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There is plenty to consider, including the cabling plant, when switching to IP telephony - Voice Networks
Matching carrier-grade voice quality is a unique challenge to the voice over IP (VoIP) services being introduced in today's market. With everything moving toward IP, can this medium be used to actually create a carrier-grade service?
A true carrier-grade service includes reliability, availability and scalability; in a VoIP environment, add in security, manageability and interoperability. The additional factors are necessary as the voice transmissions are carried over network links in conjunction with data services. Voice-grade services must have the continuous available capacity from the beginning to the end of the conversation. Latency, loss and jitter can all affect the quality of the voice service.
In a typical IP packet carrying data only, transmission through the network is fairly simple. The data packet is formed, sent and received. Small delays or retransmissions are accepted. Voice traffic, however, cannot tolerate errors. Voice packets need a mechanism to move at a higher priority so that their transmission is, in effect, guaranteed. Today, this is accomplished by setting the quality-of-service (QoS) bit in the IP header.
All IP headers have a type-of-service (ToS) byte, more recently defined as the diffServe code point field. QoS refers to a set of parameters for both connection-mode (TCP) and connectionless-mode (IP) transmissions, which provide for performance in terms of transmission quality and availability of service. It encompasses maximum delay, throughput and priority of the packets being transmitted.
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