DWDM and more ..

When SONET/SDH was developed in the mid eighties its main purpose was to provide the transport technology for voice services. Two switching levels were therefore defined. Lower order switching at 1.5/2 Mbit/s to directly support the T1/E1 voice signals and a higher order switching level at 50/150 Mbit/s for traffic engineering. Switching levels at higher bit rates were not foreseen. Over time the line rate increased while the switching rate was fixed. The gap between line rate and switching bit rate got widened. Furthermore new services at higher bit rates (IP, Ethernet services) had to be supported.

The current service providers focus mainly to provide the flexibility to transport wide variety of services at higher speeds without compromising on the bandwidths. This was achieved by DWDM technology which enables high-speed switching based on wave-lengths. The DWDM technology in itself includes challenges with respect to the end-to-end traffic switching which involves multiplexing and de-multiplexing at different wave-lengths, pre-amplification, boost-amplification, line/mid-stage amplification, wave-length conversion, management, supervision and survivability of optical channels carrying client signals.


Wavelength Division Multiplexing (WDM):

WDM combines multiple optical TDM data streams onto one fiber through the use of multiple wavelengths of light. Each individual TDM data stream is sent over an individual laser transmitting a unique wavelength of light. 

WDM

Traditional, passive WDM systems are wide-spread with 2, 4, 8, 12, and 16 channel counts being the normal deployments. This technique usually has a distance limitation of under 100 km.

CWDM

Today, coarse WDM (CWDM) typically uses 20-nm spacing (3000 GHz) of up to 18 channels. The CWDM Recommendation ITU-T G.694.2 provides a grid of wavelengths for target distances up to about 50 km on single mode fibers as specified in ITU-T Recommendations. The CWDM grid is made up of 18 wavelengths defined within the range 1270 nm to 1610 nm spaced by 20 nm.

DWDM

Dense WDM common spacing may be 200, 100, 50, or 25 GHz with channel count reaching up to 128 or more channels at distances of several thousand kilometres with amplification and regeneration along such a route.

Optical Spectrum

•      Light

–              -Ultraviolet (UV)

–              -Visible

–              -Infrared (IR)

•     Communication wavelengths

–              - 850, 1310, 1550 nm

–              - Low-loss wavelengths

•     Specialty wavelengths

–                980, 1480, 1625 nm

The following figure depicts the incorporation of TDM and DWDM to offer better flexibility and serviceability by the internet providers keeping  the future bandwidth requirements in view.

Basic DWDM Network and its components

A ) Transponders and Muxponders: 

                               The main function of the transponder originally was to translate the transmit  wavelength of a client-layer signal into one of the DWDM system's internal wavelengths in the 1550 nm band (DWDM wavelengths in accordance with the ITU-T grid) at the line port. Later on, the additional functionality of 3R (Re-time, Re-transmit, Re-shape) has been incorporated as further advancement.

                           Muxponder essentially performs some relatively simple time division multiplexing of lower rate signals into a higher rate carrier within the system (a common example is the ability to accept 4 OC-48s and then output a single OC-192 in the 1550 nm band).

B ) Multiplexer / Demultiplexer: 
                            The Multiplexer will combine all the DWDM wavelengths received from the transponders and muxponders into one composite signal in order to transmit through the optical fibre. The Demultiplexer will perform the complete reverse process of multiplexer.

C ) Optical Amplifiers:
                         The An optical amplifier is a device that amplifies an optical signal directly, without the need to first convert it to an electrical signal. They can work solely in the optical domain, performing  a 1R (Optical Re-Amplification). Optical amplifiers simultaneously amplify each wavelength of DWDM signal without the need of demultiplexing and remultiplexing. Usually following are the three types of optical amplifiers deployed in DWDM systems.
                       EDFA
                       SOA
                       RAMAN
 
                         The erbium-doped fiber amplifier (EDFA) is the most deployed fiber amplifier as its amplification window coincides with the third transmission window of silica-based optical fiber.

