Synchronization over Packet Networks Jeremy Lewis Product Line Marketing Manager Zarlink Semiconductor Agenda Why Synchronization over PSN Applicable Standards “In-Band” & “Out-of-Band” Adaptive Clock Recovery Differential Clock Recovery Combination Solution Why Synchronization Over PSN What is this technology? • Encode server clock frequency/phase information • Transmit server clock information over the PSN • Recover an accurate clock at the client nodes Benefits of Synchronization over PSN • Replaces the need for GPS receivers at client nodes • Replaces the need for synchronization cables between nodes • Replaces the need for high stability oscillators • Eliminates the need for routine calibration of high stability oscillators Standards for Synchronization E1 Standards (2.048 Mbps) Traffic interface (G.823, Table 2) T1 Standards (1.544 Mbps) Traffic interface (T1.403, section 6.3.1.2) • 18 µs over 1000s • 8.4 µs over 900s • 18 µs over 24 hours PRS PSTN T1/E1 Central Office Remote Terminal IWF IWF PSN T1/E1 PSN induced wander < 18 µs Max. end-to-end wander (traffic interface) 18 µs Customer Premises TDM Equipment Standards for Synchronization E1 Standards (2.048 Mbps) T1 Standards (1.544 Mbps) PDH synchronization interface (G.823 - Table 12) Reference interface (T1.101, section 7.2.1) • 2 µs over 2000s • 1 µs over 2000s • 2 µs over 100,000s • 5.33 µs over 100,000s PRS PSTN T1/E1 Central Office Remote Terminal IWF IWF PSN T1/E1 Network TDM Equipment PSN induced wander < 2 µs Max. end-to-end wander (synchronization interface) 2 µs PBx In-Band & Out-of-Band In-Band Synchronization over Packet • Uses information included in the standard CESoP data stream • Each T1/E1 has its own associated clock • No timing traffic overhead • Data and timing are closely coupled Out-of-Band Synchronization over Packet • Uses dedicated timing packets • Examples: NTP, IEEE 1588 • Adds some timing traffic overhead • Can be “always on” • Configurable to network conditions In-Band Timing Recovery Timing recovery for CESoPSN/TDM-over-IP • Clock & data use the same packet connection Data – Payload Clock – Header Encoded Clock Customer Premises or Central Office CESoP Carrier PSN IWF Single T1 Clock and Data Multiple Streams possible Customer Premises CESoP IWF Recovered T1 Clock and Data Encoded Data Out-of-Band Timing Recovery Timing recovery for all applications • Clock & data use separate packet connections Servers BITS SSU Primary Source Clients TDM PBX VoIP Gateway V.90 MODEM GSM / UMTS Basestation Base Station BITS SSU Secondary Source Base Station In-Band V’s Out-Of-Band In-band clock recovery advantages • No bandwidth overhead • High packet rate, equal to the CBR traffic rate • Uses already defined standards (e.g. CESoP) Out-of-band clock recovery advantages • Separation of clock recovery from the data allows for greater flexibility (e.g. always “on” operation) • Configurable for network conditions (e.g. packet rate <1pps to >1,000pps) • Far less susceptible to poor network conditions Adaptive Clock Recovery Reference clock, only available at the server • ƒ Service is frequency & phase encoded • Accurate time stamp applied & packets sent via in-band or out-of-band • Filtering of the of the arriving packets provides ƒ Service at the Client node Central Office Carrier PSN Server Node TDM Equipment T1/E1 TDM to Packet In-Band Time Stamp Primary Reference Source fReference Customer Premises Queue Out-of-Band ƒ Service Clock Encode Client Node Packet to TDM Time Stamp Filter ƒ Service TDM Equipment T1/E1 Adaptive Clock Recovery No common reference required Good jitter and wander performance is achievable • ~2µS MTIE • <15µS Frequency accuracy Can work in conjunction with QoS mechanism L2 networks offer the potential for better performance BUT more susceptible to network conditions • PDV Differential Clock Recovery Reference clock available at server & client • Only the difference between ƒ Service & ƒ Reference is transferred across the PSN Central Office Customer Premises Carrier PSN Server Node TDM Equipment T1/E1 TDM to Packet In-Band Time Stamp Queue Out-of-Band Timing Difference ƒ Service Client Node Packet to TDM TDM Equipment T1/E1 Time Stamp Difference Add Primary Reference Source fReference ƒ Service fService is asynchronous to fReference Differential Clock Recovery Good jitter and wander performance Largely unaffected by network delay, delay variation and packet loss • <1µS MTIE is achievable • <15µS frequency accuracy But requires a common reference clock at either end of the packet network Possible means of distributing a reference clock: • Central Office clock (if equipment is located in the CO) • SONET clock (e.g. if network uses Packet or Ethernet over SONET) • GPS clock Combination Clock Recovery Reference clock, only available at the server • ƒ Service is encoded using adaptive technique & sent separate to CESoP traffic • Only the difference between the T1/E1 clocks & ƒ Service transferred across the PSN Central Office Carrier PSN Server Node TDM Equipment T1/E1 Primary Reference Source Customer Premises TDM to Packet Client Node Time Stamp Queue In-Band Timing Difference Packet to TDM Time Stamp Difference Add fReference ƒ Service Out-of-Band Clock Encode Filter Time Stamp ƒ Service TDM Equipment T1/E1 Key Performance Factors PDV • The nature of the PDV needs to be understood Network loading • Average network node loading Packet Rate • Increased rate of timing packet generally increases performance Complexity of the network • Number of nodes, L2/L3 Drift Rate of the local oscillator • OCXO, TCXO … Further Performance Factors Network disturbances • Packet loss • Network congestion/modulation/bursts • Network outage • Routing changes • Night & day low frequency effects Network models/characterization • Performance models • Test bench