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LECTURE
Telephone Services
Common carrier Services & Hierarchies
o ubscribers with ANALOG services sing ANALOG networks
thinking about changing their service lines digital too
Analog Services Of the many analog services available to telephone subscribers, two are really imp
Leased Analog Services o Familiar di ntered when using a home telephone o Uses twisted pair cable to connect subscriber’s phone to the network via exchange
s usually b/w 0 and 4000 Hz
o
Telephone companies began by providing their s u o Later digital services were introduced o Nowadays carriers are even o Soon the entire n/w will be digital o For now both types of services are available and both FDM and TDM are in use
o ortant:
– Switched Analog Services
al up service most often encou
o This connection is called LOCAL LOOP o The n/w it joins is called PSTN o Signal on a local loop is Analog and BW i
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Switched Analog Services
aller dials a number and call is conveyed to a series of switches at the exchange
With switched lines, c
The switch connects two lines for the duration of the call
Analog Leased Service
the opportunity to lease a line, sometimes called a Dedicated Line
nection still passes through switches in the
Conditioned Lines
ffered by the carriers proving the quality of a line by lessening attenuation, signal
ed to Modems
To maximize efficiency, telephone companies multiplex signal from lower BW lines
o
Offers customers
Dedicated line is permanently connected to the other customer Although the con exchange, the customer experiences it as a single line because switch is always closed No dialing is needed
o Another service o o Conditioning means im distortion or delay distortion o Conditional lines are Analog but their quality makes them suitable for digital data communication when connect
Analog Hierarchy o onto higher BW lines o In this way many switched or leased lines can be combined into fewer but bigger channels FDM is used for analog lines
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Because this service is already digital, subscribers do not need modems
d by the subscriber’s device to
sceptibility to noise
a rate of 64 Kbps requires a DSU SU for this service is cheaper than switched /56 DSU because it does not
e velop a hierarchy of digital
s DS service S is a hierarchy of digital signals
They need DSU (Digital service unit) DSU changes the rate of digital data create 56 Kbps and encodes it in the format used by service provider
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 su
Bandwidth on Demand
Digital Data Service (DDS) Digital version of analog leased line Max. dat Like switched/56, DDS D need a dial pad
Digital Signal (DS) Servic Telephone companies saw the need to de services much like those used for Analog services The next step wa D
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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=9664 Kbps + 168Kbps DS-3 44.376 Mbps=67264Kbps +1.368 Mbps of overhead DS-4 274.176Mbps4032 * 64 Kbps+ 16.128Mbps overhead
T Lines
o o To imple o These are th
Telephone System
Reading Sections
Section th Edition by Behrouz A. Forouz
DS-0 DS1 are the names of the Services ment those services, telephone companies use T-lines e lines with capacities matched to the data rates of DS-0 to DS-
ServiceLine Rate(Mbps) Voice channels
DS-1 T-1 1.544 24 DS-2 T-2 6.312 96 DS-3 T-3 44.736 672 DS-4 T-4 274.176 4032
Summary The Analog Services Digital Services Fiber To The Curb (FTTC)
8.5, “Data Communications and Networking” 4 an
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