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DS34S132 DATA SHEET
19-4750; Rev1; 7/11
8 of 194
1
INTRODUCTION
The public network is in transition from a TDM Switched Network to a Packet Switched Network. A number of
Pseudowire (PW) packet protocols have been standardized to enable legacy TDM services (e.g. TDM voice, TDM
Leased-line and HDLC encapsulated data) to be transported and switched/routed over a single, unified PSN. The
legacy service is encapsulated into a PW protocol and then transported or tunneled through the unified PSN. The
PW protocols provide the addressing mechanisms that enable a PSN to switch/route the service without
understanding or directly regarding the specific characteristics of the services (e.g. the PSN does not have to
directly understand the timing requirements of a TDM voice service). The PW protocols have been developed for
use over PSNs that utilize the L2TPv3/IP, UDP/IP, MPLS (MFA-8) or Metro Ethernet (MEF-8) protocols.
PW protocols that are used for TDM services can be categorized as TDM-over-Packet (TDMoP) PW protocols. The
TDMoP protocols support all of the aspects of the TDM services (data, timing, signaling and OAM). This enables
Public (WAN) and Enterprise (LAN) networks to migrate to next generation PSNs and continue supporting legacy
voice and leased-line services without replacing the legacy termination equipment.
Legacy TDM services depend on constant bit rate data streams with highly accurate frequency, jitter and wander
timing requirements that up until recently have not been well supported by most packet switching equipment. For
public network applications the timing recovery mechanisms must achieve the jitter and wander performance that is
required by the ITU-T G.823/824/8261 standards. To accomplish this, a TDMoP terminating device must
incorporate innovative and complex mechanisms to recovery the TDM timing from a stream of packets.
Legacy TDM services also have numerous special features that include voice signaling and OAM systems that
have been developed over many years through a long list of standardization literature to provide carrier-grade
reliability and maintainability. The list of legacy functions and features is so long that today’s VoIP equipment only
supports a subset of what is used in the legacy TDM network. This, in part, has slowed the transition from a TDM to
Packet-based network. With TDMoP technology all features and services can be supported.
The TDMoP technology is similar to VoIP technology in that both provide a means of communicating a time
oriented service (e.g. voice) over a non-time oriented, packet network. TDMoP technology can be added
incrementally to the network (as needed) to supplement VoIP technology to provide an alternative solution when
VoIP price/performance is not optimal (e.g. where the number of supported lines does not warrant the infrastructure
required of a VoIP network) and where some function/features are not supported by the VoIP protocols.
The Legacy PSTN network also supports HDLC encapsulated data that is transported over TDM lines. PWs can
also be used to transport HDLC data. This form of PW could also be categorized as a TDM service since the
legacy service is carried over TDM lines. However, the fundamental aspects of an HDLC service do not depend as
much on TDM timing and the nature of the data can be described as “packetized” as with Ethernet, Frame Relay
and ATM services. For clarity the HDLC service is categorized as “HDLC over PW”. One example Legacy HDLC
service is SS7 Signaling which is used to communicate voice signaling information from one TDM switch to
another.