Sonet (Synchronous Optical NETwork) and SDH (Synchronous Digital Hierarchy)

Before reading this you may find the following tutorials useful:
Protocol Basics, Internet Protocol (IP) Optical fiber is a demanding mistress. Its vast capacity needs to be accessed by a protocol that can support high data rates of up to 10 Gbit/s per wavelength or, in the future, 40 Gbit/s. There also needs to be inherent reliability and preferably an extensive range of network monitoring and management functions. It is also desirable to be able to easily access smaller portions of the large data streams without any complicated demultiplexing techniques. The protocol used in modern networks to satisfy these cravings is Synchronous Digital Hierarchy (SDH) or the almost identical Synchronous Optical NETwork (Sonet) which is primarily used in the U.S., Canada, and Japan.

SDH is placed at the bottom of the protocol stack in the physical layer, along with the very fiber upon which it transmits its signal. Any IP traffic that is destined to be transmitted across a fiber-based SDH network will be framed by a Layer 2 protocol before being ready to take its orders from the SDH equipment.

The fundamental principle of the SDH protocol is time-division multiplexing (TDM), which works in contrast to a system such as Ethernet where data is sent in sporadic bursts. TDM ensures that there is a constant stream of data travelling through the network and taking advantage of the fiber bandwidth available. Lower bit-rate streams of information are combined, or multiplexed, up into higher bit-rate streams at the native bit rate of the SDH system. The fundamental SDH frame is known as STM1 (synchronous transport module); its Sonet counterpart is OC3 (optical container). Each provides a bit rate of 155 Mbit/s with a total frame size of around 20 kbits.

An STM-1 Frame The SDH frame has a two-dimensional structure, as shown in the diagram. Each frame is physically transmitted through the fiber row by row, each row from left to right. An STM1 frame is formed by nine rows and 270 columns of bytes of information. The first nine columns form what is known as the "section overhead" or SOH, which provides a comprehensive range of facilities such as error monitoring, network management, and automatic switching between fiber links should one be unavailable (known as "protection switching"). The remainder of the frame is termed a "virtual container" or VC, and in the case of an STM1 is known as a VC4. This contains the data – except for the first column, which is the "path overhead" or POH, whose function it is to monitor the quality of the link and indicate the type of data payload it is carrying.

SDH / SONET Bit Rates Contained within a VC4 may be multiple VC12 containers of 2-Mbit/s capacity, or VC3 containers, each giving 34 Mbit/s. These allow a greater level of "granularity," as the SDH equipment can access each of these individual streams for routing to the specific customer. The STM1 frame can also be scaled up, which is done by factors of four. An STM4 (or OC12) frame gives a bit rate of 620 Mbit/s and will still have 9 rows but 1080 (4 x 270) columns of bytes, with the overhead sections also being scaled accordingly.

Setting up a circuit of SDH frames between two locations is not as easily done as when sending out data on an Ethernet cable in a LAN (local area network). In fact, it requires human intervention to set up circuits via network management software and may take several minutes to perform. In such high-capacity links, however, the speed of configuration is not necessarily a top priority.

In a practical application, an SDH "line system" will have a multiplexer that takes its inputs from a variety of sources in different Layer-2 data formats. These are aggregated up to form frames at the line rate of the system, for example up to STM64 for a 10-Gbit/s bit-rate system. These frames are then transmitted out onto optical fiber links, and there is the possibility for multiple SDH multiplexers to each give out one wavelength of a WDM system. Along the length of the system there can be regenerators that will convert the optical signals to electrical, clean them up, and then re-transmit them out in optical format again. At the end of the system will be an SDH demultiplexer that now accesses the individual data streams from the STM64 frames as required.

There may also be an SDH add/drop multiplexer (ADM) with the ability to remove and insert lower bit-rate streams from the signal; or, alternatively, a digital crossconnect may be present with the ability to switch individual VC4’s between different fiber links. All such switching and add/drop functionality is performed purely in the electrical domain, as the optical SDH signals will always be converted to electrical signals upon arrival. It is worth noting that interworking between SDH and Sonet systems is possible at matched bit rates (e.g., STM4 and OC12), although a slight modification to the overheads is required, as they are structured a little differently.

