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A satellite can be used to in the provision of a private network and internet over a long distance, but networking using a Transmission Control Protocol over satellite is challenging due to the long latency, high bit error rate, and asymmetric bandwidth links. Moreover, the TCP performance is limited not by the satellite itself, but by the TCP's sliding window algorithm, data acknowledgement and retransmission algorithm, and slow start and congestion avoidance algorithms.
Using Sky X technology can overcome these issues. The Sky X technology increases the file transfer speed and web performance. It replaces the TCP over satellite with a high performance protocol, such as, Xpress Transport Protocol (XTP). In addition, the Sky X system is made up of Sky-X OEM (Original Equipment Manufacturer), Sky-X Client/Server and the Sky-X Gateway product. It is compatible with all TCP applications; therefor the end clients or users do not need any modifications.
The Sky X Client/Server and the Sky X Gateway systems use XTP which is a very good solution to be able to overcome the limitations that are associated with the used of TCP over satellite. However, this paper discusses the performance of TCP/IP and Sky X technology over satellite. It also focuses on the architecture, application, and merits of the Sky X technology.
Satellites are the most attractive option for transmission of internet and other internet protocol traffic from one location to others globally where there is a limit on the terrestrial options available. Sky X Technology works by replacing the Transmission Control Protocol over the link of the satellite with another protocol which is optimized for the normal high loss and bandwidth conditions which are typically associated with satellite communication [Palter 2008].
In the current access of internet technology, a satellite can be used for the provision of a private network and internet access over a long distance by networking using a Transmission Control Protocol (TCP)/ Internet Protocol (IP) [Alexander 2006]
However, this action of networking using a Transmission Control Protocol (TCP)/ Internet Protocol (IP) over satellite is challenging. It is important to note that the conditions of the satellite adversely interact and react with a variety of elements that are present in the Transmission Control Protocol architecture with the effect of constricting severely the data throughput that would have otherwise been achieved over the satellite links. Rather than attempting to summarize all the shortcomings of this sort of networking, this paper seeks to present all the aspects of using Sky X technology and also the use of TCP that limit the efficiency of transmission.
The objective of this report is to help the reader to understand how a satellite can be used to provide an internet connection. Also, this report shows the challenges of using the Transmission Control Protocol over satellite and how Sky X Technology overcomes those limitations.
Importance of the Report
This report gives the opportunity for the student to get familiar with the Sky X Technology components and the reason behind using XTP protocol in the Sky X Technology. Also, it helps the reader to get the needed acknowledgment to understand a satellite communication using Xpress Transport Protocol.
The material in this report is organised into chapters. The chapters deal with the following topics:
Chapter 2: Transmission Control Protocol (TCP) and Internet Protocol (IP)
Chapter 3: Architecture and application of the various products of the Sky X Technology Family.
Chapter 4: Conclusion
Transmission Control Protocol (TCP) and Internet Protocol (IP)
Transmission Control Protocol (TCP) and Internet Protocol (IP) technically speaking are two very distinct network protocols. However since they are so commonly used together, the term TCP/IP has been standardized to refer to both or either protocols.
Mitchell holds that internet Protocol (IP) corresponds to the Network layer of the network in use (Layer 3) in the Open Systems Interconnection (OSI) model. The Open Systems Interconnection model classifies internetworking in terms seven layers stacked vertically. The more primitive, hardware - oriented functions for example routing, addressing and flow control are implemented by the lower layers of the Open Systems Interconnection model. When data is being transmitted in this model, it starts at the top layer at the transmit side. This data then travels to the bottom layer of the stack, traverses the network connection to the layer at the bottom on the receiving side and then up its OSI stack as shown below [Mitchell 2011].
Figure 2.1 TCP/IP and OSI Model
Transmission Control Protocol (TCP) on the other hand corresponds to the Transport Layer in the Open Systems. Therefore it would be prudent to conclude that the term TCP/IP refers to the type of network communications whereby the TCP transport is used as a means of transport of data across IP addresses.
The challenges of using the Transmission Control Protocol over satellite and how Sky X overcomes them
The first challenge of using the Transmission Control Protocol over satellite is due to the long latency. Satellite connections, which are both secure and efficient, are quite challenging for the Transmission Control Protocol. The presence of long latencies and also the jittering bundling of packets causes serious reduction that the potential bandwidth that the TCP/IP link can be able to establish.
