How To C C++: SONET Network Elements

SONET protocol sits in the Physical Layer of TCP\IP, OSI model
Hardware devices that implement the SONET Protocol are called as SONET NE’s
for each SONET layer (path, line, section, and photonic) we have different network elements
optical interface layer follow a hierarchy with Path layer on the top followed by line layer, section layer and photonic layer
the group of NE’s for each layer is commonly referred as PTE(path terminating equipment), LTE(line terminating equipment) and STE(section terminating equipment)
each layer will add its own overhead bits to convey information (OH), called as POH, LOH, SOH
Path layer deals with transport of service between between PTE NE, the main function is to map the signals form a format required by the line layer, and its function include reading, interpreting and modifying the POH for performance and APS (Automatic protection switching)
Path Terminating Equipment
1. Terminal Multiplexer (TM)
2. aggregate DS1 (digital,PDH), DS3, E1s(Europe,digital,PDH), E3s, STS-Ns (Electrical,SONET), STM-Ns(SDH), into OC-N(Optical) Signals
  1. DS1, Digital Signal 1, 1.544 Mbps, called as T1 in USA, a PDH signal, contains 24 DS0(64Kbps)
  2. DS3, 44.736 Mbps, 672 DS0,
  3. E1, Digital Signal, in Europe, 2.048 Mbps, contains 32 DS0(64Kbps)
  4. STS1, Synchronous Transport Signal, Electrical Level equivalent to optical level OC1, 51.840 Mbps
  5. OC1, 51.840 Mbps with Payload 50.112 Mbps and Overhead of 1.728 Mbps
  6. STM1, Synchronous Transport Module, SDH equivalent of SONET STS3/OC3, 155.520 Mbps, 150.336 Mbps Payload, and 5.184 Mbps Overhead
3. DS1 signals are mapped into VT1.5s (Virtual Tributaries), and DS3 (44.736) signals into STS1 (51.840) SPE (Synchronous Payload Envelop)
  1. VT1.5, Virtual Tributaries, 1.728 Mbps, called as VC11 (Virtual Container) in SDH
  2. SPE, Synchronous Payload Envelop of SONET
4. EO (electrical to optical) and OE conversion takes place, during multiplexing and de-multiplexing respectively
5. TM is an ADM operating in terminal mode
Line Layer, deals with the transport of the path layer payload and it own LOH across the physical medium, main function is to provide synchronization and to perform multiplexing for the path layer, and protection switching, synchronization, line maintenance and error monitoring.
Line Terminating Equipment
1. Add Drop Multiplexer (ADM)
  1. combines (multiplex) several low bandwidth data streams and combine them into a single beam of light (optical signal)
  2. the add drop part is related to adding one or more lower bandwidth signals into an existing higher bandwidth data stream, and also extracting (dropping) low bandwidth and redirecting them
  3. a single stage Mux/Demux can multiplex various inputs into an OC-N, and then lower signals are added/dropped as required, thus providing interface between different network signals and SONET signals.
  4. SONET also enables dropping and duplicating of the signal, thus preserving the signal to be used by other nodes (drop and continue), this drop and continue is used by , cable TV , and it also provides ring survivability, by duplicating the signal and passing it along a alternate route
2. ROADM (Reconfigurable Optical Add Drop Multiplexer)
  1. Type of ADM with the options of remote reconfiguration of traffic, from a WDM (wavelength division multiplexing) system at the wavelength layer, achieved through Wavelength Selective Switching.
  2. the optical wavelengths added/dropped from a transport fiber need not be converted from Optical to Electrical to Optical (O-E-O)
  3. the bandwidth configuration can be done as and when required, without affecting the existing traffic, and allowing remote configuration
Section Layer, deals with the transport of STS-N frame across physical medium, and functions are framing, scrambling, error monitoring and section maintenance
Section Terminating Equipment
1. this can be any two adjacent SONET network elements,
2. Regenerator, or Repeater
  1. regenerate (amplify) attenuated signals
  2. needed because of the distance between multiplexers
  3. it clocks itself off the received signal, and replaces the SOH byte, LOH and POH are not changed