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40-440: AX.25 v2.0 is responsible for establishing link layer connections, transferring data encapsulated in frames between nodes , and detecting errors introduced by the communications channel . AX.25 v2.2 (1998) added improvements to improve efficiency, especially at higher data rates. Stations can automatically negotiate payload sizes larger than the previous limitation of 256 bytes. Extended sequence numbers (7 vs. 3 bits) allow

80-463: A dumb terminal and a transceiver . AX.25 has also been implemented on personal computers . For example, the Linux kernel includes native support for AX.25. The computer connects to a transceiver via its audio interface or via a simple modem. The computers can also interconnect to other computers or be bridged or routed to TNCs and transceivers located elsewhere using BPQ over Ethernet framing, which

120-666: A network segment . Despite the different semantics of layering between the Internet protocol suite and OSI model , the link layer is sometimes described as a combination of the OSI's data link layer (layer 2) and physical layer (layer 1). The link layer is described in RFC   1122 and RFC   1123 . RFC 1122 considers local area network protocols such as Ethernet and other IEEE 802 networks (e.g. Wi-Fi ), and framing protocols such as Point-to-Point Protocol (PPP) to belong to

160-552: A satellite ground station , and retransmit it to another ground station. The transceiver first appeared in the 1920s. Before then, receivers and transmitters were manufactured separately and devices that wanted to receive and transmit data required both components. Almost all amateur radio equipment today uses transceivers, but there is an active market for pure radio receivers, which are mainly used by shortwave listening operators. Analog transceivers use frequency modulation to send and receive data. Although this technique limits

200-409: A "receiver". On a mobile telephone or other radiotelephone , the entire unit is a transceiver for both audio and radio. A cordless telephone uses an audio and radio transceiver for the handset, and a radio transceiver for the base station . If a speakerphone is included in a wired telephone base or in a cordless base station, the base also becomes an audio transceiver. A modem is similar to

240-557: A hardware layer or physical layer below the link layer, and several of them adopt the OSI term data link layer instead of link layer in a modified description of layering. In the predecessor to the TCP/IP model, the ARPAnet Reference Model (RFC 908, 1982), aspects of the link layer are referred to by several poorly defined terms, such as network-access layer , network-access protocol , as well as network layer , while

280-429: A host is connected to. Such protocol packets are not routed to other networks. The link layer includes the protocols that define communication between local (on-link) network nodes which fulfill the purpose of maintaining link states between the local nodes, such as the local network topology, and that usually use protocols that are based on the framing of packets specific to the link types. The core protocols specified by

320-456: A larger window size, the number of frames that can be sent before waiting for acknowledgement. "Selective Reject" allows only the missing frames to be resent, rather than having to wastefully resend frames that have already been received successfully. Despite all these advantages, few implementations have been updated to include these improvements published more than 20 years ago. The only known complete implementation of v2.2, at this time (2020),

360-527: A lot fewer resources than reception of the same signal, thus a sufficient microprocessor might cost just US$ 5 instead of US$ 30 and a system cost might stay below US$ 50, transmitter included. However, in recent years, the ability to receive as well as send using cheap microcontrollers (such as the Atmel AVR or the Motorola 68HC08 families) has been demonstrated. It seems, however, that any new system that

400-417: A single device to reduce manufacturing costs. The term is also used for other devices which can both transmit and receive through a communications channel , such as optical transceivers which transmit and receive light in optical fiber systems, and bus transceivers which transmit and receive digital data in computer data buses . Radio transceivers are widely used in wireless devices . One large use

440-714: A transceiver in that it sends and receives a signal, but a modem uses modulation and demodulation. It modulates the signal being transmitted and demodulates the signal being received. Transceivers are called Medium Attachment Units ( MAUs ) in IEEE 802.3 documents and were widely used in 10BASE2 and 10BASE5 Ethernet networks. Fiber-optic gigabit , 10 Gigabit Ethernet , 40 Gigabit Ethernet , and 100 Gigabit Ethernet utilize GBIC , SFP , SFP+ , QSFP , XFP , XAUI , CXP , and CFP transceiver systems. Because transceivers are capable of broadcasting information over airwaves, they are required to adhere to various regulations. In

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480-512: Is a free open-source replacement for the 1980s-style TNC. It contains DSP software modems and a complete implementation of AX25 v2.2 plus FX.25 forward error correction. It can function as a digital repeater, GPS tracker, and APRS Internet Gateway (IGate) without any additional software. AX.25 is often used with a TNC that implements the KISS framing as a low-cost alternative to using expensive and uncommon HDLC controller cards. The KISS framing

520-742: Is also natively supported by the Linux kernel to facilitate more modern setups with the actual transceivers directly placed under or in the antenna mast, creating a 'low loss', shorter RF wiring need, and replacing expensive and long and thick coax cables and amplifiers with cheap fiber (RFI (both ways)/EMP/lightning resistant) or copper Ethernet wiring. BPQ Ethernet framing allows connecting entire stacks of TNC+transceiver pairs to any existing network of computers which then can all access all radio links offered simultaneously (transparently bridged), communicate with each other internally over AX.25, or with filtered routing select specific TNCs/radio frequencies. Dire Wolf

