• SONET (Synchronous Optical Network) is an optical transmission interface originally proposed by Bellcore and Standardized by ANSI

Important characteristics, similarities and differences between SONET and SDH:

  1. SONET is a synchronous
  2. SDH is also a synchronous network with optical
  3. SONET is a set of standard interfaces on an optical synchronous network of elements that conform to these
  4. SONET interfaces defines all layers, from physical to the application
  5. SDH is a set of standard interfaces in a network of elements that conform to these
  1. Like SONET, SDH interfaces define all layers, from physical to the application
    • The SONET standard addresses the following specific issues:
  2. Establishes a standard multiplexing format using any number of 51.84Mbps signals as building
  3. Establishes an optical signal standard for interconnecting equipment from different
  4. Establishes extensive operations, administration and maintenance capabilities as part of the standard.
  5. Defines a synchronous multiplexing format for carrying lower level digital

Broadband Networks

  • 8.4.1 shows SONET/SDH network services. (Refer Fig. 8.4.1 on next page).
  • Voice, video data, internet and data from LAN’S, MAN’S, and MAN’S will be transported over a SONET or a SDH
  • The SONET network is also able to transport asynchronous transfer mode (ATM) These systems, called broadband can manage a very large aggregate bandwidth or traffic.


SONET versus SDH


Some of the technical similarities between SONET and SDH are :


  1. Bit rates and frame format
  2. Frame synchronization
  3. Multiplexing and demultiplexing
  4. Error


SONET/SDH Benefits


Advantages are listed below :


  1. Reduced cost is
    1. Operation cost is
    2. Same interface for all vendors
  2. Integrated network elements :
    1. It allows for multivendor
    2. It has enhanced network element
  3. It offers network survivability
  4. It is compatible with legacy and future
  5. Remote operation capabilities. It is remotely provisioned, tested, inventoried, customized and

SONET and SDH Rates


  • The SONET specification defines a hierarchy of standardized digital data rates. SONET and SDH rates are defined in the range of 51.85 to 9953.28 Mbps and higher rates at 40 Gbps are also under study.









Data rate (Mbps)

Payload rate (Mbps)



















































  • Sts-1 = Synchronous transport signal – 1
  • OC = Optical carrier
  • STM = Synchronous transport module
  • ITU-T = International Telecommunication Union Telecommunication Standardization
    • When the SONET signal is in its electrical nature, it is known as synchronous transport signal level N (STS-N). The SDH equivalent is called synchronous transport module level N (STM-N).
    • After its conversion into optical pulses, it is known as optical carrier level N (OC-N). In SONET, N takes the value 1, 3, 12, 48 and 192 with corresponding bit rates at 51.48, 52, 622.08, 2488.32 and 9953.28 Mbps.

Why use SONET/SDH?

  • Why glass fiber is better than copper wire? Following are the benefits of glass
  1. Fiber yields thinner cable than
  2. Fiber an transmit without repeaters at longer distances as compare with
  3. Higher bandwidth per
  4. Lower bit error
  5. Higher transmission reliability. Glass fiber is not as susceptible to radio frequency or EMI as copper wire unless it is shielded and well grounded.

Optical Components

The optical components are

  1. Optical transmitter
  2. Receiver
  3. Fiber medium
  4. Optical amplifier

1.      Optical transmitter

  • It is a transducer that converts electricalplses to optical
  • The transmitter is characterized by
  1. An optical power
  2. A rise time
  3. Central wavelength
  4. Wavelength range
    • Laser diodes have better controlled parameters, higher optical power, and short times and therefore are better suited for multimega bit rates.
    • Light emitting diodes (LED) transmit a wider band of wavelengths, are more inexpensive and are better suited for lower bit rates than laser

2.      Receiver

  • It is a transducer that converts optical pulses to electrical ones. Photodetectors can be made with photoresist material or semiconductors. The response times of these technologies are very
  • For multimega bit rates, detectors must have high optical power sensitivity, very fast response to a range of wavelengths that matches the range of transmitted

3.      Fiber medium

  • Ultrapure glass fiber is the medium used to guide light pulses. Light pulses are generated by the transmitter and detected by the receiver.
  • The motivation to use glass fiber instead of copper wire is that the ability to transport a higher bit rate signal more reliably, with fewer errors and over a longer

