IITM-ISRO SATELLITE BASED HYBRID INTERNET ACCESS SYSTEM

Transcription

1 IITM-ISRO SATELLITE BASED HYBRID INTERNET ACCESS SYSTEM S. Jakir Rehman, S. Suresh Krishna, D. Jalihal TeNeT Group, Dept. of EE, IITM Chennai K. Kumar, R. Gopalakrishnan, A. Charania, V. H. Patel Space Application Center, ISRO Ahmedabad ABSTRACT This paper reports a hybrid internet access system jointly developed by IITM and ISRO whose objective is to deliver high downlink speeds efficiently and economically to LSPs [2] serving small communities in remote areas. The fact that internet access generates asymmetric traffic is used in designing a hybrid internet access system. High speed satellite broadcast on one side and a two-way low bit-rate land-based link are used to achieve an asymmetric link. Sharing of the satellite broadcast brings down cost. The geographical reach of satellite enables connecting remote areas. The paper also presents results of throughput experiments conducted on a working system. 1. INTRODUCTION A local internet service provider (LSP) providing service to a small community of 100 to 400 users finds the cost of high rate (2 Mbps) internet access prohibitive (currently a 2 Mbps connection costs approx Rs lakhs per year) thus forcing him to settle for a lower 64 kbps internet connection even when the traffic considerations may suggest a higher rate. When the LSP is located in remote areas or in rural areas at a distance of more than 4-5 km from the nearest PSTN office or the fiber presence, he is required to incur a large initial expense in order to get higher internet access speeds. He has to set up an LOS microwave radio link or lay fiber to the PSTN office in order to get higher access rates thus pushing up the cost even further. If internet has to reach a large part of the country the cost of access has to be kept low. To achieve cost reduction several ideas and technologies have to be tried out. This paper reports of one such attempt. If the access link is designed carefully taking into account the characteristics of the network traffic considerable savings in cost and greater link utilization can be achieved. It is known that the traffic generated by WWW access is asymmetric. A user sendng out a URL request generates a traffic of a few tens of bytes. In response to the request, he receives a large (several KBs) file. Typical values for the ratios of uplink to downlink traffic range from 1:4 to 1:8. A user or an LSP, then, really needs an asymmetric link. ADSL, cable based internet access and satellite based internet access are some of the technologies that allow for asymmetric internet access. A satellite based hybrid internet access system addresses these considerations, of achieving a higher rate access and an asymmetric link, very effectively. In the hybrid system, an LSP or a user maintains a low bitrate (64 kbps) connection. The URL requests from the user instead of getting serviced directly are forwarded to a transmit web proxy server which maintains a high bit-rate connection to the internet. The transmit server gets the requested file and sends it to the user on a highrate satellite link as shown in figure 1 A number of satellite based hybrid systems are available [3][4][5]. The high latency of atleast 250 ms that is inherent in satellite access has its impact on the maximum throughput that can be achieved by a single TCP connection. Studies have shown that a single TCP connection achieves a throughput of approximately only 300 kbps when used on a satellite link independent of how large the actual link speed is. Efforts are on to achieve higher throughputs by changing the parameters of a TCP connection such as its window size etc. [6]. The systems described in the previous paragraph

