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Contents Chapter 27 SONET and SDH Introduction Background Leading to SONET Development Synchronizing the Digital Signals The SONET Signal Why Bother Synchronizing? The SONET Frame Overhead Inside the STS-1 Frame SONET Overhead Section Overhead Line Overhead Path Overhead Virtual Tributaries SONET Multiplexing Functions ADD-Drop Multiplexing: A SONET Benefit SONET Topologies Point-to-Point Point-to-Multipoint Hub and Spoke Ring Evolution of SONET in the Rest of the World Synchronous Digital Hierarchy
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Copyright 2000 The McGraw-Hill Companies. Click Here for Terms of Use.
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Introduction Synchronous Optical Network (SONET) is a standard developed by the Exchange Carriers Standards Association (ECSA) for ANSI. This standard defines an optical telecommunications transport for U.S. Telecommunications. SONET standards provide an extensive set of operational parameters for optical transmission systems throughout the industry. The North American industry uses the SONET specifications, whereas the rest of the world uses a close “cousin” defined as Synchronous Digital Hierarchy (SDH). Between the two sets of standards, the industry attempted to define the roles of transport for the telecommunications providers using optical fibers as the transport medium. SONET provides more, though. It defines a means to increase throughput and bandwidth through a set of multiplexing parameters. These roles provide certain advantages to the industry, such as the following: ■
Reduced equipment requirements in the carriers’ network
■
Enhanced network reliability and availability
■
Conditions to define the overhead necessary to facilitate managing the network better
■
Definitions of the multiplexing functions and formats to carry the lower level digital signals (such as DS-1, DS-2, and DS-3)
■
Generic standards encouraging interoperability between different vendors’ products
■
A flexible means of addressing current as well as future applications and bandwidth usage
SONET defines the Optical Carrier (OC) levels and the electrical equivalent rates in the Synchronous Transport Signals (STS) for the fiber-based transmission hierarchy.
Background Leading to SONET Development Prior to the development of SONET, the initial fiber-based systems used in the PSTN were all highly proprietary. The proprietary nature of the products included the following:
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453 1. Equipment 2. Line coding 3. Maintenance 4. Provisioning 5. Multiplexing 6. Administration The carriers were frustrated with the proprietary products because of interoperability problems, sole-source vendor solutions (which held the carriers hostage to one vendor), and cost issues. These carriers approached the standards committees and demanded that a set of operational standards be developed that would allow them to mix and match products from various vendors. In 1984 a task force was established to develop such a standard. The resultant standard became SONET.
Synchronizing the Digital Signals SONET involves the synchronization of the digital signals arriving at the equipment. Keeping in mind that the signals may be introduced in one of three ways, it is important to attempt to get everything on a common set of clocking mechanisms. In digital transmission, the normal way of synchronizing traffic is to draw a common clocking reference. In the hierarchy of clocking, systems use a stratum clocking architecture. The stratum references in North America come in a four-level architecture. These are shown in Table 27-1. In a set of synchronous signals, the digital transitions in the signals occur at the same rate. There may be a phase difference between the transitions in the two signals, but this would be in specified ranges and limits. The phase differences can be the result of delay in systems, jitter across the link, or other transmission impairments. In a synchronous environment, all the clocks are traceable back to a common reference clock (the Primary Reference Clock). If two signals are almost the same, they are said to be plesiochronous. Their transitions are close (or almost the same) and variations are contained within strict limits. The clocking between the two different sources, although accurate, may be operating at a different rate.
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Stratum Reference
Location Used
Accuracy
1
Primary Reference drawn from a GPS or the National Reference Atomic clock
1 pulse in 1011
2
Toll Offices (Class 1—4)
1.6 pulses in 108
3
End Offices (Class 5)
4.6 pulses in 106
4
Customer Equipment (Multiplexer, Channel Bank, and so on)
32 pulses in 106
Finally, if two signals are randomly generated and do not occur at the same rate, they are said to be asynchronous. The difference between two clocks is much greater, possibly running from a free running clock source. Any one of these signals, synchronous, plesiochronous, or asynchronous, may arrive at a SONET multiplexer to be formulated and transmitted across the network. SONET defines the means of synchronizing the traffic for transmission.
