Practice English Speaking&Listening with: Synchronous optical networking

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Synchronous Optical Networking and Synchronous Digital Hierarchy are standardized protocols

that transfer multiple digital bit streams synchronously over optical fiber using lasers

or highly coherent light from light-emitting diodes. At low transmission rates data can

also be transferred via an electrical interface. The method was developed to replace the Plesiochronous

Digital Hierarchy system for transporting large amounts of telephone calls and data

traffic over the same fiber without synchronization problems. SONET generic criteria are detailed

in Telcordia Technologies Generic Requirements document GR-253-CORE. Generic criteria applicable

to SONET and other transmission systems are found in Telcordia GR-499-CORE.

SONET and SDH, which are essentially the same, were originally designed to transport circuit

mode communications from a variety of different sources, but they were primarily designed

to support real-time, uncompressed, circuit-switched voice encoded in PCM format. The primary difficulty

in doing this prior to SONET/SDH was that the synchronization sources of these various

circuits were different. This meant that each circuit was actually operating at a slightly

different rate and with different phase. SONET/SDH allowed for the simultaneous transport of

many different circuits of differing origin within a single framing protocol. SONET/SDH

is not itself a communications protocol per se, but a transport protocol.

Due to SONET/SDH's essential protocol neutrality and transport-oriented features, SONET/SDH

was the obvious choice for transporting the fixed length Asynchronous Transfer Mode frames

also known as cells. It quickly evolved mapping structures and concatenated payload containers

to transport ATM connections. In other words, for ATM, the internal complex structure previously

used to transport circuit-oriented connections was removed and replaced with a large and

concatenated frame into which ATM cells, IP packets, or Ethernet frames are placed.

Both SDH and SONET are widely used today: SONET in the United States and Canada, and

SDH in the rest of the world. Although the SONET standards were developed before SDH,

it is considered a variation of SDH because of SDH's greater worldwide market penetration.

The SDH standard was originally defined by the European Telecommunications Standards

Institute, and is formalized as International Telecommunication Union standards G.707, G.783,

G.784, and G.803. The SONET standard was defined by Telcordia and American National Standards

Institute standard T1.105.

Difference from PDH SDH differs from Plesiochronous Digital Hierarchy

in that the exact rates that are used to transport the data on SONET/SDH are tightly synchronized

across the entire network, using 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 they can be used to directly

support either Asynchronous Transfer Mode or so-called packet over SONET/SDH networking.

Therefore, it is inaccurate to think of SDH or SONET as communications protocols in and

of themselves; they are generic, all-purpose transport containers for moving both voice

and data. The basic format of a SONET/SDH signal allows it to carry many different services

in its virtual container, because it is bandwidth-flexible. Protocol overview

SONET and SDH often use different terms to describe identical features or functions.

This can cause confusion and exaggerate their differences. With a few exceptions, SDH can

be thought of as a superset of SONET. SONET is a set of transport containers that

allow for delivery of a variety of protocols, including traditional telephony, ATM, Ethernet,

and TCP/IP traffic. SONET therefore is not in itself a native communications protocol

per se and should not be confused as being necessarily connection-oriented in the way

that term is usually used. The protocol is a heavily multiplexed structure,

with the header interleaved between the data in a complex way. This permits the encapsulated

data to have its own frame rate and be able to "float around" relative to the SDH/SONET

frame structure and rate. This interleaving permits a very low latency for the encapsulated

data. Data passing through equipment can be delayed by at most 32 microseconds (µs),

compared to a frame rate of 125 µs; many competing protocols buffer the data during

such transits for at least one frame or packet before sending it on. Extra padding is allowed

for the multiplexed data to move within the overall framing, as the data is clocked at

a different rate than the frame rate. The protocol is made more complex by the decision

to permit this padding at most levels of the multiplexing structure, but it improves all-around

performance. The basic unit of transmission

The basic unit of framing in SDH is a STM-1, which operates at 155.520 megabits per second.

