IP OVER SATELLITE
Jarkko Viinamäki
Department of Electrical Engineering
Helsinki University of Technology
Abstract
Despite the fact that there are some problems with latency and high Bit-Error-Rate with satellite connections, IP over satellite offers an excellent way to provide Internet connections in areas where fixed broadband terrestrial networks are not be available.
Key Words
Satellite, IP, last-mile, Internet, broadband
1. Introduction
According to Global Reach statistics there were more than 720 million Internet users in March 2004 and the number of connected computers is raising rapidly. Although Internet is often considered to be a global network, there are large areas and even entire nations without the possibility to use broadband connections such as fiber optic networks, ADSL and cable modem since these terrestrial networks are relatively expensive to build and maintain. People living in areas with low population density or difficult terrain often have to look up for an answer – the satellite technology. The first satellite organizations such as COMSAT and INTELSAT were formed already in the early 1960s by many different national governments with their initial purpose to broadcast TV and radio channels and to offer wireless communication links for aviation, military and space projects. During the last 10 years a growing percentage of the satellite bandwidth has been dedicated to Internet traffic and many privately owned operators have entered the global market.
The purpose of this paper is to describe the technical architecture of satellite systems used for Internet traffic, the benefits and problems related to these systems and some comparison to other commonly used technologies. The current status and future prospects of satellite IP market niche, pricing trends and major players in the market are also covered.
2. Technology
2.1. Satellite Access Equipment
2.1.1. Spacecraft / Satellite
A communications satellite is basically a microwave repeater station revolving around the earth in a specified orbit. Satellites are meant for sending, transmitting, receiving and processing of electromagnetic signals (radio waves) of frequency larger than 1 GHz. Transmission of signals is performed via transponders. A transponder which is usually capable of providing 40-155 Mbps bandwidth consists of a transceiver and antenna tuned for a specific frequency. Most satellites have multiple transponders (even over 70) which can be used simultaneously to gain higher bandwidth on demand. Satellites are powered by solar cells and a backup battery and use propulsion jets to adjust the orbit when needed. Majority of satellites act as retransmission stations which means that they cannot regenerate any previously sent signal.
2.1.2. Service Provider Ground Station
The satellite communicates with a ground station (hub). The ground station consists of outdoor unit (ODU) which is dish transceiver of 4,5-10 m in diameter used to transmit and receive traffic between the satellite and the indoor unit (IDU). The indoor unit is responsible for demodulating, demultiplexing and reconstructing the incoming signal so that the tunneled IP datagrams can be sent to the high speed backbone Internet. The IDU also captures all packets it receives from the Internet, transforms the stream suitable for satellite transmission and sends them to the satellite.
An important component is the Network Management System which handles configuration, control, performance, alarm management, accounting and user administration with the help of Network Control Center (NCC) and Regional NCCs (RNCC).
2.1.3. Subscriber Side Ground Terminal
Each satellite subscriber has an outdoor unit which is a small (65-240 cm in diameter) low-cost dish antenna with transmit and receive components placed at the focal point of the antenna. The antenna must have a direct Line-of-Sight (LOS) to the satellite since the signal is not able to penetrate any obstacles. Larger dish usually gives better transmission power and wider receiver coverage.
The indoor unit is a satellite modem which serves as an interface between the ODU and customer equipment and controls satellite transmission. The demodulation and transmission optimization is often handled by an ASIC circuit or custom made software. The satellite modem can be connected to a PC using an USB port or DVB/MPEG-2-card depending on selected technology. Once the PC is able to receive the packets, they can be rerouted using standard Ethernet card or wireless communication techniques such as WLAN.
2.2. Positioning
The satellites orbit the earth at different altitudes and are thus categorized as GEO, MEO, LEO according to their altitude. Geosynchronous Earth Orbit (GEO) satellites are positioned at an altitude of 35786 km so that they appear to be stationary with respect to certain point on earth. GEOs offer a very wide footprint (coverage nearly 33% of earth surface) but their downside is the 200-300 ms one-way transmission delay (latency) which means that each IP datagram requires at least 400 ms just for the satellite path. Typical GEO lifetime is 15 years.
Medium Earth Orbit (MEO) satellites are positioned at 8000-20000 km and have one-way delay of approximately 50-150 ms. Low Earth Orbit (LEO) satellites operate at 350-2000 km altitudes and with an acceptable 10-30 ms one-way delay and take only about 90 minutes for one complete revolution. Finally there are so called polar satellites which operate in highly elliptical orbits at MEO altitudes.
