Last updated January 24, 1998

A special thanks to Lucent for helping with this material.

 Q. What is the difference between SONET and SDH?

A. To begin with there is no STS-1.  The first level in the SDH hierarchy is STM-1 (Synchronous Transport Mode 1) is has a line rate of 155.52 Mb/s.  This is equivalent to SONET's STS-3c. Then would come 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 VCs which they contain.  The outbound STM-Ns are then reconstructed with new overheads.  

Q. What is hair pinning, and why would I want to use it?

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 had interfaces to two IXCs on different nodes.  If one of your IXCs went down you could hair pin the other to pick the traffic, assuming the spare capacity existed 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 the interfaces.  

Q. Doesn't a  2 fiber bi-directional line switched ring waste 1/2 the line rate bandwidth?

A. No.  It can be shown that in all cases the aggregate bandwidth on a 2 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 2 fiber BDLSR can be larger than that of a path switched ring (proof left to student).

Q. 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 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 time-slot assignment (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

Q. What are some timing rules of thumb?


1. A node can only receive the synchronization referencesignal from another node that contains a clock ofequivalent or superior quality (Stratum level).

2. The facilities with the greatest availability (absence of outages) should be selected forsynchronization facilities.

3. Where possible, all primary and secondary synchronization facilities should be diverse, and synchronization facilities within the same cable should be minimized.

4. 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 the center

5. No timing loops may be formed in any combination of primary

Q. What are some advantages of timing from an OC-N line?

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 DSX interconnects with SONET interconnects and direct OC-N interfaces, OC-N distribution becomes more efficient than multiplexing DS1 references into an access facility in the CO. A previous drawback to using OC-N timing distribution was that network timing failures could not be communicated to downstream clocks via DS1 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, via 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 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, 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)

Q. 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 linerate 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

Q. Can a DS1 carried over SONET ever be used as a timing reference?

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/loop time from the DS1 signal. In addition, DLC equipment, channel banks, and PBXs will not likely have external references and may be allowed to line/loop time from a DS1 carried over SONET. Five years ago all the literature however answered no to this question.  See next question.

Q. Question: 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 LEC and 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.

Q. How many SONET NE's can I chain together in an add/drop configuration before the timing becomes degraded?

A. The Stratum level traceability of the nth node in an add/drop chain is the same as that in the first node. Also, while timing jitter will theoretically increase as the number of nodes is increased, the high quality timing recovery and filtering should allows add/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 protectionswitches occur in any of the previous n-1 nodes.

Q. 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/wander increases.

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