The SONET Signal SONET defines a technique to carry many signals from different sources and at different capacities through a synchronous, optical hierarchy. The flexibility and robustness of SONET are some of its strongest selling points. Additionally, in the past many of the high-speed, multiplexing arrangements (DS-2 and DS-3) used bit interleaving to multiplex the data streams through the multiplexers. SONET uses a byte-interleaved multiplexing format. This is a strong point because it keeps the basic DS-0 intact throughout the network, making it easier to perform diagnostics and troubleshooting. Byte interleaving simplifies the process and provides better end-to-end management. The base level of a SONET signal is called the Synchronous Transport Signal Level1 (STS-1), operating at 51.84 Mbps. The first step in using the SONET architecture is to create the STS-1. Other levels exist in multiples of the STS-n to create a full family of transmission speeds. The SONET hierarchy is shown in Table 27-2.
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Table 27-2 Summary of electrical and optical rates for SONET
Electrical Signal
The SONET hierarchy Optical Value Speed Capacity
STS-1
OC-1
51.84 Mbps
28 DS-1 or 1 DS-3
STS-3
OC-3
155.520 Mbps
84 DS-1 or 3 DS-3
STS-12
OC-12
622.08 Mbps
336 DS-1 or 12 DS-3
STS-24
OC-24
1.244 Gbps
672 DS-1 or 24 DS-3
STS-48
OC-48
2.488 Gbps
1344 DS-1 or 48 DS-3
STS-192
OC-192
9.95Gbps
5376 DS-1 or 192 DS-3
Other rates exist, but these are the most popularly implemented
Why Bother Synchronizing? In the past, transmission systems have been primarily asynchronous. Each terminal device in the network was independently timed. In a digital synchronous transmission system, clocking is all-important. Clocking uses a series of pulses to keep the bit rate constant and to help recover the ones and zeros from the data stream. Because these past clocks were independently timed, large variations occurred in the clock rate, making it extremely difficult (if not impossible) to extract and recover the data. A DS-1 operates at 1.544 Mbps 150 pps, whereas a DS-3 operates at 44.736 Mbps 1789 pps. These differences mean that one DS-1 may be transmitting at up to 300 pps different than the other (assuming that DS-1 is at –150 pps and the second one is at 150 pps). The differences can make it difficult to derive the actual data across a common receiver. Back in the section on the T-carriers (Chapter 26), we discussed the asynchronous method of multiplexing a DS-3. In that section we saw that four DS-1s were bit interleaved together to form a DS-2, and that bit stuffing occurred. From there, seven DS-2s were bit interleaved together to form the DS-3, but there were several possible steps where bit stuffing occurred at the multiplexing point. The stuff bits were random occurring in seven of 18 frames, causing confusion in delivering and demultiplexing the signal. Moreover, when a problem occurs on a DS-3 using the M13 Asynchronous Multiplexing technique, the entire DS-3 must be demultiplexed to find the problem. This is inefficient.
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Therefore, the method of synchronously multiplexing in a SONET architecture provides for better efficiency and problem resolution. Using SONET, the average frequency of all the clocks will be the same. Every clock can be traced back to a common reference, which is highly reliable and stable. Bit stuffing can be eliminated in the preparation of the STS-1 signal; therefore, the lower-speed signals are more readily accessible. The benefits outweigh the possible overhead associated with the SONET multiplexing scheme. In SONET, the hierarchy of clocking follows the master-slave clocking architecture. Higher level (stratum 1) clocks will feed the timing across the network to lower level devices. Any jitter, phase shifts, or drifting by the clocks can be accommodated through the use of pointers in the SONET overhead. The internal clock in a SONET multiplexer may also draw its timing from a Building Integrated Timing System (BITS) used by switching systems and other devices. This terminal will then serve as the master for other SONET nodes downstream, providing timing on its outgoing signal. The receiving SONET equipment will act in a slave mode (loop timing) with their internal clocks timed by the incoming signal. The standard specifies that SONET equipment must be able to derive its timing at a stratum level 3 or above.
The SONET Frame SONET also defines a frame format in which to produce the basic rate of 51.84 Mbps (the STS-1). Each of the additions to the multiplexing rates is a multiple of the STS-1. The basic format consists of a frame that is 80 bytes (columns) wide and 9 bytes high (rows). The basic STS-1 signal is then applied into this 810-byte frame. The frame is shown in Figure 27-1. The frame will occur 8,000 times per second. If we calculate the math on this we have the following: 810 bytes 8 bits/byte 8000 frames/sec 51.84 Mbps
Overhead From the 810-byte frame, overhead is allowed in several ways to handle the Operations, Administration, Maintenance, and Provisioning services (OAM&P).