Key Points

  • Synchronous Optical NETwork (SONET) in the U.S., Canada, and Japan; Synchronous Digital Hierarchy (SDH) elsewhere
  • Time-division multiplexing (TDM) transmits a constant stream of information
  • Fundamental SDH frame is STM1 (synchronous transport module); Sonet version is OC3 (optical container) – each providing 155 Mbit/s
  • STM4 provides four times the STM1 capacity; STM16 is a further fourfold increase, etc.
  • Two-dimensional frame structure with overheads for functions such as error monitoring, network management, and protection switching
  • Virtual container carries the data (a single VC4 in an STM1 frame)
  • Container may carry smaller streams as low as 2-Mbit/s VC12s that can be accessed directly at demultiplexers
  • SDH circuits set up through network management software
COMMENTS Add Comment
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vishu_ndg 12/5/2012 | 2:27:51 AM
re: Sonet (Synchronous Optical NETwork) and SDH (Synchronous Digital Hierarchy) Can any one explain what is mean't by floating SPE
and how the pointer bits of H1 and H2 (namely increment and decrement bits) are used to adjust for phase and frequency corrections ?
gpraveenji 12/5/2012 | 12:58:18 AM
re: Sonet (Synchronous Optical NETwork) and SDH (Synchronous Digital Hierarchy) Hai, I am new to SONET.
I need two clarification

Firstly, Why is that we speak about the SONET frames in terms of columns. If we want to may DS1 signal, why do we say that it fits in into 3 columns, and such kind ??

Next, what is the significance of the fixed stuff columns 30 and 59 in the SONET SPE.

Pls let me know regarding this

Thank you
dorin 12/5/2012 | 12:38:20 AM
re: Sonet (Synchronous Optical NETwork) and SDH (Synchronous Digital Hierarchy) Hello guys,
I am new into Sonet, (but I have good knowledge in PLL's)
and I have a few question?

Why do you have to synchronize 155.52MHz clock with a 8kHz clock? Why not synchronizing with 155.52?

What is the acceptable amount of jitter for the synchronized clock (the 155.52 produced from 8kHz)?


gardner 12/5/2012 | 12:00:56 AM
re: Sonet (Synchronous Optical NETwork) and SDH (Synchronous Digital Hierarchy) Can anybody tell me that how the first point to point connection is made,i.e. how does the sonet frame tells the inbetween LTEs about its destination PTE?

The short answer is that it doesn't or at least it doesn't explicitly. I think you may be misunderstanding the way SONET works. There is no addressing the way there is in networking technologies (e.g. ATM or IP). The end points don't decide where a path facility ends up. The intermediate LTEs do via cross connections. Consider an STS-1 path facility that is going from NY to SF. At each LTE (called multiplex section terminating equipment in Europe and this is more descriptive of what it really does) the collection of STS-1s in the OC-n coming in on one line can be plucked out and put on different slots on other lines. The destination of a given stream of STS-1 payload is completely determined by the cross connections on intermediate LTEs. Network connections are made by creating cross connections on network equipment that supports it (LTEs).

grapsfan 12/5/2012 | 12:00:39 AM
re: Sonet (Synchronous Optical NETwork) and SDH (Synchronous Digital Hierarchy) eallbr3853:

An asynchronous data stream maps directly into a SONET or SDH path. The legacy mappings have been standardized forever: a DS1 into a VT1.5, an E1 into a VC-12 (I think that's the right SDH frame, I'm not European), a DS3 into an STS-1.