2 Client Satellite High bit rate Satellite link Low bit rate internet link Transmit Server Figure 1: Satellite based hybrid internet access system. were designed primarily to serve isolated users located in remote areas and hence are very expensive. This paper reports a hybrid system jointly developed by Indian Institute of Technology Madras and Indian Space Research Organization whose main objective is to deliver high downlink speeds efficiently and economically to LSPs serving small communities in remote areas. Section 2 describes the architecture of the system and how its use can be economical. Section 3 describes the results of throughput experiments. Section 4 presents conclusions and future work. 2. IITM-ISRO HYBRID SYSTEM The IITM-ISRO hybrid system, shown in figure 2, consists of: one transmit proxy server (located at Delhi) with a high BW connection to the internet on one side and a high bit rate uplink to the satellite on the other side, and multiple receive proxy servers one at each LSP location with a low bit rate connection to the internet and receiving high bit-rate downlink from the satellite. Transmit proxy server running Linux has a plug-in HDLC card which is connected using RS422 interface to the satellite modulator. The modulator is connected to the transmit antenna. Receive proxy server running Linux has a plug-in HDLC card which is connected using RS422 interface to the satellite demodulator. We visualise a deployment scenario where each LSP serves clients in remote areas. A web request from a client (URL request) reach the receive proxy which instead of servicing it directly forwards it to the transmit proxy using its land-based internet link. The transmit proxy downloads the requested URL from the internet using its high speed internet-link. It in turn broadcasts downloaded file using the high-speed satellite link. This broadcast is received by all receive stations. The one to which it is intended accepts it and forwards it to the requesting client. The other receivestations ignore it. The broadcast channel is shared by all the receive stations. The satellite based hybrid system will allow each LSP to keep a fixed low-rate (64 kbps) connection to the internet and yet get the benefits of a high-speed access at minimal additional expense. It allows one large connection to the internet to be shared by a number of LSPs. The transmit proxy maintains a high-speed connection to the internet and invests in a router, modems, leased-line etc. appropriate for that speed. Sharing of the high-speed internet link is achieved by the satellite broadcast. While it is true that satellite link is expensive (a 2 Mpbs satellite link costs approx. Rs. 2 crore per year), it is being shared by a large number of LSPs. Each LSP momentarily gets to use the entire bandwidth available at the transmit station. The additional expense that each LSP has to incur because of the satellite download includes the cost of an antenna (approx. Rs. 20,000), a demodulator (approx. Rs. 50,000), an HDLC card (approx. Rs. 7,000) and a PC (which might already be there with the LSP for performing the usual LSP functions). The key cost reduction comes (i) from sharing one high-speed internet connection and sharing an expensive satellite channel and (ii) using free LINUX operating system and utilities that come with it Software and Hardware issues The satellite modulator (or demodulator) is connected to the PC using a plug-in HDLC card which has a DSP on board. The HDLC card communicates to the modulator (or demodulator) using a RS422 interface. This card was developed by Midas Communication for its cordect [1] product for a very different application. This card was modified as a programmable PPP interface that can support data rates of n 64 kbps. PPP is a two way link layer communication protocol. The LCP (Link Control Protocol) was modified to bring up the IP layer without waiting for ACKs and to work in a simplex communication situation required for a

3 SATELLITE Shared One way Satellite Link 2Mbps 2Mbps or higher Satellite Rxr. LSP 1 Client 1 2Mbps or higher Transmit Proxy Satellite Txr. INTERNET 64kbps Leased Line 64kbps Leased Line LSP n Client 2 Client n Client 1 Client 2 Client n Figure 2: One-way high-speed satellite-based download system serving multiple LSP locations. broadcast type satellite link. A Linux device driver was developed to make this card function as a PPP device. The entire system was developed using Linux (RedHat 7.2) and free web proxy server. This further contributes to keeping the overall cost low. The LSP web proxy was configured in the parent-child mode so that even when the satellite link is down, it continues to work on the two-way land line, perhaps at a lower speed. When requesting a data transfer say FTP, the user sends a request to a remote machine that is running a FTP server program. This program receives the file transfer requests and responds to them. If a hybrid terminal user wanted to receive a file from a machine running FTP server, every packet from the user terminal would take the path shown in figures 3 and 4. Since it is a one-way satellite system, the ACKs for the IP packets transmitted reach the transmitter on a slower internet link. If data gets corrupted in a satellite channel, IP packets will be received with error resulting in end-to-end retransmission of the entire IP packet. It should be noted that this would limit the RJ 45 USER FTP TELNET HTTP ETHERNET ROUTER TCP/IP PPP DEVICE DRIVER HDLC CARD SAT DEMOD RS 422 SERIAL PORT Figure 3: The data paths taking place in the user terminal at one of the receiver side.

