Access Network: Ethernet Gets Big Hug

The core of the Internet is awash with terabits of bandwidth, and metro
rings are coming up everywhere. The networks that provide subscribers access to
this huge bandwidth are woefully inadequate, both in terms of availability and
bandwidth. There is a huge set of services waiting to be offered to the
customer, services like streaming video, digital TV, high-speed Internet access,
etc, but the access network is holding up this business. An example of the
bandwidth required by these services vis-à-vis the available access bandwidth,
multiple digital TV requires access rate of up to 24 Mbps, whereas most
subscribers are stuck at sub-megabit speeds.

Why Ethernet?

Existing access solutions are inadequate in one respect or another. DSL,
cable modems and ISDN offer inadequate data rates, satellite is uni-directional,
broadband wireless solutions are not yet mature enough. In short, all existing
solutions have some or the other problems associated with them.

Given that lack of subscriber bandwidth is holding up the growth of the
Internet, leading vendors are working on providing an easy-to-implement metro
access solution, one that will bridge the bandwidth gap. Surprising as it may
seem, the protocol that made a humble beginning as a ‘Me Too’ LAN protocol
is poised to become the layer 2 protocol of choice at the metro access. While
the reasons for Ethernet rising above all others are many, there is little doubt
that Ethernet-to-the-X vendors are taking it seriously. First, Ethernet is
universally available–there are over 300 million Ethernet cards installed
today. Second, with such huge volumes, economies-of-scale have driven down the
costs. Third, the technology is stable and widely known, and a highly skilled
and well-trained talent pool is available to install and administer Ethernet
networks. Fourth, Ethernet lends itself to various types of media, with little
or no change to the MAC layer, thereby allowing existing applications to talk to
each other without re-work.

Before we get into the details of the various Ethernet solutions, we need to
study the peculiarities of the metro access network, the topologies and the
subscriber scenario’s.

ETHERNET ON MOVE
	The task force addressing these issues is the IEEE EFM working group, which is moving towards standardization of the 802.3ah specifications
	The issue is being driven by big names like Cisco and Intel, and so one is likely to see some action soon

The figure above explains the basic deployment scenarios and the associated
media/physical layers’ characteristics of the entire Ethernet metro access
solution. In order to emphasize the fact that the Internet places top priority
to the customer, the subscriber loop is called the ‘first mile’ (as opposed
to the ‘last mile’ of the core-centric telecom world), and we shall
hereafter refer to it as Ethernet in the first mile (EFM).

No single EFM solution can be deployed in all subscriber scenarios. This is
chiefly because the requirements of access subscribers vary, and because there
is a need to use the existing media infrastructure. To accommodate these
constraints, let us first study the types of access subscribers, as well as
their particular needs.

The subscriber could be a business, a residence, a large campus, hotel or
hospital. The residence itself could be a series of single-dwelling units, or a
multi-tenant apartment block. Each of these subscriber scenarios impose their
own constraints, and there is no single solution that can fit in all these
scenarios. The issue is further complicated by the fact that there could be
diverse media, and fiber may not be available in the basement or at the curb.
Investment in the existing copper cabling could be so great that the
infrastructure provider may not be able to discard his wiring. EFM is therefore
not a solution, but a solution-set that addresses all these complications.

Broadly speaking, EFM addresses two types of media, two different network
topologies, and three different physical layer specifications. The task force
addressing these issues is the IEEE EFM working group, which is moving towards
standardization of the 802.3ah specifications. These groups are being driven by
big names like Cisco and Intel, so one is likely to see some action soon.

Adapting Ethernet in the metro access centers on defining physical and MAC
layer specifications for each of the network topologies given in the figure
above, while leaving the LLC untouched. This means that, given a little
encouragement, all existing Ethernet-based devices and applications running
these devices would work seamlessly with this solution. Since the most widely
available media in the metro are copper and fiber, only these have been
targeted. Also, we observe that point-to-point and point-to-multipoint network
topologies are being worked upon, and the choice of one or the other is dictated
by bandwidth requirements, subscriber density/geographic distribution, cost, and
media availability. Let us study each of these solutions, and explore the
scenarios where they are likely to be deployed.

Point-to-Point over Copper

This technology is the way to go where required data rates are less than 15
Mbps, a large installed copper base exists, and dstance to the metro rings is
less than a mile.