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457 The first part of theoverhead is defined as the transport overhead. The transport overhead uses the first three columns and all nine rows. This creates 27 bytes of transport overhead. As shown in Figure 27-2, the transport overhead is divided into two pieces. The first three columns and the first
90 Columns
Figure 27-1 The SONET frame
9 R o w s
Figure 27-2 The transport overhead is divided into section and line overhead.
Section =3X3
Line 3X6=18
Transport O/H= 3X9= 27
SONET OC-1 Frame
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three rows (9 bytes) are used for section overhead. The remaining six rows in the first three columns (18 bytes) are used for line overhead. The remaining 87 columns and 9 rows (783 bytes) are designated as the Synchronous Payload Envelope (SPE). Inside the SPE, an additional one column, nine rows high (9 bytes) is set aside for Path Overhead. This is shown in Figure 27-3. After the Path Overhead is set aside, the resultant payload is 774 bytes. In these 774 bytes, the services are then mapped into the frame. The STS-1 payload can carry the following: ■
1 DS-3
■
7 DS-2s
■
21 E-1s
■
28 DS-1s
Combinations of the above payloads are also allowable. Two columns are reserved as fixed stuff columns; these are columns 30 and 59. The remaining 756 bytes carry the actual payload.
Figure 27-3 The SPE shown
SPE: 87x9 = 783 bytes
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Inside the STS-1 Frame The SPE can begin anywhere inside the STS-1 envelope. Normally, it begins in one STS-1 frame and ends there. However, it may begin in one STS-1 frame and end in another. The STS payload pointer contained inside the transport overhead points to the beginning byte of an SPE. The possibilities then are that the frame can carry a locked payload or a floating payload. Floating, in this regard, refers to the payload floating between two frames. The overhead associated with SONET is designed to let the receiver know where to look for its payload and extract the information at a starting point. This floating frame is shown in Figure 27-4.
Figure 27-4 A floating payload inside two frames P O H
P O H
Transport O/H
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SONET Overhead Sonet provides significant overhead, allowing the simpler multiplexing techniques and improved OAM&P operations. In the next discussion the overhead is examined to better understand why so much is dedicated to the overhead functions. In Figure 27-5, the architecture of the SONET link is shown as defined by ECSA and ANSI. In this reference the link architecture is broken down into three parts: ■
Section The section is defined as the portion of the link between two repeater functions, or between a repeater and line terminating equipment. Sufficient overhead is allowed to detect and troubleshoot errors on the link between these two points (STE).
■
Line The line overhead provides sufficient information to detect and troubleshoot problems between two pieces of Line Terminating Equipment (LTE).
■
Path The Path Overhead provides sufficient overhead to detect and troubleshoot problems between the end-to-end path terminating Pieces of Equipment (PTE).
Figure 27-5 SONET link architecture
Section
Section
Line
Path
Section
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Section Overhead The section overhead is shown in Table 27-3 and Figure 27-6. This overhead contains 9 bytes of information that is accessed, generated, and processed by the section terminating equipment. It supports functions such as the following:
Table 27-3 The individual bytes are defined in the designation and their use.
■
Performance monitoring
■
A local orderwire
■
Data communications channels for OAM&P messages
■
Framing information
Byte
Description
A1-A2
Framing bytes: These two bytes provide an indication of the start of an STS-1 frame
J0/Z0
Section Trace (J0) or Section Growth (Z0): this byte in each of the STS-1s in an STS-n was originally defined as the STS-1 ID (C-1) byte. It has been redefined as either the section trace byte (in the first STS-1 of STS-n) or as a growth byte (used in the 2nd through nth STS-1s)
B1
Section Bit Interleaved Parity code (BIP-8) byte: This is a parity code (using even parity) used to check for transmission errors over a section of the link (a repeater). The value is calculated over all bits of the previous STS-n frame after scrambling and then placed in the B1 byte of the STS-1 before scrambling. This byte only defines for STS-1 number 1 of an STS-n signal.
