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You are currently in the Satellite Internet VSAT section
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What's right for you: SHARED or DEDICATED?
One of the first technologies used in communications satellites to
deliver Internet connectivity was SCPC (Single Channel Per Carrier)
which refers to using a single signal at a given frequency and
bandwidth. In an SCPC system, satellite bandwidth is dedicated to a
single source, in other words you buy your own part of the satellite.
The primary advantage of SCPC is that the architecture allows the
complete direct and synchronous connectivity between the two remote
points on Earth (the remote customer and Internet connectivity
teleport for example), that's why it is now widely used to provide
dedicated backbone links between Global Internet access and Telephony
Service Providers worldwide.
Initially, SCPC was used to provide broadcast data and full duplex
multimedia communications. When it comes to an Internet access service
provision, an SCPC link may be compared with a terrestrial SMDS
circuits designed in 80's by Bellcore - a completely dedicated
always-on solution with guaranteed data delivery rate and usually
without any compression or sharing. So far so good, SCPC and SMDS
solutions are far from ideal to provide Internet access connectivity
for individuals and businesses - they are simply enormously expensive
for end-users, SOHO, SME, Calling Centre, start-up ISP, Hot Spot and
Internet cafe owners and similar categories of users which want to get
a broadband Internet access service for the price suitable to keep
their business alive and profitable. What makes dedicated connectivity
solutions expensive is the media - a physical resource itself: cables
are very expensive to bury and satellites are even more expensive to
launch (over 150 millions US Dollars of investment). Even nowadays,
SCPC services only make sense to businesses with thousands of
subscribers forming their networks, i.e. such environments like Global
Service and Telephony Providers.
The step forward to a consumer market was an introduction of shared
services like Frame Relay. Frame Relay circuits enable Internet
Service Providers to create a shared bandwidth resource: it was
designed to make more efficient use of existing physical resources,
thereby allowing the overprovisioning of data services to the
customers, as most of them were Home/SOHO/SME users and Internet
cafes, they are unlikely to be utilising a data service 100 percent of
time.
Just as the Frame Relay, a number of satellite Internet access systems
built on the TDMA architecture have conquered the Home/SOHO/SME, Hot
Spot and Internet cafe market. The burstable nature of an IP traffic,
combined with the technical possibility of shared resource solutions
like Frame Relay or ATM to provide several Private Virtual Circuits
(PVCs) inside the single physical resource, has offered the
opportunity for Internet Service Providers to offer a low-cost shared
broadband Internet access service by overbooking the physical
resource, as mentioned above. A term "Contention Ratio" has been
introduced, which is also known as Sharing Ratio or Oversubscription
Ratio.
There is a wide range of shared satellite Internet access services
available on the market nowadays, built on different TDMA technologies
(TDMA, MF-TDMA, D-TDMA, ...), contention ratios (from 1:50 to 1:2) and
prices (from few tens to several thousands per month) without any
clear buying guidelines for the user to understand which service is
exactly applicable to his tasks.
The first thing to understand is the concept of sharing or
subscription ratios. The fundamental idea of oversubscription is that
as ISP provides bandwidth for users, the total amount of bandwidth
assigned exceeds the actual port speed that is being delivered. The
idea is to make sure that "network" bandwidth is available when users
need it, and that the access point does not become a bottleneck that
creates a bad user experience.
As mentioned above, the fundamental reason for oversubscribing is to
safely conserve expensive bandwidth and utilize it to it's maximum
potential. Bandwidth that is unused and idle, is wasted, and it
becomes a cost which is not producing revenue for the Internet cafe or
Hot Spot operator or simply forms an enormous service price which
individuals can not afford. On the other hand, if bandwidth is far
oversubscribed and unavailable when needed by users, then customers
will be unhappy with performance and will take their business
elsewhere. So a balance must be struck, to maximize revenues and
profits, while maintaining and growing a satisfied customer base.
The intermittent, bursty nature of Internet traffic virtually
guarantees that all users are not going to require maximum bandwidth
at the same time. This is similar in concept to the telephone company,
which has been playing this same type of game for years by
oversubscribing users to the number of phone switch ports available.
The phone company knows that not all subscribers are going to be
actively making phone calls at the same time. They are able to share
or oversubscribe the switch ports based on the historical call volumes
made by subscribers. When too many people try to place a call at the
same time, a busy signal is generated. If this happens too often,
subscribers become upset with the service provider.