SONET refers to this basic unit as an STS-3c. When the STS-3c is carried over OC-3, it is

often colloquially referred to as OC-3c, but this is not an official designation within

the SONET standard as there is no physical layer difference between an STS-3c and 3 STS-1s

carried within an OC-3. SONET offers an additional basic unit of transmission,

the STS-1 or OC-1, operating at 51.84 Mbit/sexactly one third of an STM-1OC-3c carrier. This speed

is dictated by the bandwidth requirements for PCM-encoded telephonic voice signals:

at this rate, an STS-1/OC-1 circuit can carry the bandwidth equivalent of a standard DS-3

channel, which can carry 672 64-kbit/s voice channels. In SONET, the STS-3c signal is composed

of three multiplexed STS-1 signals; the STS-3c may be carried on an OC-3 signal. Some manufacturers

also support the SDH equivalent of the STS-1/OC-1, known as STM-0.

Framing In packet-oriented data transmission, such

as Ethernet, a packet frame usually consists of a header and a payload. The header is transmitted

first, followed by the payload. In synchronous optical networking, this is modified slightly.

The header is termed the overhead, and instead of being transmitted before the payload, is

interleaved with it during transmission. Part of the overhead is 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 an STS-1, the frame is 810

octets in size, while the STM-1/STS-3c frame is 2,430 octets in size. For STS-1, the frame

is transmitted as three octets of overhead, followed by 87 octets of payload. This is

repeated nine times, until 810 octets have been transmitted, taking 125 µs. In the

case of an STS-3c/STM-1, which operates three times faster than an STS-1, nine octets of

overhead are transmitted, followed by 261 octets of payload. This is also repeated nine

times until 2,430 octets have been transmitted, also taking 125 µs. For both SONET and SDH,

this is often represented by displaying the frame graphically: as a block of 90 columns

and nine rows for STS-1, and 270 columns and nine rows for STM1/STS-3c. 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. Their standards are extremely

similar in implementation, making it easy to interoperate between SDH and SONET at any

given bandwidth. In practice, the terms STS-1 and OC-1 are

sometimes used interchangeably, though the OC designation 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. SDH frame

The STM-1 frame is the basic transmission format for SDHthe first level of the synchronous

digital hierarchy. The STM-1 frame is transmitted in exactly 125 µs, therefore, there are

8,000 frames per second on a 155.52 Mbit/s OC-3 fiber-optic circuit. The STM-1 frame

consists of overhead and pointers plus information payload. The first nine columns of each frame

make up the Section Overhead and Administrative Unit Pointers, and the last 261 columns make

up the Information Payload. The pointers identify administrative units within the information

payload. Thus, an OC-3 circuit can carry 150.336 Mbit/s of payload, after accounting for the overhead.

Carried within the information payload, which has its own frame structure of nine rows and

261 columns, are administrative units identified by pointers. Also within the administrative

unit are one or more virtual containers. VCs contain path overhead and VC payload. The

first column is for path overhead; it is followed by the payload container, which can itself

carry other containers. Administrative units can have any phase alignment within the STM

frame, and this alignment is indicated by the pointer in row four.

The section overhead of a STM-1 signal is divided into two parts: the regenerator section

overhead and the multiplex section overhead. The overheads contain information from the

transmission system itself, which is used for a wide range of management functions,

such as monitoring transmission quality, detecting failures, managing alarms, data communication

channels, service channels, etc. The STM frame is continuous and is transmitted

in a serial fashion: byte-by-byte, row-by-row. Transport overhead

The transport overhead is used for signaling and measuring transmission error rates, and

is composed as follows: Section overhead

Called RSOH in SDH terminology: 27 octets containing information about the frame structure

required by the terminal equipment. Line overhead

Called MSOH in SDH: 45 octets containing information about error correction and Automatic Protection

Switching messages as may be required within the network.

AU Pointer Points to the location of the J1 byte in the

payload. Path virtual envelope

Data transmitted from end to end is referred to as path data. It is composed of two components:

Payload overhead Nine octets used for end-to-end signaling

and error measurement. Payload

User data For STS-1, the payload is referred to as the

synchronous payload envelope, which in turn has 18 stuffing bytes, leading to the STS-1

payload capacity of 756 bytes. The STS-1 payload is designed to carry a full

PDH DS3 frame. When the DS3 enters a SONET network, path overhead is added, and that

SONET network element is said to be a path generator and terminator. The SONET NE is

line terminating if it processes the line overhead. Note that wherever the line or path

is terminated, the section is terminated also. SONET regenerators terminate the section,

but not the paths or line. An STS-1 payload can also be subdivided into

seven virtual tributary groups. Each VTG can then be subdivided into four VT1.5 signals,

each of which can carry a PDH DS1 signal. A VTG may instead be subdivided into three

VT2 signals, each of which can carry a PDH E1 signal. The SDH equivalent of a VTG is

a TUG-2; VT1.5 is equivalent to VC-11, and VT2 is equivalent to VC-12.

