SCPC or TDMA, which one is right for you ?

Whether you choose for SCPC (Single Channel Per Carrierl) or TDMA (Time Division Multiple Access) each offers strengths and weaknesses, depending upon your operational objectives.

SCPC – Single Channel per Carrier

Single channel per carrier (SCPC) technology uses a dedicated carrier to each remote VSAT to receive information from a central site, and another dedicated carrier for each VSAT to transmit information back to that hub. SCPC was the first VSAT technology offered to the maritime and offshore sectors, enabling ship operators to provide high bandwidth to individual vessels.

TDMA – Time Division Multiple Access

Alternatively, a time division multiple access (TDMA) network, where bandwidth is shared and segmented across several vessels could better fit your budget. TDMA technology involves a single high speed time division multiplexing carrier transmitted from the central hub to the satellite, from which many VSATs can receive information in a shared network. To transmit data back in a TDMA network, the terminals are synchronised and transmit information in a burst mode within a series of short scheduled timeslots. Suppliers of TDMA services offer always-on high speed and flat rate VSAT connectivity, but with higher network contention ratios because the network is shared.

Am I getting what I paid for ?

Dedicated links

If you are using a dedicated satellite connection it is easy to test whether you are receiving the data connection speed you paid for.  Measure the maximum speed you can upload a large file and do the same for the download.  The time needed should be equal to near maximum speed.

Keep in mind that speeds are usually in bits per second whereas size is in bytes per second.  And as well all know there are 8 bits in 1 byte.

So, if you have a 1MB file (1 megabyte) and you have a 1 Mb (1 megabit) connection, it will theoretically take you 8 seconds to transfer the file.  This will be longer because of many technical factors (latency to start with) but it does give you an idea of the speed achieved.

Shared links

It becomes more complicated when having a shared satellite connection.  Because you can’t know what the other ships are transferring it is nearly impossible to check whether you are getting what you paid for.  Nevertheless, you can upload/download a large file and measure the time it took and see whether this calculated speed is not below your CIR (Committed Information Rate) and how close it is to your MIR (Maximum Information Rate).  The CIR should not be lower than what you paid for and the higher the MIR achieved the better.

From a cost point of view everything you achieve above the guaranteed speed  (CIR) is paid for by the other ships.

Tip from the pros: Your provider might not have as many ships on the shared connection as your contention ratio indicates … think about that for a second …

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