E1
Orderwire: This is a 64Kbps channel for a voice communications orderwire between two repeater functions or a repeater and a line terminating equipment. It allows for two technicians to talk to each other while they troubleshoot problems.
F1
Section User Byte: This byte is set aside for users’ choice. It terminates at all section terminating equipment within a line. It can be read or written at each section terminating equipment on the line. This may be used for proprietary user maintenance and diagnostic systems, or for SNMP functions on a user network.
D1—3
Section Data Communications Channel (DCC) bytes: This creates a 192 Kbps data channel for diagnostics and testing for OAM&P. It uses a message-based channel for remote locations to control alarms, monitoring, and other maintenance functions.
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A1
A2
J0/Z0
B1
E1
F1
D1
D2
D3
Line Overhead In Table 27-4 we see the line overhead, which occupies rows 4 through 9 in the first three columns. The line overhead used between two pieces of line terminating equipment provides for the OAM&P functions. This is more extensive than the section overhead. Figure 27-7 also shows its position in the overhead of the STS-1 frame.
Table 27-4 The individual bytes of the line overhead
Byte
Description
H1-H2
STS Payload Pointers: These two bytes indicate the offset bytes between the pointer and the first byte of the STS SPE. The pointer bytes are used in all STS-1s within an STS-n to align the transport overhead in the STS-n and perform frequency justification. These bytes can also be used to indicate when concatenation is used or to indicate path Alarm Indications Status (AIS)
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Description
H3
Pointer Action byte: This byte is allocated for frequency justification. The H3 byte is used in all STS-1s within an STS-n to carry the extra SPE byte to either increment or decrement the pointer.
B2
Line Bit Interleaves Parity code (BIP-8) byte: This parity code byte is used to determine if a transmission error has occurred over the line. It uses even parity and is calculated over all bits of the line overhead and STS-1 SPE of the previous STS-1 frame before scrambling. The value is placed in the B2 byte of the line overhead before scrambling.
K1-K2
Automatic Protection Switching (APS) channel bytes: These two bytes are used for protection signaling between two line terminating devices for bidirectional automatic protection switching and for detecting Alarm Indication Signal on the Line (AIS-L) and Remote Defect Indication (RDI) signals.
D4-D12
Line Data Communications Channel (DCC) bytes: This creates a 576 Kbps channel capacity for message signaling from a central location for OAM&P. These bytes are available for generic, internal, or external messages, or vendor-specific messages. The use of a protocol analyzer is needed to access the DCC.
S1/Z1
Synchronization Status Byte (S1): The S1 byte is used for synchronization status of network elements. It is located in the 1st STS-1 of an STS-n. Bits 5— 8 convey the synchronization information. The Growth (Z1) byte is located in the second through nth STS-1s of an STSn and allowed for growth.
M0-M1
STS-1 REI-L (M0) byte is defined for STS-1 in an OC-1 or for the STS-1 electrical signal. Bits 5—8 report on the Line Remote Error Indication function (REI-L, which used to be called the FEBE). STS-n REI-L (M1) byte is located in the third STS-1 (in order of appearance in the byte interleaved STS-n electrical or optical signal) in an STS-n (where n 3) and is used for the REI-L function.
Z2
Growth (Z2) byte: This byte is located in the 1st and 2nd STS-1s of an STS-3, and the 1st, 2nd and 4th through nth STS-1s of an STS-n (where 12 n 48). These bytes are reserved for future growth.
E2
Orderwire byte: This orderwire byte provides a 64-Kbps channel between line equipment for an express orderwire. It is a voice channel used by technicians, and is ignored as it passes through the repeaters.
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Z1
H1
H2
H3
B2
K1
K2
D4
D5
D6
D7
D8
D9
D10
D11
D12
S1
Z2
E2
In 2nd-nth frame
Path Overhead The final piece of overhead is the Path Overhead contained inside the SPE. Path Overhead contains 9 bytes of information starting at the first byte of the STS SPE. The Path Overhead provides for communication between two path-terminating pieces of equipment (PTE). The PTE is where the actual multiplexing and demultiplexing function takes place as the services are mapped into the SONET frame. The functions supported by the POH are as follows: ■
Performance monitoring of the STS SPE
■
Signal label the individually mapped payloads
■
Path status
■
Path trace
The Path Overhead is shown in Table 27-5 and in Figure 27-8 to correlate them in the overall function of the STS frame.
