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CS601 - Data Communication - Lecture Handout 31

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Telephone Services

Telephone Services

Common carrier Services & Hierarchies

  • Telephone companies began by providing their subscribers with ANALOG services using ANALOG networks
  • Later digital services were introduced
  • Nowadays carriers are even thinking about changing their service lines digital too
  • Soon the entire n/w will be digital
  • For now both types of services are available and both FDM and TDM are in use

Analog Services

  • Of the many analog services available to telephone subscribers, two are really important:
    • Switched Analog Services
    • Leased Analog Services
  • Familiar dial up service most often encountered when using a home telephone
  • Uses twisted pair cable to connect subscriber’s phone to the network via exchange
  • This connection is called LOCAL LOOP
  • The n/w it joins is called PSTN
  • Signal on a local loop is Analog and BW is usually b/w 0 and 4000 Hz

Analog Services

Switched Analog Services

With switched lines, caller dials a number and call is conveyed to a series of switches at the exchange The switch connects two lines for the duration of the call

Switched Analog Services

Analog Leased Service

Offers customers the opportunity to lease a line, sometimes called a Dedicated Line

Dedicated line is permanently connected to the other customer Although the connection still passes through switches in the exchange, the customer experiences it as a single line because switch is always closed No dialing is needed

Analog Leased Service

Conditioned Lines

  • Another service offered by the carriers
  • Conditioning means improving the quality of a line by lessening attenuation, signal distortion or delay distortion
  • Conditional lines are Analog but their quality makes them suitable for digital data communication when connected to Modems

Analog Hierarchy

  • To maximize efficiency, telephone companies multiplex signal from lower BW lines onto higher BW lines
  • In this way many switched or leased lines can be combined into fewer but bigger channels
  • FDM is used for analog lines

Analog Hierarchy

Digital Services

  • Digital services are largely offered nowadays
  • Digital services are less sensitive than analog ones
  • Telephone line acts as an antenna and pick up noise both in case of analog and digital TX
  • In analog, both noise and signal are analog
  • In digital, signal can easily be separated

Types of Digital Services

Switched / 56 Service

It is digital version of Analog switched line Allows data rates of up to 56 Kbps Both parties must subscribe to the service A caller with normal telephone service cannot connect to a telephone or computer with this service even with a Modem

Because this service is already digital, subscribers do not need modems They need DSU (Digital service unit)

DSU changes the rate of digital data created by the subscriber’s device to 56 Kbps and encodes it in the format used by service provider

Switched   56 Service

DSU is often included in dialing process DSU is more expensive than Modem So why would a subscriber pay for switched/56 service and DSU?Because digital line has better speed, better quality and less susceptibility
to noise Bandwidth on Demand

Digital Data Service (DDS)

Digital version of analog leased line

Max. data rate of 64 Kbps

Like switched/56, DDS requires a DSU

DSU for this service is cheaper than switched /56 DSU because it does not need a dial pad

Digital Signal (DS) Service

Telephone companies saw the need to develop a hierarchy of digital services much like those used for Analog services The next step was DS service

DS is a hierarchy of digital signals

Digital Signal (DS) Service

DS-0 resembles DDS. It is single digital channel of 64 Kbps
DS-1 is 1.544 Mbps service = 24 * 64 Kbps+8 Kbps of overhead
DS-2 􀃆6.312 Mbps=96*64 Kbps + 168Kbps
DS-3 􀃆44.376 Mbps=672*64Kbps +1.368 Mbps of overhead
DS-4 􀃆274.176Mbps4032 * 64 Kbps+ 16.128Mbps overhead

digital signals

T Lines

  • DS-0 DS1 are the names of the Services
  • To implement those services, telephone companies use T-lines
  • These are the lines with capacities matched to the data rates of DS-0 to DS-4

T Lines


  • The Telephone System
  • Analog Services
  • Digital Services
  • Fiber To The Curb (FTTC)

Reading Sections

  • Section 8.5 “Data Communications and Networking” 4th Edition by Behrouz A. Forouzan