LANs to WANs(c) The Complete Management Guide
Authors: Muller N.J.
 Published year: 2003
 Pages: 71-73/184
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8.5 Voice Compression

Even though 64 Kbps is the industry standard for encoding and transmitting voice using PCM, this is still a lot of bandwidth to allocate to voice traffic on a private network. Therefore, alternative voice-carrying capabilities can make more efficient use of the available bandwidth, which also would result in substantial cost savings. The voice compression algorithm ADPCM offers toll-quality voice at 32 Kbps, cutting the bandwidth requirement in half without sacrificing voice quality. Other implementations of ADPCM permit reliable voice transmission at 24 and 16 Kbps, which allows the user to further reduce the bandwidth required for voice.

The conversion from PCM voice to ADPCM voice can even be implemented “on the fly” without interrupting calls in progress. The conversion may be implemented through time-of-day reconfiguration, event-driven reconfiguration, or manually through keyboard command at the multiplexer’s system controller.

Other compression techniques can bring voice down to 10 Kbps or lower. These very low bit rate voice (VLBRV) techniques are typically employed in situations where the bandwidth is close to full utilization, such as during peak hours of the day, but the organization still has an additional requirement for voice and does not want to pay for more bandwidth. Because of the trade-off between quality and maximum bit compression, VLBRV schemes are more suited for internal communication than customer service lines, for example, where the detectable loss of quality may be a critical factor in providing support.


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8.6 Data Support

Multiplexers support only a limited selection of data rates because vendors typically required a different card to support each transmission rate. This practice not only inflated the initial hardware investment but compounded the cost and logistics of maintenance. Today’s multiplexers are more efficient, using only one type of data channel card to support a variety of data rates: 75 bps to 19.2 Kbps for asynchronous transmission and 75 bps to 1.544 or 2.048 Mbps for synchronous transmission. Under software control, multiple data types and a broad range of data rates are available to suit different applications.

To minimize disruption due to maintenance or failure, each channel card should have only one port. Multiport cards can leave large numbers of users stranded in case of failure. Each channel provided by a four-port card, for example, may be configured to provide up to 32 terminals with “logical” access to a cluster controller. This may appear to be very efficient, but when one channel fails, three other channels must be taken out of service until a card swap can be made.


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8.7 Clear Channel Capability

Support for the clear channel capability is another extremely desirable feature for T1 multiplexers. This option allows users to transmit DS0 or DS1 signals unconstrained by the general technical requirement that such signals not contain more than 15 consecutive zeros. This is achieved by implementing bipolar eight zero substitution (B8ZS). This line-coding technique uses bipolar violations to synchronize devices, a solution that does not require the use of extra bits, which means that a T1 circuit can use the full 64-Kbps channel for data, instead of only 56 Kbps. The scheme results in a gain of 192 Kbps of usable bandwidth on a full T1. The clear channel capability also applies to other types of T-carrier circuits, including T3.

This clear channel capability is required for ISDN, but as recently as year-end 2002, not all carriers supported it. The 64-Kbps clear channel capability is required for video applications over ISDN; carriers that provide only 56 Kbps cannot support video applications reliably.

To take advantage of the clear channel capability, the CPE must have the B8ZS capability and conform to ESF. For data applications involving frame relay or dedicated Internet access, the router or IAD will provide these features. Channel banks and multiplexers support B8ZS and ESF as well. For example, with the ESF capability integral to the multiplexer’s public network interface, bit-error rates can be picked up on a dynamic basis over the digital facility. On a static basis, the multiplexer’s transport management system comes into play by comparing error rates to predefined thresholds that are established by the network administrator when setting up the profile for each circuit.


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LANs to WANs(c) The Complete Management Guide
Authors: Muller N.J.
 Published year: 2003
 Pages: 71-73/184
Buy this book on amazon.com >>

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