Exit Zen
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`DLE STX` **1** **2** **3** **DLE** `DLE` **STX** `4` `DLE ETX`
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Amateur packet radio
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Reliability of the connection-oriented service
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Internet Protocol is the generic term for the network layer protocol in the TCP/IP protocol suite. IP version 4 is widely used but IP version 6 is being deployed globally.
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We will discuss these functions in more details when we will describe the protocols that are used in the network layer of the TCP/IP protocol suite.
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**DLE STX DLE ETX**
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Packet radio technologies have evolved in various directions since the first experiments performed at the University of Hawaii. The Amateur packet radio service developed by amateur radio operators is one of the descendants ALOHANet. Many amateur radio operators are very interested in new technologies and they often spend countless hours developing new antennas or transceivers. When the first personal computers appeared, several amateur radio operators designed radio modems and their own datalink layer protocols [KPD1985]_ [BNT1997]_. This network grew and it was possible to connect to servers in several European countries by only using packet radio relays. Some amateur radio operators also developed TCP/IP protocol stacks that were used over the packet radio service. Some parts of the `amateur packet radio network <http://www.ampr.org/>`_ are connected to the global Internet and use the `44.0.0.0/8` IPv4 prefix.
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IPv4
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Virtual circuit organization
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`DLE STX` **DLE** `DLE` **STX** **DLE** `DLE` ETX** `DLE ETX`
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Many improvements to ALOHANet have been proposed since the publication of [Abramson1970]_, and this technique, or some of its variants, are still found in wireless networks today. The slotted technique proposed in [Roberts1975]_ is important because it shows that a simple modification can significantly improve channel utilization. Instead of allowing all terminals to transmit at any time, [Roberts1975]_ proposed to divide time into slots and allow terminals to transmit only at the beginning of each slot. Each slot corresponds to the time required to transmit one fixed size frame. In practice, these slots can be imposed by a single clock that is received by all terminals. In ALOHANet, it could have been located on the central mainframe. The analysis in [Roberts1975]_ reveals that this simple modification improves the channel utilization by a factor of two.
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The request-response service
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is the version 4 of the Internet Protocol, the connectionless network layer protocol used in most of the Internet today. IPv4 addresses are encoded as a 32 bits field.
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Carrier Sense Multiple Access
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The `request-response service` is a compromise between the `connectionless service` and the `connection-oriented service`. Many applications need to send a small amount of data and receive a small amount of information back. This is similar to procedure calls in programming languages. A call to a procedure takes a few arguments and returns a simple answer. In a network, it is sometimes useful to execute a procedure on a different host and receive the result of the computation. Executing a procedure on another host is often called Remote Procedure Call. It is possible to use the `connectionless service` for this application. However, since this service is usually unreliable, this would force the application to deal with any type of error that could occur. Using the `connection oriented service` is another alternative. This service ensures the reliable delivery of the data, but a connection must be created before the beginning of the data transfer. This overhead can be important for applications that only exchange a small amount of data.
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IPv6
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ALOHA and slotted ALOHA can easily be implemented, but unfortunately, they can only be used in networks that are very lightly loaded. Designing a network for a very low utilization is possible, but it clearly increases the cost of the network. To overcome the problems of ALOHA, many Medium Access Control mechanisms have been proposed which improve channel utilization. Carrier Sense Multiple Access (CSMA) is a significant improvement compared to ALOHA. CSMA requires all nodes to listen to the transmission channel to verify that it is free before transmitting a frame [KT1975]_. When a node senses the channel to be busy, it defers its transmission until the channel becomes free again. The pseudo-code below provides a more detailed description of the operation of CSMA.
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The `request-response service` allows to efficiently exchange small amounts of information in a request and associate it with the corresponding response. This service can be depicted by using the time-sequence diagram below.
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is the version 6 of the Internet Protocol, the connectionless network layer protocol which is intended to replace IPv4. IP version 6 addresses are encoded as a 128 bits field.
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the outgoing interface for the packet
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