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2.1 Introduction
SONET and SDH define technologies for carrying multiple digital signals of different capacities in a flexible manner. Most of
the deployed optical networks are based on SONET and SDH standards. Consequently, a basic understanding of SONET and
SDH is a prerequisite for understanding the optical control plane issues discussed in later chapters.
The basis for signal transport under SONET and SDH is a technique called Time Division Multiplexing (TDM). Multiplexing in
general refers to the process of combining individual signals for transmission. The term de-multiplexing is used to denote the
process of separating individual signals from a multiplexed stream. While there are many techniques for doing these, TDM is
commonly used in telecommunication networks. This chapter begins with an introduction to TDM. Following this, the basic
features of SONET and SDH are described. The next chapter deals with more advanced features of SONET and SDH.
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2.2 Time Division Multiplexing (TDM)
TDM is a digital technology under which individual signals (i.e., a stream of binary digits or bits) are interleaved in time to
produce the multiplexed, composite signal. Under TDM, recurring time-slots or channels are created such that each channel
carries the bits from an individual signal. The total transmission bandwidth is split among the time-slots, with each
component channel getting some fixed percentage of the total signal bandwidth. This total signal includes not only the
payload bits for various component channels but also bits for performing overhead functions. TDM signals have a frame
structure that repeats at regular intervals.
TDM has a long history in telecommunication networks. Prior to the advent of TDM, telephone calls were handled in the
analog domain. Long distance calls were routed over twisted pair, coaxial cable, or analog microwave between switching
offices. In early 1960s AT&T began installing DS-1 T-carrier services between long distance switching centers. Under this
service, there were channel banks, which took 24 analog telephone lines, converted them to digital signals (called DS-0),
time-division multiplexed them into a DS-1 signal, and then transmitted them over copper wire. At the other end, the DS-1
signal was de-multiplexed into the constituent DS-0 signals, which were then converted back into analog. T-carrier reduced
the number of copper circuits required between switching centers and improved the quality of the telephone calls by
reducing noise and cross talk. As the traffic volume grew, the number of T-carrier circuits between switching centers
increased. In order to cope with the demand, higher speed T-carriers were introduced. In the late 1970s, optical
communications became feasible, enabling high-speed and high-fidelity digital communication. One of the first commercial
fiber circuits was installed in Chicago in 1977 and operated at 45 Mbps (DS-3 rate).
The frame structures of the DS-1 [ANSI95b] and the European E1 [ITU-T98a] signals are shown in Figure 2-1. The DS-1
signal consists of 24 payload channels plus overhead. The basic frame of each of these signals repeats every 125 µs, that is,
8000 times per second. With 8 bits carried in each channel, this gives rise to a basic data rate of 64 Kbps for each channel.
The requirement for this data rate stems from the need to sample the analog telephony signal 8000 times per second and
encoding each sample in 8 bits.
Figure 2-1. Basic Frame Structures for the DS-1 and E1 TDM Signals
A DS-1 frame contains 24 channels, each consisting of 8 bits, plus 1 framing/overhead bit, leading to a total of 193 bits.
Since the frame repeats every 125 µs (or 8000 times a second), the total bit rate of the DS-1 signal is 1.544 Mbps.
Similarly, the total bit rate of the E1 signal is 2.048 Mbps (32 channels of 8 bits, repeating every 125 µs).
In a TDM frame, each channel within the frame gets a fixed amount of bandwidth that is not shared with any other channel.
It is, however, necessary to determine the start of a TDM frame to de-multiplex the channels within.
With DS-1 signals, the start of frame can be identified by a particular pattern of framing/overhead bits occurring in a
consecutive series of frames. A DS-1 superframe consists of 12 regular frames as shown in Figure 2-2 [ANSI95b]. The
twelve-bit pattern, 100011011100, occurring in the superframe is used to delimit DS-1 frames. In addition to enabling this
function, the superframe structure allows the subdivision of the bits in a channel into lower rate channels.
Figure 2-2. The DS-1 Superframe Structure
In DS-1 networking, a technique called "robbed bit" signaling is used to convey signaling for each of the 24 channels within a
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DS-1 signal. Under this technique, the least significant bit of each channel (i.e., bit 8) in frames 6 and 12 of the superframe
are used to convey signaling for that channel. The bit rate of this signaling channel can be computed as follows. A single bit
out of a channel corresponds to a bit rate of 8000 bps (since the frame containing the channel occurs 8000 times per
second). Because the signaling bit is present in only two out of twelve frames, this rate is reduced by 1/6, that is, 8000/6 =
1333.3 bps. Note that robbed bit signaling prevents the use of this bit for carrying user data. Hence, only 7 of the 8 bits are
usually used for data services, leading to a 56 Kbps service over each channel.
While the DS-1 superframe allows signaling for the payload channels, there is no provision for conveying information
between the end points to aid in managing the signal. This was addressed with the DS-1 extended superframe (ESF) format
[ANSI95b]. The ESF is 24 regular frames long. Of the 24 overhead bits, 6 are used for frame alignment, another 6 are used
for error monitoring, and the remaining 12 are used to form a 4 Kbps data link between the sender and receiver.
With the E1 signal, the first octet (byte) in the frame contains overhead bits. A multiframe consisting of 16 frames is used
with the base E1 signal.
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2.3 Getting to Know the SONET and SDH Signals
North American standardization of SONET started in the mid 1980s with international standardization of SDH starting soon
thereafter. The following are the essential features of SONET [ANSI95a] and SDH [ITU-T00a]:
A base TDM frame structure along with a similarly structured set of progressively larger frames (for faster signals).