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Table 27-5
Byte
Description
The Path Overhead defined
J1
STS Path Trace byte: This user programmable byte repetitively transmits a 64 byte, or a 16-byte (E.164) format string. This is used so the receiving end of the path to verify it is still connected to the sending end.
B3
STS Path Bit Interleaved Parity code (path BIP-8) byte: This byte is even parity to check if an error has occurred across the entire path.
C2
STS Path Signal Label byte: This byte is used to identify the content of the STS SPE, including the status of the mapped payloads.
G1
Path Status Byte: This byte is used to convey path terminating status and performance characteristics back to the originating end. Bits 1—4 are used for ATA Path REI-P (was referred to as the FEBE) Bits 5, 6 and 7 are for an STS Path RDI (RDI-P) signal and bit 8 is not defined.
F2
Path User Channel Byte: The byte is used for user communications between end-to-end elements. This may be a proprietary network management system or an SNMP system to determine the status and alarms of the far end on the path.
H4
VT Multiframe Indicator byte: This byte provides a general multiframe alignment indicator for payload containers. Currently, it is only used for VT payloads.
Z3-Z5
Growth bytes: These bytes are undefined at this time.
J1
Figure 27-8 Path Overhead in the STS frame
B3 C2 G1 F2 H4 Z3 Z4 Z5 Path overhead
Payload
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Virtual Tributaries SONET is more than the STS frame; it also defines the sus-STS levels of payload that can be carried. The STS-1 payload can be subdivided into virtual tributaries, synchronous signals to transport lower-speed signals. The normal sizes defined by SONET are shown below in Table 27-6.
SONET Multiplexing Functions The primary principles of the SONET multiplexers are as follows: ■
Mapping the tributaries into the STS-n frame
■
Aligning the information by using the pointer information to determine where the first byte of the tributary is located
■
Multiplexing lower order signals are adapted to higher order signals
■
Stuffing of bits necessary to handle the various lower-speed, asynchronous channels and filling up the spare bits to keep everything in alignment
The SONET equipment provides these functions. SONET can carry very large payloads, as we have seen in the hierarchy of the data speeds. Up to now, we have seen the primary mapping and layout of an STS-1 (OC-1). SONET equipment can add the value necessary to protect investments by either lower-rate multiplexing or higher-rate multiplexing. If one looks at an OC-3, for example, the multiplexer will produce a larger STS frame. In this case, as shown in Figure 27-9, the frame is three times larger or 270 bytes (columns) wide and nine rows high for a total of 2,430 bytes. Note
Table 27-6 The values of the virtual tributaries defined for SONET
VT
Bit Rate
Equivalent DS-n level
Required Bytes (rows columns)
1.5
1.728 Mbps
DS-1
27 (9 3)
2
2.304 Mbps
E-1
36 (9 4)
3
3.456 Mbps
DS-1C
54 (9 6)
6
6.912 Mbps
DS-2
108 (9 12)
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467 STS-1
Figure 27-9 The STS-3 (OC-3) frame
STS-2
POH
STS-3
A1
A2
J0/Z0
A1
A2
J0/Z0
A1
A2
J0/Z0
J1
B1
E1
F1
B1
E1
F1
B1
E1
F1
B3
D1
D2
D3
D1
D2
D3
D1
D2
D3
C2
H1
H2
H3
H1
H2
H3
H2
H3
G1
B2
K1
K2
B2
K1
K2
B2
K1
K2
F2
D4
D5
D6
D4
D5
D6
D4
D6
H4
D7
D8
D9
D7
D8
D9
D10
D11
D12
D10
D11
D12
D10
D11
D12
Z4
S1
Z2
E2
Z2
E2
Z1
Z2
E2
Z5
Z1
H1
D7
D5 D8
D9
Z3
270 Bytes
from this figure the overhead for the STS-n is located in the beginning of the frame, whereas the Path Overhead (POH) is located at the start of each payload.