Oversubscription is deeply rooted in the fundamentals of statistical
multiplexing. Because data traffic tends to be quite intermittent and
bursty, statistical multiplexing is able to save money by optimizing
the use of the available bandwidth. Time division multiplexing (TDM)
technology permanently assigns each individual channel a portion of
the bandwidth. If a channel is idle, the bandwidth is wasted and gone
forever.
Taking a look at a simple example, think back to the good old days
before PCs, when asynchronous terminals were connected to
mini-computers. If you had eight terminals at 1200 bit/sec connecting
to the computer with time division multiplexing, they would share a
9600 bit/sec circuit (8 x 1200 = 9600). With statistical
multiplexing, the multiplexer has the intelligence to assign bandwidth
as needed. For example, in the above situation, one might set each of
the terminals to 4800 bit/sec, resulting in a four-to-one (4:1)
oversubscription (8 x 4800 = 38,400 and 38,400/9600 = 4). The odds
that each of the 8 terminals would be active simultaneously is small
enough that one would typically not see a performance degradation. In
the event that all of the 8 terminals were active at the same time,
the buffers in the multiplexer would temporarily store the data until
bandwidth became available again.
This same type of oversubscription takes place in regards to
traditional terrestrial Internet access services. ISPs "play the
odds" using traffic engineering techniques so that they can provide
services to large numbers of customers using shared, rather than
dedicated bandwidth. The challenge for network architects is to
determine how much bandwidth they need in order to optimize the use of
this expensive resource, while providing a service that will ensure
satisfied customers. This is a challenge in the terrestrial or
wireline industry, just as it is when using broadband satellite for
backhaul service.
There are basically two types of wireline services that are generally
used for Internet backhaul service:
1. Private T-1 or FT-1 (fractional T-1). These are dedicated
point-to-point links used by a single client, often to connect
a remote office location to a headquarters location, or to a
POP or Point of Presence on the Internet backbone. This type
of circuit is frequently used in private networks, but is
seldom provided for Internet access.
2. Internet T-1s. These are T-1 circuits whose bandwidth is
shared by multiple clients using statistical multiplexing
techniques. This is accomplished by aggregating or combining
multiple Internet T-1s and then connecting them to the
Internet POP, such that the total bandwidth of all the T-1s
exceeds the size of the connection to the POP. For example,
if no sharing or oversubscription takes place, 28 T-1 (1.54
Mbps) circuits could be connected to a single DS-3 (44.7 Mbps)
link to the POP. However, service providers will generally
oversubscribe Internet T-1s with 1:8 to 1:12 ratios, so that
anywhere from 200 to 300 T-1s might share bandwidth on a
single DS-3 circuit.
Many new Internet cafe, WISPs (wireless ISPs), and Hot Spot operators
are unaware that the Internet T-1 they use for Internet access is
already oversubscribed, and that this has an effect on the amount of
bandwidth that they can deliver to their clients. Most service
providers are unwilling to discuss their oversubscription ratios with
their customers, so it pays to do some research and talk to other
customers of the carrier in order to get some idea of what kind of
service to anticipate.
It is not necessary to be alarmed or overly concerned about this
practice. At the very high speeds that service providers are
aggregating Internet T-1 circuits, the statistical multiplexing is
very efficient and bandwidth may be shared by many clients quite
successfully. Generally the relative price of the circuit compared
with other carriers, is a good indication of the subscription ratio.
Similar considerations must be acknowledged when using broadband
satellite for backhaul. Dedicated bandwidth, (generally SCPC) similar
to Private T-1 circuits, will deliver the entire bandwidth to a single
client. Often times, this may be a highly inefficient use of the
bandwidth as the bursty, intermittent nature of TCP/IP traffic means
that much of the available bandwidth may be idle and wasted.
It may be more cost-effective to use shared bandwidth as a backhaul
service, since dedicated SCPC bandwidth is very expensive. An
Internet Caf? or Hot Spot (ISP) operator may only be able to afford a
low-speed SCPC circuit, for example 64 Kbps x 32 Kbps. Using shared
bandwidth, he might be able to lease a higher speed backhaul circuit,
for example 512 Kbps x 64 Kbps for the same cost.
This allows his users to receive faster downloads, and to transmit at
faster speeds.
He should understand however, that if the broadband satellite network
is very busy and the Satellite Network Operator (SNO) has
oversubscribed it too much, that he could temporarily end up during
peak traffic times with less bandwidth than a lower speed SCPC circuit
would provide. So far this is the most often admitted mistake in the
satellite broadband market, especially with the entry-level TDMA and
MF-TDMA VSAT systems when users, mislead by the lack of purchasing
guidelines, buy highly oversubscribed services and experience a poor
quality of Internet connection.
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