Timing over packet

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Contents 1 What is Timing over Packet (ToP): 2 Why we need ToP: 3 About ToP: 4 Impairments during Timing distribution:

[edit] What is Timing over Packet (ToP):

To put it in simple terms “It’s a method to transport timing [also frequency & phase] information over PSN (packet switched network)”. It’s all known that Packet switched networks are very cost effective and widely used in data communication. But due asynchronous nature of PSN’s they are not reliable for cellular backhaul networks. Timing over Packet enables time-sensitive services to be supported over packet-switched networks.

[edit] Why we need ToP:

At present we are heading towards 4G cellular devices, and the bandwidth requirements are growing exponentially in the backhaul networks. It’s not economical to still rely on our traditional backhauls (MLPP), at the same time we cant directly start using packet switched network in our backhaul, they are asynchronous. We need a cost saving, high bandwidth, time sensitive backhaul solution. Packet switched networks are cost effective and also provides high bandwidth but they have few impairments such as packet delay variation and packet loss, which results in service disruptions, impaired cell hand-offs and excessive dropped calls in cellular network. We need to overcome this timing issue. We have two options either utilize another timing based interface along with the packet switched network. (We need to bundle two different interfaces [ex: T1 + FE] to achieve our goal). Else send the timing information over the packet switched network, and here comes out Timing over Packet (ToP) into picture.

[edit] About ToP:

ToP is not dependent on physical layer technologies. It just uses a stream of packets to encode the clock information and transfer it across the packet switched network. It may use a variety of protocols to achieve timing distribution e.g. RTP, NTP, PTP(IEEE1588). The clock from the packet stream will be recovered by using adaptive clock recovery algorithm.

[edit] Impairments during Timing distribution:

   Packet Transfer Delay
   Packet Delay Variation (PDV)
      Random delay variation (e.g., queuing delays)
      Low frequency delay variation (e.g., day/night patterns)
      Systematic delay variation (e.g., store and forward mechanisms in the underlying transport layer)
      Routing changes
      Congestion effects
   Packet Loss
   Packet Error
   Extended Packet Loss (Network Outages)

All of the above impairments are addressed by adaptive clock recovery algorithm.

References:

1. ITU-T recommendation G.8261 (03/2005), Timing and synchronization aspects in packet networks.