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 In the Transmission Control Protocol, ACK packets are used to acknowledge receipt of a packet. They are used by both ends of the connection to move in between states, and are the basis of TCP's reliability.
 
  
 ADPCM is a technique for converting sound or analog information to binary information (a string of 0's and 1's) by taking frequent samples of it and expressing the value of the sampled sound modulation in binary terms.

It is a variant of DPCM that varies the size of the quantization step, to allow further reduction of the required bandwidth for a given signal-to-noise ratio. Where circuit costs are high and loss of voice quality is acceptable, it sometimes makes sense to compress the voice signal even further. An ADPCM algorithm is used to map a series of 8 bit PCM samples into a series of 4 bit ADPCM samples. In this way, the capacity of the line is doubled. The technique is detailed in the G.726 standard. Some ADPCM techniques are used in Voice over IP communications.
 
  
 ADSL is a form of DSL, a data communications technology that enables faster data transmission over copper telephone lines than a conventional modem can provide. ADSL has the characteristic that the data can flow faster in one direction than the other, i.e., asymmetrically.

For conventional ADSL, downstream rates start at 256 kbit/s up to 9 Mbit/s (about 300 m from the central office) but can go as high as 52 Mbit/s over short ranges of within 100 metres (so-called VDSL). Upstream rates start at 64 kbit/s and typically reach 256 kbit/s but can go as high as 768 kbit/s. The name ADSL Lite is sometimes used for the slower versions.

A newer variant called ADSL2 provides higher downstream rates (up to 12 Mbit/s for spans of less than 2.5 km). Higher symbol rates and more advanced noise-shaping are responsible for these increased speeds. ADSL2+ boosts these rates to up to 25 Mbit/s for spans of less than 1.5 km.

Because of the relatively low data-rate (compared to optical backbone networks) ATM is an appropriate technology for multiplexing time-critical data such as digital voice with less time-critical data such as Web traffic; ATM runs widely over ADSL technology to ensure that this remains a possibility.
 
  
 Amplitude modulation (AM) is a method used to modulate a signal, typically using radio. In the case of an analog signal to be sent, the amplitude of the radio wave is modulated to be directly proportional to the value of the analog signal at the time. This should be compared to frequency modulation (FM), which modulates the frequency rather than the amplitude.

Amplitude modulation typically produces a modulated output signal that has twice the bandwidth of the modulating signal, with a significant power component at the original carrier frequency.
 
  
 AMI is a line encoding technique (line code) for T1s which uses bipolar pulses to represent logical 1 values. It is therefore a three-level system. A logical 0 is represented by no symbol, and a logical 1 by pulses of alternating polarity. The alternating coding prevents the build-up of a DC voltage in the cable. This is considered an advantage since the cable may be used to carry a small DC current to power intermediate equipment such as line repeaters.

AMI suffers the drawback that a long run of 0's produces no transitions in the data stream (not containing sufficient transitions to guarantee lock of a DPLL). Therefore this type of encoding is not transparent to the sequence of bits being sent. For voice traffic the least significant bit of each frame (every eighth bit) is always set to '1', generating enough transitions on the line. This is acceptable for voice, but not for data.

Example of AMI encoding: The pattern of bits "1 0 0 0 0 1 1 0" encodes to "+ 0 0 0 0 - + 0".
 
  
 Distortion occurring in a system, subsystem, or device when the output amplitude is not a linear function of the input amplitude under specified conditions.

Amplitude distortion is measured with the system operating under steady-state conditions with a sinusoidal input signal. When other frequencies are present, the term amplitude refers to that of the fundamental frecuency only.
 
  
 An analog (analogue in British English spelling) signal is any continuously variable signal. It differs from a digital signal in that small fluctuations in the signal are meaningful. Analog is usually thought of in an electrical context, however mechanical, pneumatic, hydraulic, and other systems may also use analog signals.

The primary disadvantage of analog signalling is that any system has noise -that is, random variations- in it. As the signal is copied and re-copied, or transmitted over long distances, these random variations become dominant. Electrically these losses are lessened by shielding, good connections, and several cable types such as coax and twisted pair.
 
  
 Modulation in which the phase or frequency of a sinusoidal carrier is varied with the information contained in the modulating waveform. Angle modulation has two forms: phase and frequency modulation.
 
  
 The application layer is the 7th level of the seven-layer OSI model. This layer interfaces directly to and performs common application services for the application processes; it also issues requests to the Presentation Layer.

The common application services provide semantic conversion between associated application processes. Examples of common application services of general interest include the virtual file, virtual terminal, and job transfer and manipulation protocols.
 
  
 A method for finding a host's Ethernet address from its Internet address. The sender broadcasts an ARP packet containing the Internet address of another host and waits for it (or some other host) to send back its Ethernet address. Each host maintains a cache of address translations to reduce delay and loading. ARP allows the Internet address to be independent of the Ethernet address but it only works if all hosts support it.
 
  
 Automatic Repeat-reQuest is an error control technique for data transmission in which the receiver detects transmission errors in a message and automatically requests a retransmission from the transmitter. Few types of ARQ protocols are Stop-and-wait ARQ, Go-Back-N ARQ and Selective Repeat ARQ.
 
  
 Asynchronous Time-Division Multiplexing (ATDM) is a method of Time Division Multiplexing (TDM) where the time slots are allocated as needed rather than preassigned.
 
  
 ATM is a cell relay network protocol which encodes data traffic into small fixed sized (53 byte) cells instead of variable sized packets as in packet-switched networks (such as the Internet Protocol or Ethernet).

ATM was intended to provide a single unified networking standard that could support both synchronous channel networking (PDH, SDH) and packet-based networking (IP, Frame relay, etc), whilst supporting multiple levels of quality of service for packet traffic.
 
  
 The term backbone refers to the high-traffic-density connectivity portion of any communications network. In packet-switched networks, a backbone is a primary forward-direction path traced sequentially through two or more major relay or switching stations, consisting mainly of switches and interswitch trunks.
 