4.   Optical amplifier

  • An optical signal propagating in a fiber will be attenuated. The optical signal must be amplified to compensate for losses in the
  • Amplifying optical signals is a multi step process. Typically, the optical signal is converted to an electronic signal, then it is amplified, and then it is converted back to This function is known as regeneration and it is relatively expensive.
  • Another technique to amplify an optical signal is to use an all optical amplifier (OFA). It consists of a fiber segment doped with erbium and pumped with light of wavelength at 980 or 1480 nm. This pumping process excites the erbium atoms in the
  • When the optical signal with a wavelength in the range of 1530-1565 nm pass through the fiber, it causes the excited erbium atoms to yield photons of the same wavelength with the signal. This is known as stimulated emission and the result is more photons out than the photons in and thus an amplified optical
  • Amplifiers are of three types :
  1. Single wavelength digital





  1. Multiwavelength digital
  2. Amplifiers for analog applications such as




  • The SONET/SDH network consists of nodes or network elements (NE) that are interconnected with fiber cable over which user and network information is transmitted. 8.4.2 shows SONET network

  • SONET NEs may receive signals from a variety of facilities such as DS1, DS3, ATM, Internet and LAN/MAN/WAN. They also may receive signals from a variety of network
  • SONET NEs must have a proper interface to convert the incoming data format into the SONET

Network Topologies


  • Network falls into three topologies :
  1. Ring
  2. Mesh
  3. Tree

1)      Ring topology

  1. It consists of NEs interconnected with a dual fiber, the primary and secondary, to form a ring.
  2. When one of these two fibers breaks, the other fiber in the ring is This mechanism provides transmission protection and ring restoration capabilities.
  3. If both fiber break, then the network is reconfigured, forming a ring using both the primary and secondary. Information flows in all the fibers but the broken
  4. Ring topology offers fast path and is widely used in
  5. 8.4.3 shows ring topology

2)      Mesh topology

  • It consists of NEs fully
  • When an interconnecting link breaks, the adjacent NE detects the breakage and routes the traffic to another NE. this mechanism provides transmission protecton and network restoration capabilities.
  • 8.4.4 shows mesh topology

  • The mesh topology is better applicable in densely areas

3)      Tree topology

  1. it is a hierarchical distribution of NEs and is mostly used in LANs such as
  2. 8.4.5 shows tree topology.

  1. A source is connected to a distribution function as a hub, that routes the packet to its destination node. A connection between source and destination is established for the duration of the packet through
  2. This network is very efficient for asynchronous data transmission but not for real time data and voice.

SONET Multiplexing


The SONET specification defines a hierarchy of standardized digital data rates. The basic transmission rate defined in the SDH is 155.52 Mbps and is known as a synchronous transport module level 1 signal (STM-1). Higher rates of STM-4 (622 Mbps) and STM-16 (2.4Gbps) are also defined.

  • In the SONET hierarchy the term synchronous transport signal(STS) or sometimes optical signal(OC) is used to define the equivalent of an STM signal. An STM-1 signal is produced by multiplexing three such signals together and hence is equivalent to an STS-3/OC-3 signal. As with the plesiochronous digital hierarchy (PDH), the STM-1 signal is comprised of a repetitive set of frames which repeat with a period of 125 microsec. The information content of each frame can be used to carry multiple 5/2/6/34/45 or 140 Mbps streams. Each of these streams is carried in a different container which also contains additional stuffing bits to allow for variations in actual rate. To this is added some control information known as the path overhead which allow such thing as the BER of the associated container to be monitored on an end-to- end basis by network management.
  • To provide the necessary flexibility for each higher order signal, in addition to the overheads at the head of each lower level STM frame, a pointer is used to indicate the lower level STM frame’s position within the higher order frame. Multiplexing and demultiplexing operation is performed by a device known as drop and insert or add drop multiplexer (ADM).

SONET System Hierarchy


  • SONET System hierarchy has four layers as mentioned below :
  1. Photonic layer : this specifies the types of optical fibers, the minimum required laser power, sensitivity of the receivers and dispersion characteristics of This is the physical layer.
  2. Section layer : This Layer generates SONET frames and convert the electronic signals to photonic signals.
  3. Line layer : This Layer synchronizes and multiplexes the data into SONET
  4. Path layer : This layer performs end to end transport of data at the proper

Fig 8.4.7 shows the system hierarchy of SONET.

  • A section is the two basic physical building block and represents a single run of optical cable between two optical fiber transmitter or receivers. For shorter run the cable may run directly between two end For longer distances, repeaters are used. Repeater amplify the signals.
  • A line is a sequence of one or more sections such that the internal signal or channel structure of the signal remains Endpoints and intermediate switches or multiplexers that may add or drop channels terminate a line.
  • A path connects to end terminals, it corresponds to an end-to-end Data are assembled at the beginning of a path and are not accessed.



