by January 3, 2001 0 comments

If you live in any of the major cities, then you couldn’t have missed the
colorful pipes lying by the roadside everywhere. These pipes will carry fiber
optic cable that will form the backbone for providing broadband access to homes
and offices. All equipment used with this fiber-optic backbone would vary
depending upon the broadband technology used. Currently, three such
technologies, called Ethernet, cable modems, and xDSL, are being used. The
choice of technology depends upon factors like user demand and the cost of
equipment. Here, we won’t discuss which technology to choose, but explain how
their setups differ.

The three technologies use different types of cables for their communication.
Ethernet uses Cat 5 cabling, cable modems use coaxial cables, and DSL uses
single pair copper cables. Due to this, all equipment has to have the proper
termination to be able to join the fiber backbone. For each technology, let’s
look at these terminations and other equipment that’s used. For the sake of
clarity, we’ll start from the equipment used by end users and work our way
towards the broadband service provider.

Broadband over Ethernet

The setup for this is similar to a typical Ethernet setup, and the speed
would also be in the order of 10/100 Mbps. The typical subscribers for this
would be large companies needing VPN connectivity among their various office
locations. The termination at the subscriber end would be an Ethernet switch
with RJ45 ports capable of taking Cat 5 cabling. The switch would be connected
to a router, which would in turn be hooked to the broadband service provider. In
some cases, it might be more convenient to have end-to-end fiber-optic
connectivity from the broadband service provider to the subscriber.

Cable modem setup

A cable TV network was originally meant for audio and video broadcast. To
provide data over the same network, meaning Net access, extra equipment needs to
be added both at the subscriber’s and at the cable operator’s end. At the
subscriber’s end, cable modems are installed. These would connect to what’s
called an Optical Node (O/N) over coaxial cables. If there are several O/Ns,
then they would all terminate into an Optical Shelf (O/S) over optical fiber

The O/S terminates into a device called the CMTS (Cable Modem
Termination System), again over fiber cabling. This is the most important and
the costliest device in the setup as it’s responsible for both upstream and
downstream transfer of data from the Internet. As we see, this setup is a mix of
coaxial and fiber cabling. This sort of a setup is known as HFC, or Hybrid
fiber-coaxial setup. However, it can also be done purely on coaxial or fiber.

DSL setup

There are many flavors of DSL, which we’ll cover in a
separate article. Here, we’ll focus on ADSL (Asymmetric Digital Subscriber
Line), which is becoming popular among broadband service providers. In a typical
ADSL setup, the subscriber would have either a DSL modem or a router. These
devices have the same interface as used in Ethernet networks. This could be
directly connected to a computer with a network card, if it’s a home; or to a
hub or a router if it’s an office.

The other end of the DSL modem or router would connect to a
DSLAM (Digital Subscriber Line Access Multiplexer) through a plain single pair
copper cable, which is essentially an ordinary telephone line. The DSLAM would
be the termination point for all the subscribers in a particular area. This
would in turn terminate into what’s called a DSL Aggregator over a high-speed
digital fiber link called OC-3. This Aggregator in turn would connect into the
broadband service provider’s network over a fiber cable.

In all the cases mentioned above, the distribution of
bandwidth among subscribers and the placing of various equipment depend upon the
broadband service provider. An optical node in a cable modem setup, for
instance, could lie in the subscribers’ neighborhood, while the CMTS could lie
at the cable operator’s end or with the broadband service provider. A DSLAM
could lie in a subscriber’s place, such as a commercial building, where
several offices need a DSL connection. It could also lie at a telephone
exchange, from where the copper cabling that’s spread out for voice will also
start carrying data traffic.

The choice of whether to use DSL, cable modem, or Ethernet
depends largely upon the cost. For the time being, cable modems are used mostly
for homes, while DSL and Ethernet are used mostly for offices. However, as
broadband technologies become widespread, their costs will come down and they
will become affordable even for the home user. In California, for example, most
new apartment buildings are geared with DSL or other broadband technology. That
may not happen so soon here. But keep looking outside your window; maybe they’re
laying the fiber right now.

Anil Chopra with inputs from Pawan Pratap Singh of spectranet

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