  
 The original band of frequencies produced by a transducer, such as a microphone, telegraph key, or other signal-initiating device, prior to initial modulation.

In transmission systems, the baseband signal is usually used to modulate a carrier. Demodulation re-creates the baseband signal. Baseband frequencies are usually characterized by being much lower in frequency than the frequencies that result when the baseband signal is used to modulate a carrier or subcarrier.
 
  
 In telecommunication, a Block Check Character is a character added to a transmission block to facilitate error detection. In longitudinal redundancy checking (LRC) and cyclic redundancy checking (CRC), block check characters are computed for, and added to, each message block transmitted. This block check character is compared with a second block check character computed by the receiver to determine whether the transmission is error free.
 
  
 A bit (abbreviated b) is the most basic information unit used in computing and information theory. A single bit is a zero or a one, or a true or a false, or for that matter any two mutually exclusive states.

Terms for large quantities of bits can be formed using the standard range of prefixes, e.g., kilobit (kbit), megabit (Mbit) and gigabit (Gbit). Note that much confusion exists regarding these units and their abbreviations. Although it is clearer symbology to use "bit" for the bit and "b" for the byte, "b" is often used for bit and "B" for byte.
 
  
 The insertion of noninformation bits into data (it should not be confused with overhead bits).

In data transmission, bit stuffing is used for various purposes, such as for synchronizing bit streams that do not necessarily have the same or rationally related bit rates, or to fill buffers or frames. The location of the stuffing bits is communicated to the receiving end of the data link, where these extra bits are removed to return the bit streams to their original bit rates or form. Bit stuffing may be used to synchronize several channels before multiplexing or to rate-match two single channels to each other.
 
  
 In the processing or transmission of digital data, an error-control procedure that is used to determine whether a block of data is structured according to given rules.
 
  
 An error detection and/or correction code in which the encoded block consists of N symbols, containing K information symbols (K < N ) and N-K redundant check symbols, such that most naturally occurring errors can be detected and/or corrected. A neat example of a block code is the (7,4) Hamming code, which transmits N=7 bits for every K=4 source bits.
 
  
 The designation of one or more bits in a block as parity bits used to force the block into a selected parity, either odd or even. Block parity is used to assist in error detection or correction Code.
 
  
 A functional unit that interconnects two local area networks that use the same logical link control procedure, but may use different medium access control procedures.

In communications networks, it is a device that links or routes signals from one ring or bus to another or from one network to another, may extend the distance span and capacity of a single LAN system, performs no modification to packets or messages. It operates at the data-link layer of the OSI-Reference Model (Layer 2), reads packets, and passes only those with addresses on the same segment of the network as the originating user.
 
  
 To simultaneously send the same message or signal to multiple recipients.

In computer networking, a broadcast address is an IP address that allows information to be sent to all machines on a given subnet rather than a specific machine. The exact notation can vary by operating system, but the standard is laid out in RFC 919. Generally, it is found by taking the logical bit complement of the subnet mask and then logically ORing this with the IP address.
 
  
 In computer architecture, a bus is a subsystem that transfers data or power between computer components inside a computer or between computers. Unlike a point-to-point connection, a bus can logically connect several peripherals over the same set of wires.
 
  
 In computing, a client is a system that accesses a (remote) service on another computer by some kind of network.

The client-server model is used on the Internet, where a user may connect to a service operating on a remote system through the internet protocol suite. Typically, a client is a software program, such as a web browser, that runs on a user's local computer or workstation and connects to a server as necessary and retrieve web pages for display. Most people use e-mail clients to retrieve their e-mail from their ISP's mail storage servers. Online chat uses a variety of clients, which vary depending on the chat protocol being used.
 
  
 In geometry, coaxial means that two or more forms share a common axis; it is the three-dimensional analog of "concentric".

Coaxial cable, for example, has a conducting wire in the center and a second conducting layer running all the way around the exterior circumference, under the insulation. Coaxial cable is used primarily for wideband, video, or rf applications.
 
  
 In a data transmission system, the situation that occurs when two or more demands are made simultaneously on equipment that can handle only one at any given instant.
 
  
 A mathematical technique used to check for errors when sending data by modem. It is a type of hash function used to produce a checksum which is a small integer from a large block of data, such as network traffic or computer files, in order to detect errors in transmission or duplication. CRCs are calculated before and after transmission or duplication, and compared to confirm that they are the same. The most widely used CRC calculations are constructed in ways such that certain types of errors, such as those due to noise in transmission channels, are almost always detected.
 
  
 CSMA is a Media Access Control (MAC) protocol in which a node verifies the absence of other traffic before transmitting on a shared physical medium, such as an electrical bus, or a band of electromagnetic spectrum.

"Carrier Sense" describes the fact that a transmitter listens for carrier wave before trying to send. That is, it tries to detect the presence of an encoded signal from another station before attempting to transmit.
"Multiple Access" describes the fact that multiple nodes may concurrently send and receive on the medium.

In pure CSMA a transmitter does not detect collisions, and a receiver cannot distinguish between collisions and other sources of frame errors. Collision recovery relies on the ability of the receiver to detect frame error and invoke an error recovery procedure. For example, it may not send a required ACK, causing transmitters to time out and retry.

Two common modifications to CSMA, Collision Detection and Collision Avoidance, attempt to improve performance by addressing these problems.
 
  
 Carrier Sense Multiple Access with Collision Avoidance is a network control protocol in which a data station that intends to transmit sends a jam signal. After waiting a sufficient time for all stations to receive the jam signal, the data station transmits a frame, and while transmitting, if the data station detects a jam signal from another station, it stops transmitting for a random time and then tries again.
 
  
 Carrier Sense Multiple Access with Collision Detection is a network control protocol in which a carrier sensing scheme is used and a transmitting data station that detects another signal while transmitting a frame, stops transmitting that frame, transmits a jam signal, and then waits for a random time interval before trying to send that frame again.
 
  
 The condition that exists when data is unchanged from its source and has not been accidentally or maliciously modified, altered, or destroyed.
 