  • SONET frame consists of a 810 octets and is transmitted once every 125 µs, for an overall data of 51.84 Mbps. This frame is STS-1 building blocks. The frame can logically be viewed as a matrix of 9 rows of 90 octets each, with transmission being one row at a time, from left to right and top to Out of 90 columns (octet), the first three columns are allocated for transport overhead. (3 octets X 9 rows = 27 octets). Nine octets used for section overhead (3 rows, 3 columns) and 18 octets for line overhead (3 columns, 6 row) total of 27 octets of transport overhead. Fig. 8.4.8 shows frame format.

  • 87 columns and 9 rows i.e. 783 octets are called the synchronous payload enveloper (SPE). In SPE, 9 bytes (1 column, 9 row) is used for path overhead. SPE contains user data and path overhead. Path overhead used for maintenance and diagnostics at each of the circuit. Fig. 8.4.9 shows the arrangement of path overhead octets. This format is general format for higher rate frames.

  • SONET offers a standard drop-and-insert capability and it applies not just to 64 kbps channels but to higher data rates as well. SONET makes use of a set of printers that locate channels within a payload and the entire payload within a
  • Then information can be inserted, accessed and removed with a simple adjustment of Pointer information is contained in the path overhead that refers to the multiplex structures of the channels contained within the payload. A pointer in the line overhead serves a similar function for the entire payload. The synchronous payload environment





(SPE) of an STS-1 frame can float with respect to the frame. The actual payload (87 columns X 9 rows) can straddle two frames. Fig. 8.4.10 shows location of SPE in STS-1 frame. The H1 and H2 octets in the line overhead indicate the start if the payload.

  • Because even the best atomic timing sources can differ by small amounts, SONET is faced with coping with the resulting timing differences. Each node must recalculate the pointer to alert the next receiving node of the exact location of the start of the
  • The payload is allowed to slip through an STS-1 frame, increasing or decreasing the pointer value at intervals by one byte position. If the payload is higher than the local STS frame, rate, the pointer is decreased by one octet position so that the next payload will begin one octet sooner than the earlier
  • To prevent the loss of an octet on the payload that is thus squeezed, the H3 octet is used to hold the extra octet for that one frame. If the payload rate lags behind the frame rate, the insertion of the next payload is delayed by one

Virtual Tributaries (VT)


  • VTs are small containers that are used to transport used payloads. In SDH, these small containers are called virtual containers.
  • VTs come in certain predetermined
  1. A VT with a 3 column capacity, or a total of 27 bytes, is known as VT1.5.
  2. A VT with a 4 column capacity, or a total of 36 bytes, is known as
  3. A VT with a 6 column capacity, or a total of 54 bytes, is known as
  4. A VT with a 12 column capacity, or a total of 108 bytes, is known as VT12. 8.4.11 shows a virtual tributaries.
    • In SDH, a 5called a TU-11, a VT2 is called a TU-12 and a VT6 is called a TU-2.
    • The following table lists detail sof all VTs and payload rates.








VT Type





VT pasyload rate (Mbps)






























overhead Definition


Section overhead : SONET


  • The first three rows of the overhead space in an STS-1 frame, a total of 9bytes carry synchronization and section overhead information.
  • 8.4.12 shows STS-1 section overhead.

  • The first two bytes of an STS-1 frame contain a fixed pattern, known as A1 and A2. This pattern, OXF628 or in binary 1111 0110 0010 is used by the receiver to detect the beginning of the frame and thus synchronize with
  • The remaining 7 bytes in this overhead section are :
  1. A1 and A2 contain a fixed framing pattern and are set at the hexadecimal value OXF628 (1111 0110 0010 1000). A1 and A2 are not
  2. C1 is the STS-1 ID and is defined for each STS-1.
  3. B1 is a byte used for error monitoring.
  4. E1 is a 64 kbps voice communication channel for craft personnel.
  5. F1 is used by the section.
    • D1 to D3 constitute a 192 kbps communication channel between STEs. This channel is used for alarms, maintenance control, monitoring, administration and other communication
    • In an STS-N signal, this channel is defined for the first STS-1 only. The other N-1 channels are not used.

Line Overhead : SONET


  • Rows 4-9 or a total of 45 bytes, carry the line overhead information and shown in 8.4.13.