Three STS-1 signals may be multiplexed by time-division multiplexing to form the next

level of the SONET hierarchy, the OC-3, running at 155.52 Mbit/s. The signal is multiplexed

by interleaving the bytes of the three STS-1 frames to form the STS-3 frame, containing

2,430 bytes and transmitted in 125 µs. Higher-speed circuits are formed by successively

aggregating multiples of slower circuits, their speed always being immediately apparent

from their designation. For example, four STS-3 or AU4 signals can be aggregated to

form a 622.08 Mbit/s signal designated OC-12 or STM-4.

The highest rate commonly deployed is the OC-768 or STM-256 circuit, which operates

at rate of just under 38.5 Gbit/s. Where fiber exhaustion is a concern, multiple SONET signals

can be transported over multiple wavelengths on a single fiber pair by means of wavelength-division

multiplexing, including dense wavelength-division multiplexing and coarse wavelength-division

multiplexing. DWDM circuits are the basis for all modern submarine communications cable

systems and other long-haul circuits. SONET/SDH and relationship to 10 Gigabit Ethernet

Another type of high-speed data networking circuit is 10 Gigabit Ethernet. The Gigabit

Ethernet Alliance created two 10 Gigabit Ethernet variants: a local area variant with a line

rate of 10.3125 Gbit/s, and a wide area variant with the same line rate as OC-192/STM-64.

The WAN PHY variant encapsulates Ethernet data using a lightweight SDH/SONET frame,

so as to be compatible at a low level with equipment designed to carry SDH/SONET signals,

whereas the LAN PHY variant encapsulates Ethernet data using 64B/66B line coding.

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 wavelength-division multiplexing systems that currently support OC-192 SONET

signals, which can normally support thin-SONETframed 10 Gigabit Ethernet.

SONET/SDH data rates User throughput must not deduct path overhead

from the payload bandwidth, but path-overhead bandwidth is variable based on the types of

cross-connects built across the optical system. Note that the data-rate progression starts

at 155 Mbit/s and increases by multiples of four. The only exception is OC-24, which is

standardized in ANSI T1.105, but not a SDH standard rate in ITU-T G.707. Other rates,

such as OC-9, OC-18, OC-36, OC-96, and OC-1536, are defined but not commonly deployed; most

are considered orphaned rates. Physical layer

The physical layer refers to the first layer in the OSI networking model. The ATM and SDH

layers are the regenerator section level, digital line level, transmission path level,

virtual path level, and virtual channel level. The physical layer is modeled on three major

entities: transmission path, digital line and the regenerator section. The regenerator

section refers to the section and photonic layers. The photonic layer is the lowest SONET

layer and it is responsible for transmitting the bits to the physical medium. The section

layer is responsible for generating the proper STS-N frames which are to be transmitted across

the physical medium. It deals with issues such as proper framing, error monitoring,

section maintenance, and orderwire. The line layer ensures reliable transport of the payload

and overhead generated by the path layer. It provides synchronization and multiplexing

for multiple paths. It modifies overhead bits relating to quality control. The path layer

is SONET's highest level layer. It takes data to be transmitted and transforms them into

signals required by the line layer, and adds or modifies the path overhead bits for performance

monitoring and protection switching. SONET/SDH network management protocols

Overall functionality Network management systems are used to configure

and monitor SDH and SONET equipment either locally or remotely.

The systems consist of three essential parts, covered later in more detail:

Software running on a 'network management system terminal' e.g. workstation, dumb terminal

or laptop housed in an exchange/ central office. Transport of network management data between

the 'network management system terminal' and the SONET/ SDH equipment e.g. using TL1/ Q3

protocols. Transport of network management data between

SDH/ SONET equipment using 'dedicated embedded data communication channels' within the section

and line overhead. The main functions of network management thereby

include: Network and network-element provisioning

In order to allocate bandwidth throughout a network, each network element must be configured.

