IP Networks Transport TDM Voice and Data
How pseudowire emulation lets
carriers upgrade to IP networks without forcing users to buy
all new equipment.
By: John Shepler
It's no secret that packet switched networks are
the technology wave washing over the telecommunications industry.
The main obstacle to a world of everything over IP is an entrenched
infrastructure of TDM or Time Division Multiplexing networks.
It took 50 years for TDM technology to permeate every nook and
cranny of telephone and long haul data networks and it's not
about to go quietly. Why should it? TDM services such as T1,
SONET and ISDN PRI telephone trunks offer high reliability and
extremely competitive pricing.
The TDM / IP Network Dilemma
While both service providers and business users want to protect
their enormous investment in PBX and networking equipment, carriers
are looking to implement new networks as native IP networks,
including Carrier Ethernet and packet based DWDM fiber optic
networks. If TDM and other native services, such as ATM and Frame
Relay, could be readily transported on the newer MPLS networks,
then carriers would be free to migrate from their legacy TDM
technologies to a new standard protocol. A technology that enables
just this is called pseudowires.
What is a pseudowire? The name implies something that is a wire
of sorts. Indeed, this is the intention. Only pseudowires are
not physical copper wires or fiber optic cables in themselves.
Instead, a pseudowire is something of an abstraction. It is an
emulation of a hardwired connection between two points. Ideally,
you should not be able to tell whether your signal is traveling
over a pseudowire or a dedicated line. Your signal is encapsulated
by the pseudowire at the ingress point and returned to its native
format at the egress point.
You'll find pseudowires implemented on
packet switched networks, especially MPLS networks. MPLS itself
was designed to carry a variety of protocols. Hence, the name
Multi-Protocol Label Switching. Ethernet and IP networks can
also implement pseudowires to carry a variety of TDM traffic,
such as T1, E1, T3, E3. This is often referred to as TDM over
IP or TDMoIP, developed by RAD Data Communications. Pseudowire
specifications are defined by the IETF (Internet Engineering
Task Force) and the ITU (International Telecommunications Union)
Merging time division multiplexed and packet based networks presents
some formidable challenges. TDM is a strictly timed and partitioned
service with precisely multiplexed channels in the datastream.
Packet networks lack this strict clocking and are based on individual
packets rather than timeslots. TDM over IP requires that timing
information be preserved by using highly precise clocks at each
end of the line or derived from the data bit rates themselves.
The channelization structure must also be transported on the
network so that the packets can be converted back into voice
or data channels at the far end. Any packet loss must be concealed
by substituting fixed or interpolated data to fill in the bitstream.
Pseudowire emulation offers a convenient
way to re-engineer networks from switched circuit to packet based
without having to re-engineer the myriad of user services that
run on the network. This way customers can preserve their substantial
investment in PBX phone systems and other TDM network equipment
until it makeseconomic sense to convert to Enterprise VoIP or
Carrier Ethernet WANs.
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