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.