  
 The data link layer is the 2nd of the seven-level OSI model. It responds to service requests from the network layer and issues service requests to the physical layer. The Data Link Layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical Layer. Examples of data link protocols are HDLC and ADCCP for point-to-point or packet-switched networks and LLC for local area networks.
 
  
 A datagram is a self-contained packet,one which contains enough information in the header to allow the network to forward it to the destination independently of previous or future datagrams. Unlike with virtual circuit protocols, when datagrams are sent there are no call establishment or clearing procedures. Thus, the network may not be able to provide protection against loss, duplication, or misdelivery.
 
  
 The amount of time by which an event is retarded.
 
  
 A digital system is one that uses discrete values rather than a continuous spectrum of values, compared to analog. The word comes from the same source as the word digit: the Latin word for finger (counting on the fingers) as these are used for discrete counting.

A digital signal is a discrete signal which has been sampled in amplitude in addition to time.
 
  
 The Domain Name System or DNS is a system that stores information about host names and domain names in a kind of distributed database on networks, such as the Internet. Most importantly, it provides an IP address for each host name, and lists the mail exchange servers accepting e-mail for each domain.

The DNS provides a vital service on the Internet, because while computers and network hardware work with IP addresses to perform tasks such as addressing and routing, humans generally find it easier to work with host names and domain names, for example in URLs and e-mail addresses. The DNS therefore mediates between the needs and preferences of humans and of software.
 
  
 In telecommunications, duplex means "two-way" when referring to communications channels.

   - A simplex channel is a one-way channel.
   - A half-duplex channel is one that can carry information in both directions, but not at the same time.
   - A full-duplex channel is a channel which can carry information in both directions at once.
 
  
 A wave that has been reflected or otherwise returned with sufficient magnitude and delay to be perceived. Echoes are frequently measured in dB relative to the directly transmitted wave. Echoes may be desirable (as in radar) or undesirable (as in telephone systems).
 
  
 A check to determine the integrity of transmission of data, whereby the received data are returned to the source for comparison with the originally transmitted data. Synonym: loop check.
 
  
 To append redundant check symbols to a message for the purpose of generating an error detection and correction code.
 
  
 Any technique that will detect or correct errors by introducing more signal elements than are necessary to convey the basic information. Error control is divided in two main categories:

Error Detection: Redundancy allows a receiver to check whether the received data has been corrupted during transmission. He can for example request a retransmission.

Error Correction: This type of error control allows a receiver to reconstruct the original information when it has been corrupted during transmission. This is especially useful when there is only one-way communication.
 
  
 A code in which each data signal conforms to specific rules of construction so that departures from this construction in the received signal can generally be automatically detected and corrected. If the number of errors is less than or equal to the maximum correctable threshold of the code, all errors will be corrected. The two main classes of error-correcting codes are block codes and convolutional codes.
 
  
 A code in which each data signal conforms to specific rules of construction, so that departures from this construction in the received signal can generally be detected automatically. The two main classes of error-detecting codes are block codes and convolutional codes.
 
  
 Ethernet (this name comes from the physical concept of ether) is a frame based computer networking technology for local area networks (LANs). It defines wiring and signaling for the physical layer, and frame formats and protocols for the media access control (MAC)/data link layer of the OSI model. Ethernet is mostly standardized as IEEEs 802.3. It has become the most widespread LAN technology in use during the 1990s to the present, and has largely replaced all other LAN standards such as token ring, FDDI, and ARCNET.
 
  
 In ASCII, ETX is a short name for the "End of Text" control character (code 3). It is often used as a "break" character (Ctrl-C). On MS-DOS systems, however, the "end of text" is marked by the Ctrl-Z character (code 26, "Substitute").
 
  
 An online file that contains frequently asked questions with answers provided to assist new users and avoid repetitive offline inquiries.
 
  
 Is a standard for data transmission in a local area network that can extend in range up to 200 km. The FDDI protocol is based on the token ring protocol. In addition to being large geographically, an FDDI local area network can support thousands of users. The underlying medium is optical fibre (though it can be copper cable, in which case it may be called CDDI) and the topology is a dual-attached, counter-rotating token ring.
 
  
 Frequency-division multiplexing is a form of signal multiplexing where multiple baseband signals are modulated on different frequency carrier waves and added together to create a composite signal.

FDM can also be used to combine multiple signals before final modulation onto a carrier wave. In this case the carrier signals are referred to as subcarriers: an example is stereo FM transmission, where a 38 KHz subcarrier is used to separate the left-right difference signal from the central left-right sum channel, prior to the frequency modulation of the composite signal.

Where frequency-division multiplexing is used as to allow multiple users to share a physical communications channel, it is called frequency-division multiple access (FDMA).
 
  
 FDMA is the oldest and most important of the three main ways for multiple radio transmitters to share the radio spectrum. The other two methods are Time Division Multiple Access (TDMA), and Code Division Multiple Access (CDMA).

In FDMA, each transmitter is assigned a distinct frequency channel so that receivers can discriminate among them by tuning to the desired channel. TDMA and CDMA are always used in combination with FDMA, i.e., a given frequency channel may be used for either TDMA or CDMA independently of signals on other frequency channels.
 
  
 In data transmission or processing, a flag is an indicator such as a signal, a symbol, a character or a digit, used for identification. A flag may be a bit, a byte or a letter that signals the occurrence of some condition or event. For example, it can be a Boolean variable that can be set to either 1 (Set, Raised, True) or 0 (Unset, False).
 
  
 Frequency modulation (FM) is a form of modulation which represents information as variations in the instantaneous frequency of a carrier wave (Contrast this with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant).

In analog applications, the carrier frequency is varied in direct proportion to changes in the amplitude of an input signal. Digital data can be represented by shifting the carrier frequency among a set of discrete values, a technique known as frequency-shift keying.
 
  
 Forward error correction (FEC) is a system of error control for data transmission wherein the receiving device has the capability to detect and correct fewer than a predetermined number or fraction of bits or symbols corrupted by transmission errors.