DC Palter believes that a satellite in geostationary orbit can be 22,300 miles (36,000 kilometers) above the earth which can make communication through geosynchronous round trip time of about 120m/s as shown in the figure below. If the satellite is on the horizon, the distance is about 26,000 miles (42,000 km), which can make communication through geosynchronous round trip time of about 560 m/s. The round trip time can often reach 1500 m/s or more with a shared bandwidth system or a double satellite hop [Palter 2008].
Figure 2.2 Geostationary-satellite
According to [Akyildiz 2010], satellite links can introduce higher latencies than most terrestrial networking segments due to long distances from the ground stations to the satellite. Thus, the TCP/IP link's design does not work adequately in a scenario where there is high latency and noisy channels. Also, because of the high delays in geosynchronous satellite links, the TCP protocol acts to reduce the overall throughput of the link [Akyildiz 2010]. The long latency of the satellite link will increase the time and takes TCP to ramp up new connections and recover from data loss. Thus, this can be an introduction of bit errors due to the high altitudes which occurs in the atmosphere. There is a reduction of the effectiveness of the Transmission Control Protocol which is typically tailored for short hops within the low loss fiber or cables due to the before mentioned factors, in combination with the back channel which is normally smaller than the one available on the forward channel. Despite this the Transmission Control Protocol can be seen to be effective when it is used in a local network by the use of optical fibers or cables [Farrell 2004].
Figure 2.3 Internet via satellite
This challenge causes the mandatory use of acceleration, Hypertext Transfer Protocol (HTTP), which is communication protocol widely used is used to transfer web pages from a web server to a client web browser using TCP. As a consequence of using this protocols which is not optimized for the existing satellite conditions, the throughput over these existing satellite networks is not more than a bare fraction of the entire available bandwidth. Thus, The use of Sky X Technology comes in improve on this shortcoming on restriction and ensure that there is access of the maximum bandwidth. Therefore, by application of the Sky X Technology to a given satellite network, there is marginal allowance for the users to be able to take advantage of the full availability of the bandwidth, as opposed to the Transmission Control Protocol which only transmits a fraction of the entire bandwidth [Doffoh 2005].
Another challenge that is associated with the use of Transmission Protocol is its sliding window algorithm. In its operation, the Transmission Control Protocol utilizes some form of sliding window mechanism in a bid to limit the quantity of data that is in flight at a given period of time. This can be explained as a built-in windowing mechanism which is used to determine the highest likely throughput and at the same instance; it is balancing the re-transmission of any dropped packets. The moment the window is at full capacity, the sender ceases transmission until the point that it receives some new acknowledgement. In a scenario where the acknowledgements take time to return, meaning that they are slow to return, the Transmission Control Protocol window will in graduation begin to set a hard limit on the maximum rate of throughput.
Thus, The TCP uses a sliding window to buffer multiple packets of data while waiting for the acknowledgements to return. The acknowledgements can be return before the buffer becomes full, which allows the sender to send more data and keep the link fully packed with. When the buffer becomes full before the acknowledgements return due to a small window, the TCP will stop transmitting until it receives acknowledgements.
Figure 2.4 shows a TCP connection transmitting data over a link with a 600 m/s delay [Palter 2008].
with 600 ms Delay
Windows 98 SE
Windows NT 4
Mac OS 9
Mac OS X
Table 2.1 Maximum Throughputs for Various Operating Systems
Data acknowledgement is also another challenge that is faced by the application of Transmission Control Protocol. Latency in the Transmission Control Protocol/Internet Protocol is exacerbated by the sole fact that there is a requirement by the acknowledgements for packets that are sent across the link. The heuristic and simplified data acknowledgment system that is used by the Transmission Control Protocol is not adaptive to the long latency or the highly unequal conditions of the bandwidth. To ensure that there is a reliable transmission of data, there are acknowledgements for the data which is received back to the sender which are constantly sent by the Transmission Control Protocol receiver. This occurs so as to facilitate reliable communication in a situation which has uncertain and heavily congested network conditions.
Figure 2.4 The positive acknowledgement with retransmission technique
However, where there are transmissions which are short and being transmitted over high latency channels, the acknowledgement mechanism becomes a problem. It should be noted that it takes at least 600 m/s for each packet exchange to be completed. While this number may be considered a smaller factor in applications which are broad-cast only, it can notably limit the throughput for the normally interactive Internet Protocol applications such as the internet, the extranet and the intranet [Enterprises 2006].
Similar to the way the Sky X Technology works to eliminate the challenges that are brought about by the use of Transmission Control Protocol, Sky X Technology maximizes the available resources to increase the overall performance of the Internet Protocol over the satellite through the use of data compression, web pre fetching and protocol connection splitting whilst it remains transparent to the end consumer. In this scenario, the Sky X Gateway performs its work by the transparent replacement of the Transmission Control Protocol connections that emanate from the client and essentially converting the data which is acquired into Xpress Transport Protocol (XTP) so that it can be beneficial for the hop over the satellite link [Kozierok 2005].