Types of EDFA - 

    - Optical Booster Amplifier
                        The Optical Booster Amplifier will be used at the output of the multiplexer in order to amplify the DWDM wavelengths before transmission.
    - Optical Pre Amplifier
                        The Optical Pre Amplifier will be used before the demultiplexer in order to amplify the DWDM wavelengths after receiving from the fiber.
   - Optical Line Amplifier or In-Line Amplifier
                        The Optical Line Amplifier will be used as the repeater at different sections of fiber where ever the amplification is required until the signal reaches the destination.

D ) Optical Supervisory Channel:
                        This is an additional wavelength usually outside the EDFA amplification band (at 1510 nm, 1620 nm, 1310 nm or another proprietary wavelength). The OSC carries information about the multi-wavelength optical signal as well as remote conditions at the optical terminal or EDFA site. It is also normally used for remote software upgrades and user (i.e., network operator) Network Management information. It is the multi-wavelength analogue to SONET's DCC (or supervisory channel). ITU standards suggest that the OSC should utilize an OC-3 signal structure, though some vendors have opted to use 100 megabit Ethernet or another signal format. Unlike the 1550 nm band client signal-carrying wavelengths, the OSC is always terminated at intermediate amplifier sites, where it receives local information before re-transmission.


E ) Dispersion Compensation Module:
     -Dispersion:
                          Dispersion is a phenomenon related to the variation in velocity of different frequencies (wavelengths) or different modes. This can be either chromatic or polarization dispersion. The velocity of different frequencies can be different due to intrinsic properties of the medium or due to dispersive nature of the optical fiber. 
   

Polarization mode is the effect of different polarization modes (horizontal and vertical) of a signal statistically travelling at different velocities due to fiber imperfections which arise from:
                 -> The transport medium not cylindrical along its overall length.
                 -> Dopants in the fiber cladding causing high refractive index.
                 -> Fiber being twisted, tapered or bent at some points along the span. 


   -Dispersion Compensators:
                       A Dispersion compensator is a device that has the opposite chromatic dispersion effect as the transmission fiber. Various technologies are available that can compensate for all wavelengths in a band or for each wavelength. Compensating for all wavelengths greatly reduces the cost of compensation. Per-band compensation is used in some DWDM products. The various methods include:
                      - Dispersion compensation Fiber (DCF) -  A type of single-mode fiber is used
                      - Dispersion compensation Module - A module with an element which creates a                        reverse behaviour of the velocity of wavelength is used.
                      - High order mode devices
                      - Virtual image phase array (VIPA) is a free space dispersion device


F ) Fiber:
                     Fiber is one of the most critical components of a DWDM system as it provides the physical transportation medium.The two classes of fiber used in telecommunications are multimode fiber (MMF) and single-mode fiber (SMF).

       Multimode Fiber
               • MMF is a fiber that supports multiple “lanes” of light.
               • Multiple electromagnetic transmission modes are carried on  MMF.
               • Each lane is a different speed so that a pulse of light gets distorted sooner than in SMF.
      Single-Mode Fiber
              • One “lane” of light with minimum distortion
                                              MMF                                           SMF

Usually Singlemode fibers are used for the DWDM transmission because of narrow core and also less attenuation.

G ) OADMs and ROADMs:

                       The OADM based on DWDM technology is moving the telecommunications industry significantly closer to the development of optical networks. The OADM can be placed between two end terminals along any route and be substituted for an optical amplifier. Commercially available OADMs allow carriers to drop and/or add up to four STM–16/OC–48 channels between DWDM terminals. The OADM has “express channels” that allow certain wavelengths to pass through the node uninterrupted, as well as broadcast capabilities that enable information on up to four channels to be dropped and simultaneously continue as “express channels.” By deploying an OADM instead of an optical amplifier, service providers can gain flexibility to distribute revenue–generating traffic and reduce costs associated with deploying end terminals at low traffic areas along a route. The OADM is especially well-suited for meshed or branched network configurations, as well as for ring architectures used to enhance survivability. Such flexibility is less achievable with current STM64/OC–192 offerings.

Typical Optical wavelength Add/Drop scenario:

Re-configurable Optical Add Drop Multiplexers (ROADM):
                           In fiber optics, a reconfigurable optical add-drop multiplexer (ROADM) is a form of optical add-drop multiplexer that adds the ability to remotely switch traffic from a WDM system at the wavelength layer. This is achieved through the use of a Wavelength selective switching module. This allows individual or multiple wavelengths carrying data channels to be added and/or dropped from a transport fiber without the need to convert the signals on all of the WDM channels to electronic signals and back again to optical signals.

