This document is a broad outline of what Synchronous Optical NETwork
(SONET) technology is, and how it works.
There are no specific requirements for this document.
This document is not restricted to specific software and hardware
For more information on document conventions, refer to the
Cisco Technical Tips
SONET defines optical signals and a synchronous frame structure for
multiplexed digital traffic. It is a set of standards that define the rates and
formats for optical networks specified in ANSI T1.105, ANSI T1.106, and ANSI
A similar standard, Synchronous Digital Hierarchy (SDH), is used in
Europe by the International Telecommunication Union Telecommunication
Standardization Sector (ITU-T). SONET equipment is generally used in North
America, and SDH equipment is generally accepted everywhere else in the world.
Both SONET and SDH are based on a structure that has a basic frame
format and speed. The frame format used by SONET is the Synchronous Transport
Signal (STS), with STS-1 as the base-level signal at 51.84 Mbps. An STS-1 frame
can be carried in an OC-1 signal. The frame format used by SDH is the
Synchronous Transport Module (STM), with STM-1 as the base-level signal at
155.52Mbps. An STM-1 frame can be carried in an OC-3 signal.
Both SONET and SDH have a hierarchy of signaling speeds. Multiple
lower-level signals can be multiplexed to form higher-level signals. For
example, three STS-1 signals can be multiplexed together to form an STS-3
signal, and four STM-1 signals multiplexed together to form an STM-4 signal.
SONET and SDH are technically comparable standards. The term SONET is
often used to refer to either.
Each level of the hierarchy terminates its corresponding fields in the
SONET payload, as such:
A section is a single fiber run that can be terminated by a network
element (Line or Path) or an optical regenerator.
The main function of the section layer is to properly format the SONET
frames, and to convert the electrical signals to optical signals. Section
Terminating Equipment (STE) can originate, access, modify, or terminate the
section header overhead. (A standard STS-1 frame is nine rows by 90 bytes. The
first three bytes of each row comprise the Section and Line header overhead.)
Line-Terminating Equipment (LTE) originates or terminates one or more
sections of a line signal. The LTE does the synchronization and multiplexing of
information on SONET frames. Multiple lower-level SONET signals can be mixed
together to form higher-level SONET signals. An Add/Drop Multiplexer (ADM) is
an example of LTE.
Path-Terminating Equipment (PTE) interfaces non-SONET equipment to the
SONET network. At this layer, the payload is mapped and demapped into the SONET
frame. For example, an STS PTE can assemble 25 1.544 Mbps DS1 signals and
insert path overhead to form an STS-1 signal.
This layer is concerned with end-to-end transport of data.
The optical interface layers have a hierarchical relationship; each
layer builds on the services provided by the next lower layer. Each layer
communicates to peer equipment in the same layer and processes information, and
passes it up or down to the next layer. As an example, consider two network
nodes that are to exchange DS1 signals, as shown in this figure:
At the source node, the path layer (PTE) maps 28 DS1 signals and path
overhead to form an STS-1 Synchronous Payload Envelope (SPE) and hands this to
the line layer.
The line layer (LTE) multiplexes STS-1 SPE signals and adds line
overhead. This combined signal is then passed to the section layer.
The section layer (STE) performs framing and scrambling and adds
section overhead to form an STS-n signal.
Finally, the electrical STS signal is converted to an optical signal
for the photonic layer and transmitted over the fiber to the distant node.
Across the SONET network, the signal is regenerated in optical
regenerators (STE-level devices), passed through an ADM (an LTE-level device),
and eventually terminated at a node (at the PTE level).
At the distant node, the process is reversed from the photonic layer to
the path layer where the DS1 signals terminate.
A standard STS-1 frame is nine rows by 90 bytes. The first three bytes
of each row represent the Section and Line overhead. These overhead bits
comprise framing bits and pointers to different parts of the SONET frame.
There is one column of bytes in the payload that represents the STS
path overhead. This column frequently "floats" throughout the frame. Its
location in the frame is determined by a pointer in the Section and Line
The combination of the Section and Line overhead comprises the
transport overhead, and the remainder is the SPE.
For STS-1, a single SONET frame is transmitted in 125 microseconds, or
8000 frames per second. 8000 fps * 810 B/frame = 51.84 Mbs, of which the
payload is roughly 49.5 Mbs, enough to encapsulate 28 DS-1s, a full DS-3, or 21
An STS-3 is very similar to STS-3c. The frame is nine rows by 270
bytes. The first nine columns contain the transport overhead section, and the
rest is SPE. For both STS-3 and STS-3c, the transport overhead (Line and
Section) is the same.
