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H H.323 A recent standard for audio and video conferencing established by the International Telecommunication Union (ITU). H.323 is a comprehensive standard for multimedia communication among computers, terminals, network devices, and network services running on connectionless networks that do not support intrinsic quality of service (QoS) functions, such as Internet Protocol–based networks and the Internet. This standard is widely accepted by manufacturers of Voice over IP (VoIP) and Internet phone software and devices. Any hardware or software system that supports the H.323 standard can communicate with any other H.323 system. H.323 is designed to make multimedia communication over the Internet as standardized as telephone communication. For example, with telephone communication you can purchase telephone equipment from any vendor and plug it into your phone system, and the equipment works. The idea is to make Internet communication appliances just as easy to use. Half-duplex A mode of communication in which data can be transmitted or received, but cannot be transmitted and received simultaneously. The simplest example is a walkie-talkie: You have to press a button to talk and release the button to listen. When two people use walkie-talkies to communicate, at any given moment, only one of them can talk while the other listens. If both try to talk simultaneously, a collision occurs and neither hears what the other says. Communication through traditional Ethernet networks is another example of half-duplex communication. When one station on an Ethernet transmits, the other stations detect the carrier signal and listen instead of transmitting. If two stations transmit signals simultaneously, a collision occurs and both stations stop transmitting and wait random intervals of time before retransmitting. In contrast, full-duplex communication enables stations to transmit and receive signals simultaneously, with the advantage of providing twice the bandwidth of equivalent half-duplex technologies. However, full-duplex requires two communication channels to achieve these results—one to transmit and one to receive signals. A third mode of communication is called simplex, which involves transmission in one direction only, with one station tra H channel A designation for groups of channels on Basic Rate Interface ISDN (BRI-ISDN) services. H channel standards are defined by the International Telecommunication Union (ITU) and are composed of different combinations of Integrated Services Digital Network (ISDN) B channels. The most common configurations are as follows:
Header The initial portion of a packet or a frame. The header contains control information such as addressing, routing, and protocol version. The format of this information depends on the protocol being used. For example, an Internet Protocol (IP) header contains information about the version of the IP protocol, the length of the header, the type of service used, the packet’s Time to Live (TTL), the source and destination address, and so on. Headers are used to control the flow of packets through the network or over the communication link. The end of a frame sometimes has a smaller structure called a footer or trailer, but this usually contains only error-checking information. Control information is always placed in the header because this is the first portion of the packet or frame that is read by a networking device such as a switch or a router. High-bit-rate Digital Subscriber Line (HDSL) A modulation technology similar to Asymmetric Digital Subscriber Line (ADSL) that uses a group of existing copper twisted-pair subscriber telephone lines to transmit data at T1 or E1 speeds. High-bit-rate Digital Subscriber Line (HDSL) was the earliest version of Digital Subscriber Line (DSL) to be widely implemented; it is often used as a low-cost alternative to dedicated T1 links for wide area networks (WANs) and for building-to-building communication in a campuswide network High-level Data Link Control (HDLC) A standard synchronous communication protocol at the data-link layer (layer 2) of the Open Systems Interconnection (OSI) reference model that is used for wide area network (WAN) synchronous serial connections over leased lines. High-level Data Link Control (HDLC) was derived from Synchronous Data Link Control (SDLC) and was developed by the International Organization for Standardization (ISO) for point-to-point communication. It was later adapted by the International Telecommunication Union (ITU) for X.25 Link Access protocol.
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