The main advantages of the ROADM are:
           -The planning of entire bandwidth assignment need not be carried out during initial deployment of a system. The configuration can be done as and when required without affecting traffic already passing the ROADM.
           -ROADM allows for remote configuration and reconfiguration.
           -In ROADM, as it is not clear beforehand where a signal can be potentially routed, there is a necessity of power balancing of these signals. ROADMs allow for automatic power balancing.
           -The switching or reconfiguration functions of a ROADM can be achieved using a variety of switching technologies including MEMS, Liquid crystal, thermo optic and beam-steering switches in planar waveguide circuits, and tunable optical filter technology.

Measurement criteria:
            The optical and digital characteristics of all the different components must be measured before using in the DWDM network scenario. The critical optical parameters to be measured in each network component are listed below.
           Transponder:
              - Center wavelength and spectral width of emitted channel.
              - Spectral stability over time and temperature.
              - Output power (max 17dBm along with laser usage regulations) and its stability.
              - Sidemode suppression ratio (around 40dB)
           Multiplexer and demultiplexer:              
              - Passband wavelengths of different channels.
              - Pulse wavelength overlap (cross talk)
              - Channel insertion loss
              - Optical return loss (back-reflection ratio)
              - Polarization mode dispersion (PMD)
              - Polarization dispersion loss (PDL)
           Amplifier:
             - Channel center wavelength and spacing
             - Spectral stability over time and temperature.
             - Gain and wavelength dependence of the gain.
             - Noise figure
             - Output power and its stability
           Dispersion compensation modules:
             - Channel insertion loss
             - Group velocity over wavelength.
             - Chromatic Dispersion (CD)
           Receiver:
             - Back-reflection
             - Optical and electrical bandwidth
             - Sensitivity

To conclude, DWDM provides hundreds of Gbps of scalable transmission capacity today. It provides capacity beyond TDM’s capability. Supports incremental, modular growth. Transport foundation for next generation networks.

ARP and MAC

ARP stands for Address Resolution Protocol. It is used to associate a layer 3 (Network layer) address (such as an IP address) with a layer 2 (Data Link layer) address (MAC address).

ARP on a Local Host: Assuming its a TCP/IP connection, your computer will have data that it needs to send. When the data gets to the Network layer it will put on the destination IP address. All of this info (the network layer datagram, i.e packet) is passed down to the data link layer where it is taken and placed within a data link frame. Based on the IP address (and the subnet mask), your computer should be able to figure out if the destination IP is a local IP or not. If the IP is local, your computer will look in it's ARP table (a table where the responses to previous ARP requests are cached) to find the MAC address. If it's not there, then your computer will broadcast an ARP request to find out the MAC address for the destination IP. Since this request is broadcast, all machines on the LAN will receive it and examine the contents. If the IP address in the request is their own, they'll reply. On receiving this information, your computer will update it's ARP table to include the new information and will then send out the frame (addressed with the destination host's MAC address).

ARP on a remote host: If the IP is not local then the gateway (router) will see this (remember, the ARP request is broadcast so all hosts on the LAN will see the request). The router will look in it's routing table and if it has a route to the destination network, then it will reply with it's own MAC address.

This is only the case if your own computer doesn't know anything about the network topology. In most cases, your computer knows the subnet mask and has a default gateway set. Because of this, your own computer can figure out for itself that the packet is not destined for the local network. Instead, your computer will use the MAC address of the default gateway (which it will either have in it's ARP table or have to send out an ARP request for the same). When the default gateway (router) receives the frame it will see that the MAC address matches it's own, so the frame must be for it. The router will un-encapsulate the data link frame and pass the data part up to the network layer. At the network layer, the router will see that the destination IP address (contained in the header of the IP packet) does not match it's own (remember, the IP address has not been touched at all in this process since your computer created the IP packet). The router will realise that this is a packet that is supposed to be routed. The router will look in it's routing table for the closest match to the destination IP in order to figure out which interface to send the packet out on. When a match is found, the router will create a new data link frame addressed to the next hop (and if the router doesn't know the hardware address for the next hop it will request it using the appropriate means for the technology in question).