Although this can be done locally, through a craft interface, it is normally done through

a network management system that in turn operates through the SONET/SDH network management network.

Software upgrade Network-element software upgrades are done

mostly through the SONET/SDH management network in modern equipment.

Performance management Network elements have a very large set of

standards for performance management. The performance-management criteria allow not

only monitoring the health of individual network elements, but isolating and identifying most

network defects or outages. Higher-layer network monitoring and management software allows

the proper filtering and troubleshooting of network-wide performance management, so that

defects and outages can be quickly identified and resolved.

Consider the three parts defined above: Network management system terminal

Local Craft interface Local "craftspersons" can access a SDH/ SONET

network element on a "craft port" and issue commands through a dumb terminal or terminal

emulation program running on a laptop. This interface can also be attached to a console

server, allowing for remote out-of-band management and logging.

Network management system This will often consist of software running

on a Workstation covering a number of SDH/SONET network elements

TL1/ Q3 Protocols TL1

SONET equipment is often managed with the TL1 protocol. TL1 is a telecom language for

managing and reconfiguring SONET network elements. The command language used by a SONET network

element, such as TL1, must be carried by other management protocols, such as SNMP, CORBA,

or XML. Q3

SDH has been mainly managed using the Q3 interface protocol suite defined in ITU recommendations

Q.811 and Q.812. With the convergence of SONET and SDH on switching matrix and network elements

architecture, newer implementations have also offered TL1.

Most SONET NEs have a limited number of management interfaces defined:

TL1 Electrical interface The electrical interface, often a 50-ohm coaxial

cable, sends SONET TL1 commands from a local management network physically housed in the

central office where the SONET network element is located. This is for local management of

that network element and, possibly, remote management of other SONET network elements.

Dedicated embedded data communication channels SONET and SDH have dedicated data communication

channels within the section and line overhead for management traffic. Generally, section

overhead is used. 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 stack using PPP or LAP-D with tunneling functions to communicate between stacks.

To handle all of the possible management channels and signals, most modern network elements

contain a router for the network commands and underlying protocols.

Equipment With advances in SONET and SDH chipsets, the

traditional categories of network elements are no longer distinct. Nevertheless, as network

architectures have remained relatively constant, even newer equipment can be examined in light

of the architectures they will support. Thus, there is value in viewing new, as well as

traditional, equipment in terms of the older categories.

Regenerator Traditional regenerators terminate the section

overhead, but not the line or path. Regenerators extend long-haul routes in a way similar to

most regenerators, 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, regenerators have been largely replaced by optical amplifiers. Also,

some of the functionality of regenerators has been absorbed by the transponders of wavelength-division

multiplexing systems. Add-drop multiplexer

Add-drop multiplexers are the most common type of network elements. Traditional ADMs

were designed to support one of the network architectures, though new generation systems

can often support several architectures, sometimes simultaneously. ADMs traditionally have a

high-speed side, 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 network

element and sent out from the high-speed side, or vice-versa.

Digital cross connect system Recent digital cross connect systems 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 DCSs can support numerous subtending rings

simultaneously. Network architectures

SONET and SDH have a limited number of architectures defined. These architectures allow for efficient

bandwidth usage as well as protection, and are fundamental to the worldwide deployment

of SONET and SDH for moving digital traffic. Every SDH/SONET connection on the optical

physical layer uses two optical fibers, regardless of the transmission speed.

Linear Automatic Protection Switching Linear Automatic Protection Switching, also

known as 1+1, involves four fibers: two working fibers, and two protection fibers. Switching

is based on the line state, and may be unidirectional, or bidirectional.