  • These bytes are defined as
  1. H1 and H2 define the offset between the pointer and first SPE
  2. H3 defines an action byte for frequency, justification It carries valid payload if the justification is negative.
  3. BIP-8 is used for locating
  4. K1 and K2 are used for automatic protection switching. In STS-N this is defined for #1 only.
  5. D4 and D12 constitute a 576 kbps communication channel between line terminal equipment for alarms, maintenance, control, monitoring, administration and other communication
  6. Z1 and Z2 are not defined. In STS-N this is defined for #3. Z2 is only defined as line for end block
  7. E2 is an express 64 kbps communications channel between LTE. In STS-N this is defined for #1


Section Overhead : SDH


  • The first three rows of the overhead space are called the regenerator section overhead (RSOH), the fourth row is called the administrative unit pointer, and the remaining five rows are called the multiplex section overhead (MSOH).
  • The first 2 bytes of the RSOH contain a fixed pattern, known as A1 and A2. This pattern , OXF628 or in binary 1111 0110 0010 1000 is used by the receiver to detect the beginning of the frame.

Payload Pointers





  • Fig 4.14 shows the payload pointers. The two pointers, bytes H1 and H2, contain the actual pointer value. Bytes H1 and H2 contain much more information than a value

  • The first 4 most significant bits in H1 byte are known as the new data found (NDF) The NDF may be “normal =0110” or “set = 1001”.
  • The next 2 bits are known as the S-bits and indicate the size of the virtual tributary in the
  • The last 2 least significant bits of the H1 and the 8-bits of the H2 define two bit alternating S-bit words.
  • The I and D are used for incrementing or decrementing the offset.
  • Although pointer bytes H1 and H2define an offset value, the third pointer, byte H3 does not contain an actual pointer
  • 8.4.15 shows pointer H3.

·         Functions of the H1, H2 and H3 bytes

  1. Identifies that a change has occurred in the pointer value (NDF = 1001) due to an intermittent synchronization change in the node and where the new start is (I + D bits).
  2. Identifies that a change may have occurred in the pointer value (0110) due to a frequency difference between node and incoming frequency.
  3. The bits that contain the pointer values I and D, indicate whether negative or positive frequency justification is necessary.


Frequency Justification


  • When the frame rate of the STE SPE is the same as the transport overhead, the alignment of the SPE is the same as in the previous frame. This is known as no justification.
  • When the frame rate of the STE SPE is less than the transport overhead (OH), the alignment of the SPE is skipped back by a This is known as positive justification.
  • When the frame rate of the STE SPE is higher than the transport OH, the alignment of the SPE is advanced by a This is known as negative justification.
  • 8.4.16 shows no frequency justification.

  • Example : Consider that the H1, H2 and H3bytes are as in Fig. 8.4.16. In this case, the H1 and H2 contain a NDF value of 0110, indicating that no change in the pointer has

The I and D bits have not been inverted indicating no justification. The I, D value is set to 00 0010 1101 = 45. The H3 byte is 00000000.

  • 8.4.17 shows the positive justification.

H1, H2 = 0110 0010 1000 0101





H3 = 00000000


X = 1-bits inverted Next frame (n+1) :

Pnew = P + 1


or             H1, H2 = 0110 0000 0010 1110


H3 = 00000000





  • When the complete frame has been assembled, the bytes in it are Scrambling is performed to assure the receiver that a density of 1’s is maintained in the signal.
  • The A1, A2 and C1 bytes are not scrambled and the scrambling process begins with the byte right after This is shown in Fig.8.4.18. this applies to both SONET and SDH.

STS-N scrambler


  • With respect to the scrambler the following rules apply :
  1. The scrambling code is generated by the polynomial 1++
  2. The scrambler is frame synchronous at the line rate (STS-N) and it has a sequence length of 127 bits.
  3. The scrambler is set to 11111111 on the MSB of the byte following the Nth STS- 1 C4 byte.
  4. The framing bytes A1, A2 and the C1 from the first STS-1 through the Nth STS-1 are not scrambled.
  5. The scrambler runs continuously throughout the complete STS-N frame.


Layered Overhead and Transport Functions


  • The functional sequence that takes place, for example, from a DS1 signal to a SONET signal, can be summarized as follows :
  1. The incoming DS1 signal at the path layer is mapped onto a VT.
  2. The VT is mapped onto the SPE and the SPE path overhead is also constructed.
  3. The SPE is mapped onto the SONET signal and the line overhead information is
  4. The signal is mapped onto the STS-N signal and the section overhead information is At this point the complete STS SONET signal is formed and the signal is scrambled.
  5. The signal passes through the electrical to optical transducer and the optical signal with a NRZ optical coding is coupled into the optical fiber in which it travels at the speed of light.
  6. 8.4.19 shows the above process.







  1. High speed backbone
  2. Basic architecture for B-ISDN .
  3. Basic architecture for
  4. High speed optical network for data communication.