The data portion of this frame will contain the complete IP packet (where the destination IP address remains unchanged) and is sent out the appropriate interface. This process will continue at each router along the way until the information reaches a router connected to the destination network. It will see that the packet is addressed to a host that's on a directly connected network (the closest match you can get for an address, short of the packet being addressed to you). It will send out an ARP request for MAC address of the destination IP (assuming it doesn't already have it in it's table) and then address it to the destination's MAC address.

Now then, I did slightly gloss over 1 part in the above explanation and that's the part about the router finding out the hardware address for the next hop. I just didn't want to disturb the flow with entering into that there. How the router does this will depend on what type of connection (and in some cases, what protocol and/or encapsulation is used on the connection). In some cases, this will be a hard set value (like a frame relay pvc) within the configuration of the router. In some cases, you don't even need a hardware address (like any point to point connection, there's only 1 possible host you could send it to), in those cases the router will just create a data link frame appropriate for the connection and it won't even need to be addressed. This is why the OSI model is good. It's layered so that any layer can change and as long as it takes in information in a standard way (the way the layer above wants to send it) and spits out information in a standard way (the way the layer below wants to receive it), then it's all good. When Frame Relay came along nothing changed with the way you had to address IP packets, all of the changes took place at the data link and physical layers. If some new type of connection comes along in the future, only the data link and physical layers will likely change. When we go to IPv6, only the network layer should change.

Example:
Your computer has an address of 200.0.1.2, it's connected to the 200.0.1.0 network (I'm assuming a subnet mask of 255.255.255.0, we'll call this network 1) which is an ethernet network. Your default gateway is a router (router a) which has an address of 200.0.1.1. That router is connected to the 200.0.1.0 network and the 200.0.2.0 (network 2) network (the interface connected to the 200.0.2.0 network will have an address of 200.0.2.1). The network 2 is also an ethernet network. Also connected to network 2 is another router (router b) which has the address (for the interface connected to network 2 at least) of 200.0.2.2. Router b is also connected to network 3 (200.0.3.0). Router b's interface on network 3 has the address of 200.0.3.1. Here's a (probably bad) ASCII diagram to illustrate:

Router Router
a b
-----------O-------------O------------
Network 1 Network 2 Network 3
(200.0.1.0) (200.0.2.0) (200.0.3.0)

Now then, your computer (on network 1 with an address of 200.0.1.2) wants to send some data to a computer on network 3 (with an address of 200.0.3.2). We'll assume that none of the info in already cached in an ARP table on any of the machines or routers (a big assumption but it's mine to make!). Your computer will create an IP packet addressed to 200.0.3.2. That packet will be sent to the data link layer where it needs a MAC address. Based on the subnet mask, your computer will know that the destination computer isn't on the same local network. So, your computer will send out an ARP request for the default gateway's MAC address (ie, what's the MAC for 200.0.1.1?).

On receiving the MAC address, your computer will send out the IP packet (still addressed to 200.0.3.2) encapsulated within a data link frame that is addressed to the MAC address of router a's interface on network 1 (because routers have more than 1 interface they can have more than 1 MAC address, in this case each router has 2 ethernet interface each with it's own unique MAC address). Router a will receive this frame and send the data portion up to the network layer (layer 3). At the network layer, router a will see that the packet (which is addressed to 200.0.3.2) is not addressed to router a. Router a will look in it's routing table to find out where to send the packet. The routing table will show that network 3 (the closest match to 200.0.3.2) is reachable via network 2. The routing table will also show the IP address for the next hop is 200.0.2.2. Router a will send out an ARP request onto network 2 asking for router b's MAC address (well at least for the interface connected to network 2).

