1.1
Introduction
In 1984 AT&T was divested … At that time
many companies began developing their own proprietary digital communication
networks. Local telephone companies had to connect to multiple long-distance
carriers, each with different proprietary synchronous standards, so the need for
standardization became obvious. In 1985, Bellcore
began working on a standard, called SONET (Synchronous Optical NETwork) [9] . Later, CCITT joined
the effort, which resulted in a parallel set of CCITT recommendations called SDH
(Synchronous Digital Hierarchy). Today, many companies use the SONET or SDH
standards for high speed broadband communications[1,
2]. Because SDH and SONET differ in only minor ways, this paper will discuss
SONET and SDH and use the terms almost interchangeably. In this paper, we will
first begin with a historical discussion of the plesiochronous digital hierarchy to develop the setting for
SONET/SDH. A detailed overview of the SONET/SDH architecture and a discussion of
its goals will follow. Finally, we will conclude with a
element by element discussion of SDH's
STM-n.
2
Plesiochronous Digital Hierarchy (PDH)
Prior to standardizing a synchronous digital
communication network, the
Figure 2‑1: American Multiplexing Chain
European telephone companies decided on a very
similar strategy, see Figure 2. An obvious difference between these multiplexing
schemes is the number of signals that are multiplexed into the next higher order
signal. However, the real difference in these two systems lies in the
construction of their basic signal. Recall that the basic DS-1 signal consisted
of 24 channels, whereas the basic DS-1E (European)
signal consists of 32 channels (notice the common bit rate of 139.264Mbps at the
highest level of each multiplexing scheme).
Figure
2‑2:
European Multiplexing Chain
Because all of these multiplexers contain their
own clock, these systems have to allow for slight differences in bit rates at
each input stream, and compensate for these differences within the output
stream. It is also important to realize that although input streams to any
multiplexer must be homogeneous, the signals themselves are not necessarily
frame synchronized. As an example consider the DS-2 to DS-3 multiplexer; the
multiplexer receives 7 signals, all of type DS-2, but there is no requirement
for the frames of DS-21 to begin at the same time as frames within DS-22. This
lack of overall structure creates serious trouble for system
designers.
One scenario that network system designers must
consider is the ability to extract a single signal or tributary from a highly
multiplexed signal. For example, let's say that we needed access 1 of the 84
DS-1 signals from a DS-4. To access this single DS-1,
we would have to demultiplex the DS-4 into its 3 DS-3
tributaries. One of which would then be demultiplexed
into 7 DS-2 signals, etc. After the DS-1 is finally retrieved, all of the
components need to be re-multiplexed back to a DS-4 for retransmission. This
processing overhead makes these systems unnecessarily complex. As transmission
rates increase, and multiplexing into even higher order signals gets more
common, these problems would only get worse.
3
GOALS of SONET
SONET was designed to support four major design
goals. First and foremost, the synchronous architecture of SONET should make it
possible for different carriers to interwork. This
required defining common signaling standards with
respect to wavelength, timing, and framing structure. A second was to unify the
The multiplexing hierarchy intended by the
original SONET proposal would allow for any combination of input tributaries,
thereby eliminating the requirement of homogeneous tributaries of asynchronous
multiplexers. Figure 3 shows the concept of a synchronous multiplexer, where the
STS-n represents a signal that has a bit rate or frame size that is "n" times
greater than that of the basic (SONET frame) STS-1. For example, an STS-3 can be
constructed by simply Byte-by-Byte multiplexing 3 STS-1
signals.
Figure
3‑1:
Sonet Framing
The SONET architecture provides the ability to
easily extract any lower order tributary from even a very highly multiplexed
SONET signal (DROP / ADD). The ease of DROP/ADD is accomplished by incorporating
payload pointers, that immediately indicate the
position of any tributary within the overall frame.
4
SONET
Figure 4 shows the SONET layered architecture.
