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.