560-505: Is assumed that the KISS framing link is over serial with only the host computer and the TNC involved. Among other things, this makes it awkward to address multiple TNCs without having multiple (serial) data channels. Alternatives to KISS do exist that address these limitations, such as 6PACK. AX.25 has most frequently been used to establish direct, point-to-point links between packet radio stations, without any additional network layers. This

600-406: Is commonly used by police and fire departments. Digital transmissions tend to be clearer and more detailed than their analog counterparts. Many modern wireless devices operate on digital transmissions. In a wired telephone , the handset contains the transmitter (for speaking) and receiver (for listening). Despite being able to transmit and receive data, the whole unit is colloquially referred to as

640-472: Is in two-way radios , which are audio transceivers used for bidirectional person-to-person voice communication. Examples are cell phones , which transmit and receive the two sides of a phone conversation using radio waves to a cell tower , cordless phones in which both the phone handset and the base station have transceivers to communicate both sides of the conversation, and land mobile radio systems like walkie-talkies and CB radios . Another large use

680-498: Is in wireless modems in mobile networked computer devices such laptops , pads, and cellphones, which both transmit digital data to and receive data from a wireless router . Aircraft carry automated microwave transceivers called transponders which, when they are triggered by microwaves from an air traffic control radar , transmit a coded signal back to the radar to identify the aircraft. Satellite transponders in communication satellites receive digital telecommunication data from

720-492: Is not compatible with the current Bell 202 modulation is unlikely to be widely adopted. The current modulation seems to fulfill sufficient need that little motivation exists to move to a superior design, especially if the new design requires significant hardware purchases. Most recently, a wholly new protocol with forward error correction has been created by Nino Carillo, KK4HEJ, called Improved Layer 2 Protocol (IL2P). Link layer In computer networking ,

760-452: Is not part of the AX.25 protocol itself nor is it sent over the air. It merely serves to encapsulate the protocol frames in a way that can successfully pass over a serial link to the TNC. The KISS framing is derived from SLIP, and makes many of the same assumptions, such as there only being two "endpoints" involved in the conversation. With SLIP, these were the two SLIP-connected hosts; with KISS, it

800-429: Is often kludged around with varying degrees of success. Some amateurs, notably Phil Karn KA9Q, have argued that AX.25 is not well-suited to operation over noisy, limited-bandwidth radio links, citing its lack of forward error correction (FEC) and automatic data compression . However, a viable widely adopted successor to AX.25 has yet to emerge. Likely reasons may include: Despite these limitations, an extension to

840-442: Is sufficient for keyboard-to-keyboard contacts between stations and for accessing local bulletin board systems and DX clusters . In recent years, APRS has become a popular application. For tunneling of AX.25 packets over IP, AXIP and AXUDP are used to encapsulate AX.25 into IP or UDP packets. At the speeds commonly used to transmit packet radio data (rarely higher than 9,600  bit/s , and typically 1,200 bit/s),

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880-568: Is the Dire Wolf software TNC. AX.25 is commonly used as the data link layer for network layer such as IPv4, with TCP used on top of that. AX.25 supports a limited form of source routing . Although it is possible to build AX.25 switches similar to the way Ethernet switches work, this has not yet been accomplished. AX.25 does not define a physical layer implementation. In practice 1200 baud Bell 202 tones and 9600 baud G3RUH DFSK are almost exclusively used on VHF and UHF . On HF

920-503: Is used to great effect by the Automatic Packet Reporting System (APRS). A simple source routing mechanism using digipeaters is available at the datalink level. Digipeaters act as simplex repeaters , receiving, decoding and retransmitting packets from local stations. They allow multi-hop connections to be established between two stations unable to communicate directly. The digipeaters use and modify

960-743: The Internet Engineering Task Force (IETF) in this layer are the Address Resolution Protocol (ARP), the Reverse Address Resolution Protocol (RARP), and the Neighbor Discovery Protocol (NDP), which is a facility delivering similar functionality as ARP for IPv6 . The link layer of the TCP/IP model is often compared directly with the combination of the data link layer and the physical layer in

1000-534: The Open Systems Interconnection (OSI) protocol stack. Although they are congruent to some degree in technical coverage of protocols, they are not identical. The link layer in TCP/IP is still wider in scope and in principle a different concept and terminology of classification. This may be observed when certain protocols, such as ARP, which is confined to the link layer in the TCP/IP model, is often said to fit between OSI's data link layer and

1040-470: The link layer is the lowest layer in the Internet protocol suite , the networking architecture of the Internet . The link layer is the group of methods and communications protocols confined to the link that a host is physically connected to. The link is the physical and logical network component used to interconnect hosts or nodes in the network and a link protocol is a suite of methods and standards that operate only between adjacent network nodes of

