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53-487: In data networking , telecommunications , and computer buses , an acknowledgement ( ACK ) is a signal that is passed between communicating processes , computers , or devices to signify acknowledgment, or receipt of message, as part of a communications protocol . Correspondingly a negative-acknowledgement ( NAK or NACK ) is a signal that is sent to reject a previously received message or to indicate some kind of error. Acknowledgments and negative acknowledgments inform

106-756: A protocol stack , often constructed per the OSI model, communications functions are divided up into protocol layers, where each layer leverages the services of the layer below it until the lowest layer controls the hardware that sends information across the media. The use of protocol layering is ubiquitous across the field of computer networking. An important example of a protocol stack is HTTP (the World Wide Web protocol) running over TCP over IP (the Internet protocols) over IEEE 802.11 (the Wi-Fi protocol). This stack

159-503: A diverse set of networking capabilities. The protocols have a flat addressing scheme. They operate mostly at layers 1 and 2 of the OSI model. For example, MAC bridging ( IEEE 802.1D ) deals with the routing of Ethernet packets using a Spanning Tree Protocol . IEEE 802.1Q describes VLANs , and IEEE 802.1X defines a port-based network access control protocol, which forms the basis for the authentication mechanisms used in VLANs (but it

212-576: A large system would normally be accommodated by using the DTACK line to insert "wait states"—delays—into bus cycles. (The 68000 has no separate I/O address space, so all I/O devices must be memory-mapped.) The logic circuitry required to pulse DTACK high to add these delays could be fairly complex. Therefore, typically only relatively simple systems can use a design with "DTACK grounded" (i.e., DTACK always asserted). When many different peripherals with different maximum speed capabilities must be used in

265-440: A large, congested network into an aggregation of smaller, more efficient networks. A router is an internetworking device that forwards packets between networks by processing the addressing or routing information included in the packet. The routing information is often processed in conjunction with the routing table . A router uses its routing table to determine where to forward packets and does not require broadcasting packets which

318-440: A multi-port bridge. Switches normally have numerous ports, facilitating a star topology for devices, and for cascading additional switches. Bridges and switches operate at the data link layer (layer 2) of the OSI model and bridge traffic between two or more network segments to form a single local network. Both are devices that forward frames of data between ports based on the destination MAC address in each frame. They learn

371-590: A pin on the Motorola 68000 CPU that informs the CPU that data is ready to be read from memory. It stands for "Data Transfer Acknowledge." If a system has fast enough memory, this pin can be connected directly to the ground plane (or "grounded") to produce the fastest-possible memory read/write time. However, any complex system would almost certainly have several different types of devices (RAM, ROM, various peripherals) that would support different speeds of access, which in

424-544: A reference to DTACK distinctively implies the 68000 family of CPUs. In order to discourage photocopying of the newsletter, a portion of the DTACK Grounded newsletter was printed on dark red paper. This made it harder to photocopy those pages. This section of the newsletter was called "the Redlands". It typically contained the assembly source code for high performance math subroutines. A major topic in later years of

477-457: A repeater hub assists with collision detection and fault isolation for the network. Hubs and repeaters in LANs have been largely obsoleted by modern network switches. Network bridges and network switches are distinct from a hub in that they only forward frames to the ports involved in the communication whereas a hub forwards to all ports. Bridges only have two ports but a switch can be thought of as

530-436: A return packet with an ACK bit set. While some protocols send an acknowledgement per each packet received, other protocols such as TCP and ZMODEM allow many packets to be transmitted before sending an acknowledgement for the set of them, a procedure necessary to fill high bandwidth-delay product links with a large number of bytes in flight. Some protocols are NAK-based, meaning that they only respond to messages if there

