This document provides the answers to some frequently asked questions
for optical timing.
If voice traffic is still intelligible to the listener in a relatively
poor communication channel, why isn't it easy to pass it across a network
optimized for data?
A. Data communication requires very low Bit-error Ratio (BER) for high
throughput but does not require constrained propagation, processing, or storage
delay. Voice calls, on the other hand, are insensitive to relatively high BER,
but very sensitive to delay over a threshold of a few tens of milliseconds.
This insensitivity to BER is a function of the human brain's ability to
interpolate the message content, while sensitivity to delay stems from the
interactive nature (full-duplex) of voice calls. Data networks are optimized
for bit integrity, but end-to-end delay and delay variation are not directly
controlled. Delay variation can vary widely for a given connection, since the
dynamic path routing schemes typical of some data networks may involve varying
numbers of nodes (for example, routers). In addition, the echo-cancellers
deployed to handle known excess delay on a long voice path are automatically
disabled when the path is used for data. These factors tend to disqualify data
networks for voice transport if traditional public switched telephone network
(PSTN) quality is desired.
How does synchronization differ from timing?
A. These terms are commonly used interchangeably to refer to the process
of providing suitable accurate clocking frequencies to the components of the
synchronous network. The terms are sometimes used differently. In cellular
wireless systems, for example, "timing" is often applied to ensure close
alignment (in real time) of control pulses from different transmitters;
"synchronization" refers to the control of clocking frequencies.
If I adopt sync status messages in my sync distribution plan, do I have
to worry about timing loops?
A. Yes. Source Specific Multicasts (SSMs) are certainly a very useful tool
for minimizing the occurrence of timing loops, but in some complex
connectivities they are not able to absolutely preclude timing loop conditions.
In a site with multiple Synchronous Optical Network (SONET) rings, for example,
there are not enough capabilities for communicating all the necessary SSM
information between the SONET network elements and the Timing Signal Generator
(TSG) to cover the potential timing paths under all fault conditions. Thus, a
comprehensive fault analysis is still required when SSMs are deployed to ensure
that a timing loop does not develop.
If ATM is asynchronous by definition, why is synchronization even
mentioned in the same sentence?
A. The term Asynchronous Transfer Mode applies to layer 2 of the OSI
7-layer model (the data link layer), whereas the term synchronous network
applies to layer 1 (the physical layer). Layers 2, 3, and so on, always require
a physical layer which, for ATM, is typically SONET or Synchronous Digital
Hierarchy (SDH); thus the "asynchronous" ATM system is often associated with a
"synchronous" layer 1. In addition, if the ATM network offers circuit emulation
service (CES), also referred to as constant bit-rate (CBR), then synchronous
operation (that is, traceability to a primary reference source) is required to
support the preferred timing transport mechanism, Synchronous Residual Time
Most network elements have internal stratum 3 clocks with 4.6ppm
accuracy, so why does the network master clock need to be as accurate as one
part in 10^11?
A. Although the requirements for a stratum 3 clock specify a free-run
accuracy (also pull-in range) of 4.6ppm, a network element (NE) operating in a
synchronous environment is never in free-run mode. Under normal conditions, the
NE internal clock tracks (and is described as being a traceable to) a Primary
Reference Source that meets stratum 1 long-term accuracy of one part in 10^11.
This accuracy was originally chosen because it was available as a
national primary reference source from a cesium-beam oscillator, and it ensured
adequately low slip-rate at international gateways.
Note: If primary reference source (PRS) traceability is lost by the NE, it
enters holdover mode. In this mode, the NE clock's tracking phase lock loop
(PLL) does not revert to its free-run state, it freezes its control point at
the last valid tracking value. The clock accuracy then drifts elegantly away
from the desired traceable value, until the fault is repaired and traceability
What are the acceptable limits for slip and/or pointer adjustment rates
when designing a sync network?
A. When designing a network's synchronization distribution sub-system, the
targets for sync performance are zero slips and zero pointer adjustments during
normal conditions. In a real-world network, there are enough uncontrolled
variables that these targets will not be met over any reasonable time, but it
is not acceptable practice to design for a given level of degradation (with the
exception of multiple timing island operation, when a worst-case slip-rate of
no more than one slip in 72 days between islands is considered negligible). The
zero-tolerance design for normal conditions is supported by choosing
distribution architectures and clocking components that limit slip-rates and
pointer adjustment rates to acceptable levels of degradation during failure
(usually double-failure) conditions.
Why is it necessary to spend time and effort on synchronization in
telecom networks when the basic requirement is simple, and when computer LANs
have never bothered with it?
