Converging the LAN and WAN infrastructure
The next time you order a WAN link, ask for an Ethernet link
based on GFP, VCAT and LCAS. You will save a ton of money and operational headaches.
Dr Kumar Sivarajan
The data network of a typical medium-to-large enterprise needs
to satisfy three different interconnection objectives. First, it has to interconnect
users within a single building or campus. This is typically referred to as Local
Area Network or LAN. Many companies have users in multiple locations that may
be in the same city, different cities, or even in different countries. They
may also need to connect to their customer sites. Therefore, the second objective
of any enterprises network is to interconnect users across these geographically-dispersed
sites. The essential difference between this and the first requirement is that
connectivity is now through one or more service providers. This network is typically
referred to as Wide Area Network or WAN. Finally, users need connectivity to
the Internet and this means a connection to an Internet Service Provider (ISP).
This is also typically considered a part of the enterprise WAN.
The LAN is entirely based on Ethernet interfaces. Fast Ethernet (FE, 100 Mbps)
is used widely, while Gigabit Ethernet (GigE, 1,000 Mbps) is becoming increasingly
prevalent. Small LANs may consist of users connected directly to an Ethernet
switch. For larger networks, routers are used to provide additional functions,
for example firewalls between different groups, but the interfaces used in this
case are also Ethernet. Ethernet technology is thus widely deployed in enterprises
and well understood by their IT departments. It is also extremely cost-effective
due to the enormous volumes for these interfaces.
However, the WAN interfaces in the enterprise need to be compatible with the
service provider networks. These networks are almost entirely based on the Synchronous
Digital Hierarchy (SDH) or Synchronous Optical Network (SONET) standards developed
by the International Telecommunications Union (ITU). This is true for in-country
networks of all major service providers across the globe as well as the service
provider networks between countries. These SDH/SONET networks evolved from the
need to efficiently support voice that formed the bulk of the service provider
traffic until a few years ago. Thus, their interfaces are designed to efficiently
support voice but the same interfaces are now used to support data. For example,
the typical interface speeds are 2 Mbps (E1), 34 Mbps (E3) and 45 Mbps (DS3)
which are the rates used to multiplex voice circuits into SDH equipment. These
are also the data rates of leased lines that can be obtained from service providers
to connect the different sites of an enterprise, or to connect the enterprise
to an ISP. This dichotomy of interfacesEthernet at 10/100/1000 Mbps in
the LAN and E1/E3/DS3 at 2/34/45 Mbps from SDH equipment in the WANresults
in the need to maintain separate equipment for LAN and WAN with an increase
not only in capital costs but also in operational costs including training maintenance
Typically, a router is used to interface between the LAN and WAN using an Ethernet
interface to connect to the LAN and WAN interfaces at 2 or 34 or 45 Mbps to
connect to the service providers SDH network. Unfortunately, even this
is not enough in many instances as the same 2 Mbps interfaces in the router
and the SDH equipment conform to two different ITU standardsV.35 for the
router and G.703 for the SDH equipment. Thus, another converter device
is typically required to convert between these two standards.
A data rate of 2 Mbps is inadequate for many enterprises,
but at the same time a data rate of 34 or 45 Mbps is too expensive. Thus, for
someone needing, say 10 Mbps, the only option is to get five 2 Mbps interfaces,
using five ports on the router and five FCDs, and using the router to split
the Ethernet data from the LAN among these interfaces. This not only involves
more capital expenditure, but also creates operational problems.
Equipment needed to connect an enterprise LAN to the
WAN at data rates of a few Mbps, using next-generation SDH equipment from
the service provider supporting Ethernet interfaces
Recently, ITU recognised this problem and evolved a set of
standards for supporting Ethernet interfaces directly on SDH equipment. Equipment
supporting these interfaces is typically dubbed Next-Generation SDH.
It permits drastic simplification of the networking gear used to connect the
LAN to the WAN. Now the same Ethernet interface used in the LAN is used to connect
to the WAN as well. This Ethernet interface can be connected to a router in
the enterprise or to an Ethernet switch. The choice is dictated not by any requirements
imposed by the WAN infrastructure, but by the functionality needed by the different
sites being interconnected. Thus, the equipment required in the enterprise is
the same whether to connect to another user in the same floor/building or to
users at a site located across the globe.
Typical equipment needed to connect an enterprise LAN
to the WAN at data rates of a few Mbps, using multiple 2 Mbps links
Note that this convergence of the LAN and WAN network equipment
required in the enterprise does not depend on the convergence of the technologies
used to build the LAN and WAN. There are some efforts to deploy Ethernet switching
technologies in the service provider networks, but these are not relevant to
the enterprise user. What matters to businesses is not how the service provider
network is built, but what interfaces it provides to them to connect. If the
interfaces are Ethernet, businesses need not maintain two separate network infrastructuresone
for LAN and another for WAN.
What are these new ITU standards that accomplish this? There are actually three
of them that address the different components required to achieve this functionality.
The first is Generic Framing Protocol (GFP) that frames the Ethernet data into
a form suitable for transmission over SDH. The second is Virtual Concatenation
(VCAT) that allows multiple SDH channels (e.g., multiple 2 Mbps
circuits) to be combined into a single link. The last is Link Capacity
Adjustment Scheme (LCAS), which allows the size of such a GFP/VCAT link to be
varied by hitless addition and removal of one or more constituent channels.
So the next time you order a WAN link for your enterprise network from a service
provider, ask for an Ethernet link based on GFP, VCAT and LCAS. You will save
operational headaches and a ton of money.
The author is Chief Technology Officer, Tejas Networks.
He can be reached at firstname.lastname@example.org