Basically the higher the satellite is, the higher the round-trip-delay, launching cost, satellite lifetime, satellite size, footprint/coverage, bit-error-rate (BER), signal attenuation and need for transmission power. Since LEOs and MEOs are not stationary, the earth stations must either track these satellites or the satellites focus their spot beams to certain areas as they fly by. Both LEOs and MEOs can act as switching nodes and stay connected using intersatellite links (ILS) (Leon-Garcia, A., Widjaja I. 2000). They must implement signaling protocols and maintain routing information. Obviously LEO and MEO systems are much more complex than GEO installations and require a large amount of satellites to reach global coverage.
2.3. Operating Frequencies
Satellites use different frequencies for uplink (from ground station to satellite) and downlink (from satellite to ground station). These frequency bands are named S, L, X, C, Ku, Ka and V-band which are controlled by IRFB (International Radio Frequencies Board) and the FCC (Federal Communications Commission). The C-band (3,7-4,2 GHz downlink, 5,925-6,425 GHz uplink) is the most common together with Ku-band (11,7-12,2 GHz downlink, 14,0-14,5 GHz uplink), but these bands are quickly becoming congested. Latest new satellites have been tuned to use the Ka-band (17,7-21,2 GHz downlink, 27,5-31,0 GHz uplink) which currently offers almost 10 times as much capacity as the Ku-band (Fong, G.L., Nour, K., 2004), but suffers from rain attenuation and higher BER. The V-band which is designed to be in range 40-75 GHz is reserved for future use (Bem, J., Wieckowski, T., Zielinski, R., 2000).
2.4. Satellite Error Correction Schemes
For error correction satellites use FEC (Forward Error Correction), which is based on redundancy of information with the cost of lost bandwidth and ARR (Automatic Repeat Request). In ARR the packets are embedded with an error detection key and if error occurs, the transmitter will resend the packet.
2.5. Service Models
Satellites can be used in different ways to enhance or enable Internet connectivity. The first model which emerged was a so called one directional hybrid model. In 1-way model the subscriber needs an existing Internet connection which is usually implemented using low-bandwidth modem or ISDN (PSTN) or even GPRS. This uplink is used for sending requests and acknowledgements and the satellite connection works as the high bandwidth downlink. Special software must be installed on the subscriber PC in order to handle this hybrid traffic. This model works decently in practice because the uplink bandwidth requirements are usually quite low for regular web-surfing, email and messaging purposes.
The bi-directional 2-way model is becoming more and more popular and offers a pure satellite connection to the Internet. Downlink uses broadcasting and uplink is multiplexed using e.g. TDMA. The connection is still almost always asymmetric meaning that the downlink offers higher bandwidth but this depends on the deal with the satellite operator.
Finally there is the push or broadcast mode which is unidirectional providing only the downlink. The multimedia service operator broadcasts multimedia content to his subscriber base or multicasts multimedia data to a group of receiving sites. This service model is suitable for Internet-radio, video-on-demand, software distribution, news feeds and other non-interactive services aimed for large audiences.
2.6. Standards
In theory the satellite operator can freely decide how the data is transferred between subscriber transceiver, the satellite and the ground station as long as they work together. However, standardization allows better variety of satellite connectivity equipment and increases the chances of getting affordable solutions for customers.
A relatively new standard is the DVB-RCS (Digital Video Broadcast, Receive Channel via Satellite) which was specified by ETSI in 1999 (EN 301 790). DVB-RCS uses Multi-Frequency Time Division Multiple Access (MF-TDMA) to enable two-way communication and the return channel is coded using rate 1/2 convolutional FEC and Reed Solomon coding. The data to be transported may be encapsulated in Asynchronous Transfer Mode (ATM) cells, using ATM Adaptation Layer 5 (AAL-5), or use a native IP encapsulation over MPEG-2 transport. The use of DVB-RCS makes it possible to use the same satellite dish for both IP traffic and satellite TV and radio channel transmission (Bem, J., Wieckowski, T., Zielinski, R., 2000).
There has also been attempts to fit the DOCSIS (Data Over Cable Service Interface Specification) protocol developed by CableLabs (approved by ITU in 1998) for satellite communication but according to many sources this protocol is not very suitable for this purpose and thus no commercial installations using this protocol have emerged (Stevenson, J., 2003).