1080-469: The AX.25 protocol, supporting forward error correction, has been created by the TAPR . This extension is called FX.25 . Small gadget transmitters do not need to know what is being transmitted. There is only a need to monitor channel occupation by radio receiver RSSI (Received Signal Strength Indication) to know when not to send. Transmitting interleaved Reed-Solomon FEC signal in some smart modulation needs

1120-410: The TCP/IP model, do not discuss hardware issues and physical data transmission and set no standards for those aspects. Some textbook authors have supported the interpretation that physical data transmission aspects are part of the link layer. Others assumed that physical data transmission standards are not considered communication protocols, and are not part of the TCP/IP model. These authors assume

1160-581: The United States, the Federal Communications Commission oversees their use. Transceivers must meet certain standards and capabilities depending on their intended use, and manufacturers must comply with these requirements. However, transceivers can be modified by users to violate FCC regulations. For instance, they might be used to broadcast on a frequency or channel that they should not have access to. For this reason,

1200-480: The complexity of the data that can be broadcast, analog transceivers operate very reliably and are used in many emergency communication systems. They are also cheaper than digital transceivers, which makes them popular with the CB and HAM radio communities. Digital transceivers send and receive binary data over radio waves. This allows more types of data to be broadcast, including video and encrypted communication, which

1240-524: The frame address. At ITU WARC2003 the radio amateur station callsign specification was amended so that the earlier maximum length of six characters was raised to seven characters. However, AX.25 has a built-in hard limit of six characters, which means a seven-character callsign cannot be used in an AX.25 network. AX.25 lacks an explicit port (or SAP ); the SSID often assumes this role. Thus there can be only one service per AX.25 station SSID address, which

AX.25 - Misplaced Pages Continue

1280-642: The frame. AX.25 specifies that this field is subdivided into multiple addresses: a source address, zero or more repeater addresses, and a destination address, with embedded control fields for use by the repeaters. To simplify compliance with amateur radio rules, these addresses derive from the station call signs of the source, destination and repeater stations. Media access control follows the Carrier sense multiple access approach with collision recovery (CSMA/CR). AX.25 supports both virtual-circuit connected and datagram-style connectionless modes of operation. The latter

1320-424: The information in the frame's address field to perform this function. The AX.25 specification defines a complete, albeit point to point only network layer protocol, but this has seen little use outside of keyboard-to-keyboard or keyboard-to-BBS connections. NET/ROM , ROSE , and TexNet exist to provide routing between nodes. In principle, a variety of layer 3 protocols can be used with AX.25, including

1360-404: The link layer. Local area networking standards such as Ethernet and IEEE 802.3 specifications use terminology from the seven-layer OSI model rather than the TCP/IP model. The TCP/IP model, in general, does not consider physical specifications, rather it assumes a working network infrastructure that can deliver media-level frames on the link. Therefore, RFC 1122 and RFC 1123, the definition of

1400-523: The network layer. In general, direct or strict comparisons should be avoided, because the layering in TCP/IP is not a principal design criterion and in general, is considered to be "harmful" (RFC 3439). Another term sometimes encountered, network access layer , tries to suggest the closeness of this layer to the physical network. However, this use is misleading and non-standard, since the link layer implies functions that are wider in scope than just network access. Important link layer protocols are used to probe

1440-403: The next higher layer is called internetwork layer . In some modern textbooks, network-interface layer , host-to-network layer and network-access layer occur as synonyms either to the link layer or the data link layer, often including the physical layer. The link layer in the TCP/IP model is a descriptive realm of networking protocols that operate only on the local network segment (link) that

1480-435: The standard transmission mode is 300 baud Bell 103 tones . At the physical layer, AX.25 defines only a "physical layer state machine" and some timers related to transmitter and receiver switching delays. At the link layer, AX.25 uses HDLC frame syntax and procedures. (ISO 3309) frames are transmitted with NRZI encoding. HDLC specifies the syntax, but not the semantics, of the variable-length address field of

1520-450: The topology of the local network, and discover routers and neighboring hosts, i.e. functions that go well beyond network access. Transceiver In radio communication , a transceiver is an electronic device which is a combination of a radio trans mitter and a re ceiver , hence the name. It can both transmit and receive radio waves using an antenna , for communication purposes. These two related functions are often combined in

1560-437: The ubiquitous Internet Protocol (IP). This approach is used by AMPRNet , which is an amateur radio TCP/IP network using AX.25 UI-frames at the datalink layer. Traditionally, amateur radio operators have connected to AX.25 networks through the use of a terminal node controller , which contains a microprocessor and an implementation of the protocol in firmware . These devices allow network resources to be accessed using only

1600-441: The use of additional network layers with AX.25 is impractical due to the data overhead involved. This is not a limitation of AX.25 per se , but places constraints on the sophistication of applications designed to use it. HDLC protocols identify each frame by an address. The AX.25 implementation of HDLC includes sender and destination station call-sign plus four-bit Secondary Station Identifier (SSID) value in range 0 through 15 in

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