Timing distribution

Network timing distribution

DS1 signals have long been used to pass timing information through the network synchronization hierarchy. With an LNW59, LNW82, or LNW504 VLF Main, E1 timing outputs are also supported. These DS1/E1 timing references should be transmitted between master and slave clock sources over the most reliable facilities available. In some cases, these DS1/E1 signals also carry traffic. The facility of choice has evolved from T-carrier through asynchronous lightwave systems to SONET lightwave systems. As these systems are upgraded to SONET systems, timing distribution plans should be revisited to ensure the quality of the timing signals are not degraded. With proper planning, SONET can be used to improve the overall quality of the network timing.

Interoffice timing distribution

One way SONET can be used to improve the quality of interoffice network timing is through the use of OC-n (OC-3/12/48/192) timing distribution. Alcatel-Lucent 1665 DMX supports the evolution to interoffice OC-n timing distribution by providing a DS1/E1 timing output derived from the incoming OC-n signal. The DS1/E1 timing output is traceable to the clock source that times the Alcatel-Lucent 1665 DMX subnetwork and has extremely low jitter and wander. This is true regardless of the number of Alcatel-Lucent 1665 DMX systems connected in the network. This DS1/E1 can be fed to the local BITS clock which subsequently times the local Alcatel-Lucent 1665 DMX and the other equipment in the office. If a BITS clock is not available in the office, the DS1/E1 timing output can be used to time other equipment (including another Alcatel-Lucent 1665 DMX) directly. Alcatel-Lucent 1665 DMX can provide DS1/E1 timing outputs in all supported topologies (for example, add/drop and ring) in the main slots. It also supports line timing from tributary optical interfaces, allowing Alcatel-Lucent 1665 DMX to provide DS1 timing outputs even when equipped with the LNW80 headless main. E1 timing outputs are available only when an LNW59, LNW82, or LNW504 Main is used.

With OC-n timing distribution, the OC-n line signal, rather than a DS1/E1 multiplexed into the SONET payload, will provide a timing transport mechanism better suited to a complex, heavily interconnected SONET network. In this configuration, a DS1 reference from the CO BITS clock still times the OC-n transmitted to the remote site. At the remote site, a DS1/E1 output reference is created directly from the received OC-n signal, as shown in the figure below (E1 not pictured).

Figure 6-18: OC-n (OC-3/12/48/192) derived DS1 timing reference

Potential advantages

OC-n (OC-3/12/48/192) timing distribution has several potential advantages. It preserves transport bandwidth for customer services and guarantees a high-quality timing signal. Also, as the CO architecture evolves to replace DSX interconnects with SONET EC-1 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 the 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 now in place. Alcatel-Lucent 1665 DMX supports this synchronization messaging scheme. 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, Alcatel-Lucent 1665 DMX sends AIS over the DS1 interface. If the local OC-n lines fail, Alcatel-Lucent 1665 DMX outputs AIS on the DS1 output or an upstream Alcatel-Lucent 1665 DMX enters holdover.

Access network timing distribution

OC-n (OC-3/12/48/192) timing distribution can also be used in access networks or to small COs. In this configuration, a DS1 reference from the CO BITS clock still times the OC-n transmitted to the remote site. The line timing capability of Alcatel-Lucent 1665 DMX provides the ability to recover OC-n timing. The DS1 timing output feature can be used to also extend timing to customer networks or remote sites. In this case, the DS1 timing output may be used to time switch remotes, Alcatel-Lucent 1665 DMX shelves, or other local equipment directly. Ideally, the equipment can provide an external timing reference. Otherwise, the signal must be input to a traffic DS1 port on the external equipment which will tie up some of this equipment's bandwidth. In this configuration, it is important that the DS1 reference to the Alcatel-Lucent 1665 DMX in the CO be traceable to the same clock used to source the DS1s being carried to the customer site or small CO. If it is not, slips may occur.

Alternate timing sources

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. In this environment, the high-performance desynchronizer design of Alcatel-Lucent 1665 DMX allows a DS1 timing reference to be carried as a multiplexed DS1 payload.

It is recommended that, where possible, the DS1 sources (switch, PBX, or other equipment) be traceable to the same timing source used to time Alcatel-Lucent 1665 DMX SONET network. Multiplexed DS1 reference transport is also consistent with current planning and administration methods. Applications include passing synchronization from the public switched network to a PBX-based private network ( Figure 6-19, Timing from multiplexed DS1) and synchronizing an end-office remote switch to a larger office's host switch.

Important!

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. While the design of the Alcatel-Lucent 1665 DMX maintains jitter/wander within standard DS1 interface requirements, even in the presence of VT pointer adjustments, and while the DS1 is likely to be stable enough for most equipment to use as a timing reference, some equipment may have more stringent stability requirements for its timing references.