One of the major satellite market forces, Hughes Network Systems, has developed their own proprietary protocol called the DSS in order to overcome the various problems related to especially TCP/IP traffic over satellite.
2.7. Pros and Cons of Satellite IP
2.7.1. Cons
The most critical problem with especially GEO satellites is the high latency which causes more than 500 ms delay for single packet transmission. This has a direct effect on traditional unoptimized TCP, which suffers greatly from degraded performance and also makes it almost impossible to use VoIP or play Internet action games.
Second important issue is the requirement of direct line-of-sight and careful direction of the subscriber dish. LEOs and MEOs that don’t support earth-fixed approach need to be tracked and this equipment can be very expensive and prone to failures. Satellite connections also suffer from weather-related outages (rain, thunderstorms, accumulated ice on the dish) and even the sun is able drown the satellite signal when it is directly behind the satellite. The Bit-Error-Rate (BER) can climb from 10-8 to even 10-6 during bad weather conditions which is especially common for Ku and Ka-band transponders. Also the 1-way hybrid model may not seem attractive since subscriber still needs an existing connection to the Internet raising the usage costs to unbearable levels. It’s also worth noting that although the satellites have huge transmission capability, the Internet connectivity deals offered for subscribers offer very limited bandwidth (typically 400-500 kbps) with a price that is usually double compared to that of ADSL and cable. Due to these restrictions and the size of the VSAT-equipment, it’s not possible to use satellites to get Internet connection for your portable computer and handheld devices.
2.7.2. Pros
Fortunately, there are several clear advantages of satellite systems. First of all they offer a wide geographic coverage including interconnection of remote terrestrial networks and currently there are several satellite operators who can offer global coverage without any investments in expensive terrestrial networks which is good news for ISPs. The second important point is that satellite connectivity offers single-hop transmission which avoids the packet loss and congestion of terrestrial networks where data flow is often limited by different MTUs and router processing capability (IPv6 makes router load smaller by reorganizing packet fragmentation). Further, the bandwidth-of-demand (BOD) and Demand Assignment Multiple Access (DAMA) features offer tremendous flexibility for satellite users and ISPs. Also the simultaneous broadcasting ability of satellites is superior compared to terrestrial network multicasting. Satellite connectivity is also extremely reliable offering more than 99.97% service rate by nearly all operators. Finally you can use the same Direct-to-Home TV equipment and satellite dish for both IP and TV.
2.8. Optimizations for Satellite Operation
The high latency causes problems especially with TCP which interprets high delays with ACKs as network congestion. The Slow-Start algorithm prevents effective utilization of bandwidth and TCP does not recover well from frequent packet loss (Congestion Control causes window size reduction). There are several techniques which can be implemented in the ground and subscriber station equipment and software:
§ Path MTU Discovery
§ Forward Error Correction (FEC)
§ Usage of large windows
§ Larger initial window of size
§ Modified window dynamics
§ Selective acknowledgements
§ Acknowledgement (ACK) spoofing
§ Compression
§ Pre-fetching
§ Protocol proxies
§ Proprietary encapsulation
§ Content caching
Research (Astuti, D., Kojo, M., 2003) shows that these optimizations do offer noticeable performance boost to TCP traffic over satellite connection.
2.9. Comparison to Other Technologies
It’s difficult to create a reasonable table comparison of different techniques since each technology scales quite nicely. However, satellites have two advantages that no other technology has in the same scope: almost unlimited bandwidth-on-demand (BOD) and practically global coverage although the details depend on the provider and the contract with the ISP. The worst point is the latency as mentioned before and all other techniques beat satellite latency hands down. Most satellite deals aimed at private consumers offer disappointing bandwidths of less than 512 kbps at very high price (close to 100 EUR/month). DVB equipment and satellite dish are also very expensive (1000EUR and up).
High speed DSL is the most attractive choice for any broadband consumer since it offers low latency and high bandwidth at reasonable price (around ~55 EUR/month for 512 kbps ADSL link in Finland 2004). Unfortunately the ADSL signal deteriorates very quickly so subscriber needs to be relatively close to the operating point (less than 6 km) (Fong, G.L., Nour, K., 2004).
Cable modem is a good choice since it also offers limited BOD without extra cost and acceptable latency at very affordable price (~45 EUR/month for HTV Cable in Helsinki Finland – base bandwidth 512 kbps, scales up to 2 Mbps+). Like DSL, Cable modems are only available in dense residential areas.