4 RF TEST FTP TELNET HTTP HDLC Card RS422 MOD FREQ CONV NOISE DEMOD HDLC Card RS422 TCP/IP Transmit Proxy PPP DEVICE DRIVER RJ 45 ETHERNET ROUTER HDLC CARD SAT MOD RS 422 SERIAL PORT Figure 4: The data paths taking place at the transmitter side. system performance under low SNRs. At high SNRs there would be an occasional retransmission and not much degradation in the performance. 3. BENCH TESTING AND PERFORMANCE OF THE HYBRID SYSTEM Before connecting the system to the satellite, the entire system was tested in the laboratory using a WAN simulator, NISTNET. The system was initially tested for performance in the base-band as shown in figure 5. The throughput tests were performed under various channel conditions of delay, delay jitter, bandwidth, packet loss etc. RF test followed the base-band test where the output of the transmit proxy was given to the modulator and which in turn was connected to the demodulator through a frequency convertor and an external noise source. Throughput measurements were performed on the RF system under various noise conditions. The hybrid system has two kinds of links: the first is the two-way internet connection that each of the proxy sites has and the second is the one-way shared satellite link from the transmit proxy to receive proxies. The speed of internet connection of the transmit proxy should ideally be equal to the speed of satellite link. However, in the present test-case the transmit proxy located in Delhi is connected to VSNL at 128 kbps and the satellite link speed used is 1 Mbps. The internet speeds of participating receive proxies are: (i) WAN Simulator Figure 5: Bench testing of the hybrid system with a WAN simulator under base-band and RF conditions 64 kbps at Ahmedabad, connected through VSNL, (ii) 512 kbps at IITM connected through Satyam and (iii) Delhi connected through a LAN cable. The objective of the experiment was to test out (i) the internet throughput (what fraction of the 128 kbps of internet link connected to the transmitter is achieved) and (ii) the satellite throughput (what fraction of the 1 Mbps satellite bandwidth is achieved) one gets at a receive proxy station. Experiments were designed to measure both these throughputs. The throughput measurements were taken between 10:00 am and 5:00 pm and the results are shown in the form of plots in figures 6 and 7. Throughput(%) Internet Throughput IIT Madras SAC Ahmedabad DES Delhi Time of Day(Hrs) Figure 6: Throughput plots measuring the internet speed of the transmitter at each of the participating receive station. In order to measure the internet throughput, a large file of size 2.2 MB from the VSNL site [7] was downloaded once every hour at DES, IITM and SAC, Ahmedabad using the satellite link. figure 6 shows the typical internet throughput plots obtained at DES, IITM and

5 SAC. Note that the throughputs close to 85% of the transmitter internet link speed were achieved. Throughput(%) Individual Satellite Throughput Individual Scp at IIT Madras Individual Scp at SAC Ahmedabad Individual Scp at DES Delhi Multiuser(20) at IIT Madras Multiuser(20) at SAC Ahmedabad Multiuser(20) at DES Delhi Time of Day(Hrs) Figure 7: Throughput plots measuring the satellite speeds at each of the participating receive stations. In order to measure the throughputs on satellite link speed, a number of files from the transmit station were downloaded simultaneously using scp once every hour at the participating sites. figure 7 shows the typical satellite throughput plots obtained at DES, IITM and SAC. Note that throughputs close to 90% of the 1 Mbps satellite link speed were achieved while doing multiple simultaneous scp sessions. Besides the throughput plots, download rates were sampled at DES, IITM and SAC when they are downloading (i) simultaneously, and (ii) one at a time, that is, IITM alone is downloading or DES alone is downloading or SAC alone is downloading. These results are also shown in figure CONCLUSIONS The IITM-ISRO hybrid system provides a cost-efficient asymmetric internet access system. Asymmetric access is achieved by simultaneously using two links at each LSP locations - the low bit-rate two-way land based internet link and a high bit-rate shared satellite link. Low cost is achieved by sharing of all potential high-cost components by maintaining one high bit-rate internet connection at the transmit proxy station and using the satellite in the broadcast mode. The geographic reach of the satellite ensures connections to remote areas. It further ensures sharing the link among the LSP who are separated by large distances from each other. Further work is needed to determine the number of LSP stations that can effectively share a satellite link. This requires accurate models for the traffic generated at LSPs. Also, accurate information is needed on the patterns of Indian internet usage. Further cost reduction may be achieved by designing satellite demodulators to suit the end-use conditions. The current demodulators are general purpose demodulators that are designed to support rates from 9.6 kbps to 8 Mbps and all kinds of error control strategies. A careful design using the existing RF components and ICs has the potential to bring down the cost further. ACKNOWLEDGEMENT We thank Profs. Ashok Jhunjhunwala and Timothy A Gonsalves, Indian Institute of Technology Madras for suggesting the problem. We also thank Space Application Center, ISRO, Ahmedabad and Delhi Earth Station, ISRO, Delhi for providing the satellite channel and other transmit and receive systems. Without their continuous support and active involvement the system could not be put in place. REFERENCES [1] cordect Wireless Local Loop, Analog Devices Inc., USA, Indian Institute of Technology, Madras, India, Midas Communication Technologies(P) Ltd., Chennai, India, Technical Report, December [2] [3] [4] satellite/pages/xantic [5] [6] /1996/TR 96-20/TR phtml [7]