Using Telephone and Cable Networks for Data Transmission

Definition of Terms

• The telephone, which is referred to as the plain old telephone system (POTS), was originally an analog system. During the last decade, the telephone network has undergone many technical changes. The network is now digital as well as analog.
• A home computer can access the Internet through the existing telephone system or through a cable TV system. 
• The telephone network is made of three major components: local loops, trunks, and switching offices. It has several levels of switching offices such as end offices, tandem offices, and regional offices. 
• Telephone companies provide two types of services: analog and digital.
• The United States is divided into many local access transport areas (LATAs). The services offered inside a LATA are called intra-LATA services. The carrier that handles these services is called a local exchange carrier (LEC). The services between LATAs are handled by interexchange carriers (lXCs).
• A LATA is a small or large metropolitan area that according to the divestiture of 1984 was under the control of a single telephone-service provider.
• In in-band signaling, the same circuit is used for both signaling and data. In out-of band signaling, a portion of the bandwidth is used for signaling and another portion for data. The protocol that is used for signaling in the telephone network is called Signaling System Seven (SS7).
• Telephone companies provide two types of services: analog and digital. We can categorize analog services as either analog switched services or analog leased services. The two most common digital services are switched/56 service and digital data service (DDS).
• Data transfer using the telephone local loop was traditionally done using a dial-up modem. The term modem is a composite word that refers to the two functional entities that make up the device: a signal modulator and a signal demodulator.
• Most popular modems available are based on the V-series standards. The V.32 modem has a data rate of 9600 bps. The V32bis modem supports 14,400-bps transmission. V90 modems, called 56K modems, with a downloading rate of 56 kbps and uploading rate of 33.6 kbps are very common. The standard above V90 is called V92. These modems can adjust their speed, and if the noise allows, they can upload data at the rate of 48 kbps.
• Telephone companies developed another technology, digital subscriber line (DSL), to provide higher-speed access to the Internet. DSL technology is a set of technologies, each differing in the first letter (ADSL, VDSL, HDSL, and SDSL. ADSL provides higher speed in the downstream direction than in the upstream direction. The high-bitrate digital subscriber line (HDSL) was designed as an alternative to the T-l line (1.544 Mbps). The symmetric digital subscriber line (SDSL) is a one twisted-pair version of HDSL. The very high-bit-rate digital subscriber line (VDSL) is an alternative approach that is similar to ADSL.
• DSL supports high-speed digital communications over the existing telephone local loops. 
• ADSL technology allows customers a bit rate of up to 1 Mbps in the upstream direction and up to 8 Mbps in the downstream direction. 
• ADSL uses a modulation technique called DMT which combines QAM and FDM.  
• ADSL is an asymmetric communication technology designed for residential users; it is not suitable for businesses.
• ADSL is an adaptive technology. The system uses a data rate based on the condition of the local loop line.
• SDSL, HDSL, and VDSL are other DSL technologies. 
• Theoretically, the coaxial cable used for cable TV allows Internet access with a bit rate of up to 12 Mbps in the upstream direction and up to 30 Mbps in the downstream direction. 
• An HFC network allows Internet access through a combination of fiber-optic and coaxial cables. 
• The coaxial cable bandwidth is divided into a video band, a downstream data band, and an upstream data band. Both upstream and downstream bands are shared among subscribers. 
• DOCSIS defines all protocols needed for data transmission on an HFC network. 
• Synchronous Optical Network (SONET) is a synchronous high-data-rate TDM network for fiber-optic networks. 
• SONET has defined a hierarchy of signals (similar to the DS hierarchy) called synchronous transport signals (STSs). 
• Optical carrier (OC) levels are the implementation of STSs. 
• A SONET frame can be viewed as a matrix of nine rows of 90 octets each. 
• SONET is backward compatible with the current DS hierarchy through the virtual tributary (VT) concept. VT's are a partial payload consisting of an m-by-n block of octets. An STS payload can be a combination of several VT's.
• STSs can be multiplexed to get a new STS with a higher data range.
• Community antenna TV (CATV) was originally designed to provide video services for the community. The traditional cable TV system used coaxial cable end to end. The second generation of cable networks is called a hybrid fiber-coaxial (HFC) network. The network uses a combination of fiber-optic and coaxial cable.
• Communication in the traditional cable TV network is unidirectional.
• Communication in an HFC cable TV network can be bidirectional.
• To provide Internet access, the cable company has divided the available bandwidth of the coaxial cable into three bands: video, downstream data, and upstream data. The downstream-only video band occupies frequencies from 54 to 550 MHz. The downstream data occupies the upper band, from 550 to 750 MHz. The upstream data occupies the lower band, from 5 to 42 MHz.
• In a telephone network, the telephone numbers of the caller and callee are serving as source and destination addresses. These are used only during the setup (dialing) and teardown (hanging up) phases.

Three Major Components of Telephone System

• Local loops - a twisted-pair cable that connects the subscriber telephone to the nearest end office or local central office. The local loop, when used for voice, has a bandwidth of 4000 Hz (4 kHz). The existing local loops can handle bandwidths up to 1.1 MHz.
• Trunks - transmission media that handle the communication between offices. A trunk normally handles hundreds or thousands of connections through multiplexing. Transmission is usually through optical fibers or satellite links.
• Switching offices - A switch connects several local loops or trunks and allows a connection between different subscribers.

Telephone Line Bandwidth

Bandwidth division in ADSL

Summary of DSL technologies

Division of coaxial cable band by CATV

A SONET system can use the following equipment:

• STS multiplexer - combines several optical signals to make an STS signal. 
• Regenerator - removes noise from an optical signal. 
• Add/drop multiplexer - adds STSs from different paths and removes STSs from a path.

Note: You can proceed to take the multiple choice exam regarding this topic. Using Telephone and Cable Networks for Data Transmission - Set 1 MCQs

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credit: Behrouz A. Forouzan©2013 www.FroydWess.com

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