How Carrier Ethernet Interconnects
LANs Network management made easier
by a common Ethernet protocol for LAN, MAN, and WAN.
By: John Shepler
Ethernet, pretty much the default standard for
Local Area Networks, is now being touted as the coming replacement
for metro and wide area services such as ATM, Frame Relay, SONET
and even TDM (Time Division Multiplexing) networks in general.
That's pretty much out with telecommunications as we've known
it and in with something new. Or at least something new to Wide
Where Carrier Ethernet Makes Sense
Why Carrier Ethernet? It's actually quite logical, even Darwinian,
that Ethernet should swallow up competing metropolitan and long
haul digital transmission standards. That's exactly what happened
over the last 30 years as Ethernet evolved from a research paper
to the preeminent local area networking standard. Now that Ethernet
is the best understood and most widely supported network protocol,
it only makes sense to join local area networks to metropolitan
area networks to wide area networks, perhaps worldwide networks.
Why not have one ubiquitous standard used by all?
The Legacy TDM Standard
In fact, in the telephony world there has been just such a ubiquitous
standard for digital communications. It's TDM or Time Division
Multiplexing. A PBX system plugs into a T1 PRI line. Multiple
T1s are multiplexed to form T3 lines or DS3 services that are
further multiplexed into SONET fiber optic carriers. The complete
digital signal hierarchy is designed around the lowest common
denominator, the DS0 or 64 Kbps channel. Why 64 Kbps? That's
the channel size to accommodate one digitized telephone call.
The LAN / WAN Disconnect
The only problem is that computers, especially PC based computing,
is not designed around legacy analog telephone services. Local
area networking is based on Ethernet, which is based on data
packets rather than synchronized time slots. It is at the local/wide
area interface that the clash of cultures occurs. Right now that
largely means protocol conversions so that 10/100 base T and
GigE networks can send data over T1, DS3, and SONET networks.
ATM cells and Frame Relay frames can be converted to and from
TDM format. Even long haul Internet connections often employ
TDM based optical carrier services.
These accommodations certainly do work,
but it seems more efficient to have a single network protocol
from end to end. As telephony starts to convert from TDM based
telephone switching systems to VoIP equipment, it makes sense
to migrate toward Ethernet as the future standard. The challenge
is to engineer wide area Ethernet networks that offer the same
reliability and protections designed into TDM networks.
Ethernet Goes Carrier Grade
That's where Carrier Ethernet Services come into play. Carrier
Ethernet is a fairly new development being standardized by the
Metro Ethernet Forum. The MEF has defined five attributes that
are essential for carrier grade Ethernet service. They are standardized
services, scalability, reliability, quality of service and service
Standardization of services must include
provisions to carry legacy TDM traffic that is highly time sensitive.
In effect, flipping the current situation where TDM circuits
transport Ethernet packets from one LAN to another by fitting
them into available TDM channels. Instead, carefully controlled
packet networks will carry TDM data without the latency and packet
loss that can occur on "best effort" networks.
Carrier Ethernet is Available Now
Carrier Ethernet Services are already available in major metropolitan
areas from competitive carriers such as XO Communications. You
can order 10 Mbps Ethernet, 100 Mbps Fast Ethernet or Gigabit
Ethernet connections to match your current network bandwidth.
Standard 10BaseT, 100BaseTX, and 1000BaseSX/LX interfaces are
supported at the WAN edges. It's almost a plug-and-play situation
allowing you to treat your interconnected LANs as one large network,
even when spread across the country.
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