This solution is likely to be deployed in MxU’s and campuses. MxU is a
generic term for large apartment complexes (multi-dwelling units), commercial
complexes (multi-tenant units), hotels (multi-hospitality units), etc. The
services that would be offered would typically be convergent services like voice
and video and high-speed Internet access. This technology would typically be
chosen where initial investment is constrained; quick installation is a
pre-requisite and reasonable speed is desired.

The EFM sub-group at the IEEE that is looking into defining a suitable
physical layer specification has not yet converged on the physical layer
standards that need to be adopted for Ethernet over copper. The current trend
seems to be veering towards adopting a compromise between the ANSI VDSL PHY and
the SHDSL PHY. In the meanwhile, vendors are pressing ahead with their
development.

Point-to-Point over Optical Fiber

Where fiber is available or future proofing is required, point-to-point
Ethernet over fiber would be a good choice.

Ethernet over fiber offers a new business model, and a large set of
revenue-generating services, tremendous scalability in terms of bandwidth and
subscriber density, and an extremely flexible service provisioning model and
longevity of infrastructure. While all of the above are properties of optical
fiber, Ethernet as a layer 2 protocol adds a new dimension ie low-cost 1000BaseX
cards, which have been in production for years now.

Service providers who are looking towards installing a new network
infrastructure will find this an interesting choice, for aforementioned reasons.
The lifetime costs of the network would be reduced greatly, while ensuring a
high degree of protection against obsolescence, though initial investments could
be high.

Point-to-Multipoint over Fiber

While P2P solutions over copper offer a cost-effective, low-investment,
high-speed access to the metro network, the level of services that this can
carry is limited. For instance, a multi-channel digital TV requires 24 Mbps per
subscriber, while EoVDSL delivers up to 15 Mbps. The alternate solution,
point-to-point over fiber, solves the problem of bandwidth, but involves high
initial investments, and delivers more bandwidth than most subscribers require
(up to 1 Gbps). Ethernet passive optical network (EPON) solves this problem. It
provides reasonable amount of bandwidth at low costs. EPON can deliver bandwidth
of up to 30 Mbps, and could provide higher bandwidth for short periods of time
(a subscriber could, for instance, ask for and obtain a burst of 75 Mbps
bandwidth for downloading large amounts of data, and revert back to 30 Mbps when
done).

In addition to delivering adequate bandwidth at low costs, EPON offers
several technical and business advantages to the infrastructure provider.
Instead of drawing a fiber from the service provider PoP to each customer
premises, the fiber is pulled till a convenient point close to a cluster of
subscribers, and an optical splitter is used to transport data to subscribers
from that point onwards. This not only saves the length of fiber to be
installed, but also reduces the density of the fiber to be managed at the PoP.
This results in lesser number of transceivers in the central office, lower power
requirements, lower space requirements, and lower operations and management
costs. Because field devices are passive in nature (of the order of one per 32
customers ), they do not need power and are free of electronics. This translates
into significantly low outdoor plant costs and maintenance costs.

Implementation Issues

However, it’s not roses all the way. Since an Ethernet network was to be
administered by a single entity confined to a single LAN, the protocol did not
define the elaborate operation and maintenance mechanisms, which are vital to
running large networks. Nor are there any robust self-recovery mechanisms. To
address these issues, the EFM working group has formed a sub-group for the
purpose of defining an operation, administration and management (OAM) scheme to
enable metro-wide Ethernet networks to be managed. The importance of this group
cannot be emphasized enough, suffice it to say that Ethernet could be well on it’s
way to becoming a true carrier-class protocol if large networks can be managed
and administered remotely.

There seemed to be two schools of thought about how to embed OAM into
Ethernet. One school was for defining a special MAC layer OAM data unit, the
other was for using the PHY preamble (largely redundant) to carry OAM data. The
group seems to be veering towards the preamble solution, being pushed by the big
guns in the industry.

In all, Ethernet in the first mile offers a broad range of solutions for all
needs, reduces costs, increases bandwidth, and is poised to be the engine of
growth for the Internet. In these lean times, this could mean the difference
between life and death. All the three solutions will be deployed in some measure
or the other though one may suspect if EPON has enough merit to warrant special
attention.

The day when a humble LAN protocol runs the Internet is not far.

Arif Shouqi, Cisco Systems Global R&D
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