es per second, or once every 125 ms. Each one of The terms FXS and FXO were originally used within these samples consists of an 8-bit measurement for telephone company networks to describe provision of a total of 64,000 bits per second to be transmitted. As a telephone service from a central office other than final step, companding is used to provide greater accu- normally assigned.) racy of low-amplitude components. In North America, Within FXS and FXO interfaces, it is also necessary to this is u-law (mu-law), while elsewhere it is typically emulate variants in supervision. Typical telephones A-law. For international interworking purposes, it is operate in a loop start mode. The telephone normally agreed that the North American side will make the presents a high impedance between the two wires. When conversion. the receiver goes off-hook, a low-impedance closed cir- To construct a T1, 24 channels are assembled for cuit is created between the two wires. The switch, a total of 1.536 Mbps, and an additional 8 bits are Network infrastructure CHAPTER 3.2 added every 125 ms for framing, resulting in a rate traffic can change drastically from hour to hour. In some of 544 Mbps. Often, T1 frames are combined into systems, a different scheme, known as frequency division larger structures called SuperFrames (12 frames) and multiplexing (FDM), is preferred. Extended-SuperFrames (24 frames). "Robbing bits" from the interior frames can then transmit additional signaling. Voice over ATM Basic T1 and El interfaces emulate a collection of analog voice trunks and use robbed bit signaling to Many companies have ATM backbone infrastructure transfer supervisory information similar to the E&M and deploying convergence within their network and analog model. As such, each channel carries its own sig- outside their network. It is crucial to understand how naling, and the interface is called channel-associated an ATM network is configured before even considering signaling (CAS). A more efficient method uses convergence for voice/video/d