FEC is accomplished by adding redundancy to the transmitted information using a predetermined algorithm. Each redundant bit is invariably a complex function of many original information bits. The original information may or may not appear in the encoded output; codes that include the unmodified input in the output are systematic, while those that do not are nonsystematic.

The two main categories of FEC are block coding and convolutional coding. Block codes work on fixed-size blocks of bits or symbols of predetermined size, while convolutional codes work on bit or symbol streams of arbitrary length.
 
  
 In telecommunications, a frame is a packet which has been encoded for transmission over a particular link:

  - In data transmission, the sequence of contiguous bits delimited by, and including, beginning and ending flag sequences.
  - In pulse-code modulation (PCM) systems, a set of consecutive time slots in which the position of each digit can be identified by reference to a frame-alignment signal.
  - In a time-division multiplexing (TDM) system, a repetitive group of signals resulting from a single sampling of all channels, including any required system information, such as additional synchronizing signals.
  - In ISDN, a block of variable length, labeled at the Data Link Layer of the OSI-Reference Model.
 
  
 Frame Relay is a high-performance WAN protocol that operates at the physical and data link layers of the OSI reference model. It was originally designed for use across Integrated Services Digital Network (ISDN) interfaces. Today, it is used over a variety of other network interfaces as well. Frame Relay is an example of a packet-switched technology, which enables end stations to dynamically share network resources.
 
  
 FTP or file transfer protocol is an application protocol, part of the TCP/IP protocol stack, used for transferring files between network nodes. FTP is defined in the RFC 959.
 
  
 A fully connected topology, also called complete topology or full mesh topology, is a network topology in which there is a direct link between all pairs of nodes. In a fully connected network with n nodes, there are n(n-1)/2 direct links. Networks designed with this topology are usually very expensive to set up, but have a high amount of reliability due to multiple paths data can travel on.
 
  
 In telecommunications, the term gateway has the following meanings:

 - In a communications network, a network node equipped for interfacing with another network that uses different protocols.
    - A gateway may contain devices such as protocol translators, impedance matching devices, rate converters, fault isolators, or signal translators as necessary to provide system interoperability. It also requires the establishment of mutually acceptable administrative procedures between the two networks.
    - A protocol translation/mapping gateway interconnects networks with different network protocol technologies by performing the required protocol conversions.
 - Loosely, a computer configured to perform the tasks of a gateway.

Gateways, also called protocol converters, can operate at any layer of the OSI model. The job of a gateway is much more complex than that of a router or switch. Typically, a gateway must convert one protocol stack into another.
 
  
 GSM is a standard for mobile phones. The ubiquity of the GSM standard makes international roaming very common with "roaming agreements" between operators. It differs significantly from its predecessors in that both signalling and speech channels are digital, which means that it is seen as a second generation (2G) mobile phone system. GSM is an open standard which is developed by the 3GPP.

GSM has retained backward-compatibility with the original GSM phones. At the same time, the GSM standard continues to develop and packet data capabilities were added in the Release'97 version of the standard with GPRS. Higher speed data transmission has been introduced by providing a new modulation scheme with EDGE (Enhanced Data rates for GSM Evolution).
 
  
 H.323 is an ITU VOIP protocol. It was created at about the same time as SIP, but was more widely adopted and deployed earlier. Today, most of the world's VoIP traffic is carried over H.323 networks, with billions of minutes of traffic being carried every month.

H.323's strengths lie in its ability to serve in a variey of roles, including multimedia communication (voice, video, and data conferencing), as well as applications where interworking with the PSTN is vital. H.323 was designed from the outset with multimedia communications over IP networks in mind, making it the perfect solution for real-time multimedia communication over packet-based networks.

**
 
  
 An error-detecting and error-correcting binary code, used in data transmission, that can detect all single- and double-bit errors and correct all single-bit errors. Note: A Hamming code satisfies the relation 2m n 1, where n is the total number of bits in the block, k is the number of information bits in the block, and m is the number of check bits in the block, where m = n- k
 
  
 The number of digit positions in which the corresponding digits of two binary words of the same length are different. Note 1: The Hamming distance between 1011101 and 1001001 is two. Note 2: The concept can be extended to other notation systems. For example, the Hamming distance between 2143896 and 2233796 is three, and between toned and roses it is also three. Synonym signal distance.
 
  
 High-level data link control - A Link-Level protocol used to facilitate reliable point-to-point transmission of a data packet.
 
  
 Half-duplex operation - Operation in which communication between two terminals occurs in either direction, but in only one direction at a time. Half-duplex operation may occur on a half-duplex circuit or on a duplex circuit, but it may not occur on a simplex circuit. Synonyms one-way reversible operation, two-way alternate operation.
 
  
 A collision that occurs on a communications channel when two or more users begin to transmit on the channel at approximately the same instant.
 
  
 The portion of a message that contains information used to guide the message to the correct destination. Examples of items that may be in a header are the addresses of the sender and receiver, precedence level, routing instructions, and synchronizing bits.
 
  
 Hypertext Transfer Protocol In the World Wide Web, a protocol that facilitates the transfer of hypertext-based files between local and remote systems.
 
  
 A combination of any two or more network topologies. Instances can occur where two basic network topologies, when connected together, can still retain the basic network character, and therefore not be a hybrid network. For example, a tree network connected to a tree network is still a tree network. Therefore, a hybrid network accrues only when two basic networks are connected and the resulting network topology fails to meet one of the basic topology definitions. For example, two star networks connected together exhibit hybrid network topologies. A hybrid topology always accrues when two different basic network topologies are connected.
 
  
 Internet Control Message Protocol - An Internet protocol that reports datagram delivery errors. ICMP is a key part of the TCP/IP protocol suite.The packet internet gopher (ping) application is based on ICMP.
 
  
 1. The meaning that a human assigns to data by means of the known conventions used in their representation.
2. In intelligence usage, unprocessed data of every description which may be used in the production of intelligence.
 
  
 The process of moving messages containing user information from a source to a sink. The information transfer rate may or may not be equal to the transmission modulation rate.
 
