I've been designing VSAT systems in the military for a few years, so hopefully I can answer most of your questions. Hughes-Olivetti (among others) offers VSAT service here in Europe, and the VSAT service we have with them uses primarily 3 types of medium access. When your transceiver sees that you have only a few small packets to transmit, it uses a protocol known as slotted ALOHA. It just transmits a packet during synchronized time slots and hopes there's no collision; if so, it just keeps trying after random wait times just like CSMA/CD. The second method, used for larger uploads, is called transaction reservation. Just like a cell phone, the transceiver lets the satellite know that it has a large chunk of data to send, and the satellite gives it a time frame in which to do it, usually all in one lump. The third method is to request a dedicated channel out of your guaranteed leased bandwidth, but I don't think that's applicable for what we're talking about here.
As far as security goes, I'm not sure it's something to worry about (but I'm one of those weird trusting types!) To intercept your uplink, someone would have to be right in front of the dish; the side lobes aren't that strong. On the downlink, which is all TDM, an eavesdropper would have to know the IP address of the person they are trying to target, not to mention the specific freq of the transponder the target is on. It would be less trouble to just tap their phone line! I must admit I don't know what the service providors have in mind for security, but I'd like to think they have something in the works.
Ever heard of INMARSAT? More commonly known as those SEAL magnaphones you see in some action flicks. I can get a solid 64kbps up and down with one, and the latest models are no bigger than a laptop computer (with a fold-out flat "dish"). Those are direct connections though. Most typical VSAT setups use a 1.8 meter dish and a ~3watt head for uplink with a GEO satellite, and I've run those up to 512kbps down and 128Kbps up.
The problem with uplinking however is the MAC protocol. The VSATs I've used employ slotted ALOHA, which is like CMDSA/CD only without the carrier sense. Essentially, everyone transmit at random and hope there's no collision! Count me out!
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I've been designing VSAT systems in the military for a few years, so hopefully I can answer most of your questions. Hughes-Olivetti (among others) offers VSAT service here in Europe, and the VSAT service we have with them uses primarily 3 types of medium access. When your transceiver sees that you have only a few small packets to transmit, it uses a protocol known as slotted ALOHA. It just transmits a packet during synchronized time slots and hopes there's no collision; if so, it just keeps trying after random wait times just like CSMA/CD. The second method, used for larger uploads, is called transaction reservation. Just like a cell phone, the transceiver lets the satellite know that it has a large chunk of data to send, and the satellite gives it a time frame in which to do it, usually all in one lump. The third method is to request a dedicated channel out of your guaranteed leased bandwidth, but I don't think that's applicable for what we're talking about here. As far as security goes, I'm not sure it's something to worry about (but I'm one of those weird trusting types!) To intercept your uplink, someone would have to be right in front of the dish; the side lobes aren't that strong. On the downlink, which is all TDM, an eavesdropper would have to know the IP address of the person they are trying to target, not to mention the specific freq of the transponder the target is on. It would be less trouble to just tap their phone line! I must admit I don't know what the service providors have in mind for security, but I'd like to think they have something in the works.
Ever heard of INMARSAT? More commonly known as those SEAL magnaphones you see in some action flicks. I can get a solid 64kbps up and down with one, and the latest models are no bigger than a laptop computer (with a fold-out flat "dish"). Those are direct connections though. Most typical VSAT setups use a 1.8 meter dish and a ~3watt head for uplink with a GEO satellite, and I've run those up to 512kbps down and 128Kbps up. The problem with uplinking however is the MAC protocol. The VSATs I've used employ slotted ALOHA, which is like CMDSA/CD only without the carrier sense. Essentially, everyone transmit at random and hope there's no collision! Count me out!