On receiving this, router a will send the IP packet (still addressed to 200.0.3.2, nothing's changed here) encapsulated in a data link frame addressed to router b's MAC address. When router b receives this frame it will do the same thing that router a did, it will send the IP packet up to the network layer and see that the packet is not addressed to router b (the packet is still addressed to 200.0.3.2). Router b will then look up in it's routing table for the closest match and see that it is directly connected to network 3, so there isn't a next hop router to send it to. Router b will send out an ARP request to learn the MAC address for 200.0.3.2.       When this is received, router b will send out the IP packet (again, this is still addressed to 200.0.3.2) encapsulated within a data link frame that is addressed to the MAC address of the destination computer. The destination computer will see that the data link frame is addressed to it and will pass the IP packet to the network layer. At the network layer, the IP address will also match that of the computer and the data from the IP packet will be passed up to the transport layer. Each layer will examine the header and determine where to pass it up to until eventually, the data reaches the application running on the destination computer that has been waiting for the data.

What you'll notice through this whole process is that the IP address never changes. The IP packet is always addressed to 200.0.3.2. However, at the data link layer, the address used changes at each hop (it's always addressed to the next hop). As you go up through the layers, you get more and more specific about where the data is supposed to be going. At the data link layer this is very vague, it's basically just, "here's who to pass it on to, they should know what to do with it". At the network layer you get more specific (this is the exact computer I want to send this to). Above that you get more specific (is it TCP or UDP?, what port?, etc)

Physical Layer - SONET/SDH

The Physical Layer is the first and lowest layer in the seven-layer OSI model of computer networking. The implementation of this layer is often termed PHY.The Physical Layer consists of the basic hardware transmission technologies of a network. It is a fundamental layer underlying the logical data structures of the higher level functions in a network.The Physical Layer defines the means of transmitting raw bits rather than logical data packets over a physical link connecting network nodes. The bit stream may be grouped into code words or symbols and converted to a physical signal that is transmitted over a hardware transmission medium. The Physical Layer provides an electrical, mechanical, and procedural interface to the transmission medium. The shapes and properties of the electrical connectors, the frequencies to broadcast on, the modulation scheme to use and similar low-level parameters, are specified here.Within the semantics of the OSI network architecture, the Physical Layer translates logical communications requests from the Data Link Layer into hardware-specific operations to affect transmission or reception of electronic signals.

The Plesiochronous Digital Hierarchy (PDH) is a technology used in telecommunications networks to transport large quantities of data over digital transport equipment such as fibre optic and microwave radio systems. The term plesiochronous is derived from Greek plēsios, meaning near, and chronos, time, and refers to the fact that PDH networks run in a state where different parts of the network are nearly, but not quite perfectly, synchronised.

PDH allows transmission of data streams that are nominally running at the same rate, but allowing some variation on the speed around a nominal rate. By analogy, any two watches are nominally running at the same rate, clocking up 60 seconds every minute. However, there is no link between watches to guarantee they run at exactly the same rate, and it is highly likely that one is running slightly faster than the other. Synchronization is required to get rid of these drifts.

Synchronous Optical Networking (SONET) and Synchronous Digital Hierarchy (SDH) are standardized multiplexing protocols that transfer multiple digital bit streams over optical fiber using lasers or light-emitting diodes (LEDs). Lower data rates can also be transferred via an electrical interface. The method was developed to replace the Plesiochronous Digital Hierarchy (PDH) system for transporting larger amounts of telephone calls and data traffic over the same fiber without synchronization problems. SONET generic criteria are detailed in Telcordia Technologies Generic Requirements document GR-253-CORE.[1]

STS-1 Synchronous Transport Signal rate 1.
- fundamental bit rate within SONET hierarchy
SONET rate =51.840 Mbs. When transmitted via light called Optical Carrier rate 1, orOC-1
STS-1 typically can be a DS3 signal within a SONET frame
Frame Rate=9 Rows X 90 Columns X 8 bits/sec X 8000 frames/sec = 51.84 Mbs
Payload = 50.112 Mbs, Transport Overhead = 1.728 Mbs

STS-N frames are formed by formed by byte-interleaving lower rate STS modules
– 3 STS-1 are muxed to create an STS-3 (156 Mbps)
– Have 3 sets of TOHs and 3 SPEs.


