The legacy public switched telephone network (PSTN) is organized into a tightly synchronized (10 parts per trillion) structure to allow for the quality of service we have come to expect from telecom services.
New services utilize the unsynchronized Gig-E packet network (100 parts per million) that cannot support constant bit rate services, nor recover timing from line. The convergence of these two networks breaks the traceability of primary reference sources (PRS) throughout the system. A new need exists to reconstitute timing stability at the network edge to ensure reliable operation of remote network elements such as remote switches, GR-303 DLCs, mobile telephone switching stations (MTSO), base stations, enterprise PBXs/routers, and other such aggregation devices to the PSTN.
Several strategies exist for ensure timing stability of remote network elements including filtering, retiming and GPS synchronization with a PRS.
Filtering
Filtering is a simple method to reduce the impact of payload DS1/E1 frequency deviations without the expense of installing a PRS. Filtering is applicable in networks where the DS1/E1is directly or indirectly traceable to the PRS of the PSTN and transported over conventional TDM networks such as SONET, SDH, HDSL or similar technologies.
Filtering disciplines the payload DS1/E1 line to that of the average frequency of the received DS1/E1 line. Placed in the receive side of the DS1/E1, the buffering of the TEK smoothes or eliminates momentary deviations thus allowing the edge element to track the D1/E1.
Filtering benefits remote switching centers, mobile telephone switching offices (MTSO) and cellular base stations by eliminating breaks in PRS tracking, clock alarms and nuisance line/reference faults, and by maintaining performance of non-facility associated bonded circuits/calls.
Retiming
For applications where a BITS or local PRS is available, retiming can be used to bring a payload DS1/E1 to PRS performance even after traversing packet network topologies. Inserted into the path of a DS1/E1 and referencing a timing port from a BITS or local PRS, a retimer regenerates the payload signal at the PRS rate.
Restoring a traffic bearing DS1/E1 to PRS performance upon egress of packet based transport will inevitably result in some slips. To mitigate the slip events, a 250usec frame slip mechanism allows for absorption capability before introducing a slip. Reducing the number of slips improves customer QoS while ensuring the edge element remains at PRS performance.
A Retimer can be employed at any time, such as in preparation of an eventual packet conversion, when the transport network topology is unknown or when QoS is considered paramount.
Synchronizing SONET/SDH
As telecommunications service providers deploy and maintain SONET/SDH systems for large enterprise customers, the demand for quality of service (QoS) may dictate the need for an on-premise PRS. Although an excellent transport system, a SONET system makes VT pointer adjustments in order to carry various payloads within its synchronous line rate. These pointer adjustments inject phase steps into its DS1 or E1 payloads. This can cause some customer network elements, such as ATM switches and PBXs, to lose synchronization momentarily, causing dropped calls, data errors, and the recurring need to troubleshoot and reset communication links. While SONET equipment often supplies a derived DS1 from the optical line rate, service providers and/or enterprise customers may not be able to use this output as a timing source.
Higher QoS can be achieved by installing a Stratum-1 PRS with a timing output directly connected to premises network equipment, such as an ATM switch. If several premises network elements require timing, a distribution amplifier is used to provide 12 outputs or more.
Products for Telecom Synchronization
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Retimers and Filters
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Our new Timing Enhancement Kits (TEK™) offer the lowest cost alternative for simply reconstituting timing stability at the network edge. TEK retimer / filters provide a simple solution for mitigating the effects of clock aberrations and SONET pointer adjustments.
Features: Compact, stand-alone unit, DS1 and E1 versions, retiming buffer option to accept timing reference
Benefits: Improved QoS, fewer dropped calls, eliminate clock alarms, mitigates slips, enables bonding of circuits, easy-to-install |
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Primary Frequency References
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Master Oscillators are highly accurate primary reference sources (PRS) locked to GPS. Use for T1/E1 syncronization for enterprise or independent networks to provide slip-free operation. Timing outputs are DS1 or CEPT framed all 1's, or RS-485 at selectable frequencies.
Features: GPS synchronization, Stratum-1
Benefits: high accuracy, reliability, versatile |
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Time and Frequency Distribution
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Distribution of timing from one source to many network elements is a Spectracom specialty. Applications include: communications center timing distribution from an incoming T1 to MUXs and PBX, SONET timing distribution from an add-drop MUX into customer premises separately from the traffic lines, redundant clock selection, timing source with stratum-3 accuracy to distributes optional built-in master clock to 12 network elements.
Features: Economical clock distribution, failover detection
Benefits: Redundancy, easy system integration |
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Telecom Clock Converter
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VersaTap™ is a unique device that converts or synthesizes a clock from a frequency signal.
Features: 10 MHz input to (x2)DS1 or (x2)E1 outputs
Benefits: Versatility, leverages frequency reference to telecom testing |
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Synchronization Supply Unit (SSU)
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The Epsilon™ Synchronization Supply Unit (SSU) is a fully modular and flexible SSU that can be customized to provide the most cost-effective solution to all network synchronization requirements, from Primary Reference Clock to Central/Transit/Local nodes.
Features: Meets ITU G.811/G.812 and ETSI specs
Benefits: Cost-optimized solution, flexible, fully manageable |
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Network Time Servers
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NetClock/GPS systems provide reliable and secure time synchronization across the IP-network. Ouputs include NTP via Ethernet, serial time codes, 1PPS, IRIG and 10 MHz.
Features: GPS synchronization for IP networks and devices through NTP, 1PPS & 10MHz options
Benefits: Secure, accurate, easy-to-manage |