WOLFGANG FISCHER TDM MIGRATION
TDM Migration TO PACKET NETWORKS
Native time division multiplexing now accounts for only a small fraction of tra c so integrated networks must be based on IP/MPLS over optical systems.
by point-to-point connections across packet networks. The use of MPLS for pseudowires carrying TDM tra c has become most popular because it allows the highest degree of end-to-end control and protection. TDM-based transmission, and thus, Circuit Emulation requires clock synchronisation between the ingress and egress nodes. Clock distribution has been an inherent feature of TDM transmission networks, but packet networks are inherently asynchronous. Clock distribution over Ethernet-based transmission systems is, therefore, an obvious requirement because of Ethernet’s prominent role. ITU-T in cooperation with IEEE have specified Synchronous Ethernet (SyncE) in Recommendations G.8261, G.8262, G.8264. For mobile backhaul to move away from TDM-based
WOLFGANG FISCHER T elecommunication services have been on this technology. Modern services, however, are mostly using packet-based transmission. Therefore, most of the tra c in our networks is packet-based, but often still rides on a TDM-based infrastructure. based on Time Division Multiplexing (TDM) technology for a long time and much of today’s infrastructure is based As systems for PDH and SDH (the standards for TDM technology) are becoming obsolete, there is a clear trend towards unified networks built exclusively on the basis of packet (Ethernet/IP/ MPLS) technologies. Consequently, also native TDM applications either have to be migrated to native packet applications, or they have to be mapped appropriately so they can be supported by packet networks. DRIVERS FOR MIGRATING SDH NETWORKS TO PACKET Since native TDM constitutes only a small fraction of today’s tra c, truly integrated networks have to be based on IP/MPLS over an optical transport infrastructure. Sophisticated optical transport technology is integrated into packet switches and routers, with common management across the layers. Because of the high volumes in which routers and switches are deployed, and because of the ever increasing bitrates, packet technology has evolved to become much more cost e ective and uses significantly less power than TDM technology when compared at the same speeds. These savings become even more expressed when network simplification is taken into account. Existing PDH/ SDH-based transmission networks are composed of Add-Drop-Multiplexers (ADM) operating primarily on VC-4 granularity, and Digital Cross-connect Systems (DXC or DCS) operating on lower
granularity down to E1/VC-12. Packet- based systems are independent of such granularity considerations. All modern broadband access technologies allow voice to be transported as Voice over IP (VoIP), so that the transport of voice in the baseband of a DSL line, or even on a separate telephone line is no longer required. Consequently, most service providers are migrating their fixed
telephony customers to VoIP. Eventually, in most countries TDM voice switches will completely disappear from the networks. While 2G mobile networks were mainly focused on voice communication with TDM-oriented backhaul, starting
technologies clocking and timing is required. 2G and 3G networks require frequency synchronisation among all the base stations, 4G (LTE-A) networks, in addition, require phase synchronisation. For this purpose, IEEE have specified the Precision Timing Protocol (PTP,
Corporate networks are mostly using packet-based communications
from 3G the focus was increasingly on data communication, and, therefore, the backhaul became packet-oriented. Corporate networks are mostly using packet-based communication. Where companies still use legacy PBXs there is a need to carry TDM tra c on leased lines. In all the above examples there has to be a careful consideration if and when the TDM-based applications will be migrated to native packet-based services, eliminating the need for a conversion of the native TDM tra c into packet. In those cases, where the native TDM applications cannot be migrated, or only parts of the network will be migrated, the TDM tra c has to be converted into streams of packets using Circuit Emulation (CE). TECHNOLOGIES Much of the emulated TDM tra c is mission-critical. Therefore, protection mechanisms have to be implemented to meet the requirements for sub-50ms switchover times introduced with SDH. Pseudowires emulate physical wires
IEEE 1588v2). Based on IEEE 1588v2 ITU-T has created standards and profiles (G.826x and G.827x) for the transfer of timing and synchronisation in packet networks. CIRCUIT EMULATION Many legacy TDM-based services will be migrated towards native packet-based services, like, for example, TDM voice to VoIP, TDM-based leased lines to MPLS VPNs, etc. However, a number of native TDM-based services will remain to be supported by universal packet networks. Circuit Emulation converts the TDM tra c arriving on PDH and SDH interfaces into packets transported via MPLS pseudowires. Support for clocking is important because both SDH technology and wireless backhaul require very precise clocks, and in the case of 4G radio networks, also precise phase information. Sophisticated protection switching mechanisms inside the packet networks are required to meet the sub-50ms switchover times introduced with SDH technology.
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| ISSUE 7 | Q3 2016
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