, often, will move traffic between municipalities.
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SONET Synchronization

Like management, Synchronization of SONET and SDH networks is a difficult and arcane subject. Remember that a SONET NE will transport and/or multiplex traffic that has originated from a variety of different clock sources. In addition, a SONET NE may have a number of different synchronization options to choose from, which in some cases it will do so dynamically based on Synch Status Messages and other indicators.

As for Synchronization sources available to a SONET NE, these are:

  • Local External Timing. This is generated by an atomic Cesium clock or a satellite-derived clock by a device located in the same central office as the SONET NE. the interface is often a DS1, with Sync Status Messages supplied by the clock and placed into the DS1 overhead.
  • Line-derived timing. A SONET NE can choose (or be configured) to derive its timing from the line-level, by monitoring the S1 sync status bytes to ensure quality.
  • Holdover. As a last resort, in the absence of higher quality timing, a SONET NE can go into "holdover" until higher quality external timing becomes available again. In this mode, a SONET NE uses its own timing circuits to time the SONET signal.

An interesting and hard-to-troubleshoot issue in SONET Networks is the existence of "timing loops". With a timing loop, SONET NEs in a network are each deriving their timing from another NE, and back again to initial NE, like a snake biting its own tail. This network loop will eventually see its own timing "float away" from any external SONET networks, causing mysterious bit errors, the source of which can be hard to find (unless the presence of the timing loop is detected). In general, a SONET Network that has been properly configured will never find itself in a timing loop, but it is sometimes hard to avoid this without sophisticated network management tools.

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Next Generation SDH

SONET/SDH was originally developed primarily to transport multiple DS1s (ie T1s), DS3s (ie, T3s), and other groups of multiplexed 64 kbit/s pulse-code modulated voice traffic. The ability to transport ATM (Asynchronous Transfer Mode) traffic was another early application. In order to support large ATM bandwidths, the technique of concatenation was developed, whereby smaller SONET multiplexing containers (eg, STS-1) are inversely multiplexed to build up a larger container (eg, STS-3c) to support large data-oriented pipes. SONET is therefore able to transport both voice and data simultaneously.

One problem with traditional concatenation, however, is inflexibility. Depending on the data and voice traffic mix that must be carried, there can be a large amount of unused bandwidth left over, due to the fixed sizes of concatenated containers. For example, fitting a 100 Mbit/s Fast Ethernet connection inside a 155 Mbit/s STS-3c container leads to considerable wastage.

Virtual Concatenation (VCAT) allows for a more arbitrary gluing-together of lower order multiplexing containers to build larger containers of fairly arbitrary size (e.g. 100 Mbit/s), without the need for intermediate SONET NEs to support that particular form of concatenation. Virtual Concatenation now often leverages X.86 or Generic Framing Procedure (GFP) protocols in order to map payloads of arbitrary bandwidth into the virtually concatenated container.

Link Capacity Adjustment Scheme (LCAS) allows for dynamically changing the bandwidth via dynamically virtually concatenating multiplexing containers based on short-term bandwidth needs in the network.

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See also

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External links

de:Synchronous Optical Network es:SONET fa:سلسله همگاه رقمی hu:Szinkron digitális hierarchia ja:Synchronous Digital Hierarchy nl:Synchrone Digitale Hiërarchie pl:Synchronous Digital Hierarchy zh:同步数字体系

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