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 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.

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 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.