• Concatenated STS-N frames are called as STS-Nc where “c” denotes concatenation
• Also called a “super-rate” payload
• Instead of multiple slow rate SPEs combined into one SPE, have 1 high-capacity and “unchannelized” SPE
• Useful for sending traffic that is bigger than STS-1 payload


SONET transmission from end-to-end is steered at three segments with overhead maintenance. The overhead bytes are processed/added/removed at the respective segment.

This is how a typical sonet frame spans and further classified:


Section Overhead: 1st 3 rows of the TOH
– 9 Bytes

• Main functions include

– Monitor STS-N Performance

– Local Orderwire

– Data communication channels for OAM&P info

– Framing







Line Overhead: Last 6 Rows of TOH
– 18 Bytes
• Main functions
– Locating the SPE in the frame

– Muxing or Concatenating Signals

– Performance monitoring

– Automatic Protection Switching

– Line Maintenance








Path Overhead: 1st Column of the STS SPE is Path Overhead
– an SPE can begin anywhere in the STS-1 envelope and overlap the adjacent frame.
• Main functions
– Performance monitoring

– Signal label, i.e. STS SPE content, including status of mapped payloads

– Path status

– Path trace


Higher order POH:

Lower Order POH:

SDH (Synchronous Digital Hierarchy):
. The Plesiochronous Digital Hierarchies (PDH) line rates across the world all start at DS-0/E-0 level (64 kbps) but vary after that
– NADH (N. America), JDH (Japan), EDH (Europe)
. Original SONET proposal did not accommodate all PDH systems
. SDH is essentially a SONET adjusted to accommodate the slight differences between NADH and the rest of the world’s digital hierarchies

SONET & SDH are more similar than different
– SONET and SDH equipment are fully interoperable
– SDH is the “international version of SONET”
• Differences are relatively minor:
– Basic Frame size and line rate
• Base SDH container, STM-1 (OC-3) is 3 times the size of SONET STS-1, I.e. 9 rows by 270 columns and 3 times the line rate, i.e. 155.52 Mbps
Nomenclature differences
• For example, Virtual Tributaries = Virtual Containers
• Section = Regenerator Section, Line = Multiplex Section
– Some differences in overhead
• C2 POH Payload byte mapping differs between SONET & SDH.

Line Rates: SDH base line-rate is 155.52 Mbps STM-1
• Higher order line rates are multiples of STM-1

SDH is the International Standard and SDH standards are used for international links
• Efforts for standardizing management systems for interoperable optical environments led by ITU
– ITU defines standard for Telecommunications Management Networks (TMN)
• Demand for bandwidth has made OC-3 prevalent in U.S. which aligns well with STM-1
. Equipment vendors tend to emphasize SDH over SONET to address the global standard.

Equipments:
• Terminal Multiplexer: Basically combines PDH signals into an STS-N signal for transport onto an OC-N or disassembles an STS-N signal into lower rate PDH signals. Almost always at the edge of a SONET/SDH network

• Add/Drop Multiplexer (ADM): The ADM is the most prolific SONET/SDH equipment. “Grooms” SONET STS-N, I.e. Add/Drop DS-n channels. Allows access to transmission signals down to the DS-0 level without having to demultiplex the whole SONET channel.
Typical ADM configurations:
– Terminal Multiplexer mode: As above
– Matched Node: Used when survivability of a inter-ring link is desired.
These typically participate in sub-tending ring topologies.
– Drop and Repeat Nodes: For broadcasting from a SONET/SDH ring (Cable TV)

• Digital Cross Connect (DCS): Two types
- Broadband DCS: A Broadband Digital Cross connect (DCS) accepts SONET rate signals, accesses STS-1 and switches at this level. Also terminates DS1 and DS3. Mainly used for STS-1 grooming and broadband traffic management.

- Wideband DCS:Similar to Broadband DCS but switching is done at VT levels. Mainly used for DS1 level grooming particularly at hub locations. Use wideband DCS over DS3/1 cross connect to minimize mux/demux events.


• Regenerator: Regenerates a signal weakened by attenuation over long distance.
e.g, 1310nm optical signal needs regeneration every 26 mi.
• Digital Loop Carrier (DLC): Concentrates low-speed services before they are brought into the local Central Office for distribution. Economical when demand is in between 200 and 2000 lines.

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.