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).

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.

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,
• Line Layer,
• Section Layer,
• Photonic Layer.

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:

• reads,
• interprets,
• modifies teh path overhead for performance and automatic protection switching.

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:

• Protection Switching,
• Synchronization,
• Multiplexing,
• Line maintenance,
• Error Monitoring.

Section Layer

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

• Framing,
• Scrambling,
• Error Monitoring,
• Section Maintenance.

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:

• Wavelenght,
• Pulse Shape,
• Power Levels.

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).

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:

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:

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:

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

Section Overhead

• Framing A1 and A2 are the two framing bytes, are dedicated to each STS-1 to indicate the beginning of a STS-1 frame. The A1, A2 bytes pattern is F628 hex(this F628 is never scrambled). When 4 consecutive errored framing patterns have been received, an OOF (Out Of Frame) condition is declared. When 2 consecutive error free framing patterns have been received, an in frame condition is declared.
• STS-ID C1, is a number assigned to each STS-1 signal in a STS-N frame in according to the order of its appearance, ie the C1 byte of the first STS-1 signal in a STS-N frame is set to 1, the second STS-1 signal is 2 and so on. The C1 byte is assigned prior to byte interleaving and stay with the STS-1 until deinterleaving.
• Section BIP-8 B1, is allocated from the first STS-1 of a STS-N for section error monitoring. The B1 byte is calculated over all bits of the previous STS-N frame after scrambling using a bit interleaving parity 8 code with even parity. Each piece of section equipment calculates the B1 byte of the current STS-N frame and compares it with the B1 byte received from the first STS-1 of the next STS-N frame. If the B1 bytes match, there is no error. If the B1 bytes do not match and the threshold is reach, then the alarm indicator is set.
• Orderwire E1, is allocated from the first STS-1 of a STS-N frame as local orderwire channel for voice communication channel. One byte of a SONET frame is 8 bits/125 usec or 64 Kbps which is the same rate as a voice frequency signal.
• User F1, is set for the user purposes. It is passed from one section level to another and is terminated at all section equipment.
• Data Communication D1, D2 and D3 are allocated from the first STS-1 of a STS-N frame. This 192 kpbs message channel can be used for alarms, maintenance, control, monitoring, administration and communication needs between two section terminating equipments.

Line Overhead

• Pointer H1 and H2, in each of the STS-1 signals of a STS-N frame is used to indicate the offset in the bytes between the pointer and the first byte of the STS-1 SPE. The pointer is used to align the STS-1 SPE in an STS-N signal as well as to perform frequency justification. The first pointer bytes contain the actual pointer to the SPE, the following pointer bytes contain the linking indicator which is 10010011 11111111.
• Pointer Action H3, in each of the STS-1 signals of a STS-N frame is used for frequency justification purpose. Depending on the pointer value, the byte is used to adjust the fill input buffers. It only carries valid information. But it is not defined for negative justification.
• BIP-8 B2, in each of the STS-1 signal of a STS-N frame is used for line error monitoring function. Similar to B1 byte in the Section overhead, but the B2 byte uses bit interleaving parity 8 code with even parity. It contains the result from the calculation of all the bits of line overhead and STS-1 envelope capacity of the previous STS-1 frame before scrambling.
• Automatic Protection Switching (APS) K1 and K2, are allocated for APS signaling between line level entities for line level bi-directional APS. These bytes are defined only for STS-1 number 1 of an STS-N signal.
• Data Communication D4-D12(9 bytes), are allocated for line data communication and should be considered as one 576-kbps message-based channel can be used for alarms, maintenance, control, monitoring, administration and communication needs between two section terminating equipments. The D4-D12 bytes of the rest of the STS-N frame are not defined.
• Growth, Growth/FEBE Z1 and Z2, are set aside for functions not yet defined.
• Orderwire E2, is allocated for orderwire between line entities. This bytes is defined only for STS-1 number 1 of a STS-N signal.

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:

• Trace J1, class A, is used by the receiving terminal to verify the path connection.
• BIP-8 B3, class A, is assigned for path error monitoring. The path B3 byte is calculated over all bits of the previous STS SPE before scrambling using bit interleaved parity 8 code with even parity.
• Signal Label C2, class A, is assigned to indicate the construction of the STS SPE. The following hex values of the C2 byte has been defined:

0x00 -- Unequipped signal,

0x01 -- Equipped signal,

0x02 -- Floating VT mode,

0x03 -- Locked VT mode,

0x04 -- Asynchronous mapping for DS3,

0x12 -- Asynchronous mapping for 139.264 Mbps,

0x13 -- Mapping for ATM,

0x14 -- Mapping for DQDB,

0x15 -- Asynchronous mapping for FDDI.

• Path Status G1, class A, is assigned to carry back an originating STS PTE the path terminating status and performance. This allows a complete duplex path to be monitored at either end.
• User Channel F2, class C, is allocated for user communications between path elements.
• Indicator H4, class C, provides a generalized multiframe indicator for payload.
• Growth 3 bytes, Z3-Z5 are class D, is reserved for future functions.

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:

See the report for some more details.