For an STS-3 frame, the SPE contains three separate payloads and three
separate path overhead fields. In essence, it is the SPE of three separate
STS-1s packed together, one after another.
In STS-3c, there is only one path overhead field for the entire SPE.
The SPE for an STS-3c is a much larger version of a single STS-1 SPE.
STM-1 is the SDH (non-North American) equivalent of a SONET (North
American) STS-3 frame (STS-3c to be exact). For STM-1, a single SDH frame is
also transmitted in 125 microseconds, but the frame is 270 bytes long by nine
rows wide, or 155.52 Mbs, with a nine-byte header for each row. The nine-byte
header contains the Multiplexer and Regenerator overhead. This is nearly
identical to the STS-3c Line and Section overhead. In fact, this is where the
SDH and SONET standards differ.
SDH and SONET are not directly compatible, but only differ in a few
overhead bytes. It is very unlikely that Cisco will ever use a framer that does
not support both.
SONET is very widely deployed in telco space, and is frequently used in
a ring configuration. Devices such as ADMs sit on the ring and behave as
LTE-layer devices; these devices strip off individual channels and pass them
along to the PTE layer.
All current Cisco line cards and Port Adapters (PAs) act as PTE-layer
devices; these devices terminate the full SONET session and L2 encapsulation.
They are Packet Over SONET (POS) cards, which indicate serial transmission of
data over SONET frames. There are two RFCs that describe the POS process: RFC
1619, PPP over
, and RFC 1662,
These Cisco products cannot sit directly on a
SONET or SDH ring. One of them must hang off of some LTE-layer device, such as
an ADM. Equipment such as an Integrated SONET Router (ISR) has both PTE and LTE
functionality, so it can terminate and pass through data.
These parameters affect configuration of SONET devices:
Clocking—The clocking default value is line, and is
used whenever clocking is derived from the network. The clock
source internal command is typically used when two Cisco 12000
Series Internet Routers are connected back-to-back, or are connected over dark
fiber where no clocking is available. In either case, each device must have its
clock source set to internal. For a more detailed explanation, refer to
Clock Settings on POS Router Interfaces.
Loopback—Loopback is a line and internal (DTE)
value. This is a SONET section loopback if done on the controller. If done on
the individual interface, these are individual path loopbacks.
Framing—Most Cisco framers support both SONET and
Payload scrambling—This value is normally set to On.
S1S0 flag—This value must be between 0 and 3; the
default value is 0. With SONET, s1so must be set
to 0, and with SDH it must be set to 2. Value 3 corresponds to the received
Alarm Indication Signal (AIS).
J0 flag - 0-255—This setting is the section trace
identifier. It is only required for section tracing.
C2 flag - 0-255—This setting specifies the STS path
signal label (5 to 7 are configured with the pos
Alarm reporting—Alarm reporting allows you to
specify which alarms are reported. The permitted values are b1-tca, b2-tca,
sf-ber, sd-ber, los, lof, ais-l, and rdi-l. (This value is configured with the
pos report command).
Alarm thresholds—The alarm threshholds setting
specifies the Bit Error Rate (BER) thresholds that signal an alarm. (This value
is configured with the pos threshold command).
Provided in this section is a screen capture from the show
controllers pos x/y command that displays the status of the SONET
If the link is down/down, check for active alarms and defects.
Troubleshooting in this case is essentially the same as serial troubleshooting.
If you look at the SONET controller (refer to the example given), it can
provide plenty of L1 and SONET information. Defects and alarms in SONET are
similar to the same alarms when you troubleshoot and diagnose T1/E1 and T3/E3
(LOS, LOF, AIS (Blue Alarm), and so on) issues.
Active defects and active alarms fields show the current status of the
POS controller, and point to the problem.
The numbers for errors under the Section, Line, and Path are
accumulators, and tell you the number of times the condition has occurred;
these numbers do not indicate if the error is currently happening.
Bit Interleaved Parity (BIP) errors are parity errors that correspond
to a specific SONET layer: BIP(B1) corresponds to Line, BIP(B2) to the Section,
and BIP(B3) to the Path layer parity errors.
When you look at the output of the show controllers pos
x/y command, pay attention to which SONET layers accumulate
errors: SONET Line, Section, or Path. When you troubleshoot SONET problems or
errors, the first thing to do is to isolate the bad section.