A. The requirement for PRS traceability of all signals in a synchronous
network at all times is certainly simple, but it is deceptively simple. The
details of how to provide traceability in a geographically distributed matrix
of different types of equipment at different signal levels, under normal and
multiple-failure conditions, in a dynamically evolving network, are the
concerns of every sync coordinator. Given the number of permutations and
combinations of all these factors, the behavior of timing signals in a
real-world environment must be described and analyzed statistically. Thus, sync
distribution network design is based on minimizing the probability of losing
traceability while accepting the reality that this probability can never be
How many stratum 2 and/or stratum 3E TSGs can be chained either in
parallel or series from a PRS?
A. There are no defined figures in industry standards. The sync network
designer must choose sync distribution architecture and the number of PRSs and
then the number and quality of TSGs based on cost-performance trade-offs for
the particular network and its services.
Is synchronization required for non-traditional services such as
A. The answer to this topical question depends on the performance required
(or promised) for the service. Usually, Voice-over-IP is accepted to have a low
quality reflecting its low cost (both relative to traditional PSTN voice
service). If a high slip-rate and interruptions can be accepted, then the voice
terminal clocks could well be free-running. If, however, a high voice quality
is the objective (especially if voice-band modems including Fax are to be
accommodated) then you must control slip occurrence to a low probability by
synchronization to industry standards. You must analyze any new service or
delivery method for acceptable performance relative to the expectations of the
end-user before you can determine the need for synchronization.
Why is a timing loop so bad, and why is it so difficult to
A. Timing loops are inherently unacceptable because they preclude having
the affected NEs synchronized to the PRS. The clock frequencies are traceable
to an unpredictable unknown quantity; that is, the hold-in frequency limit of
one of the affected NE clocks. By design, this is bound to be well outside the
expected accuracy of the clock after several days in holdover, so performance
is guaranteed to become severely degraded.
The difficulty in isolating the instigator of a timing loop condition
is a function of two factors: first, the cause is unintentional (a lack of
diligence in analyzing all fault conditions, or an error in provisioning, for
example) so no obvious evidence exists in the network's documentation.
Secondly, there are no sync-specific alarms, since each affected NE accepts the
situation as normal. Consequently, you must carry out trouble isolation without
the usual maintenance tools, relying on a knowledge of the sync distribution
topology and on an analysis of data on slip counts and pointer counts that is
not usually automatically correlated.
What is the difference between SONET and SDH?
A. There is no STS-1. The first level in the SDH hierarchy is STM-1
(Synchronous Transport Mode 1) has a line rate of 155.52 Mb/s. This is
equivalent to SONET's STS-3c. Then comes STM-4 at 622.08 Mb/s and STM-16 at
2488.32 Mb/s. The other difference is in the overhead bytes which are defined
slightly differently for SDH. A common misconception is that STM-Ns are formed
by multiplexing STM-1s. STM-1s, STM-4s and STM-16s that terminate on a network
node are broken down to recover the virtual circuits (VCs) they contain. The
outbound STM-Ns are then reconstructed with new overheads.
What is hair pinning, and why would I want to use
A. Hair pinning is bringing traffic in on a tributary and instead of
putting it on the high speed OC-N line you direct it out another low speed
tributary port. You might want to do this if you have interfaces to two
interexchange carriers (IXCs) on different nodes. If one of your IXCs goes
down, you can hair pin the other to pick the traffic, assuming the spare
capacity exists on the tributary. Hairpin cross-connections allow local drop of
signals, ring extensions supported by a ring host node, and allow passing
traffic between two ring interfaces on a single host node. In this case, no
high speed channel is involved and the cross-connections are entirely within
Doesn't a two fiber bi-directional line switched ring (BDLSR) waste half
of the line rate bandwidth?
A. No. It can be shown that in all cases the aggregate bandwidth on a two
fiber BDLSR is no less than the aggregate bandwidth on a path switched ring. In
some cases that exemplify an inter-office transport ring, it can actually be
shown that the aggregate bandwidth of a two fiber BDLSR can be larger than that
of a path switched ring.
What is the difference between TSA and TSI?
A. Time Slot Assignment (TSA) allows for flexible assignment for
add-dropped signals but not for through path signals. Once a signal is
multiplexed onto a time slot it stays in that time slot until it is dropped.
Time Slot Interchange (TSI) is more flexible in that it allows a signal passing
through a node to be placed in another time slot if desired. Equipment that
provides neither TSA or TSI is said to be hard wired. This pass-through
grooming, which is not supported by systems limited to TSA, allows in-transit
bandwidth rearrangements for maximum facility utilization. This grooming is
most useful for networks with intersite routing (for example, interoffice or
private networks) and networks with significant churn (service removal as well
as new service installation).
