Compander in Pulse Code Modulation

Compander is a Signal processing technique which uses both compression and expansion to improve dynamic range and signal-to-noise ratio.

            Before the conversion of analog to digital in PCM, the compander boost up the low amplitudes of the voice.

In leaner quantization we give a low signal to distortion ratio and high ratio for high amplitudes. This is not useful, because if we get a low signal to distortion ratio, it let know the distortion is higher, compare to signal’s amplitude. Therefore we use compander for overcome that problem. What compander do is it only boost up the low amplitudes of the voice.

And also receiving end compander (expander) reduce the amplitude value of the boosted up signal.

But when we use nonlinear quantization there is a solution for this matter. It has lower gaps in quantized levels for low amplitudes and higher gaps in quantized levels for high amplitudes. Therefore we can achieve high signal to distortion ratio in both cases. And it is useful.

Analog over Digital

               The fundamental transmission concept is to transmit the minimum possible frequency or the bandwidth with maximum possible number of channels or the bit rate.

When we consider the analog transmission we can’t transmit much distance than digital transmission. Therefore we need to use small bandwidth. Using that small bandwidth can transmit much distance. Therefore we use modulation techniques to reduce the bandwidth. FDM multiplexing system is combining number of channels to convey over common transmission medium. Major advantage of analogue transmission is occupied band width is low When we use frequency modulation in low pass filter there are two, out come upper side bandwidth and lower side bandwidth. By choosing the lower side bandwidth we can reduce bandwidth of the signal.

             In digital transmission we can have more than solutions compare to analog.

Theoretically we know Nyquist Theorem, it tells us relationship between bandwidth and channel capacity.

                             C = 2Blog₂M

C- Channel capacity   ,    B – Bandwidth       ,    M – signal levels

If we want increase channel capacity for a value of bandwidth need to increase signal levels.

              In digital transmission we can use time division multiplexing (TDM).using this technique we can reduce bandwidth. In this TDM large numbers of channels are put into one line and transmit. . In the digital transmission can use the available bandwidth of the channel much more efficiently as compared to analogue system.

The methods used for data communication over the PSTN

Normally the earlier PSTN is designed for carry voice, but after has been developed to carry data as well as voice.

There are many methods used for data communication over the PSTN. They are PSDN, ISDN, Dial up and  X DSL

       

PSDN (Public switched data Network) is runs over normal PSTN line. It is a Packet Switched data network that based on packet switching as opposed to circuit switching that used in PSTN. When we are connecting to the internet (WAN) we use normal PSTN twisted pair cabled. In this technology packets are routing according to the source and destination address.

 Integrated Services Digital Network (ISDN) turns the local loop into a TDM digital connection. It is purely digital network but runs on analog twister pair lines. Speed is one advantage ISDN has over telephone line connections. There are two ISDN implementations. They are,

 Basic Rate Interface (BRI)

 B channels – Bandwidth of 64kbps

D channels – Bandwidth of 16 kbps

          Designed as 2B + D

Primary rate Interface (PRI)

  B & D – Bandwidth of 64kbps

 The design varies according to the nation.

 Ex – North America & Japan 23B+D      

  

 

SDN Carries voice calls and data simultaneously on one line. It uses time division multiplexing (TDM) over two wire pairs. TDM Divides single channel into multiple channels.      

                         Dial-up connection also uses the PSTN for connect with WAN. It provides a low cost need based access. We can achieve bit rate up to 56kbps.In this method Modem is connected to a Telephone Line on the Customer End. Remote Access Server (RAS) is connected to Telephone Lines (33.6 Kbps connectivity) or E1/R2 Line (56 Kbps connectivity) on the Service Provider End. Then RAS provide dialing connectivity, authentication and metering. Disadvantage of this technology is achievable bandwidth depends on the line quality and the traffic of the network.

 

Digital Subscriber Line (DSL) technology is a broadband technology that uses existing twisted-pair telephone lines to transport high-bandwidth data to service subscribers. The term XDSL covers a number of similar yet competing forms of DSL technologies.DSL technology allows the local loop line to be used for normal telephone voice connection and an always-on connection for instant network connectivity. The two basic types of DSL technologies are asymmetric (ADSL) and symmetric (SDSL).  All forms of DSL service are categorized as ADSL or SDSL and there are several varieties of each type. Asymmetric service provides higher download or downstream bandwidth to the user than upload bandwidth. Symmetric service provides the same capacity in both directions.

Multiple DSL subscriber lines are multiplexed into a single, high capacity link by the use of a DSL Access Multiplexer (DSLAM) at the provider location. DSLAMs incorporate TDM technology to aggregate many subscriber lines into a less cumbersome single medium, generally T3/DS3 connection techniques to achieve data rates up to 8.192 Mbps.