Introduction to SONET Network connections SONET Layers Frame Structure (STS-1) Scrambling

Introduction to SONET

SONET stands for Synchronous Optical NETwork. The SONET format allows different types of formats to be transmitted on one line. SONET is a long term solution for a mid-span-meet between vendors. The other major advantage is that SONET allows ADDING and DROPING signals with a single multiplexer.

Network connections

Communication between various localized networks is costly because differences in digital signal hierarchies, encoding techniques and multiplexing strategies. For example, the DS1 signals consist of 24 voice signals and one framing bit per frame. It has a rate of 1.544 Mbps. DS1 uses the AMI encoding scheme, it robs a bit from an eight bit byte for signaling. Therefore, it has a rate of 56 kbps per channel. But with the B8ZS bipolar violation encoding scheme, every bit is used for transmission. Therefore, it has a rate of 64 Kbps per channel. The CEPT-1(E1) signal consist of 30 voice signals and 2 channels for framing and signaling, its rate is 2.048 Mbps.

Therefor communication between different networks requires complicated multiplexing/demultiplexing, coding/decoding process to convert a signal from one format to another format. To solve this problem SONET standardize the rates and formats. The Synchronous Transport Signal (STS) is the basic building block of SONET optical interfaces with a rate of 51.84 Mbps. The STS consists of two parts, the STS ayload(data, carries the information) and the STS overhead(carries the signaling and protocol information).

All different types of formats are multiplexed to form a single SONET 51.48Mbits/s.

At the other ends of a communication system, it involves signals with various rates and different formats. A signal is converted to STS and travel through various SONET networks in the STS format until it terminates. The terminating equipment converts the STS to the user format.

Typical End-to-End SONET connection.

Path Terminating Equipment (PTE)

The STS path terminating equipment is a network element that multiplex/demultiplex the STS payload. The STS path terminating equipment assembles 28 1.544Mbps DS1 signals and inserts path overhead to from a 51.84 Mbps STS-1 signal.

Line Terminating Equipment (LTE)

The LTE is the network element that originate and/or terminates line signal.

Section Terminationg Equipment (STE)

STE can be a terminating network element or a regenerator. Its able to access, modify, terminate the overhead, or originate.

SONET Layers

SONET has four optical interface layers. They are:

Path Layer

The Path Layer deals with the transport of services between the PTE. The main function of the Path Layer is to map the siganls into a format required by the line layer. Its main functions are:

Line Layer

The line layer deals with the transport of the path layer payload and it overhead across the physical medium. The main function of the Line Layer is to provide synchronization and to perform multiplexing for the path layer. Its main functions are:

Section Layer

The Section layer deals with the transport of an STS-N frame across the physical medium. Its main functions are:

Photonic Layer

Photonic Layers main mainly deals with the transport of bits across the physical medium. Its main function is the conversion between STS signal and OC signals. Its main functions are:

Frame Structure (STS-1)

The STS-1 (Synchronous Transport Signal level 1) is the basic signal rate of SONET. The SONET adopts the frame length of 125 usec or frame rate of 8000 frames/sec. Each frame has 9-rows by 90-columns of octects(bytes) structure, a total of 810 octects(9-rows * 90-columns = 810 octects).

STS-1 Frame Structure.

Line Rate and Derivation of Line Rate

SONET line rate is synchronous and is flexible enough to support many different signals. The STS-1/OC-1 line rate is 51.84Mbps that accommodates 28 DS1 signals and 1 DS3 signal.

The first 3 columns are the transport overheads = 27 bytes.

Overhead bytes: 9rows * 3columns = 27bytes.

SONET has a an actual line of 51.84Mbps.

Actual Line Rate: 90columns * 9rows *8bits/byte * 8000frames/sec = 51.84Mbps

Columns 4 to 90 are the Synchronous Payload Envelope(SPE). Therefor the actual data rate is 50.112Mbps.

Actual Data Rate:

87columns * 9rows * 8bits/byte * 8000frams/sec = 50.112Mbps.

The STS-1 frame is transmited starting from the byte in row 1 column 1 to the byte in row 9 column 90. The most significant bit of a byte is transmitted first.

Higher line rates are obtained by synchronous multiplexing the lower line rates. The OC-1 signal can be repeted by N'th times(where the values of N are 1, 3, 12, 48 and 192. Therefor the OC-192 has a line rate of:

51.84Mbps * 192 = 9953.28Mbps

But as the line rates increases the percentage of overhead increases and in-tern the percentage of useful capacity for payload decreases. The additional overheads are used for control, parity, stuffing, alarm and signaling.

The actual percentage of SONET overhead is:

4columns * 100% / 90columns = 4.44%

The following figure shows the transport overheads and the path overheads of the STS-1 frame.

Transport Overhead

Section Overhead

Line Overhead

Path Overhead

The path overhead is assigned to, and transported with the payload. It is created by the PTE as part of the SPE until the payload is demultiplexed at the terminating path equipment. The path overhead supports the following four classes of operations:

  • Class A: payload independent functions, required by all payload type,
  • Class B: mapping dependent functions, not required by all payload type,
  • Class C: application specific functions,
  • Class D: undefined functions, reserved for future use.

  • Scrambling

    Scrambling and descrambling is necessary to make the data appear more random and guarantee the users privacy. The scrambling and descrambling is made possible with the following figure:

    Scrambler and Descrambler

    The technique used to scrambl and descrambl is a 127 bit synchronous frame operating at the line rate. The generating polynomial is 1+x^6+x^7. The scrambler resets to "1111111" on the most-significant bit of the byte following the STS-1 number N C1 byte. The scrambling is done after the multiplexing step, but before the C1 byte insertion and the electrical to optical convertion. Therefore the Frame byte(A1, A2) and the C1 bytes are NOT scrambled. The scrambler exclusive ORs the STS-N frame starting from the byte right after the Nth C1 byte with the 127 bit sequence. The sequence, generated by the pseudo random number generator is obtained by dividing the binary 111111 by 10000011. But the first 127 bits generated are always:

    111111100000010000011000010100 0111100100010110011101010011110100000111 00010010011011010110 110111101100011010010111011100 0101010

    See the report for some more details.

  • Nortel (northern Telecom)'s documentation,
  • PMC-Sierra's documentation,
  • "SONET Basic", Telephonoy,
  • "Digital Synchronization Network Plan",
  • Computer Communications; K.G. Beauchamp & G.S.Poo, International Thompson Computer Press,
  • PDH, Broadband ISDN, ATM & all that: "A guide to modern networking and how it evolved; P.Reilly, Silicon Graphics.