The case for a shift to PTN

Wan Junqiao
19 Jan 2015

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Synchronous Digital Hierarchy (SDH)/Synchronous Optical Networking (SONET) products are currently widely deployed and comprise a big part of the modern transport network.

These products are synchronized, with fixed hierarchy and with strict SLA. However, when the operator’s main traffic shifts from voice to data, a decoupling of revenue and traffic consumption occurs.

Classic SDH-based networks or MSTP (multi-service transfer platform) are unfit to address this problem. Upgrading the network bandwidth from STM16 to STM64 is more expensive than upgrading from Gigabit Ethernet to 10GE.

The burst-character feature of the packet data transition does not match the fixed bandwidth pipe of the SDH. There is too much wasted timeframe when the allocated end-to-end link is silent. The right choice would be MPLS -TP (multi-protocol label switching-transport profile).

Smartphone applications and HD/UHD videos services, global mobile and fixed dramatically increase data traffic, challenging the traditional TDM transport far beyond its capabilities. And when data traffic on the network changes from TDM to packet, so should the SDH network evolve into a packet transport network (PTN).

E1/STM-1 circuits are not only inefficient for transporting packet-based traffic via EoS/X.85 (ITU-T), they are also significantly more expensive — in terms of per port or per bit — than packet-based alternatives such as Ethernet.

In the long run, sticking with TDM is untenable for operators. The solution is to define a set of protocols and procedures that inherit all the efficiencies of the packet networks, and to meet the OAM requirements of the transport network as SDH.

MPLS-TP has simplified dynamic routing but enhanced transport profile features. MPLS packets transport in the PTN network through label switching, which is much efficient than the destination IP address lookup forwarding.

SDH was designed to set up end-to-end circuits in large-scale networks with nature features of end-to-end QoS. MPLS-TP adapts this transport profile features but changes the TDM transmit to packet transmit, allowing bandwidth to be shared.

Compared to traditional IP/MPSL, MPLS-TP simplifies the dynamic routing protocol. MPLS-TP is enhanced to support static provisioning, in-band OAM, fault detection and switchover to backup path within 50 ms, and a network management system interface to configure and manage the network.

With enhanced the transport profile features, FiberHome Packet Transport Network CiTRANS 600s can provide the same quality SDH circuit service. The difference is that in the STM-1 service in SDH network, the pipe is fixed and bandwidth cannot be shared even when there is no traffic at all. Whereas in PTN, the pipe is flexible and bandwidth can be shared with QoS management strategy.

TDM-based SDH/MSTP networks are no longer fit for the packet services as voice service revenue will keep decreasing each year. In building a PTN network, replacing or keeping aside a SDH network is a good call.

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