IP is NOT the World Wide Web
In order to understand how the airborne Internet can successfully work with airplanes, it is beneficial to understand some elements of how networks work.
In basic computer networks, each computer utilizes a network device, such as a network interface card (NIC), as its physical interface onto the network wire, or Local Area Network (LAN). The network wire (or cable) connects directly to the NIC in the back of the PC. At the next level higher, network protocols are applied and bound to the NIC. Typically and most commonly used is TCP/IP. The Internet Protocol (IP) is a network-layer protocol that contains addressing information and some control information that enables packets to be routed. The IP component provides routing from the department to the enterprise network, then to regional networks, and finally to the global Internet. TCP is responsible for verifying the correct delivery of data from the client to the destination server. Use of TCP/IP is not necessarily the same as using the World Wide Web (www).
The use of the www involves using a web browser that utilizes another higher protocol layer: hypertext transfer protocol (HTTP). The web browser allows you to enter a web site, such as http://www.abc.com and using TCP/IP, connects to the server that serves up that web page. The browser interprets the programming language of hypertext markup language (HTML) and presents the information to you in a way that the user understands.
TCP and IP are actually underneath HTTP. Using TCP/IP, a network computer can do many other useful things such as transfer files, send and receive email, and log on to a network server remotely. The Airborne Internet concept is based on using TCP/IP, but not necessarily HTTP.
Uniquely Being Identified:
Each technology has its own convention for transmitting messages between two machines within the same network. On a LAN, messages are sent between machines by supplying the six byte unique identifier (the media access control address: the "MAC" address). More specifically, the MAC address is embedded into the network card (NIC). Each NIC has a different MAC address that is applied during the manufacturing process. TCP/IP assigns a unique number to every workstation (actually its NIC) in the world. The IP address and the MAC are married together. This "IP number" is a four byte value that, by convention, is expressed by converting each byte into a decimal number (0 to 255) and separating the bytes with a period. For example, the www.abc.com web site server is 220.127.116.11.
The enterprise network is built using commercially available TCP/IP router boxes. Each router has tables to translate the name into a destination IP address. Think of it in terms of how mail is delivered to homes. A computer’s IP address is similar to the address of a residence, which starts in a large context and eventually is defined at the exact residence location. The residence address might really be Earth, United States of America, New Jersey, Atlantic City, Massachusetts Ave, and then house number 405. Most of the top level is assumed when it is only being routed within the U.S. Postal Service, but if a person from Germany were to send a letter to an address in the United States, it would have to be addressed as follows:
John Doe 405 N. Massachusetts Ave. Atlantic City, NJ. 08405 U.S.A.
As the post offices use zip codes to help them route the mail and then eventually sorted and delivered properly, so do network routers. They use IP to help them accomplish it. The router must also determine where to send it next, or which “route” to use to ensure it arrives at its destination. Every time a message arrives at an IP router, it makes an individual decision about where to send it next. Routing decisions are not necessarily complex. More sophisticated routing measures traffic patterns and sends data through the least busy link. If one communications line in this network breaks down, traffic can still reach its destination through an alternate path. This provides continued service with possible degraded performance. This kind of recovery is the primary design feature of IP. The connectivity loss is detected by the routers at the origin and destination. Each network adopts some Router Protocol which periodically updates the routing tables throughout the network with information about changes in route status.
If the size of the network grows, then the complexity of the routing updates will increase as will the cost of transmitting them. Building a single network that covers the entire US would be unreasonably complicated. Fortunately, the Internet is designed as a Network of Networks. This means that loops and redundancy are built into each regional carrier. The regional network handles its own problems and reroutes messages internally. Its Router Protocol updates the tables in its own routers, but no routing updates need to propagate from a regional carrier to the spine or to the other regions.
When data arrives at a congested router, there is no
place to send the overflow. Excess packets are simply discarded. It becomes the
responsibility of the sender to retry the data a few seconds later and to
persist until it finally gets through. This recovery is provided by the TCP
component of the Internet protocol. TCP was designed to recover from node or
line failures where the network propagates routing table changes to all router
nodes. Since the update takes some time, TCP is slow to initiate recovery. The
TCP algorithms are not tuned to optimally handle packet loss due to traffic
congestion. Instead, the traditional Internet response to traffic problems has
been to increase the speed of lines and equipment in order to say ahead of
growth in demand. The receiver can detect missing or incorrectly sequenced
packets. TCP acknowledges data that has been received and retransmits data that
has been lost. The TCP design means that error recovery is done end-to-end
between the Client and Server machine.
Why all this detail?
The rather lengthy and detailed explanation just provided is to illustrate how the use of IP can very dependably be relied on to deliver network communications. Aircraft use of communication and navigation information must be nearly real time, highly dependable and it must have backup redundancy. IP has inherent redundancy in its digital delivery system, making it an excellent candidate for aircraft use. The reason IP has never been used in an aircraft context before is because until now there has not been a method proposed to keep the aircraft connected to the network, so that the IP connection is never lost. Now it is appropriate to examine how aircraft currently operate so we can draw both analogy and cite the differences between present day aircraft “networks” and an IP based aviation network (Airborne Internet). NEXT
Copyright©2002 Ralph Yost, All Rights Reserved.