Synchronous optical networking
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Synchronous optical networking, is a method for communicating digital information using lasers or light-emitting diodes (LEDs) over optical fiber. The method was developed to replace the Plesiochronous Digital Hierarchy (PDH) system for transporting large amounts of telephone and data traffic and to allow for interoperability between equipment from different vendors.
There are multiple, very closely related standards that describe synchronous optical networking:
- SDH or synchronous digital hierarchy standard developed by the International Telecommunication Union (ITU), documented in standard G.707 and its extension G.708
- SONET or synchronous optical networking standard as defined by GR-253-CORE from Telcordia
Both SDH and SONET are widely used today: SONET in the U.S. and Canada and SDH in the rest of the world. Although the SONET standards were developed before SDH, their relative penetrations in the worldwide market dictate that SONET now be considered the variation.
Synchronous networking 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 synchronization system allows entire inter-country networks to operate synchronously, greatly reducing the amount of buffering required between elements 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/SDH (POS) networking. As such, it is inaccurate to think of SDH or SONET as communications protocols in and of themselves, but rather as generic and all-purpose transport containers for moving both voice and data.
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[edit] 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 - 1).
- for SDH is a signal that operates at 155.52 Mbit/s, exactly three times that of STS-1, designated STM-1 (Synchronous Transport Module level - 1) .
[edit] Framing
In standard packet or frame oriented data transmission, a frame usually consists of a header and a payload, with the header of the frame being transmitted first, followed by the payload (and possibly a trailer, such as a CRC). In synchronous optical networking, this is modified slightly. The header is termed the overhead and the payload still exists, but the overhead is not all transmitted before the payload - instead the transmission is interleaved, with part of the overhead being transmitted, then part of the payload, then the next part of the overhead, then the next part of the payload, until the entire frame has been transmitted. In the case of SONET, the frame is 810 octets in size; whereas in SDH the frame is 2430 octets in size. In the case of SONET, the frame is transmitted as 3 octets of overhead, followed by 87 octets of payload, nine times over until 810 octets have been transmitted, taking 125 microseconds. In the case of SDH, 9 octets of overhead are transmitted, followed by 261 octets of payload, also nine times over until 2430 octets have been transmitted, also taking 125 microseconds. For both SONET and SDH this is normally represented by the frame being displayed graphically as a block: of 90 columns and 9 rows for SONET; and 270 columns and 9 rows for SDH. This representation aligns all the overhead columns, so the overhead appears as a contiguous block, as does the payload.
The internal structure of the overhead and payload within the frame differs slightly between SONET and SDH, and different terms are used in the standards to describe these structures. However, the standards are extremely similar in implementation, such that it is easy to interoperate between SDH and SONET at particular bandwidths.
It is worth noting that the choice of a 125 microsecond interval is not an arbitrary one. What it means is that the same octet position in each frame comes past every 125 microseconds. If one octet is extracted from the bitstream every 125 microseconds, this gives a data rate of 8 bits per 125 microseconds - or 64 kbits/s, the basic DS0 telecommunications rate. This relation allows an extremely useful behaviour of synchronous optical networking, which is that low data rate channels or streams of data can be extracted from high data rate streams by simply extracting octets at regular time intervals - there is no need to understand or decode the entire frame. This is not possible in PDH networking. Furthermore, it shows that a relatively simple device is all that is needed to extract a datastream from an SDH framed connection and insert it into a SONET framed connection and vice versa.
[edit] SONET structures
The two major components of the STS-1 frame are:
- Transport overhead; and
- Synchronous payload envelope (SPE)
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.
[edit] Transport overhead
The transport overhead (27 bytes) comprises:
- Section overhead
- Line overhead
These bytes are used for signalling and measuring transmission error rates.
[edit] Synchronous payload envelope
The SPE is comprised of two components:
- Payload overhead (9 bytes) - used for end to end signalling and error measurement.
