NGN Seminar Report

by Shiv Narayan, Trumala Seven Hills, Kolkata





Present Network Convergence Trend and Scenario :


       In packet switched networks multiplexing and routing is asynchronous (statistical).

       This asynchronous nature must have consequences regarding timing-critical network functions.



Synchronization Needs in NGN Networks :


       Examples of synchronization needs in the access network domain:

       Mobile Base Stations (GSM, UMTS, LTE, cdmaX, WiMax)

       Cable networks (DOCSIS)

       PONs: BPON (Broadband PON), EPON (Ethernet PON), GPON (Gigabit PON)

       Specific needs in Customer Premise Networks

       Some need frequency synchronization, some also phase and even time-of-day synchronization





Case1: Sync over asynchronous networks


Problem: How to do network synchronization over completely asynchronous networks?



















Case 2:  E1/T1 traffic over asynchronous networks


       NGN and legacy TDM networks will co-exist for at least a decade.

       E1 and T1/DS1 traffic to be carried over NGN packet networks: Circuit Emulation Services (CES), Pseudo-Wire Emulation (PWE)

       Problem: How can the required E1/T1 timing transparency (G.8261 wander limits) be achieved?















Case 3:  Assymetry


       When the path is different between the back and forth link

       When there is any change of data link frequency (e.g. from FE to GbE)

       When the data link ferquency is different between the back and the forth path (e.g. ADSL)

       Network elements own design can introduce difference between back and forth way

       Cable can introduce delay if the back and the forth links have not the same length




A Simple Model of NGN Network :





A. Network Synchronization Solutions Carried over the network

B. Network Synchronization Solutions Not carried over the network




A. Network Synchronization Solutions Carried over the network


       Carried over the physical layer (Layer 1)

       Synchronous Ethernet (Sync-E)

       Frequency synchronization

       Over higher protocol layers

       Proprietary Solutions

       Vendor dependant technology; not standardized

       PTP IEEE 1588-2008 (v2)

       Frequency, Phase and Time-of-day Sub-microsecond accuracy

       NTP (only Time-of-day)

       Frequency, Phase and Time-of-day Sub-millisecond precision within a LAN





       Physical signal carries synchronization (in the line code)

       Only frequency synchronization, but not phase nor time-of-day synchronization

       Needs compatible Newtorks Elements

       SSM (Synchronization Status Message)

       Ethernet Equipment Clock (EEC) similar to SDH Equipment Clock (SEC):

       Same performance specifications

       External timing inputs and outputs

       Resembles SDH-type synchronization (physical layer, same equipment clocks, SSM)

       ITU-T Standards (G.8261, G.8262, G8264)

       Requires PRCs and SSUs





Fact Sheet PTP v2


       PTP (Precise Time Protocol) is an IEEE standard:

       IEEE 1588 v1: for LAN applications

       IEEE 1588 v2: broader application space, incl. telecom

       Protocol for sub-microsecond synchronization of real-time clocks over frame and packet switched networks

       Uses ‘Two-way Time Transfer’ (TWTT) and ‘Hardware Assistance’

       Mappings for various lower layer protocols (UDP/IP/Ethernet, Ethernet, others)

       The main idea is to mitigate the protocol stack delay with appropriate electronic hardware and a modified TWTT protocol.

       A hardware « Precision Time Stamp Generator » measures the frame receive and transmit times at or close to the Physical Layer (Layer 1).

       The measured frame transmit times are communicated to the other end system with a second message.

       The TWTT calculation expoits the time values measured by the Precise Time Stamp Generator.

       The configuration of the clock hierarchy can be done manually or automaticaly using the Best Master Clock (BMC) algorithm.





Fact Sheet NTP


       NTP: Network Time Protocol

       Distribution of Time-of-Day (UTC) over IP networks using Two-way Time Transfer

       Accurcay: 0.5-2ms in LAN; 10-100ms in WAN

       Hierarchical NTP clock trees, several operation modes

       Optional ‘MD5’ authentication mechanism

       NTP standards:

       NTP v3: RFC 1305

       NTP v4: RFC 5905



B. Network Synchronization Solutions Not carried over the network:


       Off-the air GNSS (Global Navigation Satellite System)


       Provides Frequency, Phase and Time of day



Fact Sheet GNSS :






1. Base Stations: GNSS














2. Base Stations: Synchronous Ethernet






















3. Base Stations: PTP v2 (IEEE 1588)
























4. PTP application: mobile networks (e.g. 3G)





5. PTP application: Smart Grid (Power utilities)








6. Sync Architecture for NGN




[Courtesey of Vodafone]