  
 A worldwide interconnection of individual networks operated by government, industry, academia, and private parties.The Internet originally served to interconnect laboratories engaged in government research, and has now been expanded to serve millions of users and a multitude of purposes.
 
  
 The process of interconnecting two or more individual networks to facilitate communications among their respective nodes. The interconnected networks may be different types. Each network is distinct, with its own addresses, internal protocols, access methods, and administration.
 
  
 Internet protocol - A DOD standard protocol designed for use in interconnected systems of packet-switched computer communication networks. The internet protocol provides for transmitting blocks of data called datagrams from sources to destinations, where sources and destinations are hosts identified by fixed-length addresses. The internet protocol also provides for fragmentation and reassembly of long datagrams, if necessary, for transmission through small-packet networks.
 
  
 An Internet Protocol (IP) which is designed to be an evolutionary step from IPv4. It is a natural increment to IPv4. It can be installed as a normal software upgrade in internet devices and is interoperable with the current IPv4. Its deployment strategy is designed to not have any flag days or other dependencies. IPng is designed to run well on high performance networks and at the same time is still efficient for low bandwidth networks. In addition, it provides a platform for new Internet functionality that will be required in the near future. IPng is intended to support Internet traffic for many years into the future by providing enhancements over the capabilities of the existing IPv4 service. IPng Implementation and Specifications IPv6 Security vs. SSL
 
  
 Integrated services digital network - An integrated digital network in which the same time-division switches and digital transmission paths are used to establish connections for different services. ISDN services include telephone, data, electronic mail, and facsimile. The method used to accomplish a connection is often specified: for example, switched connection, nonswitched connection, exchange connection, ISDN connection.
 
  
 International Organization for Standardization - An international organization that consists of member bodies that are the national standards bodies of most of the countries of the world, is responsible for the development and publication of international standards in various technical fields, after developing a suitable consensus, is affiliated with the United Nations, and has its headquarters at 1, rue de Varembé, Geneva, Switzerland.
 
  
 International Telecommunication Union - A civil international organization established to promote standardized telecommunications on a worldwide basis. The ITU-R and ITU-T are committees under the ITU. The ITU headquarters is located in Geneva, Switzerland. While older than the United Nations, it is recognized by the U.N. as the specialized agency for telecommunications.
 
  
 Local area network - A data communications system that lies within a limited spatial area, has a specific user group, has a specific topology, and (d) is not a public switched telecommunications network, but may be connected to one. LANs are usually restricted to relatively small areas, such as rooms, buildings, ships, and aircraft. A interconnection of LANs within a limited geographical area, such as a military base, is commonly referred to as a campus area network. An interconnection of LANs over a city-wide geographical area is commonly called a metropolitan area network (MAN). An interconnection of LANs over large geographical areas, such as nationwide, is commonly called a wide area network (WAN). LANs are not subject to public telecommunications regulations.
 
  
 link access procedure D - A link protocol used in ISDN.
 
  
 In telecommunications networks and open systems architecture, a group of related functions that are performed in a given level in a hierarchy of groups of related functions. Note: In specifying the functions for a given layer, the assumption is made that the specified functions for the layers below are performed, except for the lowest layer.
 
  
 Logicallink control sublayer - In a local-area-network/metropolitan-area-network (LAN/MAN) system, the part of the link level that supports medium-independent data link functions and uses the services of the medium access control sublayer to provide services to the network layer.
 
  
 Longitudinal redundancy check - A system of error control based on the formation of a block check following preset rules. The block check formation rules are applied in the same manner to each character.
 
  
 Medium access protocol - In a communications network, the part of the data link layer that supports topology-dependent functions and uses the services of the physical layer to provide services to the logical link control.
 
  
 Metropolitan area network - A data communications network that covers an area larger than a campus area network and smaller than a wide area network (WAN), interconnects two or more LANs, and usually covers an entire metropolitan area, such as a large city and its suburbs.
 
  
 A code in which data and clock signals are combined to form a single self-synchronizing data stream, each encoded bit contains a transition at the midpoint of a bit period, the direction of transition determines whether the bit is a 0 or a 1, and the first half is the true bit value and the second half is the complement of the true bit value. Contrast with non-return-to-zero.
 
  
 The maximum rate, in bits per second, at which binary information can be transferred in a given direction between users over the telecommunications system facilities dedicated to a particular information transfer transaction, under conditions of continuous transmission and no overhead information.
 
  
 1. In telecommunications, the transmission path along which a signal propagates, such as a wire pair, coaxial cable, waveguide, optical fiber, or radio path.
2. The material on which data are or may be recorded, such as plain paper, paper tapes, punched cards, magnetic tapes, magnetic disks, or optical disks.
 
  
 A network topology in which there are at least two nodes with two or more paths between them.
 
  
 In telecommunications record information expressed in plain or encrypted language and prepared in a format specified for intended transmission by a telecommunications system.
 
  
 A method of handling message traffic through a switching center, either from local users or from other switching centers, whereby the message traffic is stored and forwarded through the system.
 
  
 A station in the mobile service intended to be used while in motion or during halts at unspecified points.
 
  
 The process, or result of the process, of varying a characteristic of a carrier, in accordance with an information-bearing signal.
 
  
 In a network, a technique that allows data, including packet form, to be simultaneously transmitted to a selected set of destinations. Note: Some networks, such as Ethernet, support multicast by allowing a network interface to belong to one or more multicast groups.
 
  
 The combining of two or more information channels onto a common transmission medium. In electrical communications, the two basic forms of multiplexing are time-division multiplexing (TDM) and frequency-division multiplexing (FDM). In optical communications, the analog of FDM is referred to as wavelength-division multiplexing (WDM).
 
  
 A negative-acknowledge character - A transmission control character sent by a station as a negative response to the station with which the connection has been set up. In binary synchronous communication protocol, the NAK is used to indicate that an error was detected in the previously received block and that the receiver is ready to accept retransmission of that block. In multipoint systems, the NAK is used as the not-ready reply to a poll.
 
  
 Network control program - In a switch or network node, software designed to store and forward frames between nodes. An NCP may be used in local area networks or larger networks.
 
