This document explains why the output of the show
controller pos command on a Packet Over SONET (POS) interface can
display a non-zero value for the Positive Stuff Event (PSE) and Negative Stuff
Event (NSE) counters. The value continuously increases. These events increase
when the POS link experiences clocking problems. Therefore, this document also
There are no specific requirements for this document.
This document is not restricted to specific software and hardware
Technical Tips Conventions for more information on document
Here is a sample output of the show controller
pos command, captured on a Cisco 12000 Series Internet
LOF = 0 LOS = 0 BIP(B1) = 0
AIS = 0 RDI = 0 FEBE = 0 BIP(B2) = 0
AIS = 0 RDI = 0 FEBE = 967 BIP(B3) = 26860037
LOP = 0 NEWPTR = 205113 PSE = 295569 NSE = 18
Note: The NEWPTR error counter also can increase when NSE and PSE events
A simple view of a physical network link is that it defines a one-way
transmission path from a sending device or transmitter to a receiving device or
receiver. In other words:
A source device communicates pulses of voltage or light waves to
transmit a binary 1 or 0.
A destination device receives a binary 1 or 0. For this, the
receiving device measures the signal level on the physical wire at a specific
rate (frequency) and at a specific time (phase).
Both devices use a clock in order to determine when to perform the
task. Ideally, bits must arrive at the receiver in a very precise and concise
manner. The receiver must know the exact time that a binary 1 or 0 manifests
itself at the receiver interface. A transmitter and a receiver are perfectly
synchronized when they are in phase and in frequency.
Accurate clocking becomes more important with high-speed interfaces
like SONET because there is an inverse relationship between the number of bits
on a physical link in a second and the length of time that a bit manifests
itself at the receiver. For example, a SONET OC-3 interface can transmit
155,000,000 bits per second. Use this formula in order to calculate the time on
the wire of each bit:
1 / 155000000 = .000000006 seconds
Compare this value with the time on the wire of a bit on a T1 link:
1 / 1544000 = .000000648 seconds or 648 microseconds
Therefore, if the receiver experiences even a slight inaccuracy in the
timing of its sampling clock, it cannot detect a bit or even several bits in
succession. This problem leads to clock slips, which are the loss of timing,
and the resultant loss of the detection of bits. Clock slips can also result in
an incorrect interpretation of the binary 1s and 0s, and therefore lead to
parity and Cyclic Redundancy Check (CRC) errors.
Timing is not carried explicitly. Instead, a receiving interface
derives the frequency and phase of the transmitting interface. For this, the
receiving interface tracks the incoming signals and the transitions from 0 to 1
and 1 to 0.
You first need to understand how SONET uses H1 and H2 bytes in the line
Each Synchronous Transport Signal (STS-1) consists of 810 bytes, which
includes 27 bytes for the transport overhead and 783 bytes for the Synchronous
Payload Envelope (SPE). The format of an STS-1 frame and the nine rows by 90
columns are illustrated in .
Figure 1 – The Format of an STS-1 Frame
The transport overhead section breaks down into section overhead and
line overhead. The line overhead includes the H1 and H2 bytes. The SONET
protocol uses these bytes to identify the position of the payload in the SPE
portion of the frame. This table illustrates the location of the H1 and H2
D1 Data Com
D2 Data Com
D3 Data Com
C2 Signal Label
H3 Pointer Action
G1 Path Status
F2 User Channel
D4 Data Com
D5 Data Com
D5 Data Com
D7 Data Com
D8 Data Com
D9 Data Com
D10 Data Com
D11 Data Com
D12 Data Com
S1/Z1 Sync Status/Growth
M0 or M1/Z2 REI-L Growth
Z5 Tandem Connection
While SONET networks exhibit very accurate timing, some variations are
inevitable. Although the variation is very small, the small time on the wire of
each bit necessitates strict timing accuracy.
Synchronous networks can use several methods in order to resolve timing
problems. SONET networks use byte stuffing and pointer adjustments. Before you
study these concepts, you first need to understand underflows and overflows.
Fundamentally, a network device accepts traffic on an input line, and
writes it into a buffer based on the frequency of the incoming signal. A
locally generated clock determines the read frequency of the bits from the
buffer. The read rate determines when the contents of the frame (the binary 1s
and 0s) are placed onto an output line.