SONET layers are similar to other layered protocols in that the layers
communicate with other layers of like type. SONET sections are defined as links
between any two SONET network elements. A line represents a link between any two
SONET multiplexers, a path represents the entire portion of the network during
which a signal is being transported by SONET. Each section, line, and path layer
add and check for header information, which may include
alarms, pointers, and maintenance information.
Figure
4‑1:
Layering
Ideally SONET is intended to be a physical
layer multiplexing system which may be mapped into the physical layer of the OSI
architecture. However, because sections, lines and paths, perform error checking
(not normally an operation performed at the physical layer of the OSI
architecture), SONET also comprises some of the OSI data link
layer.
The basic SONET frame, the STS-1, consists of
810 Bytes with a continuous transmission rate of 8000 frames/second. Within this
frame, 27 Bytes are dedicated to transport overhead, while up to 783 Bytes can
be used for payload. The structure of the basic STS-1 frame can most easily be
understood by viewing it as a 90 column wide by 9 row high rectangle of Bytes,
such as the one shown in Figure 5 (2 frames are illustrated here). The rectangle
consists of all 810 Bytes. Within each of the 9 rows, the first 3 Bytes are
reserved for header information. During transmission, the Byte at row 1-column 1
(top left) is sent into the network first, followed sequentially by all other
Bytes within row 1. All Bytes within row 2 are transmitted next, …etc., the last Bye to be transmitted is Byte #810
(bottom right), subsequent frames are transmitted in this same
order
Figure
4‑2:
Basic SONET Frame
As we stated earlier, Synchronous multiplexing
is accomplished by Byte-multiplexing of lower order synchronous signals. If the
frame structure in figure 5 was synchronously multiplexed with two other STS-1
signals, the resulting STS-3 frame would be a 270 column by 9 row rectangle of
Bytes, with the first 9 Bytes of each row dedicated as header Bytes. Notice that
that the individual synchronous signals need to be frame aligned before
multiplexing, however, there is no such requirement on the initial tributaries.
Because of differences that were cited between US and European communications
equipment, STS-1 signals are not defined in SDH, the European equivalent of
SONET. Recall that there is no commonality in transmission rates until DS-4
(139.264Mbps), refer to Figures 1 and 2. Because of
this, the basic European synchronous signal (STM-1) is defined to be equal to
STS-3 (155.52Mbbs).
Figure 4-3 summarizes nomenclature and various
bit rates of selected synchronous signals. Gross and SPE bit rates of an STS-n
or OC-n signal can be easily calculated by using the following
equation.
Figure
4‑3:
Data rates
To investigate header area in greater detail,
see Figure 6. Here it is easy to see that 9 Bytes are allocated to section
header, 15 Bytes are reserved for line overhead, the first 3 of which are
defined to be payload pointers.
Figure
4‑4:
Overhead & Payload
The individual overhead Bytes, as defined by
Kumar[10], are defined as
follows:
Section Overhead:
Figure 4‑5: Section Overhead
A1-A2 Frame alignment.
C1 This byte
represents the order of this frame in the Byte interleaved STS-n frame
B1 Section error monitor.
E1 This byte provides
a local order wire channel for voice communications between the regenerators and
network elements.
F1 Defined by user, and terminated at all
section-level equipment.
D1-D3 These bytes
provide a data communications channel for administration, alarms, maintenance,
and any other communication needs of a 192 Kbps channel between section
termination equipment.
Line Overhead:
Figure
4‑6:
Line overhead
H1-H3 These three
Bytes point to the starting locations of the payload information within the
STS-1 frame.
B2 Provides line error monitor.
K1-K2 These two Bytes
provide APS signaling between line termination
equipment.
D4-D12 These bytes
provide a data communications channel for administration, alarms, maintenance,
and any other communication needs of a 576 Kbps channel between line termination
equipment
Z1-Z2 Reserved for future growth
E3 This byte provides
a local orderwire channel for voice communications
between line termination equipment.