There's new standards being adopted for new technologies. Most of these involve mapping Ethernet payloads into SONET frames, via something call Virtual Concatenation. There's way too much with that to get into here, I guess...if LR doesn't have an online tutorial on the subject, drop them a note and threaten to write your congressman or something.
grapsfan 12/5/2012 | 12:00:38 AM
re: Sonet (Synchronous Optical NETwork) and SDH (Synchronous Digital Hierarchy) eallbr3853:

In terms of SONET rings, there's two Bellcore-standard methods. The simpler of the two is called Unidirectional Path Switched Rings, or UPSR. In this architecture, a node inserting traffic on a ring puts it in one STS timeslot on each of the two fibers on the ring. The node dropping that traffic does a comparison (called a path selection) between the two, and takes the better of the two signals. So in case of a fiber cut, for example, the drop node is going to see good signal on one path, Loss Of Signal on the other, and take the good one. The rules for what makes the signal "better", along with everything else about UPSR, is defined in GR-1400-CORE.

The other ring algorithm is called Bidirectional Line Switched Rings, or BLSR. In BLSR, the K1 & K2 bytes in the SONET line overhead are used to communicate any problems in the traffic on the ring (just like they're used in 1+1 linear protection). So, going back to our fiber cut example, when a node detects the cut, the K1 & K2 bytes in its outgoing signal are going to tell the other nodes on the ring that there's a failure, and that traffic needs to be rerouted (through a procedure called squelching) from working paths to protection paths. It's an order of magnitude more complicated than UPSR, but it also provides a large improvement in the survivability of the traffic for multiple failures on that ring. All of this is defined, not very well might I add, in GR-1230-CORE.

Hope this is what you're looking for, and that I didn't screw everything up. I'm including 11 years (off and on) of SONET rings into two paragraphs.
cdo 12/4/2012 | 11:37:40 PM
re: Sonet (Synchronous Optical NETwork) and SDH (Synchronous Digital Hierarchy) would the oc48 channelized be able to transport accross the oc48 concatenation interface? Would the OC48c/OC48 port automatically detects the concatenantion indicator of H1 and H2 bytes and transport hte signal accordingly? Which vendor and product supports this capability?

Many thanks,

stephenpcooke 12/4/2012 | 11:37:37 PM
re: Sonet (Synchronous Optical NETwork) and SDH (Synchronous Digital Hierarchy) As you have posted your question in an area that deals with BOTH SONET and SDH I will answer both.

SONET and SDH both use the same pointer format for the H1 & H2 bytes. The concatenation indicator (valid NDF - usually 1001 - SS bits - both are 'don't care' for this situation - and 10 '1s') is identical for both formats. The relevant standards are for SONET GR-253-CORE Issue 3, and for SDH G.707 October 2000 (there were a couple of addendums in Dec 2002 but these do not change anything to do with pointer processing relating to the concatenation indication).

To specifically answer your question, virtually ANY piece of equipment that is designed to handle OC48 can also handle OC48c. Similarly, for SDH, any piece of equipment designed to handle STM-16 can handle STM-16c. In addition, all OC-192 equipment can handle upto 4 STS-48c's on its high-speed side (STM-64 equipment can handle upto 4 STM-16c's).

If you are buying equipment to handle such traffic you might insist on seeing the results of compliance tests to ensure that they do what they are supposed to do or even do them yourself. SONET and SDH test sets are available in all levels for rent as well as purchase.

I hope that I have been able to answer your question. Good Luck!
glowingduck 12/4/2012 | 11:02:04 PM
re: Sonet (Synchronous Optical NETwork) and SDH (Synchronous Digital Hierarchy) Sure, that's easy. Get two identical SDH muxes. Connect the "outputs" of one mux to the "inputs" of the other mux with suitable lenghts of fiber. Turn on the muxes. Voila! You should be sending 8000 SDH frames per second!

As for network mangement tools, everybody makes one, and most of them suck. I suggest you bone up on TL-1 and Procomm Plus and roll your own.
debasish71 12/4/2012 | 10:49:39 PM
re: Sonet (Synchronous Optical NETwork) and SDH (Synchronous Digital Hierarchy) what are the standards for transmissions above 10gbps(stm-64)...like in the terabits per sec context.
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