  
 An interconnection of usually passive electronic components that performs a specific function (which is usually limited in scope), e.g., to simulate a transmission line or to perform a mathematical function such as integration or differentiation. Note: A network may be part of a larger circuit.
 
  
 Layer 3. This layer responds to service requests from the Transport Layer and issues service requests to the Data Link Layer. The Network Layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks while maintaining the quality of service requested by the Transport Layer. The Network Layer performs network routing, flow control, segmentation/desegmentation, and error control functions.
 
  
 The execution of the set of functions required for controlling, planning, allocating, deploying, coordinating, and monitoring the resources of a telecommunications network, including performing functions such as initial network planning, frequency allocation, predetermined traffic routing to support load balancing, cryptographic key distribution authorization, configuration management, fault management, security management, performance management, and accounting management.
 
  
 The specific physical, i.e., real, or logical, i.e., virtual, arrangement of the elements of a network. Note 1: Two networks have the same topology if the connection configuration is the same, although the networks may differ in physical interconnections, distances between nodes, transmission rates, and/or signal types.
 
  
 Is software that controls a network and its message (e.g. packet) traffic and queues, controls access by multiple users to network resources such as files, and provides for certain administrative functions, including security.
 
  
 Non-return-to-zero - A code in which 1s are represented by one significant condition and 0s are represented by another, with no neutral or rest condition, such as a zero amplitude in amplitude modulation (AM), zero phase shift in phase-shift keying (PSK), or mid-frequency in frequency-shift keying (FSK). Contrast with Manchester code, return-to-zero. For a given data signaling rate, i.e., bit rate, the NRZ code requires only one-half the bandwidth required by the Manchester code.
 
  
 Communication in which information is always transferred in only one preassigned direction. One-way communication is not necessarily constrained to one transmission path. Examples of one-way communications systems include broadcast stations, one-way intercom systems, and wireline news services.
 
  
 A telecommunications cable in which one or more optical fibers are used as the propagation medium. The optical fibers are surrounded by buffers, strength members, and jackets for protection, stiffness, and strength. A fiber-optic cable may be an all-fiber cable, or contain both optical fibers and metallic conductors. One possible use for the metallic conductors is the transmission of electric power for repeaters. Synonyms optical cable, optical fiber cable.

*
 
  
 Open Systems Interconnection Pertaining to the logical structure for communications networks standardized by the International Organization for Standardization (ISO). Adherence to the standard enables any OSI-compliant system to communicate with any other OSI-compliant system for a meaningful exchange of information.
 
  
 A cable made up of one or more separately insulated twisted-wire pairs, none of which is arranged with another to form quads.
 
  
 The structure of data, address, and control information in a packet. (188) Note: The size and content of the various fields in a packet are defined by a set of rules that are used to assemble the packet.
 
  
 The process of routing and transferring data by means of addressed packets so that a channel is occupied during the transmission of the packet only, and upon completion of the transmission the channel is made available for the transfer of other traffic.
 
  
 Pulse-amplitude modulation - Modulation in which the amplitude of individual, regularly spaced pulses in a pulse train is varied in accordance with some characteristic of the modulating signal. The amplitude of the amplitude-modulated pulses conveys the information.
 
  
 A test that determines whether the number of ones or zeros in an array of binary digits is odd or even. Odd parity is standard for synchronous transmission and even parity for asynchronous transmission.
 
  
 In communications, patches may be made by means of a cord, i.e., a cable, known as a
 
  
 In automated systems, patches may be made electronically.
 
  
 Pulse-code modulation - Modulation in which a signal is sampled, and the magnitude (with respect to a fixed reference) of each sample is quantized and digitized for transmission over a common transmission medium. In conventional PCM, before being digitized, the analog data may be processed (e.g., compressed), but once digitized, the PCM signal is not subjected to further processing (e.g., digital compaction) before being multiplexed into the aggregate data stream. PCM pulse trains may be interleaved with pulse trains from other channels.
 
  
 Protocol data unit - In layered systems, a unit of data that is specified in a protocol of a given layer and that consists of protocol-control information of the given layer and possibly user data of that layer.
 
  
 In layered systems, one of a set of entities that are in the same layer or the equivalent layer of another system.
 
  
 Layer 1. The lowest of seven hierarchical layers. The Physical layer performs services requested by the Data Link Layer. The major functions and services performed by the physical layer are: establishment and termination of a connection to a communications medium participation in the process whereby the communication resources are effectively shared among multiple users, e.g., contention resolution and flow control and, ersion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel.
 
  
 Phase modulation - Angle modulation in which the phase angle of a carrier is caused to depart from its reference value by an amount proportional to the instantaneous value of the modulating signal.
 
  
 In a communications network, a point at which signals can enter or leave the network en route to or from another network.
 
  
 Layer 6. This layer responds to service requests from the Application Layer and issues service requests to the Session Layer. The Presentation Layer relieves the Application Layer of concern regarding syntactical differences in data representation within the end-user systems. Note: An example of a presentation service would be the conversion of an EBCDIC-coded text file to an ASCII-coded file.
 
  
 The dispatching, for reception elsewhere, of a signal, message, or other form of information. In communications systems, a series of data units, such as blocks, messages, or frames.
 
  
 Quadrature amplitude modulation - Modulation in which the two carriers are amplitude modulated.
 
  
 A code using more signal elements than necessary to represent the intrinsic information.The redundancy may be used for error-control purposes.
 
  
 A digital device that amplifies, reshapes, retimes, or performs a combination of any of these functions on a digital input signal for retransmission. The term repeater originated with telegraphy and referred to an electromechanical device used to regenerate telegraph signals. Use of the term has continued in telephony and data communications.
 
  
 In data transmission, the content of the control field of a response frame advising the primary station concerning the processing by the secondary station of one or more command frames.
 
  
 A digital code having two information states, e.g. , 0 and 1 or mark and space, in which code the signal returns to a rest state during a portion of the bit period.
 
  
 A network topology in which every node has exactly two branches connected to it.
 