After the above discussion of the challenges that the use of the Transmission Control Protocol faces in satellite transmission it would be imprudent not to discuss the steps that have been taken in a bid to minimize the challenges that have been previously been discussed. Clearly most of the Transmission Control Protocols are saddled with technical shortcomings which are evident in both networks of satellite high speed and terrestrial but feasible solutions do exist and there is extensive research geared towards discovering those solutions.
Worth of mention is the challenge in the use of Transmission Control Protocol is the congestion avoidance algorithm. On his part [Sunil 2009] says that in an effort to curb the problem or even the possibility of a congestive network meltdown, the Transmission Control Protocol typically makes the assumption that each and every data loss that occurs is as a result of congestion and its consequent response to this anomaly is to reduce the rate at which transmission is occurring [Sunil 2009].
However, there are some instances whereby the Transmission Control Protocol misinterprets the action of the long time taken to make the round trip together with bit errors as a sign of congestion and thus responds, albeit inappropriately. Similarly, the Transmission Control Protocol algorithm referred to as 'Slow Start" TCP uses a process called Slow Start to ramp up each connection. Slow Start works relatively efficiently on a LAN where acknowledgements return in a few milliseconds. However, on a satellite link, Slow Start causes new connections to ramp up very slowly. These congestion avoidance mechanisms outlined above are very vital in any routed environments but are ill- suited for the single - path satellite links. This can also be explained in this manner: The "slow start" upon the initial connection setup discovers the throughput capacity of the Transmission Control Protocol [Palter 2008]. [Patrick 2011] believes that this slow start subsequently sends a packet across the physical connection and then it waits for a response. If it receives a response, then it means that subsequent packet which is sent is a bit faster [Patrick 2011]. There is a repeat of this procedure up until the speed of the link in question is discovered. When you cater for the half second delay which is evident between the responses, the throughput is to a great extent slowed. Therefore there can be a significant drag on the Transmission Control Protocol/Internet Protocol which can be removed if a system to by-pass the slow start can be found.
In an effort to overcome the problem of slow start, there has been the development of a Transmission Control Protocol Acceleration whose purpose is to work over the satellite link. This system works by reducing rates of it error to rates which are compatible with good and efficient Transmission Control Protocol/ Interest Protocol performance. Here there is a termination of all Transmission Control Protocol and then they are made to re-originate at the both ends of the satellite link. A new protocol that is transparent to Transmission Control Protocol/Internet Protocol and is optimized for the satellite environment is used over the satellite link. There is no use of slow start in this satellite link protocol. Since there is control over the link and the schedules capacity, the Transmission Control Protocol connection's throughput is known before transmission and there is no requirement of a discovery process. Without further round trip delays; the data is then sent at a rate which is already predetermined over the link. The most appropriate window size can be selected because the characteristics of the rates of bit error and satellite's latency on the link have already been established. Where there exists clear sky conditions, this system will perform at a 10-10 or better. Where this is added to the system's ability to adjust power automatically in a link, it drops the Bit Error Rates to 10-9 or better. There is effective removal of throughput degradation because of the window size because at these performance levels the possibility of big windows exists. There can also be increased network efficiency without the sacrifice of reliability because of the reduced frequency of acknowledgements [Sunil 2009] [Palter 2008].
These performance limitations of the Transmission Control Protocol can be easily overcome by the Sky X Technology. Through a combination of actions that include data compression, web pre-fetching, protocol connection splitting the Sky X products can be able to increase the overall performance of the Internet Protocol over the satellite.
In addition to Sky X Technology's ability to greatly improve the overall performance of Transmission Control Protocol, to accelerate web downloads; the Technology also has products which include Hypertext Transfer Protocol (HTTP) specific optimizations. By the reduction of the handshaking that is required in the establishment of a new Hypertext Transfer Protocol connection, fast start is able to save one full round trip time for each and every connection [Doffoh 2005].
High Bit Error
Another challenge that is associated with the used of Transmission Control Protocol is the high bit error rate. The problem is that TCP has no way to differentiate between packet loss due to congestion or due to a bit error. Thus, TCP will slow down the transmission rather than retransmitting quickly .TCP assumes that any data loss is caused by congestion.
