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An Introduction to the SDH Protocol| Category | | | Summary | What is covered in this article?
This article looks at the basics of the SDH protocol. This includes a basic introduction, transmission rates, the STM-1 frame structure and the section overhead structure. The aim of this article is to give the |
| | Body | What is covered in this article?
This article looks at the basics of the SDH protocol. This includes a basic introduction, transmission rates, the STM-1 frame structure and the section overhead structure. The aim of this article is to give the reader a basic understanding of the SDH protocol and lead on to further, more advanced, articles.
What is SDH?
SDH stands for Synchronous Digital Hierarchy. It is a synchronous transmission protocol that has been developed for use in high speed communication systems. There are several standards that set out how SDH works and how it works with other standards such as the ANSI SONET standard. When the SDH standards were defined, the opportunity to address most of the long-standing problems of PDH was taken. The features and advantages of SDH are summarized as follows:
- SDH is apparently synchronous. All the SDH recommendations are based upon the use of a SINGLE reference clock.
- SDH permits the mixing of existing North American (ANSI) and European (ETSI) PDH systems.
- SDH provides much simpler extraction and insertion of any lower-order Tributary stream from a high-order Tributary stream or frame structure without the need to completely de-multiplex all the lower-order Tributaries. [The Drop And Insert Admux Function]
- SDH provides cross-connection of any lower-order Tributary stream to any other lower-order stream without the need to completely de-multiplex. [The Cross-Connect (DXC) Function]
- SDH provides an optical interface standard thus allowing the inter-working of different manufacturers' equipment.
- SDH dramatically reduces the quantity of equipment and inter-connections.
- SDH is compatible with existing PDH networks.
- SDH allows for integrated network management and performance-monitoring using centralised network control.
- SDH provides higher order data rates by BYTE interleaving without extra FAW and justification.
- SDH standards have allowed for, and are prepared for, future applications such as Asynchronous Transfer Mode (ATM), High Definition Television (HDTV) and Metropolitan Area Networks (MAN).
SDH Transmission Rates
The initial SDH transmission rate is defined as 155.52 Mbit/s Synchronous Transport Module (STM-1). A Synchronous Transport Module (STM-n) is a precisely defined signal frame structure and transmission rate, used to carry information between SDH network elements. This article will focus on STM-1, although there are others: further SDH transmission rates are direct multiples of 155.52 Mbit/s. At present there are five:
- STM-1 - 155.520 Mbit/s (155 Mbit/s)
- STM-4 - 622.080 Mbit/s (622 Mbit/s)
- STM-16 - 2488.320 Mbit/s (2.5 Gbit/s)
- STM-64 - 9953.280 Mbit/s (10 Gbit/s)
- STM-256 - 39813.120 Mbit/s (40 Gbit/s)
STM-1 Frame Structure
The STM-1 frame is nominally 125 microseconds long and consists of 270 bytes by 9 rows (or segments). As each byte within the STM-1 frame occurs once per 125 microseconds, they each represent a data channel of 64 kbit/s. The aggregate data rate is:
64 x 1000 x 270 x 9 bit/s = 155.52 Mbit/s
An STM-1 frame is shown below.
The first 9 bytes of a row are used for overheads, the remaining 261 bytes for payload.
A frame is usually represented on a drawing as an assembly of colunms and rows, where a row is shown horizontally in a vertical stack of nine rows. The frame commences at the top left corner of the stack and finishes 125 microseconds later at the bottom right corner; the overhead bytes therefore align vertically as the first nine columns of the structure with the payload being contained in columns 10 to 270. These overhead bytes are named the Section Overhead (SOH), and are sub-divided into the regenerator and multiplex section overheads.
Figure 1 : STM-1 Frame
The payload, however does not have any fixed relationship to the Section Overheads, as illustrated below. The AU Pointer bytes indicate the matrix address of the first byte in the STM-1 payload in each STM-1 frame:
Figure 2: STM-1 Frame showing typical payload offset
STM-1 Section Overhead
Most of the bytes in the section overheads have been allocated by the ITU-T, their usage is shown below:
Figure 3: STM-1 Section Overheads
A1, A2 Framing bytes. the A1 bytes have the Hex value F6, and the A2 bytes have the hex value 28.
J0 Regenerator Section Trace (16 byte frame including CRC7) supports continuity testing between transmitting and recieving device on each regenerator section.
B1 An 8-bit Byte-interleaved parity (BIP-8) check sum which is monitored and calculated by every regenerator and multiplexer on an SDH line. This check sum is calculated on the entire STM-1 frame after scrambilng and is inserted into the B1 byte of the following frame before scrambling occurs.
E1 Order Wire for the repeater section. this byte, repeated 8000 times per second like all the other bytes in the frame, provides a 64 kbit/s channel. It is intended for voice transmission in regenerator-maintanance applications.
F1 User Channel. This byte is intended for transmission of digital information in regenerator-maintanance applications.
D1 - D3 These 3 bytes provide a 192 kbit/s Data Communications Channel (DCC) for operation and management of the regenerators on an SDH line.
B2 3 bytes that carry the 24-bit byte-interleaved parity (BIP-24) checksum which is calculated and inserted into the B2 bytes by the multiplexer that transmits the SDH signal. The multiplexer that recieves the signal will also calculate the check sum and compare it to the contents of the B2 bytes. This checksum is calculated before scrambling, on the entire STM-1 frame except for the bytes contained in the RSOH. The value is then placed into the B2 bytes of the following frame before scrambling.
K1, K2 (1-5) - MS Automatic Protection Switch signalling
K2 (6-8) - Used to indicate MS Remote Defect Indication (MSRDI).
D4 - D12 These 9 bytes provide a 576 kbit/s Data Communications Channel (DCC) for operation and management of multiplexers on an SDH line.
E2 Order Wire for the multiplex section. This byte is similar to the E1 byte for the repeater section.
M1 MS Remote Error Indication, MS-REI.
S1 (1-4) MSH.
S1 (5-8) SMA.
Z Bytes Reserved for future use.
Nat Bytes reserved for National Applications.
* Unscrambled Bytes.
Other Articles
- SDH Protocol Structure: Complete structure including the structure of containers, virtual containers, tributary units, tributary unit groups, administrative units, and administrative unit groups.
- SDH Intermediate topics: Pointers, justification, clocks, synchronisation.
- SDH Networks: Management, maintenance and standards.
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This article was imported from zZine. (original author: Tacheon)
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