583-524: A sender of the receiver's state so that it can adjust its own state accordingly. The ASCII code point for ACK is 0x06 (binary 0000 0110). By convention a receiving device sends an ACK to indicate it successfully received a message. ASCII also provides a NAK code point (0x15, binary 0001 0101) which can be used to indicate the receiving device cannot, or will not, comply with the message. Unicode provides visible symbols for these ASCII characters, U+2406 (␆) and U+2415 (␕). ACK and NAK symbols may also take

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636-418: A single failure can cause the network to fail entirely. In general, the more interconnections there are, the more robust the network is; but the more expensive it is to install. Therefore, most network diagrams are arranged by their network topology which is the map of logical interconnections of network hosts. Common topologies are: The physical layout of the nodes in a network may not necessarily reflect

689-420: A small system, another solution to keep DTACK grounded is to slow down the clock rate of the CPU so that all memory and peripheral devices can transfer data at the (reduced) maximum speed of the CPU. In turn, using a lower CPU clock frequency enables a simpler, less expensive circuit board design, with fewer layers, to be used successfully. This, too, like limiting the system to a few peripherals of low diversity,

742-403: A standard voice telephone line. Modems are still commonly used for telephone lines, using a digital subscriber line technology and cable television systems using DOCSIS technology. A firewall is a network device or software for controlling network security and access rules. Firewalls are inserted in connections between secure internal networks and potentially insecure external networks such as

795-481: A time slot for an acknowledgment bit after each byte. Data networking A computer network is a set of computers sharing resources located on or provided by network nodes . Computers use common communication protocols over digital interconnections to communicate with each other. These interconnections are made up of telecommunication network technologies based on physically wired, optical , and wireless radio-frequency methods that may be arranged in

848-877: A transmission medium. Power line communication uses a building's power cabling to transmit data. The following classes of wired technologies are used in computer networking. Network connections can be established wirelessly using radio or other electromagnetic means of communication. The last two cases have a large round-trip delay time , which gives slow two-way communication but does not prevent sending large amounts of information (they can have high throughput). Apart from any physical transmission media, networks are built from additional basic system building blocks, such as network interface controllers , repeaters , hubs , bridges , switches , routers , modems, and firewalls . Any particular piece of equipment will frequently contain multiple building blocks and so may perform multiple functions. A network interface controller (NIC)

901-406: A variety of network topologies . The nodes of a computer network can include personal computers , servers , networking hardware , or other specialized or general-purpose hosts . They are identified by network addresses and may have hostnames . Hostnames serve as memorable labels for the nodes and are rarely changed after initial assignment. Network addresses serve for locating and identifying

954-495: A variety of different sources, primarily to support circuit-switched digital telephony . However, due to its protocol neutrality and transport-oriented features, SONET/SDH also was the obvious choice for transporting Asynchronous Transfer Mode (ATM) frames. Asynchronous Transfer Mode (ATM) is a switching technique for telecommunication networks. It uses asynchronous time-division multiplexing and encodes data into small, fixed-sized cells . This differs from other protocols such as

1007-657: A virtual system of links that run on top of the Internet . Overlay networks have been used since the early days of networking, back when computers were connected via telephone lines using modems, even before data networks were developed. The most striking example of an overlay network is the Internet itself. The Internet itself was initially built as an overlay on the telephone network . Even today, each Internet node can communicate with virtually any other through an underlying mesh of sub-networks of wildly different topologies and technologies. Address resolution and routing are

1060-522: Is computer hardware that connects the computer to the network media and has the ability to process low-level network information. For example, the NIC may have a connector for plugging in a cable, or an aerial for wireless transmission and reception, and the associated circuitry. In Ethernet networks, each NIC has a unique Media Access Control (MAC) address —usually stored in the controller's permanent memory. To avoid address conflicts between network devices,

1113-518: Is a design strategy that is typically applied to small or embedded systems but would not be appropriate for high-performance workstations. Many other microprocessors, including the Intel 8086 , 8088 , 80286 , 80386 , 8080 , 8085 , Zilog Z80 , and 6502 family , each have an input pin that serves a similar function to DTACK, but it is usually named READY or RDY. (On the Z80, it is named WAIT.) Therefore,