ADD-Drop Multiplexing: A SONET Benefit Another major benefit of the SONET specification is the ability to perform add-drop multiplexing. Even though network elements are compatible at the OC-N level, they may still differ from vendor to vendor. SONET doesn’t attempt to restrict vendors to all providing a single product, nor does it require that they produce one of every type out there. One vendor may offer an add-drop multiplexer with access to the DS-1 level only, whereas another may offer access to DS-1 and DS-3 rates. The benefit of an add-drop multiplexer on a Wide Area Network is to drop (demultiplex) only the portions of the optical stream required for a location and let the rest pass through without the demultiplexing process. It would be extremely inefficient to have to demultiplex an entire OC-12, for example, only to drop out one DS-1. The ability to extract only what is necessary helps to
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prevent errors, loss of data, and other delays inherent with older technologies. The add-drop multiplexer makes this attractive for carriers to use in rural areas where they may bundle many lower-speed communications channels onto a single OC-1 or OC-3 to carry the information back to the central metropolitan areas. Moreover, beyond just dropping a digital signal out of a higher-speed OC-n, the carrier can fill in what has been vacated. (For example, if a DS-1 is dropped off along the optical path, a new DS-1 can be multiplexed back into the OC-3 in its place.) This allows the carriers considerable flexibility. In Figure 27-10 an add-drop multiplexer is shown. Here portions of the bandwidth can be dropped off and additional new signals can be added in place of the data stream dropped out of the higher-speed signal. A single stage add-drop multiplexing function can multiplex various inputs into an OC-n signal. At an add-drop site, only those signals that need to be accessed are dropped and inserted. The remaining traffic continues through the network switching system without requiring special processing.
SONET Topologies Several different topologies can be used in a SONET network by using the various multiplexers. These include the normal topologies most networks have been accustomed to over the years. They include the following:
Figure 27-10 Add-drop multiplexing with SONET
■
Point-to-point
■
Point-to-multipoint
OC-1
OC-1
DSI DS1
OC-1
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Hub and spoke
■
Ring
It is the variations that allow the flexibility of SONET in the Wide Area Networks built by the carriers, but now becoming the method of choice at many large organizations. In each of the topologies, larger organizations are finding the benefits of installing highly reliable, interoperable equipment at the private network interfaces and access to the public networks.
Point-to-Point The SONET multiplexer, the entry level PTE for an organization (or the equipment installed by the LEC at the customer’s premises to access the network), acts as a concentrator device for the multiple lower-speed communications channels, such as DS-1 and DS-3. In its simplest form, two devices are connected with an optical fiber (with any repeaters as necessary) as a point-to-point circuit. As the entry-level point into a SONET architecture, the inputs and outputs are identical. In this environment, the network can act as a stand-alone environment and not have to interface with the public switched networks. See Figure 27-11 for the point-to-point multiplexing arrangement.
Point-to-Multipoint The next step is to consider the point-to-multipoint arrangement. This will use a form of add-drop multiplexing to drop circuits off along the way. In a large corporate network spanning the country (or any subset), a single, highspeed link may be employed. The SONET Add-Drop Multiplexer (ADM) is
Figure 27-11 Point-to-point service with SONET
OC-N
OC-N
OC-N
OC-N
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used for the task at hand, dropping circuits out without demultiplexing the entire high-speed signal. In Figure 27-12, the ADM is installed between two far-end locations so that signals can be added or dropped off as necessary. This is a better solution than renting three different circuits between points A-B, A-C, and B-C, which adds to the complexity and cost. By using a circuit from A-B-C with ADMs, the service can usually be more efficiently accommodated.
Hub and Spoke The hub and spoke method (sometimes referred to as the star network) allows some added flexibility in the event of unpredicted growth or constant changes in the architecture of the network. SONET multiplexers can be hubbed into a Digital Cross-Connect where it is concentrated and then forwarded on to the next node (Figure 27-13). This is used in many larger orga-
Figure 27-12 ADMs installed along the way
OC-N
OC-N
OC-N
OC-N
Insert 1 OC-1 Drop off 1 OC-1
Figure 27-13 Hub and spoke in a SONET multiplexer network
OC-1 OC-1
OC-1
OC-1
OC-3
OC-1
OC-1
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471 nizations where regional offices are located and district or branch offices are tied into the network through the hub. Once again the flexibility is there if a major change occurs in the network architecture or in the event of major campaigns in the organization. Hubs will act as the cross-connect points to link the various echelons in the network together. These may be developed in a blocking or nonblocking manner. Typically, some blocking may be allowed.