5 Elements of an STM-n
Figure
5‑1:
Multiplexing
5.1
Containers
The container (C-11, Figure 7) is the most
elemental unit of the synchronous multiplexing (SM) structure in the sense that
all of the North American and European tributaries have to be mapped into their
respective containers before they can proceed with the SM process and emerge as
a part of the STM-1.
5.2
Virtual
Containers
The VC's function is to support the connections
between different path layers in synchronous transmission. The VC (VC-11, Figure
7) consists of the payload and the path overhead (POH). To construct a VC a
single Byte pointer is added to the container.
5.3
Tributary Units
Once a Virtual Container has been formed, the
next step is to make a Tributary Unit (TU). The TU (TU-11, Figure 7) was
designed to provide adaptability between higher-order and lower-order path
layers. For instance, lower-order VCs can be mapped into higher-order VCs
through a TU or a TU Group (TUG). A TU is created by attaching a TU pointer to a
lower-order VC. (such as a VC-11) Here the pointer is
used to indicate the degree of offset of the lower-order VC relative to the
starting position of the higher-order VC's frame.
5.4
Tributary Unit Groups
The role of the tributary unit group (TUG,
Figure 7) is to collect one or more TUs and load them
onto a fixed location on the payload of a higher-order VC. No overhead is added
when a TUG is formed from the TUs. For example, a
TUG-2 is built by multiplexing four TU-11s together.
5.5
Administrative
Units
The last two unique types in a STM-n frame are
administrative units (AU, Figure 7) and administrative unit groups (AUGs). The AU functions as an adapter between the
higher-order path layer and the multiplexer section layer. As with the tributary
unit, the AU consists of payload and the AU pointer. The payload carries a
higher-order VC, and the AU pointer indicates the relative offset between the
starting positions of the AU payload and the frame of the multiplexer section
layer.
For example, the two types of AU, namely, AU-3
and AU-4, carry VC-3 and VC-4, respectively, and the AU pointer indicates the
degree of offset of VC-3 or VC-4 with respect to the STM-n frame. [8]
5.6
Administrative Unit
Groups
One or more AUs
occupying a fixed location on an STM payload is called an administrative unit
group (AUG, Figure 7). An AUG can consist of three AU-3s or a single AU-4. Like
a tributary unit group (TUG), the role of the AUG is to collect one or more
AUs and load them onto a fixed location on the payload
of the STM frame. No overhead is added when a AUG is
formed from the AUs.
5.7
STM-n
The STM is the final product of the SM
structure and is the signal that is actually transmitted over the synchronous
digital transmission network. STM-n is formed by byte-interleaving n AUGs with the addition of section overhead (SOH) at the
beginning of its frame.
5.8
Conclusions:
Manufacturers are eagerly developing equipment
to support 9.95Gbps (OC-192), because of Sonnet's reliability as a high speed
broad band digital hierarchy. Undersea fiber optic
cables as well as over-land cables make use of SONET rings for increased
bandwidth, speed, and system reliability[1,2].
6
References
[1] Forbes, "Sprint's Edge", May 8, 1995
[2]
Computer World, "Sprint Runs Rings to Protect Service", July 3, 1995
[3] IEEE
Communications Magazine, "Introduction to SDH/SONET", Sept 1993
[4] IEEE
Communications Magazine, "SONET Implementation", Sept 1993
[5] IEEE
Communications Magazine, "Network Synchronization-A Challenge for SDH/SONET?",
Sept 1993
[6] IEEE Communications Magazine, "Implementing a Flexible
Synchronous Network", Sept 1993
[7] IEEE Communications Magazine, "The Impact
of G.826", Sept 1993
[8] LEE, B.G., KANG, M., LEE, J.: Broadband
Telecommunications Technology, Boston: Artech House,
1993
[9] TANENBAUM, A.S.: Computer Networks 3rd Edition, Englewood Cliffs,
NJ: Prentice Hall, 1996
[10] KUMAR, B.: Broadband Communications, New York:
McGraw-Hill, 1994
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