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1166-420: Is a problem. Examples include many reliable multicast protocols which send a NAK when the receiver detects missing packets or protocols that use checksums to verify the integrity of the payload and header . Still other protocols make use of both NAKs and ACKs. Binary Synchronous Communications (Bisync) and Adaptive Link Rate (for Energy-Efficient Ethernet ) are examples. The acknowledgement function

1219-422: Is a virtual network that is built on top of another network. Nodes in the overlay network are connected by virtual or logical links. Each link corresponds to a path, perhaps through many physical links, in the underlying network. The topology of the overlay network may (and often does) differ from that of the underlying one. For example, many peer-to-peer networks are overlay networks. They are organized as nodes of

1272-566: Is also found in WLANs ) – it is what the home user sees when the user has to enter a "wireless access key". Ethernet is a family of technologies used in wired LANs. It is described by a set of standards together called IEEE 802.3 published by the Institute of Electrical and Electronics Engineers. Wireless LAN based on the IEEE 802.11 standards, also widely known as WLAN or WiFi, is probably

1325-478: Is an electronic device that receives a network signal , cleans it of unnecessary noise and regenerates it. The signal is retransmitted at a higher power level, or to the other side of obstruction so that the signal can cover longer distances without degradation. In most twisted-pair Ethernet configurations, repeaters are required for cable that runs longer than 100 meters. With fiber optics, repeaters can be tens or even hundreds of kilometers apart. Repeaters work on

1378-412: Is inefficient for very big networks. Modems (modulator-demodulator) are used to connect network nodes via wire not originally designed for digital network traffic, or for wireless. To do this one or more carrier signals are modulated by the digital signal to produce an analog signal that can be tailored to give the required properties for transmission. Early modems modulated audio signals sent over

1431-404: Is not sending packets, the link can be filled with packets from other users, and so the cost can be shared, with relatively little interference, provided the link is not overused. Often the route a packet needs to take through a network is not immediately available. In that case, the packet is queued and waits until a link is free. The physical link technologies of packet networks typically limit

1484-433: Is the process of selecting network paths to carry network traffic. Routing is performed for many kinds of networks, including circuit switching networks and packet switched networks. DTACK Grounded Hal was the editor of the newsletter and wrote almost all of its content. The ostensible purpose of the newsletter was to promote Hal's company's line of 68000-based hardware and software. However, Hal never let that get in

1537-459: Is used between the wireless router and the home user's personal computer when the user is surfing the web. There are many communication protocols, a few of which are described below. The Internet protocol suite , also called TCP/IP, is the foundation of all modern networking. It offers connection-less and connection-oriented services over an inherently unreliable network traversed by datagram transmission using Internet protocol (IP). At its core,

1590-455: Is used in the automatic repeat request (ARQ) function. Acknowledgement frames are numbered in coordination with the frames that have been received and then sent to the transmitter. This allows the transmitter to avoid overflow or underrun at the receiver, and to become aware of any missed frames . In IBM Binary Synchronous Communications , the NAK is used to indicate that a transmission error

1643-462: The Institute of Electrical and Electronics Engineers (IEEE) maintains and administers MAC address uniqueness. The size of an Ethernet MAC address is six octets . The three most significant octets are reserved to identify NIC manufacturers. These manufacturers, using only their assigned prefixes, uniquely assign the three least-significant octets of every Ethernet interface they produce. A repeater

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1696-477: The World Wide Web , digital video and audio , shared use of application and storage servers , printers and fax machines , and use of email and instant messaging applications. Computer networking may be considered a branch of computer science , computer engineering , and telecommunications , since it relies on the theoretical and practical application of the related disciplines. Computer networking

1749-440: The Internet protocol suite or Ethernet that use variable-sized packets or frames . ATM has similarities with both circuit and packet switched networking. This makes it a good choice for a network that must handle both traditional high-throughput data traffic, and real-time, low-latency content such as voice and video. ATM uses a connection-oriented model in which a virtual circuit must be established between two endpoints before

