Satellite Networks Essay
For three decades, satellite communications have been used for public switched telephone network (PSTN) and video broadcasting applications. However, with daily technology advancements made in society and the requirement for global data connectivity, satellites are increasingly being used to provide data communication links at all data rates ranging from 64 Kbps to 155 Mbps (Buchsbaum). Wireless technology allows data to be sent out into space to a satellite and back to earth to provide connectivity among many different network clients.
The network can be designed to reach geographically remote sites, where installing land-based lines is not feasible. The only restriction is that the footprint of the satellite must match the area it covers. Satellite networks can use either Ku-band satellites, which use a 3-meter dish (this type of satellites used most commonly for cable television broadcasting) transmitting at 1 GHz, or Ka-band satellites, which uses a 1-meter dish (used less commonly for television broadcasting) transmitting at 2.5 to 3.5 GHz (Shelly).
Frame relay is a means of providing a high-speed service that supports LAN interconnectivity, Internet access, file transfer and bandwidth-hungry applications including corporate intranets and desktop video conferencing. It enables devices to be allocated bandwidth on a Committed Information Rate (CIR) and also allows for rapid applications (Invsat.com). In a frame relay network, a geosynchronous satellite (GEOs) is used as the link between frame relay switches, or routers, and the other devices on the network (Shelly).
In a remote land or maritime environment both operators and subcontractors require independent communication support for voice and data applications between the site or ship and the onshore corporate offices (Invsat)
A satellite is a repeater in the sky. A source or terminal on the earth transmits a radio signal to the satellite, receives, processes, and retransmits it to, where another terminal receives the signal. These transmit and receive terminals are also referred to as earth stations (Buchsbaum).
The altitude of the satellite above the earth’s surface has a direct impact on its characteristics and performance. Geostationary satellites (GEO) are at an altitude of 35,786 kilometers above the equator. Satellites at this altitude rotate around the earth at the same speed of rotation of the earth. Medium earth orbiting satellites (MEO) are at altitudes between 10,000 and 20,000 kilometers. Low earth orbiting satellites (LEO) have orbital altitudes between 750 and 1500 kilometers. However, MEO and LEO satellites are new technologies in the commercial communication services (Buchsbaum).
A satellite is frequently referred to as broadcast / multicast medium. An earth station sends an uplink signal to the satellite. The satellite receives and retransmits this signal to a certain coverage area. Any receiving station in the coverage area can receive the signal. Multicast is a subset of broadcast where an addressing scheme is used to select a target group of receivers. Point-to-point links are achieved when only one receiver accepts the broadcast carrier. Broadcasting is usually unidirectional while multicasting can be both unidirectional and bi-directional. Point-to-point links are usually bi-directional and are the equivalent of the typical terrestrial point-to-point links (Buchsbaum).
Satellite channel costs are based on the speed of transmission and how long one uses the satellite. The owner of the satellite sets the monthly prices, which may range from $1,000 to $10,000. In addition, to these monthly charges, a company must have a satellite dish to transmit and receive data. A shared earth station can be used to receive the data transmitted from the satellite at a cost of at least $3,500 per month. Some companies allow you to rent transponder time on a satellite for a single transmission of a fixed length of time (Shelly).
SATELLITE COMMUNICATIONS – Frequency Bands
Frequency bands are allocated to satellite communications based upon broad classifications of the services provided, such as fixed satellite service (FSS) or broadcast satellite service (BSS).
Examples for the FSS band allocations are:
Satellite to EarthEarth to Satellite
C — Band3,700 — 4,200 MHz5,925 — 6,425 MHz
Ku — Band10,700 — 11,700 MHz13,750 — 14,300 MHz
The frequency bands allocated for a satellite are usually divided into smaller segments known as transponders. A transponder is also used to refer to a physical path through the satellite that goes from the receive antenna on board the satellite through the different processing stages and exiting through the transmit antenna. The transponders contain many carriers, which are signals modulated by the information signal. Individual satellite system operators designate the center frequency of these carriers (Buchsbaum).
A carrier in the satellite world is a continuous wave (CW) signal, which is modulated by the information signal. Phase shift keying (PSK) is the most common modulation technique used for data communications over satellite. The center frequency of the carrier is designated by the satellite system operator and is used to set both the transmitter and receiver center frequencies (Buchsbaum).
