In the face of this global era internet, satellite systems can also be developed to support the services it. Among the weaknesses possessed by the satellite system when compared to terrestrial networks, there are several unique advantages which can be utilized and a high potential to support multimedia applications such as for broadcasting and multicast.
Disadvantages satellite system
The majority of the backbone network and access the internet using terrestrial networks, such as: copper and optical cables, the bandwidth varies from hundreds to kilobyte megabyte. Despite dominating the cable internet access, but the study of the use of mobile networks / wireless for wide-bandwidth applications also accomplished.
Among the network of mobile / wireless, GEO satellite network has a high potential of the multimedia applications with the ability to broadcast and multicast large amounts of data at very wide area so as to give the possibility of global relations. Distribution of Internet via satellite in GEO satellite in particular, provide several benefits / advantages:
Wide bandwidth: satellite operating at Ka-band frequencies (20-30 GHz) can deliver throughput will be in the order of gigabits per second.
Relatively inexpensive: a satellite system is relatively cheaper because there are no deployment costs and one satellite can cover large areas.
Network topology is simple: compared with a mesh interconnect models on the terrestrial network, GEO satellites have a simpler configuration. With a simple topology, the network performance is more easily controlled.
On the other hand, satellite communication raises a major problem for Internet applications, the latency between the earth station / terminal. For a GEO satellite communication systems, its latency of at least 250 m-sec. Sometimes it is also coupled with the framing process, queing, as well as on-board switching so that its latency can range to 400 milli-seconds. Latency is approximately 10 times higher than that of fiber-optic point-to-point. Latency is not much effect on data transfer applications or broadcasting, but very influential on interactive applications that require handshaking, and unfortunately TCP requires the interaction.
LEO and MEO satellites can also provide broadband capacity as well as global. Latency in LEO approximately two times greater than the terrestrial network. Due to the nature of satellites in LEO and MEO are not fixed to the earth it is necessary constellation to provide a global service. With this constellation raises a high level of complexity in the satellite control and network management (as required handoff, tracking and routing) compared to GEO satellites.
Internet Applications
Common Internet applications including: Web browsing, file transfer protocol (FTP), remote login (Telnet), video conferencing, e-mail, and broadcasting. The above applications have different requirements in terms of bandwidth, speed of response, tolerance to noise that would cause different performance. For example: Remote logins are very sensitive to delay. Normally the user / user expects a quick response during the login session, while videoconferencing can usually receive a packet is lost or an error due to the transmission.
Figure 1 below shows the variation of the need for bandwidth and speed of response. Because of these differences, the implementation will be very different techniques. Some applications require distribution information (using TCP and sensitive to latency). Others can use UDP or other real-time protocol that can tolerate delay so it can work well via satellite.
Currently many Internet uses TCP to its main applications. TCP performance on high-delay network Air have direct influence on the performance of Internet access that uses GEO satellites. This delay will be especially slow when transferring large files. One-way delay in GEO approximately 250-270 milliseconds, while the TCP protocol requires that the receiver must send an acknowledgment to the sender to inform that segment sent has been received and waiting for the next segment. So for the delivery of the required segment approximately 500-540 milliseconds, the value of this will be a very slow process of data transmission.
If we have a T1 link (1.544 Mbps) and transmits each segment 64 KB TCP window, it will take 339 milliseconds to transmit segment. Receiver will receive a segment after 270 milliseconds and the sender will receive an acknowledgment after the next 270 milliseconds, so it takes 879 milliseconds to transmit a complete segment.
The following will be delivered performance issues that are important in the use of TCP.
window Size
TCP flow-control stems from the concept of "Window Size" .. Currently the maximum TCP Window Size is 64 Kb. In the high-delay network will air many packages that can not be acknowledge. To maximize utilization of the bandwidth in satellite networks, TCP requires a larger window size. For example, a satellite link with the round-trip delay of 0.8 seconds and a bandwidth of 1.54 Mbps then theoretically optimal window size is 154 kb (Far above the existing standard is 64 Kb).
A TCP-Extention, known as TCP-LW (Large-Window), is defined to widen the window size from 216 to 232 thereby allowing utilizes the bandwidth optimally and to obtain better performance through a satellite link.
