06-11-2012, 05:34 PM
Time Synchronization Aspects in Mobile Backhaul Networks
1Time Synchronization.pdf (Size: 367.17 KB / Downloads: 254)
Technology Options* for Distributing Phase/Time
• GPS Receiver based (satellite-based distribution)
– Accurate, traceable, dependable and robust
– Mostly used in North American deployments
– Cost and deployments difficulties for many other wireline/wireless operators
– Requires additional antenna and cabling, leasing & installation
– Limited indoor coverage, holdover required during GPS failure event
• IEEE1588v2* without support from network nodes (network-based distribution)
– Typically used for frequency distribution between a Master and Slave ITU-T developing
1588 PTP profile for frequency distribution
– Difficult to distribute phase/time due to unknown latency characteristics at all layers
– Timestamping done in end network elements only, no support from network
• IEEE1588v2* with support from network nodes (network-based distribution)
– Accuracy primarily dependent only on physical layer characteristics
– Timestamping done in every network element, implemented via boundary and/or transparent
clocks which removes unwanted latency impairments
• IEEE 802.1AS is an example that uses 1588/timestamps & support from network
– ITU-T Phase Synchronous Ethernet proposal is another example which does not use 1588
support from network but requires timestamps
Summary of Mobile Backhaul Aspects
- Initial discussion items for TSSG and PAR definition
• Time sync in mobile networks driven by requirement of radio interfaces (eg., LTE system)
– Interfaces that require timestamping are FE/GE/10GE and those that might are 40GE/100GE
• Various impairments contribute to time error - careful impairment budget allocation necessary
• Various measurement points - must be taken as close to the physical layer as possible
– MII used today as meas. point in IEEE1588, applicable for FE/GE interfaces, difficult for integrated MAC/PHY
– MAC level: latency performance might not be deterministic
– PCS: read/write timestamps triggered based on using/defining unused special codegroups
– PCS/PMA layer: replace nearest idle ordered set with a “timing” ordered set, NTT/Nihon published paper,
demonstrated 2.5nsec accuracy over 5km fiber
• TSSG could work towards specifying for instance
– Timestamping based on distinct and easily recognizable patterns
– TX/RX timestamping function error (resolution)
– Delay and delay variation such PHY latency, PHY clock jitter, etc.
– Wait times, read-to-transmit process
– etc.
• IEEE 802.3 TSSG work is relevant to other standards development
– ITU-T: starting to study network phase/time distribution and performance
– IETF TICTOC: accurate time & frequency distribution over packet switched networks
• Other challenges
– Passing timestamps to upper layers, security of timestamps
– Higher speed interfaces: xMII, multi-lane distribution, deskew mechanism, oscillator frequency
– Network architecture: underlying OTN structure, ETH interfaces mapped into SDH/SONET