Unidirectional path-switched ring In unidirectional path-switched rings, two

redundant copies of protected traffic are sent in either direction around a ring. A

selector at the egress node determines which copy has the highest quality, and uses that

copy, thus coping if one copy deteriorates due to a broken fiber or other failure. UPSRs

tend to sit nearer to the edge of a 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, in an OC-3 ring with

3 STS-1s used to transport 3 DS-3s from ingress node A to the egress node D, 100 percent of

the ring bandwidth would be consumed by nodes A and D. Any other nodes on the ring could

only act as pass-through nodes. The SDH equivalent of UPSR is subnetwork connection protection;

SNCP does not impose a ring topology, but may also be used in mesh topologies.

Bidirectional line-switched ring Bidirectional line-switched ring comes in

two varieties: two-fiber BLSR and four-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 on the protection

fibers. BLSRs trade cost and complexity for bandwidth efficiency, as well as the ability

to support "extra traffic" that can be pre-empted when a protection switching event occurs.

In four-fiber ring, either single node failures, or multiple line failures can be supported,

since a failure or maintenance action on one line causes the protection fiber connecting

two nodes to be used rather than looping it around the ring.

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. In the best case,

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. The SDH equivalent of BLSR is called Multiplex Section-Shared Protection

Ring. Synchronization

Clock sources used for synchronization in telecommunications networks are rated by quality,

commonly called a stratum. Typically, a network element uses the highest quality stratum available

to it, which can be determined by monitoring the synchronization status messages of selected

clock sources. Synchronization sources available to a network

element are: Local external timing

This is generated by an atomic cesium clock or a satellite-derived clock by a device in

the same central office as the network element. The interface is often a DS1, with sync-status

messages supplied by the clock and placed into the DS1 overhead.

Line-derived timing A network element can choose 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 network element can go into a holdover mode until higher-quality external

timing becomes available again. In this mode, the network element uses its own timing circuits

as a reference. Timing loops

A timing loop occurs when network elements in a network are each deriving their timing

from other network elements, without any of them being a "master" timing source. This

network loop will eventually see its own timing "float away" from any external networks, causing

mysterious bit errorsand ultimately, in the worst cases, massive loss of traffic.

The source of these kinds of errors can be hard to diagnose. In general, a network that

has been properly configured should never find itself in a timing loop, but some classes

of silent failures could nevertheless cause this issue.

Next-generation SONET/SDH SONET/SDH development was originally driven

by the need to transport multiple PDH signalslike DS1, E1, DS3, and E3along 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, concatenation

was developed, whereby smaller multiplexing containers are inversely multiplexed to build

up a larger container to support large data-oriented pipes.

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.

More important is the need for all intermediate network elements to support newly introduced

concatenation sizes. This problem was overcome with the introduction of Virtual Concatenation.

Virtual concatenation allows for a more arbitrary assembly of lower-order multiplexing containers,

building larger containers of fairly arbitrary size without the need for intermediate network

elements to support this particular form of concatenation. Virtual concatenation leverages

the X.86 or Generic Framing Procedure protocols in order to map payloads of arbitrary bandwidth

into the virtually concatenated container. The Link Capacity Adjustment Scheme 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 that enable Ethernet transport is referred

to as Ethernet over SONET/SDH. See also

List of device bandwidths Routing and wavelength assignment

Multiwavelength optical networking Optical mesh network

Optical Transport Network G.709

Notes

References

External links Understanding SONET/SDH

The Queen's University of Belfast SDH/SONET Primer

SDH Pocket Handbook from Acterna/JDSU SONET Pocket Handbook from Acterna/JDSU

The Sonet Homepage SONET Interoperability Form

Network Connection Speeds Reference Next-generation SDH: the future looks bright

The Future of SONET/SDH Standards

Telcordia GR-253-CORE, SONET Transport Systems: Common Generic Criteria

Telcordia GR-499-CORE, Transport Systems Generic Requirements: Common Requirements

ANSI T1.105: SONET - Basic Description including Multiplex Structure, Rates and Formats

ANSI T1.119/ATIS PP 0900119.01.2006: SONET - Operations, Administration, Maintenance,

and Provisioning - Communications ITU-T recommendation G.707: Network Node Interface

for the Synchronous Digital Hierarchy ITU-T recommendation G.783: Characteristics

of synchronous digital hierarchy equipment functional blocks

ITU-T recommendation G.803: Architecture of Transport Networks Based on the Synchronous

Digital Hierarchy

The Description of Synchronous optical networking