- Payload (774 bytes)
The STS-1 payload is designed to carry a full PDH DS-3 frame. When the DS-3 enters a SONET network, path overhead is added, and that SONET network element (NE) 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 Regenerator terminate the section but not the path or line.
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.
[edit] 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.
[edit] SONET/SDH data rates
| SONET Optical Carrier Level | SONET Frame Format | SDH level and Frame Format | Payload bandwidth[1] (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-96 | STS-96 | STM-32 | 4,810,752 | 4,976,640 |
| 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 |
- ^ Payload bandwidth is the line rate less the bandwidth of the line and section overheads. User throughput must also deduct path overhead from this, but path overhead bandwidth is variable based on the types of cross-connects built across the optical system.
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.
[edit] SONET physical layer
The physical layer in SONET 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 come with a host of features for isolating and identifying signal defects and their origins.
[edit] SONET/SDH Network 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 on a "craft port" and issue commands through a dumb terminal or terminal emulation program running on a laptop. This interface can also be hooked-up to a console server, allowing for remote out-of-band management and logging.
- SDH has dedicated Data Communication Channels (DCC)s for management traffic. According to ITU-T G. 7712, there are three modes used for management:
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.
[edit] 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.
[edit] 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.
[edit] SONET add-drop multiplexer (ADM)
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.
[edit] 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 simultaneously.
[edit] 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". Because the same data is sent around the ring in both directions. The total capacity of a UPSR is equal to the line rate N of the OC-N ring. For example if we had an OC-3 ring with 3 STS-1s used to transport 3 DS-3s from ingress node A to the egress node D. Then 100% of the ring bandwidth (N=3) would be consumed by nodes A and D. Any other nodes on the ring, say B and C could only act as pass through nodes.
- 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. Because a BLSR does not send redundant copies from ingress to egress the total bandwidth that a BLSR can support is not limited to the line rate N of the OC-N ring and can actually be larger than N depending upon the traffic pattern on the ring. The best case of this is that all traffic is between adjacent nodes. The worst case is when all traffic on the ring egresses from a single node, i.e. the BLSR is serving as a collector ring. In this case the bandwidth that the ring can support is equal to the line rate N of the OC-N ring. This is why BLSRs are seldom if ever deployed in collector rings but often deployed in inter-office rings.
[edit] SONET synchronization
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.
[edit] Next-generation SDH
SONET/SDH development was originally driven by the need to transport multiple DS1s (ie T1s), DS3s (ie, T3s) along with other groups of multiplexed 64 kbit/s pulse-code modulated voice traffic. The ability to transport ATM 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 waste.
Virtual Concatenation (VCAT) allows for a more arbitrary assembly of lower order multiplexing containers, building larger containers of fairly arbitrary size (e.g. 100 Mbit/s) without the need for intermediate SONET NEs to support this particular form of concatenation. Virtual Concatenation increasingly 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 dynamic virtual concatenation, multiplexing containers based on the short-term bandwidth needs in the network.
The set of next generation SONET/SDH protocols to enable Ethernet transport is referred to as Ethernet over SONET/SDH (EoS).
[edit] See also
[edit] External links
- Generalized Multiprotocol Label Switching IETF Internet Draft on Common Control and Measurement Plane (ccamp)
- RFC4257 describing a framework for Generalized Multi-Protocol Label Switching (GMPLS)-based control of (SDH/SONET) Networks
- Ethernet-over-Sonet Tutorial: Part 2
- Understanding SONET/SDH
- Next-generation SDH and MSPP
- The Future of SONET/SDH (pdf)
- ITU-T defining standards
- The Queen's University of Belfast SDH/SONET Primer
- SDH Pocket Handbook from Acterna/JDSU
- SONET Pocket Handbook from Acterna/JDSU
- Network Connection Speeds Reference
- Synchronous Optical Network / Synchronous Digital Hierarchy Encoding for Link Management Protocol Test Messages
- The Sonet Homepage