Ring Next, the glory of SONET! In a ring architecture, where SONET automatic protection switching is employed, the best of all worlds comes to fruition. In this topology, ADMs are used throughout the network, and a series of point-to-point links are installed between adjoining neighbors. The bidirectional capability places the most robustness into the network; however, unidirectional services can also be installed. The primary advantage of the ring architecture is survivability in the event of a cable cut or a failure in a network node. The multiplexers have sufficient intelligence to reroute or reverse direction in the event of a failure. If more than one fiber link is installed, the systems could use alternate paths, but they must recover in milliseconds (which APS on SONET is designed to do). Figure 27-14 shows the ring topology with dual fibers run (bidirectional service) between the ADMs.
Figure 27-14 Ring architecture of SONET multiplexers ADM
ADM
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Evolution of SONET in the Rest of the World Following the work by ANSI in developing the SONET standards, the rest of the world began to move in tandem. The ITU-TS decided to define a synchronous standard that would address internetworking between the ITU and ANSI transmission hierarchies. In 1989, the ITU published the Synchronous Digital Hierarchy (SDH) standards. SDH is a world standard, whereas SONET is a North American standard. However, SONET is considered a subset of the SDH standards. Prior to the publication of the SDH standard, different rates and speeds were available in the North American community (which the United States, Canada, and Japan) compared to the rest of the world. The ITU was looking for a means of accommodating the differences in a standardized package so that both standards could coexist. SDH marks one of the first steps in making that happen. However, although SDH sets a rate of multiplexing and the derived speeds (and the means of achieving those speeds), some differences between SONET and SDH exist. The packaging of the tributaries is different. In Table 27-7, a comparison of the SONET and SDH architectures is shown. SDH defines the Synchronous Transport Module (STM) level N, whereas SONET defines the STS-n and the OC-n.
Table 27-7
SONET Signal
SDH Signal
SONET Payload
SDH Payload
Comparison of SONET and SDH rates
OC-1
51.84 Mbps
STM-0
28 DS-1 or 1 DS-3
21 E-1
OC-3
155.52 Mbps
STM-1
82 DS-1 or 3 DS-3
63 E-1 or 1 E-4
OC-12
622.08 Mbps
STM-4
336 DS-1 or 12 DS-3
252 E-1 or 4 E-4
OC-48
2.488 Gbps
STM-16
1344 DS-1 or 48 DS-3
1008 E-1 or 16 E-4
OC-192
9.953 Gbps
STM-64
5376 DS-1 or 192 DS-3
4032 E-1 or 64 E-4
Rate
A. The OC-n also has a compatible STS-n rate. B. Even though the rates are the same, the packaging of the frames for SDH and SONET is different.
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Synchronous Digital Hierarchy Ever since the standards bodies approved the recommendations for the SDH (and SONET), the services have been effectively used to improve and revolutionize the industry. Significant cost efficiencies and performance improvements have been shown. SDH provides a means for the rest of the world to use the capabilities of fiber-based transport systems and multiplexing architectures to improve upon the older Plesiochronous Digital Hierarchy (PDH), which was inefficient and expensive. Digital networks continue to expand in complexity and penetration within the carriers’ networks, now moving closer to the consumer’s door. High-speed communications prior to the formulation of SDH in 1990 operated at speeds of up to 139.364 Mbps. However, the carriers implemented coaxial and radio-based systems operating at 140 Mbps– 565 Mbps. The networks were severely constrained due to the high cost of the transmission medium (coaxial cable, especially). The multiplexing rates used plesiochronous rates, which led to the European PDH. After the development of fiber and the enhancements of integrated circuitry, the newer transmission speeds and complex networking architectures became realistic. In Europe the evolution and deployment of ISDN also led to the proliferation of the B-ISDN standards, which allow a simple multiplexing technique. In the United States, the Bell breakup prompted the local carriers to look for interoperability and improvements in network management because of the proliferation of the number of carriers providing long distance services. The ITU-TS agreed that something had to be done to improve and standardize the multiplexing and the interoperability, while at the same time taking advantage of the higher capacity of optical fiber. Older bit interleaving of multiplexers should be replaced by byte interleaving to afford better network management. The new standard appeared as SONET in the North Americas, drafted by Bellcore. Later, this same standard developed into the SDH/SONET standard, as approved by the ITU. Although SONET and SDH were initially drafted in support of a fiber, radio-based system, supporting the same multiplexing rates became available.