As discussed above the heuristic and simple data acknowledgement mechanism that is employed by the Transmission Control Protocol is not compatible with the high levels of latency and the equally high conditions of asymmetrical bandwidths. In order to enable the provision of reliable data transmission, there is a constant sending of acknowledgements back to the sender by the receiver. Only when the round trip has lapsed a number of times without getting any acknowledgement, does the sender assume the position that data is lost and/or corrupted. Where in a satellite network the round trip time is very long, this algorithm will not respond well and the error rates will definitely be high. There is wastage of back channel bandwidths which is plenty precious where there is such constant stream of acknowledgements; and the acknowledgements to the sender can easily become a bottleneck where the back channel the size of the back channel in comparison to the size of the channel where the bandwidth is originating from, is small. In such a scenario, there is dominance of error messages and acknowledgements and consequently there is a reduction of the rate of flow.The Sky X Technology overcomes this short coming which is the high sensitivity of the Transmission Control Protocol to the bit error of the link [Sunil 2009] [Palter 2008]. The superiority of the Sky X Technology over the Transmission Control Protocol can be will illustrated by an example. At a very low rate of error, the Transmission Control Protocol can be able to deliver a mere 1.5 Mbps, but where there is a different rate of error of 1x10-5; the Transmission Control Protocol's throughput will become less than 0.03 Mbps. On the other hand, the Sky X Technology fully saturates the link which is at very low rates of error that even at the rate of error of 1x10-5, there will be an achievement of an amazing 5.1 Mbps where there is no compression and where there data is compressed, an astounding 15.8 Mbps [Sunil 2009]. The essence of Sky X Gateway is to do away with the dependency which the Transmission Control Protocol has on the round-trip of the link. The level of the Transmission Control Protocol drops rapidly with the increment of time it takes to make a round-trip. In contrast, the Sky X Technology has the ability to maintain not a fraction, but the full link's usage immaterial of how long it takes to make the round trip. Where compressible text is in play, Sky X Technology will consistently deliver throughput rates which are greater than the actual bandwidth [Doffoh 2005].
Architecture and application of the various products of the Sky X Technology family
The next part of this paper shall discuss the Architecture and application of the various products of the Sky X Technology family. Currently, Sky X Technology products can be classified as the leading forefront implementers of a class of transmission products which are also known as Protocol Gateway Transmission Control Protocol Performance Enhancing Proxy. It should be noted that the Sky X Technology has been designed to replace the Transmission Control Protocol satellite link and in its place there is a protocol which is optimized for long latency, the ridiculously high loss and the unequal/asymmetrical bandwidth conditions that are typically associated with communication via a satellite. The Sky X Technology is made up of the Sky X OEM, Sky Client Server or user and Sky X Gateway.
Figure 3.1 Sky X Protocol
Sky X Technology family
Sky X OEM
The Sky X OEM is software which is made available to various computer users and also real time operating systems. The Sky X Technology family essentially increases the performance over the working satellite by way of transparency replacing [Sunil 2009].
Sky X Gateway
One of the Sky X Technologies' product , the Sky X Gateway can be described as a hardware solution which is designed to accommodate being introduced into any satellite network and it furnishes performance enhancement for all the devices which are connected to the network. It performs its designated work by intercepting the Transmission Control Protocol Connection that is originating from the client and then converts it into Sky X Protocol ready for transmission [PSA n.d].
Figure 3.2 Sky X Gateway
Sky X Client or Server
The operation of the Sky X Client or Server is similar with the exception that the Sky X Client can be installed on each and every end users of a Personal Computer (PC). The connection from the applications being run by the PC is then intercepted and by the use of the Sky X Protocol, it is sent over the satellite.
It should also be noted that the Sky X Gateway XR10, Xh45 and the XH155 are all in availability in a hardware solution which is ready to be installed. This means that they can be added to any satellite network in their original form. The Sky X Client software can be directly installed by end users in the Personal Computers. This product and Sky X Server, ss10 or ss45 hardware which is located the network hub work together. It is also possible to use the Sky X Gateway in configurations of multilink where one Sky X Gateway hub is used in improving the performance of many separate outbound links which emanate from one uplink facility [Sunil 2009].
Sky X 250
Sky X 250 is best used as a remote, or even a hub a device that is used to lower the bandwidth of a network which has a very high network performance [PSA n.d].
