Optical transport networks interconnect switches and routers in the network core, or connect highspeed,
typically business/enterprise, users to the network (access). these networks are almost entirely
based on the synchronous Digital Hierarchy (SDH) standards today (or the equivalent synchronous optical
Network (SONET) standard in North America).
Traditionally, SDH has been the sole transport network and thus has carried all of the
telecommunication service provider’s traffic, including voice and data. so, SDH has, in a sense, provided “converged” transport, all along. However, the interfaces provided by SDH equipment for the first 10
years or so (since about 1990 to 2002) were restricted to the rates used by the older plesiochronous
digital hierarchy (PDH) systems, namely, 2/34/140 mbps, or the standard SDH rates, 155/622/2488 mbps.
Data typically originates within an enterprise local area network (LAN), invariably ethernet at 10, 100 or
1000 mbps, and had to adapted to a “standard” pDH or sDH data rate, typically using a router within the
enterprise. (this application of a router is so prevalent that the PDH interface on a router is called the
wide-area network, WAN, interface.)
The first step that the SDH standards, and equipment vendors, took to support a data-friendly
network was to provide ethernet interfaces (10/100/1000 mbps) on the SDH equipment in addition to
PDH and SDH interfaces. the provision of ethernet interfaces on SDH equipment is based on the generic
framing procedure (GFP), Virtual Concatenation (VCAT) and link Capacity Adjustment scheme (LCAS)
standards that were published and refined in 2001-03, and which are collectively referred to as the “ethernet-over-SDH/SONET” or EoS standards. “Next-generation” SDH equipment supporting these EoS
standards has been deployed since early 2003; tejas’ first shipment of standards-based eos equipment
to carry commercial traffic was in January 2003.
ETHERNET OVER SDH
SDH frames carry lower speed client traffic such as
E1 (2 Mbps) or E3/DS3 (34/45 Mbps) in “virtual
containers” which can be thought of as time-slots of
fixed capacity. A different-sized virtual container is
defined for each type of traffic, e.g., a VC-12 for E1
or 2 Mbps. Since the bandwidth allotted to Ethernet
traffic can vary widely (upto the maximum rate), a
method of combining, or concatentating, multiple
virtual containers into a single group was needed. Such
a method is termed Virtual Concatentation, or VCAT,
and the group of virtual containers created using this
method is termed a Virtually Concatenated Group, or
VCG, and has a user-defined capacity. E.g., for transport
of Fast Ethernet traffic, the capacity can be defined in
multiple of 2 Mbps, from 2-100 Mbps, and for transport
of Gigabit Ethernet traffic, the capacity can be defined
in multiples of 48 Mbps upto 1 Gbps. VCAT is defined
by the ITU-T in G.707 [1].
Ethernet traffic needed to be mapped onto VCGs
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Paper No 36A; Copyright © 2006 by the IETE. |
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and while more than one method is defined, the most
prevalent today is the Generic Framing Procedure or
GFP that is defined by the ITU-T in G.7041 [2]. GFP
allows mapping of variable length, higher-layer, client
signals which can be both protocol-data-unit oriented
(like IP/PPP or Ethernet) or can be block-code oriented
(like Fibre Channel).
Finally, the user may wish to add or delete members
from a VCG, or this may be required to be done in
response to network faults. The protocol for adding and
removing VCG members automatically is specified by
the ITU-T in G.7042 [3] and is termed Link Capacity
Adjustment Scheme (LCAS). LCAS also provides a
reasonable protection mechanism for Ethernet circuits
without physically consuming the bandwidth as in
the case of Layer-1 SDH protection mechanisms like
SNCP (Subnetwork Connection Protection) or MSP
(Multiplex Section Protection). In fig 1, Customer A is
provided a 10 Mbps Ethernet leased line connection to
Customer B with 6 Mbps allocated along the clockwise
direction and 4 Mbps in the anticlockwise direction. In
case of a fibre-cut and with the traffic as shown in Fig 1
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