There are some cases where small satellite networks are required, which are referred to as VSAT, which stands for “very small aperture terminal.” This refers to receive-only and receive/transmit terminals installed at geographically dispersed sites, and connected to a hub via satellite (Buchsbaum). A VSAT terminal offers a selection of antenna sizes, ranging from 1.2 – 4.6 M for C, Ku, and K-band VSAT networks. Hub stations are provided from 1.8 – 1.9m. Each terminal comprises of an antenna, outdoors transceiver, indoor modem, mux and optional frame relay/multiplexor with channel interfaced. Each terminal also incorporates a monitoring and alarm facility (Multipoint).
LNA – Noise Amplifier
An un-cooled low noise amplifier (LNA) and integrated up and down converters with a solid-state power amplifier (SSPA) are standard packages available today in a box known as a transceiver. VSATs do not require antenna-tracking equipment, due to the tight station keeping of current satellite series and the relatively wide beam width of small antennas. These features make VSATs easy to deploy, install and maintain, and a very cost effective solution to bringing instant communications to any part of the globe. The small terminal size also allows installation on the customer’s premises in most cases. This leads to additional savings since no terrestrial access circuits are needed (Buchsbaum).
SATELLITE CHARACTERISTICS – Data transfer rates
VSATs typically operate at data rates ranging from 64 Kbps to two Mbps through satellite modems making use of standard interfaces such as RS-449 and V35. Data rates of up to 8.448 Mbps can be accommodated by VSATs, and are sufficient in meeting the requirements of most end users accessing frame relay circuits. The data rates coupled with the relatively low cost of accessing VSAT earth stations provide an economic and fast deployable solution that can extend global connectivity to any remote location in the world. Data rates higher than eight Mbps typically require larger antennas
and facilities that are usually more readily available from larger telecom operators (Buchsbaum).
Frame losses in an end-to-end connection can occur due to losses over the communication links, losses associated with congestion events in the terrestrial nodes. In extreme congestion conditions, the buffers in the switch are flushed with corresponding loss of data. Losses due to traffic policing done by the network for customers exceeding their agreed or contracted traffic rates with the network provider.
Latency – time delay
Since frame relay does not have an acknowledgment mechanism for the user’s data frames, throughput of native frame relay data is not affected by a fixed transit delay in the end-to-end connection. Geostationary satellites introduce a constant one-way delay of 260 ms, affecting delay sensitive applications running on frame relay (Buchsbaum).
Frame Relay Jitter
Jitter is defined as the variation in an end-to-end delay of frame arrival in any given virtual circuit (PVC or SVC). A geostationary satellite link in the end-to-end path does not contribute to jitter. Satellite links can be used to bypass a terrestrial route, which could include a large number of hops that can lead to high values of jitter (Buchsbaum).
VSAT and Frame Relay
Frame Relay Access over Satellite
A VSAT can be used to extend a frame relay network to anywhere in the world in a very short time. A typical block diagram showing the deployment of VSATs to extend services to remote sites is shown in the above diagram. Routers are very effective in filtering LAN traffic and reserving the use of satellite bandwidth only for traffic destined across the satellite link (Buchsbaum).
IMPACT ON APPLICATIONS
The main difference between a terrestrial and hybrid (terrestrial plus satellite) frame relay network is the propagation delay. Applications are the top layers in the protocol stack, which rely on the services of the layers below them. The network layer (IP) is connectionless and stateless. IP data grams are forwarded from one node to another with each node making routing decisions about the next hop depending on the current state of the routing table in the node. There are two categories of transport protocol: user data gram protocol (UDP) and transmission control protocol (TCP). UDP is connectionless and has no acknowledgment mechanism. TCP is connection oriented, relies on the sliding window mechanism for flow control, and depends on acknowledgments from the receiving end of the connection (Buchsbaum).
Acknowledgment-based systems limit throughput on connections, which have a large speed-bandwidth product. The sending (TCP) end cannot send an amount of data greater than the window size without receiving new acknowledgments. The maximum throughput is calculated as TCP window size divided by the round trip time (RTT) (Buchsbaum).
Voice over Frame Relay
There is no direct impact (other than adding a 260 msec one-way delay) of satellite delay on standard connections of voice over frame relay (VoFR) service. VoFR is a native frame relay application and does not run over the TCP/IP stack. VoFR is a real time application and has acceptable performance over hybrid terrestrial and satellite frame connections (Buchsbaum).