1802-574: The Internet. Firewalls are typically configured to reject access requests from unrecognized sources while allowing actions from recognized ones. The vital role firewalls play in network security grows in parallel with the constant increase in cyber attacks . A communication protocol is a set of rules for exchanging information over a network. Communication protocols have various characteristics. They may be connection-oriented or connectionless , they may use circuit mode or packet switching, and they may use hierarchical addressing or flat addressing. In

1855-627: The actual data exchange begins. ATM still plays a role in the last mile , which is the connection between an Internet service provider and the home user. There are a number of different digital cellular standards, including: Global System for Mobile Communications (GSM), General Packet Radio Service (GPRS), cdmaOne , CDMA2000 , Evolution-Data Optimized (EV-DO), Enhanced Data Rates for GSM Evolution (EDGE), Universal Mobile Telecommunications System (UMTS), Digital Enhanced Cordless Telecommunications (DECT), Digital AMPS (IS-136/TDMA), and Integrated Digital Enhanced Network (iDEN). Routing

1908-468: The association of physical ports to MAC addresses by examining the source addresses of received frames and only forward the frame when necessary. If an unknown destination MAC is targeted, the device broadcasts the request to all ports except the source, and discovers the location from the reply. Bridges and switches divide the network's collision domain but maintain a single broadcast domain. Network segmentation through bridging and switching helps break down

1961-437: The form of single bits or bit fields depending on the protocol data link layer definition or even as a dedicated wire at physical layer . Many protocols are acknowledgement-based, meaning that they positively acknowledge receipt of messages. The internet's Transmission Control Protocol (TCP) is an example of an acknowledgement-based protocol. When computers communicate via TCP, received packets are acknowledged by sending

2014-668: The literature as the physical medium ) used to link devices to form a computer network include electrical cable , optical fiber , and free space. In the OSI model , the software to handle the media is defined at layers 1 and 2 — the physical layer and the data link layer. A widely adopted family that uses copper and fiber media in local area network (LAN) technology are collectively known as Ethernet. The media and protocol standards that enable communication between networked devices over Ethernet are defined by IEEE 802.3 . Wireless LAN standards use radio waves , others use infrared signals as

2067-665: The means that allow mapping of a fully connected IP overlay network to its underlying network. Another example of an overlay network is a distributed hash table , which maps keys to nodes in the network. In this case, the underlying network is an IP network, and the overlay network is a table (actually a map ) indexed by keys. Overlay networks have also been proposed as a way to improve Internet routing, such as through quality of service guarantees achieve higher-quality streaming media . Previous proposals such as IntServ , DiffServ , and IP multicast have not seen wide acceptance largely because they require modification of all routers in

2120-415: The most well-known member of the IEEE 802 protocol family for home users today. IEEE 802.11 shares many properties with wired Ethernet. Synchronous optical networking (SONET) and Synchronous Digital Hierarchy (SDH) are standardized multiplexing protocols that transfer multiple digital bit streams over optical fiber using lasers. They were originally designed to transport circuit mode communications from

2173-412: The network needs to deliver the user data, for example, source and destination network addresses , error detection codes, and sequencing information. Typically, control information is found in packet headers and trailers , with payload data in between. With packets, the bandwidth of the transmission medium can be better shared among users than if the network were circuit switched . When one user

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2226-420: The network topology. As an example, with FDDI , the network topology is a ring, but the physical topology is often a star, because all neighboring connections can be routed via a central physical location. Physical layout is not completely irrelevant, however, as common ducting and equipment locations can represent single points of failure due to issues like fires, power failures and flooding. An overlay network

