1. Chapter 15. Satellite Services and the Internet
第十五章 衛星服務與網際網路
亞洲大學
資訊工程學系碩士班
呂克明教授
二○○六年十二月十八日
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2. Chapter 15. Satellite Services and the Internet (第十五章衛星服務與網際網路)
Introduction (緒言)
Network Layers (網路層)
The TCP Link (TCP 鏈路)
Satellite Links and TCP (衛星鏈路與TCP)
Enhancing TCP Over Satellite Channels Using Standard Mechanisms (使用標準機制的加強TCP衛星頻道, RFC-2488)
Requests for Comments (特邀工作小組評論)
Split TCP Connections (分割TCP之連接)
Asymmetric Channels (非對稱頻道)
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3. Introduction (緒言)
In 1995, Federal Networking Council (FNC, 美國聯邦網絡會議) in US defined the Internet as a global information system:
Is logically linked together by a globally unique address space based on the Internet Protocol (IP, 網際網路通訊協定)
Is able to support communication using Internet Transmission Control Protocol/Internetworking Protocol (TCP/IP, 傳輸控制協定/網際網路協定)
Provides uses or makes accessible, high level services layered on the communications
Protocol (協定;約定;規程;議定;通訊協定) - A set of formal rules describing how to transmit data, especially across a network. Low level protocols define the electrical and physical standards to be observed, bit- and byte-ordering and the transmission and error detection and correction of the bit stream. High level protocols deal with the data formatting, including the syntax (造句法) of messages, the terminal to computer dialogue (對話) , character sets, sequencing of messages etc.
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4. Introduction (緒言)(continued)
The key elements are TCP and IP. Both protocols are usually lumped together as TCP/IP and embedded in the software for operating systems and browsers such as Windows and Netscape.
Access to the Internet follows well-defined rules.
Users connect to Internet service providers (ISP, 網際網路供應商), who in turn connect to network service providers (NSP , 網絡供應商) complete the connections to other users and to servers.
Servers are computers dedicated to the purpose of providing information to the Internet.
Internet applications:
, discussion groups, long-distance computer, and file transfers.
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5. Introduction (緒言)(continued)
Routers are computers that form part of the communications net and that route or direct the data along the best available paths in the network.
The World Wide Web Consortium (W3C, 全球資訊網協會) was founded in 1994 and oversees a number of special interest groups and coordinates its efforts with IETF (網際網路工程工作特別小組,是一個大型的開放性國際社群，成員包括對網路網路架構進化和平順運作有興趣的網路設計者、營運商、代理商和研究人員。而且它也對所有有興趣的個人開放。) and with other standards bodies.
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6. Network Layers (網路層)
The uplink and downlink between satellite and earth stations from what is known as the physical layer in data communication system.
The data are transmitted in packets.
Many separate functions have to be performed in packet transmission, such as:
packet addressing, routing, and
coping with packet congestion (對付封包的擁塞) .
The modern approach is to assign each function to a layer in what is termed the network architecture.
The layers are conceptual in the sense that they may consist of software or some combination of software and hardware.
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7. Network Layers (網路層)(continued)
The layers of the layered structure:
Physical layer: Physical connectors, signal format, modulation, and the uplink and downlink in a satellite communication
Data-link layer: To organize the digital data into blocks (or frames) such as Asynchronous Transfer Mode (ATM) technology.
Network Layer: This strictly an IP layer. The packets are passed along the Internet from router to router and to the host stations. The problems of lost packets or packets arriving out of sequence are not a concern of the IP layer, and for this reason, the IP layer is called connectionless. These problems are taken care of by the transport layer.
Transport layer: Two (2) sets of protocol are provided in this layer.
TCP – information is passed back and forth between transport layers such as the correct sequencing of packets, replacement of lost packets, and adjusting the transmission rate of packets to prevent congestion.
UDP – The User Datagram Protocol (用戶數據報協議) provides a connectionless service, similar to IP.
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8. Network Layers (網路層)(continued)
Packet Terms (IPv4):
Data: Application layer the packet is simply referred as data
TCP Segment: Transport layer, TCP header + data
UDP Segment: Transport layer, UDP header + data
IP datagram: Network layer, IP header + TCP (or UDP) header + data
Frame: Data link, Frame header + IP header + TCP (or UDP) header + data
Packet Terms (IPv6): IP datagram become IP packet.
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9. The TCP Link (TCP 鏈路)
A virtual communications link exists between corresponding (對應) layers in a network.
The header in the TCP segment carriers instructions that enable communication between send and receive TCP layers.
TCP/IP suite:
TCP end-to-end connections: WWW, News, FTP, SMTP, Telnet, and NFS
UDP single message – NFS, DNS, and SNMP
Buffer memories
Receive window: the amount of received buffer space available at any given time.
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10. Satellite Links and TCP (衛星鏈路與TCP)
Congestion control
Bit error rate (BER, 位元錯誤率,誤碼率 ) – Typically the satellite link BER without error control coding is around 1.E-6, whereas a level of 1.E-8 or lower is needed for successful TCP transfer.
Round-trip time (RTT,封包來回時間 ) – the round-trip propagation path is ground station to satellite to ground station and back again.
Bandwidth-delay product (BDP,帶寬延遲產物 ) – A satellite bandwidth of 36 MHz carrying a BPSK signal could handle a bit rate as 30 megabits per second (Mb/s) = 3.75x E-6 bytes per second. If the sender transmits at this rate, the largest packet it can send within the RTT of 0.55 second is 3662 x 0.55 = 2014 Kb.
Variable round-trip time – LEO and MEO will be much less than that for the GEO.
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11. Enhancing TCP Over Satellite Channels Using Standard Mechanisms (使用標準機制的加強TCP衛星頻道, RFC-2488)
Forward error correction (前置糾錯碼技術, FEC) - methodology that uses error correction coding to transmission. This is the opposite of ARQ (automatic repeat request) which uses retransmission of data.
FEC CODE: RATE 1/3, A simple FEC code that repeats each bit three times.
FEC CODE: RATE 2/3, A more robust FEC method using a (15,10) shortened Hamming code (漢明碼,These are the first class of linear binary codes used for error correction in long-distance telephony. ).
Slow start and congestion avoidance
Fast retransmit and fast recovery
TCP large windows
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12. Requests for Comments (特邀工作小組評論)
RFC-2760: Ongoing TCP Research Related to Satellite, February 2000
RFC-2488: Enhancing TCP Over Satellite Channels Using Standard Mechanisms, January 1999
RFC-2018: TCP Selective Acknowledgment Options, October 1996
RFC-1323: TCP Extensions for High Performance, May 1992
RFC-1072: TCP Extensions for Long Delay Paths, October 1988
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13. Split TCP Connections (分割TCP之連接)
TCP provides end-to-end virtual link between layers at the sender and receiver.
If it is assumed that the end-to-end connectivity can be split, new possibilities are opened up for introduction of satellite links as part of overall Internet.
Breaking the network in this way is termed spoofing (欺騙,愚弄) . This refers to the fact that the TCP source thinks it is connected to the TCP destination, whereas the inter-working unit (IWU) performs a protocol conversion.
Spoofing - the creation of TCP/IP packets using somebody else's IP address.
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14. Asymmetric Channels (非對稱頻道)
The term asymmetry applies in two (2) senses to an Internet connection. It can refer to the data flow, which is often asymmetric in nature.
A short request being sent for a Web page and the returned Web page may be a much larger document.
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