A limited variety of "payload envelopes" (SONET) or "virtual con-tainers" (SDH) of fixed capacity (bandwidth) that
are synchronously multiplexed into the base signal (or multiples of the base signal).
A flexible byte interleaving synchronous multiplexing method for placing the payload envelopes (or virtual
containers) into the base signal such that short-term rate variations are accommodated and latency is minimized.
An ever-increasing set of standardized payload mappings indicate how payloads as diverse as DS-3 and IP packets are
mapped into the payload envelopes or virtual containers. These features are described in detail in this and in the next
chapter.
2.3.1 The Base SONET and SDH Signals
Let us start looking at SONET/SDH signals from their most concrete aspect, that is, the basic signal types for which optical
interfaces exist. We can then examine the structure of these signals until complete familiarity with their constituent parts is
achieved. The basic physical SONET signal is known as an Optical Carrier–level 1 (OC-1) signal [ANSI95a]. This is an optical
signal with a bit rate of 51.840 Mbps and a host of physical parameters defined by national and international standards. For
the purposes of this book, the frame structure, rather than the physical layer signal itself, is more relevant.
SONET uses the terminology Synchronous Transport Signal—level 1 (STS-1) to denote the logical signal contained within the
optical signal, that is, the recurring TDM frame structure. Because the TDM frame structure in SONET/SDH is much longer
compared with those of the DS-1 and the E1 signals, a compact "row and column" representation is used when describing
SONET/SDH signals. Figure 2-3 shows the representation of the SONET STS-1 signal, which has 90 columns and 9 rows of
bytes. The basic frame repeats every 125 µs and the transmission order relative to this representation is from left to right,
and top to bottom. That is, the bytes in the first row are transmitted first, from left to right, followed by the bytes in the
second row, and so forth. The SONET frame contains both overhead bytes and a Synchronous Payload Envelope (SPE) that
carries the payload. The purpose of the overhead bytes is described in section 2.4.
Figure 2-3. Format of a SONET STS-1 Frame
Let us now compute the bit rate of a SONET STS-1 signal. The STS-1 frame consists of 810 bytes (90 columns times 9
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rows). With 8000 frames per second and 8 bits in each byte, the total bit rate is 51.84 Mbps. The bit rate of the SPE portion
can be computed as follows. Given that 3 columns of the frame are dedicated to section and line overhead, there are 87
columns and 9 rows used for the SPE in each frame. This gives 783 bytes per frame, and with 8000 frames per second and 8
bits/byte, the bit rate of the SPE is 50.112 Mbps. Thus, the line and section overhead shown in Figure 2-3 add approximately
3.33 percent overhead.
The basic structure of the higher rate SONET signals, the STS-3, -12, -48, -192 and -768 is very similar to the STS-1
structure to permit easy multiplexing of the constituent signals. In fact, these higher rate SONET signals are obtained by
byte-interleaving N STS-1 signals. Figure 2-4 shows the basic structure of an STS-N signal. The overhead and the payload
envelope capacity of this signal are N times the size of the corresponding fields of the STS-1 signal. These are, however, N
distinct signals. Also, there are slight differences in the definition and placement of the overhead bytes. It is actually the
SONET SPEs that are completely preserved during the byte interleaving when lower rate SONET signals are multiplexed into
a higher rate signal. Reference [ANSI95a] has separate diagrams detailing the overhead structure for each of the STS-1,
STS-3, STS-12, STS-48, STS-192, and STS-768 signals.
Figure 2-4. The STS-N Frame Structure (from [ANSI95a])
Figure 2-5 shows the overhead (line and section) associated with an STS-3 signal. Here, the framing pattern bytes (A1 and
A2) are repeated three times (once for each STS-1 interleaved signal). Similarly, there are three separate sets of pointer
bytes (H1–H3) that are used to locate the "floating" SPEs associated with each STS-1. Some overhead bytes such as section
trace (J0) need not be repeated and hence the corresponding bytes of the second and third STS-1 are either reserved for
growth or left undefined.
Figure 2-5. Overhead Bytes in an STS-3 Frame
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The bit rates of the standard SONET optical carrier signals, that is, OC-3, OC-12, OC-48, and OC-192, can be computed as
follows. First, since the optical carriers are just the optical version of their STS-N counterparts, their bit rates are the same
as the corresponding STS-N signals. Because an STS-N frame is obtained by byte interleaving N STS-1 frames, we get the
following signal bit rates: (a) OC-3 (3 x 51.84 Mbps) = 155.52 Mbps, (b) OC-12 = 622.08 Mbps, (c) OC-48 = 2.48832 Gbps,
(d) OC-192 = 9.95328 Gbps. Note that the rates (c) and (d) are typically rounded up and referred to as 2.5 Gbps and 10
Gbps in the literature.
Under the Synchronous Digital Hierarchy (SDH) definition, the base signal is known as a Synchronous Transport Module
(STM). SDH does not differentiate between the optical and electrical signals as SONET does with its OC-N/STS-N
terminology. The frame structure of the STM-N signal is shown in Figure 2-6. The STM-N frame is very similar in structure to
the STS-N frame except that it is three times larger. For example, an STM-1 signal is very similar to an STS-3 signal.
Figure 2-6. Frame Structure of an STM-N (from [ITU-T00a])
2.3.2 Payload Envelopes and Virtual Containers
Instead of just laying a user's signal into the payload of and STS-N or STM-N signal, the user's signal first gets placed into a
payload envelope. This is called the SPE in SONET or a Virtual Container (VC) in SDH. Earlier, we saw the entire SPE as part
of a SONET frame. The structure of an STS-1 SPE itself is shown in Figure 2-7. This is also referred to as an STS-1 path layer
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