El concepto y la utilidad de las medidas tecnológicas de protección

The ICE solution utilizes frame relay as a transport protocol. The use of the frame relay is intended to address several of the shortcomings of TCP/IP over a

geosynchronous satellite system. TCP/IP often struggles over such a link because of the large latency times involved with such a system. The frame relay protocol eliminates some of the three-way handshakes and acknowledgements utilized in TCP/IP in an effort to decrease retransmissions and increase the effective throughput of the system. [Ref. 19, p.213]

a. Frame Relay Protocol

Frame relay protocol is a network transportation protocol that is utilized to provide high-speed wide area network services. The protocol is often used for service in the long-distance telephone carriers’ networks. The protocol works to accept and deliver blocks of data on the network. Each block of data can be as large as 8,000 octets of data. The frame relay protocol is a connection-oriented service. This means that a connection must be formed and maintained between two nodes of the network before any information can be transmitted. This type of connection-oriented service is designed to best handle and deliver continuous data at a fixed rate. It is not the most efficient protocol for a burst-transmission traffic network. [Ref. 19, p.236]

b. Geosynchronous Satellite Latency

Latency can simply be defined as the time it takes a packet to travel from source to destination in the system. This is viewed as delay in a communications system and is one of the basic parameters when considering a system’s quality of service (QOS). Several applications cannot properly operate in situations of increased latency.

For example, latency in voice communications becomes

noticeable when delays begin to exceed 100-200 msec. While small latency times will exist on the network, the average caller will not be able to distinguish a delay of smaller than about 100 msec in a phone conversation. For this reason, telephony applications demand that the system latency not exceed these parameters.

The latency is, in fact, the time it takes the signal to travel the entire distance of the network from the source node to the destination node. In terrestrial networks, this may be on the magnitude of several miles or even a few thousand miles. With these distances, today’s transmission lines are able to transmit signals over the required distances with only minimal delays (on the order of tens of milliseconds). Problems arise when the signal is required to transmit via a geosynchronous satellite as a part of the network. The geosynchronous satellite is located approximately 36,000 kilometers above the earth’s surface. This equates to an up and down travel distance of approximately 75,000 kilometers and incurs a system latency of approximately 250 msec. This latency already exceeds that required for telephony applications. In addition to this latency, further delay incurred by terrestrial network routing and connection solutions. For example, if one ship were calling another ship, only 500 yards away, via a standard INMARSAT terminal, the minimum delay under perfect conditions would be 250 msec. This incurs a noticeable and awkward time delay for voice communications. The problem is caused by the fact that the first ship is not simply calling the second. Each transmission must travel 36,000 kilometers to the satellite and 36,000 kilometers back to

the receiver. This is one of the downfalls of a geosynchronous satellite network.

The sizable latency incurred by a geosynchronous system also affects other aspects of a communications network. Latency has severe affects on any network client/server and transport protocols. Most network protocols are designed with a LAN in mind. By using the LAN as a network model, the protocol is designed under the assumption of only minimal round-trip delays. For this reason, the protocols are often designed using a series of low bandwidth request and acknowledgement exchanges. The sending and receiving nodes both transmit several signals either requesting data transmission or acknowledging the receipt of the data. Many protocols utilize a store and forward method of transport. This means that the sending node stores the data to be transmitted before transmitting.

The node then transmits the data, retaining a stored copy.

The sending node waits for a signal from the receiving node signifying that the data was received in an acceptable format. The original node waits a set time for the response message. If it does not receive the acknowledgement within that time it starts the entire process again. The original node will do this until it receives the acknowledgement message. It will also not move on to the next piece of data until it successfully sends the first.

With the above process in mind, it becomes obvious why the latency caused by a geosynchronous satellite system could cause severe problems for these protocols. Each transmission also requires at least one

response. That equates to 75,000 kilometers the packet must travel and 75,000 kilometers the acknowledgement must travel. These delays equate to a minimum system latency of 500 msec before the sender receives an acknowledgement.

TCP/IP is the protocol with which the internet and most LAN’s operate. It works similar to the generic protocol described above. For this reason, TCP/IP is often ineffective over a geosynchronous satellite network. One solution to the problem is to modify the protocol to wait a longer time for the acknowledgement message. The problem with that solution is TCP/IP is an end-to-end protocol.

That means that you would have to modify the protocol on every computer that will communicate over the network.

That essentially renders the solution infeasible.

Another solution is to translate the protocol to one that does not exhibit the same responses to extended latency. An example of such is the frame relay protocol used by the ICE technology. The problem with this solution is that any data or network accessed outside the NOC needs to be translated to the new protocol. This, unfortunately, eliminates the reliability aspect of the TCP and could potentially result in the delivery of invalid data.