Clock slips, and the resultant overflows and underflows, lead to PSE
and NSE events within the network because a byte in the transmission stream is
deleted or repeated. Fundamentally, clock slips indicate that the clock rate on
the incoming interface is not synchronized somehow with the clock rate on the
Write into buffer is performed faster than read from buffer.
NSE—Move frame backward by one byte location.
Write into buffer is performed slower than read from buffer.
PSE—Move frame forward by one byte location, add an artificial
byte to compensate for failure of the writes.
A need for bit-stuffing occurs when the buffer is empty at a time when
a bit must be read. Stuff bits make up for a shortfall in the number of bits in
A PSE occurs on an Add/Drop Multiplexer (ADM) when the incoming signal
runs slightly behind with respect to the clock of the outgoing interface where
that data is cross connected. A PSE also occurs when the payload data rate is
slow with respect to the STS frame rate. In these conditions, the byte position
after the H3 byte is stuffed (skipped), and the pointer value in the H1 or H2
bytes is increased.
An NSE is precisely the opposite. When the input signal arrives too
quickly with respect to the frequency of outgoing interfaces, the data is not
buffered. Instead, the pointer value decreases by one, and the payload starts
one byte position earlier. Specifically, one payload byte is placed in the H3
byte, also known as the Pointer Action Byte. Normally, this byte is empty.
NSE and PSE events typically increase due to synchronization problems
on a link or incorrect clock settings. These events also increase in these
The received signal is very degraded, and the SONET framer on the
router reports what appears to be NSE and PSE events because of the highly
A back-to-back configuration uses internal - line, and there are
sufficient differences in the accuracy of the oscillator at each end.
The physical fiber is not sufficiently clean.
The transmitter overdrives the remote receiver, and there is
insufficient attenuation on the link.
The link experiences an alarm or badly-errored condition. While the
router clears this state, the router detects some valid NEWPTRs, and counts
these incorrectly as NSEs or PSEs.
It is important to note that Cisco POS interfaces do not generate PSE
or NSE counters because they send a fixed value in the H1 or H2 bytes. Cisco
POS interfaces only report what they see from the cloud.
This table lists the maximum allowable NSE and PSE rates for different
Stratum clock accuracy levels:
Maximum NSE and PSE Rate
11.2 stuffs per day
12.44 stuffs per minute
59.6 stuffs per second
259 stuffs per second
These numbers assume absolute worst case, end-of-life specifications
for the various clocks. They also assume that the two clocks are at opposite
ends of their ranges (that is, one is at the maximum while the other is at the
minimum), which is very unlikely in a production environment. Therefore,
typical numbers in a real network must be one or two orders of magnitude less
than these numbers.
Here are the PSE and NSE rates, if you assume the presence of two
Telcos with independent Stratum clocks:
Stratum 1 accuracy = +/- 1x10-11
Therefore, the worst-case offset between two Stratum 1 clocks is
STS-1 rate = 51.84x10+6 bits/second
Worst-case offset between two STS-1s that run off independent Stratum 1
(51.84x10+6) x (2x10-11)
= 103.68 x10-5 bits/second
= (103.68/8) x 10-5 bytes/second
= 12.96 x 10-5 bytes/ second
Each STS-1 pointer adjustment (or stuff) accommodates one byte of data.
Therefore, the number is also the NSE or PSE rate. Thus, the maximum NSE or PSE
rate when you assume the existence of Stratum 1 clocks is:
= 12.96 x 10-5 stuffs per second
= (12.96x10-5) x (60x60x24) stuffs per day
= 11.2 stuffs per day
Remember these points when you troubleshoot NSE and PSE events:
The rate of PSE and NSE events must not increase with load.
Cisco POS line cards generate a fixed pointer value of 522.
Therefore, you must not see any PSE or NSE events when you connect two POS line
cards back to back.
Some NEWPTR events can be reported when an interface clears an alarm
or during a badly-errored condition.
When you open a case with the Cisco Technical Support for help to
resolve the increase in the number of PSE and NSE events, please be prepared to
provide this information:
Whether the topology is back to back or across a SONET network of
Hardware platform and line card you use.
Brief description of the history of the problem and any steps that
you took to troubleshoot the problem.
Output of the show tech command from the
router that reports the events.