Mentat SkyX Gateway 250 - gateway
Figure 3.3 Sky X 250
Xpress Transport Protocol
Using a TCP over satellite is challenging due to the long latency, high bit error rate, and asymmetric bandwidth links, thus, there is one solution which can overcome these issues which is to use different protocol, such as , Xpress Transport Protocol (XTP).The XTP is a transport layer protocol which allows XTP packets to run over IP networks .It is designed for high-speed networks and has been promoted by the XTP Forum which developed it so as to replace the Transmission Control Protocol. XTP includes a NACK-based retransmission algorithm, window sizes of up to 2^64 bits, streamlined connection set-up, and rate control protocol option for flow control, error control and multicast are provided by XTP. XTP is charged with controlling patterns as to facilitate production of different models. This is the option used instead of having separate protocols for each and every type of communication. The general characteristics of XTP are that control error, they control flow, they facilitate Byte ordering, they have a multicast mode, multi-packet handshaking and selective retransmission. The Sky X Protocol Design performs these functions: efficient acknowledgment algorithm, rate control, data compression and dynamic window sizing [Sunil 2009] [Palter 2008].
Figure 3.4 Xpress Transport Protocol
Sky X multicast fan-out
There is also a function referred to as Sky X multicast fan-out. Computer networking relies traditionally on unicast transfers of data which leads to the establishment of point-to-point connections in between the devices. This process takes up very most of the bandwidth resources and it is equally time consuming. With the use of the multicast technology multiple recipients receiving only one data stream is a possibility. In case this multicast transfer can use up an underlying layer of broadcast, this technology becomes a powerful tool for use by satellite networks or any other similar architecture. This technology is the most convenient and simple solution where one wants to multicast over a relatively wide area of networks. The Sky X multicast fan-out which is a major component of the Sky X gateway performs the job of converting Transmission Control Protocols unicast connections into convenient multicast transfers. The server must systematically send to each recipient a copy of the same file in situations whereby the same data is required to be sent to multiple users [Palter 2008].
Merits of Application of the Sky X Technology
There are a numerous merits of the application of the Sky X Technology as discussed below:
Because the internet protocols which are currently in use are not typically optimized for efficient satellite conditions and therefore the throughput over these satellite become restricted to a mere fraction of the bandwidth which is available, we need the Sky X Technology to reduce these losses and enable the access of maximum bandwidth.
The architecture of Sky X Gateway which converts data to Xpress Transport Protocol by intercepting the Transmission Control Protocol (TCP) connection that emanates from the client for transmission over to the satellite, seems to offer performance which is greatly improved while at the same time remains transparent to its end user and in regards to the internet infrastructure, it is fully compactable.
It should be noted that when one decides to use the Sky X Technology there are no changes which are required to be implemented by the client or the on the server. In other word, none of the applications in the network need be modified.
Another product of the Sky X family whose merits are worth being mentioned is the Sky X Gateway XR10 which is most compatible with networks affected by high latency, bit error which is high and links which are asymmetrical. It is designed as an edge or a remote device, and its performance is at an optimum at maximum bandwidth whereby there is a single, very large File Transfer Protocol (FTP). This product supports a number of industry best simultaneous sessions along with the connection turnover rate. Suffice to say, there is no other device which comes close to the prowess of this product.
Another merit of the use of Sky X Technology is that it increases the performance of the web and allows it to be three times faster than when the use of Transmission Control Protocol is employed by accelerating file transfers (there is faster, more efficient, more reliable multicast file transfer), irrespective of whether they are large or small. Further, the speed of Hypertext Transfer Protocol (HTTP) sessions, disaster recovery, database sync and backups is markedly improved and there is also an increase in link utilization on Wide Area Network (WAN) links which are fat. The switch to using Sky X Technology will minimize retransmission and the recovery from errors is swifter. [Sunil 2009] [Doffoh 2005] [Palter 2008].
In conclusion therefore, Sky X Technology works by replacing the Transmission Control Protocol over the link of the satellite with another protocol which is optimized for the normal high loss and bandwidth conditions which are typically associated with satellite communication. When one adds the Sky X System or Technology to a satellite network, that person is able to utilize the available bandwidth. The other advantage is that Sky X Technology enhances the performance of every user that is present on a satellite network, and it does so transparently. There is enhanced performance with the use of Sky X Technology. It increases the performance of the web by 3 or more times and also increases the speed of file transfers by 10 or even 100 times. Lastly, Sky X Network is compatible with all Transmission Control Protocol therefore no modifications are required to the end servers and clients.
The use of the Sky X Technology has gained quite a market as a solution to overcoming the limitations that are faced by the use of Transmission Control Protocol/ Internet Protocol over satellite. Many organizations and even militaries are relying heavily on this technology for the enhancement of their satellite networks performance. There are confirmed test results by NASA that show that the use of this Technology, and especially Sky X Gateway improves the performance for private access and internet usage over satellite communication.