  
 In data communications, a functional unit used to interconnect two or more networks. Routers operate at the network layer (layer 3) of the ISO Open Systems Interconnection Reference Model. The router reads the network layer address of all packets transmitted by a network, and forwards only those addressed to another network.
 
  
 The process of determining and prescribing the path or method to be used for establishing telephone connections or forwarding messages.
 
  
 A matrix associated with a network control protocol, which gives the hierarchy of link routing at each node.
 
  
 The number of samples taken per unit time, i.e., the rate at which signals are sampled for subsequent use, such as for modulation, coding, and quantization. Deprecated synonym sampling frequency.
 
  
 Service access point - In an Open Systems Interconnection (OSI) layer, a point at which a designated service may be obtained.
 
  
 A satellite system or a part of a satellite system, consisting of only one satellite and the cooperating Earth stations.
 
  
 Service data unit - In layered systems, a set of data that is sent by a user of the services of a given layer, and is transmitted to a peer service user semantically unchanged.
 
  
 Signaling in which the whole or a part of one or more channels in a multichannel system is used to provide for supervisory and control signals for the message traffic channels. (188) Note: The same channels, such as frequency bands or time slots, that are used for signaling are not used for message traffic. Contrast with common-channel signaling.
 
  
 A network device that provides service to the network users by managing shared resources. Note 1: The term is often used in the context of a client-server architecture for a local area network (LAN). Examples are a printer server and a file server.
 
  
 In the Open Systems Interconnection--Reference Model (OSI--RM), a capability of a given layer, and the layers below it, that is provided to the entities of the next higher layer and for a given layer, is provided at the interface between the given layer and the next higher layer.
 
  
 A time-dependent variation of a characteristic of a physical phenomenon, used to convey information.
 
  
 Signaling in which two conductors are used for a single channel, and a center-tapped coil, or its equivalent, is used to split the signaling current equally between the two conductors. SX signaling may be one-way, for intra-central-office use, or the simplex legs may be connected to form full duplex signaling circuits that function like composite (CX) signaling circuits with E & M lead control.
 
  
 SIP, the session intitation protocol, is the IETF protocol for VOIP and other text and multimedia sessions, like instant messaging, video, online games and other services.

The Session Initiation Protocol (SIP) is an application-layer control (signaling) protocol for creating, modifying, and terminating sessions with one or more participants. These sessions include Internet telephone calls, multimedia distribution, and multimedia conferences. It is a text-based protocol, similar to HTTP and SMTP.

SIP invitations used to create sessions carry session descriptions that allow participants to agree on a set of compatible media types. SIP makes use of elements called proxy servers to help route requests to the user's current location, authenticate and authorize users for services, implement provider call-routing policies, and provide features to users. SIP also provides a registration function that allows users to upload their current locations for use by proxy servers. SIP runs on top of several different transport protocols.
 
  
 A variable-duration window that allows a sender to transmit a specified number of data units before an acknowledgement is received or before a specified event occurs. An example of a sliding window in packet transmission is one in which, after the sender fails to receive an acknowledgement for the first transmitted packet, the sender slides the window, i.e., resets the window, and sends a second packet. This process is repeated for the specified number of times before the sender interrupts transmission. Synonym (loosely) acknowledgement delay period.
 
  
 In a distributed-queue dual-bus (DQDB) network, a protocol data unit (PDU) that consists of 53 octets used to transfer segments of user information has the capacity to contain a segment of 52 octets and a 1-octet access control field, and may be either a pre-arbitrated (PA) slot or a queued arbitrated (QA) slot.
 
  
 In networks using carrier sense multiple access with collision detection (CSMA/CD), the length of time that a transmitting station waits before attempting to retransmit following a collision. Note: Slot time varies from station to station.
 
  
 The Transmission Control Protocol/Internet Protocol (TCP/IP) standard protocol that facilitates transfer of electronic-mail messages, specifies how two systems are to interact, and specifies the format of messages used to control the transfer of electronic mail.\r\n
 
  
 Simple Network Management Protocol (SNMP): The Transmission Control Protocol/Internet Protocol (TCP/IP) standard protocol that is used to manage and control IP gateways and the networks to which they are attached, uses IP directly, bypassing the masking effects of TCP error correction, has direct access to IP datagrams on a network that may be operating abnormally, thus requiring management, defines a set of variables that the gateway must store, and specifies that all control operations on the gateway are a side-effect of fetching or storing those data variables, i.e., operations that are analogous to writing commands and reading status.
 
  
 Signal-to-noise ratio - The ratio of the amplitude of the desired signal to the amplitude of noise signals at a given point in time. SNR is expressed as 20 times the logarithm of the amplitude ratio, or 10 times the logarithm of the power ratio. SNR is usually expressed in dB and in terms of peak values for impulse noise and root-mean-square values for random noise. In defining or specifying the SNR, both the signal and noise should be characterized, e.g., peak-signal-to-peak-noise ratio, in order to avoid ambiguity.
 
  
 start-of-text character A transmission control character that precedes a text and may be used to terminate the message heading.
 
  
 In an Internet Protocol (IP) address, an extension that allows users in a network to use a single IP network address for multiple physical subnetworks. The IP address contains three parts: the network, the subnet, and host addresses. Inside the subnetwork, gateways and hosts divide the local portion of the IP address into a subnet address and a host address. Outside of the subnetwork, routing continues as usual by dividing the destination address into a network portion and a local portion.
 
  
 A collection of equipment and physical transmission media that forms an autonomous whole and that can be used to interconnect systems for purposes of communication.
 
  
 A comparison of checksums on the same data on different occasions or on different representations of the data in order to verify data integrity. Synonym sum check.
 
  
 In communications systems, a mechanical, electro-mechanical, or electronic device for making, breaking, or changing the connections in or among circuits.
 