What are some timing rules of thumb?
A. Here are some basic points:
A node can only receive the synchronization reference signal from
another node that contains a clock of equivalent or superior quality (stratum
The facilities with the greatest availability (absence of outages)
should be selected for synchronization facilities.
Where possible, all primary and secondary synchronization facilities
should be diverse, and synchronization facilities within the same cable should
The total number of nodes in series from the stratum 1 source should
be minimized. For example, the primary synchronization network would ideally
look like a star configuration with the stratum 1 source at the center. The
nodes connected to the star would branch out in decreasing stratum level from
No timing loops may be formed in any combination of primary.
What are some advantages of timing from an OC-N
A. OC-N timing distribution has several potential advantages. It preserves
transport bandwidth for customer services and guarantees a high-quality timing
signal. Also, as the network architecture evolves to replace Digital Signal
Cross Connect (DSX) interconnects with SONET interconnects and direct OC-N
interfaces, OC-N distribution becomes more efficient than multiplexing DS1
references into an access facility. A previous drawback to using OC-N timing
distribution was that network timing failures could not be communicated to
downstream clocks via DS1 Alarm Indication Signal (AIS), since the DS1 signal
does not pass over the OC-N interface. A standard SONET synchronization
messaging scheme to convey synchronization failures is in place. With this
option, clock stratum levels can be passed from NE to NE, allowing downstream
clocks to switch timing references without creating timing loops, if a network
synchronization failure occurs. If a quality timing reference is no longer
available, the NE sends AIS over the DS1 interface. If the local OC-N lines
fail, the NE outputs AIS on the DS1 output or an upstream NE enters holdover.
Although an ideal source of timing, OC-N timing distribution, through a DS1
timing output, cannot be used to provide timing in all applications. In cases
where the local equipment is not provided with an external timing reference
input, or in some private networks where the timing is to be distributed from
another private network location, timing may be distributed via
traffic-carrying DS1s. In these applications, a stable DS1 timing source can be
achieved by ensuring that all elements in the SONET network are directly
traceable to a single master clock via line timing.
Note: Synchronous operation via line timing eliminates the generation of
virtual terminal (VT) pointer adjustments, thus maintaining the phase stability
needed for a high-quality DS1 timing reference. Cross-connecting at the STS-1
level also eliminates the VT pointer adjustments. It is recommended that, where
possible, the DS1 sources (switch, private branch exchange [PBX], or other
equipment) be traceable to the same timing source used to time the SONET NE.
Multiplexed DS1 reference transport is also consistent with current planning
and administration methods (but you better know exactly what is happening to
that multiplexed DS1).
What is the advantage of using the DS1 timing output instead of a
multiplexed DS1 as the timing reference?
A. The DS1 timing output is derived from the optical line rate and is
superior because the DS1 is virtually jitter-free. Synchronization messages
guarantee the traceability of the timing. Administration of traffic DS1s for
timing is eliminated
Can a DS1 carried over SONET ever be used as a timing
A. Yes. In many applications there is no other choice. Most switch
remotes, for instance, obtain their timing from a specific DS1 signal generated
by their host switch; so these remotes must line or loop time from the DS1
signal. In addition, digital loop carrier (DLC) equipment, channel banks, and
PBXs are not likely to have external references and may be allowed to line or
loop time from a DS1 carried over SONET. Five years ago all the literature
however answered no to this question. See the next question for more
Are there any specific concerns when using a DS1 carried over SONET to
time equipment such as a switch remote or DLC?
A. Yes. The major concern is to make sure all the equipment is synchronous
to each other to prevent pointer adjustments. For example should you have an
OC-N that goes through multiple carries, a LAN Emulation Client (LEC) and
interexchange carrier (IXC) for example, and one of clock is a stratum 1 while
the other is being timed from some stratum 3 holdover source, you will have
pointer adjustments that will translate into DS1 timing jitter.
How many SONET NEs can I chain together in an add or drop configuration
before the timing becomes degraded?
A. The stratum level traceability of the nth node in an add or drop chain
is the same as that in the first node. Also, while timing jitter theoretically
increases as the number of nodes is increased, the high quality timing recovery
and filtering should allows add or drop chains to be extended to any practical
network limit without detectable increases in jitter levels. In practice, the
only effects on timing at the nth node will occur whenever high-speed
protection switches occur in any of the previous n-1 nodes.
Why are there more issues related to timing with SONET equipment than
there is with asynchronous equipment?
A. SONET equipment was designed to work ideally in a synchronous network.
When the network is not synchronous, mechanisms such as pointer processing and
bit-stuffing must be used and jitter or wander increases.