2279-729: The network. On the other hand, an overlay network can be incrementally deployed on end-hosts running the overlay protocol software, without cooperation from Internet service providers . The overlay network has no control over how packets are routed in the underlying network between two overlay nodes, but it can control, for example, the sequence of overlay nodes that a message traverses before it reaches its destination . For example, Akamai Technologies manages an overlay network that provides reliable, efficient content delivery (a kind of multicast ). Academic research includes end system multicast, resilient routing and quality of service studies, among others. The transmission media (often referred to in

2332-564: The newsletter was the Halgol interpreter, later converted to the DBASIC interpreter. Hal produced a fast Basic interpreter for the Atari ST computer. It was written in hand-tuned assembly language, and ran very fast compared to other Basics. But it was non-standard: it couldn't run Microsoft BASIC programs, and it couldn't read or write MS-DOS format floppy disks. Unfortunately, as a result, it

2385-494: The nodes by communication protocols such as the Internet Protocol . Computer networks may be classified by many criteria, including the transmission medium used to carry signals, bandwidth , communications protocols to organize network traffic , the network size, the topology, traffic control mechanisms, and organizational intent. Computer networks support many applications and services , such as access to

2438-520: The physical layer of the OSI model but still require a small amount of time to regenerate the signal. This can cause a propagation delay that affects network performance and may affect proper function. As a result, many network architectures limit the number of repeaters used in a network, e.g., the Ethernet 5-4-3 rule . An Ethernet repeater with multiple ports is known as an Ethernet hub . In addition to reconditioning and distributing network signals,

2491-456: The protocol suite defines the addressing, identification, and routing specifications for Internet Protocol Version 4 (IPv4) and for IPv6 , the next generation of the protocol with a much enlarged addressing capability. The Internet protocol suite is the defining set of protocols for the Internet. IEEE 802 is a family of IEEE standards dealing with local area networks and metropolitan area networks. The complete IEEE 802 protocol suite provides

2544-556: The same message multiple times in hopes that at least one copy of the message gets through. Some computer buses have a dedicated acknowledge wire in the control bus used to acknowledge bus operations: DACK used for ISA DMA ; DATACK used in the STEbus , the data transfer acknowledge pin of the Motorola 68000 that inspired the title of DTACK Grounded , etc. Some computer buses do not wait for acknowledgement of every transmission, see for instance posted write . The I²C serial bus has

2597-515: The sharing of files and information, giving authorized users access to data stored on other computers. Distributed computing leverages resources from multiple computers across a network to perform tasks collaboratively. Most modern computer networks use protocols based on packet-mode transmission. A network packet is a formatted unit of data carried by a packet-switched network . Packets consist of two types of data: control information and user data (payload). The control information provides data

2650-493: The size of packets to a certain maximum transmission unit (MTU). A longer message may be fragmented before it is transferred and once the packets arrive, they are reassembled to construct the original message. The physical or geographic locations of network nodes and links generally have relatively little effect on a network, but the topology of interconnections of a network can significantly affect its throughput and reliability. With many technologies, such as bus or star networks,

2703-404: The way of telling a good story or explaining how to design or build a fast computer. He was a hardware engineer and a businessman who also knew how to write software. His newsletter was full of amusing and educational stories about the way the personal computer hardware business worked, and strong opinions about the best way to design personal computer hardware and software. "DTACK" is the name of

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2756-526: Was detected in the previously received block and that the receiver is ready to accept retransmission of that block. Bisync does not use a single ACK character but has two control sequences for alternate even/odd block acknowledgement. ACK and NAK based methodologies are not the only protocol design paradigms. Some protocols such as the RC-5 , User Datagram Protocol (UDP), and X10 protocols perform blind transmission with no acknowledgement, often transmitting

2809-462: Was influenced by a wide array of technological developments and historical milestones. Computer networks enhance how users communicate with each other by using various electronic methods like email, instant messaging, online chat, voice and video calls, and video conferencing. Networks also enable the sharing of computing resources. For example, a user can print a document on a shared printer or use shared storage devices. Additionally, networks allow for

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