  
 A communications network, such as the public switched telephone network, in which any user may be connected to any other user through the use of message, circuit, or packet switching and control devices
 
  
 synchronous idle character A transmission control character used in synchronous transmission systems to provide a signal from which synchronism or synchronous correction may be achieved between data terminal equipment, particularly when no other character is being transmitted.
 
  
 synchronization pulse - A pulse used to achieve or maintain synchronism. Note: The term synchronization pulse is usually applied to analog signals, whereas the term synchronization bit is usually applied to digital data streams. Synonym sync pulse.
 
  
 A multiplexing scheme in which timing is obtained from a clock that controls both the multiplexer and the channel source.
 
  
 In the Internet Protocol suite, a standard, connection-oriented, full-duplex, host-to-host protocol used over packet-switched computer communications networks. TCP corresponds closely to the ISO Open Systems Interconnection--Reference Model (OSI--RM) Layer 4 (Transport Layer). The OSI--RM uses TP-0 or TP-4 protocols for transmission control.\r\n
 
  
 Transmission Control Protocol/Internet Protocol - Two interrelated protocols that are part of the Internet protocol suite. TCP operates on the OSI Transport Layer and breaks data into packets. IP operates on the OSI Network Layer and routes packets. TCP/IP was originally developed by the U.S. Department of Defense.
 
  
 Time-division multiplexing - Digital multiplexing in which two or more apparently simultaneous channels are derived from a given frequency spectrum, i.e. , bit stream, by interleaving pulses representing bits from different channels. Successive pulses represent bits from successive channels, e.g., voice channels in a T1 system.
 
  
 Any transmission, emission, or reception of signs, signals, writings, images, sounds, or information of any nature by wire, radio, visual, or other electromagnetic systems.
 
  
 The branch of science devoted to the transmission, reception, and reproduction of sounds, such as speech and tones that represent digits for signaling. Transmission may be via various media, such as wire, optical fibers, or radio. Analog representations of sounds may be digitized, transmitted, and, on reception, converted back to analog form.Telephony originally entailed only the transmission of voice and voice-frequency data. Currently, it includes new services, such as the transmission of graphics information.
 
  
 The TCP/IP standard network virtual terminal protocol that is used for remote terminal connection service and that allows a user at one site to interact with systems at other sites as if that user terminal were directly connected to computers at those sites.
 
  
 A code used for the transmission and identification of time signals. In telecommunications systems, the format of the time code must be specified.
 
  
 Period of time during which certain activities are governed by specific regulations
 
  
 A network parameter related to an enforced event designed to occur at the conclusion of a predetermined elapsed time.
 
  
 In certain local-area-network protocols, a group of bits that serves as a symbol of authority, is passed among data stations, and is used to indicate the station that is temporarily in control of the transmission medium.
 
  
 A network access procedure in which a token passes from station to station and the only station allowed to transmit information is the station with the token.
 
  
 A quantitative measurement of the total messages and their length, expressed in CCS or other units, during a specified period of time.
 
  
 Any material substance, such as fiber-optic cable, twisted-wire pair, coaxial cable, dielectric-slab waveguide, water, and air, that can be used for the propagation of signals, usually in the form of modulated radio, light, or acoustic waves, from one point to another. By extension, free space can also be considered a transmission medium for electromagnetic waves, although it is not a material medium.
 
  
 In data communications systems, transmit flow control is control of the rate at which data are transmitted from a terminal so that the data can be received by another terminal. It may occur between data terminal equipment (DTE) and a switching center, via data circuit-terminating equipment (DCE), or between two DTEs. The transmission rate may be controlled because of network or DTE requirements.

Transmit flow control can occur independently in the two directions of data transfer, thus permitting the transfer rates in one direction to be different from the transfer rates in the other direction.

Transmit flow control can be either stop-and-go or use a sliding window.
 
  
 Layer 4 This layer responds to service requests from the Session Layer and issues service requests to the Network Layer. The purpose of the Transport Layer is to provide transparent transfer of data between end users, thus relieving the upper layers from any concern with providing reliable and cost-effective data transfer.
 
  
 In the Internet Protocol suite, a standard, low-overhead, connectionless, host-to-host protocol that is used over packet-switched computer communications networks, and that allows an application program on one computer to send a datagram to an application program on another computer. Note: The main difference between UDP and TCP is that UDP provides connectionless service, whereas TCP does not.\r\n
 
  
 virtual circuit A communications arrangement in which data from a source user may be passed to a destination user over various real circuit configurations during a single period of communication. : Virtual circuits are generally set up on a per-call basis and are disconnected when the call is terminated however, a permanent virtual circuit can be established as an option to provide a dedicated link between two facilities. Synonyms logical circuit, logical route.
 
  
 A network that provides virtual circuits and that is established by using the facilities of a real network.
 
  
 Vertical Redundancy Check In synchronized parallel bit streams, a redundancy check that is based on the formation of a block check following preset rules, in which the check-formation rule applied to blocks is also applied to characters, and in which the check is made on parallel bit patterns.
 
  
 wide area network A physical or logical network that provides data communications to a larger number of independent users than are usually served by a local area network (LAN) and is usually spread over a larger geographic area than that of a LAN. WANs may include physical networks, such as Integrated Services Digital Networks (ISDNs), X.25 networks, and T1 networks. A metropolitan area network (MAN) is a WAN that serves all the users in a metropolitan area. WANs may be nationwide or worldwide.
 
  
 A material medium that confines and guides a propagating electromagnetic wave. In the microwave regime, a waveguide normally consists of a hollow metallic conductor, usually rectangular, elliptical, or circular in cross section. This type of waveguide may, under certain conditions, contain a solid or gaseous dielectric material.
 
  
 wavelength-division multiplexing In optical fiber communications, any technique by which two or more optical signals having different wavelengths may be simultaneously transmitted in the same direction over one fiber, and then be separated by wavelength at the distant end.
 
  
 World Wide Web An international, virtual-network-based information service composed of Internet host computers that provide on-line information in a specific hypertext format. WWW servers provide hypertext metalanguage (HTML) formatted documents using the hypertext transfer protocol (HTTP). Information on the WWW is accessed with a hypertext browser such as Mosaic, Viola, or Lynx. No hierarchy exists in the WWW, and the same information may be found by many different approaches.