Basic Telephony

Telephones, a mode of voice communication were originally connected directly together in pairs. Each user had separate telephones wired to the various places he might wish to reach. This became inconvenient when people wanted to talk to many other telephones, so the telephone exchange was invented. Each telephone could then be connected to other local ones, thus inventing the local loop and the telephone call. Soon, nearby exchanges were connected by trunk lines, and eventually distant ones were as well. This is how always need paves the way for invention.

Central Office (CO) connects two divisions both sides:

Plain Old Telephone Service (POTS) basically connects two telephone systems through central office using twisted copper wire. These are the typical 10 digit dialing calls.
Phone Line <----------> Digital Switch in CO <----------------->Phone line

Special Circuits: These comprise Analog Phone, ISDN Phone, computer circuits, DS1 units and other utility circuits. These are customer requested circuits to connect to Network.
Special Circuits(SC) <--> Transmission Equipment(TE) <-----> Network<---->TE<--->SC .

Its an Analog Signal that goes from telephone line to CO which gets converted to digital representation in binary form 1's and 0's. This digital signals travel along the network to other CO, again received as analog at the telephone line.

Analog to Digital: The analog signals are filtered and passed over low pass filters. The human voice is audible at the maximum frequency of 4000Hz. These signals are then sampled according to the Nyquist's sampling theorem. Sampling is the process of converting a signal (continuous representation of time) into a numeric sequence (discrete representation). According to Nyquist,

"Bandlimited analog signal can be perfectly reconstructed after sampling, provided the sampling rate exceeds twice the maximum frequency of the original signal".

As per the theorem, the maximum frequency is 8000Hz and each signal is sampled at 125microsecond time space which is 8000 samples per sec.

Now, this numeric sequence is represented in terms of 1's and 0's using pulse code modulation. On representing each sample using 8-bit binary code, voice signal is digitized as 8000X8-bit = 64000bits/sec ~64kbps. This is the base digital signal called digital signal level 0 or DS0. This kind of digital representation of an analog signal reduces noise problems.

In this way, 8-bit word is represented at 125microsec time space. This on further squeezing can accommodate other pulses in the same 125microsec time space. This resulted in time division mutliplexing.

Time-division multiplexing (TDM) is a type of digital or (rarely) analog multiplexing in which two or more signals or bit streams are transferred apparently simultaneously as sub-channels in one communication channel, but are physically taking turns on the channel. The time domain is divided into several recurrent timeslots of fixed length, one for each sub-channel. A sample byte or data block of sub-channel 1 is transmitted during timeslot 1, sub-channel 2 during timeslot 2, etc. One TDM frame consists of one timeslot per sub-channel plus a synchronization channel and sometimes error correction channel before the synchronization. After the last sub-channel, error correction, and synchronization, the cycle starts all over again with a new frame, starting with the second sample, byte or data block from sub-channel 1, etc.

The pulse widths of bits in 24 channels are squeezed to pull all 24 channels into one high-speed channel (24chan X 8-bit) + 1 framing bit = 193 bits/frame.
193bits/frame X 8000 frames/sec = 1544000 frames/sec = 1.54Mbps , which is the DS1 frame.
Analog->Digital conversion <--Time Division mux--> Digital-->Analog conversion
In this way of multiplexing 28DS1 signals, DS3 frames are constructed at rate 44.5Mbps.

Signaling is the process by which two or more telephone offices communicate between each other to setup and take down a telephone call. Inband signaling works as..,

phone line(A) <---> CO <-------> CO <------> phone line(B)
A informs B of incoming call.
B checks line for on-hook or off-hook condition.
B informs A of status of line.
B applies ringing to the line.
When phone answers, voice path is created between A & B.
If off-hook, a busy tone is sent from B to A.
When a phone is again on-hook, path is dropped.

DS1 Super Frame (DS1 SF): 12 DS1 frames grouped together forms super frame.

DS1 Extended Super Frame (DS1 ESF): 24 frames grouped together forms extended super frame. As always, framing bits are used to communicate the signaling information.

DS1- low voltage levels, typically 5-12 volts.Limitation of 400 ft in an office.
T1- has added DC power component used to power line repeaters.

DSX-3 jacks - terminating DS3 cables, providing cross-connections between DS3 circuits. Connects to M13 muxes & FOT’s.
M13 Multiplexer – TDM device combining 28 DS1’s into one DS3.
Fiber Optic Terminal (FOT) – TDM device combining lower speed DS3 (and DS1) circuits together into a high speed circuit. This high speed circuit is converted into light pulses and connected to a fiber cable.

Wave length Division Multiplexing (WDM): WDM is used to place multiple wavelengths of light on a single fiber. a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths (colours) of laser light. This technique enables bidirectional communications over one strand of fiber, as well as multiplication of capacity.
A WDM system uses a multiplexer at the transmitter to join the signals together, and a demultiplexer at the receiver to split them apart. With the right type of fiber it is possible to have a device that does both simultaneously, and can function as an optical add-drop multiplexer.
WDM systems are popular with telecommunications companies because they allow them to expand the capacity of the network without laying more fiber. By using WDM and optical amplifiers, they can accommodate several generations of technology development in their optical infrastructure without having to overhaul the backbone network. Capacity of a given link can be expanded by simply upgrading the multiplexers and demultiplexers at each end.
Fiber optic lasers traditionally operated at 1310 nm, 1550 nm.

Dense wavelength division multiplexing, or DWDM for short, refers originally to optical signals multiplexed within the 1550 nm band so as to leverage the capabilities (and cost) of erbium doped fiber amplifiers (EDFAs), which are effective for wavelengths between approximately 1525-1565 nm (C band), or 1570-1610 nm (L band). EDFAs were originally developed to replace SONET/SDH optical-electrical-optical (OEO) regenerators, which they have made practically obsolete. It uses finer increments of wavelengths – 0.1 nanometers. DWDM uses wavelengths such as 1557.1, 1557.2, 1557.3, 1557.4, and higher.