Bandwidth adaptation
TCP is able to adapt to network bandwidth by performing his set window size. Adaptation speed is directly proportional to the latency. In satellite networks, adaptation requires a longer time and as a result the TCP congestion control becomes ineffective.
slow Start
Slow Start was introduced in 1988 by Jacobson and declared as one of the requirements in 1989 Slow start is used to gradually increase the data rate to the network. It starts by sending a single segment and wait for an acknowledgment from the receiver. For each acknowledgment received by the sender, the two segments will be sent to the network, in order to obtain an exponential increase in data transmission. Slow start stops when the window is reached or detect a loss of data.
Due to the time it takes a slow start to reach the available bandwidth is a function of round-trip time, the satellite links are very sensitive to the limitations of available throughput during slow start.
On one side of a slow start before obtained congesti prevent bandwidth availability assessment, on the other TCP bandwidth utilization becomes not optimal for its assessment process. So that the shorter ends of TCP slow start, the better the achieved performance.
Approaches for Improved Performance
There have been many techniques developed to reduce the effects of latency. The first alternative is to adopt a version of TCP that provides good performance in satellite and through terrestrial does not reduce performance. The second approach is to rely on satellite gateways to perform specific functions in accelerating TCP session. While the third approach is to develop a better implementation on general applications use TCP to obtain a more efficient and more sensitive. Here are some alternatives to increase TCP performance.
TCP Extention
Some of the problems that exist in the current GEO satellite system will appear also in the high-speed terrestrial network fiber future. Issues such as the width of the window size, slow-start period is longer, and inefficient bandwidth adaptation will affect both the network. Various techniques have been mentioned above, such as TCP-LW can be used.
middleware
Performance improvement in some things can be done by working directly on the level of infrastructure without the need to make modifications to its TCP, which is known as the middleware layer. If you make modifications to the TCP required changes to the operating system of each end host, while this technique is only necessary to change a little or even without any changes. There are two types of middleware, namely:
Split TCP
The idea is to split the Split TCP connection end-to-end TCP into two or three segments. Each segment is a complete TCP connection. Forwards the data flow from one segment to another segment (if necessary buffering). If the TCP Split performed on the satellite link, the middle segment bridging the satellite link, while the other segment connecting the routers that connect the internet terrestrial and satellite links to the endpoints.
TCP Spoofing
In this technique, intermediate gateway (typically using a satellite link) considers receives TCP segment is in good condition without the need to wait for an acknowledgment from the receiver. This gives the illusion of a network with low latency at the sender so that the TCP slow start phase can be accelerated, as shown in Figure 3 Intermediate buffer gateway TCP segment. When the acknowledgment is actually received by the gateway, the gateway will be removed by the acknowledgment in order to prevent a double in the sender side. If acknowledgment is not accepted and the gateway also experience time-out, then it will be done from the retransmit buffer. As Split TCP, TCP spoofing to break the concept of semantic end-to-end TCP because the sender thinks that a segment has arrived at the destination when in fact it is still on the way. These techniques can be accepted by many applications such as WWW browsing through a proxy, but will cause problems if an application built using end-to-end semantics.
Potential Use of Satellite
Network used by companies essentially asymmetric in other words most of the time data flows in one direction and slightly in the opposite direction. For example: software distribution to remote areas, sending financial data to the central office, distribution of multimedia files etc.. For these types of applications over satellite provides an alternative solution. It also assures that communications to the unreached areas of terrestrial telecommunications infrastructure. Broadcast satellite communications can offer a channel for data communication that is economical and efficient. In a simple broadcast technology enables the delivery of information to customers at the same time. But in some cases, the information is not disseminated to all the customers, but only to some customers. So the operator must be able to disseminate information selectively to the purpose.
Satellites in GEO are very suitable as a medium MBONE (Virtual network via the Internet for multicast applications). Currently for terrestrial use MBone, the data must go through the links in huge quantities and replicate itself on routers that very much. (See Figure 4 below). This takes up substantial bandwidth and increase the likelihood of congestion at each router along the path.
Tidak ada komentar:
Posting Komentar