FTP & HTTP
Web browsing is a delay sensitive application and works well over satellite networks because most service providers are designed to open multiple TCP connections allowing simultaneous transmission of packets, and minimizing the delay problems with satellite transmission. (Shelly). File transfer protocol (FTP) runs over TCP and is therefore delay sensitive. FTP is not necessarily a real time application since the user can initiate a file download in the background and continue working on the network or locally in the foreground. The TCP window size set on the client is one of the major determining factors in the maximum throughput achievable in a file download. The default window size of 8760 bytes found in Windows 95, Windows 98 and NT implementations of TCP/IP is sufficient for users of an effective end-to-end rate of 128 Kbps or less. For power users with more effective end-to-end bandwidth, the TCP window size should be set properly on the client to utilize more bandwidth (Buchsbaum).
E-mail and news services work well over a satellite network because these applications are not real-time and thus some delay is not a problem. (Shelly) Both simple mail transfer protocol (SMTP) and network news transfer protocol (NNTP) use TCP as a transport protocol and the throughput is sensitive to delay. However, both news and mail processes are run in the background and there is no perceptible impact on the users (Buchsbaum).
However, Videoconferencing is a real-time application, which might use frame relay over a satellite, but if transmission delays were present, problems would occur with viewing the video (it would make the picture look choppy). The potential delays can be introduced by the compression of the signal regardless of the transmission medium, but the fixed delay introduced by satellite transmission has been tested and found to cause no noticeable changes in the signal (Shelly).
INTEGRATION OF FRAME RELAY NETWORKS
Frame relay is implemented in both private and public networks. Public networks can are used to interconnect private networks. Although, Virtual private networks (VPN) are beginning to become a popular service in many companies and businesses.
The configuration in the diagram above represents the typical use of a VSAT to provide access to a public frame relay network. The model used is that of a branch office, connecting to its headquarters, using a satellite link to a frame relay POP and the results are based on actual measurements done on this configuration using a live satellite link. A native frame relay load is used to characterize the frame loss ratio and FTP is used as the measurement tool for TCP/IP applications (Buchsbaum).
The test equipment and data logging used in this test-enabled segregation of the total frame loss rates into satellite induced and public network induced (Buchsbaum). To achieve more accurate results, a reverse link was used when connecting the hub to the VSAT system.
Test Results and findings
FTP tests showed that full bandwidth utilization was achieved for rates up to 256 Kbps using a TCP window size of 23,360 bytes. The public networks rather than the satellite link predominantly caused the frame losses encountered during the test. The frame loss events in the public network lasted less than one minute per event with two to three events occurring per day. For networks behaving in this manner, overlying TCP connections are affected only for a very short time. Most of the time connections will not have retransmissions and throughput will depend mostly on the available bandwidth. The satellite link increases the end-to-end delay but does not contribute to congestion. In fact, satellite links can be used to avoid congested parts of the network and connect a local frame relay cloud directly to the network backbone. This can result in major savings because satellite links are not distance sensitive (i.e., the network provider does not need to have a terrestrial point of presence near the customer) and can be quickly deployed and provide a high quality of service anywhere on the globe (Buchsbaum).
Customers should test the ability of the network to deliver the agreed-upon CIR at the time of circuit commissioning. Tariffs for satellite capacity are typically based on occupied bandwidth and power. The customer needs to verify that the bandwidth resources are available end-to-end and that the network provider is not oversubscribing his circuits to the point of degrading the effective bandwidth available for the customer (Buchsbaum).
Customers should also research or ask about a continuous monitoring (and logging) of their link utilization and statistics. The measuring devices could be independent probes or probes embedded in their CSU/DSU. It is also easier to deploy such measures when considered from the planning and design phases rather than as an afterthought. Dedicated protocol analyzers usually provide more detailed analysis capabilities but are usually expensive to operate and maintain. Protocol analyzers are usually not required for regular operations unless a complicated problem arises requiring a more thorough troubleshooting (Buchsbaum).
We have come a long way over the years with network configurations. Today, you will find satellite networks to be more common throughout businesses. However, the downside to having a satellite link connecting to a frame network is the delay in continuous data transmission. Although, he satellite can help to alleviate congestion on a network due to the wide bandwidth available over the satellite link. Incorporating a satellite link into your network is a cost-effective way to extend a frame relay network to areas not covered by a terrestrial network provider. Consequently, the satellite can provide high-quality service to any geographic location on earth for a reasonable price.
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